Properties of synthetic monosulfate as a novel material for arsenic removal

Assessment and optimization of As(V) adsorption on hydrogel composite integrating chitosan-polyvinyl alcohol and Fe3O4 nanoparticles and evaluation of their regeneration and reusable capabilities in aqueous media

A modified chitosan-polyvinyl alcohol (PVA) hydrogel was developed by incorporating Fe₃O₄ nanoparticles. Four chitosan-Fe₃O₄ (ChFe) hydrogel types were developed based on chitosan:Fe₃O₄ ratio as 1:0, 1:1, 1:0.5 and 1:0.25. Batch sorption experiments were conducted with different pH, dosage, kinetics, and isotherms. The exhausted ChFe hydrogels were evaluated for their regeneration and reuse capability with different acids and bases. The best hydrogel for arsenic (V) [As(V)] adsorption was 1:0.5 ratio ChFe hydrogel. The highest As(V) adsorption (89 %) was at pH 4 and the adsorption capacity gradually decreased with increasing solution pH. Within the pH 4-6 range, the hydrogel surface became positively charged due to protonation of NH₂ and OH groups in the polymer chain and the positive surface attracted H₂AsO₄^- and HAsO₄^2- oxyanions. The experimental kinetic data was well-fitted to the Elovich model (R² of 0.99) while the Freundlich isotherm model best described the isotherm data (R² of 0.97). The models predicted chemisorption mechanisms on ChFe hydrogel composites. Electrostatic attractions with NH₃⁺ and OH₂⁺, ligand-exchange inner-sphere complexes formation and bidentate corner-sharing (²C) and bidentate edge-sharing (²E) trimetric surface complexes formation have been proposed as the adsorption mechanism of As(V) into ChFe hydrogel. 0.1 M CH₃COOH showed the best regeneration pattern with 75, 96, 81, 53 and 43 % of 1^st, 2^nd, 3^rd, 4^th and 5^th adsorption respectively. Because of this re-usable capability, the As(V) adsorption capacity is not a single value from one adsorption cycle, but a cumulative value of several adsorption cycles and it was 17.39 mg/g for five adsorption cycles. Open for regeneration and reuse, no post-treatment is needed for adsorbent-water separation, allow applications of the ChFe hydrogel composite in a wide range of applications such as water filtration and purification systems. The modification with ChFe further expands the application capacity since the ChFe can remove other contaminants as well.

Solidification/stabilization of highly toxic arsenic-alkali residue by MSWI fly ash-based cementitious material containing Friedel's salt: Efficiency and mechanism

Arsenic-alkali residue (AAR) and MSWI fly ash (MFA) are hazardous wastes, which still lack effective treatment methods. In this study, a novel solidification/stabilization (S/S) method for AAR with MFA-based cementitious material (MFA-CM) containing Friedel's salt was proposed. The efficiency and mechanism of S/S was mainly focused. Abundant Friedel's salt as well as a few C-S-H gel and ettringite (AFt) were found as hydration products of MFA-CM. 12% of AAR was well solidified/stabilized by MFA-CM, accompanied by As leaching concentration reducing from 10,687 mg/L to less than 5 mg/L. In order to investigate S/S mechanism of As, removal mechanism of As during co-precipitation synthesis of Friedel's salt was studied. During co-precipitation process, As was successively removed by formation of calcium arsenate precipitates, formation of As-Friedel's salt (replacement of Cl^- by AsO₄^3-), and adsorption of Friedel's salt. The S/S mechanism of As by MFA-CM was found to be similar to the removal mechanism of As during co-precipitation. With the prolonging of curing time, As was mainly solidified/stabilized by formation of calcium arsenate precipitates and As-Friedel's salt, and adsorption of Friedel's salt. Thus, this study provides a novel harmless treatment method for highly toxic arsenic-containing wastes by "treating the wastes with wastes".

