Toward Realizing Multifunctionality: Photoactive and Selective

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Toward Realizing Multifunctionality: Photoactive and Selective Adsorbents for the Removal of Inorganics in Water Treatment Published as part of the Accounts of Chemical Research special issue “Water for Two Worlds: Urban and Rural Communities”. Lauren N. Pincus,†,‡ Amanda W. Lounsbury,‡,§ and Julie B. Zimmerman*,†,‡,§ †

School of Forestry and Environmental Studies, Yale University, 195 Prospect Street, New Haven, Connecticut 06511, United States Nanosystems Engineering Research Center for Nanotechnology-Enabled Water Treatment (NEWT), Yale University, New Haven, Connecticut 06511, United States § Department of Chemical and Environmental Engineering, Yale University, 17 Hillhouse Avenue, New Haven, Connecticut 06511, United States

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‡

S Supporting Information *

CONSPECTUS: Persistent and potentially toxic inorganic oxoanions (e.g., arsenic and selenium) are one class of contaminants of concern in drinking water for which treatment technologies must be improved. Effective removal of these oxoanions is made difficult by the varying adsorption affinity of the different oxidation states, as well as the presence of background ions with similar chemical structure and behavior that strongly compete for adsorption sites, greatly reducing removal efficiencies. Recent studies pointing to the negative health effects of inorganic oxoanion contaminants have resulted or are expected to result in new regulations lowering their allowable maximum concentration level (MCL) in drinking water. While these regulations are intended to protect human and environmental health, they must also allow for balanced economic costs. As such, the MCLs are often set at levels that are not as health protective due to high treatment costs that continue to present a significant challenge for small (500−3300 people) to very small (25−500 people) communities. In this Account, we focus on the development of novel cost-effective, sustainable, and efficient multifunctional and selective adsorbents that offer solutions to the above challenges through two platforms: nanoenabled and transition-metal cross-linked chitosan (TMCC) and crystal facet engineered nanometal oxides (NMO). These complementary platforms offer treatment solutions at different scales and flow rates (e.g., in a point-of-use device versus a small-scale community system). Multifunctional adsorbents combine processes that traditionally require multiple steps offering the potential for reducing treatment time and costs. Development of selective adsorbents can greatly increase removal efficiencies of target contaminants by either promoting their adsorption or hindering adsorption of competitive ions. The following sections describe (1) synthesis of novel nanoenabled waste sourced bioadsorbents; (2) development of multifunctional adsorbents to simultaneously photo-oxidize arsenite and adsorb arsenate; (3) development of a selective adsorbent for removal of arsenate and selenite over phosphate; (4) investigations of the conventional wisdom that increased surface area yields increased oxoanion removal using selenium sorption on nanohematite as a case study; and (5) crystal engineering of nanohematite to promote selenite adsorption. The novel technologies developed through these research efforts can serve as templates for the creation of future adsorbents tailored for use targeting other oxoanion contaminants of interest.

1. INTRODUCTION Providing access to clean water was one of the Grand Challenges for Engineering in the 21st Century developed by © XXXX American Chemical Society

Received: December 28, 2018

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DOI: 10.1021/acs.accounts.8b00668 Acc. Chem. Res. XXXX, XXX, XXX−XXX

