Magnetic Hierarchically Macroporous Emulsion-Templated Poly

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Article Cite This: Langmuir 2019, 35, 8996−9003

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Magnetic Hierarchically Macroporous Emulsion-Templated Poly(acrylic acid)−Iron Oxide Nanocomposite Beads for Water Remediation Muhammad Ahmad Mudassir,†,‡,§ Syed Zajif Hussain,† Asim Jilani,⊥ Haifei Zhang,§ Tariq Mahmood Ansari,*,‡ and Irshad Hussain*,†

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†

Department of Chemistry and Chemical Engineering, SBA School of Science and Engineering (SBASSE), Lahore University of Management Sciences (LUMS), Lahore 54792, Pakistan ‡ Institute of Chemical Sciences, Bahauddin Zakariya University, Multan 60800, Pakistan § Department of Chemistry, University of Liverpool, Oxford Street, Liverpool L69 3BX, U.K. ⊥ Center of Nanotechnology, King Abdulaziz University (KAU), Jeddah 21589, Saudi Arabia S Supporting Information *

ABSTRACT: Tainting of waterbodies with noxious industrial waste is the gravest environmental concern of the day that continues to wreak inevitable havoc on human health. To cleanup these hard-to-remove life-threatening water contaminants, we have prepared hierarchically porous poly(acrylic acid) beads by emulsion templating. These emulsion-templated macroporous polymer beads not only mediate the synthesis of Fe3O4 nanoparticles inside their porous network using a coprecipitation approach but, in turn, create diverse anchoring sites to immobilize an additional poly(acrylic acid) active layer onto the nanocomposite beads. These post-synthetically modified nanocomposite beads with macropores and abundant acrylic acid moieties offer the ready mass transfer and fair advantage of relatively higher overall negative charge to efficiently adsorb lead [Pb(II)] and crystal violet with impressive performanceeven superior to many of the materials explored in this regard so far. Furthermore, the strong entanglement of nanoparticles in the porous polymeric scaffolds tackles the curb of trade-off between all-round effective remediation and secondary pollution and the millimeter size eases their processing and recovery during the adsorption tests, thereby making these materials practically worthwhile.

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INTRODUCTION

Pb(II) poses grave threat to humans because of its inherent toxicity to the brain tissue, central nervous system, reproductive system, and children’s intellectual development while accumulating in the human body through food chain and/or drinking water polluted with the disposal of storage batteries, painting pigments, ammunitions, textiles, steels industries, etc. Likewise, CV is highly toxic, mutagenic, and carcinogenic, which is commonly found in effluent water of the paint, printing ink, paper, and textile industries, and causes cyanosis, tissue necrosis, shock, jaundice, and quadriplegia in human beings.5−8 Many new materials have been explored to offer economic and effective solutions to decontaminate water, but Fe3O4 nanoparticles (NPs) have received wider interest due to their low cost, good biocompatibility, easy surface modification, and interesting magnetic properties. The easier surface modification of Fe3O4 nanoparticles is generally exploited to tune their

Global water use continues to grow by ∼1% per annum since 1980s on account of population explosion, socio-economic development, and changing consumption patterns. The United Nations (UN) report anticipates the continuity of a similar rise in water demand until 2050, thus accounting for an upsurge of 20−30% above the present-day water consumption, owing to its escalating industrial and domestic demand. Three out of 10 people globally are deprived of access to potable water, and projections also reveal that about two-thirds of the world’s population will have inadequate access to safe water by 2050.1,2 Only around 2.5% of all the water on this planet i.e., Earth, is fresh. These finite sources of drinkable water are being poisoned with nearly 2 million tons of unprocessed industrial, sewage, and agricultural discharge per day across the globe. The failure to manage such waste containing the inorganic and organic pollutants is not only jeopardizing public health but also causing deaths of ∼14,000 individuals on a daily basis. Of these, many heavy metals and dyes including lead (Pb) and crystal violet (CV) tend to be the notoriously dangerous because of their severe toxicity and low degradability.3,4 © 2019 American Chemical Society

Received: April 16, 2019 Revised: May 22, 2019 Published: June 12, 2019 8996

DOI: 10.1021/acs.langmuir.9b01121 Langmuir 2019, 35, 8996−9003

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finally in IPA for about 12 h. The porous oil-free PAA macrobeads were dried in a drying oven at 80 °C. Preparation of PAA−Fe3O4 NC Beads. An in situ chemical precipitation approach was utilized to generate Fe3O4 NPs inside the macroporous PAA beads.30 For this purpose, an aqueous mixture of ferrous and ferric salts was prepared by adding 0.374 g of FeSO4· 7H2O and 0.72 g of FeCl3·6H2O in 20 mL of ultrapure deionized water under continuous stirring for complete dissolution and gradual heating to attain 80 °C temperature. Afterward, ∼10 mg of PAA beads was soaked in the resultant warm mixture for 15 min. The precursorloaded PAA beads were then immersed in 13.33 mL of precipitating agent (25% ammonium hydroxide solution) to precipitate Fe2+ and Fe3+ ions. The black-colored PAA−Fe3O4 NC beads were filtered out, washed with plenty of deionized water, and placed in a drying oven at 100 °C. Post-Synthetic Modification of PAA−Fe3O4 NC Beads. The carboxylate moieties were immobilized onto the surface of iron oxide NPs by soaking and gently shaking the PAA−Fe3O4 NC beads (∼10 mg) in 20 mL of 2% (w/v) aqueous solution of poly(acrylic acid) overnight.31 The post-synthetically modified PAA−Fe3O4−PAA NC beads were then dried in a vacuum oven at 100 °C (Figure 1).

