Preparation Of Modified Biochar And Its Remediation Of Cadmium Contaminated Soil

Heavy metal pollution has become the main problem of soil environmental quality of agricultural land.It has become a research hotspot to explore efficient and economical remediation technology of heavy metal contaminated soil.Biochar has been widely used in the remediation of heavy metal contaminated soils because of its excellent passivation of heavy metals and rich nutrient elements.In this paper,pine branch biochar was modified to improve the adsorption capacity of Cd and enhance the passivation effect of Cd in soil.A pot experiment was conducted to study the effects of modified biochar on passivation remediation of Cd contaminated soil,soil physical and chemical properties,enzyme activity and Cd accumulation and absorption of pakchoi（Brassica chinensis L）.The main contents and results are as follows:（1）Using pine twig as raw material,biochar（BC）was prepared by oxygen-limited pyrolysis at 350℃.A pot experiment was conducted to study the effects of pine branch biochar on the transformation of soil Cd forms and the accumulation and absorption of Cd by mung bean（Vigna radiate（Linn.）Wilczek.）.It was found that biochar could improve soil p H,promote the transformation of Cd from weak acid extraction state with high activity to residue state with low activity,and significantly reduce the bioavailability of Cd.The application of biochar increased the biomass of mung bean,of which 5%biochar treatment was the best,with a maximum increase of 13.47%.Compared with the control group,the Cd content in the aboveground and underground parts of mung bean decreased by 81.86%and 43.86%respectively.（2）The biochar was modified by acid（H₃PO₄,HNO₃）,alkali（KOH）and oxidation（H₂O₂,KMn O₄）,and the adsorption properties of Cd in water were studied.Among them,the maximum adsorption capacity of Cd by BC-KOH and BC-Mn is101.1 mg·g^-1 and 128.91 mg·g^-1,respectively.The quasi-second-order kinetic model and Freundlich isothermal adsorption model can better simulate the adsorption process of BC-KOH,while BC-Mn is more consistent with the quasi-second-order kinetic model and Langmuir isothermal adsorption model.The relative contribution of adsorption mechanism shows that mineral precipitation（40.1%）is the main adsorption mechanism of BC,while oxygen-containing functional complexation and mineral precipitation of BC-Mn and BC-KOH are the main mechanisms for the removal of Cd in water（69.3%,74.8%）.The chemical stability of the two adsorption mechanisms of complexation and precipitation is high,which is beneficial to the passivation of heavy metals in soil,and it is difficult to reactivate heavy metals when the soil environment changes.（3）The effects of different application rates of BC,BC-Mn and BC-KOH on the morphological distribution of soil Cd and soil physical and chemical properties were studied by soil culture experiment.The results showed that compared with biochar BC,modified biochar（especially BC-Mn）had better passivation effect on Cd in soil.The application of biochar significantly reduced the bioavailability and mobility of cadmium by transforming soil Cd from highly active acid extracted and reducible to weakly active residue,among which the application of 3%BC-Mn had the best effect（effective Cd reduced 48.05%by 56.27%）,and the passivation remediation effect of low concentration Cd contaminated soil(4 mg·kg^-1)was better than that of high concentration Cd contaminated soil(20 mg·kg^-1).The application of biochar can increase the activities of soil p H,neutral phosphatase and urease by 19.26%,56.60%and 37.69%,respectively;at the same time,it can also improve soil fertility and increase the content of soil available P,available K and ammonium N.（4）A pot experiment was conducted to explore the effect of BC-Mn on the growth of pakchoi in Cd contaminated soil.The growth parameters and Cd content of pakchoi were determined on the 20th and 40th day.The results showed that the application of BC-Mn,significantly increased the total chlorophyll content and leaf photosynthetic rate,thus increased leaf length,leaf width and yield,and increased by up to 53.31%.The contents of soluble sugar and soluble protein in pakchoi increased by 0.33-0.47,0.47-0.49 times respectively,and the quality of pakchoi was improved.At the same time,the Cd content of each part of pakchoi was significantly reduced（47.11%-74.93%）,the activity of superoxide dismutase was increased,and the content of malondialdehyde was decreased.