Study On The Effect Of The Variable Charge Soil In South China On Cadmium Release From River Sediments

Commonly, the emergency measure of sudden heavy metal pollution accident in waters is to dose the alkali and coagulant, by means of adsorption and sedimentation, making the heavy metal concentration up to water quality standard for water supply. However, in high-concentration cadmium pollution incident, the conventional coagulation technology for emergency treatment is difficult to ensure the removal efficiency. Meanwhile, the sediments have potential risk to cause secondary pollution. Variable charge soil is named for the surface charge number and characteristics that altering with change of environment. This kind of soil is widely found in South China. The surface of variable charge soil contains a large quantity of iron and aluminium oxides with latent adsorption capacity of heavy metal ions. To explored the feasibility that variable charge soil applied to emergency water treatment for cadmium pollution, this study investigated the effect of variable charge soil enhanced coagulation, simulated the formation of sediment in the emergency treatment process of heavy metal pollution accidents, and characterized speciation distribution of cadmium in sediment, to study the function of variable charge soil and the cadmium fixed and release kinetics of the sediment involving variable charge soil during environmental geochemical processes. Moreover, this research investigated the effect of environmental factors on cadmium release from sediment involving variable charge soil by regulating the p H, dissolved oxygen, humic acid content and salinity of the overlying water and disturbing the sediment.(1) The result of the coagulation experiment on high-concentration cadmium-containing water indicated that variable charge soil and alkali could improve the cadmium removal efficiency. With the same dosage of alkali, adding 5 g/L variable charge soil could reduce the residual cadmium concentration from 1.892 mg/L to 30 μg/L. The flocs of the coagulation experiment remained stable in a certain time.(2) The release experiments of the sediments that was simulated in the emergency treatment process of cadmium pollution accidents showed variable charge soil would stimulate the cadmium release from the sediment more. The sequential oscillation experiment showed that the cadmium release concentration of the sediment involving variable charge soil(variable charge soil : sediment=1 : 3) was almost twice that of the sediment which holds the similar cadmium content to the mixed sediment. The static release experiment showed that the cadmium release concentrations under different concentration of dissolved oxygen had a great difference. However, the Cd release concentrations of the simulated sediments in the emergency treatment process involving variable charge soil, expecially the sediment involving variable charge soil slurry water, were always higher than that of the simple sediment. The variable charge soil in slurry water with small particle size, poor settling characteristics, easy resuspension, and easy transferability made the cadmium release from the mixed sediments. The cadmium speciation distribution experiment on the sediments before static release experiment showed that the compositions of acid-extractable cadmium from large to low are as follows: the sediment involving variable charge soil slurry water, the sediment involving variable charge soil passing through the 60 mesh screen, the sediment simply. This order was opposite to the compositions of reducible and oxidizable cadmium. After the static release experiment, the acid-extractable cadmium composition of these three sediments reached similar.(3) Environmental factors affected the cadmium release of the sediment involving variable charge soil greatly. The overlying water under low concentration of dissolved oxygen(DO), at a high p H value, with low salinity decreased the migration of cadmium by fixating and inactivating. However, the effects of humic acid and sediment disturbance on cadmium release were complicated. The effect of DO on cadmium release could be ordered as follow: weak oxidizing environment> oxidizing environment> anoxic environment. At low p H values, the cadmium released rapidly from sediment. The cadmium release accelerated with the salinity increased. Under weak oxidizing environment and oxidizing environment, humic acid stimulated the release of cadmium. However, under anoxic condition, humic acid inhibited the cadmium release to a certain extent. Sediment disturbance promoted the cadmium release from the sediment involving variable charge soil slurry water, but made the cadmium release from the sediment involving variable charge soil reduced.In short, during the emergency treatment of sudden cadmium pollution accident in waters, the addition of variable charge soil to enhance coagulation could effectively reduce the residual cadmium concentration. However, the addition of variable charge soil changed the component of the sediment. Also the sediment mixed with variable charge soil had a risk to secondary pollution that threaten the environment.