LEAD SORPTION CHARACTERISTICS OF BENTONITES FROM JAPAN AND U.S.A IN VIEW OF THEIR MINERALOGY AND CHEMISTRY

摘要

The mineralogical, chemical, and physical properties of bentonites vary to a great extent according to their parent rock and the location of mines. In this study, characteristics of lead sorption have been assessed for 16 bentonites from 5 localities in Japan and U.S.A in terms of their mineralogy and chemistry to gain more substantive information about the differences in their lead sorption capacities. In batch sorption tests, most of the Na-bentonites from Japan sorbed higher amounts of lead than the ones from Wyoming in US, and the Na-bentonites from Gunma exhibited the highest levels of lead sorption. Calcium carbonate minerals in the Na-bentonites from Yamagata and Gunma function effectively as buffers against pH reduction caused by the addition of acid solution containing lead and the protons released by surface precipitation of lead on clay minerals and hydroxides in the samples, resulting in the enhancement of lead sorption. Multiple regression analyses showed that CaCO_3 content in all the sampled bentonites contributes the most to maximum lead sorption, followed by exchangeable sodium percentage and cation exchange capacity. Selective sequential extraction for lead-added bentonites indicated that lead is sorbed through carbonate and exchangeable phases. Cerussite (PbCO_3) was identified by X-ray diffraction in the Na-bentonites from Yamagata and Gunma after sorption of lead, indicating that lead is mainly sorbed in the form of cerussite.