| At present, the mercury pollution has become more and more serious in our country. A number of technologies have been developed over the years to remove mercury from water and soil. However, these traditional treatment methods are expensive, and also have other disadvantages, such as low removal performance, destruction of soil properties and so on. Therefore, efficient technologies for the mercury treatment from water and soil should be studied. Recently, biochars, as a kind of new environment materials have been focused on. Literatures show their good adsorptive properties for heavy metals in water and soil. Biochars also have great potential on improving the soil environment quality and reducing the greenhouse gases emission. In this study, four types of waste/raw materials including sawdust, rice straw, rice husk and water caltrop were used to make biochars at 600℃ by oxygen-limited pyrolysis. By using the scanning electron microscopy and fourier transform infrared (FTIR), morphological structure and chemical characteristics of four biochars were investigated. The difference of adsorption characteristics of four biochars were investigated by adsorption experiment. The chemical fractions of mercury in soil with and without biochars were compared by a series of experiments under the simulant conditions. Additionally, the adsorption kinetics behavior and removal mechanisms of mercury were discussed. The main results are as follows:(1) Four biochars show a similar tendency in production rate, ash content and pH value, as water caltrop char> rice husk char> rice straw char> sawdust char. While for the carbon content, the trend was inversed. The biochar derived from sawdust had the highest carbon content, while its mineral nutrient content was the lowest compared with other materials. The SEM analysis results showed that the morphological structure of biochas displayed distinct properties. However, abundant pore structure was observed, which had an important effect on improving the water-holding capacity of the soil. The characteristic peaks of-COO-,-C=O and -OH were shown in biochars from different materials. There was similar difference in the adsorption strength of these peaks. The amount of acidic functional groups of biochars was sawdust char> rice straw char> rice husk char> water caltrop char.(2) The pH value plays a significant role on mercury removal by biochars. The optimum pH value for Hg2+ adsorption by biochars was approximately 5. The adsorption of Hg4+ onto the biochar from sawdust was a slower process compared with other materials. Langmuir isotherm model fitted the isotherm data of Hg2+ adsorption by biochars produced from sawdust, straw and water caltrop, suggesting a functional group-limited sorption process. The sawdust char reached 60.2 mg g-1 maximum adsorption, while other biochars showed lower Qm values under the same experimental conditions. Kinetics of Hg2+ adsorption by four biochars could be well described by the pseudosecond-order model. The intraparticle diffusion model illustrated that the adsorption of Hg2+ by four biochars was controlled by internal diffusion and membrane diffusion. The pristine and mercury loaded biochars were characterized by scanning electron microscopy and fourier transform infrared. Results indicated that the ion exchange and complexation reaction were major process for the Hg2+ removal. Meanwhile, reduction and cation-π interaction may also responsible for Hg2+ sorption.(3) The pH value, cation exchange capacity (CEC) and total organic carbon (TOC) contents in soil increased markedly after the addition of biochars, indicating that biochars can improve soil acidity and increase the soil fertility. When 50 mg kg-1 of Hg2+ was added, biochars did not show obvious stabilization effect. While 500 mg kg-1 of Hg2+ was added, the remediation effect of biochars on Hg was obvious. The exchangeable and carbonate Hg were transformed into residue Hg. The stabilization effect of water caltrop char on Hg in soil is superior to other three materials. Not obvious differences were found among other three biochars.(4) In solid/liquid system, the classic kinetics adsorption models could not accurately describe adsorptive effect and the parameter values were unstable, which is unhelpful for practical use. In the Hg2+/biochar adsorption system, SCA-second-order kinetic equation can be used for describing adsorption behavior. The related rate constants were independent of adsorptive effect. Adsorption capacity on the different time points was calculated by SCA-second-order prediction equation, which agreed with our experimental data, indicating a better practical use. |
PDF Full Text Request