Selenite Uptake by Ca-Al LDH: A Description of Intercalated Anion Coordination Geometries

Layered double hydroxides (LDHs) are anion exchangers with a strong potential to scavenge anionic contaminants in aquatic environments. Here, the uptake of selenite (SeO₃^2-) by Ca-Al LDHs was investigated as a function of Se concentration. Thermodynamic modeling of batch sorption isotherms shows that the formation of SeO₃^2--intercalated AFm (hydrated calcium aluminate monosubstituent) phase, AFm-SeO₃, is the dominant mechanism controlling the retention of Se at medium loadings. AFm-Cl₂ shows much stronger affinity and larger distribution ratio (R_d ∼ 17800 L kg^-1) toward SeO₃^2- than AFm-SO₄ (R_d ∼ 705 L kg^-1). At stoichiometric SeO₃^2- loading for anion exchange, the newly formed AFm-SeO₃ phase results in two basal spacing, i.e., 9.93 ± 0.06 Å and ∼11.03 ± 0.03 Å. Extended X-ray absorption fine structure (EXAFS) spectra indicate that the intercalated SeO₃^2- forms inner-sphere complexes with the Ca-Al-O layers. In situ X-ray diffraction (XRD) shows that basal spacing of Ca-Al LDHs have a remarkable linear relationship with the size of hydrated intercalated anions (i.e., Cl^-, SO₄^2-, MoO₄^2-, and SeO₃^2-). Contrary to AFm-SeO₃ with inner-sphere SeO₃^2- complexes in the interlayer, the phase with hydrogen-bonded inner-sphere complexed SeO₃^2- is kinetically favored but thermodynamically unstable. This work offers new insights about the determination of intercalated anion coordination geometries via XRD analyses.

Synthesis and thermodynamic properties of arsenate and sulfate-arsenate ettringite structure phases

Arsenic is a toxic and carcinogenic contaminant of potential concern. Ettringite [Ca₆Al₂(SO₄)₃(OH)₁₂·26H₂O] has the ability to incorporate oxyanions as a solid solution with SO₄²⁻, which could lower the soluble oxyanion concentrations. Therefore, ettringite containing SO₄²⁻ and AsO₄³⁻ has been synthesized. Results indicated that AsO₄³⁻ could substitute for SO₄²⁻ inside the channels of ettringite in the form of HAsO₄²⁻, and a linear correlation existed between X_{initial solution} and X_solid. The thermodynamic characterization of the solid samples was investigated by means of Visual MINTEQ, a freeware chemical equilibrium model, and the solubility product logK of -48.4 ± 0.4 was calculated for HAsO₄–ettringite at 25°C. The Lippmann phase diagram and X_HAsO4–X_HAsO4,aq plot showed that the solid solution series containing arsenate has HAsO₄-poor aqueous solutions in equilibrium. These findings can be helpful to arsenate solidification and arsenate leaching modeling projects.

Evidence of Multiple Sorption Modes in Layered Double Hydroxides Using Mo As Structural Probe

Layered double hydroxides (LDHs) have been considered as effective phases for the remediation of aquatic environments, to remove anionic contaminants mainly through anion exchange mechanisms. Here, a combination of batch isotherm experiments and X-ray techniques was used to examine molybdate (MoO₄^2-) sorption mechanisms on CaAl LDHs with increasing loadings of molybdate. Advanced modeling of aqueous data shows that the sorption isotherm can be interpreted by three retention mechanisms, including two types of edge sites complexes, interlayer anion exchange, and CaMoO₄ precipitation. Meanwhile, Mo geometry evolves from tetrahedral to octahedral on the edge, and back to tetrahedral coordination at higher Mo loadings, indicated by Mo K-edge X-ray absorption spectra. Moreover, an anion exchange process on both CaAl LDHs was followed by in situ time-resolved synchrotron-based X-ray diffraction, remarkably agreeing with the sorption isotherm. This detailed molecular view shows that different uptake mechanisms-edge sorption, interfacial dissolution-reprecipitation-are at play and control anion uptake under environmentally relevant conditions, which is contrast to the classical view of anion exchange as the primary retention mechanism. This work puts all these mechanisms in perspective, offering a new insight into the complex interplay of anion uptake mechanisms by LDH phases, by using changes in Mo geometry as powerful molecular-scale probe.