Multifunctional photoactive and selective adsorbent for arsenite and arsenate: Evaluation of nano titanium dioxide-enabled chitosan cross-linked with copper

Nanoscience and nanotechnology for water remediation: an earnest hope toward sustainability

Water pollution and the global freshwater crisis are the most alarming concerns of the 21st century, as they threaten the sustainability and ecological balance of the environment. The growth of global population, climate change, and expansion of industrial processes are the main causes of these issues. Therefore, effective remediation of polluted water by means of detoxification and purification is of paramount importance. To this end, nanoscience and nanotechnology have emerged as viable options that hold tremendous potential toward the advancement of wastewater treatment methods to enhance treatment efficiency along with augmenting water supply via utilization of unconventional water sources. Materials at the nano level have shown great promise toward water treatment applications owing to their unique physicochemical properties. In this focus article, we highlight the role of new fundamental properties at the nano scale and material properties that are drastically increased due to the nano dimension (e.g. volume-surface ratio) and highlight their impact and potential toward water treatment. We identify and discuss how nano-properties could improve the three main domains of water remediation: the identification of pollutants, their adsorption and catalytic degradation. After discussing all the beneficial aspects we further discuss the key challenges associated with nanomaterials for water treatment. Looking at the current state-of-the-art, the potential as well as the challenges of nanomaterials, we believe that in the future we will see a significant impact of these materials on many water remediation strategies.

Mechanistic Insights into the Selectivity for Arsenic over Phosphate Adsorption by Fe3+-Cross-Linked Chitosan Using DFT

Fe3+-cross-linked chitosan exhibits the potential for selectively adsorbing arsenic (As) over competing species, such as phosphate, for water remediation. However, the effective binding mechanisms, bond nature, and controlling factor(s) of the selectivity are poorly understood. This study employs ab initio calculations to examine the competitive binding of As(V), P(V), and As(III) to neat chitosan and Fe3+-chitosan. Neat chitosan fails to selectively bind As oxyanions, as all three oxyanions bind similarly via weak hydrogen bonds with preferences of P(V) = As(V) > As(III). Conversely, Fe3+-chitosan selectively binds As(V) over As(III) and P(V) with binding energies of -1.9, -1, and -1.8 eV for As(V), As(III), and P(V), respectively. The preferences are due to varying Fe3+-oxyanion donor-acceptor characteristics, forming covalent bonds with distinct strengths (Fe-O bond ICOHP values: - 4.9 eV/bond for As(V), - 4.7 eV/bond for P(V), and -3.5 eV/bond for As(III)). Differences in pKa between As(V)/P(V) and As(III) preclude any preference for As(III) under typical environmental pH conditions. Furthermore, our calculations suggest that the binding selectivity of Fe3+-chitosan exhibits a pH dependence. These findings enhance our understanding of the Fe3+-oxyanion interaction crucial for preferential oxyanion binding using Fe3+-chitosan and provide a lens for further exploration into alternative transition-metal-chitosan combinations and coordination chemistries for applications in selective separations.

A review on chitosan/metal oxide nanocomposites for applications in environmental remediation

A cleaner and safer environment is one of the most important requirements in the future. It has become increasingly urgent and important to fabricate novel environmentally-friendly materials to remove various hazardous pollutants. Compared with traditional materials, chitosan is a more environmentally friendly material due to its abundance, biocompatibility, biodegradability, film-forming ability and hydrophilicity. As an abundant of -NH2 and -OH groups on chitosan molecular chain could chelate with all kinds of metal ions efficiently, chitosan-based materials hold great potential as a versatile supporting matrix for metal oxide nanomaterials (MONMs) (TiO2, ZnO, SnO2, Fe3O4, etc.). Recently, many chitosan/metal oxide nanomaterials (CS/MONMs) have been reported as adsorbents, photocatalysts, heterogeneous Fenton-like agents, and sensors for potential and practical applications in environmental remediation and monitoring. This review analyzed and summarized the recent advances in CS/MONMs composites, which will provide plentiful and meaningful information on the preparation and application of CS/MONMs composites for wastewater treatment and help researchers to better understand the potential of CS/MONMs composites for environmental remediation and monitoring. In addition, the challenges of CS/MONM have been proposed.

