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

Abstract

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.

Mechanistic study of simultaneous arsenic and fluoride removal using granular TiO2-La adsorbent

“…This is because two sites (mixed Ti and La sites) were available for As(III) adsorption, whereas only La site was responsible for F adsorption (vide infra). The q m for present studied TiO 2 -La composite is higher than other reported adsorbents with simultaneous As and F removal, such as E33P, Al 2 O 3 , La-Al 2 O 3 (LAA), Ti-La impregnated activated carbon (TLAC), Al/Fe-doped adsorbents, and modified cellulose fibers, with adsorption capacities in the range of 7.3-40 mg/g for As, and 2.3-100 mg/g for F [15,[19][20][21][22].…”

“…S8). The other site was on La-oxide that favorable at high pH range of 7-10 with its pHpzc at 9.2 [22]. The F adsorption behaviors with increasing initial concentrations followed the characteristic of anion adsorption on single metal oxide (LaCO 3 OH), which is decreasing with the increasing pH (Fig.…”

“…EXAFS analysis resolved 1.8 Ti atoms at a distance of 3.36 Å and 0.7 La atoms at 3.42 Å (Fig. 7, Table S5), indicating that As(III) adsorbed in bidentate configuration on Ti sites and in monodentate configuration on La sites [22,24]. The contribution of La sites for As(III) adsorption at pH 10 can be attributed to the fact that less electronic repulsion exists between the negatively charged As(III) species, H 2 AsO 3…”

“…However, a limited number of studies have paid attention to developing effective adsorbents for the simultaneous removal of As(III) and F [19][20][21]. Recently, La-impregnated activated alumina (LAA) has been synthesized for As(III) (9.23 mg/g) and F (16.9 mg/g) adsorption, with both As(III) and F occupying the sites on La rather than on Al [15,22]. This result indicates that the impregnation of amorphous La oxide on AA may mask the Al sites, and thus As(III) and F have to compete for limited La sites.…”

Oxidation of Arsenite by Epoxy Group on Reduced Graphene Oxide/Metal Oxide Composite Materials

“…The collection and treatment of the spectra for As(III) and As(V) adsorption on rGO/MOs were similar to the procedure in the previous publications. [15,21] Briefly, the adsorbent film was deposited on the ZnSe crystal by applying 500 µL of the adsorbent suspension (1.5 g L −1 ) and drying at room temperature. Prior to use, the film was rinsed with DI water to remove loosely deposited particles.…”