An Experimental Study on Groundwater Pollution Prevention of Sexivalent Chromium Leached from Chromium Containing Wastes by Providing Cement-Bentonite Cut-off Barriers

Mamoru Kashiwaya*, Hiroshi Deguchi*, Seiichi Ueoka** and Hiromu Iwasa***

* Faculty of Science and Technology, Science University of Tokyo
** Raito Kogyo Co. Ltd. (Formerly, Graduate Student of Civil Engineering Course, Science University of Tokyo)
*** Technology Research Institute, Raito Kogyo Co. Ltd.

+ Correspondence should be addressed to Mamoru Kashiwaya:
(2641 Yamazaki, Noda, Chiba 278 Japan)

Abstract

An experimental study using both laboratory permeability testing apparatus and permeating soft groundwater contaminated by sexivalent chromium (Cr6+ groundwater) was carried out to develop a measure on groundwater pollution prevention (caused by Cr6+ compound leached from chromium containing wastes such as scoriae dumped out from smelting works in the past), by providing cut-off barriers which were made from a mixture of blast furnace slag cement and a sodium type bentonite (CB) cut-off barrier. From the laboratory tests, it was necessary to maintain the permeability coefficient of 2X10-9m/s or less, as soon as possible after the barrier provision. Through Test I, in which MgCl2270mg/L and some amounts of Ca (OH) 2 for pH adjustment were added to the permeating Cr6+ groundwater, it was found that adjusting the pH within a range of 9.0 to 9.5 was an appropriate approach. Through Test II, in which MgCl2270mg/L and some amounts of either Poly-aluminum chloride (PAC) or Al2 (SO4) 3 were added to the permeating Cr6+ groundwater, it was also found that the addition of PAC brings a good result due to the reduction of permeability and no breakthrough of T-Cr containing leachate. It was confirmed from Test III that the minimum requirement of PAC was 30mg/L approximately as Al2O3. No ettringite existed in CB test pieces which had had PAC added, while much ettringite was found in inlet and outlet sides of the test piece which had had Al2 (SO4) 3 added. The ettringite generated in the test piece did not exert a favorable influence upon the reduction of permeability coefficient.

Key words: chromium containing wastes, CB cut-off barrier, permeating Cr6+ groundwater, permeability test, void filler