Study On The Redistribution Of Heavy Metals In The Solid Phases Of The Aquatic Environment

The existing configuration of heavy metals in the natural aquatic environment is complex, once they enter into the natural aquatic environment, most of them will be adsorbed by the solid phases (biofilm, sediment, suspended solid) in natural water. Although the main trend of heavy metals'transformation is that they enter into the relatively stable solid phases, this doesn't mean that the process of heavy metals'transformation will not be changed again. With the development of heavy industries in recent years, the massive waste water discharded into the natural water and the falling acid rain have caused the acidity and the alkalinity of the most of lakes and heavy metals'content in the water undergoing the great change, which have changed the orginial water's acid- alkali equilibrium. Once the conditions of the aquatic environment have changed, it will cause heavy metals of the original aquatic environment's equilibrium system transferring. In the natural water, because of the different quantity of the components in the solid phases, it will cause the redistribution of heavy metals among the solid phases, after a period of time the system will achieve the adsorption equilibrium again.Although many researches have been undertaken to investigate the solid phases'adsorption of heavy metals in the natural aquatic environment at present, these researches are often limited to a solid phase, and there are few comprehensive researches on the interaction of the coexisting solid phases themselves (including sediment, suspended solid, biofilm ) and the solid phases with the trace metals in the natural aquatic environment, especially the researches on heavy metals which have achieved the adsorption equilibrium will be redistributed after the conditions being changed in the natural water's solid phases are fewer.The experiment aimed at researching the redistribution process of heavy metals onto the natural water's solid phases. The pH value, the ionic strength, the concentration of heavy metals and the content of the solid phase were changed through simulating the natural aquatic environment, and the research on the adsorption - desorption kinetics under the different conditions was undertook to analyse the content change and the transfer's characteristics which Pb, Cd and Cu achieved to a new adsorption equilibrium from the original adsorption equilibrium.The results showed that under the conditions of the change of the pH value, ionic strength and the concentration of heavy metals, the change characteristics conformed to the Langmuir kinetic equation and the modified formulas in most of the time, but the Langmuir kinetic equation and the modified formulas didn't reflect heavy metal content's change characteristics well under the condition of the change of the solid phases'content.Under the condition of the pH value decrease, the adsorption of Cu onto the suspended solid, sediment and biofilm present the consistent change tendency, that was they all present constant reducing and then approached to the adsorption equilibrium, and the adsorption quantity of Pb onto the biofilm and the sediment had the similar change with Cu, but the adsorption quantity of Pb onto the suspended solid had the similar change with Cd, that was they all present increase after the decreasing trend. This was possibly due to the recomplexing, the coacervation, the adsorption or the redeposit of heavy metals which caused the change in the deportion process. Under the condition of the pH value increase, the adsorption quantity of Cd and Cu onto the three soild phases all present increasing trend, but the change trend of Pb had difference. Therefore, we can see from this, the influence of the pH value to Pb was the most remarkable, this might relate with the manganese oxide which combine with Pb onto the soild phase medium. In the whole process, regardless of the pH value increase or decrease, the change process of the content of Pb and Cu was in relatively rapid in the sediment.When the ionic strength changed, the adsorption quantities'change of Pb, Cd, Cu onto the biofilm displayed large differences under two situations, the adsorption quantities'change of Pb, Cd, Cu onto the sediment was in similar by and large, which present decrease after increase in different time, and the one of Pb, Cu onto the suspended solid also prenent some comparabilities, but the one of Cd present decreasing trend on the whole. This might due to existing the competition effect of the electrostatic adsorption and the specific adsorption, and existing the mutual influence among heavy metals, which caused different change characteristics of Pb, Cd, Cu in the reaction process. Thus it could be seen, the influence of the ionic strength's increasing extent was the largest in the biofilm, and the one was the smallest in the sediment. At the same time, the desorption speed of Cd became quickly along with the ionic strength increasing in the sediment and suspended solid, and the transfer's speed of Cu became quickly also along with the ionic strength increasing onto the sediment and suspended solid.When the concentration of the heavy metals increased, the adsorption kinetic process of Pb, Cd and Cu onto solid phases often experienced two stages, i.e. preliminary fast adsorption process and continued relative slow adsorption process, and during the process, Cu was adsorbed the most slowly, and the content of the three heavy metals onto the suspended solid were the most when arriving the adsorption equilibrium, and the one onto the sediment were the least. But because of the three heavy metals coexisting in the same solid phases, the content's increasing extent of the heavy metals had some influences to the mutual adsorption competition among them, thus under the two conditions, the adsorption characteristic of Pb, Cd, Cu onto the solid phases also represent certain differences. Along with the concentration increasing of the heavy metals, the adsorption speed of Cd onto the biofilm and sediment became quickly, and the adsorption speed of Pb, Cu onto the suspended solid also accelerated, but the one of Pb, Cu onto the sediment slowed down.When the content of the solid phases increasing, the content of the heavy metals onto the unit mass biofilm decreased in the preliminary stage, and then increased later, this was mainly because of"silt effect of heavy metal adsorption". In the preliminary stage, the suspended solid adsorbed massively, and the content became fluctuated and decreased. The content onto the sediment was much fewer than the biofilm and the suspended solid, and the change was slow relatively. In the entire process, the content order was suspended solid > biofilm > sediment.Compare with the kinetic process of the heavy metals onto the solid phases medium in above each reaction condition, the content of the iron oxides were much higher than the manganese oxides in all solid phases. Seeing about the content change of the heavy metals, the content of Cd onto the solid phases kept the fewest all the time, and compare to Pb, Cu, the content of Cd onto the suspended solid appeared relatively strong fluctuation in most cases. There were adsorption-desorption and diffusion effects in the entire kinetic reaction process, and the two effects existed all through this simulating process, and they were principal and subordinate by turns, and therefore the content of the heavy metals onto the solid phases appeared to different change process in different phases, and the heavy metals onto the solid phases also appeared different redistribution processes under the different conditions.