Interference of Phosphate in Adsorption of Arsenate and Arsenite over Confined Metastable Two-Line Ferrihydrite and Magnetite

Contamination of groundwater by arsenic (As­(III/V)) is a serious global issue, and phosphate (P­(V)) is known to be the biggest interference in adsorption-based remediation methods. The present study is focused on understanding the interaction between phosphate and iron oxides/oxy-hydroxides with two well-known classes of potential adsorbents in the important pH range of 5–9 and the effect of such interactions on the uptake of arsenite and arsenate. Spectroscopic studies such as X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared (FTIR) spectroscopy were used to understand the binding of various oxyanions of phosphorous and arsenic with the iron oxides/oxy-hydroxides, exploring the core levels of P 2p and Fe 2p. Materials used for adsorption experiments were magnetite (MAG) and a nanocomposite, confined metastable two-line ferrihydrite (CM2LF); CM2LF is used for arsenic remediation in the affected states in India. Further, we studied the interference of P­(V) in As­(III/V) adsorption. The kinetics of adsorption was quantified using ion chromatography (IC), where P­(V) alone followed a pseudo-second-order model. In the case of mixed solutions, namely, APmix1 (P­(V) + As­(III)) and APmix2 (P­(V) + As­(V)), kinetics data suggested that P­(V) or As­(III/V) oxyanions partially follow the pseudo-second-order model. Results also confirmed that CM2LF performed better than magnetite (MAG) for As­(III/V) uptake in the presence of P­(V). As­(III) and As­(V) species are more competitive than P­(V) at neutral pH. A model for the adsorption of P­(V) species in water on ferrihydrite particles was developed using density functional theory (DFT). This accounted for phosphate complexation at various pH values. The study is highly useful in developing an affordable solution for sustainable arsenic remediation. Various aspects of sustainability are discussed.