| Biochar is the carbon-rich product obtained when biomass, such as wood, straw, or animal manures, is heated in a closed container with little or no available air. Nowadays, biochar is paid more attention for its environmental values. Biochar can not only improve the carbon content in soil but also alleviate the greenhouse effect. In addition, it is not easy for soil microorganism to disassimilate the biochar so that it can keep stable for hundred or thousand years. However, thermal properties and the effect of feedstocks in biochar preparation were not clear. And it was not in-dept investigated that the relationships and corresponding mechanisms between heavy metal adsorption and compositions (or structures) of biochars. Thus, rice straw, rice bran, dairy and chicken manures were chosen as the biomass and a series of different pyrolytic:temperature biochars were produced in muffle in this study. Some chemical methods and spectrum technologies were used to characterize their composition and structure properties. The thermodynamic parameters were calculated by some thermal equations. Furthermore, adsorption capacities and mechanisms of cadmium on various biochars derived from rice bran and chicken manure were investigated. At the same time, fast and slow adsorption of cadmium was found on different temperature rice bran and chicken manure biochars. The relative contribution of different composition (organic matter, water soluble minerals, HC1soluble minerals and Si) to cadmium adsorption onto biochars was also studied. And the corresponding mechanisms were explained by the infrared spectroscopic analysis (FT-IR) and scanning electron microscopy with an energy spectrum analysis (SEM-EDX). The knowledge on the thermal-chemical transformation of solid raw matter and then thermodynamic parameters in the conversion of biomass into biochar are important for proper design of a pyrolysis reactor in large-scale pyrolysis process. And the optimized pyrolytic conditions can be determined with the investigation of cadmium onto different biochars. The main original conclusions of this work are drawn as follows.(1) Thermodynamic parameters of the conversion of two companion materials (rice straw vs dairy manure and rice bran vs chicken manure) to biochars were systematicly investigated. And the role of minerals in pyrolysis was also studied. Water evaporation, transition, organic matter degradation, and carbonization processes were included for the biomass pyrolysis. The peaks of water, hemicelluloses, cellulose, and lignin were successively observed in the DTG curves. The appearant activation energies (Ea) were calculated by the Flynn-Wall-Ozawa equation. The Ea values increased steeply from about 120to180kJ/mol at the mass conversion (a)(0.2-0.4), decomposing of hemicelluloses and some ammonium salts. The Ea kept a relatively steedy at0.4<a<0.65, cellulose degrading. At a>0.65, Ea showed a quick increase, especially for two manures. The higher contents of minerals in manures resulted in the larger Ea as a thermal barrier. And the lignin or proteins were degraded in this process. In addition, the entropy changes (△S°) were also calculated and showed a decrease in the biomass pyrolysis, indicating the evolution of biomass to biochar from disorder to order. The FTIR results showed that aliphatic fraction gradually transformed to the aromatic fraction in pyrolysis.(2) Fast and slow sorption of cadmium was found onto different temperature biochars derived from rice bran and chicken manure, which was dominated by the compositions and structures of biochars. A quite slow adsorption was observed on the300℃rice bran and chicken manure biochars which showed the heterogeneous composition. The equilibrium time was at least10days. And the solution pH gradually increased with the reacting time increasing. However, a very fast adsorption of cadmium was observed on the700℃rice bran and chicken manure biochars with reaching the appearant equilibrium within48hours. And the solution pH also increased and kept steedy change within a relatively short time. In addition, the fast adsorption of cadmium was also observed onto the100℃chicken manure and two ash samples. For100℃biochar, cadmium can react with its surface carboxyl (—COOH) and hydroxy (—OH) functional groups by ion exchange. And this process was very fast with the release of proton. For300℃biochar, cadmium can react with its deprotonized functional groups (-COO-and-O-) by complexation, with the slow release of OH-. Higher temperature biochar (700℃) showed a quite fast adsorption of cadmium and the precipitation occurred between cadmium and the inorganic minerals.(3) The main compositon and sites for cadmium adsorption onto biochars were firstly distinguished. Adsorption mechanism and relative contributions were preliminarily elucidated. The organic functional groups, water soluble minerals, HC1soluble minerals, and Si containing compounds were taken into account in cadmium adsorption. About two times decrease of adcadmium sorption capacities after water washing due to the dissolving out of water soluble minerals. Adsorption capacities significantly decreased after aid treatment. And the relative contribution of HC1soluble minerals (Ca, Mg, A1, etc) was above90%while the role of SiO2particles could be ignored in cadmium adsorption. |
PDF Full Text Request