Preferential adsorption of selenium oxyanions onto {1 1 0} and {0 1 2} nano-hematite facets

Liquid-liquid extraction of selenium (IV) ions from hydrochloric acid solution using Aliquat 336 dissolved in kerosene

Solvent extraction of selenium(IV) ions from highly concentrated hydrochloric acid using 0.4 mol/L Aliquat 336 dissolved in kerosene was investigated. As a modifying agent, 1-octanol (10% v/v) was added to the organic phase to avoid the third phase formation. The effect of different parameters affecting the liquid-liquid extraction of selenium(IV) such as the acid concentration, shaking time, metal ion concentration in the aqueous phase, loading capacity, diluents, and temperature, was studied. The results indicate that selenium(IV) is extracted efficiently by 0.4 mol/L Aliquat 336 dissolved in kerosene. It was noticed that the extraction increased with the increase in the acid and Aliquat 336 concentrations, reaching an extraction percentage of about 92% at 8 mol/L HCl and 97.1% at 1 mol/L extractant. The extracted organic species is postulated to be [H2SeO2Cl2.2R4NCl]org by using the slope analysis method, and the value of Kex for selenium(IV) extraction was found to be 26.17 ± 2 M- 2. The structure of the extracted organic species was confirmed by FT-IR. The effect of diluents using various aliphatic and aromatic diluents indicated that kerosene is the most preferred diluent. This is owing to safety ground purpose, economic consideration, the lower cost, availability, and lower toxicity. Thermodynamic parameters indicate the endothermic nature for the solvent extraction of selenium(IV) for the investigated system according to the positive value obtained of the enthalpy change (ΔH). Depending on the obtained results, the method was used to recover selenium(IV) from a simulated solution synthesized in hydrochloric acid medium, which is expected in anode slime leach liquor solution.

Extreme accumulation of ammonia on electroreduced mackinawite: An abiotic ammonia storage mechanism in early ocean hydrothermal systems

An increasing amount of evidence suggests that early ocean hydrothermal systems were sustained sources of ammonia, an essential nitrogen species for prebiotic synthesis of life's building blocks. However, it remains a riddle how the abiotically generated ammonia was retained at the vent-ocean interface for the subsequent chemical evolution. Here, we demonstrate that, under simulated geoelectrochemical conditions in early ocean hydrothermal systems ([Formula: see text][Formula: see text] V versus the standard hydrogen electrode), mackinawite gradually reduces to zero-valent iron ([Formula: see text]), generating interlayer [Formula: see text] sites. This reductive conversion leads to an up to 55-fold increase in the solid/liquid partition coefficient for ammonia, enabling over 90% adsorption of 1 mM ammonia in 1 M NaCl at neutral pH. A coordinative binding of ammonia on the interlayer [Formula: see text] sites was computed to be the major mechanism of selective ammonia adsorption. Mackinawite is a ubiquitous sulfide precipitate in submarine hydrothermal systems. Given its reported catalytic function in amination, the extreme accumulation of ammonia on electroreduced mackinawite should have been a crucial initial step for prebiotic nitrogen assimilation, paving the way to the origin of life.

Elucidating the Role of Capping Agents in Facet-Dependent Adsorption Performance of Hematite Nanostructures

Organic capping agents are a ubiquitous and crucial part of preparing reproducible and homogeneous batches of nanomaterials, particularly nanocrystals with well-defined facets. Despite studies reporting surface ligands (e.g., capping agents) having a non-negligible role in catalytic behavior, their impact is less understood in contaminant adsorption, an important consideration given their potential to obfuscate facet-dependent trends in performance. To ascribe observed behaviors to the facet or the ligand, this report evaluates the impact of poly(N-vinyl-2-pyrrolidone) (PVP), a commonly utilized capping agent, on the adsorption performance of nanohematite particles of varying prevailing facet in the removal of selenite (Se(IV)) as a model system. The PVP capping agent reduces the available surface area for contaminant binding, thus resulting in a reduction in overall Se(IV) adsorbed. However, accounting for the effects of surface area, {012}-faceted nanohematite demonstrates a significantly higher sorption capacity for Se(IV) compared with that of {001}-faceted nanohematite. Notably, chemical treatment is minimally effective in removing strongly bound PVP, indicating that complete removal of surface ligands remains challenging.

