Synthesis Of Soda Residue Modified Biochar And Its Study In Removing Mn²⁺ From Acid Mine Wastewater

Acid mine drainage（AMD）contains a large amount of manganese,which can cause immeasurable harm to the quality of life and safety of residents around polluted water bodies.In this paper,agricultural waste sweet sorghum straw was used as the raw material for biochar,and industrial waste soda residue was used as the modifier for biochar.The two solid wastes were combined in different mass ratios by blending method,then pyrolysed in a tubular furnace to obtain soda residue modified biochar（SRB）.Through a series of batch adsorption experiments and characterization analysis,the main adsorption mechanism of modified biochar towards Mn²⁺from aqueous solution was determined.Select the optimal modified biochar,where the material with a mass ratio of soda residue to sweet sorghum straw（SRB 1:2）exhibits the best manganese adsorption ability.The results are as follows:The number of oxygen-containing functional groups on the surface of modified biochar significantly increased compared to unmodified biochar,especially with a significant O-H peak at 3444 cm^-1.After adsorption,a large amount of manganese hydroxide precipitates are distributed on the surface of SRB 1:2.It can be inferred that the main mechanism of removing Mn²⁺by soda residue modified biochar is the combination of Ca O and Ca S on the surface of the material with H₂O in the solution.Free Mn²⁺combines with OH^-on the biochar to generate manganese precipitation,which is agglomerated on the surface of the biochar under electrostatic attraction,so as to achieve the purpose of adsorbing Mn²⁺in the form of chemical precipitation.Soda residue modified biochar exhibits excellent removal ability for Mn²⁺in weakly acidic solutions,especially with a removal rate of 91.72%for SRB 1:2.The adsorption isotherm data of soda residue modified biochar are more consistent with the Langmuir model,indicating that its adsorption behavior is a single layer adsorption occurring on a uniform surface.The maximum adsorption capacity of SRB 1:2 reached 125.97 mg/g,about 7 times that of unmodified biochar.According to the experiment on the influence of adsorption time,it can be seen that the adsorption capacity of modified biochar towards Mn²⁺increased sharply within the initial 120 minutes,and at 120 to 540 minutes,the removal rate slowed down.At this point,the entire adsorption approached saturation,and finally reached an equilibrium state.The adsorption kinetics of soda residue modified biochar is more consistent with a pseudo second order model,indicating that the adsorption mechanism of modified biochar towards Mn²⁺is mainly chemical adsorption.From the intra particle diffusion model,it can be seen that the adsorption process of Mn²⁺on soda residue modified biochar can be divided into two stages:external diffusion and internal diffusion.It is calculated by thermodynamic formula thatΔH⁰ is the positive value andΔG⁰is the negative value,indicating that the adsorption process is a spontaneous endothermic reaction.Compared with a single metal solution system,the adsorption capacity of SRB 1:2 for Mn²⁺in Fe-Mn coexisting solution systems has decreased,but still maintains a removal rate of more than half,indicating that SRB 1:2 has a higher adsorption priority for Fe than Mn.With the increase of adsorption time,the removal rate of Mn²⁺from AMD by SRB 1:2 showed a trend of rapid growth in the early stage and gradually kept stable after 180 minutes,finally reaching an equilibrium state.As the influent Mn²⁺concentration increases,the removal rate of Mn²⁺from AMD by SRB 1:2 gradually decreases,and the adsorption capacity gradually increases.When the influent Mn²⁺concentration is 50 mg/L,the removal rate of Mn²⁺by SRB1:2 is the highest,reaching 76.33%.In low p H AMD,the removal rate of Mn²⁺by SRB 1:2 is only 4.3%.In weakly acidic AMD,the removal rate of Mn²⁺by SRB 1:2 reached 58.79%.In summary,it can be seen that SRB 1:2 exhibits good removal ability for low concentration Mn²⁺in high-iron and weak-acid AMD.