Layered Double Hydroxide-Bismuth Molybdate Hybrids toward Water Remediation via Selective Adsorption of Anionic Species

The steady release of anthropogenic toxins into the biosphere is compromising water security globally. Herein, CoAl layered double hydroxide, a clay-like layered material with a cationic surface charge, was organically modified and used to template the growth of Bi₂MoO₆. The resulting nanohybrid selectively removed the anionic dye methyl orange from aqueous solution and showed an enhancement of greater than 300% in the maximum adsorptivity (1.95 mmol/g) compared to modified CoAl layered double hydroxide (0.42 mmol/g). Interestingly, the observed improvement in adsorption occurs without any significant increase in the surface area of the hybrids. Furthermore, these hybrids exhibit increased broadband visible light absorption, and their photoactivity is slightly improved compared to CoAl layered double hydroxide. This study demonstrates that composites of clay-like materials with Aurivillius oxides are promising sorbent materials for water decontamination and photocatalytic antifouling membranes and shows that the synthetic strategy that was first established with an anionic layered silicate nanoclay can be generalized to other ionic layered materials.

Polymeric biomolecules based nanomaterials: Production strategies and pollutant mitigation as an emerging tool for environmental application

The ever-exploding global population coupled with its anthropogenic impact has imparted unparalleled detrimental effects on the environment and mitigating them has emerged as the prime challenge and focus of the current century. The niche of nanotechnology empowered by composites of biopolymers in the handling of xenobiotics and environmental clean-up has an unlimited scope. The appositeness of biopolymer-nanoparticles (Bp-NPs) for environmental contaminant mitigation has received unique consideration due to its exclusive combination of physicochemical characteristics and other attributes. The current review furnishes exhaustive scrutiny of the current accomplishments in the development of Bp-NPs and biopolymer nanomaterials (Bp-NMs) from various polymeric biomolecules. Special attention was provided for polymeric biomolecules such as cellulose, lignin, starch, chitin, and chitosan, whereas limited consideration on gelatin, alginate, and gum for the development of Bp-NPs and Bp-NMs; together with coverage of literature. Promising applications of tailored biopolymer hybrids such as Bp-NPs and Bp-NMs on environmentally hazardous xenobiotics handling and pollution management are discussed as to their notable environmental applications.

Parasitic Light Absorption, Rate Laws and Heterojunctions in the Photocatalytic Oxidation of Arsenic(III) Using Composite TiO2 /Fe2 O3

Composite photocatalyst‐adsorbents such as TiO₂/Fe₂O₃ are promising materials for the one‐step treatment of arsenite contaminated water. However, no previous study has investigated how coupling TiO₂ with Fe₂O₃ influences the photocatalytic oxidation of arsenic(III). Herein, we develop new hybrid experiment/modelling approaches to study light absorption, charge carrier behaviour and changes in the rate law of the TiO₂/Fe₂O₃ system, using UV‐Vis spectroscopy, transient absorption spectroscopy (TAS), and kinetic analysis. Whilst coupling TiO₂ with Fe₂O₃ improves total arsenic removal by adsorption, oxidation rates significantly decrease (up to a factor of 60), primarily due to the parasitic absorption of light by Fe₂O₃ (88 % of photons at 368 nm) and secondly due to changes in the rate law from disguised zero‐order kinetics to first‐order kinetics. Charge transfer across this TiO₂‐Fe₂O₃ heterojunction is not observed. Our study demonstrates the first application of a multi‐adsorbate surface complexation model (SCM) towards describing As(III) oxidation kinetics which, unlike Langmuir‐Hinshelwood kinetics, includes the competitive adsorption of As(V). We further highlight the importance of parasitic light absorption and catalyst fouling when designing heterogeneous photocatalysts for As(III) remediation.

