Kinetic Release Of Arsenic And Zinc From River Sediment Contaminated By Gold Mining

Gold mining and smelting processes can cause serious heavy metal pollution to rivers and sediments. The sediments contaminated by heavy metals, as the source of pollution, can cause ecological risk. So it is important to indicate the release rules of heavy metals. This study collected the surface sediments from Jiehe River in Zhaoyuan, Shandong, which was contaminated by mine drainage. Chemical speciations of arsenic(As) and zinc(Zn) were analyzed by BCR sequential extraction. The kinetics release of As and Zn were determined by simulated leaching carried out with the stirred-flow reactors. The impacts of grain sizes and pH of the background solutions on the release of heavy metals were analyzed respectively. The results of experiments were simulated by the two-site equilibrium-kinetic model. The specific contents as follows:(1) Physicochemical properties of the four sediments were different. The lowest pH was 4.2, the highest pH was 7.8.The grain size of sediments LS12, LS16 from the upstream were smaller than the sediments JH26, JH27 from midstream. The contents of As and Zn from the collected samples were high, which exceeded the mean values of Chinese rivers. The weak acid extractable As contents were low, from 2.6% to 9.9%. The residual forms were high. The weak acid extractable Zn contents were high except sediment LS16. The contents were all above 60%.(2) The leaching experiments were conducted by leaching the original sediments. Two intervals were set when the time reached to 60 min and 210 min. The time increased with the decrease of the As and Zn’s contents. When the time was less than 200 min, As and Zn were released quickly. Then with the time increased, the release of the two heavy metals was gradually slow. The heavy metals’ contents increased after the two stop-flow, which means the kinetic release was controlled by time. Two-site equilibrium-kinetic model could be used to simulate the whole release process.(3) The sediments were classified into four grades:<0.048 mm, 0.048 mm~0.18 mm, 0.18 mm~0.9 mm, 0.9 mm~2 mm. The release capacity of As in four different grain sizes were 5.8 mg/kg, 4.9 mg/kg, 6.8 mg/kg, 8.8 mg/k; while the release capacities of Zn were 933.7 mg/kg, 894.3 mg/kg, 741.7 mg/kg, 629.4 mg/kg. The release capacity of Zn was much higher than As. With the increase of grain size, the release content of As was also increased, while the release content of Zn was decreased gradually.(4) Three pH grades(pH=3,6,9) of background solution were set. pH could affect the release of heavy metals, and the release rules of the two heavy metals were significantly different in the four sediment samples. It is because that the morphologies of the two heavy metals were different, some were of weak adsorbed state, Others were of precipitated state. When pH was exchanged, the heavy metals of precipitated state were released from sediments, resulting in the high content of the heavy metals.