Structural-controlled formation of nano-particle hematite and their removal performance for heavy metal ions: A review

Hematite is ubiquitous in nature and holds great promise for a wide variety of applications in many frontiers of environmental issues such as heavy metal remediation in environment. Over the past decades, numerous efforts have been made to control and tailor the crystal structures of hematite to improve its adsorption performance for heavy metal ions (HMIs). It is now well established that the adsorption behavior of hematite nanocrystals is strongly affected by their particle sizes, crystal facet contributions, and defective structures. This review examined the size- and facet-dependent hematite, as well as the defective hematite according to their fabrication methods and growth mechanisms. Furthermore, the adsorption performance of various hematite particles for HMIs were introduced and compared to clarify the structure-active relationships of hematite. We also overviewed the advances in charge distribution (CD)-multisite complexation (MUSIC) modeling studies about the HMIs adsorption at the hematite-water interface and the binding parameters. The Present review systematically describes how the formation conditions impact the structural and surface properties of hematite particles, thereby providing new strategies for enhancing the performance of hematite for environmental remediation.

Exceptional removal and immobilization of selenium species by bimetal-organic frameworks

Binary metallic organic frameworks can always play excellent functions for pollutants removal. One binary MOFs, UiO-66(Fe/Zr)), was newly synthesized and applied to remove aquatic selenite (SeIV) and selenate (SeVI). The adsorption behaviors and mechanisms were investigated using batch experiments, spectroscopic analyses, and theoretical calculations (DFT). The characterization results showed that the material inherited the topological structure of UiO-66 and excellent thermal stability. The large specific surface area (467.52 m²/g) and uniform mesoporous structures of the synthesized MOFs resulted in fast adsorption efficiency and high adsorption capacity for selenium species. The adsorbent kept high adsorption efficiency in a wide pH range from 2 to 11 with good anti-interference ability. The maximum adsorption capacity for Se(IV) and Se(VI) reached as high as 196 mg/g at pH 3 and 258 mg/g at pH 5, respectively. The process was conformed to fit pseudo-second-order kinetics and Langmuir isotherm, and could be explained by the formation of Fe/Zr-O-Se bond on the material surface, which was interpreted by the results of XPS, FTIR and DFT calculation. The regeneration and TCLP experiments demonstrated that UiO-66(Fe/Zr) could be regenerated for five cycles without obvious decrease of efficiencies, and the leaching rate of the adsorbed Se(IV) and Se(VI) in the spent adsorbent were only 4.8% and 2.3%. More than 99% of original Se(IV) and Se(VI) in the lake and tap water samples (1.0 mg/L of Se) could be removed in 2.0 h.

Impact of dissolved oxygen and pH on the removal of selenium from water by iron electrocoagulation

Removing dissolved selenium (i.e., selenate and selenite) from wastewater is a challenging issue for a range of industries. Iron electrocoagulation can produce Fe(II)-containing solids that can adsorb and chemically reduce dissolved Se. In a series of bench-scale experiments we investigated the effects of dissolved oxygen (fully oxic, partially oxic, and strictly anoxic) and pH (6 and 8) on the rate and extent of dissolved selenate and selenite removal by iron electrocoagulation. These studies combined measurements of the aqueous phase with the direct characterization of the resulting solids. Among the conditions studied the rate and extent of dissolved selenium (Se) removal were highest at pH 8 and strictly anoxic conditions. X-ray absorption spectroscopy demonstrated that in the absence of oxygen, Se was primarily transformed to elemental selenium (Se0) and selenide. Green rust that formed in the suspension during electrocoagulation played a key role as a reductant and sorbent of Se. At pH 6 dissolved oxygen did not affect the rates and extents of dissolved Se removal. Under all the conditions studied, dissolved Se removal was more effective with iron electrocoagulation than with the direct addition of pre-synthesized green rust or ferrous hydroxide. The most rapid and substantial dissolved Se removal was achieved by freshly-formed green rust and ferrous hydroxide, which are both Fe(II)-bearing solids. With an improved understanding of the products and mechanisms of the process, iron electrocoagulation can be optimized for removal of Se from wastewater.