surface properties to remove a variety of pollutants, and their magnetic properties are very helpful to recycle them after magnetic separation and regeneration. Moreover, the incorporation of Fe3O4 NPs into the macroporous polymeric scaffolds also offers more value to their practically viable water treatment solutions. In this connection, the emulsion templating technique has recently gained high acclaim for the easier and scalable preparation of millimeter-sized complex shapes including the monoliths, membranes, and beads, with inherent macroporosity.9−17 The large interconnected pores of such emulsion-templated porous materials offer convective mass transfer, which is the driving force for the excellent separation and purification of water pollutants, and the macrosize is highly useful for their facile handling and recovery. By way of example, these emulsion-templated porous materials have been extensively applied for the remediation of heavy metals, particulate matters, volatile organic compounds, and organic dyes, for oil/water separation, for enzyme immobilization, and for microbial disinfection.12,18−29 In this regard, we have prepared post-synthetically modified hierarchically macroporous poly(acrylic acid)−Fe3O4 nanocomposite (NC) beads. These materials are highly favorable for the adsorption of cationic inorganic and organic pollutants such as Pb and CV and can easily be processed and subsequently separated based on their macrosize. Furthermore, decent mechanically stability and the absence of the leached iron even after the mechanical shaking of the nanocomposite beads containing aqueous solutions exclude the chance of secondary contamination, thereby making these materials practically viable.

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Figure 1. Schematic view of the preparation of PAA (2.28−2.22 mm), PAA−Fe3O4 NC (2.26−2.20 mm), and PAA−Fe3O4−PAA NC (2.26−2.21 mm) beads.

EXPERIMENTAL SECTION

Materials. Acrylic acid (AA, CH2CHCOOH, anhydrous, 99.0%), poly(acrylic acid) (PAA, Mw ≈ 250,000, 35 wt % in H2O), N,N′-methylenebisacrylamide (BAM, 99%), N,N,N′,N′-tetramethylethylenediamine (TEMED, 99%), iron(III) chloride hexahydrate (FeCl3·H2O, chunks, reagent grade, ≥98%), iron(II) sulfate heptahydrate (FeSO4·7H2O, ACS reagent, ≥99.0%), ammonium peroxodisulfate (AP, 98%), Triton X-405 (70% in H2O), acetone (99.9%), isopropyl alcohol (IPA, 99%), hexane (95%), ammonium hydroxide (NH4OH, ≥25%) solution, and crystal violet (CV, ACS reagent, ≥90%) were purchased from Sigma-Aldrich (Germany). Lead(II) nitrate [Pb(NO3)2, ACS reagent grade, 99.5%] was obtained from Scharlau. Poly(vinyl alcohol) (PVA, Mw = 16,000) was taken from Acros Organics, Fisher Scientific. The commercially available cooking oil (sunflower) was purchased from a local market in Lahore. A Milli-Q Plus system (Millipore, Bedford, MA, USA)-purified deionized water was used for the entire research. Synthesis Steps. Synthesis of Poly(acrylic acid) (PAA) Beads. A slightly modified O/W/O emulsion templating technique was used to prepare macroporous PAA beads.25 The monomer (AA, 1 mL), crosslinker (BAM, 240 mg), cosurfactant (PVA, 60 mg), and deionized water (2 mL) were mixed together and sonicated to obtain a clear solution. The clear solution was further mixed with a surfactant (Triton X-405, 650 μL) using a vortex mixer to obtain a homogeneous mixture. Then, an oil phase (sunflower oil, 3 mL) containing a redox initiator (TEMED, 12 μL) was added to the above mixture and vigorously vortexed to attain an oil-in-water (O/W) emulsion. Subsequently, an aqueous solution of an initiator (20 mg of AP/600 μL of water) was evenly mixed with the O/W emulsion to initiate polymerization. The resultant O/W emulsion was injected dropwise into the hot cooking oil (∼85 °C) and baked for ∼3 h to complete the polymerization process. Afterward, the PAA beads were separated from the oil column and soaked in n-hexane for ∼24 h, alternatively in acetone and n-hexane for 1 h (repeated thrice), and

