Screening And Mechanism Of Heavy Metal Adsorption Materials

Soil heavy metal pollution is one of the most important environmental problems in food safety.When uptaken by plants into the food chain,or leaked into underground water,heavy metal ion can pose a fatal threat to human beings as well as the ecological system.Thus it has become such a pollution that should be handled properly.After contaminating the soil,Pb,Cd and Cu can stay there for a long time,and become a real threat when accumulated to a certain level.In recent years,researchers have done a lot of work to develop new materials to remediate the soil contaminated by heavy metals.Because of its excellent ability of heavy metal adsorption,water-retaining agent has been considered to be a good choice for remediation of soil contaminated by heavy metals.In addition,attapulgite,diatomite,biochar and other environmental friendly minerals have also been used for remediation materials for heavy metals removal in soil.Firstly,to better analyze the adsorption behavior of water-retaining agent(sodium polyacrylate),attapulgite,diatomite and biochar,electron microscopy is applied to characterize the surface structure and elemental distribution.The sorption capacity of above-mentioned sorbents for Pb,Cd and Cu has been compared with batch sorption/desorption experiment.for their heavy metal sorption.Secondly,different rates of water-retaining agent,attapulgite,diatomite and biochar are added to Pb,Cd and Cu contaminated soil in pots with growth of ryegrass.This work investigated the effects of these sorbents on plant uptake and morphological transformation of heavy metals.The results can offer a theoretical reference to the application of those materials for soil remediation.The main conclusions are as follows:1.Scanning electron microscopy(SEM)shows that the smooth surface of water-retaining agent becomes unsmooth after adsorbing heavy metal ion,while the surface morphology of attapulgite,diatomite and biochar was not changed evidently.2.The batch adsorption experiments revealed the sorbents have different sorption capacity for Pb,Cd and Cu.The maximum adsorption capacity of Pb are water-retaining agent>attapulgite>diatomite>biochar,which are 461.6 mg·g-1,98.0 mg·g-1,123.3 mg·g-1 and 73.3 mg·g-1 respectively.The maximum adsorption capacity of Cd are as follows:water-retaining agent>biochar>attapulgite>diatomite,which are,131.3 mg·g-1,30.9 mg·g-1,19.6 mg·g-1 and 19.4 mg·g-1 respectively.The maximum adsorption capacity of Cu are as follows:water-retaining agent>attapulgite>biochar>diatomite,which are77.6 mg·g-1,23.6 mg·g-1,13.6 mg·g-1 and 1.5 mg·g-1,respectively.The CEC value of the materials also confirmed that water-retaining agent has the maximum adsorption capacity of Pb,Cd and Cu.3.Desorption experiment suggested that heavy metals are dominantly adsorbed by chelation and complexation mechanisms,while only a small portion is adsorbed through ion exchange.4.Water retaining agent can promote the transformation of Pb,Cd and Cu from soluble state into insoluble state,while ryegrass can promote the dissolution of insoluble Pb,Cd and Cu.Both of them contribute to a significant reduction of Pb,Cd and Cu in insoluble state.The dissolved Pb,Cd and Cu was mainly absorbed by water-retaining agent,which reduced uptake of ryegrass as well as,concentrations of Pb,Cd and Cu in the above-groundtissues.5.Attapulgite can change the forms of Pb,Cd and Cu in soil,but has no effect on the passivation process of metal ions.Ryegrass can promote the transformation of insoluble residual Pb,Cd and Cu to oxidizable,reducible an acid soluble states.The application of attapulgite in soilevidently inhibited uptake of Pb,Cd and Cu by ryegrass.6.Diatomite can change the forms of Pb,Cd and Cu in soil,but has no effect on the passivation process of metal ions.Ryegrass can promote the transformation of insoluble residue residual of Pb,Cd and Cu to be transformed to oxidizable,reducible and acid soluble states.Diatomite inhibited plant uptake of Pb and Cd evidently,but Cu significantly.7.Biochar can change the forms of Pb,Cd and Cu in soil,but has no effect on the passivation process of metal ions.Ryegrass can promote the transformation of insoluble residue residual of Pb,Cd and Cu to be transformed to oxidizable,reducible and acid soluble form.The concentrations of Pb,Cd and Cu in plant tissues decreased with increased biochar applciation rates.iBiochar significantly inhibited plant uptake of Pb and Cd.