Effects of oxalate and phosphate on electrokinetic removal of arsenic from mine tailings

Electrokinetic remediation process can be used to separate arsenic from mine tailings and prevent it from leaching into the environment. In this process, oxalate and phosphate solutions might serve as mobility-enhancing agents. The influence of oxalate and phosphate ions was investigated using three approaches. First, desorption tests showed that oxalate is more effective extractant than phosphate. Secondly, equilibrium modeling was used to predict the electric charge distributions of As and co-existing elements under the prevailing pH and redox gradient. Modeling results suggested that oxalate affects the fate of As indirectly by mobilizing As-bearing minerals such as Fe oxyhydroxides, while phosphate influences mainly the speciation of Ca, Cu, Mg and Mn. Thirdly, electrokinetic experiments showed that in the tailings section near the anode, As removal was 30% with oxalate and 48% with phosphate, after 20 d. Due to the accumulation of As in the middle section, the respective overall removals in the entire tailings material were only 6% and 12%. The results encouraged to use equilibrium modeling for selecting enhancement agents and their concentration and for predicting accumulation of As between electrodes. However, the co-influence of all major elements and desorption/adsorption kinetics have to be taken into account.