Arsenic Adsorption from Geothermal Water

Abstract

Previous studies of arsenic adrorption have ofien identified a discrepancy betweeti obsented and theoretically predicted arsenic behaviour in fresh and marine waters; a discrepancy which &o occurs in geothermal bore waters. On the basis of published experimental dnta and model predictiotu using MIhTEQA2, complete arsenic adrorptioti onto hydrous Fe-oxide (HFO) may be expected under optiniutri pH and HFO concentratiom. Experinrental adrorption of both A?' and Asv from the bore water of Wairakzi Flashplatit 10 showed that both p' atid Asv were rapidly &orbed onto HFO. However, only Asv at pH <5 approached the 90% adrorption predicted by MIhTEQA2. At pH <9, A?' adsorption was consistently < 50 5% of the predicted value, while Asv adsorption ranged froni 33- 95% of the predicted value. It is tentatively concluded that theoretically calculated As adrorption failsto correctly account for either the changes iti HFO sugace potential and/or bonding site cottyetitioti which arises through the adrorption of rnajor ions, or for the @ect of dissolved arsenic conyler fortnatioti.