Extracellular polymeric substances and mineral interfacial reactions control the simultaneous immobilization and reduction of arsenic (As(V))

Extracellular polymeric substances (EPS) play a crucial role in controlling the mobility and bioavailability of heavy metal(loid)s in water, soils, and sediments. The formation of EPS-mineral complex changes the reactivity of the end-member materials. However, little is known about the adsorption and redox mechanisms of arsenate (As(V)) in EPS and EPS-mineral complexes. Here we examined the reaction sites, valence state, thermodynamic parameters and distribution of As in the complexes using potentiometric titration, isothermal titration calo-rimetry (ITC), FTIR, XPS, and SEM-EDS. The results showed that similar to 54% of As(V) was reduced to As(III) by EPS, potentially driven by an enthalpy change (Delta H) of - 24.95 kJ/mol. The EPS coating on minerals clearly affected the reactivity to As(V). The strong masking of functional sites between EPS and goethite inhibited both the adsorption and reduction of As. In contrast, the weak binding of EPS onto montmorillonite retained more reactive sites for the reaction with As. Meanwhile, montmorillonite facilitated the immobilization of As to EPS through the formation of As-organic bounds. Our findings deepen the understanding of EPS-mineral interfacial reactions in controlling the redox and mobility of As, and the knowledge is important for predicting the behavior of As in natural environments.

Automated summary

Extracellular polymeric substances (EPS) are important in controlling the mobility and bioavailability of heavy metal(loid)s in natural environments, but the adsorption and redox mechanisms of arsenate (As(V)) in EPS and EPS-mineral complexes are not well understood. In this study, we investigated the reaction sites, valence state, thermodynamic parameters, and distribution of As in the complexes. We found that EPS could reduce As(V) to As(III) and the EPS coating on minerals influenced the reactivity to As(V), with different effects on goethite and montmorillonite.