Nanoporous Sorbents for the Removal and Recovery of Phosphorus

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Nanoporous Sorbents for the Removal and Recovery of Phosphorus from Eutrophic Waters: Sustainability Challenges and Solutions Ali Othman, Eduard Dumitrescu, Daniel Andreescu, and Silvana Andreescu*

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Department of Chemistry and Biomolecular Science, Clarkson University, 8 Clarkson Ave., Potsdam, New York 13699-5810, United States ABSTRACT: Increased industrial and agricultural activity has affected the availability of phosphorus (P) in nature and has caused a significant imbalance in the P cycle with long-term consequences on ecosystem health and sustainability. While P is an essential element for food production as well as plant and animal nutrition, it is also a limited nonrenewable resource whose availability is expected to decrease in the next century. Widespread application of P-based fertilizers and their excessive accumulation in water bodies leads to eutrophication which is associated with overgrowth of harmful algal blooms and degradation of water quality. This paper provides an overview of contemporary challenges and methodologies for improving P use efficiency and sustainability in the environment. Technologies and processes for the removal of P-containing compounds from water through the use of functional nanomaterial sorbents with tailored surface properties for capture, removal, and recycling are described. Various classes of materials including carbon-based, zeolites, mesoporous silica, metal organic frameworks, metal oxides and hydroxides, biomass-derived materials, and P-binding receptors are reviewed along with their properties, binding affinity, and adsorption capacity. The potential of these materials to control the amount of P in the environment and create decision support tools for water resource management is also discussed, with examples of applications. KEYWORDS: Sustainable water management, Water resources, Phosphate, Nutrient reuse and recovery

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P that are limited and nonrenewable,6,7 and (iv) the inefficient mobilization of P resources across the globe and the associated economic impact. Managing and controlling the P cycle is essential for ensuring an overall nutrient balance and achieving equilibrium between beneficial and harmful effects. With the growing incidence of eutrophication, the scientific community is faced with the challenge to develop solutions to the problems associated with the limited supply, increasing demand, and damaging effects of P in the environment. This review discusses the latest advances in materials science and engineering, with a focus on nanoporous sorbents that can provide sustainable solutions to capture, recycle, and reuse P-containing nutrients from the environment and address sustainability challenges associated with the P cycle in the environment. Nanoporous sorbents represent a new wave of materials that can be engineered at the nanoscale level to monitor and control the amount of P-containing fertilizers. The surface of these materials can be modified to provide binding sites and form stable and reversible complexes to capture, remove, and recycle P in their nanoporous network. On the basis of these principles, nanoporous sorbents can be used in water treatment, filtration, and sensing. This paper outlines the varying types of nanoporous materials that can be used to bind and remove P and develop technology and decision support systems for water resources management.

INTRODUCTION The management of water resources is one of the most challenging problems faced at local and global levels. The water circuit has implications for both the availability of clean water sources for human consumption as well as for the normal functioning of natural ecosystems. Human activities and advances in industrial and agricultural processes have greatly affected the quality of water resources and impacted the nutrient cycle, which is essential for all forms of life. The main effects are observed in the nitrogen (N)- and phosphorus (P)-cycles in the ecosystem, two elements that are vital for living organisms. P is part of the building blocks of cells and plays an important role in bioenergetic processes and in the storage and processing of genetic information.1 While P is an essential element for all forms of life and its supply is needed to sustain crop production, its excessive use and release in the environment leads to unwanted consequences. When used constantly as a fertilizer to improve crop production, excess P can cause extreme growth of plants and toxic algal blooms in bodies of water,2 essentially acting as a pollutant when present in high concentrations. Enrichment of water by nutrients, e.g., eutrophication, is a major societal and economic problem that has gained renewed interest in the past decade.3−5 Main concerns are (i) ensuring the global agricultural needs and food production for an increasing population which is highly dependent on P resources, (ii) counteracting the environmental damages caused by the excess release of P nutrients (e.g., leaching from agricultural fields leading to eutrophication), (iii) depletion of natural reserves of © XXXX American Chemical Society

Received: April 22, 2018 Revised: June 19, 2018 Published: August 9, 2018 A

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P can be found in different forms, as inorganic phosphates26 and as a constituent of organic compounds, some of which are commonly used as pesticides in agriculture, e.g., organophosphates such as paraoxon, diclorvos, chlorpyrifos, and diazinon.27−29 Inorganic phosphates exist as PO43−, HPO42−, H2PO4−, or H3PO4, depending on pH.21 The different ionic forms can be found as neutral species in solution (H3PO4, pKa1 = 2.1) at low pH (2.1 ± 0.5) and as negative ions such as H2PO4− and HPO42− (pKa2 = 7.2 and pKa3 = 12.3, respectively) at pH values between 3 and 10. When the pH is above 12.3, phosphate is fully deprotonated, i.e., PO43−.30 The inorganic P forms may be present as both soluble or particulate matter.31 Particulate inorganic P is found in sediments combined with calcium, iron, or aluminum.32 In ocean water, soluble inorganic P represents the largest share of the total P.33 Speciation of P in smaller water bodies (lakes and rivers, ponds, etc.) is harder to predict, as variable composition of soluble or particulate, inorganic, or organic P are possible depending on the source of release. Variability is observed as well in the reported P concentrations for different water and waste streams. Mean concentrations of total P between 0.1 and 30 mg/L were reported for several different anthropogenic sources in the U.K.34 Estimations based on data collected in periodic national surveys conducted by the U.S. EPA suggest that increasing concentrations of total P are ubiquitous, with the median total P being 0.056 mg/L (2014 survey) in streams and 0.037 mg/L in lakes (2012 survey).35 While water streams contain relatively low concentrations of total P, animal waste streams may contain as much as 2.4 mg/L total P.36 The overall lack of homogeneity in the speciation pattern and concentration range of P species in water and waste streams is an impediment in the search for an universal approach for recapture and reuse of P.

PHOSPHORUS SOURCES, SPECIATION, AND CONCENTRATION Global food production relies on the use of fertilizers and pesticides containing P for the treatment of crops.8,9 The rapid population growth in the 20th century led to an increased use of P-based crop additives in order to sustain the food demand.10,11 Fertilizers and pesticides are produced using raw materials obtained from the mining of phosphate rock. On the basis of the available estimations, the global phosphate rock reserves are expected to decrease significantly in the next century.7,12 The latest estimates indicate that Morocco and the western Sahara have the vast majority of the available phosphate resources, followed by China, Algeria, Syria, and the United States among others.3,13 There is a general consensus within the scientific community that the current practices in the P supply chain are not sustainable in the long term.7 At the same time, the intensive use of P-containing fertilizers and pesticides has resulted in a significant increase in the global flux of P and a growing demand for industrial production.10,11 P is extracted from phosphate rock deposits, which are used as the primary source for the production of fertilizers. P is found in the runoff and waste streams originating from mining, agricultural industrial activities,14−16 and domestic wastewaters, e.g., food waste and sewage from domestic users.17,18 While about 80% of mined P ends up in fertilizers, pesticides, and animal feeds, almost half of this amount is lost in agricultural activities through soil leaching and erosion.8 Food production and consumption are also responsible for the loss of P in the food supply chain through food and human waste. It has been estimated for the U.S. that 85% of the mined P is lost in the environment somewhere along the food supply chain.19,20 While fertilizers and pesticides are needed in agriculture to maximize crop production, their presence causes excessive growth of biota in environments in which they are released, affecting aquatic ecosystems from the smallest streams to oceans.21 The excessive accumulation of P- and N-containing species in surface waters leads to eutrophication,2,16,22,23 which has severe consequences on the environment and human health (Table 1).24,25