Characterization. Fourier transform infrared (FTIR) spectra were obtained by an attenuated total reflectance−infrared (ATRFTIR, Alpha, Bruker) spectrometer. Thermogravimetric analysis curves were taken with a TA Instruments model Q600-SDT under air in the temperature range of 27−984 °C with a ramp rate of 10 °C/ min. Magnetic properties were measured with a vibrating sample magnetometer (VSM, Lake Shore 7404). Surface areas were measured by multipoint Brunauer−Emmett−Teller (BET) data taken with a surface area analyzer (Quantachrome Nova 2200e) using N2 gas as the adsorbate. Scanning electron microscopy (SEM) images, energydispersive X-ray (EDX) spectra, and elemental mapping were recorded using a field emission scanning electron microscope (NovaNano). The particle and pore size distributions were calculated by ImageJ software using SEM images. The bead size was measured by a vernier caliper. The iron leaching and Pb adsorption analyses were carried out by an inductively coupled plasma-optical emission spectrometer (ICPE-9000, Shimadzu), whereas the adsorption analysis of CV was performed by an ultraviolet−visible (UV−vis) light spectrophotometer (Shimadzu UV-1800). Swelling Experiment. The water swelling behavior of PAA, PAA−Fe3O4 NC, and PAA−Fe3O4−PAA NC beads was examined gravimetrically for different time periods (0.25, 0.5, 1, 2, and 4 h) at room temperature. Magnetic Property. The magnetic characters of PAA, PAA− Fe3O4 NC, and PAA−Fe3O4−PAA NC beads were determined through magnetization curves at 300 K. The extent of magnetization under the applied magnetic field was further exploited to certify the immobilization of PAA onto the surface of Fe3O4 NPs. Leaching Analysis. To verify the risk of secondary pollution through any leaching of Fe3O4 NPs, both the PAA−Fe3O4 NC and PAA−Fe3O4−PAA NC beads were soaked in water for 24 h under mechanical agitation. ICP-OES analyses were then performed to analyze the amount of leached iron in the leachate samples. 8997

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Figure 2. (A) FTIR spectra and (B) TGA curves of (a) PAA, (b) PAA−Fe3O4 NC, and (c) PAA−Fe3O4−PAA NC beads. Adsorption Test. Experimental Study. Key operational parameters including the pH [2, 4, 6, and 8 for CV and 2, 3, 4, and 5 for Pb(II)], adsorbent dose (8.22 ± 0.72, 17.75 ± 0.49, 24.43 ± 1.07, 35.33 ± 0.76, and 43.80 ± 1.23 mg), pollutant concentration (5.02 ± 0.10, 10.30 ± 0.35, 15.47 ± 0.79, 19.72 ± 1.03, and 24.95 ± 0.02 mg/ L), and adsorption time (15, 30, 60, 120, and 240 min) were applied to find the optimum values of the pH [8 for CV and 5 for Pb(II)], adsorbent dose [17.75 ± 0.49 mg for CV and 24.43 ± 1.07 mg for Pb(II)], pollutant concentration (24.95 ± 0.02 mg/L), and adsorption time (240 min) to achieve the best possible adsorption results of PAA, PAA−Fe3O4 NC, and PAA−Fe3O4−PAA NC beads using 50 mL of CV and Pb(II) at a shaking speed of 250 rpm at room temperature. The following equation was applied to determine the adsorption capacity of the tested adsorbents at equilibrium

qe =

(Co − Ce)V W

The transient behavior of the adsorbates’ adsorption process was investigated by using the pseudo-first-order and pseudo-second-order models.32,33 The pseudo-first-order rate equation is expressed as i k y log(qe − qt) = log qe − jjjj 1 zzzzt k 2.303 {

The pseudo-second-order rate equation is denoted as

t 1 t = + qt qe k 2qe 2

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RESULTS AND DISCUSSION Synthesis. Herein, a very simple chemical coprecipitation approach was utilized to generate Fe3O4 NPs inside the macroporous PAA beads prepared via O/W/O emulsion templating technique followed by the immobilization of an additional PAA active layer onto the Fe3O4 NP-laden PAA beads, taking into account the strong chelation and H-bonding interaction of their carboxyl moieties with the surface hydroxyl groups of Fe3O4 NPs as well as the probability of the interchain H-bond formation among the neighboring carboxyl groups. The PAA beads mediate the synthesis and entanglement of Fe3O4 NPs inside their macroporous network, which, in turn, create the diverse anchoring sites and improve the BET surface area of the resultant PAA−Fe3O4 NC beads. Even though the immobilization of an additional PAA layer onto the Fe3O4 NPladen PAA beads causes a negligible reduction in the surface area, it still offers abundant AA moieties and bestows fair advantage of relatively higher overall negative charge to the resultant PAA−Fe3O4−PAA NC beads without much affecting their magnetic properties. The size, surface area, and zeta potential values of PAA, PAA−Fe3O4 NC, and PAA−Fe3O4− PAA NC beads are presented in Table S1. Characterization. The surface chemistry of PAA, in situ precipitation of Fe3O4 NPs, and immobilization of an additional active layer of PAA were ascertained by FTIR spectra. In the PAA spectrum [Figure 2A(a)], two highly weak signals at 3129 and 3054 cm−1 corroborate the carboxylic group-originated O−H stretching mode, whereas the relatively broad and distinct signals centered at 2928 and 2857 cm−1 could be assigned to the asymmetric and symmetric CH2 stretching vibrations, respectively.25,26,34,35 The appearance of

