Reactive behavior of natural manganese oxides toward the adsorption of phosphate and arsenate

Traces of arsenic can be selectively removed from drinking water by adsorption processes using natural manganese oxides. Monocomponent adsorption of phosphate and arsenate has been studied by means of column experiments with two natural manganese oxides. A simple technique was proposed to deduce the concentration variations at the column outlet from the conductivity and pH measurements. These macroscopic-scale data enabled us to obtain phenomenological information on the type of surface reactions involved. Two behaviors were encountered with the oxides: (i) only surface complexation was involved, the developed technique could be applied, and adsorption isotherms could be rapidly and accurately measured by a series of column experiments with very few analyses; or (ii) surface complexation was coupled with anion exchange. The latter behavior was far more complex, and the direct deduction of arsenate breakthrough from the conductivity and pH signals was no longer possible.