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PHOSPHORUS IN THE ENVIRONMENT: CAUSES, CONSEQUENCES, AND EUTROPHICATION EFFECTS Well-known examples of water ecosystems affected by eutrophication are the Lake Erie basin,37,38 the Gulf of Mexico,39 the Great Barrier Reef lagoon,40 and the Baltic Sea.41 Excess nutrients in water determine an increased production and development of phytoplankton and micro- and macroalgae.42 As a consequence, aquatic ecosystems may suffer changes in species productivity and composition due to competition for light and reduced oxygen availability, leading to the formation of “dead or hypoxic zones” in water bodies.43,44 Vascular plants, coral reefs, and finfish species are mostly affected by poor health, migration, or death and extinction.40,45 From a health perspective, exposure to or ingestion of water with high content of phosphate can lead to impaired renal function, rhabdomyolysis, tumor lysis syndrome, or even death in humans.46 Another consequence of eutrophication is the release of toxins by blue-green algae formations (cyanobacteria).47−49 Exposure to cyanobacteria is associated with sickness in humans50 and wildlife mortality.51 The lack of clean water affects primarily communities with limited sources of drinking water and developing regions that rely on untested or untreated water sources. Poor water quality has direct economic and societal implications such as increased drinking water treatment costs, restoration of biodiversity in affected ecosystems, altered tourism and recreational activities in the region of affected lakes and water bodies, and decreased value of lakefront properties.52 The estimated total economic cost of eutrophication mitigation in the U.S. is around $2.2 billion, but

Table 1. Potential Effects of Eutrophication Caused by Excessive Loading of N and P in Lakes, Reservoirs, Rivers, and Coastal Oceansa Effects of eutrophication Increased biomass of phytoplankton and macrophyte vegetation Increased biomass of consumer species Shifts to bloom-forming algal species that might be toxic or inedible Increases in blooms of gelatinous zooplankton (marine environments) Increased biomass of benthic and epiphytic algae Changes in species composition of macrophyte vegetation Declines in coral reef health and loss of coral reef communities Increased incidence of fish mortality Reduction in species diversity Reduction in harvestable fish and shellfish biomass Decreased water transparency Oxygen depletion Taste, odor, and drinking water treatment issues Decreased drinking water resources Altered recreational and aesthetic values of the water bodies a

Adapted from ref 2. B

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this is most likely a conservative assessment.52 In the current state, the global P cycle is not sustainable without adopting measures for recovery and reuse. Strategies to prevent the excessive loss of nutrients in the environment3 and improve conservation and recycling are needed to reduce the harmful consequences of eutrophication (Figure 1) and achieve a sustainable P supply.26

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TECHNOLOGIES FOR PHOSPHORUS REMOVAL

Chemical precipitation, crystallization, biological processes, and adsorption are reviewed here as the most commonly known technologies for removal and recovery of P from wastewater. The charged forms of phosphate can be involved in electrostatic interactions with charged sorbents and participate in ligand exchange mechanisms with ionic species from the surface. Phosphates can also form complexes with a variety of metal (M) ions, as MPO4. These interactions constitute the basis of most P-removal technologies. Chemical Precipitation. A common method to remove P from wastewaters and reduce effluent concentrations below 1.0 mg P/L is through chemical precipitation. The process involves the addition of metal salts of aluminum, calcium, magnesium, or iron to react with soluble phosphate and form solid precipitates.57 In the chemical precipitation techniques, 80%−99% P can be removed and recovered from wastewater streams in the form of fertilizer (magnesium ammonium phosphate or struvite).58 Karapinar et al.59 utilized ammoniumloaded zeolite as a seeding material for precipitating calcium phosphate and suggested the possibility of using the precipitate ammonium-loaded zeolites as fertilizer. The efficiency of chemical precipitation is affected by alkalinity, organic matter content, and the coexistent metals.60 The process can generate a large amount of sludge due to the precipitation of P-bonded chemicals and release magnesium and aluminum that could have secondary effects on the environment. The method is sensitive to seasonal and diurnal variations in temperature and changes in feed concentrations which make this approach a less attractive option for wastewater treatment. Crystallization. An alternative strategy is to crystallize P by inducing supersaturation, nucleation, and crystal growth on seeding materials such as calcite, sand, magnetite, and a variety of Ca phosphate crystals.61−63 The distinction between precipitation and crystallization is generally based on the speed of the process and the size of the solid particles produced. Furthermore, instead of bulky sludge produced by precipitation, the crystallization process generates high purity phosphate crystal pellets that can be reused as an alternative to mined phosphate.64,65 This method is the basis of the Phosnix process, a commercially available technology that utilizes a column reactor with ancillary chemical dosing equipment for nucleation and growth of struvite crystals followed by ion exchange−precipitation. Biological Processes. Biological treatment thorough the use of photosynthetic organisms (algae, plants, and planktonic bacteria) which uptake P as an energy reserve represents a sustainable alternative for the removal of P. Soluble P is assimilated as polyphosphate by phosphorus accumulating organisms (PAOs) which utilize P within their cells for growth.66 The P can be recovered after separation of the accumulated biomass from the treated water.66 Biological aerated filters (BAF) are widely employed in wastewater treatment for nitrogen and phosphorus removal in a single reactor, with reduced space and cost requirements. To increase efficiency for phosphorus removal, BAFs are often used as a secondary treatment, after a pretreatment step using coagulants to chemically precipitate phosphorus. Addition of the coagulant directly into the BAF reactor may induce filter clogging and inhibition of the microbial activity. Other limitations of the biological treatment method are the sensitivity to seasonal variations in temperature and changes

Figure 1. Schematic of the supply chain of P from mining, industry, and agricultural use to human consumption. Phosphate rock is mined as a phosphorus source to produce fertilizers and pesticides for agricultural use. Inefficient use at each step of the extraction, production, and use, as well as food and human waste, represent other major sources of P. Excess nutrients impact water bodies by forming toxic blue-green algae (cyanobacteria) blooms with harmful consequences on human health and aquatic ecosystems.

While complete loss of P in the environment is unavoidable, steps can be taken to manage the effects of eutrophication in the most affected areas and provide a more sustainable and responsible use.53,54 Strategies to bring the lost P back in use from each step of its supply chain, while at the same time preventing release during food production, distribution, and consumption, and human waste treatment, are also needed.6 Recycling approaches are essential toward shifting from reliance on mined phosphate rock to a more effective utilization of P fertilizers obtained from sources within the food supply chain. Recycling methods can be developed to capture P from waste streams originating from industries relying on P use, runoff and erosion streams from agricultural and postharvest activities, crop residues and dairy operations, final food products waste, human waste, and sewage.36,55,56 Considering the importance of recycling P toward ensuring a sustainable food supply chain in the future, there is a need to develop technologies that can efficiently capture and recover P, enabling its reuse in the fertilizer industry. Progress of nanotechnology has stimulated the development of new nanoporous material sorbents with unique surface sorption properties that can be used in the treatment of P-rich water streams, waste streams, or runoff for P capture and recycling. C

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Figure 2. Proposed mechanisms for the removal of phosphate in acidic and alkaline conditions.

the environment, and therefore, functionality under relevant conditions of pH, temperature, and ionic composition should be demonstrated. Generally, the adsorption capacity is greater for higher surface area materials due to increasing pore volumes. Porosity and surface area characteristics are commonly measured using the Brunauer−Emmett−Teller (BET) method by nitrogen adsorption/desorption isotherms at 77 K. The adsorbent efficiency also depends on the surface charge of the material. Therefore, sorbents should be selected based on their isoelectric point (pHpzc) to ensure charge complementarity. Typically, adsorption involves weak electrostatic, Lewis acid-base, or hydrophobic interactions. These interactions are pH dependent, and binding can vary with the pH of the environment. The dominant species of phosphates, for example, HPO42− and H2PO4−, are negatively charged over a wide pH range. When pH is below the pHpzc of the adsorbent (i.e., pH < pHpzc), its surface is positively charged enhancing adsorption. On the other hand, at higher pH (i.e., pH > pHpzc), the adsorbent surface is more likely negatively charged, leading to decreased adsorption affinity (Figure 2). The binding affinity is another important consideration. The binding affinity is usually determined by fitting the data using the Langmuir isotherm equation (eq 1) through an overall consideration of the adsorption isotherms.

