Study On Magnetized Modified Pig Manure Hydrothermal Charcoal And Its Adsorption Performance For Cr(Ⅵ)in Water

In recent years,with the increasing living standards of the people,the demand for meat and egg products represented by pork also increases.With the continuous expansion of livestock and poultry breeding scale,a large number of fecal pollution problems also occur.The harmless and resource treatment of pig manure has gradually become the focus of pollution treatment of livestock and poultry.In addition,the development of China’s industry has also led to the generation of a large number of wastewater containing heavy metal ions,these wastewater is not treated to the standard is discharged into the water and soil,causing great harm to human and the environment.Using waste biomass hydrothermal carbon as adsorbent for the treatment of heavy metal wastewater has the advantages of green efficiency,low cost and simple operation,which has good environmental and economic benefits.Based on the above background,this paper took pig manure as raw material,carried out hydrothermal carbonization treatment at different temperatures and magnetized modification by chemical co-precipitation method,explored the adsorption and desorption of hexavalent chromium in water by hydrothermal carbon under different conditions,and analyzed the adsorption mechanism of removing hexavalent chromium in water by hydrothermal carbon and its modified hydrothermal carbon.To provide data support and theoretical basis for the study of the feasibility of removing hexavalent chromium from water and its reuse,and to find a new way for the resource utilization of pig manure.The main research contents are as follows:（1）Hydrothermal carbonization of pig manure raw materials at different temperatures is used to prepare hydrothermal carbon.By analyzing the element analysis,specific surface area,and iodine adsorption value of hydrothermal carbon at different temperatures,it is found that when the hydrothermal carbonization temperature is220℃,the prepared hydrothermal carbon has the largest specific surface area and iodine adsorption value,it is named HC.Using Fe Cl₃solution to magnetize HC at different concentrations and solid-liquid ratios,it is concluded that when the Fe Cl₃solution concentration is 0.5mol/L and the solid-liquid ratio is 1:10,the adsorption capacity for Cr（VI）is the strongest.The magnetic hydrothermal carbon prepared under this condition is named MHC.（2）Through BET,SEM,FTIR,iodine adsorption value,Zeta potential and VSM analysis of HC and MHC,the results show that MHC has larger specific surface area and more abundant pore structure than HC,and the average pore diameter is smaller.Because the surface of MHC loaded with magnetic substances is rougher,the iodine adsorption value is also increased,so the magnetized hydrothermal carbon has stronger physical adsorption capacity.The phenolic hydroxyl group,carboxyl group and other oxygen-containing functional groups of MHC increased,and the polarity was enhanced,and Fe-O characteristic absorption peak appeared in the FTIR spectrum,indicating that the magnetic modification process changed the functional group properties on the surface of hydrothermal carbon.The results of Zeta potential showed that the MHC potential tended to move to a positive direction on the whole,and the p H_ZPC-HCwas 3.06,and the p H_ZPC-MHCwas 4.11.After the magnetization modification,the isoelectric point of the hydrothermal carbon became larger,and the surface charge properties changed,which was more beneficial to the adsorption of Cr（VI）in water on the surface of the hydrothermal carbon.VSM analysis results show that the saturation magnetization is20.15 emu/g,which can ensure the use of permanent magnets to quickly separate MHC from the aqueous solution,indicating that the magnetic material is successfully loaded on the hydrothermal carbon.The risk assessment of heavy metals showed that,on the whole,HC and MHC had no increase in environmental toxicity and were in the low risk level with low ecological toxicity.（3）By studying the influence of different conditions on the adsorption of HC and MHC on Cr（Ⅵ）,it can be concluded that the removal rate of Cr（Ⅵ）per unit mass of HC and MHC is proportional to the dosage,but the amount of adsorption decreases with the increase of the dosage.With the increase of p H,the adsorption of HC and MHC on Cr（Ⅵ）gradually decreases.When p H=2,the reaction time is 24h and the temperature is30℃,the adsorption capacity of HC and MHC is the strongest,and the adsorption capacity is 6.58mg/g and 12.34mg/g,the removal rate is 52.63%and 98.75%.The correlation coefficient R₂of the quasi-second-order kinetic equations of HC and MHC is the largest,HC and MHC have higher correlation when fitting Langmuir isotherm adsorption model under the conditions of 20℃,30℃and 40℃.By comparing the Langmuir equation calculated K_Land R_Lvalue,and the value of n K_fin the Freundlich equation fitting parameters,it can be seen that MHC has a better adsorption effect on Cr（Ⅵ）than HC.The calculation of adsorption standard thermodynamic function shows that the adsorption of HC and MHC to Cr（Ⅵ）is a spontaneous and endothermic entropy increase process.The order of coexisting ions’influence on adsorption is:CO₃^2->SO₄^2->H₂PO₄^->Cl^-.It shows that the adsorption process is affected by electrostatic gravity.Although coexisting ions will reduce the adsorption capacity,overall HC and MHC can still maintain good adsorption performance,and the effect is limited when the concentration is not high,and the adsorption effect and stability of MHC are better than HC.Through the analysis of hydrothermal carbon FTIR before and after adsorption,it can be seen that during the adsorption process of HC and MHC on Cr（Ⅵ）,they are subjected to the combined effects of physical adsorption and chemical adsorption,including electrostatic attraction,ion exchange,and surface complexation,coordination and redox,etc.（4）Through the desorption treatment of HC and MHC,it is found that the desorption effect of different desorption agents from strong to weak is Na OH>Na₂CO₃>Na HCO₃>Na NO₃.The amount of desorption of HC and MHC by Na OH solution is the largest,reached 1.44mg/g and 5.88 mg/g.As the concentration of Na OH in the solution increases and the desorption time increases,the desorption effect of HC and MHC gradually increases.The optimal desorption concentration is 0.5mol/L Na OH solution,and the desorption time is 12h,the desorption amount of MHC is always greater than that of HC.As the number of reuses increases,the adsorption capacity of HC and MHC will decrease.After 3 times of repeated use,the removal rate of MHC was 81.79%,still retaining a large adsorption capacity.After 5 times of repeated use,the removal rates of HC and MHC were 17.08%and 52.38%,respectively.The adsorption capacity is much greater than before magnetization,indicating that magnetization increases the reusability of hydrothermal carbon.From the experimental results of this study,it can be seen that the magnetized modified hydrothermal carbon has better adsorption,stability and reusability for Cr（Ⅵ）,and because the magnetic hydrothermal carbon has magnetism,it can be more convenient Recycle and reuse from water.The hydrothermal charcoal prepared by hydrothermal carbonization of pig manure can adsorb Cr（VI）,which can not only realize the resource utilization of pig manure,but also reduce the problem of heavy metal pollution in water.