The Optimization Of Modification Methods For Biochar And The Research On The Adsorption Performance Of Dissolved Cr(?) In Wastewater

Increasingly serious heavy metal pollution and biomass wastes are two major environmental problems fascinating great attention all around the world.Biochar derived from biomass wastes is one kind of environmental-friendly materials.As is known,modification can effectively enhance adsorption capacity of the biochar,however,the study on the processes of modification is rarely reported.Therefore,in this study,two different modification processes?pre-pyrolysis modification and post-pyrolysis modification?were selected as critical factor for the preparation of modified biochar.In the meantime,a three-factored,four-leveled orthogonal experiment was carried out to prepare the modified biochar.Three sources of biomass?tree branch,pomelo peel and cornstalk?,modification reagents?citric acid,sodium hydroxide and ferric chloride?,concentration of reagents?0.1 mol/L,0.3 mol/L and 0.5 mol/L?and the ratio of solid to liquid?1:5,1:10 and 1:20?were chosen as the experimental factors.Furthermore,the optimal modification parameters,the properties and mechanism of Cr???adsorption by the optimized biochar were explored.The main achievements were given in this following:?1?The optimal modification parameters of pre-pyrolysis modification and post-pyrolysis modification based on Cr???adsorption were cornstalks,ferric chloride solution,solution concentration of 0.3 mol/L and solid-liquid ratio of 1:5.The optimal biochar denoted as MBCpre and MBCpost,respectively.The results of pre-pyrolysis modification were summarized as follows:Modification reagents played a primary role of affecting the yield,pH and electrical conductivity of modified biochar.The yield ranged from 25%to 50%,and the yield of MBCpre was optimal.The sequence and level of influence factors on Cr???adsorption were ferric chloride>citric acid>sodium hydroxide,1:5>1:10>1:20,cornstalks>pomelo peel>garden branches,0.3 mol/L>0.5mol/L>0.1 mol/L,respectively.The results of post-pyrolysis modification were summarized as follows:The biochar yield of the three sources of biomass was about30%,and modification reagents and biochar sources played primary roles of affecting the pH and electrical conductivity of biochar,respectively.The sequence and level of influence factors on Cr???adsorption were 0.3 mol/L>0.5 mol/L>0.1 mol/L,cornstalks-biochar>garden branches-biochar>pomelo peel-biochar,ferric chloride>citric acid>sodium hydroxide,1:5>1:10>1:20,respectively.?2?The adsorption capacity of MBCpre was better than that of MBCpost,the maximum adsorption gap reached 48.33%with Cr???initial concentration of 50 mg/L.The saturated adsorption capacity of MBCpre and MBCpost were 21.70 mg/g and 18.89mg/g when adsorption time reached 24 h,respectively.The adsorption properties of MBCpre were less affected by the temperature,and the optimal adsorption capacity was22.9 mg/g when the temperature was 25?.The adsorption capacity of MBCpost increased with the increase of temperature,which reached 22.5 mg/g at 45?.There was no significant difference both MBCpre and MBCpost in different pH?P>0.05?.The optimal dose 0.4 g/L of MBCpre was lower than 1 g/L of MBCpost.Sodium nitrate had no significant effect on Cr???adsorption,and there was no significant difference between MBCpre and MBCpost in different ion intensities?P>0.05?.MBCpre was more stable than that of MBCpost on Cr???adsorption,and MBCpost was easier to desorption and recovery by sodium hydroxide solution.?3?The heterogeneity of MBCpost surface energies by multilayer adsorption was described by Freundlich isotherm,the adsorption of MBCpre on Cr???was complex process.The mechanism of chemical adsorption and pore mass transfer played dominant roles of MBCpre and MBCpost on Cr???adsorption,and more active sites existed on MBCpre surface.Both of MBCpre and MBCpost on Cr???adsorption were endothermic process.The thermal demand of MBCpost was more significant,and the adsorption process of MBCpost were more spontaneous in the conditions of low Cr???concentration and high temperature.Pre-pyrolysis modification can obtained better pore structure and specific surface area,and the formation of Fe₃O₄ particles played a dominant role of MBCpre on Cr???adsorption.The relatively stable positive charge was provided by complexing Fe³⁺and functional groups,and the rich oxygen containing functional groups?C=O,O-H,C-O and O-C=O?on MBCpost surface promoted the adsorption properties of Cr???.