in feed concentrations and low efficiency for achieving removal of low P concentrations. Adsorption. The most simple and cost-effective method for P removal is through adsorption on porous materials.67 The method relies on the P-sorption properties of certain materials to bind and capture P in a targeted manner. Most adsorbents are carbon based, but other materials such as zeolites, metal oxides, functional polymers, and composites have been reported.68 Adsorption can involve weak physical forces through electrostatic or van der Waals interactions or chemical processes via formation of chemical bonds between the adsorbed P and the active functional sites of the sorbent. In contrast to earlier discussed techniques, adsorption is a highly efficient process, particularly for low P concentration. The success of the adsorption method depends on the selection of a suitable sorbent and the presence (or possibilities for grafting) of P-binding sites to its surface. Parameters to consider include the adsorption/removal capacity, kinetics, recyclability/reuse, compatibility, cost, and local availability.69 Furthermore, an ideal adsorbent should have large surface area, pore volume, and abundant accessible functionalities to sorb and desorb P, as well as increased stability and lifetime. Additionally, sorbents should be selective for P and able to remove P down to 50 μg/L, which is the U.S. EPA recommended limit to prevent algae blooms. Indeed, it is challenging to find a material that could meet all of the requirements of an ideal sorbent. More effective materials are needed to improve removal efficiency while also providing possibilities for recovery and reuse. Nanoporous materials could provide alternative sorbents with increased sorption efficiency due to their high surface area. In the next section, we discuss recent research on the development of nanoporous materials for adsorption, removal, and recovery of P

Ce C 1 = + e qe KLqmax qmax

(1)

where Ce (mg/L) and qe (mg/g) are the aqueous concentration and adsorbed concentration at adsorption equilibrium, respectively; qmax (mg/g) is the maximum adsorption capacity, and KL is the affinity coefficient. The KL can be further used for calculating the dimensionless separation factor (RL), which is defined as

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CHARACTERISTICS OF ADSORBENTS FOR REMOVAL AND RECOVERY Adsorption Characteristics. The properties of the adsorbent which include surface area, porosity (specific surface area, pore volume, and pore size distribution), surface charge, and the availability of P-binding sites strongly influence removal efficiency. Ideally, the sorbent should be regenerated for reuse, while conserving the same sorption characteristics for multiple cycles. At the same time, the captured P should be easily detached and freed from materials in order to be reused as a fertilizer. Adsorption can vary significantly depending on

RL = 1/(1 + KLC0)

(2)

where C0 is the initial concentration of the metal ions. The RL value indicates the shape of the isotherm. RL values between 0 and 1 indicate favorable adsorption.70 Also RL values can be either unfavorable (RL > 1), linear (RL = 1), or irreversible (RL = 0). RL also can be plotted versus C0. The decrease of RL with the increase in the initial C0 concentration is indicative of favorable adsorption of phosphate onto the adsorbent.71 The value varies with the C0 concentration. Examples of reported RL values for phosphate binding affinity are 0.3 to 0.6 at C0 D

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ACS Sustainable Chemistry & Engineering between 5 and 150 mg P/L72 and between 0.3 and 0.8 at C0 between 9−982 mg P/L onto Fe−Al−bentonite (FAB) following a monolayer adsorption model.73 The Freundlich adsorption isotherm can be plotted to determine the sorption capacity for nonideal sorption on heterogeneous surfaces and multilayer adsorption, which is defined as log qe = (1/n)log Ce + log K f

destabilize surface charge. However, addition of a high concentration of salt may result in high salinity to the soil that might have an adverse effect on plant growth if the phosphate recovered is reused as a fertilizer.82 Challenges of Designing Effective Sorbents. Several challenges exist when designing sorbents for P removal. These include interferences from other competing species, potential biofouling, large scale manufacturing, stability, and toxicity considerations of the sorbent material. Ideal sorbents should enable effective removal of P from streams that usually have a variable and complex composition. In general, wastewater contains significant concentrations of solid matter, dissolved and particulate matter, nutrients, cations and anions, and organic matter, as well as microorganisms.83 The concentrations of P species are likely much smaller than concentrations of typical cations and anions found in water bodies. Typical cations (e.g., calcium, magnesium, potassium, sodium) are present in concentrations of 4−70 mg/L, while typical anions (e.g., carbonate, bicarbonate, chloride, nitrate, sulfate) reach concentrations of 5−100 mg/L.84 Salt concentrations are also known to affect the removal capabilities of sorbents.85 Interference from cations and anions are usually reported for new materials developed for P adsorption; however, there are multiple other wastewater composition parameters not assessed.86 P sorbents should demonstrate performance in realistic environmental samples at relevant concentrations of P species and coexisting ions. Many matrix constituents can also cause fouling of sorbents, a major problem for filtration systems in wastewater treatment. Fouling can be caused by inorganic or organic matter, particulate or colloidal matter, or microbial/biological-derived matter (biofouling).87 The issue of fouling is rarely discussed in regard with P capture and wastewater treatment. Development of P sorbents would benefit from studies to understand the fouling mechanisms in wastewater systems, as well as the implementation of materials with antifouling properties. To ensure environmental sustainability, development of P sorbents should consider principles of green chemistry when dealing with eutrophication and nutrient pollution while trying to tackle the challenging issues related to P recovery, reuse, and pollution prevention. While the principles of green chemistry are generally applicable to all aspects of research and development,88 achieving sustainability in nanotechnology is still an area that needs to be addressed.89 Only a few reports have highlighted green approaches for the synthesis of P-capture materials.90−93 Implementation of nanotechnology-enabled materials in wastewater treatment is not risk free. The potential environmental impact of engineered materials represents a concern for toxicologists.94 From this perspective, it is important to assess toxicity characteristics of developed P sorbents, evaluate potential leaching and dissolution during use, and consider the overall environmental impact of disposed materials. Many of the reported approaches do not consider the environmental health and safety implications of these materials. Variable levels of toxicity have been associated with the exposure to carbon-based nanomaterials,95 magnetic iron oxide nanoparticles,96 titanium dioxide nanoparticles,97 nanosized aluminum and aluminum oxide,98 manganese oxide micro- and nanoparticles,99 and nano- and microsized lanthanum oxide.100 There is limited information about risks associated with exposure to more novel materials such as nanostructured metal−organic frameworks and nanocomposites.101 Although these materials have been successfully

(3)

where Ce is the equilibrium concentration (mg/L), qe is the amount adsorbed at equilibrium (mg/g), Kf is the partition coefficient (mg/g), and n is the intensity of adsorption. High percent removal is generally obtained for low phosphate concentrations, but as the concentration increases the percentage removal decreases. When a large surface area sorbent is used, as in the case of nanoporous sorbents, numerous adsorption sites are available that can catch most phosphate when this is present at lower concentrations, whereas at high concentrations the adsorption sites become limited. Experimental data obtained from both the kinetics (e.g. pseudo-first- and pseudo-secondorder and intra-particle diffusion (IPD)) models) and adsorption isotherm study (e.g. Langmuir and Freundlich) can be used to describe the type of adsorption, e.g. chemisorption or physisorption, monolayer or multilayer. A composite material of magnesite and bentonite clay was able to remove more than 99% of phosphate between the range from 2 to 200 mg/L at pH 10 after 30 min incubation following a monolayer adsorption model, fitted to the Langmuir adsorption isotherm.74 Porous materials can serve as sorbents by themselves or they can be modified with organic or inorganic species to enhance P binding and improve selectivity.67 Specific P-binding receptors can be incorporated within/onto porous materials to facilitate separation and recovery. Therefore, the surface of nanosorbents can be carefully designed to provide binding sites for P which in addition to their porosity and large surface area provide promising features for enhancing recovery of P.75 For example, graphene alone is unable to retain phosphate due to electrostatic repulsion. However, reduced graphene that has been modified with positively charged metal ions (e.g., La3+) enabled removal of phosphate and nitrate through coordination with the carboxylate groups of the modified graphene.76 High performance adsorbents with P separation and recovery can be developed by incorporating P-binding receptors to supporting materials such as mesoporous silica structures, magnetic particles, or polymers. The rapidly growing class of supramolecular phosphatebinding receptors provides opportunities for enhancing sorption activity and selectivity for phosphate binding.77 The recyclability of the adsorbent defined by its ability to bind and release P compounds for multiple cycles is another characteristic that should be considered in order to ensure efficient recycling of the sorbent and enable the reuse of the bound P. Various regeneration techniques have been reported such as thermal (high temperature calcination),78 chemical, electrochemical,79 biological, and oxidizing80 methods. Thermal and chemical treatment methods are the most common due to their cost effectiveness and practicality. In the thermal process, the temperature and the regeneration time of the sorbent can affect the percent recovery in further cycles. Chemical treatment using acids and bases can desorb phosphate as a function of pH and provide an effective strategy to recover the bound phosphate by changing the pH environment. However, some adsorbents can be affected under extreme acidic and basic conditions81 and may leach into the environment. Salts have been also used as desorbing agents to E

