The Effect Of Solution PH On The Kinetics Of Pb And Cd Adsorption And Desorption By Natural Surface Coatings

Heavy metals, a kind of important pollutant, in the aquatic systems have prompted great interests due to their nonbiodegradable properties, bioaccumulation effects and toxicities even at low concentrations, and are affected by the characteristics of natural waters such as the solution pH, redox, and the speciation and the concentrations of the ions in the waters. Additionally, the speciation and transformation of heavy metals in the aquatic environments are affected significantly by the solid phases in the water, and reactions occurring at solid surfaces play a dominant role in determining metal partitioning between the solid and solution phases, and thus, the potential impact of metals in the aqueous environments. Natural surface coatings composed of microorganisms, oxide minerals and many other things are the composite solids existing widely in the natural aquatic environments, and they occur on all kinds of solid surfaces and affect the behaviors of heavy metals significantly. Due to the high surface reactivity and large surface areas, the most important role of surface coatings in the aquatic environments is the adsorption or enrichment of pollutants in the water. Besides, the adsorbed pollutants may be degraded or detoxified due to the bioactivity of microorganisms. While the solution pH is the most important factor in determining the binding force between the heavy metals and the solid surfaces, which directly influences the speciation of electric charge and components in the solid surfaces, the distribution of anions and cations between the solutions and solid surfaces, and their removal, reaction activity, bioavailability, bioaccumulation, and toxicity.The adsorption rate of heavy metals to the surface of solid phase in aquatic environment can directly reflect their transference characters in the aquatic environment. Solution pH is the most important variable to control the binding force of trace heavy metals to the solid phase in the aquatic environment. Pb and Cd are the most common heavy metal pollutants in natural waters, which can be easily adsorbed to surface coatings. Therefore, take Pb and Cd as the example, this research work mainly focused on simulating the effects of solution pH to the heavy metals adsorption kinetics to surface coatings, and providing the assessment and predication of the natural waters theoretically. The adsorption process of pollutants to the surface coatings is a dynamic equilibrium process containing adsorption and desorption. The heavy metals in the natural waters can be adsorbed by surface coatings, and can also produce the secondary pollution by releasing to the aquatic environments because of the change of environment conditions or the effects of the organisms. Therefore, this research work investigated the heavy metals desorption dynamics to surface coatings under different solution pH, hoping to give some help to the further understandings of the adsorption mechanisms to surface coatings and the effects of pH to the phase distribution of heavy metals in the aquatic environment. This research work has discussed the effect of solution pH on the kinetics of Pb and Cd adsorption and desorption by natural surface coatings. The surface coatings are put into the MMS solutions which containing 2.0 μmol/L Pb and Cd . The solution pH are 5.0,6.0,7.0,8.0,9.0 ± 0.1, respectively. Samples are taken from the solutions at certain time, the Pb and Cd concentrations in which are measured, and the Pb and Cd adsorbed are calculated by subtraction. The desorption is investigated as followed. The surface coatings are put into the MMS solutions which containing 2.0 μmol/L Pb and Cd to adsorb Pb and Cd for 24 h at pH 6.0 ± 0.1 and 20 ± 1oC. After the adsorption equilibrium is reached, the surface coatings are transferred to other MMS solutions which contain no Pb and Cd, stirring magneticly for 13 h. The solution pH are 5.0,6.0,7.0,8.0,9.0 ± 0.1, respectively. Samples are taken from the solutions at certain time, and the Pb and Cd concentrations are measur...