Establishing the state-of-the-art on the adsorption of coexisting pnictogens in water: A literature review

The occurrence of pnictogens, namely phosphorus, arsenic, and antimony, can be observed in soils, sediments and mining areas, and their coexistence requires a multifaceted approach to the design of adsorption systems to maximize their simultaneous removal efficiency. Therefore, this work aims to provide an extensive literature review of P, As, and Sb adsorption in multicomponent systems and the statistical treatment of the quantitative results. Binary As-P systems have been the most studied in the literature. The oxidation state did not significantly affect the P influence in As adsorbed amount (p = 0.955), but this influence was correlated with the As:P ratio (p < 0.05). A few works have explored As-Sb and Sb-P systems, demonstrating that effective treatments for As do not always reveal a good removal efficiency of the other pnictogens. The Sb adsorbed amount was significantly less affected in the trivalent than in the pentavalent state in both As-Sb and Sb-P systems (p < 0.05). Most of the interactions were competitive, with a few studies reporting synergistic effects for Sb due to the presence of the other elements. Many topics have been identified as lacking in-depth research: ternary As-Sb-P systems, the effect of concentration ratios, pH, and redox conditions (namely those that lead to trivalent species' prevalence), the surface interactions with materials other than iron oxides, and the influence of other aqueous components. This review provides a first step in gathering the relevant literature and approaching the study of adsorption treatment methodology as a complex subject involving many factors.

Advances in biopolymer composites and biomaterials for the removal of emerging contaminants

Domestic, agriculture, and industrial activities contaminate the waterbodies by releasing toxic substances and pathogens. Removal of pollutants from wastewater is critical to ensuring the quality of accessible water resources. Several wastewater treatments are often used. Researchers are increasingly focusing on adsorption, ion exchange, electrostatic interactions, biodegradation, flocculation, and membrane filtration for the efficient reduction of pollutants. Biopolymers are a combination of two or more products produced by the living organisms used to give the desired finished product with a unique attribute. Biomaterials are also similar to traditional polymers by having higher flexibility, biodegradability, low toxicity, and nontoxic secondary byproducts producing ability. Grafting, functionalization, and crosslinking will be used to enhance the characteristics of biopolymers. The present chapter will illustrate some of the important biopolymers and its compos that will impact wastewater treatment in the future. Most commonly used biopolymers including chitosan (CS), activated carbon (AC), carbon-nanotubes (CNTs), and graphene oxide (GO) are discussed. Finally, the opportunities and difficulties for applying adsorbents to water pollution treatment are discussed.

Hybrid Beads of Zero Valent Iron Oxide Nanoparticles and Chitosan for Removal of Arsenic in Contaminated Water

Water contaminated with highly hazardous metals including arsenic (As) is one of the major challenges faced by mankind in the present day. To address this pressing issue, hybrid beads were synthesized with various concentrations of zero valent iron oxide nanoparticles, i.e., 20% (FeCh-20), 40% (FeCh-40) and 60% (FeCh-60) impregnated into a polymer of chitosan. These hybrid beads were employed as an adsorbent under the optimized conditions of pH and time to facilitate the efficient removal of hazardous arsenic by adsorption cum reduction processes. X-ray Diffraction (XRD), Scanning Electron Microscopy (SEM), Fourier Transform Infrared Spectroscopy (FTIR), Brunauer- Emmett-Teller BET, a porosity test and wettability analysis were performed to characterize these hybrid beads. The porosity and contact angle of the prepared hybrid beads decreased with an increase in nanoparticle concentration. The effects of various adsorption factors such as adsorbent composition, contact period, pH value and the initial adsorbate concentration were also evaluated to study the performance of these beads for arsenic treatment in contaminated water. FeCh-20, FeCh-40 and FeCh-60 have demonstrated 63%, 81% and 70% removal of arsenic at optimized conditions of pH 7.4 in 10 h, respectively. Higher adsorption of arsenic by FeCh-40 is attributed to its optimal porosity, hydrophilicity and the presence of appropriate nanoparticle contents. The Langmuir adsorption kinetics described the pseudo second order. Thus, the novel beads of FeCh-40 developed in this work are a potent candidate for the treatment of polluted water contaminated with highly toxic arsenic metals.

