By the 1970s, Cr(VI) sludge was illegally dumped around Komatsu-gawa, Edogawa-Ku, Tokyo, in the metropolitan area of Japan. Even now, high level of Cr(VI) pollution is observed in water in street inlet. Although Cr(VI) is easily reduced to Cr(III) under reducing conditions, the concentration is very high (>150 mg/L) under the high pH (>11) and low Eh (<0 mV) condition in the inlet. In order to explain why such high concentration of Cr(VI) has been detected under the reductive condition, influence of pH, Eh on redox reaction of Cr(VI) was investigated by batch experiment using Cr(VI) solution prepared from chemicals and the contaminated water sample collected from the polluted inlet. Iron sulfate (II) was used as reductant. When the Cr(VI) chemical was used, Cr(VI) was reduced less with higher pH condition. When contaminated water sample was used, the redox reaction was also suppressed in the higher pH condition (pH>9). The results of batch experiment also showed that, with the lower Eh condition, the more Cr(VI) was reduced when sample water collected form the polluted inlet was used. And it is also showed that pH is a stronger factor which influence on concentration of Cr(VI) than Eh. The reason why high concentration of Cr(VI) was detected under low Eh condition at Komatsu-gawa seemed to be the suppression of reducing reaction by the high pH.
