Immobilization Of Pb²⁺ In Contaminated Waters And Soils By Biohcars

Biochar is a group of carbon-rich materials, produced from abandoned biomassthrough thermal decomposition in a closed system with little or no oxygen. Due to itsstability and special physicochemical properties (e.g., surface oxygen-containingfunctional group, pore structure and ash components), biochar as an environment-friendly material has been applied in sequestrating carbon, reducing greenhouse-gasemissions (e.g., carbon dioxide, CO2and nitrous oxide, N2O), improving soil qualityand increasing crop yields, has generated great interest for scientists and policymakers. In this study，ten biochars were prepared from an agricultural by-product(peanut shells) and medical waste (Chinese medicine material residue, the mixture ofpolygonatum sibiricum and other herbs after pharmaceutical production) at differentpyrolysis temperatures (300-600°C), and the physical and chemical properties ofbiochars were studied, and thus analysed the intrinsic relationship between thepyrolysis temperature and the different properties of biochars. Aiming at theincreasingly serious heavy metal (lead) pollution in waters and soils, the adsorptionprocesses, adsorption rate and adsorption capacity of Pb2+onto the peanutshells-derived biochars and the Chinese medicine material residue-derived biocharswere also investigated and analysed. Based on the Pb2+sorption by the untreatedbiochars and the deashed biochars, the thesis confirmed the involved mechanisms ofPb2+sorption, and the contribution of each adsorption mechanism to the total sorptionwere estimated. Considering the preparation cost of the biochars, these physical andchemical properties and adsorption ability of Pb2+by the biochars, the right peanutshells-derived biochars or Chinese medicine material residue-derived biochars wereadded to the lead-contaminated soils to analyse the immobilization of Pb2+bybiohcars, the growth of Indian mustard and enrichment of lead in the shoots and rootsof Indian mustard after the remediation with biochars, and determine the influence ofthe roots of Indian mustard on the lead speciation in the contaminated soils at theaddition with the biochars. The results of this study will be of great importance in biomass utilization, screening a new biochar material as adsorbent and establishmentof the situ repair technology system by biochar for lead-contaminated soil, and thusultimately convert to the sustainable development of economy with low energyconsumption, low pollution, low emission of success. The main results of this thesisare as following:(1) To investigate the effect of pyrolysis temperature on basic properties,biochars were produced from peanut shells and Chinese medicine material residue at300-600°C and characterized for their physical and chemical properties. Withincreasing temperatures, C content increased, while H, O and N content decreased;ash content and pH gradually increased, whereas total surface acidic functionalgroups decreased; π conjugate aromatic structures increased; more pore structuredeveloped, and specific surface area increased; inorganic elements (K, Ca, Mg, P andS) accumulated in pyrolysis, but soluble Ca2+, Mg2+and PO43-content reduced.（2）The sorption of Pb2+on the biochars was tested at different pH. Withincreasing solution pH values (pH:2-5), Pb2+sorption onto the biochars increasedgradually. When the pH value was6, the amount of Pb2+sorption on some biochars(e.g., PBC300and MBC300) reduced. The optimum pH for Pb2+sorption onto thebiochars was5.（3）Adsorption of Pb2+on the biochars at different times was also studied. Pb2+sorption on the biochars included fast and slow adsorption stages, and the distributionproportion and adsorption rate of these two stages depended on the types and contentof acidic functional groups, the content of minerals and mineral components. Pb2+sorption onto these biochars could be divided into three stages as follows:(i) Pb2+from solution quickly spread to the surface of biochar across the boundary layer;(ii)Pb2+was adsorbed to the external sites of biochar;(iii) Pb2+diffused through thepores and reacted with the internal sites of biochar. Pb2+sorption on the biochars wascontrolled by film and intraparticle diffusion and chemical reactions.（4）The adsorption of Pb2+on the biochars had great heterogeneity. Pb2+sorption on the low-temperature peanut shells-derived biochars (≤400°C) wasstronger, and the amount of Pb2+sorption onto the high-temperature biochars (≥500°C) from Chinese medicine material residues was greater than the low-temperature biochars. However, in a high concentration of Pb2+in the solution (Ce≥1000mg L-1),Pb2+sorption on MBC400was stronger than MBC500and MBC600.（5）Based on the adsorption of Pb2+on the untreated biochars and thedemineralized biochars, the thesis confirmed the adsorption mechanisms for Pb2+, andestimated the contribution of each adsorption mechanism to the total adsorption ofPb2+on the biochars. The involved adsorption mechanisms for Pb2+included theoxygen-containing functional groups complexation, Pb2+-π interaction with πelectrons of C=C and co-precipitation with minerals. With increasing temperature,co-precipitation was always dominant for all biochar samples, Pb2+-π interaction wasstrengthened, but the complexation controbutinon was reduced. The sorptionattributed to the co-precipitation was depengding on the content of minerals in thebiochars and their speciation, and the mineral crystallization above500°C reducedthe sorption due to the co-precipitation between Pb2+and the minerals.（6）The mixs of PBC400and MBC400at a ratio of1:1were added to thedifferent degree of lead-contaminated soils. After applying BC400, the pH values ofsoils increased significantly, and the higher amount of biochar addition (5%), thegreater rise of the pH values. BC400treatment significantly reduced the NH4NO3extractable Pb2+concentrations in the lead-contaminated soils, and the reductionassociated with the adding amount of BC400. After the remediation by BC400, theplant height and leaf area of Indian mustard was obviously improved. In lowlead-contaminated soils (200mg kg-1) added with5%BC400, biochar had apromoting effect on the biomass of Indian mustard significantly, especially thebiomass of the roots. Whereas in higher lead-contaminated soils (1000mg kg-1) addedwith1%BC400, BC400had no significant effect on the growth of Indian mustard.When BC400was added into the lead-contaminated soils, the Pb concentrations inshoots and roots of Indian mustard both declined dramaticlly. In the lead-contaminated soils repaired with BC400, the pH values of rhizosphere soil werelower than the non-rhizosphere soil, and the NH4NO3extractable Pb2+concentrationsof rhizosphere soil (1000mg kg-1) was significantly greater than the non-rhizospheresoil, implying that the improved roots of Indian mustard possibly activated theimmobilized lead by BC400. Using the biochars as remediation reagents, the activation mechanisms of the sorbed lead on soil particles or biochars in therhizosphere micro-domain due to the improvement of plant roots is still not clear.Therefore, more studies are need to explore these activation mechanisms.