Removal of Arsenic and Phosphate from Aqueous Solution by Metal (Hydr-)oxide Coated Sand

Abstract

Arsenic contamination is a major concern in many drinking water supplies around the world. One low-cost approach for removing arsenic and other oxyanions such as phosphate is to use metal (hydro-)oxide coated sands. In this study, solution pH, sand grain size, coating efficiency, and mineral type for iron-coated sand (ICS), manganese-coated sand (MCS), and iron- and manganese-coated sand (IMCS) were evaluated. Scanning electron microscopy (SEM) and X-ray diffraction (XRD) analysis were used to investigate the surface properties of the coated layer. The mineral type of ICS prepared at different solution pH was identified as a mixture of FeOOH and Fe2O3. The mineral type of MCS prepared at pH 4 and 7 was identified as MnO2 and changed to ramsdellite (γ-MnO2, a mixture of α-MnO2 and β-MnO2) at pH 10. ICS exhibited a greater phosphate removal capacity, and phosphate removal increased with increasing iron oxide on the sand. ICS was also shown to have a greater removal capacity for phosphate than chemical precipitation methods by FeCl3, especially below neutral pH. In contrast, IMCS(Fe:Mn = 7:3) was better for removing As(III), which occurs through oxidation to As(V) and adsorption of As(III) and As(V) and can reach >98% removal rate (residual concentration <0.02 mg/L) at neutral pH. IMCS(7:3) is an efficient adsorbent for arsenic, as >50% can be removed within 60 min; 66% adsorption was reached after 24 h. Removal of arsenic and phosphate by these metal (hydro-)oxide coated sand adsorbents followed a pseudo-second-order rate and Langmuir as well as Freundlich adsorption isotherms. Removal under different conditions (e.g., pH, ionic strength) was also evaluated to provide information to optimize the process.