ARSENIC, BORON, SELENIUM, AND MOLYBDENUM DISPLACEMENT AND TRANSPORT IN A FLY ASH AMENDED SOIL LEACHED WITH CALCIUM PHOSPHATE SOLUTION

Abstract

Substantial amounts of arsenic (As), selenium (Se), molybdenum (Mo), and boron (B) can be added with fly ash (FA) in FA amended soils. Leaching of these potential contaminants and the effect of soil solution anion composition on their retention and displacement is not well documented in the literature. The objective was to compare the interactions between phosphorus (P) in the leaching solution and As, B, Se, and Mo on FA surfaces and to estimate the mobility of As, B, Se, and Mo in a FA amended Cecil topsoil (Ultisol). Two FAs, A (216 mg As kg⁻¹) and B (51 mg As kg⁻¹), were used. Each ash was mixed with soil at the equivalent rates of 2 and 41 kg As ha⁻¹. These mixtures were used to pack the columns, which were leached with 1,66 mmol_(c) calcium phosphate [Ca(H₂PO₄)₂] L⁻¹ solution. The results show that the concentrations of As, B, Mo, and Se in the leachate and their total amounts removed from the surface horizon (0–20 cm) of a FA amended soil depend on the initial contents of these elements in FA and the rate of FA application. As appears much faster and in higher concentrations in the leachate of treatments that received the highest As rate. The maximum As concentration and the total As amount released in the leachate, were much higher in the low As FA (B) than in the high As FA (A). The low As ash, therefore, seems to be more hazardous than the high As ash when both applied in rates to yield 41 kg As ha⁻¹. The differences observed between treatments in As, B, Mo, and Se mobility are also due to the ratio soil/FA in different treatments. Because under acid conditions some variable charge minerals in the soil and FAs develop positive charge and anion exchange capacity, larger amounts of As, B, Mo, and Se may be retained and their movement through the soil column may be retarded. This could be another reason why lower amount of As, B, Mo, and Se were released in the A (acid FA) than the B treatments (alkaline FA).

Acknowledgments