where qe is the adsorption capacity (mg/g), Cο and Ce are the initial and equilibrium concentrations (mg/L) of adsorbate entities, respectively, W is the mass (g) of the tested adsorbent, and V is the volume (L) of the adsorbate solution. Theoretical Modeling. The equilibrium adsorption characteristics were explained by the Langmuir and Freundlich isotherms models. The Langmuir isotherm is represented by the following equation

(2)

where Ce represents the equilibrium concentration (mg/L) of adsorbate entities in the solution, and qe stands for the amount of the targeted adsorbate entities adsorbed per unit mass of the tested adsorbent (mg/g) at equilibrium, whereas qmax denotes the maximum adsorption capacity (mg/g) of the tested adsorbent, and b (L/mg) is the Langmuir constant that defines the affinity of binding sites. The feasibility of adsorption was determined by the separation factor or equilibrium parameter (RL) using the following equation

RL =

1 bCo + 1

(3)

where RL is a dimensionless constant that demonstrates the feasibility of adsorption to be either unfavorable (RL > 1), linear (RL = 1), favorable (0 < RL < 1), or irreversible (RL = 0). The Freundlich isotherm is expressed as follows log qe = log K f +

1 log Ce n

(6)

where qt is the adsorption capacity (mg/g) at time t, and k1 (min−1) and k2 (g mg−1 min−1) are known as equilibrium rate constants of the pseudo-first-order and pseudo-second-order models, respectively. The linear regression coefficient (R2) values are used to predict the appropriate isotherm/kinetics model for the approximation of the adsorption process.

(1)

Ce C 1 = + e qe qmax b qmax

(5)

(4)

where Kf is the Freundlich constant or uptake factor, and 1/n is known as Freundlich’s intensity or heterogeneity factor. The values of 1/n in 0.1 < 1/n < 1 and 1/n > 2 ranges indicate favorable and unfavorable adsorption, respectively. 8998

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Figure 3. (A) Time-wise swelling capacities and (B) magnetization curves of (a) PAA, (b) PAA−Fe3O4 NC, and (c) PAA−Fe3O4−PAA NC beads.

a peak at 1702 cm−1 and its shoulder at 1652 cm−1 due to the CO stretch are characteristic of PAA.26,36 The medium intensity peak at 1533 cm−1 and a very weak signal at 1395 cm−1 refer to the asymmetric and symmetric stretching vibrations of −COO− groups, respectively,37,38 while a shoulder at 1230 cm−1 corresponds to the O−H bending vibrations of −COOH groups.39 Conversely, a prominent peak at 553 cm−1 that arose from the Fe−O stretch in the PAA− Fe3O4 NC bead spectrum [Figure 2A(b)] reveals the successful in situ generation of Fe3O4 NPs,40 while the broadening and amplification of bands at 3118 and 3026 cm−1 can be associated with the O−H stretches41,42 and may perhaps appear from the hydrogen bonding between the −COOH moieties of PAA and the surface −OH groups of Fe3O4 NPs.38,43 Altogether, the shifting of a peak from 2857 to 2808 cm−1, suppression of the signals at 1702 and 1652 cm−1, sharpening of a signal at 1395 cm−1, and fading of a peak at around 1168 cm−1 indicate the interaction of Fe3O4 NPs with the PAA surface. However, the coating of PAA−Fe3O4 NC beads with additional PAA may just be proven by the obvious suppression of almost all peaks of Fe3O4 NPs [Figure 2A(c)]. TGA curves show a stepwise weight change in PAA [Figure 2B(a)], PAA−Fe3O4 [Figure 2B(b)] , and PAA−Fe3O4−PAA [Figure 2B(c)] NC beads. The weight losses of 7.59, 11.98, and 5.83% at ∼100 °C may be attributed to water evaporation. The steep weight changes starting in the temperature range of 150−300 °C in all samples are most probably associated with decarboxylation decomposition of the polymer. The ∼100% mass loss at 534 °C in the PAA sample is due to the complete degradation of the polymer, while 83.22 and 88.18% weight losses in PAA−Fe3O4 and PAA−Fe3O4−PAA NC beads samples at around 984 °C indicate an average 14.30 ± 3.51 wt % loading of iron oxide. The water swelling capabilities of PAA [Figure 3A(a)], PAA−Fe3O4 NC [Figure 3A(b)], and PAA−Fe3O4−PAA NC beads [Figure 3A(c)] were evaluated in terms of time at room temperature. The swelling capacities of all materials gradually increase with the passage of time of up to 4 h. The swelling nature of PAA may be attributed to the electrostatic repulsion among the acrylate groups probably formed by the ionization of the acrylic acid groups in aqueous media.11 By comparison, the swelling capacities decrease in the order PAA > PAA− Fe3O4 NC > PAA−Fe3O4−PAA NC beads may be correlated with their decreasing hydrophilicity and rising swellingresistance ability, perhaps due to the hydrogen bonding between PAA and Fe3O4 NPs,43 as manifested by the FTIR bands associated with the hydrogen-bonded hydroxyl groups.

