| The threat of heavy metal pollution in the environment is becoming increasingly serious, efficient and convenient methods of treating heavy metal pollution are the current research focus. Biochar is a kind of carbon-rich by-product synthesized through pyrolysis of plant and animal based biomass under oxygen depleted atmosphere at low temperature(<700 ℃), it has a large specific surface area, rich in oxygen-containing functional groups and mineral ingredients. Since the materials are available and the preparation is simple, biochar is widely used in heavy metal pollution amendment. This paper selects corn straws, wheat straws and corn cobs prepared biochar to study adsorption properties. Through different biochar dosage, initial metal ion concentration, p H value and temperature conditions, and place Cu2+ ions Cd2+ ions in single and multiple systems to carry on batch experiments. In addition, adding humic acid(HA) in the experiment to simulate the natural environment. Adsorption kinetics analysis and isothermal adsorption properties were carried on for biochar, combined with scanning electron microscopy(SEM-EDS), X-ray diffraction(XRD), infrared spectroscopy(FT-IR), surface area(BET) and other analytical characterization to study adsorption mechanisms of heavy metals as well as the impact of humic acid. On the other hand, the soil will be applied by biochar in the soil column experiment, explore the impact of biochar on soil fixation of heavy metals. The main conclusions are as follows:(1) Physical and chemical properties of biochar. Corn straw biochar and wheat straw biochar have much larger specific surface area than that of corn cob biochar. Corn straw biochar and Corn cob biochar contain more dissolved organic matter content than wheat straw biochar. The p H values of three kinds of biochar are above 9.0 which can be used to improve acidic soils. In addition, heavy metal contents in three kinds of biochar are all below the pollution limit, which will not cause secondary pollution to the environment.(2) Adsorption properties of biochar. Corn straw biochar and wheat straw biochar are much better than corn cob biochar in heavy metal adsorption capacity, the removal rate of the former two can reach more than 95 percent, while the latter only reach about 40 percent. This may be related to the specific surface area and related ions content in materials.(3) Heavy metal competitive adsorption. While Cu2+ions and Cd2+ions are coexist in the system, Cu2+ions will compete with Cd2+ions for the same adsorption sites, resulting in less adsorption amount of Cd2+ions by biochar. This may be related to the effective radius of the two hydrated ions.(4) Adsorption mechanisms of biochar and the effect of humic acid. Adsorption mechanisms of biochar include physical adsorption and chemistry adsorption, they have relationships with biochar’s specific surface area, functional groups content and ionic components, etc. Humic acid acts a carrier for the transfer of metal ions in the adsorption process, result in the ions rapid adsorption.(5) Biochar helped fixed heavy metals in soil. Under simulated acid rain conditions, applying biochar would enhance the buffer capacity of the soil, adjusting p H value more stable, Cu2+ions and Cd2+ions in leachate is reduced 6.49% ~ 28.95%, 12.74% ~ 37.99%, respectively. Thus can mitigating the harm caused by heavy metal migration. |
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