ARSENIC MOBILIZATION IN SOIL BY ROCK PHOSPHATE AMENDMENTS AND FERTILIZERS

Phosphate is an important nutrient widely used in agriculture, and it also has been used as an amendment for contaminated soils to reduce lead (Pb) mobility. However, the application of phosphate can potentially mobilize arsenic (As); this is because arsenate and phosphate share the similar chemical structure and compete for the same adsorption sites on mineral surfaces. This study intends to understand and quantify the degree to which As is mobilized by different types of rock phosphates as well as a triple super phosphate (TSP) fertilizer. In an early trial, three columns were set up where collected rainwater was let dripping through low-As soils with or without rock phosphate amendments. Arsenic concentrations in the effluent were 2-5 times higher for the phosphate-amended columns (20-45 µg/L) than the control column (1-15 µg/L). The addition of rock phosphate also reduced infiltration rates noticeably - standing water or poor drainage may lead to anoxic conditions which facilitate As mobilization.

Batch experiments were then conducted to compare the mobilization of As under aerobic and anaerobic conditions for two types of soils: one from an urban garden (52 ppm As and 2200 ppm Pb) and the other from an old orchard (802 ppm As and 7800 ppm Pb). Four types of rock phosphates (Espoma, Rabbit Hill, Green Sense and Bone Meal) or two different amounts of TSP fertilizer were used. TSP led to faster release of As from the soil and the soluble As concentrations were as high as 10,000 µg/L. Anaerobic conditions appear to favor As mobilization markedly than aerobic conditions. Among the rock phosphates, Bone Meal has the largest effect on As mobilization, followed by Espoma, Rabbit Hill and Green Sense.