Selenium (Se) recovery for technological applications from environmental matrices based on biotic and abiotic mechanisms

Selenium (Se) is an essential element with application in manufacturing from food to medical industries. Water contamination by Se is of concern due to anthropogenic activities. Recently, Se remediation has received increasing attention. Hence, different types of remediation techniques are listed in this work, and their potential for Se recovery is evaluated. Sorption, co-precipitation, coagulation and precipitation are effective for low-cost Se removal. In photocatalytic, zero-valent iron and electrochemical systems, the above mechanisms occur with reduction as an immobilization and detoxification process. In combination with magnetic separation, the above techniques are promising for Se recovery. Biological Se oxyanions reduction has been widely recognized as a cost-effective method for Se remediation, simultaneously generating biosynthetic Se nanoparticles (BioSeNPs). Increasing the extracellular production of BioSeNPs and controlling their morphology will benefit its recovery. However, the mechanism of the microbial production of BioSeNPs is not well understood. Se containing products from both microbial reduction and abiotic methods need to be refined to obtain pure Se. Eco-friendly and cost-effective Se refinery methods need to be developed. Overall, this review offers insight into the necessity of shifting attention from Se remediation to Se recovery.

Effects of Al substitution on sorption of diclofenac to Fe(III) (hydr)oxides: roles of phase transition and sorption mechanisms

Fe(III) (hydr)oxides commonly contained many metal impurities such as Al. The incorporation of Al might change the properties of minerals and consequently affect sorption behaviors of pollutants with polar functional groups (e.g., diclofenac (DCF)). In this study, batch experiments and microscale characterization were conducted to investigate the DCF sorption mechanisms to goethite and Al-substituted minerals. Goethite and Al-substituted products (including Al-goethite, Al-goethite-hematite, and Al-hematite) were synthesized with different Al contents (i.e., 0%, 5%, 10%, and 15% (in mol)) by co-precipitation method. Due to difference of ionic radius between Al and Fe and formation of excessive -OH, Al substitution resulted in deviation of cell parameters from the Vegard line. Al substitution caused increasing -OH in Al-goethite and phase transformation caused decreasing -OH in Al-hematite. The total -OH in minerals was positively related to DCF sorption capacity. In the lower initial concentration range (0.4-9 mg/L), the sorption distribution coefficient (Kd) values of goethite, Al-goethite, and Al-hematite were 21.98, 22.25, and 21.18 L/kg, respectively. Desorption characteristics and ion strength effects indicated that DCF sorption to minerals occurred mainly through outer-sphere complexation. Fourier transform infrared analyses revealed that H-bonds could be formed through -OH of minerals and -COOH of DCF, and the H-bond strength on Al-hematite was stronger than that on goethite/Al-goethite. In the normal environmental pH (e.g., 6.0 to 8.0), Kd values of DCF decreased linearly with increasing pH. These findings are helpful for understanding of DCF migration in environment involving Al-substituted minerals.

Selenium removal from water using adsorbents: A critical review

Selenium (Se) has become an increasingly serious water contamination concern worldwide. It is an essential micronutrient for humans and animals, however, can be extremely toxic if taken in excess. Sorption can be an effective treatment for Se removal from a wide range of water matrices. However, despite the synthesis and application of numerous adsorbents for remediation of aqueous Se, there has been no comprehensive review of the sorption capacities of various natural and synthesized sorbents. Herein, literature from 2010 to 2021 considering Se remediation using 112 adsorbents has been critically reviewed and presented in several comprehensive tables including: clay minerals and waste materials (presented in Table 1); zero-valent iron, iron oxides, and binary iron-based adsorbents (Table 2); other metals-based adsorbents (Table 3); carbon-based adsorbents (Table 4); and other adsorbents (Table 5). Each of these tables, and their relevant sections, summarizes preparation/modification methods, sorption capacities of various Se adsorbents, and proposed model/mechanisms of adsorption. Furthermore, future perspectives have been provided to assist in filling noted research gaps for the development of efficient Se adsorbents for real-world applications. This review will help in preliminary screening of various sorbent media to set up Se treatment technologies for a variety of end-users worldwide.

