Assessment of adsorption potential of alluvial soils for arsenic removal under competing anion conditions

This study investigated arsenate (As(V)) retention in natural alluvial soils and examined the effects of soil properties and competing silicate (SiO 4 4− ), sulphate (SO 4 2− ) and phosphate (PO 4 3− ) ions on As(V) adsorption. Sandy loam (soil-1), sandy clay loam (soil-2), and clay loam (soil-3) soils were selected for the adsorption study. Texture, pH, Electrical Conductivity, organic carbon, Cation Exchange Capacity, available phosphorus, BET surface area, Scanning Electron Microscope - Energy-dispersive X-ray spectroscopy, Fourier Transform Infrared spectroscopy and X-Ray diffraction of those soils were analyzed. Kinetic, pH dependent adsorption, adsorption isotherm and competitive adsorption studies were carried out. Concentration of As(V) was analyzed in Atomic Absorption Spectrophotometry with Vapour Generation Assembly. Adsorption was preferred in acidic pH range (pH 4.0–pH 6.0). The Langmuir model estimated maximum As(V) adsorption capacities of 0.531, 0.612, and 0.662 mg g⁻¹ for soil-1, soil-2, and soil-3, respectively. When 30 mg L − 1 of SiO₄⁴⁻, SO₄²⁻, or PO₄³⁻ was added with 20 mg L − 1 As(V), the adsorption of As(V) significantly decreased in all soils. The competitive effect on As(V) adsorption followed the order PO₄³⁻ > SiO₄⁴⁻ > SO₄²⁻. The progressive decline in the inhibitory effects of PO₄³⁻, SiO₄⁴⁻, and SO₄²⁻ from soil-1 to soil-3 indicates that competitive adsorption is governed by both the relative affinity of competing anions for specific mineral surface and the abundance and heterogeneity of reactive adsorption sites in the soils.