Quantification of arsenic release from arsenic rich wastes and soil in presence of cowdung

Abstract

To combat the arsenic crisis, a number of household and community based arsenic removal technologies have been developed and a number of such technologies are currently being used in many arsenic affected areas in Bangladesh. All arsenic removal units generate some form of arsenic-rich wastes. Currently, disposal of such wastes in cow-dung bed is widely practiced. It has been suggested that biochemical processes in cow-dung bed transform inorganic arsenic into volatile forms and release them into air. In this study elimination of arsenic from arsenic solution, arsenic rich wastes and soil was evaluated in the laboratory. Role of phosphate in displacing arsenic from soil was also observed. Batch experiments were done to assess the elimination of arsenic from arsenic solution (unbound arsenic), arsenic rich wastes and soil (bound arsenic) in the presence of cowdung. Different concentrations of arsenic were mixed with different weight of fresh cowdung and the elimination of arsenic was studied under both cap-open condition (for simulating aerobic condition) and cap-closed condition (for simulating anaerobic condition). Results of batch experiments performed on a\\'ide range of arsenic aqueous solution suggest that, in general, significant elimination of arsenic (III) occurred from aqueous solution in the presence of fresh cowdung. The range of elimination was 13% to 96%. Majority of this elimination appeared to take place during the first few days. The results of this study suggest that the elimination of arsenic is not proportional to the amount of cowdung added to the aqueous solutions and initial concentration of arsenic in aqueous solution. Similar results were obtained for the batch experiments with As (V) aqueous solution. With arsenic rich wastes (Steven's Technology for arsenic Removal unit and Bucket Treatment Unit), the range of elimination was 9% to 69%. The elimination of arsenic from soil was observed 2% to 25% without using cowdung whereas the range of arsenic elimination was 52% to 89% with using cowdung. This study also suggest that elimination of unbound arsenic (aqueous solution) is quite high compared to bound arsenic (arsenic in wastes and soil.) In some cases it has been found that with increasing initial arsenic concentration elimination of arsenic decrease but more study is needed to ascertain this. The possible reason for great variation in the elimination of arsenic from solutions and wastes are variability in the number and type of microorganisms present in cowdung and in the availability of nutrients. Again, it has been found that As (V) reduced to As (III) before elimination, which has been suggested in literature. The ability of phosphate in displacing arsenic from soil was also observed in laboratory. From the study it can be suggested that bio-chemical processes eliminate arsenic from arsenic rich wastes and soil in the presence of cowdung but the reaction involved, type of volatile arsine produced, characteristics of microorganism present in the cowdung should be studied in more detail for better understanding of the elimination process.