Experimental Study And Mechanism Of Magnetic Biochar Purification Of High Arsenic Mine Acidic Wastewater

Arsenic（As）is highly toxic,carcinogenic and widespread.Mining activities are one of the important causes of arsenic pollution.Some coal mining areas in Xingren County,Guizhou province are typical high-arsenic coal mining areas in China.In the process of high-arsenic coal mining,acidic water will dissolve arsenic from coal into water,causing serious harm to the environment.At present,there are chemical precipitation methods,biological methods and adsorption methods to treat heavy metals in water.Compared with other methods,the adsorption method has the characteristics of simple operation,high efficiency and low cost.Biochar is an effective adsorbent for removing heavy metals from water due to its large number of functional groups and abundant pore structure.However,the particles of biochar are too fine to be separated from water,so this study adopts a new method to prepare magnetic biochar which is easy to be separated,and studies the adsorption effect and mechanism of magnetic biochar on pentavalent As（V）in acidic wastewater.The magnetic biochar was prepared by"co-precipitation+microwave assisted"method using lotus seedpod,solanum solanum and straw as raw materials.Compared with the commonly used"co-precipitation"method,the preparation method was introduced into the microwave environment,and the operation was faster and the material was saved.From the magnetic biochar prepared from three different raw materials,the lotus seed magnetic biochar（L-BC@Fe₃O₄）with the best adsorption effect on As（V）in acidic wastewater was selected,and combined with single factor adsorption experiment,the initial mass concentration of arsenic,the dosage of L-BC@Fe₃O₄,the initial p H of arsenic-containing acidic wastewater and the reaction temperature on the adsorption of As（V）by L-BC@Fe₃O₄were studied.Kinetics and isotherm model fitting of the adsorption process of As（V）by L-BC@Fe₃O₄.The adsorption mechanism was investigated by specific surface area and porosity（BET）,magnetic characterization（VSM）,scanning electron microscopy（SEM）,X-ray diffraction（XRD）,fourier transform infrared spectroscopy（FTIR）,X-ray photoelectron spectroscopy（XPS）and other characterization analysis.The main results are as follows:（1）The single factor adsorption experiment shows that when the initial mass concentration of arsenic is 1 mg·L^-1,the dosage of L-BC@Fe₃O₄is 2.0 g·L^-1,the initial p H value of arsenic-containing acidic wastewater is 2,and the ambient temperature of the experimental reaction is 25°C,L-BC@Fe₃O₄has the highest removal rate of As（V）,which can reach more than 98%.The p H of water increased after L-BC@Fe₃O₄adsorbed As（V）,indicating that magnetic biochar can not only remove As（V）in acidic water,but also improve the p H value of water,effectively solving the two problems of arsenic pollution and low p H of water.（2）The kinetic and adsorption isotherms were in accordance with the quasi-second-order kinetic equation and Langmuir model respectively,which indicated that the adsorption was mainly monolayer adsorption controlled by chemical action and limited by the adsorption sites on the surface.（3）Characterization analysis showed that the specific surface area of biochar increased after magnetization,and Fe₃O₄particles were uniformly attached to the surface of biochar.The magnetic strength of L-BC@Fe₃O₄was up to 3.30 emu·g^-1,which was conducive to the separation of L-BC@Fe₃O₄from water under the mode of external magnetic field.The surface of L-BC@Fe₃O₄is rich in hydroxyl,unsaturated carbon bonds,and newly formed Fe-O functional groups,which are favorable for the adsorption of L-BC@Fe₃O₄to As（V）.Combined with FTIR and XPS,the changes of functional groups on L-BC@Fe₃O₄and the morphology distribution of Fe and As on biochar surface before and after adsorption were compared,revealing that the adsorption process of L-BC@Fe₃O₄on As（V）is the joint action of multiple mechanisms.The adsorption mechanism mainly includes electrostatic attraction,complexation reaction between surface functional groups,and Fe-As co-precipitation.