长江河口悬浮物对几种金属吸附的pH效应

EFFECTS OF pH ON ADSORPTION OF SEVERAL METALS TO SUSPENDED SEDIMENT IN THE CHANGJIANG RIVER ESTUARY

在实验室中控制吸附反应体系的pH ,研究了长江河口现场悬浮颗粒物质对添加金属Cu、Pb、Zn和Cd吸附作用。结果表明 ,反应体系pH值的增大使得金属在悬浮颗粒物质上的吸附百分率增大。在长江河口水体pH变化范围内 ,pH的变化对Cu和Pb在悬浮颗粒物上的吸附作用影响较小 ,而对Zn和Cd的影响则非常显著。根据固 -液界面吸附模型 ,长江河口悬浮物质对Cu、Pb、Zn和Cd的吸附平衡常数 (lgK)分别为 - 8 87、- 6 5 1、- 1 3 6 8和 - 1 3 6 5。长江河口特定的水化学环境和水、沙特征决定了金属Zn和Cd的环境行为显著受到pH变化的控制 。

Riverine suspended sediment has important effects upon partitioning of trace metals between solid phase and solution in the estuary mixing zone. With a water discharge of 928.2×10 9 m 3/a, the Changjiang is the fourth largest aquatic system, after the Amazon, Zaire and Orinoco, draining into the world ocean. The annual sediment load of the river is 0.5×10 12kg/a with an average suspended sediment level of 544mg/L, so adsorption or desorption of suspended sediments in the Changjiang River estuary for trace metals is one of the most important biogeochemical processes that control the flux of trace metals into the East China Sea. The objective of this study is to understand the behaviors of four metals Cu, Pb, Zn and Cd in the Changjiang River estuary by simulating adsorption of natural suspended sediments in the estuary for these metals in laboratory, especially considering pH effect. Because the riverine suspended sediments of the Changjiang River is a major source of the suspended sediments in the estuary, the sediment used in the simulating experiment were sampled at Xuliujing, located at river end-member of the Changjiang River estuary, in July 2001. About 50 liters surface water was collected with an acid-washed polyethylene barrel. The sediment settled on the bottom of barrel was transferred to a pre-acid-washed 5 liters polyethylene container and preserved in refrigeratory at 4℃ before experiment. The concentration of the preserved suspended sediment was determined by method of filtering of pre-weighted 0.45μm nuclepore polycarbonate membrane. The sediment concentration in simulating adsorption suspension was obtained by dilution. Metals of Cu, Pb, Zn and Cd added to experimental adsorption suspension were prepared in concentrated solutions (1000 mg/L) with salts (Guarantee reagent). The distribution and range of pH were surveyed along salinity gradient in the Changjiang River estuary in July 2001. The range of pH in the Changjiang River estuary is about 7.5-8.3. The pH range in the simulation experiment is controlled within 5.0-9.0, to cover the pH range of the estuary. To eliminate the influence of adsorption of bottles employed in experiment, amount of metals in solution and particle-phase were both determined in all experimental suspensions. After separating suspended sediments from each experimental suspension by centrifugation, adsorbed metals were rinsed off from sediments by 4mol HNO 3 before determined by ICP-AES. The adsorption percentages of Cu, Pb, Zn , Cd in each experimental suspension under different pH were calculated based on the determinations. An increase of sorption percentages with equilibrium pH was observed for Cu, Pb, Zn and Cd in this study. The sorption percentages for Zn and Cd varied respectively from 46% to 90% and 12% to 60% over the pH range in the Changjiang River estuary, whereas that was over 85% for both Cu and Pb. A simple model was used to fit experimental data, and conditional equilibrium constants (lgK) for adsorption of Cu, Pb, Zn and Cd on natural suspended particles in the Changjiang River estuary were derived from this adsorption model. The lgK are -8.87, -6.51,-13.68 and -13.65 for Cu, Pb, Zn, Cd, respectively, indicating that the affinity of natural suspended particles in the Changjiang River estuary is stronger for Cu and Pb than for Zn and Cd. Hence, variation of water pH may have little effect on partitioning of Cu, Pb between suspended matter and solution, but will significantly influence partitioning of Zn and Cd in the Changjiang River estuary.