Multiple kinetic Langmuir modeling to predict the environmental behaviour of As(v) in soils

Abstract

A soil with a relatively high Fe content (2.82% [w/w]) was loaded for up to one year with As(V) by equilibrating it with a solution containing 1000 mg l⁻¹As(V) at a soil mass-to-solution ratio of 0.1 kg l⁻¹. The incorporation of As(V) into the soil and its distribution over the soil phases were monitored by sampling at strategic time intervals using an operationally defined five-step sequential extraction procedure (Wenzel et al., Anal. Chim. Acta, 2001, 436, 309) and subsequent As measurement. A multiple kinetic Langmuir model was developed to retrieve the dynamic parameters (adsorption and desorption rate constants, capacities and Langmuir equilibrium constants) for each of the soil phases by numerical fitting of the experimental adsorption data to the model. Under the equilibration conditions used the adsorption rate constants for all five operationally defined soil phases were very similar but the desorption rate constants decreased by a factor of ca. 150 from soil phase 1 (non-specifically sorbed As) to 5 (residual phases). This implies that As(V) incorporation “deeper” into the soil leads to stronger binding which is associated with the Langmuir equilibrium constants (adsorption rate constants/desorption rate constants). Equilibration of the soil with As(V) was complete in ca. 10 days with As(V) predominantly bound to soil phase 2 (specifically sorbed As) and soil phase 3 (amorphous and poorly crystalline hydrous oxides). X-Ray absorption spectroscopy techniques revealed that these binding characteristics may be related to adsorption of As(V) on Si- and/or Al-containing structures and natural hydrous iron oxide (HFO) surface sites, respectively. Since the model is independent of the initial As(V) concentration in the solution and the soil mass-to-solution ratio, the behaviour of the thus characterized soil–As(V) system can be predicted for a range of conditions. Simulations showed that in an accidental As(V) spill the soil studied would actively scavenge As(V) by instantaneous adsorption onto all soil phases followed by redistribution of As(V) from weaker binding sites to stronger ones over time.