ROLE OF PHOSPHATE IN THE MOBILIZATION OF ARSENIC FROM SOIL AND AQUIFER

Correlation between the concentrations of arsenic and phosphate are shown to be more significant than the correlation between that of arsenic and iron /manganese in three watersheds of east coast and our laboratory experiment. Even though sulfide reducing mobilization of arsenic is considered more prominent traditionally, arsenic release by phosphate may actually surpass the arsenic release by a sulfide reducing condition in some situations. A reducing condition makes Fe³⁺ become Fe²⁺, and makes both the iron-phosphate and iron arsenic complex more soluble and cause them to release into solution. The arsenic release in a sulfide reducing condition is common in groundwater aquifers contaminated by chlorinated or petroleum hydrocarbon and in groundwater aquifers with potent levels of organic layers. However, in an oxidized environment where pH is averaged around 7.5, and HPO₄^- and H₂PO₄^2- exist, phosphate mobilization of arsenic may be more prominent. This is apparent in a watershed where large agriculture areas exist and fertilizers are applied regularly. Phosphate from fertilizers mobilizes arsenic from soil by its competition of the absorption sites with arsenic due to their similar chemical properties. The significantly higher (about 1.5 times) dissolution rate of As by H₃PO₄ (10%) than that of concentrated HNO₃ (75%) in a laboratory arsenic leaching experiment also corroborates the notion that phosphate plays an important role in mobilizing arsenic from the soil and aquifer.