Interaction of As(III), As(V) and PO4 with Fe Oxide Impregnated Activated Carbons: Modeling Multisorbate Adsorption using the Surface Complexation Approach

The adsorption behavior of As (III) as a function of pH on an iron oxide impregnated activated carbon (FeAC, 7% Fe(III) amorphous iron-oxide) at different adsorbate/adsorbent concentrations was modeled using the surface complexation approach. The surface complexation constants developed from single sorbate experiments successfully predicted competition between As(V) and As(III) and the surface complexation modeling (SCM) predictions were verified experimentally. The monoprotic surface site representation described the experimental data better than the diprotic representation. Based on surface complexation modeling simulations, L-Act had a higher affinity for As(V) than As(III). Significant As(III) oxidation to As(V) was observed at pH > 8. As(V) and As(III) removal by an iron oxide impregnated carbon (L-Act, 9% Fe(III) amorphous iron-oxide) over a range of environmental conditions using SCM approach was also investigated. L-Act surface complexation constants determined from single adsorbate systems were effectively used to predict pH dependent removal of As(V), As(III) and PO4 across a range of adsorbent/adsorbate ratios in single, binary and tertiary adsorbate systems and from a `National Sanitation Foundation' challenge water. As(V) and PO4 complexes were modeled as bidentate binuclear species at low pH and monodentate species at high pH; using the two monoprotic surface site representations with the diffuse electric double layer model (MDLM). For the bidentate-binuclear complexation, two approaches based on the value of the exponent (n) on [ NOH] in the mass action law (n = 1 or 2), produced similarly good results. In the tertiary system, the effect of PO4 on As(III) removal was more pronounced than on As(V) removal. As(III) oxidation to As(V) was observed for L-Act at pH > 8.5. The MDLM successfully predicted As(III)/As(V)/PO4 behavior for other amorphous iron-based adsorbents that were reported in the literature. The MDLM was also used to address other arsenic-related environmental questions such as the cost of L-Act required to treat water to target concentrations as a function of pH. The framework for a mechanistic model that describes the competitive adsorption between arsenic species and other compounds present in arsenic laden waters (e.g., PO4) has been developed and can be used by other researchers in describing their experimental data