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promising sorbents for the removal of P. These materials have high specific surface area and abundant binding sites, oxygen functionalities, and high thermal and chemical stability and can be easy functionalized to enhance adsorption/separation processes for the removal of pollutants.112−115 However, the nonspecific interactions between GR/GO nanosheets and the adsorbate might result in poor selectivity. To enhance the adsorption capacity and improve selectivity for phosphate anions over other interfering ions present in water, these materials are often modified with organics, metals, metal oxides/hydroxides, or magnetic particles.76,81,116,117 Examples of GR/GO-based adsorbents that have demonstrated P-removal capabilities include zirconia-functionalized GO (Zr-GO)117 and expanded graphite (EG) loaded with lanthanum oxide (EG-LaO).118 Zirconia-functionalized GO (Zr-GO)117 was synthesized, and its adsorption performance for the removal of phosphate in water was evaluated by batch and column tests. The use of nanosized ZrO2 dispersed on the graphite oxide surface enhanced the phosphate adsorption capability on Zr-GO compared with GO. Phosphate binding was suppressed by increasing pH. Regeneration and reuse of the sorbent was demonstrated over 11 consecutive adsorption−desorption cycles using alkali treatment by a 0.1 M NaOH solution. The equilibrium data fitted with the Langmuir adsorption isotherm indicated monolayer adsorption of phosphate on the surface of GO-Zr. Zirconia-functionalized graphite oxide demonstrated good performance for the removal of phosphate but the process required the use of toxic anhydrous toluene, which made the preparation procedure complex and difficult to implement at a large scale. An alternative one-step hydrothermal preparation method of zirconium-loaded reduced graphene oxide (RGO-Zr) sorbent was demonstrated with high adsorption ability for phosphate removal.119 Expanded graphite (EG) is another possible material that demonstrated high adsorption capacity. The maximum phosphate adsorption capacity of EG loaded with lanthanum oxide (EG-LaO)118 calculated using the Langmuir model was 12.6 mg/g. In the presence of interfering ions such as F−, Cl−, NO3−, CO32−, and SO42−, adsorption of phosphate decreased due to competition of these ions for the same binding sites as phosphate. Mahdavi et al. studied the removal of phosphate from aqueous solutions by using CuO and CNTs.120 The phosphate sorption capacities in the absence of competing anions were 15.4 and 23.9 mg/g (PO43−-P) for CNTs and CuO, respectively. Despite promising characteristics in water decontamination, the potential release of CNTs and multiwalled CNTs (MWCNTs), NPs, and nanocomposites could alter the diversity of the bacterial community in wastewater,121 and therefore, toxicity should be further evaluated when considering the use of these sorbents for large scale applications. Biomass-Derived sorbents. Along with conventional carbonaceous nanomaterials, biomass-derived materials have received a great deal of interest as a cost-effective and environmentally friendly approach to remove phosphate from wastewaters.66 Biomass-derived carbons or biochar have high porosity and a large surface area. These are commonly synthesized by pyrolysis from widely available and renewable feedstock. Common sources of carbon from biomass that have been explored for P- removal include refined aspen wood fiber,122 coir path,123 eggshell waste,124 and Staphylococcus xylosus biomass.125 Engineered biochar prepared from Mg-enriched tomato tissues as sorbents for P removal from aqueous solutions has been also reported.126 Dolochar, a solid waste produced in

proven as viable sorbents for P, there is still a need to assess potential hazards associated with their use within water and wastewater treatment technologies.102 In the following section, we review the main classes of porous sorbents, their properties, and applications for the removal, recovery, and conversion of P from environmental wastewaters.

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NANOPOROUS SORBENTS FOR THE REMOVAL AND RECOVERY OF P The many different types of nanoscale materials that can be created with defined pore structure (size, shape, connectivity, etc.) and controllable physicochemical properties can be used as adsorbents with high sorption capacity and improved Premoval efficiency. These can be used in a variety of configurations, as sorbents, catalysts, membranes, or filters and can be used in urban, municipal, domestic, and agricultural wastewater.103 Advantages of nanoporous materials include high surface area, large pore volume, and the possibility to control charge, accessibility, and availability of surface functional groups,104,105 all of which enhance chemical activity and adsorption capacity of contaminants within their structure. The surface of nanosorbents can be carefully designed or modified to provide binding sites for P making these materials promising candidates for the removal and recovery of P-containing compounds. Carbon-Based Materials. Carbonaceous-based materials, such as activated carbon (AC), graphene (GR), carbon nanoparticles (CNPs), and carbon nanotubes (CNTs) are some of the most promising materials for contaminant adsorption and wastewater treatment. Their applications can be extended to adsorption and removal of P. Activated carbon is the most widely used adsorbent to remove contaminants from water matrices.106 The highly porous adsorptive structure, high specific surface area and pore volumes, relatively low cost, and fast adsorption kinetics provide excellent properties for water treatment processes, with demonstrated capabilities for removal of organic pollutants, volatile organic compounds, dyes, and heavy metals.107 AC is produced by activation of carbon-containing materials using chemical agents (e.g., potassium hydroxide) or by physical processes (high temperature steam). Agricultural waste can also be used as a sustainable source.108 Following application, AC can be regenerated or disposed as waste.107 To increase P-binding selectivity over other anions that may be present in water such as Cl−, NO3−, CO32−, and SO42−, the carbon is typically modified with metals or metal oxides/hydroxides that have high P-binding capacity. Lanthanum (La) has high affinity toward phosphates by forming La phosphates at the surface, and therefore, La is one of the most commonly used surface modifiers to increase P-binding affinity.109 ACF-LaO adsorbents are characterized by good chemical and physical stability. Examples of sorbents for P removal include activated carbon modified with La oxide (ACF-La)109 and hydroxide (ACF-LaOH).110,111 Incorporation of La hydroxide into the framework of activated carbon fibers was shown to enhance phosphate adsorption. The strategy can be applied to other carbon-based materials. The content of La in ACF-LaOH determined by inductively coupled plasma (ICP) was 52.9 mg/g. After adsorption of phosphate, the La content decreased to 47 mg/g indicating some loss of La by precipitation. Long-term use of the sorbent may also result in damage and potential release of fibers from the adsorbent surface. Other forms of carbon, like GR and its oxidized form, graphene oxide (GO), as well as CNTs, are emerging as F

