Distribution Characteristics of Lead and Cadmium in Aquatic Microcosm
|Keywords||aquatic microcosm lead cadmium water phase surface coatings deposited sediment|
Lead and Cadmium are heavy metals commonly found in the environment. They are mainly from mining, metal smelting, chemical industry, coal and so on. When heavy metals enter into the water environment, its behavior are significant influenced by a variety of solid-phase in water environment. Different solid-phase has different effects on behavior of heavy metals in water environment. Various solid-phase transformations between themselves will also affect the migration and transformation of heavy metals. Aquatic microcosm is Model Aquatic Ecosystem. It is the most application simulated ecosystems of environmental study in foreign. At present, aquatic microcosm laboratory methods have been widely used to study heavy metals and other pollutants’ fate in aquatic ecosystems.They are only select a different solid material (surface coatings (SC) or deposited sediment (DS) or suspended particulate matter (SPM)) and water mixed in a simple container (e.g. beaker, etc.), or use multi-phase system to study the adsorption and desorption of pollutants in aqueous phase by solid-phase. Then they compared the different law of pollutants’ adsorption and desorption in solid-phase. This experiment can not be achieved in large measure on the natural conditions of the water environment simulation of multi-phase system, and can not study on pollutants of the longitudinal movement in the sediments. In natural water, SC, SPM, DS is the co-existence among the aqueous phase, with mutual links and interaction, and the existence of the three have their own state. Distribution of pollutants in the three phase and aqueous phase is different between a mixture of various solid-phase co-exist with a single aqueous phase and the coexistence of solid and multi-phase system at the same mixing device.This study uses the method of Microcosm. We acquire water and a variety of solid-phase in South Lake in Changchun City. Then use them establish some laboratory aquatic microcosm and cultured SC. When SC grown up, we use aquatic microcosm to study distribution characteristics of Pb and Cd in multi-phase system in the natural water system, as well as impact of heavy metal concentration and pH on the distribution of Pb, Cd in the aqueous phase, SC and DS.The results showed that the concentration of heavy metals in the water phase decreased rapidly early in the beginning of experiment, followed by a gradual slowing down the rate of decline and to maintain a certain concentration. When we increases the total amount of heavy metals in the system, heavy metals in the water phase again after going through the process to maintain a certain extent, and is no less than before. The changes of pH in the system will affect the concentration of heavy metals in the water phase, when the pH reduced, the concentration of heavy metals in the water phase increased, on the contrary, it decrease.When the concentration of heavy metals in the water in the 0.5-16μmol / L concentration range increases, heavy metal content in solid-phase will increase; when concentrations of heavy metals in the water phase are higher, the heavy metal in the water is rapidly enriched by solid-phase through the phase of ions exchange, adsorption or precipitation. The speed of enrichment slow down with time-rich and the lower the concentration of heavy metals in aqueous phases. When concentration of heavy metals in the water phase down to a smaller value, the heavy metals will be part of released from solid-phase medium. The heavy metals in the water - solid-phase system would to be stabilizing finally.When the system increases the total amount of heavy metals, the heavy-metal contents in solid-phase medium will increase, and the increase step is similar with process. When the system stabilized, the heavy metal content in solid-phase compared with process to an increase, and in high concentrations system it is more obvious.Changes in pH value can significantly affect the solid-phase on the accumulation of heavy metals, and the effects of Pb and Cd is similar to. When the pH value decreased, the solid-phase on the ability to enrich the heavy metals decreased, on the contrary it increased. But when the pH value too high, the phenomenon of heavy metal hydrolysis increased, so that the content of heavy metals in SC reduced, but content of heavy metals in DS increased.In aquatic microcosm, the SC on the ability of accumulating heavy metals is higher than the DS. While the impact of the ability of accumulating heavy metals to SC by the changes in external conditions is more than to DS.Enrichment of heavy metals in surface DS are mainly distributed in the depth of 0～1cm layer, as time goes by, high concentration system, the depth of 1～2cm of DS were also found higher concentrations of heavy metals. In the experiment lasted more than 2000 hours, the depth of 2～5cm of the DS were found without significantly higher content of heavy metals. Comparison the content of Pb and Cd in the DS at different depths, Cd in DS found in the vertical migration is higher than that of Pb.