Selective adsorption of arsenic over phosphate by transition metal cross-linked chitosan

The ability of transition metal chitosan complexes (TMCs) of varying valence and charge to selectively adsorb As(III) and As(V) over their strongest adsorptive competitor, phosphate is examined. Fe(III)-chitosan, Al(III)-chitosan, Ni(II)-chitosan, Cu(II)-chitosan, and Zn(II)-chitosan are synthesized, characterized via Attenuated Total Reflectance-Fourier Transform Infrared spectroscopy (ATR-FTIR) and X-ray Diffractometry (XRD), and their selective sorption capabilities towards As(III) and As(V) in the presence of phosphate are evaluated. It was found that the stability of the metal-chitosan complexes varied, with Al(III)- and Zn(II)-chitosan forming very unstable complexes resulting in precipitation of gibbsite, and Wulfingite and Zincite, respectively. Cu(II)-, Ni(II)-, and Fe(III)-chitosan formed a mixture of monodentate and bidentate complexes. The TMCs which formed the bidentate complex (Cu(II)-, Ni(II)-, and Fe(III)-) showed greater adsorption capability for As(V) in the presence of phosphate. Using the binary separation factor ∝ t / c , it can be shown that only Fe(III)-chitosan is selective for As(V) and As(III) over phosphate. Density Functional Theory (DFT) modeling and extended X-ray adsorption fine structure (EXAFS) determined that Fe(III)-chitosan and Ni(II)-chitosan adsorbed As(V) and As(III) via inner-sphere complexation, while Cu(II)-chitosan formed mainly outer-sphere complexes with As(V) and As(III). These differences in complexation likely result in the observed differences in selective adsorption capability towards As(V) and As(III) over phosphate. It is hypothesized that the greater affinity of Fe(III)- and Ni(II)-chitosan towards As(V) and As(III) compared to Cu(II)-chitosan is due to their forming less-stable, more reactive chitosan complexes as predicted by the Irving Williams Series.

Sustainable Removal of Contaminants by Biopolymers: A Novel Approach for Wastewater Treatment. Current State and Future Perspectives

Naturally occurring substances or polymeric biomolecules synthesized by living organisms during their entire life cycle are commonly defined as biopolymers. Different classifications of biopolymers have been proposed, focusing on their monomeric units, thus allowing them to be distinguished into three different classes with a huge diversity of secondary structures. Due to their ability to be easily manipulated and modified, their versatility, and their sustainability, biopolymers have been proposed in different fields of interest, starting from food, pharmaceutical, and biomedical industries, (i.e., as excipients, gelling agents, stabilizers, or thickeners). Furthermore, due to their sustainable and renewable features, their biodegradability, and their non-toxicity, biopolymers have also been proposed in wastewater treatment, in combination with different reinforcing materials (natural fibers, inorganic micro- or nano-sized fillers, antioxidants, and pigments) toward the development of novel composites with improved properties. On the other hand, the improper or illegal emission of untreated industrial, agricultural, and household wastewater containing a variety of organic and inorganic pollutants represents a great risk to aquatic systems, with a negative impact due to their high toxicity. Among the remediation techniques, adsorption is widely used and documented for its efficiency, intrinsic simplicity, and low cost. Biopolymers represent promising and challenging adsorbents for aquatic environments’ decontamination from organic and inorganic pollutants, allowing for protection of the environment and living organisms. This review summarizes the results obtained in recent years from the sustainable removal of contaminants by biopolymers, trying to identify open questions and future perspectives to overcome the present gaps and limitations.

Selective removal of arsenic in water: A critical review

Selective removal of arsenic (As) is the key challenge for any of As removal mechanisms as this not only increases the efficiency of removal of the main As species (neutral As(III) and As(V) hydroxyl-anions) but also allows for a significant reduction of waste as it does not co-remove other solutes. Selective removal has a number of benefits: it increases the capacity and lifetime of units while lowering the cost of the process. Therefore, a sustainable selective mitigation method should be considered concerning the economic resources available, the ability of infrastructure to sustain water treatment, and the options for reuse and/or safe disposal of treatment residuals. Several methods of selective As removal have been developed, such as precipitation, adsorption and modified iron and ligand exchange. The biggest challenge in selective removal of As is the presence of phosphate in water which is chemically comparable with As(V). There are two types of mechanisms involved with As removal: Coulombic or ion exchange; and Lewis acid-base interaction. Solution pH is one of the major controlling factors limiting removal efficiency since most of the above-mentioned methods depend on complexation through electrostatic effects. The different features of two different As species make the selective removal process more difficult, especially under natural conditions. Most of the selective As removal methods involve hydrated Fe(III) oxides through Lewis acid-base interaction. Microbiological methods have been studied recently for selective removal of As, and although there have been only a small number of studies, the method shows remarkable results and indicates positive prospects for the future.

