Arsenic Adsorption from Geothermal Bore Water. Theory and Practice

Abstract

Arsenic adsorption onto hydrous iron oxide (HFO)is viewed as a viable option for the removal of arsenic from geothermal bore waters. On the basis of published experimentaldata and model predictionsusing MINTEQM, completearsenic removal may be expected under optimum and HFO concentrations. However, previous studies of arsenic in natural freshwater and saline waters have indicated an often large discrepancy between observed and theoretically predicted arsenic behaviour; a discrepancy which also occurs in geothermal bore waters. Experimental adsorption of both and from the bore water of Wairakei Flashplant 10 showed that, while both and were rapidly adsorbed onto HFO, only at approached the 90% adsorption predicted by At adsorption was consistently of the predicted value, while As" adsorption ranged from 33-95% of the predicted adsorption. It is tentatively concluded that theoretically calculated As adsorption fails to correctly account for either the changes in HFO surface potential and/or bonding site competitionwhich arises through the adsorption of major ions, or for the effect of dissolved arsenic complex formation.