Selenite capture by MIL-101 (Fe) through FeOSe bonds at free coordination Fe sites

Selective immobilization of SeO₃^2- is highly desired for the remediation of Se-contaminated water. Thus, the irreversible sorption of SeO₃^2- ions by adsorbents through unique coordination bonds with high affinity is needed. Herein, we demonstrated that Fe-based metal-organic framework (MOF) (Fe-MIL-101) with free coordination sites (FCSs) enabled selective and irreversible capture of SeO₃^2- ions from aqueous solution with fast kinetics and a high uptake capacity of 183.7 mg∙g^-1, owing to large MOF apertures and substantial numbers of FCSs as capture sites through forming Fe-O-Se bonds. Meanwhile, Fe-MIL-101 maintained excellent performance in a broad pH range (4-11) and high selectivity for SeO₃^2- ions in the presence of excessive competitive anions (e.g., CO₃^2-, PO₄^3-). Density functional theory (DFT) calculation, extended X-ray absorption fine structure (EXAFS), and Mössbauer fittings confirmed that the capture on Fe-MIL-101 was through the Fe-O-Se coordination bonds between FCSs and SeO₃^2-. Moreover, Fe-MIL-101 could effectively remove SeO₃^2- in simulated natural water and sewage by overcoming the influence of co-existing ions and organic matters. This study highlights new opportunities for the design of MOF-based materials for removing toxic and radioactive anions with irreversibility and high selectivity from natural and waste water.

Questioning the suitability of available microplastics models for risk assessment - A critical review

The rise of microplastic (MP) pollution in the environment has been bolstering concerns regarding MPs' unknown environmental fate, transport, and potential toxicity toward living forms. However, the use of real environmental plastics for risk assessment is often hindered due to technical and practical challenges such as plastics' heterogeneity and their wide size distribution in the environment. To overcome this issue, most available data in the field is generated using plastic models as surrogates for environmental samples. In this critical review, we describe the gaps in risk assessments drawn from these plastic models. Specifically, we compare physicochemical properties of real environmental plastic particles to synthesized polymeric micro-beads, one of the most commonly used plastic models in current literature. Several surface and bulk characteristics including size, surface chemistry, polymer type, and morphology are shown to not only be inherently different between environmental MP's and synthesized micro-beads, but also drive behavior in fate, transport, and toxicity assays. We highlight the importance of expressing real-world physicochemical characteristics in representative MP models and outline how current state-of-the-art models are limited in this regard. To address this issue, we suggest future areas of research such as combinations of mechanical, photochemical, and thermal degradation processes to simulate real-world weathering, all in an effort to increase realism of plastic modeling and allow more robust and reliable environmental MP risk assessment in the future.

Facet-dependent surface charge and Pb2+ adsorption characteristics of hematite nanoparticles: CD-MUSIC-eSGC modeling

Accurate prediction of the environmental fate of Pb depends on the understanding of Pb coordination to mineral surfaces. Here, the proton and Pb adsorption and speciation on hematite nanocrystals with different exposed crystallographic facets were investigated. High-resolution transmission electron microscopy images revealed that hematite nanoplates (HNP) were of 75.3 ± 9.5% (001) facets and 24.6 ± 9.3% (012) facets, while hematite nanocubes (HNC) were of 76.0 ± 11.1% (012) facets and 24.0 ± 3.2% (110) facets. Our modeling results revealed that the proton affinity constant (log K_H) of ≡FeOH^-0.5 and ≡Fe₃O^-0.5 was 7.8 and 10.8 on hematite (012) facets, and changed to 7.7 and 11.7 on (110) facets, respectively. Owing to the different atomic arrangements, (012) facets not only have higher adsorption performance for Pb, but also present a greater dependence on pH than (110) facets. Additionally, our modeling further indicated that (012) facets bind Pb via both bidentate and tridentate complexes, while (110) facets bind Pb only through bidentate complexes at pH 3.0-6.5. These results facilitate a more detailed understanding of the complex species of Pb on hematite surface while also provide new insight into the reactivity mechanism of individual hematite facets.

Critical Review of Advances in Engineering Nanomaterial Adsorbents for Metal Removal and Recovery from Water: Mechanism Identification and Engineering Design

Nanomaterial adsorbents (NAs) have shown promise to efficiently remove toxic metals from water, yet their practical use remains challenging. Limited understanding of adsorption mechanisms and scaling up evaluation are the two main obstacles. To fully realize the practical use of NAs for metal removal, we review the advanced tools and chemical principles to identify mechanisms, highlight the importance of adsorption capacity and kinetics on engineering design, and propose a systematic engineering scenario for full-scale NA implementation. Specifically, we provide in-depth insight for using density functional theory (DFT) and/or X-ray absorption fine structure (XAFS) to elucidate adsorption mechanisms in terms of active site verification and molecular interaction configuration. Furthermore, we discuss engineering issues for designing, scaling, and operating NA systems, including adsorption modeling, reactor selection, and NA regeneration, recovery, and disposal. This review also prioritizes research needs for (i) determining NA microstructure properties using DFT, XAFS, and machine learning and (ii) recovering NAs from treated water. Our critical review is expected to guide and advance the development of highly efficient NAs for engineering applications.