Starch, cellulose, pectin, gum, alginate, chitin and chitosan derived (nano)materials for sustainable water treatment: A review

Natural biopolymers, polymeric organic molecules produced by living organisms and/or renewable resources, are considered greener, sustainable, and eco-friendly materials. Natural polysaccharides comprising cellulose, chitin/chitosan, starch, gum, alginate, and pectin are sustainable materials owing to their outstanding structural features, abundant availability, and nontoxicity, ease of modification, biocompatibility, and promissing potentials. Plentiful polysaccharides have been utilized for making assorted (nano)catalysts in recent years; fabrication of polysaccharides-supported metal/metal oxide (nano)materials is one of the effective strategies in nanotechnology. Water is one of the world's foremost environmental stress concerns. Nanomaterial-adorned polysaccharides-based entities have functioned as novel and more efficient (nano)catalysts or sorbents in eliminating an array of aqueous pollutants and contaminants, including ionic metals and organic/inorganic pollutants from wastewater. This review encompasses recent advancements, trends and challenges for natural biopolymers assembled from renewable resources for exploitation in the production of starch, cellulose, pectin, gum, alginate, chitin and chitosan-derived (nano)materials.

Agro-Waste Derived Biomass Impregnated with TiO2 as a Potential Adsorbent for Removal of As(III) from Water

A novel type of adsorbent, TiO2 impregnated pomegranate peels (PP@TiO2) was successfully synthesized and its efficacy was investigated based on the removal of As(III) from water. The adsorbent was characterized using Scanning Electron Microscopy (SEM), Energy Dispersive X-ray Spectrometer (EDS), X-ray Diffraction (XRD) analysis, and Fourier Transform Infrared (FTIR) Spectroscopy, to evaluate its morphology, elemental analysis, crystallinity, and functional groups, respectively. Batch experiments were conducted on PP@TiO2 for As(III) adsorption to assess the adsorption isotherm, effect of pH, and adsorption kinetics. Characterization data suggested that TiO2 was successfully impregnated on the biomass substrate. The equilibrium data better fitted to the Langmuir isotherm model having a maximum adsorption capacity of 76.92 mg/g and better distribution coefficients (KD) in the order of ~103 mL/g. The highest percentage of adsorption was found at neutral pH. The adsorption kinetics followed the pseudo-2nd-order model. X-ray Photoelectron Spectroscopy (XPS) of the adsorption product exhibited that arsenic was present as As(III) and partially oxidized to As(V). PP@TiO2 can work effectively in the presence of coexisting anions and could be regenerated and reused. Overall, these findings suggested that the as-prepared PP@TiO2 could provide a better and efficient alternative for the synergistic removal of As(III) from water.

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.

Megamerger of biosorbents and catalytic technologies for the removal of heavy metals from wastewater: Preparation, final disposal, mechanism and influencing factors

Heavy metal pollution, because of its high toxicity, non-biodegradability and biological enrichment, has been identified as a global aquatic ecosystems threat in recent decades. Due to the high efficiency, low cost, satisfactory recyclability, easy storage and separation, biosorbents have exhibited a promising prospect for heavy metals treatment in aqueous phase. This article comprehensively summarized different types of biosorbents derived from available low-cost raw materials such as agricultural and forestry wastes. The raw materials obtained are treated with conventional pretreatment or novel methods, which can greatly enhance the adsorption performance of the biosorbents. The suitable immobilization methods can not only further enhance the adsorption performance of the biosorbents, but also facilitate the process of separating the biosorbents from the wastewater. In addition, once biosorbents are put into large-scale use, the final disposal problems cannot be avoided. Therefore, it is necessary to review the currently accepted final disposal methods of biosorbents. Moreover, through the analysis of the adsorption and desorption mechanisms of biosorbents, it is not only beneficial to find the better methods to improve the adsorption performance of the biosorbents, but also better to explain the influencing factors of adsorption effect for biosorbents. Especially, different from many researches focused on biosorbents, this work highlighted the combination of biosorbents with catalytic technologies, which provided new ideas for the follow-up research direction of biosorbents. Finally, the purpose of this paper is to inject new impetus into the future development of biosorbents.

