ARSENIC ADSORPTION AND DESORPTION BY WATER TREATMENT RESIDUALS: PRELIMINARY RESULTS

Arsenic is a naturally occurring element and has found widespread use in agriculture and industry through the use of arsenical pesticides, crop desiccants and even livestock feed additives. Most of these uses have been discontinued. However, residues from such activities, together with the ongoing generation of arsenic wastes from the smelting and mining of various ores have left a legacy of soils contaminated with unacceptably high levels of arsenic. There is a great need for cost effective technologies capable of reducing arsenic in soils to environmentally acceptable levels. In recent years, attention has focused on the development of in-situ immobilization methods that are generally less expensive and disruptive to the natural landscape. One of the most interesting and promising of these in-situ techniques is the use of Water Treatment Residuals (WTRs). WTRs are the by-products from drinking water purification plants and contain sediments, organic material from the raw water, Al/Fe oxides and activated C. These WTRs are amorphous and have an affinity for oxyanions (e.g., arsenate and arsenite), due to the high positive surface charge they generally possess. Recent studies have suggested that WTRs retain arsenic and decrease arsenic mobility. As mobility of Arsenic is controlled by adsorption/desorption reactions, knowledge of adsorption and desorption of arsenic in WTRs is of relevance. This study examined the adsorption and desorption characteristics of arsenic in three types of WTRs, namely Fe-WTR, Al-WTR, and Ca-WTR. All adsorption experiments were carried out in batch mode and arsenic adsorption on WTRs was investigated as a function of soil/solution ratio, adsorbent dose, equilibration time and arsenic concentration. All the above parameters were varied one at a time to study their effect on the arsenic adsorption process. Isotherm modeling was performed and desorption studies were carried out immediately after each adsorption step. Preliminary results show that the Fe-WTR and the Al-WTR have high arsenic adsorption capacity.