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Accounts of Chemical Research the National Academy of Engineers in 20081 and was reiterated in 2018.2 Exposure through contaminated drinking water to persistent and toxic oxoanions, such as arsenic (As) and selenium (Se), poses potentially serious human and environmental health risks.3 As of 2000, more than 100 million people in the United States were served by groundwatersourced public supply systems and greater than 40 million people utilized self-supplied groundwater, primarily from private domestic wells.4,5 These metalloids are released into the environment naturally through weathering of inorganic minerals3 or from anthropogenic sources such as coal combustion, mining, agriculture, and petroleum refining.6−8 Removing oxoanions from aqueous systems is challenging due to adsorption affinity varying with oxidation state. For example, arsenic commonly exists in aqueous environments as arsenite (As(III)) and arsenate (As(V)).3 With pKa’s of 2.3, 6.8, and 11.6, at common environmental pH (5−7) arsenate is negatively charged (H2AsO4− and HAsO42−). In contrast, with pKa’s of 9.2 and 12.7, arsenite, the more toxic form, is usually present as uncharged H3AsO3 making it more difficult to remove.9,10 Similarly, selenium exists in the environment predominantly as selenite (Se(IV), HSeO3−, pKa’s 2.7 and 8.5) and selenate (Se(VI), SeO42−, pKa 1.8).8 Selenate is more difficult to remove due to its weaker complexation with adsorbents.8,11 Regulating priority metals in drinking water must balance treatment costs with the goal of health protection.12 For example, lowering of the (total) As maximum contaminant level (MCL) in drinking water from 0.05 to 0.01 mg/L in 2001 was more influenced by the high treatment costs that would be borne by small groundwater-based communities, than by the level that would be health protective.13 Recent epidemiologic studies suggest that human toxicity of Se in drinking water occurs at much lower levels than previously surmised,14 raising concerns over the treatment costs that will be incurred to meet new standards.8 Despite MCLs often being set at levels that are not sufficiently health protective, the cost still presents a significant challenge, particularly for small (500−3300 people) to very small (25−500 people) communities.8,15 As such, if we can reduce treatment costs, MCLs can move closer to health protective goals. Traditional treatment technologies for oxoanion removal include chemical coagulation/precipitation, ion exchange, membrane separation, and adsorption.3,16,17 Adsorption techniques generally have higher removal efficiencies, are more facile in field conditions, and are potentially regenerable.3,18 Challenges associated with traditional adsorbents include lack of selectivity and the incorporation of UV light into flow-through systems when an adsorbent relies on photoactivity for performance. The former can be addressed through the design and development of selective sorbents and the latter through the use of electrochemical, rather than photochemical, transformations. In this Account, we detail efforts to develop more costeffective, efficient, and sustainable water treatment technologies for oxoanion removal, using arsenic and selenium as examples, through two platforms: nanoenabled and transitionmetal cross-linked chitosan (TMCC) and crystal facet engineering of nanometal oxides (NMOs). These platforms offer different solutions in terms of providing technologies for water treatment at different scales and flow rates (e.g., in a point-of-use device versus a small-scale community system). By developing multifunctional adsorbents for different platforms

as described in this Account, traditionally multistep treatment processes can be combined (e.g., photo-oxidation and adsorption), reducing treatment time and costs.

2. DEVELOPMENT AND OPTIMIZATION OF CHITOSAN EMBEDDED WITH NANOMETAL OXIDES Nanometal oxides present emerging opportunities for improving treatment systems due to their extremely high adsorption capacity and, in some cases, photoactivity. Metal oxides, specifically iron and aluminum oxides, have traditionally been recognized as sinks for inorganic aqueous contaminants in natural systems.19 Further, some nanomaterials such as nanohematite and nanotitanium dioxide (n-TiO2) exhibit photo-oxidative properties that can be exploited to transform arsenic to its more readily removed oxidation state.9,20−23 While NMOs are attractive adsorbents due to their high surface area, they can be difficult to recover due to the need for an energy intensive post-treatment filtration process.17,21 In order to avoid this costly filtration process, nanoparticles can be embedded in a porous support material such chitosan.9,11,21,22 Early research efforts in our laboratory focused on the successful incorporation of NMOs into chitosan beads for the removal of As and Se in various oxidation states.9,11,21,22 Chitosan, a natural biopolymer and industrial waste product, was chosen due to its abundance, biocompatibility, biodegradability, and cost effectiveness. Chitosan is easily converted from a solid to a gel via pH change and can be readily formulated into beads for use in a packed-bed reactor.9,11,16,20−22 While chitosan itself has limited adsorptive capacity for oxoanions,21,22 it can readily interact with NMOs and can be easily functionalized through the amine and hydroxyl groups on the polymer backbone. 2.1. Removal of H3AsO3 and H2AsO4− by Multifunctional Photoactive n-TiO2−Chitosan