Rapid Preparation of TiO2-x and Its Photocatalytic Oxidation for Arsenic Adsorption under Visible Light

A rapid preparation method of TiO_2-x by thermal treatment using H₂TiO₃ as the precursor was proposed compared with the static hydrogenation thermal treatment process. Its adsorption properties of arsenic under visible light were explored and investigated as well. Various colors of TiO_2-x were prepared and characterized via XRD, TEM, BET, and so on. The results indicate that the method of rapid preparation is feasible. The TiO_2-x exhibits a larger particle size that varied from 10 nm to 2 μm, and deeper color products were obtained as the treatment temperature increased from 600 to 900 °C. Light yellow TiO_2-x was prepared after increasing the temperature from 600 to 900 °C, Ti₄O₇ and Ti₆O₁₁ with a dark color were formed under a H₂ atmosphere at 1500 °C. The arsenic adsorption performances of some samples under visible light were tested, and reveal a high efficiency of TiO_2-x in the photocatalytic oxidation arsenic adsorption under visible light, the conversion ratio of As(III) photocatalytic oxidation fluctuates around 2.85 mgAsg^-1 h^-1. In the absence of visible light, the adsorption capacities for As(III) and As(V) are 3.7 and 42.7 mg/g, respectively, at pH = 3. Under visible light condition, the adsorption capacity of As(III) increases sharply to 15.6 mg/g, which provides the foundation for a new application of TiO_2-x in the field of arsenic adsorption.

Chitosan-TiO2: A Versatile Hybrid Composite

In recent years, a strong interest has emerged in hybrid composites and their potential uses, especially in chitosan–titanium dioxide (CS–TiO₂) composites, which have interesting technological properties and applications. This review describes the reported advantages and limitations of the functionalization of chitosan by adding TiO₂ nanoparticles. Their effects on structural, textural, thermal, optical, mechanical, and vapor barrier properties and their biodegradability are also discussed. Evidence shows that the incorporation of TiO₂ onto the CS matrix improves all the above properties in a dose-dependent manner. Nonetheless, the CS–TiO₂ composite exhibits great potential applications including antimicrobial activity against bacteria and fungi; UV-barrier properties when it is used for packaging and textile purposes; environmental applications for removal of heavy metal ions and degradation of diverse water pollutants; biomedical applications as a wound-healing material, drug delivery system, or by the development of biosensors. Furthermore, no cytotoxic effects of CS–TiO₂ have been reported on different cell lines, which supports their use for food and biomedical applications. Moreover, CS–TiO₂ has also been used as an anti-corrosive material. However, the development of suitable protocols for CS–TiO₂ composite preparation is mandatory for industrial-scale implementation.

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.

Design and synthesis of a calcium modified quaternized chitosan hollow sphere for efficient adsorption of SDBS

In this study, a new pleated hollow sphere material (CS@Ca@CTA) was prepared by mixing chitosan (CS) and calcium chloride (CaCl₂) in acetic acid solutions followed by crosslinking with glutaraldehyde (GA), and quaternized using (3-chloro-2 hydroxypropyl) trimethylammonium chloride (CTA). The natural biopolymer chitosan (CS) was used to prepare an environmentally friendly adsorbent material which can efficiently adsorb sodium dodecylbenzene sulfonate (SDBS). The adsorption of SDBS on CS@Ca@CTA can be efficiently improved by CTA at different pH values and Ca can promote the precipitation. The adsorption process of SDBS is as follows. First, SDBS can be adsorbed by electrostatic attraction and Cl^- of CTA is used to ion exchange DBS^- of SDBS. Then DBS^- can bind stably with the Ca²⁺ in a bidentate form. Furthermore, CS contains NH₂ and OH groups, which can provide enormous vacant active sites to adsorb SDBS, and a pleated surface have an ability to capture SDBS. The results indicated that the saturated adsorption capacity of CS@Ca@CTA up to 2300 mg g^-1 at pH 3.0 within 240 min. Additionally, adsorption kinetics, isotherm and thermodynamic parameters were discussed. The aim of this article is to present CS@Ca@CTA, which has a great effect on adsorbing the SDBS.