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demonstrated high sorption performance for phosphate anions in batch experiments.145 The adsorption of phosphate was pH dependent, increasing with increasing pH from 1 to 6.5 and decreasing above pH of 6.5. The material displayed a rapid sorption rate (99% of phosphate removal within 1 min) and the ability to remove low concentrations of phosphate to ∼10 μg/L, which is lower than the U.S. EPA’s established freshwater contaminant level for phosphorus (20 μg/L). Rapid phosphate sorption was explained by the rigid and open pore structure of the mesoporous silica, allowing easy access of phosphate anions to the binding sites.145 The adsorption was affected by high ionic strength conditions (0.1 M NaCl), but it was relatively unaffected by presence of bicarbonate, sulfate, and citrate anions. In another work, La-doped mesoporous hollow SiO2 spheres with an ordered hexagonal mesoporous structure enabled removal of phosphate with a removal capacity of 47.9 mg P/g when silica was loaded with 22.4% La. The mesoposous structure provided fast adsorption kinetics with phosphate removal capacity of 99.7%.30 High removal rates were also obtained with SiO2 NPs deposited on activated carbon nanocomposites.146 Nanomaterials of magnesium hydrosilicate Mg3Si2O5(OH)4 having an interlamellar structure enabled recovery of phosphate and ammonium from wastewater147 with adsorbing capacities of ∼19 mg/g for phosphate and 7.8 mg/g for ammonium. The phosphate sorption characteristics of mesoporous silica-based, metal oxides/hydroxides/sulfate, and carbon-based materials have been reviewed recently by Huang et al.148 Zeolites. Zeolites are porous aluminum silicates with an opened framework architecture of [SiO4]4− and [AlO4]5− and high cation exchange and ion adsorption capacity allowing for the removal of inorganic ions, trace, and radioactive elements in water purification applications.149,150 Zeolites are obtained from natural sources at low cost. Surface modification with La or other ions increases their P-sorption properties making them valuable materials for remediation of P-enriched waters.151,152 A zeolitic material synthesized from fly ash and its calcium-modified form demonstrated phosphate-sorption capacities of 57 ± 5 and 203 ± 11 mg P-PO4/g for the native and Ca-zeolite sorbent, respectively.153 The removal mechanism involves complexation through the AlOH surface groups of the zeolitic structure originally present as Al oxides or through formation of Ca-phosphate phases, mainly brushite. The process is pH dependent and showed a maximum sorption capacity at pH 8.153 Phosphate removal efficiency of a synthetic Al3+-activated zeolite (HUD) was shown to be affected by coexisting ions.69 Using the Al-HUD system, ions like Cl−, SO42−, and NO3− slightly improved removal rate through outersphere complexation with P-binding sites, while fluorides (F−) showed reduced removal capacity. Hybrid adsorbents of a zeolite from coal fly and lanthanum hydroxide (La-ZFA) reached an absorption capacity of 66.09 mg P/g through phosphate binding to LaOH and showed 97.3% removal efficiency of phosphate from lake water and 97.9% from a wastewater effluent. The process was affected by the presence of anions (Cl−, NO3−, SO42− ,and HCO3−).93,152 The phosphate-sorption mechanism on zeolites and the sorbent−phosphate interactions involving charged AlOH sites for hydroxylated, neutral, and protonated forms at different pH values can be depicted as69

the sponge iron industry, has demonstrated high P-removal efficiency.127 Other promising adsorbents include modified wheat residue,128 activated fruit juice (Citrus limetta) residue, rice husk,129 and agro-waste rice husk ash,130 as well as activated sewage sludge.131 The main limitation of biochar is the leakage of P under different environmental conditions which can affect eutrophication.132−135 For example, the release of P from biochar by two types of Lysinibacillus strains ( Lysinibacillus sphaericus D-8 and Lysinibacillus fusiformis A-5) reached 54% and 47%, respectively. In addition, the surface and physicochemical properties of the biochar have changed during the adsorption process which may be caused by microbial activity.136 About 2.2 mg g−1 P was released from a raw biochar (contained 4.7 mg P g−1) at an initial pH of 9.0 in the first 8 h. The release of P (orthophosphate) was significantly enhanced by coexisting anions Cl−, NO3− ,or SO42− due to competitive ion exchange effects.133 Adsorption on these materials is achieved by specific interactions between the phosphate molecules and carbonaceous network, modified with P-selective sorbents, as discussed earlier. These include composites with layered hydroxides, nanostructured metal oxides, or the carbon network impregnated with La to increase sorption capacity.137−140 Figure 3

Figure 3. Schematic representation summarizing possible mechanisms for phosphate adsorption on pristine and modified nanoporous sorbents.

summarizes possible mechanisms for phosphate adsorption on pristine and modified porous carbonaceous materials, selected here as a model sorbent. Similar mechanisms occur on other sorbents such as double layer hydroxides, metal oxides, and polymer composites. Silica-Based Materials: Mesoporous Silica. Ordered mesoporous silica (SiO2) with pore sizes between 2 and 50 nm and silica-ceramic materials have attracted significant interest as porous sorbents for environmental applications due to their low toxicity, structural stability, and chemical resistance.141 Mesoporous silica functionalized with metal-chelate ligands showed excellent anion exchange behavior for arsenate, molybdate, and chromate.142,143 Silica gels have been modified with siloxane (Si−O−Si) or silanol groups (Si−OH) to enhance hydroxyl functionalities and provide coordination sites for binding of phosphate. Functionalization with amine groups (mono, di, or tri, i.e., N-silane) has also been shown to enhance adsorption of phosphate on mesoporous silica.144 Cationic metal− EDA complexes anchored inside mesoporous silica MCM-41 supports (Cu(II)−EDA-SAMMS and Fe(III)−EDA-SAMMS)

A1OH + H+ ↔ A1OH 2+ A1OH + OH− ↔ A1O− + 2H 2O

pH < pH pxc

(4)

pH < pH pxc (5)

G

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polymer was used to stabilize the particles.168 A sandwiched 2D MXene-iron oxide (MXI) material, prepared by selectively exfoliating an Al layer followed by magnetic ferric oxide intercalation, exhibited high capture ability for trace phosphate sequestration.169 Compared with commercial adsorbents, the MXI nanocomposite demonstrated fast separation ability within 120 s and high treatment capacities of 2100 and 2400 kg per kg in simulated and real phosphate wastewaters. The high performance was explained by the structural morphology ferric oxide intercalation within the layers of MXene, widening the layer distance, and stimulating strong complexation of phosphate onto the embedded magnetic nanoparticles. Manganese oxides have been also explored for phosphate removal. Hydrous manganese oxide (HMO) is generally negatively charged at neutral pH and cannot effectively remove anionic pollutants such as phosphate. Pan et al. proposed a new strategy to enhance HMO-mediated phosphate removal by immobilizing nano-HMO within a polystyrene anion exchanger (NS).170 The HMO@NS nanocomposite exhibited substantially enhanced phosphate removal in the presence of sulfate, chloride, and nitrate. The increased performance was attributed to the pHpzc shift from 6.2 for the bulky HMO to 10.5 for the encapsulated HMO nanoparticles, where HMO NPs are positively charged at neutral pH. The ammonium groups of NS also favor phosphate adsorption. Cerium oxide (CeO2) has received increasing interest as a promising sorbent for P removal. The interest in CeO2 is due to its high surface charge density,171 rich functionalities, intrinsic surface reactivity, and rapid formation of cerium-ligand complexes to their surface.172,173 CeO2 is a relatively inexpensive oxide, and it has one of the lowest solubilities in acidic and basic media among other oxides. Binary oxides with properly modulated ratios can display higher catalytic activities and sorption characteristics as compared to single component nanoparticles.174 Su et al. developed a solvothermal process to synthesize a series of Ce/Zr binary oxide nanoadsorbents with high phosphate sorption characteristics. The highest phosphate adsorption capability of 112.2 mg/g was obtained for the mixed Ce0.8Zr0.2O2 oxide.175 Phosphate desorption was achieved by washing with NaOH solution. Additional magnetic capabilities were obtained with mixed oxides core−shell CeO2-functionalized (Fe3O4@SiO2−CeO2)176 and (Fe3O4@ SiO2@mCeO2)177 for achieving magnetic separation and recovery. Metal Organic Frameworks (MOFs). Metal−organic frameworks (MOFs) are an emerging class of crystalline organic−inorganic hybrid materials with high porosity and tunable morphology.178 Their versatile three-dimensional framework with tunable pore size, interconnected pores, and surface functionality provides a large specific surface area that can facilitate rapid and high capacity adsorption for removal and decontamination applications. While most MOFs are employed for gas sorption, investigations of their porous structure for the removal of pollutants from aqueous solutions are growing.179,180 To impart P-removal characteristics, MOFs have been modified with La, Zr, or Fe, which provide specific active sites that can act as hosts receptors for P. Removal is attributed to electrostatic attraction between the La3+ and its complexation and ligand exchange reaction with H2PO4− in the pores of the MOFs structure with a maximum adsorption capacity of 142 mg/g from water.181 The adsorption kinetics followed a pseudo-second-order model and is pH dependent. The sorbent was recycled by elution with NaOH solution,