In water treatment, it is common to have a pretreatment step to transform oxoanions into preferred oxidation states for facile sorption. Arsenite is oxidized to arsenate, which is less toxic and more easily removed. Neat and modified (nano)metal oxides of manganese,24 iron,25 and titanium21,22,26 have been used as photo-oxidants for this transformation. For n-TiO2, the photo-oxidation in UV light of H3AsO3 proceeds in two main steps: (1) H3AsO3 reacts with superoxide radicals generated at the TiO2 surface to create As(IV) radicals,26,27 and (2) As(IV) radicals are then rapidly oxidized to H2AsO4− through reaction with molecular oxygen, hydroxyl radicals, or a trapped hole.26,27 Thus, by incorporating n-TiO2 into chitosan, the photo-oxidation pretreatment step can be combined with adsorption into a single treatment step, resulting in a multifunctional adsorbent (Figure 1A).21,22 Use of multifunctional adsorbents to combine or eliminate treatment steps enables reduction of treatment costs and increased sustainability of the treatment process.28 Chitosan embedded with n-TiO2 was synthesized as follows: (1) a chitosan gel was formed by dissolving chitosan in 1% HNO3; (2) n-TiO2 (90% reduction in removal efficiency for As/Se on a variety of sorbent materials.31−33 In order to work toward solving this beguiling challenge, we approached development of selective adsorbents through two distinct approaches: (1) cross-linking chitosan with various transition metals (TMCC) and (2) crystal facet engineering of

α12 =

q1C2 q2C1

where ion 1 is the target contaminant such as arsenic or selenium, ion 2 is the competitor phosphate, q is the equilibrium adsorbed-phase concentration, and C is the equilibrium aqueous-phase concentration.16 When α12 > 1, then the adsorbent was selective for the target contaminant.16 To examine the effect of copper loading on selective adsorption performance, α12 was plotted against mol of Cu(II)/mol of chitosan (Figure 2). Selective adsorption of H2AsO4− over H2PO4− was observed at a loading of >0.25 mol of Cu(II)/mol of chitosan.16 For HSeO3−, selectivity was observed at >0.4 mol of Cu(II)/mol of chitosan.16 These calculations confirmed that the type 2 bidentate complex favored adsorption of H2AsO4− and HSeO3− over H2PO4−.16 D

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relatively high adsorption capacity for H3AsO3.9,20 Loading of Cu(II) controls performance in systems requiring selectivity for arsenic over phosphate.20 The optimal Cu(II) loading was 0.4 g of Cu(NO3)2·3H2O/g of chitosan, a high enough loading to develop the selective type 2 bidentate Cu(II)-complex and provide ample Cu(II) adsorption sites but not too high that Cu(II) precipitates as CuO, resulting in less chitosan-complex formation.20 A synergistic effect was observed in the system requiring both photo-oxidation and selective adsorption where a necessary ratio of both active components was required in order to maximize both photo-oxidative (by n-TiO2) and selective adsorption (by Cu(II)) performance and maximum loadings of both components does not yield optimal selectivity or removal efficiency.20 The optimal adsorbent was determined statistically to contain 0.3 g of n-TiO2/g of chitosan and 0.4 g of Cu(NO3)2·3H2O/g of chitosan. Once the optimal loading of both active components was determined, this adsorbent was tested against nonselective nanometal oxides n-Al2O3 and n-TiO2 (Figure 3).20 For

Figure 2. Quantifying the degree of selectivity for H2AsO4− and HSeO3− over H2PO4− using the binary separation factor, α12, as a function of mol Cu(II)/mol chitosan. Adapted with permission from ref 16. Copyright 2016 Elsevier.

