Immobilization And Remediation Of Cadmium Contaminated Soil With Biochars

With the development of urbanization and industrialization, heavy metal soil contamination, especially cadmium (Cd) contamination, has become a global concern because of its adverse effects on ecosystem health and food security. Biochar is of high specific surface area, cation exchange capacity (CEC) and rich oxygen containing functional groups, which can increase the adsorption capacity of various metal ions and thus reduce the bioavailability and biotoxicity of heavy metals. Hence, it is important to strengthen the research on the application of biochar in remediation of the soil contaminated by heavy metals. In present study, four kinds of local agricultural wastes in Guangxi Zhuang Autonomous Region, including sugarcane top, cassava straw, rice straw and silkworm excrement were selected to prepare biochars (BCst, BCcs, BCrs and BCse for short, respectively) at 500℃ with limited oxygen supply. The structural properties of biochars from different materials were analyzed by means of scanning electron microscopy, elemental analysis and Boehm titration. Batch experiments were used to investigate the effects of biochars on Cd chemical forms in Cd contaminated soil (5 mg±kg-1) which was incubated over Od,15d,30d and 45d. Furthermore, BCrs and Bcse, which had better deactivation effects on Cd, were selected for some pot experiments in order to study their capacities of Cd fixation. The main results were as follows:1) All types of the biochars applied in the study were of high pH value, CEC, organic carbon content, oxygen-containing functional group and the surface polarity, which made them have good absorption for heavy metals. The soil pH, CEC, and oxygen-containing functional group of BCse were all the highest among the four kinds of biochars, but the organic carbon content of BCse was on the low side, with 54.3% of the highest content. On the other hand, for BCst, the soil pH, CEC and oxygen-containing functional group were the lowest while the organic carbon content was the highest of the biochars studied.2) All of the four biochars could effectively reduce the bioavailability of Cd in the contaminated soil. The contents of weak acid extractable and reducible Cd decreased while the contents of oxidized and residual Cd increased when adding biochars into the Cd contaminated soil. The inactivation effect of Cd after adding biochars varied with the experiment time, and the effect obtained the highest value at 45d. The remediation effects of different biochars on the polluted tidal soil, paddy soil and latosolic red soil were all decreased in the order:BCse>BCrs>BCcs> BCst. At the same time, for each kind of biochar, there were significant differences among the three soil types, the best remediation effect was found in the tidal soil, followed by the effect in the paddy soil and finally in the latosolic red soil. After 45 days, when compared to the control experiment, the contents of weak acid extractable Cd and reducible Cd were found to reduce 62.39% and 24.73%, the contents of oxidized Cd and residual Cd increase 178.95% and 216.76% respectively when adding BCse into the tidal soil.3) Biochars could influence the chemical forms of Cd by surface adsorption and the changes of the soil properties. All of the four kinds of biochars promoted the soil pH, CEC and organic carbon content. There were significant negative correlations between the soil pH and CEC and the contents of weak acid extractable and reducible Cd. In addition, significant positive correlations were found between the soil pH, CEC and the contents of oxidized and residual Cd. No significant correlations between organic carbon content and the chemical forms of Cd were detected in present work.4) Adding BCse and BCrs into the polluted paddy soil could significantly enhance the plant height and fresh weight of maize, reduce the accumulation of Cd in maize plant, decrease the bioaccumulation coefficient and prevent the translocation of Cd from the roots to aboveground parts. Compared with the control, the contents of Cd in the plant roots reduced 27.78% and 16.67%, the bioaccumulation coefficient was 19.63% and 9.11% respectively when 0.5% BCse and BCrs were separately added into the paddy soil, and the content of Cd reduced 50.00% and 33.33%, the bioaccumulation coefficient was 24.77% and 13.56% respectively by the 1% addition of BCse and BCrs. In a word, the remediation effects increased with the increasing addition amounts of the two biochars, and the effect achieved by adding BCse into the Cd contaminated soil was better than that achieved by adding BCrs into the soil.In conclusion, good performance had been showed when biochars were added into the Cd contaminated soil, and the remediation technique of heavy metal polluted soil through adding biochars into the soil was feasible to some extent. Biochars could effectively prevent the heavy metal Cd transporting from the soil to the plant system, and then the toxicity of Cd was declined. Of the remediation experiments of Cd polluted soil, the effect that obtained through adding BCse into the tidal soil was the best.