Kinetic Studies on the Lime Sulfurated Solution (Calcium Polysulfide) Process for Removal of Heavy Metals from Wastewater

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Abstract
A new process has been developed for wastewater purification, utilizing “Lime Sulfurated Solution” as a coagulant. Calcium polysulfide, the main component of this solution, decomposes in water on contact with a gas (atmospheric air or CO2 gas) and reacts with heavy metals. The heavy metals are removed as practically insoluble precipitates. The kinetics of this process were studied in a batch stirred tank vessel to obtain the fundamental data needed for development of a commercial continuous process. The findings are as follows: (1) Processing of bivalent metals Hg2+, Cd2+, Zn2+, Pb2+, and Cu2+ are completed in a few minutes to achieve the water quality standards in Japan. (2) Cr3+ is flocculated as Cr(OH)3 by the strong alkaline base of CaSx solution. It takes about 60 minutes for the precipitation with CO2 blowing, and 30 min with aeration. Cr(VI) is quickly reduced to Cr3+ and removed as Cr(OH)3. (3) A simulated wastewater simultaneously contaminated with 6 kinds of heavy metals can be brought well within the standards within 20 min of processing time.
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