Kinetic Behavior and Mechanism of Arsenate Adsorption by Loam and Sandy Loam Soil

In the present work, the arsenate [As(V)] adsorption by loam and sandy loam soil was carried out as a function of initial As(V) concentration, pH, contact time, and adsorbent dose to ensure As adsorption and its mobilization in the soil of arsenic-affected areas. The As(V) adsorption with varying time and initial As(V) concentrations were analyzed using the linear and nonlinear forms of the adsorption isotherms and kinetic models. Based on the comparison of R2 values and calculated qe values, the linear pseudo-second order and non-linear pseudo-first order and second-order kinetics showed better applicability of S1 (loam) than S2 (sandy loam) soil samples for arsenate adsorption. The loam soil (S1) also exhibited maximum adsorption capacity and low As(V) mobility due to the presence of soil organic matter and low phosphorus content. The FTIR results showed that soil surface functional groups (─COOH, O─H and Fe─O group) were predominantly involved in As(V) binding via surface complexation or coordination. Results derived from equilibrium adsorption isotherms showed that the data fitted well to both the linear and non-linear Langmuir adsorption isotherms. The monolayer surface adsorption (qmax) in S1 sample (29.41 µg/g) was better than that in the S2 soil (27.39 µg/g), indicating strong surface affinity of S1 toward As(V) than S2 soil sample. This observation was also supported by the separation factor (RL values). Results on thermodynamic parameters (ΔH°, ΔS° and ΔG°) showed spontaneous and endothermic nature of adsorption. The activation energy (Ea) calculated for the surface binding of As(V) onto S1 (4.56 kJ mol–1) and S2 soil (7.15 kJ mol–1) indicated that the surface binding of As(V) by both the soils was an energetically favorable physico-chemisorption. The results on adsorption characteristics of both soil samples indicated that adsorption efficiency and retention ability of loam soil (S1) was better than sandy loam (S2) soil.