Groundwater Arsenic Adsorption on Granular TiO₂: Integrating Atomic Structure, Filtration, and Health Impact

A pressing challenge in arsenic (As) adsorptive filtration is to decipher how the As atomic surface structure obtained in the laboratory can be used to accurately predict the field filtration cycle. The motivation of this study was therefore to integrate molecular level As adsorption mechanisms and capacities to predict effluent As from granular TiO₂ columns in the field as well as its health impacts. Approximately 2,955 bed volumes of groundwater with an average of 542 μg/L As were filtered before the effluent As concentration exceeded 10 μg/L, corresponding to an adsorption capacity of 1.53 mg As/g TiO₂. After regeneration, the TiO₂ column could treat 2,563 bed volumes of groundwater, resulting in an As load of 1.36 mg/g TiO₂. Column filtration and EXAFS results showed that among coexisting ions present in groundwater, only Ca²⁺, Si­(OH)₄, and HCO₃^– would interfere with As adsorption. The compound effects of coexisting ions and molecular level structural information were incorporated in the PHREEQC program to satisfactorily predict the As breakthrough curves. The total urinary As concentration from four volunteers of local residences, ranging from 972 to 2,080 μg/L before groundwater treatment, decreased to the range 31.7–73.3 μg/L at the end of the experimental cycle (15–33 days)

Competing Interactions of As Adsorption and Fe(III) Polymerization during Ferric Coprecipitation Treatment

This study revealed the effect of As on the formation and dissolution of iron (hydr)­oxides and its further impact on the As removal efficacy of FeCl₃ treatment. Adding 6.7 mg/L FeCl₃ into 325 μg/L As solution (coprecipitation) resulted in more As removal (99% As­(V) and 75% As­(III)) at 2 min than adding As into aged FeCl₃ solution (preaged, 52–87% As­(V) and 7–42% As­(III)) at pH 7. However, soluble As gradually increased in the coprecipitation system and decreased in the preaged system to give similar concentrations during 800 h aging. The particle size of the iron (hydr)­oxides increased more slowly in the coprecipitation than in the preaged systems. These results suggest the rapid adsorption of As on Fe polymer during the initial polymerization process, which delays the growth of iron (hydr)­oxides. Thermodynamically, quantum chemical calculations implied that iron ions adsorption on iron (hydr)­oxide polymer was more stable than As adsorption, which is the main driving force for the As release during aging process. This study improved our understanding of the kinetic and thermodynamic processes of As adsorption and iron (hydr)­oxide precipitation in the coprecipitation treatment of As, and the potential for As release during aging of sludge generated in the treatment

Arsenic Adsorption on Lanthanum-Impregnated Activated Alumina: Spectroscopic and DFT Study

Rare earth-modified adsorbents (REMAs) have been widely used to remove oxyanion pollutants from water, including arsenic (As). However, the molecular-level structural information and reactions at the liquid/solid interface are still murky, which limits the design of applicable REMAs. Herein, a lanthanum-impregnated activated alumina (LAA) was synthesized as a representative REMA, and its As uptake mechanisms were explored using multiple complementary characterization techniques. Our adsorption experiments showed that LAA exhibited 2–3 times higher As adsorption capacity than AA. In contrast to the bidentate configuration formed on most metal oxide surfaces, our EXAFS and DFT results suggest that As­(III) and As­(V) form monodentate surface complexes on LAA through As-O-La coordinative bonding. In situ flow cell ATR-FTIR observed a strong dependence of As-O peak positions on pH, which could be interpreted as the change in the fractions of As­(V) surface complexes with zero- to double-protonation on LAA, AA, and LaOOH. As­(V) on LAA existed as singly and doubly protonated surface species, and the pK_a of transition from double to single protonation (∼5.8) was lower than that for its soluble counterpart (6.97). The surface reaction and structural configuration were incorporated in a CD-MUSIC model to satisfactorily predict macroscopic As adsorption behaviors. The insights gained from the molecular-level reactions shed light on the design and application of REMAs in environmental remediation for As and its structural analogues

Effect of Arsenic on the Formation and Adsorption Property of Ferric Hydroxide Precipitates in ZVI Treatment

Treatment of arsenic by zerovalent iron (ZVI) has been studied extensively. However, the effect of arsenic on the formation of ferric hydroxide precipitates in the ZVI treatment has not been investigated. We discovered that the specific surface area (ca. 187 m²/g) and arsenic content (ca. 67 mg/g) of the suspended solids (As-containing solids) generated in the ZVI treatment of arsenic solutions were much higher than the specific surface area (ca. 37 m²/g) and adsorption capacity (ca.12 mg/g) of the suspended solids (As-free solids) generated in the arsenic-free solutions. Arsenic in the As-containing solids was much more stable than the adsorbed arsenic in As-free solids. XRD, SEM, TEM, and selected area electron diffraction (SAED) analyses showed that the As-containing solids consisted of amorphous nanoparticles, while the As-free solids were composed of micron particles with weak crystallinity. Extended X-ray absorption fine structure (EXAFS) analysis determined that As­(V) was adsorbed on the As-containing suspended solids and magnetic solid surfaces through bidentate binuclear complexation; and As­(V) formed a mononuclear complex on the As-free suspended solids. The formation of the surface As­(V) complexes retarded the bonding of free FeO₆ octahedra to the oxygen sites on FeO₆ octahedral clusters and prevented the growth of the clusters and their development into 3-dimensional crystalline phases