Nano-structural effects on Hematite (α-Fe2O3) nanoparticle radiofrequency heating

Nano-sized hematite (α-Fe₂O₃) is not well suited for magnetic heating via an alternating magnetic field (AMF) because it is not superparamagnetic-at its best, it is weakly ferromagnetic. However, manipulating the magnetic properties of nano-sized hematite (i.e., magnetic saturation (Ms), magnetic remanence (Mr), and coercivity (Hc)) can make them useful for nanomedicine (i.e., magnetic hyperthermia) and nanoelectronics (i.e., data storage). Herein we study the effects of size, shape, and crystallinity on hematite nanoparticles to experimentally determine the most crucial variable leading to enhancing the radio frequency (RF) heating properties. We present the synthesis, characterization, and magnetic behavior to determine the structure-property relationship between hematite nano-magnetism and RF heating. Increasing particle shape anisotropy had the largest effect on the specific adsorption rate (SAR) producing SAR values more than 6 × greater than the nanospheres (i.e., 45.6 ± 3 W/g of α-Fe₂O₃ nanorods vs. 6.89 W/g of α-Fe₂O₃ nanospheres), indicating α-Fe₂O₃ nanorods can be useful for magnetic hyperthermia.

Efficient removal of selenate in water by cationic poly(allyltrimethylammonium) grafted chitosan and biochar composite

The discovery of cheap and eco-friendly functional materials for the removal of anionic heavy metal ions is still challenging in the treatment of heavy metal-contaminated water. Herein, a new poly(allyltrimethylammonium) grafted chitosan and biochar composite (PATMAC-CTS-BC) was introduced for the removal of selenate (SeO₄^2-) in water. Results suggest that the PATMAC-CTS-BC showed a rapid removal of SeO₄^2- with efficiency of >97% within 10 min and it followed a pseudo-second-order model. High capacity of SeO₄^2- adsorption by the composite was achieved, with maximum value of 98.99 mg g^-1 based on Langmuir model, considerably higher than most of reported adsorbents. The thermodynamic results reflected the spontaneous and exothermic nature of SeO₄^2- adsorption onto the composite. The composite could be applied at a wide initial pH range (2-10) with high removal efficiency of SeO₄^2- because of permanent positive charges of quaternary ammonium groups (=N⁺-). The removal mechanisms of SeO₄^2- were mainly attributed to electrostatic interactions with =N⁺- and protonated -NH₃⁺ groups, and redox-complexation interactions with -NH₂, -NH-, and -OH groups. Besides SeO₄^2-, the hexavalent chromium (Cr₂O₇^2-) was considered as example to further demonstrate the anion removal capability of cationic hydrogel-BC composite. The study outcomes open up new opportunities to efficiently remove anionic heavy metal ions (e.g., SeO₄^2- and Cr₂O₇^2-) in water using these materials.

Toward Informed Design of Nanomaterials: A Mechanistic Analysis of Structure-Property-Function Relationships for Faceted Nanoscale Metal Oxides

Nanoscale metal oxides (NMOs) have found wide-scale applicability in a variety of environmental fields, particularly catalysis, gas sensing, and sorption. Facet engineering, or controlled exposure of a particular crystal plane, has been established as an advantageous approach to enabling enhanced functionality of NMOs. However, the underlying mechanisms that give rise to this improved performance are often not systematically examined, leading to an insufficient understanding of NMO facet reactivity. This critical review details the unique electronic and structural characteristics of commonly studied NMO facets and further correlates these characteristics to the principal mechanisms that govern performance in various catalytic, gas sensing, and contaminant removal applications. General trends of facet-dependent behavior are established for each of the NMO compositions, and selected case studies for extensions of facet-dependent behavior, such as mixed metals, mixed-metal oxides, and mixed facets, are discussed. Key conclusions about facet reactivity, confounding variables that tend to obfuscate them, and opportunities to deepen structure-property-function understanding are detailed to encourage rational, informed design of NMOs for the intended application.

