Conditions for the Precipitation of Heavy Metals from Wastewater by the Lime Sulfurated Solution (Calcium Polysulfide) Process


A new chemical precipitation process utilizing CaSx solution as a single coagulant was tested for the removal of heavy metals from a batch stirred vessel. The precipitation of heavy metals from the simulated wastewater was investigated under varing conditions of pH, temperature, and heavy metal concentrations. Several agitating speeds were studied for the gases (atmospheric air and C02 gas) blown into the water to decompose the coagulant (CaS*). The findings are as follows: Heavy metal removal in keeping with the water quality standards in Japan was achieved for the simulated wastewater having pH's between 3 and 11, and the temperatures from 10 to 85 °G. The agitating speed for optimum removal was found to be 10 Hz. More than 250 ppm of the initial heavy metal concentration gives sufficient removal at stoichiometric quantities of coagulant, while less than 50 ppm of the initial concentration results in rather poor removal owing to the colloidal properties of the metal sulfide or hydroxide. For low concentrations of heavy metals, several units of the coagulant over stoichiometric amount are necessary for sufficient precipitation to achieve the water quality standards.

The effluent from the industry and the cities known as wastewater has various pH's, temperatures and concentrations of contaminants. Therefore, in designing and operating wastewater treatment plants, information about the properties of the wastewater to be treated and the precipitating conditions are of great importance. For the removal of heavy metals, there have been many processes with high efficiency.1* Among these, the chemical precipitation processes are most widely put into practice.2'3* The temperature and pH of the wastewater, the initial concentration of heavy metals, the amount of coagulant to be injected, and the agitating speed of the stirrer are known to adversely affect the heavy metal removal by chemical precipitation.1)

In the previous work,4) a new chemical precipitation process has been developed to cope with the severe water quality standards in Japan for Hg2+, Cd2+, and Cr(VI). This process utilizes 'Lime Sulfurated Solution' as a single coagulant to precipitate the heavy metals from wastewater as metal sulfides or hydroxides. Kinetic studies on this process have already been done for the removal of heavy metals: Hg2+, Cd2+, Cu2+, Zn2+, Pb2+, Cr3*, andCr(VI).4'5> In the present study, optimum precipitating conditions have been discussed. The conditions for effective coagulation were investigated experimentally in a batch stirred vessel for the removal of Cd2+ and Cr(VI) from simulated wastewater at various pH's, temperatures, and initial concentrations of the heavy metals.

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