Metal Oxides and Hydroxides. Metal oxides and hydroxides of iron,154,155 zirconium,156,157 aluminum,158 and manganese(IV)159 are some of the most widely studied materials for P adsorption, removal, and recovery via Lewis acid-base interactions and ion exchange mechanisms. The interest in these sorbents originates from their amphoteric characteristics, chemical stability, and cost-effective synthesis.82 While these materials can be used a sorbents by themselves, their capacity can be increased by loading them on high surface area materials like graphene, mixing with other oxides, or by doping with La. These composite structures can remove phosphates through combined surface adsorption, complexation, and ligand exchange mechanisms.30 Removal of phosphates with La-doped oxides can be shown as La−OH + H 2PO4 − ↔ La−H 2PO4 + OH−

(8)

La(OH)2 + HPO4 2 − ↔ LaH 2PO4 + 2OH−

(9)

La(OH)3 + HPO4 3 − ↔ LaPO4 + 3OH−

(10)

The adsorption of phosphate on manganese dioxide (δMnO2) in a simple electrolyte solution (0.7 M NaCl) and seawater was shown to occur through a surface complexation model with formation of outer-sphere phosphate complexes on the surface of δMnO2. Performance of this system in seawater varied with pH, temperature, and salinity.159 SO42− and humic acid were found to suppress adsorption at low pH but had no effects in the pH of seawater, while Ca2+ and Mg2+ enhanced adsorption at pH > 4 due to changes in surface charge and phosphate speciation at different pH values. High adsorption capacity of 588.4 mg/g was obtained with nanosized magnesium hydroxide impregnated on high surface area graphitized mesocarbon obtained from waste, which promoted multilayer adsorption.160 Precipitation of the metal ion with the phosphate could limit recyclability and reuse of these complexes for agricultural purposes.161 Safety considerations related to the potential release of transition metals in the environments are lacking and should be further studied. Iron oxide NPs are another class of oxides that have shown high P-adsorption capacity. These oxides provide added magnetic capabilities which facilitate facile removal and recovery through magnetic action. P removal can be achieved on zerovalent iron162 and on magnetic NPs coated with humic acid163 impregnated within nanosized La hydroxides164,165 or mixed with other oxides such as core−shell Ce−Ti@Fe3O4 NPs.166 Phosphate adsorption capacities of 28.9 mg/g (at pH 6) were obtained with humic acid-coated magnetic NPs and 116.3 mg/g (at pH 6−8) with nanosize La-doped magnetic graphene nanocomposites.167 Functionality of the La-doped composite was demonstrated for P removal from river and sewage water. Adding polymeric coatings on magnetic cores (i.e., nanomagnetic polymers) (NMPs) has been shown to enhance stability of nanodispersions and improve adsorption capacity. For example, a series of tetraethylenepentaminefunctionalized core−shell nanomagnetic Fe3O4 (TEPA-Fe3O4NMPs) demonstrated more effective P removal when the H

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bentonite (CSBent) with multivalent metal ions like Zr4+, Fe3+, and Ca2+).191 In addition to polyelectrolytes, a variety of polymeric hydrogels have been developed for removal of contaminants through electrostatic interactions. Fast removal of ammonium ions in a pH range from 3 to 8 was achieved using a hydrogel consisting of poly(vinyl alcohol), acrylic acid (AA), and tourmaline (Tm) with an entangled tridimensional network.192 Adsorption of both ammonium and phosphate ions in a pH range between 4 to 9 was demonstrated using a protein-grafted poly(potassium acrylate)/poly(vinyl alcohol) (FP-g-PKA/PVA) hydrogel produced through graft copolymerization.193 Several hybrid polymer nanocomposites have been reported for phosphate removal, e.g., a hybrid polymer nanocomposite (HPN-Pr) biofilm reactor,194 polyaniline/ Ni0.5Zn0.5Fe2O4 magnetic nanocomposite,195 alginate/Fe(III) capsules,196 and alginate beads.197,198 Examples of other commonly used sorbents with their sorption characteristics are presented in Table 2. Supramolecular Receptors. Supramolecular receptors engineered to bind and release nutrients are a growing class of materials with promising P-binding properties that could be incorporated in porous sorbents. Several types of P-binding receptors have been reported. Johnson, Hayley, and collaborators have synthesized an anion-modulated supramolecular switch that selectively binds dihydrogen phosphate, H2PO4−.199 The receptor consists of a bisurea-based anion bearing a bipyridine core unit that is capable to preferentially coordinate H2PO4−. Flood and collaborators have reported an acid-based multistate switching of organophosphates in cyanostar complexes, which could be used in the future as potential receptors for the binding and release of organophosphates.200 Phosphates oligomerization in cyanostar macrocycles induced formation of stackable higher order assemblies.201 The principle concept can be expanded in the future to create molecular assemblies for phosphates recognition upon complexation. Other materials such as high surface mesoporous cross-linked β-cyclodextrin polymers developed by Dichtel et al.202 for removal of organic micropollutants featuring easier and cheaper regeneration could be adapted to P-containing compounds with potential for low-energy, flowthrough water purification. Pierre and collaborators developed a gadolinium-based supramolecular receptor that has the ability to catch and release phosphate from water.77 The complex has two open coordination sites that bind phosphate in a pH-dependent manner, providing opportunities for the design of pH-dependent catch-and-release systems for the removal of phosphate from surface waters. In water at neutral pH, the complex binds phosphate with high affinity (Ka = 1.3 × 104) via the formation of a ternary complex in which one phosphate replaces both inner-sphere water molecules. Under strong acidic conditions, the complex releases phosphate (Figure 4). This pH variability has been used as a recycling scheme, demonstrating reusability for at least 10 cycles. The complex was selective for phosphate over other anions, including HCO3−, HCO2−, CH3CO2−, SO42−, NO3−, NO2−, BrO3−, AsO4−, F−, Cl−, and Br− and to a lesser extent for ClO3−. Figure 5 provides a summary of the different types of nanomaterials that can be used as sorbents for P removal and their desired properties. These include but are not limited to C-based and activated carbonaceous materials like graphene and carbon nanotubes (CNTs), zeolites, nanostructured metal oxides and hydroxides, MOFs, and functional polymeric materials.

