No abstract

Purpose Understanding the mechanisms and kinetics controlling the release of metals from sediment is a prerequisite for evaluating the risk of sediment contaminated by toxic elements. The objectives of this research were to determine the leaching properties of arsenic (As) from stream sediment contaminated by gold mining and to quantify the kinetic rate of As release using a leaching experiment and numerical modeling. Materials and methods In this study, we collected surface stream sediment from Jiehe River which was contaminated by gold mining in Zhaoyuan, Shandong Province. Chemical speciation of As was analyzed by a modified three-step BCR sequential extraction procedure. The sediment microscopic morphological characteristics and elemental composition on the surface of the sediment were analyzed by scanning electron microscope-energy dispersive spectrometer (SEM-EDS). Release kinetics of As were studied by a simulated leaching experiment using a stirred-flow reactor and a two-site equilibrium-kinetic model. Results and discussion The sediments we studied were significantly contaminated by As, with maximum concentrations of 777.8-3389 mg/kg. Sequential extraction analysis suggested that As contents in weak acid extractable, oxidizable, reducible, and residual forms were 2.6-9.8, 18-79, 2.4-7.1, and 8.7-75 %, respectively. SEM-EDS analysis showed that As on the surface of the sediment was higher than its overall content in all four sediments especially sediment JH27, which has high content of TOC, indicating that abundant As was sorbed or precipitated on the sediment surface. Our two-site equilibrium-kinetic model fits the As release data well and can reproduce the stop-flow experimental results. Kinetic rates obtained from curve fitting showed large variation among sediments for As, indicating different reaction mechanisms. Conclusions The release of As from stream sediment contaminated by gold mining was nonequilibrium and timedependent and might cause long-term pollution to river water. Our two-site model was demonstrated as an effective tool to describe the kinetic release of As from stream sediment to waters.