Study On Adsorption Mechanism Of KOH/KMnO₄ Modified Biochar For Hg²⁺ In Aqueous Solution

The discharge of mercury-containing wastewater has harmed the water environment and seriously threatened the safety and health of human beings.Finding economic,effective and stable sewage treatment methods is the top priority of water pollution control.However,the agricultural waste had a huge output but is also rich in nutrients,so a clean treatment method was urgently needed to improve the resource utilization rate of agricultural waste.Therefore,in this study,corn cob and litchi shell,which were inexpensive,widely sourced,and easy to collect,were selected to prepare modified biochar with stable physicochemical characteristic and excellent adsorption capacity.The specific surface area and surface functional group types of biochar were improved by chemical modification with Potassium hydroxide and potassium permanganate,so as to improve the adsorption performance of modified biochar for mercury ion.The main results are as follows:（1）Using corn cob and litchi shell as raw materials,to explore the impacts of activation temperature,activation time,alkali to carbon ratio on modified biochar adsorption capacity and specific surface area through single factor experiment and orthogonal experiment,and the best preparation process was selected.The optimal preparation conditions of KOH modified corn cob biochar（CCAC）were as follows:activation time was 2 h,activation temperature was 800℃,alkali to carbon ratio was2.5:1.The optimal preparation conditions of KOH modified litchi shell biochar（LCAC）were as follows:activation time was 1.5h,activation temperature was 800℃,alkali to carbon ratio was 1.5:1.The adsorption capacities of mercury ion were 184.76 mg/g and 200.38 mg/g,and the specific surface areas were 2797.75 m²/g and 2152.96 m²/g,respectively.In order to further enhance the adsorption capacity,CCAC and LCAC were modified with KMn O₄,and the adsorption capacity of KOH+KMn O₄modified corn cob biochar（CKAC）and litchi shell biochar（LKAC）was increased to 222.22 mg/g and 239.02 mg/g,the dosage of KMn O₄was 0.14 mol/L and 0.06 mol/L,respectively.（2）The effects of four preferred modified biochar on the adsorption capacity of mercury ion were investigated under different initial concentration of mercury ion,adsorbent dosage,p H and other factors.The optimal initial concentration was 100 mg/L,the dosage of adsorbent was 0.40 g/L.The optimal adsorption p H of CCAC,CKAC,LCAC and LKAC were 4,3,4 and 3,respectively.And the optimal adsorption time was 120 min,60 min,90 min and 90 min,respectively.The experimental data showed that the adsorption rate was the fastest in the initial stage of adsorption,which was due to the sufficient number of adsorption sites on the modified biochar in the initial stage.With the increase of time,the rate gradually slowed down,that was because the number of adsorption sites reached saturation and the adsorption amount of mercury ion reached dynamic balance.Through the experiment of different ion interference,it was found that there was a competitive adsorption relationship between different metal ions,and the selective adsorption of mercury ion on modified biochar was better,and the competitive order was Hg²⁺>Pb²⁺>Cu²⁺>Ni²⁺>Cd²⁺>Zn²⁺.（3）The adsorption kinetics and isotherm of mercury ion were analyzed by using the adsorption test results of four modified biochar.It was found that the best fitting results were obtained with the pseudo second order kinetic model and Langmuir adsorption isotherm model,which meant that the process was chemisorption on the monolayer.Langmuir isotherm fitting results showed that the maximum adsorption capacity of CCAC,CKAC,LCAC and LKAC were 175.88 mg/g,227.32 mg/g,198.31 mg/g and 239.94 mg/g,respectively,indicating that the four kinds of modified biochar had good adsorption performance for mercury ion.In addition,SEM-EDS,BET,FTIR,XPS and other characterization methods were used to analyze the biochar before and after modification and adsorption,and the adsorption mechanism was explored.The results showed that the specific surface area,micropore ratio and the number and species of oxygen-containing functional groups such as C-O,C=O and O-H increased,all of which promoted the adsorption of mercury ion.