Exploring the Mechanisms of Selectivity for Environmentally Significant Oxo-Anion Removal during Water Treatment: A Review of Common Competing Oxo-Anions and Tools for Quantifying Selective Adsorption

Development of novel adsorbents often neglects the competitive adsorption between co-occurring oxo-anions, overestimating realistic pollutant removal potentials, and overlooking the need to improve selectivity of materials. This critical review focuses on adsorptive competition between commonly co-occurring oxo-anions in water and mechanistic approaches for the design and development of selective adsorbents. Six "target" oxo-anion pollutants (arsenate, arsenite, selenate, selenite, chromate, and perchlorate) were selected for study. Five "competing" co-occurring oxo-anions (phosphate, sulfate, bicarbonate, silicate, and nitrate) were selected due to their potential to compete with target oxo-anions for sorption sites resulting in decreased removal of the target oxo-anions. First, a comprehensive review of competition between target and competitor oxo-anions to sorb on commonly used, nonselective, metal (hydr)oxide materials is presented, and the strength of competition between each target and competitive oxo-anion pair is classified. This is followed by a critical discussion of the different equations and models used to quantify selectivity. Next, four mechanisms that have been successfully utilized in the development of selective adsorbents are reviewed: variation in surface complexation, Lewis acid/base hardness, steric hindrance, and electrostatic interactions. For each mechanism, the oxo-anions, both target and competitors, are ranked in terms of adsorptive attraction and technologies that exploit this mechanism are reviewed. Third, given the significant effort to evaluate these systems empirically, the potential to use computational quantum techniques, such as density functional theory (DFT), for modeling and prediction is explored. Finally, areas within the field of selective adsorption requiring further research are detailed with guidance on priorities for screening and defining selective adsorbents.

Nanocrystal facet modulation to enhance transferrin binding and cellular delivery

Binding of biomolecules to crystal surfaces is critical for effective biological applications of crystalline nanomaterials. Here, we present the modulation of exposed crystal facets as a feasible approach to enhance specific nanocrystal–biomolecule associations for improving cellular targeting and nanomaterial uptake. We demonstrate that facet-engineering significantly enhances transferrin binding to cadmium chalcogenide nanocrystals and their subsequent delivery into cancer cells, mediated by transferrin receptors, in a complex biological matrix. Competitive adsorption experiments coupled with theoretical calculations reveal that the (100) facet of cadmoselite and (002) facet of greenockite preferentially bind with transferrin via inner-sphere thiol complexation. Molecular dynamics simulation infers that facet-dependent transferrin binding is also induced by the differential affinity of crystal facets to water molecules in the first solvation shell, which affects access to exposed facets. Overall, this research underlines the promise of facet engineering to improve the efficacy of crystalline nanomaterials in biological applications.

Modulation of exposed crystal facets enhances transferrin binding to chalcogenide nanocrystals and their subsequent delivery into cancer cells. Facet-dependent protein binding occurs through inner-sphere thiol complexation and is affected by the structure of the first solvation shell.

Facet-Specific Photocatalytic Degradation of Organics by Heterogeneous Fenton Chemistry on Hematite Nanoparticles

Hematite nanoparticles are abundant in the photic zone of aquatic environments, where they play a prominent role in photocatalytic transformations of bound organics. Here, we examine the photocatalytic degradation of rhodamine B by visible light using two different structurally well-defined hematite nanoparticle morphologies. In addition to detailed solid characterization and aqueous kinetics measurements, we also exploit species-selective scavengers in electron paramagnetic resonance spectroscopy to sequester specific reaction channels and thereby assess their impact. The photodegradation rates for nanoplates dominated by {001} facets and nanocubes dominated by {012} facets were 0.13 and 0.7 h^-1, respectively, and the turnover frequencies for the active sites on {001} and {012} were 7.89 × 10^-3 and 3.07× 10^-3 s^-1, yielding apparent activation energies of 17.13 and 24.94 kcal/mol within the energetic span model, respectively. Facet-specific differences appear to be directly not linked with the simple aerial cation site density but instead with their extent of undercoordination. By establishing this linkage, the findings lay a foundation for predicting the photocatalytic degradation efficiency for the myriad of possible hematite nanoparticle morphologies and more broadly help unveil key reactions at the interface that may govern photocatalytic organic transformations in natural and engineered aquatic environments.

Toward Realizing Multifunctionality: Photoactive and Selective Adsorbents for the Removal of Inorganics in Water Treatment

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.