Study On The Mechanism Of Manganese And Cadmium Releasing From Scale Of Water Supply Pipeline Network

Scale in water supply network is the main place for accumulation of heavy metal elements.When water quality or hydraulic conditions change suddenly,it may lead heavy metal pollution releasing and cause serious drinking water safety problems.In this paper,two typical heavy metals,manganese and cadmium are selected as the research objects.The release process of heavy metals from pipe scale is simulated by static release experiment.The mechanism of manganese and cadmium releasing from pipe scale is studied from two aspects of influencing factors and kinetics.The main conclusions are as follows:（1）The concentration of manganese and cadmium in water will increase significantly after a period of release under stagnant flow condition.The lower the pH and alkalinity,the higher the temperature and sulfate radical concentration,the higher the concentration of manganese and cadmium releasing from pipe scale.（2）Because of the strong oxidation of disinfectants,disinfectants can inhibit the release of manganese and cadmium from pipe scale.The inhibitory effects of three disinfectants on the release of manganese and cadmium were ranked as follows:ClO₂>NaClO>monochloroamine.（3）The results of orthogonal experiment showed that the order of the four factors affecting the release of manganese in pipe scale was SO₄^2-concentration>alkalinity>pH>type of disinfectant.The order of factors affecting the release of cadmium in pipe scale was SO₄^2-concentration>alkalinity>pH.The type of disinfectant had no significant effect on cadmium releasing.（4）The kinetics of release of manganese and cadmium from scale can be well described by Elovich equation,two-constant rate equation and quasi-second-order kinetics equation at 5℃,15℃and 25℃.For the kinetics process of manganese releasing from scale,the quasi-second-order kinetics equation is the best,while for the kinetics process of cadmium releasing from scale,Elovich equation is the best.（5）The effect of temperature on release kinetics of manganese and cadmium in pipe scale:the higher the temperature,the larger the release amount of manganese and cadmium,and the faster the release rate,but the longer it takes to reach equilibrium.