Arsenate adsorption onto hydrous ferric oxide: Insights from triple layer surface complexation modeling

Abstract

This study investigates arsenate [As(V)] adsorption dynamics along soil depth profiles in natural soils from Gurukhwind village, Tarn Taran district, Punjab, using batch adsorption experiments in an arsenic (As) affected region. Kinetic experiments showed rapid As(V) adsorption within the first 4 h, reaching equilibrium at 12 h across all soil layers. As(V) adsorption was substantially higher in surface soil layers than in bottom layers, largely due to variations in surface area, organic matter content, pH, and mineralogical components such as hydrous ferric oxide (HFO). Adsorption decreased with increasing pH, with maximum uptake occurring under acidic conditions due to enhanced electrostatic forces of attraction between protonated surface sites and negatively charged As(V) species. The pseudo-first-order (PFO) model best described the kinetic data, while equilibrium data followed the Langmuir isotherm, indicating monolayer adsorption, with R² values exceeding 0.9 across all soil layers. The 2-pK Triple Layer Model (TLM) successfully simulated the pH-edge and isotherm trends for As(V) adsorption on HFO, suggesting the formation of dominant inner-sphere complexes = Fe(H₂AsO₄), =Fe(HAsO₄)⁻ and = Fe(AsO₄)²⁻ along with minor contribution from outer-sphere complexes FeOH₂⁺_ H₂AsO₄⁻ and FeOH₂⁺ _ HAsO₄²⁻. The good agreement between modeled and experimental data was supported by reasonable RMSE values for isotherms and pH-edge data. The combined findings underscore the importance of iron (Fe) oxide rich surface horizons in controlling As mobility, with direct implications for the development of site specific management and mitigation strategies in As affected regions.