In the case of H2AsO4−/H2PO4− competition, H2AsO4− is electrostatically preferred over H2PO4− (Figure 1C).16 As Cu(II) binds to the amine groups of chitosan, the complex gains cationic character that can electrostatically attract oxoanions.16,20,38,39 In the monodentate type 1 complex, we hypothesized this positive charge is more concentrated and thus more similar to the polarized diffusion of charge between the δ+ on P and δ− on O in H2PO4−, where δ is the partial charge.16 In a bidentate type 2 complex, the positive charge is dispersed over a larger area, so it is more diffuse and similar to the diffusion of charge between the δ+ on As and δ− on O in H2AsO4−. For selenite/phosphate, sterics likely are the driver for selectivity exhibited by the type 2 complex.16 Selenite has a significantly smaller hydrodynamic radius and hydration number than phosphate, so in a type 2 complex where access to the Cu(II) binding site is more sterically restricted, HSeO3− will more easily adsorb.16 While selanite/sulfate competitive adsorption was studied, no development of selective adsorption of selenate over sulfate was observed by the type 2 Cu(II)−chitosan potentially due to Cu(II) precipitating as copper sulfate in this system.16

Figure 3. Selective adsorption performance for As over H2PO4− by nTiO2−Cu(II)−chitosan (0.40 g of Cu(NO3)2·3H2O and 0.30 g of nTiO2) vs traditionally used neat, nonselective NMOs in various system conditions. Initial concentrations were 3.8 ppm arsenic, 15.7 ppm phosphate when present, 25 mM acetate buffer pH 6. Adapted with permission from ref 20. Copyright 2018 Elsevier.

adsorption of arsenite over phosphate in UV light, the most complex system condition, n-TiO2−Cu(II)−chitosan was approximately seven times more effective at arsenic removal than n-Al2O3 and four times more effective than n-TiO2.20

3.2. Optimization of a Multifunctional Selective Adsorbent for H3AsO3 and H2AsO4− Removal in the Presence of H2PO4− Using n-TiO2−Cu(II)−Chitosan

Once it was observed that Cu(II)−chitosan prefers H2AsO4− adsorption over H2PO4−, research efforts next focused on how to incorporate photoactivity into this adsorbent and how to optimize selective removal performance. n-TiO2−Cu(II)− chitosan was synthesized with n-TiO2 added to photo-oxidize H3AsO3 to H2AsO4− in UV light9,20−22 and Cu(II) added to develop selective adsorption ability for H2AsO4− over H2PO4− (Figure 1D). 16,20 To determine the optimal sorbent composition, loading of Cu(II) and n-TiO2 was varied.20 Performance was tested through batch experiments with pH held constant at 6 using a 25 mmol, pH 6, acetate buffer to examine arsenic removal performance in UV light/darkness, arsenite/arsenate, and presence/absence of phosphate at 10× As on a molar basis.20 It was found that in systems requiring H3AsO3 photo-oxidation, arsenic removal performance is controlled by n-TiO2 where at least 0.3 g of n-TiO2/g of chitosan was needed to effectively photo-oxidize arsenite into arsenate.20 If H3AsO3 was not photo-oxidized, performance increased with higher loadings of n-TiO2 as n-TiO2 has a

3.3. Crystal Facet Engineering of Nanohematite to Promote Adsorption of Selenium

Our second approach to developing selective adsorbents is based on crystal facet engineering of NMOs. Crystal facet engineering is utilized in numerous fields where there is a need for facet-dependent preferential adsorption, such as in surfacecontrolled catalysis.41−43 We applied the same principles to inorganic aqueous adsorption to determine first if facets are similarly important in aqueous remediation44 and second how different facets affect the binding environment.8 Given the synthetic challenges associated with crystal facet engineering, there is a significant opportunity to use our insight of molecular-scale analysis in conjunction with advanced quantum modeling to inform the design of nanomaterials with facets that are selective for the target sorbates. We utilized nanohematite (n-α-Fe 2 O 3 ), due to its thermodynamic stability, facile synthesis into multiple morphologies, natural affinity for inorganic contaminants, and lack of toxicity.23 In our first study on Se adsorption E

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Figure 4. Similar and irregular shape of particles n-α-Fe2O3−C (a) and α-Fe2O3−N (b). Particles A (c) and B (d) were indexed using the atomic structure (top left inset) and SAED pattern (top right inset) with HR-TEM. Both these particles were found to have prevalent facets in the {110} family (bottom insets). For particles C (e) and D (f), prevalent facets were determined based on data from ref 47. Copyright 2019 Elsevier. Panels a and b adapted with permission from ref 44. Copyright 2016 Elsevier. Panels c and d adapted with permission from ref 47. Copyright 2019 Elsevier.

