A novel, integrated treatment system for coal waste waters. Quarterly report, March 2, 1994--June 1, 1994

Abstract

The aims of this study are to develop, characterize and optimize a novel treatment scheme that would be effective simultaneously against the toxic organics and the heavy metals present in coal conversion waste waters. A specific goal of the study is to remove and recover cationic and anionic heavy metals from aqueous solutions and coal conversion waste waters using modified-clay adsorbents developed in this study. To this end, a multi-step adsorption/desorption process has been carried out with hectorite-CBDA-DT (HCDT) as the adsorbent and Cr(VI) as the adsorbate. Adsorption was carried out at pH 4.0 in 0.02 M buffer, while desorption was effected at the same pH and in the same buffer with either 0.5 M NaCl or 0.02 M Na{sub 2}SO{sub 4} as the desorbates. Multi-step involves cycling the same adsorbent through these two sets of operating conditions with a washing step after each adsorption/desorption sequence. The authors results indicate that, during the first two cycles, the potency of the adsorbent remains unchanged, but it diminishes after the third and the fourth cycles. The total decrease in potency is, however, only 15% even after 4 cycles of adsorption/desorption. Addition of 20% isopropyl alcohol (IPA) to the reaction medium, however, diminishes the potency even more after 4 cycles of adsorption and desorption. Both the desorbates yielded identical results, and the overall mass balance on Cr(VI) was between 95 and 102%. Continuous leaching experiments on HCDT revealed that DT bound to HCDT is mobilized to the extent of only 10% after 44 hrs in aqueous medium while in 20% IPA-water mixtures the extent of dissolution of DT from the surface is close to 16%. Thus, the loss of potency of HCDT is attributed partly to the loss of DT from the surface and partly to the incomplete washing of the adsorbent between each adsorption/desorption step.