achieving 90% regeneration after three adsorption−desorption cycles. Fe-based MOFs MIL-101 and NH2-MIL-10171 and hollow magnetic Fe3O4@NH2-MIL-101(Fe)182 demonstrated high sorption capacity for phosphate removal from eutrophic waters with maximum adsorption capacities of 107.7 and 124.4 mg/g−1 for MIL-101(Fe) and NH2-MIL-101(Fe), respectively. To prepare the Fe3O4@NH2-MIL-101(Fe), hollow porous magnetic (Fe3O4) was used as a template which was then coated with NH2-MIL-101(Fe) on its surface. The porous surface functionalized hollow structure has enabled selective removal and magnetic recovery of phosphates from aqueous solutions to concentrations of 45 ppb in 50 minutes.182 Another MOF, La-MOF-500, was designed to have a hierarchical micro/nanostructure183 which provided a phosphate capture capacity of 173.8 mg P/g due to the high acessibility of lanthanum active sites. The La-MOF-500 was shown to remove phosphate to less than 10 μg P/L. Polymeric Materials. Surface charged polymeric materials can be used as sorbents on supporting materials to enhance adsorption properties and permeability of phosphate ions. Addition of polymeric coatings on magnetic cores (i.e., nanomagnetic polymers) (NMPs) has been shown to enhance stability of nanodispersions and improve phosphate adsorption capacity of a series of tetraethylenepentamine-functionalized core−shell nanomagnetic Fe3O4 (TEPA-Fe3O4-NMPs).184 Polymers can also be used to prevent aggregation effects102 and prepare charged nanocomposites with increased removal efficiency. A nanocomposite based on magnetite nanoparticles (Fe3O4-NPs) immobilized on the surface of a cationic exchange polymer, C100, demonstrated an adsorption capacity of 4.9 mg PO4-P/g at pH 7.165 In another work, hydrated La(III) oxide (HLO) nanoclusters immobilized inside the pores of a polystyrene anion exchanger, D-201, exhibited enhanced phosphate adsorption in the presence of competing anions (Cl−, SO42−, NO3−, HCO3−, and SiO32−), with 2− 4 times greater performance than a commercial Fe(III) oxidebased nanocomposite HFO-201.185 Other polymeric materials have been used as sorbents in adsorptive filtration media obtained by coating a filter or column materials with polymers, so that it can act simultaneously as a separation media and an adsorbent. Du et al. reported a functional polymeric ligand exchanger (PLE) for the removal of nitrate and phosphate from water down to 0.1 ppm, obtained by incorporation of La3+ into a Dowex ion-exchange resin.186 The column was regenerated by elution with 0.1 M hydrochloric acid. In another study, mixed metal oxide (Fe-Ti bimetal oxide) was coated on sulfonated polymer beads for enhanced phosphorus removal from membrane bioreactor effluents.187 The adsorption capacity of the coated beads was substantially enhanced upon Ti doping, which may be due to the presence of more positively charged surfaces. The selectivity for phosphate ions against Cl−, NO3−, and SO42− was high, with a selectivity factor higher than 25, but less selective against bicarbonate and organic materials. A commonly used biopolymer, chitosan that contains multiple reactive functional groups has been used to prepare polymeric composites, beads, and resins for phosphate removal. Examples include polymeric composites with chitosan and poly(vinyl alcohol) and polyethylene glycol (PEG/chitosan and PVA/chitosan),188 chitosan beads cross-linked with glutaraldehyde (GA) and epichlorohydrin (EP),189 chitosan-melamineglutaraldehyde resins (QCMGR),190 and chitosan-supported I

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ACS Sustainable Chemistry & Engineering Table 2. Examples of Adsorbents with Their Characteristics for P Adsorption Adsorbent

Surface area (m2/g)

Capacity (mg/g)

Adsorbent dosage (g/L)

pH

Equilibrium time

% Removal

Regeneration

ref

Coir-pith activated carbon Magnetic iron oxide Magnetite (Fe3O4) Fe−Zr binary oxide Iron oxide nanoparticle-chitosan Nanobimetal ferrites CuFe2O4 Amorphous zirconium oxide Hydrous niobium oxide Fe3O4@PAH−Gu ZrO2·nH2O core−shell Fe3O4@LDHs 3D Graphene−nanoparticle aerogel 3D graphene−La2O3 composite Cerium oxide nanoparticles Iron oxide NPs (IONP) La-doped mesoporous SiO2 ZrO2/SiO2 nanofibrous membranes Synthesized La(OH)3 Modified iron oxide-based sorbents PEI impregnated MOF (UiO-66) Cubic zeolitic imidazolate framework-8 Porous MgO Mesoporous magnesium oxide modified diatomite Zirconium(IV) loaded lignocellulosic butanol residue (LBR-Zr) lanthanum(III)-immobilized aminated lignin Poly(vinyl alcohol) (PVA) hydrogel beads Zirconium-loaded magnetic interpenetrating network chitosan/poly(vinyl alcohol) hydrogels Cross-linked chitosan bead Brown algae modified by molybdate (algae−Mo)

727.4 82.2 − 106.2 2.4 69.1 327 46 − − 133 210 − 121 − 278 17.4 153.3 140 1157 − 181 34.7 76.4

7.7 5.0 27.2 13.7 0.06 13.5 99.0 15 3.7−4 66 36.9 350 82.6 0.3−0.4 18.7 23.1 43.8 107.5 38.8 78.4 38.2 236 161 8.8

4 0.6 1.0 1 30 3 0.1 2 0.5 2 2 0.2 2 0.3 2 0.3 0.2 2.5 1−15 0.25 0.5 0.1 0.3 0.6

6.0 6.0 7.0 4.0 5−9 2.6 6.2 2 5−10 2 3 88 >80 99 >85 80 100 96.9 97.3 ∼100 85 >99 − 80 >90 − 72.8 −

− 1 M NaOH 1 M NaOH 0.1 M NaOH 5 mM NaOH 0.001N NH4OH 0.1 M NaOH H2O (pHs 5 and 12) 10 mM NaCl H2O (pHs 5 and 12) − − − − − 5% HNO3 0.1 M NaOH 3 M NaOH 0.1 M NaOH − − − − −

203 204 205 206 207 91 208 209 210 211 212 213 214 215 216 217 218 219 220 221 222 223 224 225

85.8 − −

65.8 11.5 50.8

1.25 50 0.25

6 3−9 6.5

1h 480 min 20 h

94 90 79

5 M NaOH 0.1 M HCl 0.05 M NaOH

226 227 228

− 1.44

52.1 149.3

0.4 10

8.5 5

30 min 60 min

− 98.8

229 230

Ferric oxides loading on granular biochar Mg/Al-LDHs modified biochar Silicate material (CSH@SiO2@MgO) Metal (hydro-)oxide coated sands Fe/Mn composites (TS-N) Copper-substituted zeolites (Cu-ZSM-5) Amphoteric straw cellulose (ASC) adsorbent Core−shell bioceramic/Zn-LDHs composites

219 12.3 33.9 3.5 68.9 376 0.57 −

0.96 81.8 − 0.32 26 33 38.6 0.36−0.77

20 2.5 0.4 5 1 30 2 100

− 3 8 7 5 8.6 4−7 7

24 h 60 min 1h 24 h 60 min 1.5 h 30 min 8−14 h

− >99 93.9 ∼100 − 98.9 − ∼72

synthetic hydrotalcite

600

47.3

−

7.8

24 h

−

La(III)-modified zeolite adsorbent (LZA)

−

24.6

0.8

6

3.5 h

99.5

− recycled without any modifications − − HCl (1 mol) − − NaCl (10 g/L) − 5 M NaCl + 0.1 M NaOH 10% NaCl + 3% NaOH 0.8 M NaCl

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MATERIALS FOR ORGANOPHOSPHATE PESTICIDES (OPPS) REMOVAL While this review focused primarily on materials for the removal of inorganic P, adsorbents for removal of organic P are also of interest as many P-containing compounds such as the class of organophosphorus pesticides (OPPs) are used widely in agriculture. Adsorption of organophosphates (paraoxon, diazinon, and structurally related compounds) was achieved on porous hierarchical organosilicate sorbents such as diethylbenzene-bridged organosilicate.27 Gu et al. fabricated Zr-based MOFs of UiO-67 and showed effective adsorption and removal of OPPs, glyphosate (GP) and glufosinate (GF), from aqueous solutions.241 Nearly 100% adsorption in 1 h of OPs was reported with humic acid-modified silica gel used as adsorbents for the removal of OPs in hexane.242 Adsorption capacities of

231 232 233 234 235 236 237 238 239 240

456 and 57.1 mg/g OPP dimethoate was achieved by adsorption onto Au nanospheres and nanorods, respectively.243 Nanostructured Ti/Ce oxides and their composites enabled effective reactive adsorption of toxic OP parathion methyl and dimethyl methylphosphonate. 244 OP flame retardants (OPFRs) removal from water was reported using ACs245 through a sorption process involving hydrophobic, electrostatic, hydrogen bonding, and π−π interactions. The ACs were successfully regenerated and reused at least four times with stable sorption capacity. Other studies report the use of an NaX zeolite for adsorption of trimethyl phosphate (TMP),246 graphene-coated silica for adsorption of 11 different OPPs,247 cellulose/graphene composite (CGC) for triazine,248 graphene-coated SiO2 NPS for phenanthrene,249 a graphene@ SiO2@Fe3O4 nanocomposite for the extraction of OPs J

