Environmental Adsorption Behavior Of Biochar And Its Application In Remediation Of Cadmium Contaminated Soil

Cadmium, a kind of trace metal with the strongest biotoxicity, due to its high chemical activity and persistent toxicity in soil, poses a great potential chance to cause hazards to human health via food chains. China’s soil contaminated by Cd is in "grim situation". According to the "National Soil Pollution Investigation Communique" released by Ministry of Land and Resources in April2014, Cd was formally identified as the soil primary pollutant in China for its high exceeding standard rate by measuring the point position, thus the remediation and control technology is urgent needed for Cd contaminated soil. Biochar, due to its well-developed micropore, large numbers of functional groups and negative charges on its surface, has a strong adsorption and fixation capacity to trace metals. Biochar can also enhance the complexing ability of soil with Cd through altering soil physical and chemical properties, reducing the degree of soil contamination with Cd effectively, thus has broad application prospects in control and remediation of soil cadmium contamination.This thesis focused on the problems existing in the present research, the structure of herb biochar, woody biochar and activated carbon and their different adsorption ability on polar and nonpolar substances were studied systematically, thus to characterize the adsorption ability and adsorption performance parameters of different carbon materials. By applying biochar and related passivation systems in simulated Cd contaminated soil, the contamination remediation effect was explored through the planting of rice, lactuca sativa L. and oilseed rape. The experimental results were applied in the remediation of copper tailings, The intelligent on-line monitoring system for soil environmental parameters was applied, offered real-time monitoring on temperature, humidity and pH of soil, realized the remediation process informatization. The scientific basis was provided for the application of biochar on soil Cd pollution remediation. The main results are as follows:(1) Affected by carbonization temperature and raw materials, the characteristics of three kinds of carbon materials were demonstrated as follows:the specific surface area of three kinds of carbon materials were in order of:coconut shell activated carbon (AC-Y)>woody biochar (BC-M)>herb biochar (BC-C); the amount of basic functional groups:AC-Y>BC-C>BC-M, the density of basic functional groups:AC-Y>BC-M>BC-C; number of acid functional groups:BC-C>AC-Y>BC-M, the density of acid functional groups:BC-C>BC-M>AC-Y and total functional groups:BC- OAC-Y>BC-M. AC-Y was nonpolar, with highest adsorption capacity for Phe almost up to94.40%in6hours, while BC-C and BC-M was polar, with relatively higher adsorption ability for Cd2+in solution than AC-Y. To characterize the polar or nonpolar property and the polarity degree of charcoal, the density of basic and acid functional groups on them was more accurate than their numbers on them.(2) The adsorption isotherm of three kinds of carbon materials could fit well with the Freundlich multilayer adsorption model, and their adsorption process could be well described by Pseudo-second-order kinetics model, moreover, the saturated adsorption of BC-M, BC-C and AC-Y for Cd2+was7.89mg/g,7.12mg/g and6.81mg/g respectively at25℃, which indicated that the adsorption of Cd2+was a combined effect of physical and chemical process. The results showed that the parameter of saturated adsorption amount (qe) for Cd2+calculated by Pseudo-second-order kinetic equation is closer to the saturated adsorption amount of different carbon materials characterized by the density of total functional groups. Based on factors of specific surface area, amount and density of functional groups, pore size distribution and so on, especially the adsorption stability, BC-M was more suitable for application in the remediation of Cd contaminated soil.(3) When adding biochar to long-term Cd contaminated soil, there was an "activation" effect on Cd in the soil in the short term, the amount of bioavailable form Cd increased in the soil. However it turned out to be an "inactivation" effect in the long time scale. The passivation system, in core of biochar, had a significant effect on remediation of the Cd contaminated soil. The combined system consisting of1%biochar associated with B-CSSH passivant (5times to the molar mass of total Cd in the soil) had an additive effect on the "inactivation" effect. This system was the best passivation system. Adding order should be:1%biochar was added first, then B-CSSH was added after3d when the system was stable. Compared to the control group, the concentration of bioavailable form Cd in SD-H soil fell83.93%, while residual Cd content increased by78.62%; the concentration of bioavailable form Cd in SD-L soil decreased by68.89%and residual form Cd content increased by58.97%after120d treatment.(4) Black soil system had an obvious suppressing impact on the Cd absorption procedure by Lactuca sativa L.. Compared to the control group, the BCF decreased by57.35%as the maximum scale after50d. Different adding proportion of black soil system made different absorption suppressing impact, compared to the control group, the absorption amount of Cd by the Lactuca sativa L. growing in YMC-L soil with2%of black soil decreased by55.09%after70d; the one in YMC-H soil with2%black soil system decreased by22.45%. At different growth stages of Lactuca sativa L.,0.11mol/L HAc had higher extraction amount of bioavailable form Cd in soil than1mol/L NH4Ac and0.01mol/L CaCl2. The correlation coefficient, R2, between extraction amount of effective soil Cd and enrichment amount by plant of these three extractants were0.6308,0.3831and0.1884, separately. Among them, the HAc solution had the best extraction performance.(5) Covering the copper tailings soil with biochar, mushroom planting soil and farmland soil from the bottom up, it was equal to multistage suppression system, trace metals released from copper tailings layer were mainly adsorbed in mushroom soil layer. Compared to covering the copper tailings with farmland soil directly, total amount of trace metals in topsoil under the treatment of this multistage suppression system decreased by53.26%. Moreover, the enrichment amount of Cu in oilseed rape which were planted in this multistage suppression system decreased by67.62%, compared to S1group. Oilseed rape grew normally, did not show characters of trace metal poisoning, the treatment system showed an obvious effect on inhibition of trace metals migration from copper tailings layer to topsoil layer.Biochar, which is low-cost, with wide sources of raw materials, significant efficient in the remediation of trace metal contaminated soil and capable of application in new areas such as phytoremediation of tailings, is expected to be one of the most promising material for environmental remediation in the future.