Magnetization curves show no response of the PAA bead sample to the applied magnetic field. On the contrary, the PAA−Fe3O4 NC bead sample exhibits the highest magnetization under the applied magnetic field, whereas the relatively lower magnetization shown by the PAA−Fe3O4−PAA NC bead sample due to the shielding effect also certifies the immobilization of PAA onto the surface of Fe3O4 NPs. Nevertheless, PAA−Fe3O4 and PAA−Fe3O4−PAA NC beads possess sufficient magnetization to be easily separated from the aqueous pollutant solutions by the help of a permanent magnetic field (Figure 3B). The cross-sectional view of SEM images reveals the hierarchically porous structure of PAA, PAA−Fe3O4 NC, and PAA−Fe3O4−PAA NC beads with abundant macropores that range in sizes from ∼0.7 to 43.95 μm (Figures S1A and S2A), 0.38 to 43.43 μm [Figure 4A,B and Figure S2B), and 0.38 to 38.93 μm (Figures S1B and S2C), respectively, could be the core reason of their lower BET surface areas.

Figure 4. Cross-sectional SEM images of PAA−Fe3O4 NC beads reveal (A, B) the hierarchically macroporous structure and confirm (C, D) the formation and higher loading of the Fe3O4 NPs.

The higher loading and subsequent aggregation of the Fe3O4 NPs (Figure 4C,D, mostly of 9−12 nm in size (Figure S2D), inside the porous PAA matrix also lower the overall surface area (12.46 m2/g, Table S1) of PAA−Fe3O4 NC beads, but it is still higher than the surface area (2.77 m2/g, Table S1) of simple PAA beads. However, the negligibly smaller surface area (10 m2/g, Table S1) of PAA−Fe3O4−PAA NC beads may be linked to the coverage of the NP surface with an additional PAA layer. The presence as well as uniform distribution of iron over polymer matrix in PAA−Fe3O4 NC beads and PAA over iron in PAA−Fe3O4−PAA NC beads is obviously evident from varying intensities of the characteristic peaks and colors in 8999

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Figure 5. Effect of operational parameters including (A) pH, (B) adsorbent dose, (C) pollutant concentration, and (D) time on the adsorption capacities of (a, d) PAA, (b, e) PAA−Fe3O4 NC, and (c, f) PAA−Fe3O4−PAA NC beads for (a−c) CV and (d−f) Pb(II) at room temperature.

23.80 ± 0.78 mg. This implies that the improvement in adsorption capacities upon increasing the adsorbents doses is associated with the enhanced available active binding sites, whereas further increments in the adsorbent doses may probably promote the sedimentation and impede the diffusion of CV and Pb(II) to the surface and pores/channels of the adsorbents and thus results in lessening of adsorption capacities (see Figure 5B).45 The increasing concentrations (5, 10, 15, 20, and 25 mg/L) of both the CV and Pb(II) momentously affect the adsorption potentials of PAA, PAA−Fe3O4 NC, and PAA−Fe3O4−PAA NC beads. However, all of these materials adsorb CV in a gradual fashion while removing Pb(II) somewhat faster. These boosts in the adsorption capacities can be elaborated by the concentration-based thermodynamic force that, in fact, commands the adsorbate mass transfer into the macroporous networks of all adsorbents (Figure 5C). The time-wise uptakes of CV as well as Pb(II) seem faster during the first 30 min, then become steady until 120 min, and finally slow down up to 240 min. These time-dependent changes in the adsorption capabilities of all adsorbents may be concomitant with their time-reliant swelling that allows the infiltration of more amountsof the adsorbates into the pores/ channels of these materials (Figure 5D).25 Comprehending the Adsorption Mechanism. The behavior of PAA, PAA−Fe3O4 NC, and PAA−Fe3O4−PAA NC beads to adsorb CV and Pb(II) was theoretically evaluated through the adsorption isotherms and kinetics models. The comparatively higher linear regression coefficient (R2) values support the Langmuir isotherm to be an appropriate model that verifies the existence of a monolayer adsorption phenomenon. The equilibrium parameters (RL) for the Pb(II) adsorption are closer to unity than the CV values; however, all of these favor the process of adsorption. The maximum

