Effects of pH and phosphate competition on the transport of arsenate

The effects of pH and phosphate (PO4) competition on arsenate (AsO4) sorption by soils and aquifer materials have been previously investigated in batch studies, however, little has been reported on processes that influence AsO4 transport through porous media. Consequently, the objective of this study was to determine the effects of pH and PO4 on the transport of AsO4 under conditions where reaction and mass transfer rates may be controlling pore-water AsO4 concentrations. Saturated column transport experiments (pore water velocity = 1 cm h(-1)) were performed using a sand containing free Fe oxides in which a pulse of (AsO4)-As-73 (133 mu M) was applied in the presence of 0, 13.4, 134, and 1340 mu M PO4 at pH values of 4.5, 6.5, and 8.5. Breakthrough curves (BTCs) of AsO4 exhibited low recovery and significant tailing at pH values of 4.5 and 6.5 in the absence of PO4. Competition between PO4 and AsO4 for sorption sites resulted in increased AsO4 mobility; however, even in the presence of PO4 levels representing more than 100% of maximum sorption capacity for the columns, significant amounts of AsO4 remained sorbed to the sand. Application of a continuous PO, pulse to a column containing sorbed AsO4 resulted in an increase in AsO4 recovery (>35%); however, even after total PO4 loading exceeded the column capacity by more than two orders of magnitude, approximately 40% of the applied AsO4 remained sorbed to the sand indicating that rates of AsO4 desorption play an important role in transport of AsO4 through porous media.