Effects of pH and phosphate on metal distribution with emphasis on As speciation and mobilization in soils from a lead smelting site

Arsenic in soils from the Asarco lead smelter in East Helena, Montana was characterized by X-ray absorption spectroscopy (XAS). Arsenic oxidation state and geochemical speciation were analyzed as a function of depth (two sampling sites) and surface distribution. These results were compared with intensive desorption/dissolution experiments performed in a pH stat reactor for samples from the site with the highest degree of As heterogeneity. The objectives of the study were to investigate the solid-phase geochemical As speciation, assess the speciation of As in solutions equilibrated with the solids under controlled pH (pH=4 or 6) and Eh (using hydrogen or air) environments, observe the effects of phosphate on the release of As into solution, and examine the effects of phosphate on metal mobility in the systems. Arsenic was predominantly found in the As(V) valence state, though there was evidence that As(III) and As(0) were present also. The dominant geochemical phase was scorodite (FeAsO4·2H2O). The pH was controlled in the pH stat experiments by the addition of equinormal solutions of monoprotic (HNO3), diprotic (H2SO4), or triprotic (H3PO4) acids. For many of the divalent metal cations, solution concentrations greatly decreased in the presence of phosphate. Solutions were also analyzed for anions. Evidence exists for sulfate release into solution. More As was released into solution at lower pH. A slight increase in solution arsenate occurs with the addition of phosphate, but the risk posed from the increased desorption/dissolution of As must be weighed against the decrease in solution concentrations of many metals especially Pb. If tailings from this site underwent acidification (e.g., acid mine drainage), in situ sequestration of metals by phosphate could be combined with placement of subsurface permeable reactive barriers for capture of As to reduce the risk associated with arsenic and trace metal mobilization. Results from this study could be used in risk management plans for sites similar to the Pb smelter site examined here.