EDX spectra (Figure S3) and elemental mapping (Figure S4) results, respectively. Furthermore, the nondetectable levels of iron shown by the leaching tests also confirm the strong entrapment of Fe3O4 NPs in the porous poly(acrylic acid) scaffolds. This, in fact, eliminates the risk of any secondary contamination from the component of the adsorbent and makes it highly useful for practical applications. Adsorption Study. Factors Affecting Adsorption. To avoid any precipitation of the pollutant solutions at higher pH values,11,44 the adsorption tests of PAA, PAA−Fe3O4 NC, and PAA−Fe3O4−PAA NC beads were performed at pH values of 2, 4, 6, and 8 for CV and 2, 3, 4, and 5 for Pb(II). Due to the lower concentration of H+ ions and, thus, the reduced competition with the cationic pollutants for the highly dissociated functional groups of all adsorbents, the uptake of the Pb(II) and CV was found to be the highest at pH values of 5 and 8, respectively.11 Owing to their diverse binding sites (−COO− and Fe−O−), PAA−Fe3O4 NC beads exhibit higher capacities to adsorb both Pb(II) and CV than the PAA beads possessing only carboxylate moieties, whereas the highest uptake potential of PAA−Fe3O4−PAA NC beads compared with the other adsorbents is attributed to Fe−O− and plenty of −COO− anchoring sites. However, the better removal of Pb(II) than CV may probably be due to its better complexation with the carboxylic acid groups (see Figure 5A).38 The varying adsorbent doses of PAA, PAA−Fe3O4 NC, and PAA−Fe3O4−PAA NC beads also influence the adsorptive removal of the CV and Pb(II) pollutants. The overall adsorption of CV reaches to the maximum levels by increasing the dosages of all the adsorbents from 7.97 ± 1.01 to 18.10 ± 0.40 mg and starts decreasing afterward with the further additions in their dosages even up to 40.90 ± 0.20 mg. In the same way, the highest Pb(II) removal is achieved by using 9000

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Table 1. Isotherm Parameters for the Adsorptive Removal of Pb(II) and CV by PAA, PAA−Fe3O4, and PAA−Fe3O4−PAA NC Beads PAA

PAA−Fe3O4−PAA NCs

PAA−Fe3O4 NCs

type of isotherm

parameter

Pb(II)

CV

Pb(II)

CV

Pb(II)

CV

Langmuir

qmax (mg/g) b (L/mg) RL R2 Kf 1/n R2

257.23 1.43 × 10−03 0.9986 0.9999 0.36 0.0545 0.9438

60.84 3.13 × 10−02 0.9696 0.9906 1.74 0.0812 0.9601

272.14 4.02 × 10−04 0.9996 0.9876 0.11 0.0742 0.9796

71.42 1.02 × 10−02 0.9899 0.9879 0.65 0.1352 0.9201

290.69 2.48 × 10−04 0.9998 0.9880 0.08 0.0762 0.9742

80.20 5.82 × 10−03 0.9942 0.9868 0.42 0.1561 0.9048

Freundlich

Table 2. Performance Comparison of the Tested PAA, PAA−Fe3O4 NC, and PAA−Fe3O4−PAA NC Beads for Pb(II) Removal with Those of the State-of-the Art Materials Cited Recentlya operational parameter material

Pb(II) adsorption capacity (mg/g)

g/L of adsorbent

C0 (ppm)

pH

h

reference

aromatic polyamidoximes magnetic GO@PNB BPB-mesoporous organosilica MPA@PM NPs α-FeOOH hollow spheres composite chitosan/graphene oxide urchin-like Fe3O4 MSs Fe3O4/TiO2/polypyrrole nanofibers porous tablet ceramic CHT/ALG/Fe3O4@SiO2 beads CS−MA−DETA microspheres magnetic porous polymers MnO2−biochar PAA beads PAA−Fe3O4 NC beads PAA−Fe3O4−PAA NC beads

0.967 11.76 21.92 68.41 80 112.35 112.8 126 215.52 234.77 239.23 257 268 257.23 272.14 290.69

0.1 1.5 2 0.5 0.5 1 1 0.45 1 0.83 1 0.15 0.4 0.50 ± 0.0014 0.50 ± 0.0005 0.49 ± 0.0011

20 50 10−50 10−1000 10−1500 50 100 0.2−40 10−800 20−500 250 30−300 0.5−210 4.95−24.93 4.95−24.93 4.95−24.93

6 5 6 6.5

3 2/6 1 6 3 7 10

46 6 47 48 49 50 11 51 52 53 54 12 45 this work

5 5 6 4.2 5 5.5 5 5 5 5

8 6 24 12 4 4 4

a

MSs and NC stand for microspheres and nanocomposite, respectively.

