| Biochars are materials produced by pyrolyzing biomass under limited oxygen. Due to its promising feature in recycling biomass waste, attenuating climate change, and improving soil fertility, biochar has quickly become a research hotspot as a novel functional material with multiple applications. Due to the highly porous structure and aromaticity, biochars show great adsorptive capacity for various pollutants, and hence influencing their environmental behavior, fate, and eco-risk.The present thesis systematically studied the impacts and mechanisms behind on the adsorption, hydrolysis, and biodegradation of two typical pesticides, carbaryl and atrazine, by different biochars. To do so, different biochars with varied composition and structure, were derived from pig-manure, maize straw, and rice straw under different pyrolysis temperatures, and the present biochars were further deashed by acid. The structure characteristics, such as the content and constitutes of organic and inorganic moieties, specific surface area and pore structure, and chemical surface functional groups,were measured. Moreover, the adsorption of the two pesticides on the original biochars and deashed biochars were studied, and relationships between adsorption affinity and structural parametersof biochars were analyzed to elucidate their functions in adsorption. The hydrolysis of carbaryl and atrazine were studied in suspensions of different biochars. The mechanism of the catalytic hydrolysis were discussed by analyzing the effects of elevated solution pH, mineral surface, and dissolvedmetal ions step by step. Finally, the degradation of pesticides in soil amended withbiochars were studied under different designs. Exogenous mixed bacteriawere added to strengthen the biodegradation of the pesticides. The influence and pathways behind of biochars on pesticide removal were discussed. Through theresearchabove, some main results obtainedwere as followed:(1) The biochars consisted of organic moiety including aliphatic carbon and aromatic carbon and inorganic moiety containing phosphate, carbonate, and other minerals. In addition, chemical functional groups, such as hydroxyl, carboxyl and carbonyl, also bound on the surface of bio chars. Great difference existed in composition and structure of various biochars derived from different feedstocks and temperatures. Biochars derived from pig-manure contained high content of ash, however, biochars derived from straws had low ash contents, and biochars derived from maize straw contained lower ash than that those derived from rice straw. The ash contents of biochars increased with elevated pyrolyzing temperature. Porosity and specific surface area also increased with increasing temperature.(2) Carbaryl could be adsorbed efficiently by biochars. Adsorption isotherms were fitted by Freundlich equation with distict nonlinearity. In prisent biochars, the distribution coefficients normalized by organic carbon content (Koc) of biochars were102.65-104.48L/kg at low solute concentration. When the ash was removed, adsorption increased generally,the distribution coefficients normalized by organic carbon content (Koc) of biochars were103.64-104.74L/kg at low solute concentration Differences in composition and structure of biochars derived from different feedstocks and temperatures led to the differences in adsorption affinity and mechanism of the various biochars. Inorganic moiety of the biochars derived from pig-manure (PBC) had an apparently negative effect on adsorption. The inorganic moiety of biochars derived from maize straw (MBC) and rice straw (RBC) under low pyrolyzing temperature had negative effect on adsorption, however, that of MBC and RBC obtained under high pyrolyzing temperature had positive effect. Overall, the adsorption of carbaryl on biochar was controlled by hydrophobic effect, pore-filling effect, effects with inorganic ash and specific effect. No significant correlation could be found between adsorption affinity with any of single parameters of polarity, aromaticity, and hydrophobic functional groups.(3) Adsorption isotherms of atrazine on biochars were fitted in Freundlich equation and displayed nonlinear trends. The Koc of biochars werel01.90-103.10L/kg at high solute concentration. After deashing treatment, The Koc of deashed biochars were102.63-103.10L/kg at high solute concentration. Mechanism of adsorption of atrazine on biochars was also a combination of hydrophobic effect, pore-filling effect, effects with inorganic moiety and specific effects. In addition, no significant correlation between polarity of biochars and adsorption affinity could be found for atrazine either.(4) Rapid hydrolysis of carbaryl occurred in alkaline (pH=9.1) background solution with over80%lost in18h, however, it was stable in neutral background solution. Hydrolysis of carbaryl in suspensions of biochars varied a lot with7d hydrolysis ratio ranging from21.9%to90.6%, and biochars influenced hydrolysis through different ways. We found a positive correlation between carbaryl hydrolysis and the pH of the suspensions. Beside pH, surface minerals, metal ions, and adsorption of biochars also influenced hydrolysis ratio, with the former two factors having catalyzing effect and the latter one showing inhibitory effect. The hydrolysis of carbaryl in majority of the tested systems were fitted in pseudo-first order kinetic equation.(5) Hydrolysis of atrazine only occurred in strong alkaline background solution, and only16.1%of atrazine was hydrolyzed at pH of9.1. The hydrolysis of atrazine in suspensions of biochars were generally weaker than those of carbaryl with7d hydrolysis ratio ranging from2.6%to63.4%. The hydrolysis of atrazine was positive correlated with the quantity of inorganic moiety, and surface minerals and carboxylic groups could enhance the OH" catalyzed hydrolysis of atrazine by forming H-bond with atrazine. The hydrolysis of atrazine in the suspensions of biochars were mainly influenced by mineral surface and solution pH, and the contribution of metal ions was very low. The hydrolysis of atrazine in majority of the tested systems were fitted in pseudo-first order kinetic equation.(6) Enhanced hydrolysis of the pesticides were the dominating process for the degradation of pesticides in sterile systems of biochar-soil mixture. The order of degradation ratio of the pesticides in biochar-soil mixing system was PBC>RBC> MBC, which was the same with the hydrolysis of pesticides in biochar suspensions. In unsterile systems, the chemical degradation and biodegradation co-existed, and the impact of biochar addition on the pesticide removal varied a lot. Generally, the addition of biochars enhanced pesticides removal, however, the enhancement extent and mechanism were different. Enhanced hydrolysis of pesticides was the main degradation in the system containing higher dose of biochars pyrolyzed at higher temperature due to the elevated pH of the system, which is favorable for hydrolysis and unfavorable for the growth of microorganisms. However, degradation of pesticides were reduced due to the lowered bioavailability by adsorption in the system containing higher dose of biochars pyrolyzed at lower temperature. However, biodegradation were increased in the system amended with MBC containing more organic carbon, which could supply more nutrition to microorganisms and did not change pH too much. Mixed floras were obtained by screening from activated sludge, and the degradation of carbaryl by the mixed flora was stronger than that of atrazine. The40d degradation ratio of carbaryl was enhanced to79.8%in the soil bioaugmented with the mixed flora; while the degradation ratio ranged in64.9%-82.9%when amended by biochars. Both enhancement and inhibitory occurred by different biochars, and similar results were acquired for atrazine. This is due to that biochars could simultaneously affect the hydrolysis, the growth of microorganisms, and the bioavailability of the pesticides. |
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