The Research On The Effects And Mechanism Concerned Of The Soil Heavy Metal Of Biodegradable Polymer Materials

Biodegradable polymer materials are a new type of fertilizer,their application is increasingly concerned owing to their unique advantages,such as varieties of nutrients,long degradation cycle,continuous nutrients supply,green degradation products,reducing the negative environmental effects and so on.Soil heavy metal pollution affects the growth of crops and ultimately hazards human health through food chain,existing serious eco-environment risks.The chemical remediation is one of the main technologies for the remediation of soil heavy metal pollution.Biodegradable polymeric materials can not only provide nutrients for crops,but also immobilize the heavy metal ions and reduce their activity by enhancing the soil pH value and increasing the soil available phosphorus?AP?,organic matter contents?OM?in the degradation process.In the current study,the biodegradable polymeric materials containing various nutritive elements produced by Research Center for Engineering Technology of Polymeric Composites of Shanxi Province are choosed as the research objects,we investigated the environmental effect of biodegradable polymer materials on soil and assessed environmental risk,provided the theoretical basis for development of economical and practical chemical remediation techniques for heavy metal contaminated soils via leaching experiment and two-year pot experiment.The main results of study are as follows:?1?The application of biodegradable polymer materials had a significant effect on the pH of the leachate.The pH of the leachate increased at first and then stabilized at 7.80-8.05.With the increase of the leaching times,the concentrations of Zn and Pb in the leachate treated with biodegradable polymer materials gradually had shown a downward trend and then leveled off.However,the concentration of Cu and Cd in the leachate fluctuated obviously and didn't exceed the Farm Irrigation Water Standard and didn't pose a risk to groundwater contamination.The available Pb and Cd mainly accumulated in the surface layer of soil and were a trend of migration to the subsoil.The mobility of Pb in the top soil layer was lower than that of Cd.The available contents of copper and zinc were also high in the upper soil.But the contents of the available four elements all decreased with the increase of soil depth,which indicated that the biodegradable polymer materials effectively reduced the risk of groundwater pollution.?2?And by contrasting restorative effects of the biodegradable polymer materials with different carbon chain structures type in the multi-metal contaminated soils,the results indicated that the biodegradable polymer materials containing carboxyl functional groups with the function of water absorption?SRPFWA?was better than other materials,such as the biodegradable polymer materials containing nutrient elements NPK without the function of water absorption?PRF??the biodegradable polymer materials containing nutrient elements silicon without the function of water absorption?GRF??the biodegradable polymer materials containing nutrient elements boron without the function of water absorption?BRF??the biodegradable polymer materials containing nutrient elements sulphur without the function of water absorption?SRF?,even better than organic-inorganic compound fertilizers?OM+INF?,polyvinyl alcohol-inorganic compound material?PVA+INF?.Besides,the biodegradable polymer materials with low solubility were more effective than high solubility.Under the second year's pot experiments,the bioavailable concentrations of heavy metal decreased,the soil microorganism activity raised and the amount of nutrient supply enhanced more markedly in the same treatment soil than the first years.The bioavailable Pb concentrations?CaCl₂extracted?in the soils reduced by 95.13%⁹9.20%of applying SRPFWA compared with the control soils?CK?,at the same time,the bioavailable Cu,Zn,Cd concentrations?CaCl₂extracted?decreased by 63.72%⁸4.36%and 93.47%⁹9.12%,81.96%⁹6.31%,respectively.Furthermore,the microbial enzyme activities were markedly improved.Correlated analysis showed that the soil available phosphorus?AP?,the soil oxidation-reduction potential?ORP?and organic matter contents?OM?were negatively correlated with the available Cu,Zn,Cd and Pb contents in soils.Soil catalase activity was significantly negatively correlated with CaCl₂ extracted Cu,Zn,Cd,and Pb contents.The results proved that the biodegradable polymer materials immobilized the heavy metal ion of soil by improving soil physicochemical performance?AP?OM?ORP,etc?,and that the improvement of the soil enzymic activities were greatly related to the contents of the easily mobile heavy metal.The application of the biodegradable polymer materials significantly reduced the toxicity of heavy metal ions,thereby increasing microbial enzyme activity in soil.In particular,in the SRPFWA treatment,the effect fixation of heavy metal ions was even more significant.Because SRPFWA was the water absorbent and retention material with interpenetrating networks structure and contained carboxyl functional groups,which was synthesized by chemical bond.Compared with other biodegradable polymer materials,SRPFWA had some definite advantages,such as,its degradation period was longer and it had much influence on the soil available phosphorus and organic matter contents.Consequently,the heavy-metal toxicities of SRPFWA treated soils significantly declined,which might be related to the soil had more adsorption complexation reactive sites with heavy metal.?3?Based on the two-year pot experiments,the biodegradable polymer materials were applied in the soils polluted by heavy metals,the result showed that the accumulation of heavy metals in the tissues of tomatoes significantly reduced,and the plant height,the biological yield and average weight of tomato significantly increased.Because the application of the biodegradable polymer materials not only continuously supplied nutrition?nitrogen,phosphorus,and potassium?for the growth of tomatoes,but also increased the contents of available phosphorus?AP?and organic matter?OM?in the soil.At the same time,the soil had more adsorption complexation reactive sites with heavy metal and their mobility significantly reduced.The most striking result was that the concentrations of heavy metal in tomatoes fruit treated by the biodegradable polymer materials significantly reduced.Specially,there were not detected Cu and Cd in the fruits treated by the biodegradable polymer materials containing carboxyl functional groups with the function of water absorption?SRPFWA?and the biodegradable polymer materials containing nutrient elements NPK with low solubility?PRFL?.And the contents of Cu and Cd in the fruits were lower than the food safety standards and the maximum allowable amounts of contaminants in foods after applying two materials.