Table 3. Kinetic Parameters for the Adsorptive Removal of Pb(II) and CV by PAA, PAA−Fe3O4, and PAA− Fe3O4−PAA NC Beadsa PAA order of reaction

parameter

pseudo-first-order

qe (mg/g)

pseudo-second-order

k1 (min−1) R2 qe (mg/g) K2 (g mg−1 min−1) R2

PAA−Fe3O4 NCs

PAA−Fe3O4−PAA NCs

Pb(II)

CV

Pb(II)

CV

Pb(II)

CV

28.36 (E) 23.77 (T) 2.89 × 10−02 0.9686 28.36 (E) 32.40 (T) 6.69 × 10−04 0.9753

9.54 (E) 7.01 (T) 3.17 × 10−02 0.9771 9.54 (E) 11.75 (T) 1.38 × 10−03 0.9861

40.08 (E) 23.09 (T) 1.38 × 10−02 0.9724 40.08 (E) 44.09 (T) 9.33 × 10−04 0.9992

18.50 (E) 13.56 (T) 3.38 × 10−02 0.9887 18.50 (E) 21.95 (T) 9.48 × 10−04 0.9936

45.09 (E) 24.72 (T) 1.36 × 10−02 0.9959 45.09 (E) 48.60 (T) 9.87 × 10−04 1.00

24.01 (E) 18.51 (T) 3.38 × 10−02 0.9848 24.01 (E) 29.02 (T) 6.95 × 10−04 0.9977

a

E stands for experimental values, and T stands for theoretical values.

theoretical adsorption capacities of PAA, PAA−Fe3O4 NC, and PAA−Fe3O4−PAA NC beads come out to be 257.23, 272.14, and 290.69 mg/g for Pb(II), respectively, and 60.84, 71.42, and 80.20 mg/g for CV, respectively (Table 1). These Langmuir capacities for the Pb(II) adsorption are even superior to many recently reported state-of-the-art materials (see Table 2). On the other hand, the relatively higher R2 values and the reasonable closeness between the theoretical (T) and experimental (E) qe values favor the pseudo-second-order to be a more suitable kinetics model, which further suggests that the underlying adsorption phenomena are chemical in nature (Table 3).55

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CONCLUSIONS

The present study demonstrates the production of new hierarchically macroporous PAA beads using an emulsion templating technique. These porous polymeric scaffolds were then loaded with Fe3O4 NPs via a very facile in situ chemical coprecipitation method. The synthesis of Fe3O4 NPs inside the macroporous network of PAA beads does not only improve the BET surface area of the resultant PAA−Fe3O4 NC beads a bit but also creates the diverse anchoring sites to be used to immobilize an additional poly(acrylic acid) layer. Even though the immobilization of an additional PAA layer onto the Fe3O4 9001

