The Efficacy And Mechanism Of Cadmium (Ⅱ) Removal By Hydrous Manganese Dioxide

Cadmium pollution of water resource become severer and cadmium sudden pollution of surface water happened frequently in China in recent years. Ministry of Health of PR China has revised the maximum contaminant level (MCL) for Cd in drinking water from 0.01mg/L to 0.005mg/L in 2007. The conventional water treatment process can not remove trace cadmium in water efficiently. And chemical precipitation, ion exchanger, adsorption and membrane for cadmium removal are not in point when cadmium dissolved in large volume of solution of relatively low concentration. So, the search of new methods for inexpensive, effective cadmium removal is becoming very urgent. The thesis focuses on solving this problem; this research prepared hydrous manganese dioxide by reducing KMnO4 with the addition of a Na2S2O3 solution. The removal of cadmium in water by hydrous manganese dioxide was conducted. The research lays stress on the effects of co-existence ions on the cadmium elimination. The study of primary mechanisms during cadmium removal was conducted.Coagulation experiments were conducted with deionized water spiked with a certain amount of cadmium, with fixed ionic strength and alkalinity. Hydrous manganese dioxide prepared in beaker under the condition that the reaction time is 2 minutes and the KMnO4/ Na2S2O3 mole ratio is 2. Then the coagulation experiments carried out with two minutes'coagulation and then for fifteen minutes'flocculation and twenty minutes'precipitation. The results show the hydrous manganese dioxide has a good effectiveness on cadmium removal. When initial cadmium concentration in the raw water of 1mg/L, hydrous manganese dioxide dosages is 20mg/L, pH=6.92, the removal efficiency of cadmium is 85.04%. The hydrous manganese dioxide also has high removal efficiency for different kinds of background waters.The removal efficiency of cadmium is highly depended on pH; the higher pH values the better removal efficiency. Turbidity and magnesium do not have great effect on cadmium removal. Ion strength and calcium restrain the elimination of cadmium by hydrous manganese dioxide. Silicate, phosphate and humid acid enhance the cadmium removal at a lower ions concentration (<0.2mmol/L). However, cadmium removal decreases markedly at a higher ions concentration (>0.2mmol/L). Sulfate and phosphate enhance the cadmium removal in acidic conditions. But they have little effect on removal efficiency in alkalinity conditions. In the range of pH=5~10, silicate has little effect on the removal efficiency of cadmium.Adsorption on hydrous manganese dioxide is the mainly mechanisms for cadmium removal. Hydrous manganese dioxide has better effective on cadmium removal than commercial solid manganese dioxide, ferric and alumina coagulants. The removal efficacy of cadmium by hydrous manganese dioxide synthesized by Na2S2O3 is not as good as that synthesized by MnSO4 and Na2SO3, however it performed better on turbidity and concentration of residual manganese of the effluent.Conclusively, the hydrous manganese dioxide synthesized by mixing Na2S2O3 and KMnO4 exhibits good efficacy of removing cadmium form drinking water. On the other hand, this technology, being highly effective, convenient to be employed, is promising to treat source water contaminated by trace cadmium.