Enhanced remediation of As(iii) and As(v) by new zirconium-loaded attapulgite and its mechanisms in the aquatic environment

Abstract

Arsenic pollution in waters and sediments has severely threatened the aquatic ecology and natural environment. Arsenite [As(III)] is more hazardous and active than As(V), and hence, its removal is a great challenge to the remediation of As pollution in waters. A new zirconium-loaded attapulgite (Zr-ATP) clay mineral was prepared by a simple and low-cost method to effectively adsorb As(III) and As(v) simultaneously. Its adsorption capacities to As(III) and As(V) were 2.9 and 4.6 mg g⁻¹, respectively. The kinetics, isotherm model and fitting demonstrated that Zr-ATP adsorption to As(III) and As(V) was mainly chemical and multilayer adsorption. Their adsorption was uninfluenced by pH (3–10), and the concentrations of SO₄²⁻ (0.05–4.50 g L⁻¹), Cl⁻ (0.03–15.00 g L⁻¹), HCO₃⁻ (4.00–120.00 mg L⁻¹), PO₄³⁻ (0.05–1.00 mg L⁻¹), SiO₃²⁻ (5.00–10.00 mg L⁻¹), Na⁺ (25.00–100.00 mg L⁻¹), K⁺ (10.00–40.00 mg L⁻¹), and humic acid (1.00–18.00 mg L⁻¹). There occurred the breaking of Zr–O–H bonds and the generation of Zr–O–As bonds during the adsorption process. Zr-ATP application for 30 days could significantly curtail the concentration gradients of dissolved and DGT (diffusive gradients in thin films)-labile As for diffusion in the vertical profiles of three As-polluted sediments, with the maximum reduction rates of 72.9, 80.4, and 80.0% for dissolved As, and 71.8, 91.8 and 95.8% for DGT-labile As. The simulation of the DGT-induced fluxes in sediments (DIFS) model further proved that Zr-ATP capping in the sediments resulted in a low resupply of DGT-labile As from the solid phase to pore water. The As fractionations in the sediments revealed that Zr-ATP coverage promoted the transition from adsorbed As to Fe–Al hydrous oxide-bound As in the sediments; thus reduced the bioavailability of As species. The results provided strong evidence that the new Zr-ATP is an alternative adsorbent and capping agent of As in the water environment. In addition, this study has proposed a comprehensive method to evaluate the remediation effects of materials and explore the mechanism of As adsorption in water and immobilization in sediments.