Effect Of Biochar On The Bioavailability And Redistribution Of Heavy Metals In Soil

Recently, with the development of industrial and agricultural production, wastewater irrigation and other activities have increased the number of agricultural soils contaminated with heavy metals in many parts of the world. Heavy metals are non-degradable and easily accumulate in soils, which may pose a risk to the environment and to human health when they entered the food chain through plants. Previous studies showed that biochar may be used as adsorbent for adsorption of heavy metals and subsequently reduce their bioavailability in contaminated soils. In the current study, bamboo biochar and rice straw biochar were selected as the soil amendments. In order to investigate the feedstock material, application rates(0, 1% and 5 %) and mesh sizes(< 0.25 and <1 mm) of biochar on the effect of the heavy metal(cadmium(Cd), copper(Cu), lead(Pb), and zinc(Zn)) extractability and enzyme activity(urease, catalase and acid phosphatase) in co-contaminated soil, a 1-year incubation experiment was carried out. After incubation, the physicochemical properties, extractable heavy metals, available phosphorus, and enzyme activity of soil samples were analyzed.In addition, a pot experiment was conducted to explore the influence of tobacco stalk biochar and dead pig biochar on the bioavailability of heavy metals and the growth of tobacco plants in co-contaminated soil(Cd and Zn). The underground biomass of tobacco and extractable heavy metal in soil were determined. The bioavailability of heavy metal was analyzed by the uptake of tobacco plants. The main result are as following.1. The incubation experiment clearly showed significant(p < 0.05) changes in soil properties, including increases in soil p H, EC, CEC and available P, after application of rice straw biochar, especially at the higher application rate and finer particle size. Significant increases in the soil CEC and available P were also observed at the 5% fine bamboo biochar addition rate. The 5 % application rate of fine rice straw biochar resulted in the greatest reductions of extractable Cu and Zn, 97.3 and 62.2%, respectively. These results indicated that rice straw biochar was more suitable as a soil amendment than bamboo biochar. In addition, the biochars investigated in this study also had the potential to significantly affect the activities of urease and catalase in the contaminated soil. Urease activity increased by 143 and 107% after the addition of 5% coarse and fine rice straw biochars, respectively. Significant correlations were observed between the concentration of extractable heavy metals and activity of enzymes.2. The pot experiment demonstrated that tobacco stalk biochar and dead pig biochar significantly increased the p H of soil, but there was no significant effect on electricity conductivity with biochar addition. The content of Ca Cl2-extractable Cd and Zn decreased with the increase of the application rates. At 5% application rate, the concentration of extractable Cd and Zn decreased by 64.2% and 94.9% for tobacco stalk biochar treatment and 45.8% and 61.8%for dead pig biochar treatment. In the 5%tobacco stalk biochar treatments, the exchangeable fractions of Cd and Zn were transformed to carbonates bound and Fe-Mn oxides bound fractions. With the addition of dead pig biochar, the exchangeable and carbonates bound fractions of Cd were transformed to Fe-Mn oxides bound fraction, exchangeable and organically bound fractions of Zn were transformed to carbonates bound and Fe-Mn oxides bound fractions. The underground biomass of tobacco was significantly increased after the addition of biochar, and the contents of Cd and Zn decreased with the increase of the application rates. In the 2.5% dead pig biochar treatment, the above ground and underground biomass of tobacco increased by 43.5% and 40.9%, respectively.In conclusion, rice straw biochar was more suitable to be used as soil amendment for remediation of contaminated soils. The impact of biochar treatment on heavy metal extractability and enzyme activity varied with the biochar type, application rate and particle size. The tobacco stalk biochar had greater potential as an amendment for reducing the bioavailability of heavy metals in soil than that of the dead pig biochar. This can help enhance soil productivity, plant growth rate and crop quality.