Study On The Treatment Of Low Concentration Of As(Ⅲ) Containing Wastewater By Iron-Manganese Modified Biochar

The main sources of arsenic pollution in water are mining activities and the use of chemical fertilizers and pesticides,long-term exposure to arsenic-contaminated water sources and food can cause serious health damage.It is estimated that the health of about 200 million people worldwide is at risk from the presence of arsenic in water.Arsenic in water mostly exists in the form of 3-valent and 5-valent,and the toxicity of3-valent arsenic is 25 to 60 times that of 5-valent arsenic.Because of its high toxicity,the World Health Organization（WHO）sets the maximum concentration of arsenic in drinking water at 0.01 mg/L.Among the existing arsenic removal methods,adsorption has attracted more and more attention due to its advantages of low cost and easy operation,especially the adsorption technology based on biochar materials has become a research hotspot in recent years.In this study,a biochar sorbent with low cost and high adsorption capacity was prepared by using agricultural waste corn stover as raw material,using sintering carbonization and iron-manganese modification,which was used to adsorb and remove As（III）in water.Firstly,the optimum preparation conditions of Fe-Mn modified biochar（FMBC）were determined by single factor experiment.The effects of adsorption time,initial As（Ⅲ）concentration,adsorption temperature,initial p H,ionic strength and co-existing substances on the adsorption properties of FMBC were investigated under the optimum preparation conditions.In addition,theoretical discussion was made from the perspectives of reaction kinetics model,intra-particle diffusion model,adsorption isotherm model and adsorption thermodynamics model.Based on this,the modified biochar was characterized and its adsorption mechanism was discussed.Meanwhile,the adsorption performance of FMBC on actual water containing As（Ⅲ）under different influencing factors was investigated through dynamic adsorption experiments.The Thomas model,Yoon-Nelson model and Adams-Bohart model were fitted.Finally,the recycling performance and economy of FMBC adsorbent were analyzed,which provided some basis and reference for the treatment of low concentration actual wastewater containing As（Ⅲ）.The main conclusions of the study are as follows:（1）Through single-factor test,it was determined that FMBC had the best adsorption effect under the condition that the sintering temperature was 600°C,the sintering time was 2 h,and the mass ratio of biochar:Fe:Mn was 1.5:0.75:0.125.At an initial As（III）concentration of 10 mg/L,the removal rate can reach 81.5%.（2）The adsorption process of As（Ⅲ）by FMBC conforms to the pseudo-second-order kinetic model,which is chemical adsorption,which is divided into three stages:outer surface diffusion,particle internal diffusion and adsorption equilibrium.The isothermal adsorption process conforms to the Freundlich model,and the maximum adsorption capacity is 20.63 mg/g at 25°C,respectively.Adsorption thermodynamic studies have shown that the process of adsorbing arsenic by FMBC is entropy-increasing,spontaneous and endothermic.（3）FMBC has good adaptability to water p H value,and can maintain good adsorption performance when p H value is 4.0～10.0.The ionic strength of the solution has little effect on the adsorption effect of the adsorbent,and it can be judged that the mechanism of inner sphere complexation is suitable.In the coexisting substances,PO₄^3-and Si O₃^2-have a great influence on the adsorption effect,and other anions and cations and humic acid have little effect on the adsorption effect.At the same time,the removal rate of actual As（III）containing water in FMBC treatment is more than 89%,which has good application value.（4）Fe and Mn elements were successfully loaded onto the surface of biochar through analysis and characterization.Fe-Mn modification increases the specific surface and average pore volume pore size of biochar.There are more particles on the surface and pore,and the surface is rough and uneven.Fe-O and Mn-O functional groups exist on FMBC surface,which provide oxidation and adsorption sites for As（Ⅲ）.Based on the analysis of valence states and functional groups before and after adsorption by X-ray photoelectron spectroscopy,the mechanism of As（Ⅲ）adsorption by FMBC was determined to be oxidation and chemisorption of the inner layer complexation.（5）The dynamic adsorption column test shows that the influent flow rate has a strong linear relationship with the effective bed volume,and the adsorption column can be controlled by controlling the flow rate.The influent concentration of As（Ⅲ）is inversely proportional to the effective bed volume of the adsorption column.The maximum adsorption capacity（1.41 mg/g）was obtained when the concentration of As（Ⅲ）was 200μg/L and the flow rate was 9 m L/min.The fitting results of the three models showed that low flow rate was favorable for arsenic adsorption by FMBC.By comparing the maximum adsorption capacity of RO（Reverse Osmosis）water with that of the actual water,it shows that other ions and coexisting substances in the actual water have a certain influence on the adsorption effect of the adsorption column,but compared with RO water,the adsorption effect can still reach about 70%.（6）FMBC has good recycling performance.0.5 mol/L Na OH solution was used to regenerate FMBC that reached adsorption equilibrium,and the removal rate remained about 50%after four times of recycling.Through economic analysis,the total cost per kilogram of iron-manganese modified biochar can be controlled at about 11.9 yuan,which has a certain cost competitiveness.In this study,the idea of"treating waste with waste"was adopted,and iron-manganese modified straw biochar was used to treat wastewater containing As（Ⅲ）,and good results were obtained.The experimental results can provide certain theoretical and data support for the practical application of treating similar wastewater.