Study On Adsorption Of Mn²⁺in Polluted Groundwater From Electrolytic Manganese Slag Field By Alkali Modified Pomelo Peel Biochar

Electrolytic manganese slag is a solid waste produced during the preparation of electrolytic manganese by hydrometallurgy.It has a large yield,a relatively high proportion of soluable divalent manganese ions(Mn2+)and a low resource utilization rate.At present,it is mainly disposed by landfill.In recent years,there have been frequent leakage accidents in the electrolytic manganese slag field and Mn2+ has entered the groundwater with the seepage,causing adverse effects on the environment and the treatment of Mn2+ pollution in the water body is imminent.Pomelo is a unique fruit in China,with abundant production.Its peel is often discarded or burnt at will,which pollutes the environment and wastes resources.Based on this,this study used pomelo peel as the raw material to prepare alkali modified biochar with slow pyrolysis method,and explored the adsorption effect towards Mn2+ in aqueous solution through batch adsorption experiments,characterization methods and column experiments.The main results were as follows.1)The yield of pomelo peel biochar was 25.5%.The surface was relatively smooth.There was no obvious pore structure.The biochar pH was 10.2.The surface functional groups were aromatic hydrocarbons C=C,-CH/-CH3,C-O-C,C-O and-CH.The yield of alkali modified biochar was 32.8%.Tunnels and holes appeard on the surface.The specific surface area and total pore volume increased by 9.3 and 71.6 times,respectively.The pore was mainly mesopores.The biochar pH was 11.4.The main surface functional groups were-COO-and CO32-.The yield,microscopic morphology and physichemical properties of pomelo peel biochar changed significantly before and after alkali modification.2)The adsorption effect of alkali modified biochar on Mn2+ in water was significantly better than that of pomelo peel biochar.When the concentration of Mn2+was 50 mg·L-1,the removal rate of pomelo peel biochar and alkali modified one was 78.6%and 99.6%,respectively.As the initial Mn2+ concentration increased,the removal rate of biochar showed a downward trend.With the extension of the reaction time,the removal rate increased rapidly in a short time,and then tended to the equilibrium.When pH>2.0,the removal rate of alkali modified biochar reached 99.6%.Alkali modified biochar had a certain ability to remove co-existing ammonia nitrogen ions,the removal rate was 21%.The adsorption kinetics of pomelo peel biochar and alkali modified one followed Elovich and pseudo-second-order models,respectively,indicating that the adsorption rate of both depended on the chemical reaction;the isotherms followed the Freundlich model,indicating that the adsorption was uneven and there was no saturated adsorption capacity.When the Mn2+ concentration was 3000mg·L-1,the adsorption capacity of alkali modified biochar reached 163.194mg·g-1.3)Alkali modified biochar adsorbed Mn2+ with functional groups-COO-and CO32-through ion exchange and surface precipitation.Mn2+ finally existed in the form of-COOMn+and MnCO3.After adsorption,the specific surface area,total pore volume and average pore size of the alkali mofied biochar increased.4)Reducing the flow rate and the initial concentration of influent,and increasing the height of the fix bed could extend the penetration time.Within a certain range of conditions,the Thomas,Yoon-Nelson and BDST models fitted the penetration curve of alkali modified biochar(R2=0.811～0.927).The Thomas model showd that the column adsorption process could ignore axial diffusion,radial dispersion,intraparticle diffusion and mass transfer resistance.The Yoon-Nelson model could better fit the penetration time.Within a certain range of conditions,the BDST model could predict the saturation time at different flow rates and initial influent concentrations.5)The soil had a blocking effect on the leaching of Mn2+,and the soil sample in Chongqing was superior to that in Hainan;the soil column loaded with alkali modified biochar had a better blocking effect on the leaching of Mn2+ than the column without biochar;the blocking effect with Hainan soil was more affected by alkali modified biochar.