Figure 5. Freundlich isotherms of n-α-Fe2O3 of varying size and surface area for HSeO3− (A) and SeO42− (B). Particle A is the smallest with largest surface area, and particle D is the largest with the 2nd smallest surface area. Adapted with permission from ref 47. Copyright 2019 Elsevier.

different facets and this results in different binding sites for Se adsorption,44 with α-Fe2O3-C comprising more of the reactive undersaturated singly and triply coordinated hydroxyl groups than the less reactive saturated doubly coordinated hydroxyl groups. However, the specific facet for these particles is unmeasurable due to the irregular morphology of the particles (Figure 4a,b).47 To test the aforementioned hypothesis and to determine the impact of facet on binding environment, we synthesized, without capping agents, particles A, B, C, and D (Figure 4c,d,e,f) with (110) and (012) exposed facets and, therefore, different proportions of coordinated hydroxyl groups.47 Capping agents are organic molecules, typically surfactants, that preferentially bind to specific facets, thereby promoting growth of the crystal along noncapped facets. However, capping agents change the binding environment, and it is unclear how well they can be removed postsynthesis, potentially interfering with subsequent sorption studies. For this reason, we chose to avoid any organic material or surfactant that could change the n-α-Fe2O3 active sites unintentionally.

onto n-α-Fe2O3 we synthesized particles that were the same size but had different surface areas by adapting a forced hydrolysis synthesis process.45 n-α-Fe2O3-C used FeCl3 for a precursor salt, while n-α-Fe2O3-N used Fe(NO3)3 for its salt. n-α-Fe2O3-C and n-α-Fe2O3-N were characterized for size, morphology, surface area, pore and aggregate size, point of zero charge (PZC), and chemical composition. Se adsorption was evaluated at the environmentally relevant pH = 6.0 ± 0.5. These particles differ negligibly in all characteristics except surface area. n-α-Fe2O3-N have a larger surface area than n-αFe2O3-C by ca. 25%.44 Se adsorption between the dried particles was similar to a maximum adsorption (qmax) of 7.47 ± 0.473 mg/g and 6.01 ± 0.64 mg/g for SeO42− and 17.9 ± 0.66 mg/g and 17.3 ± 0.59 for HSeO3− on α-Fe2O3-C and αFe2O3-N, respectively, despite the difference in surface area. Thereby we countered the long-held belief in aqueous remediation literature that an increase in surface area results in an increase in sorption capacity.46 We hypothesized that the reason for the observed differences in adsorption capacities was a result of different sorption sites. Specifically, we inferred that the use of the different counterions promotes the presence of F

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4. CONCLUSIONS AND AREAS OF FUTURE RESEARCH Ten years of research into multifunctional and selective adsorbents has resulted in the successful development and optimization of six adsorbents tailored for use removing the oxoanions As and Se in a variety of competitive systems conditions. Current research efforts within the Zimmerman Lab include developing adsorbents capable of selectively removing As and Se over other strong oxoanion competitors such as silicate and sulfate, as well as targeting other oxoanions of interest, such as Cr(VI), W, and V. We are also currently exploring the possibility of utilizing quantum calculations and DFT modeling in conjunction with advanced spectroscopy (e.g., EXAFS and ATR-FTIR) to exploit the natural affinities to certain surface structures toward selective adsorption of priority contaminants. In addition to these mechanistic studies, another critical research priority is to scale up these technologies and tailor them for use in flow-through and bench-scale systems. Through these research efforts, we aim to continue to be at the forefront of the development of sustainable, selective, and multifunctional water treatment technologies.

Through a statistical analysis relating adsorption capacity, pH = 3.5 ± 0.5 (to maximize adsorption capacities), (Figure 5) to particle size, surface area, frontier facet, and crystallinity, we confirmed our postulate that frontier facet is a larger determinant in adsorption capacity than surface area.47 We then sought to determine how facet is related to binding environment. Extended X-ray adsorption fine structure (EXAFS) spectroscopy (Figure 6) examining how Se binds

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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.accounts.8b00668.