DOI: 10.1021/acs.langmuir.9b01121 Langmuir 2019, 35, 8996−9003

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Frameworks and Perspective on Their Industrial Applications. ACS Sustainable Chem. Eng. 2019, 7, 4548−4563. (4) Bolisetty, S.; Peydayesh, M.; Mezzenga, R. Sustainable Technologies for Water Purification from Heavy Metals: Review and Analysis. Chem. Soc. Rev. 2019, 48, 463−487. (5) Luo, J.; Sun, M.; Ritt, C. L.; Liu, X.; Pei, Y.; Crittenden, J. C.; Elimelech, M. Tuning Pb(II) Adsorption from Aqueous Solutions on Ultrathin Iron Oxychloride (FeOCl) Nanosheets. Environ. Sci. Technol. 2019, 53, 2075−2085. (6) Pan, L.; Zhai, G.; Yang, X.; Yu, H.; Cheng, C. Thermosensitive Microgels-Decorated Magnetic Graphene Oxides for Specific Recognition and Adsorption of Pb(II) from Aqueous Solution. ACS Omega 2019, 4, 3933−3945. (7) Mousavi, S. H.; Shokoofehpoor, F.; Mohammadi, A. Synthesis and Characterization of γ-CD-Modified TiO2 Nanoparticles and Its Adsorption Performance for Different Types of Organic Dyes. J. Chem. Eng. Data 2019, 64, 135−149. (8) Singha, N. R.; Dutta, A.; Mahapatra, M.; Roy, J. S. D.; Mitra, M.; Deb, M.; Chattopadhyay, P. K. In Situ Attachment of Acrylamido Sulfonic Acid-Based Monomer in Terpolymer Hydrogel Optimized by Response Surface Methodology for Individual and/or Simultaneous Removal(s) of M(III) and Cationic Dyes. ACS Omega 2019, 4, 1763−1780. (9) Zhou, J.; Wang, L.; Qiao, X.; Binks, B. P.; Sun, K. Pickering Emulsions Stabilized by Surface-Modified Fe3O4 Nanoparticles. J. Colloid Interface Sci. 2012, 367, 213−224. (10) Zhou, J.; Qiao, X.; Binks, B. P.; Sun, K.; Bai, M.; Li, Y.; Liu, Y. Magnetic Pickering Emulsions Stabilized by Fe3O4 Nanoparticles. Langmuir 2011, 27, 3308−3316. (11) Yu, Y.; Li, Y.; Wang, Y.; Zou, B. Self-Template Etching Synthesis of Urchin-Like Fe3O4 Microspheres for Enhanced Heavy Metal Ions Removal. Langmuir 2018, 34, 9359−9365. (12) Zhu, H.; Tan, X.; Tan, L.; Zhang, H.; Liu, H.; Fang, M.; Hayat, T.; Wang, X. Magnetic Porous Polymers Prepared via High Internal Phase Emulsions for Efficient Removal of Pb2+ and Cd2+. ACS Sustainable Chem. Eng. 2018, 6, 5206−5213. (13) Zhang, H.; Cooper, A. I. Synthesis of Monodisperse EmulsionTemplated Polymer Beads by Oil-in-Water-in-Oil (O/W/O) Sedimentation Polymerization. Chem. Mater. 2002, 14, 4017−4020. (14) Zhang, H.; Hardy, G. C.; Rosseinsky, M. J.; Cooper, A. I. Uniform Emulsion-Templated Silica Beads with High Pore Volume and Hierarchical Porosity. Adv. Mater. 2003, 15, 78−81. (15) Zhang, H.; Hardy, G. C.; Khimyak, Y. Z.; Rosseinsky, M. J.; Cooper, A. I. Synthesis of Hierarchically Porous Silica and Metal Oxide Beads Using Emulsion-Templated Polymer Scaffolds. Chem. Mater. 2004, 16, 4245−4256. (16) Zhang, H.; Hussain, I.; Brust, M.; Cooper, A. I. Synthesis of Hierarchically Porous Inorganic−Metal Site-Isolated Nanocomposites. Chem. Commun. 2006, 2539−2541. (17) Zhang, H.; Hussain, I.; Brust, M.; Cooper, A. I. EmulsionTemplated Gold Beads Using Gold Nanoparticles as Building Blocks. Adv. Mater. 2004, 16, 27−30. (18) Pribyl, J. G.; Taylor-Pashow, K. M. L.; Shehee, T. C.; Benicewicz, B. C. High-Capacity Poly(4-vinylpyridine) Grafted PolyHIPE Foams for Efficient Plutonium Separation and Purification. ACS Omega 2018, 3, 8181−8189. (19) Liu, J.; Li, M.; Wang, P.; Liu, K.; Fang, Y. Gel-Emulsion Templated Polymeric Monoliths for Efficient Removal of Particulate Matters. Chem. Eng. J. 2018, 339, 14−21. (20) Chakrabarty, A.; Maiti, M.; Miyagi, K.; Teramoto, Y. Gel Emulsion Based on Amphiphilic Block Copolymer: A Template to Develop Porous Polymeric Monolith for the Efficient Adsorption of Volatile Organic Compounds. ACS Appl. Nano Mater. 2018, 1, 1569− 1578. (21) Zhang, S.; Fan, X.; Zhang, F.; Zhu, Y.; Chen, J. Synthesis of Emulsion-Templated Magnetic Porous Hydrogel Beads and Their Application for Catalyst of Fenton Reaction. Langmuir 2018, 34, 3669−3677.

NP-laden PAA beads causes a negligible reduction in the surface area, it offers surplus AA moieties and bestows fair advantage of relatively higher overall negative charge to the resultant PAA−Fe3O4−PAA NC beads without much affecting their magnetic properties. Interestingly, these materials are good to effectively adsorb the model hazardous cationic inorganic and organic water pollutants [e.g., Pb(II) and CV] and can easily be operated and subsequently separated based on their macrosize. Furthermore, the good mechanically stability and the absence of leached iron even after the mechanical shaking of the PAA−Fe3O4 and PAA−Fe3O4−PAA NC beads containing aqueous solutions exclude the chance of secondary contamination and thus makes these materials practically viable.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.9b01121. Size, surface area, zeta potential, and iron leaching results of PAA, PAA−Fe3O4 NC, and PAA−Fe3O4− PAA NC beads; SEM micrographs (cross-sectional view) of PAA and PAA−Fe3O4−PAA NC beads; pore size distributions of PAA, PAA−Fe3O4 NC, and PAA− Fe3O4−PAA NC beads and particle size distribution of Fe3O4 NPs; EDX spectra of PAA, PAA−Fe3O4, and PAA−Fe3O4−PAA NC bead samples; and SEM and EDX elemental (C, N, O, and Fe) mapping images of PAA, PAA−Fe3O4, and PAA−Fe3O4−PAA NC bead samples (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (T.M.A.). *E-mail: [email protected] (I.H.). ORCID

Muhammad Ahmad Mudassir: 0000-0002-6624-007X Syed Zajif Hussain: 0000-0002-3834-6061 Irshad Hussain: 0000-0001-5498-1236 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This article is part of the Ph.D. thesis of M.A.M., who acknowledges funding from the Higher Education Commission (HEC) of Pakistan under the Indigenous PhD 5000 Fellowship Program (Phase-II) to pursue his Ph.D. in Pakistan at BZU Multan and LUMS Lahore and under the International Research Support Initiative Program (IRSIP) to support his 6 month research visit to the University of Liverpool, U.K.

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DOI: 10.1021/acs.langmuir.9b01121 Langmuir 2019, 35, 8996−9003