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p-tert-butylcalix[4]arene-based magnetic sporopollenin, CalixEPPTMS-MS (4) for the removal of chlorpyrifos and diazinon,251 and the adsorption of organophosphate esters (OPEs) on CNTs.252 A MOF/GO hybrid nanocomposite (UiO-67/GO) enabled high-affinity capture of OPPs.253 Figure 6 shows an example of a fabrication procedure of a magnetic MOF-based adsorbent, Fe3O4@SiO2@UiO-67254, via a layer by layer assembly approach and its application for the simultaneous recognition, detection, and removal of glyphosate. A sorbent prepared by immobilizing zirconia and magnetite Fe3O4 NPs in chitosan enabled rapid adsorption of OPs from juice and water samples.255 Activated carbon derived from sieve-like cellulose/graphene oxide composites (ACCE/ G),256 and poly(ionic liquid) immobilized magnetic NPs (PILMNPs) synthesized via copolymerization of a 1-vinyl-3hexylimidazolium-based ionic liquid were also reported as effective sorbents for the removal of OPPs.245

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Figure 4. Representation of the pH-dependent recycling scheme for catch and release of phosphate using Gd-TREN-MAM complex (gadolinium-2,2′,2′′-(((nitrilotris(ethane-2,1-diyl))tris(azanediyl))tris(carbonyl))tris(4-oxo-4H-pyran-3-olate). Reprinted (adapted) with permission from ref 77. Copyright 2017 The American Chemical Society.

CONCLUSIONS AND FUTURE OUTLOOK One of the most challenging sustainability problems faced by society today is the quality and safety of water resources. Application of P-based fertilizers and pesticides, and their excessive accumulation by ecosystems, has generated increased occurrence of water eutrophication which generates an overgrowth of harmful algal blooms and degradation of water quality. The management and control of P-containing compounds has important environmental and economic implications. Over 50% of the P used in fertilizers (∼8 million tons per year) is lost through soil leaching and erosion from farms.257 Moreover, food wastes accounts for 1 million tons loss of P. Nationally, more than 80,000 miles of streams and rivers are impaired due to nutrient pollution, which is considered a substantial underestimate because only 25% of these water bodies have been assessed. Similarly, over 2.5 million acres of U.S. lakes, reservoirs, and ponds are impaired due to nutrient pollution, but only ∼43% of these areas have been assessed.257 The prevention of algal growth by removing the excess of P in freshwater and marine water would be beneficial in securing the safety of water ecosystems and reducing the cost of water treatment processes. Employment of P-capturing systems in regions with oversupply and the treatment of P-rich industrial effluents may help both reduce their damaging effects and create a source of recycled P. Recent technological advances promise improved efficiency of phosphate rock mining.258 However, the inefficient use of P-based products in agricultural activities calls for improved fertilization approaches.259,260 It was suggested that agricultural activities can take advantage more efficiently of the P already present in the soil.261 Breeding of crops with lowered fertilization requirements or the development of fertilizers that can release P locally can also minimize the need for P and limit runoff in water bodies.262

Figure 5. Summary of commonly used material sorbents and their properties for P removal.

residues (malathion, chlorpyrifos, isocarbophos, fenamiphos, profenofos) in environmental water,250 amino-substituted

Figure 6. Schematic illustration for the fabrication of the Fe3O4@- SiO2@UiO-67 MOF-based adsorbent and the mechanism of adsorption for the removal of OPPs. Reprinted (adapted) with permission from ref 254. Copyright 2018 The Royal Society of Chemistry. K

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This review summarized emerging classes of nanoporous materials that can be used as sorbents to capture, remove, and recover P-containing compounds released during industrial and agricultural activities. Reducing the amount of these species in waste or runoff streams could (i) result in improved water quality and (ii) minimized risk, causes, and consequences of water eutrophication and (iii) provide opportunities for recycling and reuse of P-containing nutrients and fertilizers and (iv) conservation of the limited P-resources. Nanoporous sorbents can be used to develop more efficient high performance water filtration and purification technologies for the recycling and management of P-containing nutrients in the environment. Their high sorption capacity can be used in the future to develop decision support systems for fieldmonitoring and rapid screening of region-specific P-content and determine a risk factor for implementing pollution prevention measures related to the excessive presence of these species. Recently reported supramolecular receptors that have shown to selectively bind phosphate at neutral pH and releasing it under acidic conditions77 could be grafted onto nanoporous sorbents to enhance P-removal capacity and increase selectivity. This area of research is still in its infancy, and we anticipate that future work will study systems and devices based on nanoporous sorbents, some of which may contain selective receptors incorporated within or grafted on their surface. Preparation of new materials and a closer investigation of the adsorption mechanisms in realistic conditions are also needed to improve efficiency and speed up implementation of these new technologies in the environmental and agricultural fields. Other considerations are the cost, availability, and large scale manufacturing of the adsorbent materials and that of the P-binding receptors for large scale applications. Research is also needed to establish stability and the potential leaching risk of the adsorbent, its recyclability, and reuse and to evaluate the quality of the P recovered from separation and filtration operations to permit further reuse in agriculture. In addition to strategies for the recovery of P from wastewaters, there is growing interest to identify alternative fertilizers and nutrient delivery systems.259 Popular approaches include the development of nutrient-based or nutrient-loaded nanomaterials that are engineered to provide slow and localized real-time release of nutrients. Increased interest is expected in designing nanoporous materials with controlled pore size, connectivity, and functionality to increase nutrient loading, as well as developing more effective release mechanisms for the recovery and reuse. Future development of green and environmentally friendly sorbents and nutrient delivery systems is also of interest, as well as considerations of recyclability and reuse. There is also a need in the future for engineering solutions to integrate these materials into filtration membranes, columns, and separation systems and to test these systems in realworld conditions in order to demonstrate efficiency and enable practical implementation. Successful implementation of these materials can reduce further contamination of water streams and potentially increase efficiency of agricultural practices.

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Eduard Dumitrescu: 0000-0003-2277-4085 Silvana Andreescu: 0000-0003-3382-7939 Notes

The authors declare no competing financial interest. Biographies

Ali Othman is currently a Ph.D. candidate in the Department of Chemistry and Biomolecular Science at Clarkson University. He completed his M.Sc. from Jordan University of Science & Technology in Jordan. His research interests focus on controlled synthesis and characterization of nanostructured materials and their applications for sensing and environmental remediation.

Eduard Dumitrescu is currently a Ph.D. candidate in the Department of Chemistry and Biomolecular Science at Clarkson University. He obtained his B.S. from the University of Bucharest, Romania, in 2014. His current research focuses on the characterization of metal and metal oxide nanoparticles and their toxicity and environmental impact.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Fax: +(315) 268 6610. Tel: +(315) 268 2394.

Daniel Andreescu received his Ph.D. in Chemistry from the University of Bucharest, Romania, in 2002. In 2003, he joined Clarkson L

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ACS Sustainable Chemistry & Engineering

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University where he currently works on the synthesis, characterization, and applications of nanosize metallic, metal oxides and composites and their interactions.

Silvana Andreescu is the Egon Matijević Endowed Chair in Chemistry in the Department of Chemistry and Biomolecular Science at Clarkson University. She received her P.h.D in 2002 from the University of Bucharest, Romania, and University of Perpignan, France. Her research interests include the development of advanced materials and methods for sensing and environmental applications.

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ACKNOWLEDGMENTS This work was supported in part by NSF Grant No. 1610281. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation. Funding provided by the New York State Pollution Prevention Institute through a grant from the New York State Department of Environmental Conservation is also acknowledged. Any opinions, results, findings and/or interpretations of data contained herein are the responsibility of author(s) and do not necessarily represent the opinions, interpretations or policy of the State.

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