Figure 6. Se k-edge EXAFS of HSeO3− (A) and SeO42− (B) bound onto facet defined n-α-Fe2O3. The modeled data is represented by the dotted line whereas the solid line is the processed EXAFS data. Spectra A−D are the respective particles, which vary by size and surface area, and spectra E are the corresponding reference spectra. The second set of peaks represent binding to the iron oxide. The location of those peaks with corresponding shoulders suggest the presence of bi- and mononuclear species. Adapted with permission from ref 47. Copyright 2019 Elsevier.

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Presenting more details on NMO and TMCC adsorbents (PDF)

AUTHOR INFORMATION

Corresponding Author

*Telephone: +1 (203) 432-9703. E-mail: julie.zimmerman@ yale.edu. ORCID

to particles A−D47 showed SeO42− formed primarily outersphere complexes with some proportion of inner-sphere complex onto n-α-Fe2O3. Alternately, HSeO3− binds primarily through edge sharing, inner-sphere bidentate mononuclear complexes on the (012) facet, whereas on the (110) facet, HSeO3− binds primarily via corner sharing, bidentate binuclear complexes. The capacity data shows greater adsorption of HSeO3− to the (012) facet than the (110) facet. Compared to other reports such as Huang et al., it appears more generally that bidentate binuclear complexes prefer the (110) facet.47,48 We also observed enhanced adsorption capacity of SeO42− onto the (012) facet, in agreement with Sugimoto and Wang, who saw that SO42− (similar in structure to SeO42−) also had increased adsorption on the more reactive (012) facets.49 Based on these results, we believe we can harness this information to design selective nanoparticle-based adsorption systems. As a simple example, it may be possible to selectively adsorb HSeO3− over SeO42− through engineered NMOs with facets selective for bidentate binuclear complexes. This approach can be expanded to engineer crystal facets preferential to binding inorganics of interest over other background oxoanions present in the system.

Julie B. Zimmerman: 0000-0002-5392-312X Notes

The authors declare no competing financial interest. Biographies Lauren N. Pincus is a 4th year Ph.D. candidate in the Zimmerman Lab through the Yale School of Forestry and Environmental Studies and the Yale Center for Green Chemistry and Green Engineering. Lauren’s Ph.D. dissertation focuses on the development of multifunctional and selective biomaterial adsorbents for removal of oxoanions. She received her B.A. in Chemistry and Geology from Middlebury College. Amanda W. Lounsbury is a postdoctoral associate in Chemical and Environmental Engineering at Yale University under Professor Julie Zimmerman, where she completed her Ph.D. as an EPA STAR Fellow. Her Ph.D. work focused on mechanistic understanding of nanometal oxides towards the sustainable design of water treatment systems. Julie Beth Zimmerman is a Professor jointly appointed to the Department of Chemical and Environmental Engineering and the School of Forestry and Environment at Yale University. Dr. Zimmerman is the Deputy Director for Research at the Yale Center for Green Chemistry and Green Engineering. Her research interests G

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focus broadly on green chemistry and green engineering with an emphasis on renewable chemicals, materials, and fuels, sustainable water treatment, and sustainability assessments of emerging technologies.

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ACKNOWLEDGMENTS This work was supported by the NSF Nanosystems Engineering Research Center for Nanotechnology-Enabled Water Treatment (ERC-1449500). We thank Dr. Min Li and the Yale Materials Characterization Core, Dr. Jonas Karosas and the Yale Analytical and Stable Isotope Center, and Dr. Make Rooks and the Yale Institute for Nano and Quantum Engineering. Zeta potential and DLS measurements were performed in the Yale Facility for Light Scattering. Research described in this work was performed at the Canadian Light Source, which is supported by the Natural Sciences and Engineering Research Council of Canada, the National Research Council Canada, the Canadian Institutes of Health Research, the Province of Saskatchewan, Western Economic Diversification Canada, and the University of Saskatchewan. This research used resources of the Advanced Photon Source, a U.S. Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory under Contract No. DE-AC0206CH11357.

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DOI: 10.1021/acs.accounts.8b00668 Acc. Chem. Res. XXXX, XXX, XXX−XXX