Study On Adsorption And Passivation Of Cadmium And Arsenic By Fe-Mn Oxide Biochar

Cadmium（Cd）and arsenic（As）have become environmental heavy metal pollutants of global concern.Excessive discharge of Cd and As not only threatens ecosystem functions,but also causes potential harm to human health,leading to various organ diseases and even cancer.Thus,it is urgent to control Cd and As pollution.Biochar,as an environmentally friendly substance with extensive sources,low cost and stable structure,has attracted increasing attention in the field of heavy metal pollution remediation.In particular,the modified biochar combined with metal oxides has good adsorption or passivation performance for Cd and As pollution.Therefore,in this study,the agricultural waste Pennisetum giganteum z.x.lin straw with abundant yield was used as raw material to obtain biochar（BC）,and KMn O4 and Fe（NO3）3 were added to prepare the iron manganese oxide biochar（FMBC）by means of agitator impregnation sintering.Through material characterization and solution adsorption experiment,the adsorption performance and influence mechanism of biochar on Cd and As were explored.BC and FMBC were applied to the actual Cd and As co-contaminated farmland soil to carry out a 90-day passivation experiment,to evaluate the remediation effect on Cd and As composite contaminated soil.The main conclusions of this thesis were as follows:（1）SEM-EDS,XRD,FTIR and XPS showed that FMBC successfully loaded Fe oxide and Mn oxide,and Fe and Mn elements mainly existed as Fe2O3 and Mn2O3.FMBC had more oxygen-containing functional groups than BC and had good stability.（2）The adsorption of Cd（II）and As（Ⅲ）by BC and FMBC were more suitable for pseudosecond-order kinetics,which belonged to the heterogeneous diffusion system dominated by chemisorption,with the adsorption were spontaneous endothermic processes.The adsorption of Cd（II）was more suitable for Langmuir model,while the adsorption of As（Ⅲ）was more suitable for Freundlich model.The maximum adsorption capacity of FMBC for Cd（II）and As（Ⅲ）were 141.1 and 31.79 mg/g,respectively,which were 2.71 and 3.89 times of that of BC.Response surface methodology showed that the optimal conditions for FMBC adsorption of Cd（II）were as follows: FMBC dosage = 0.91 g/L,p H = 5.55,Cd（II）initial concentration =180 mg/L.The optimal conditions for As（Ⅲ）adsorption were as follows: FMBC dosage =0.89 g/L,p H = 4.85,As（Ⅲ）initial concentration = 100 mg/L.（3）When FMBC simultaneously adsorbed Cd（II）and As（Ⅲ）,competition and synergy existed between the two metals.The presence of Cd（II）enhanced the adsorption capacity of FMBC to As（Ⅲ）by 5.1%-123.9%,while As（Ⅲ）adding suppressed Cd（II）adsorption by0.7%-24.8%.The competition mechanism between Cd（II）and As（Ⅲ）was mainly through ion exchange and the complexation of surface oxygen-containing functional groups,while the synergy mechanism was mainly due to the electrostatic interaction and the formation of ternary surface complexes（Fe/Mn-Cd-As）.（4）After applying 2% BC and FMBC,soil p H value increased by 0.18-0.43 and 0.94-1.19 units,respectively,soil organic matter（SOM）increased by 29.56-40.86% and 19.07-28.88%,respectively,soil cation exchange capacity（CEC）increased by 11.65-31.72% and47.23-62.98%,respectively.Both BC and FMBC could reduce soil available Cd by 0.178-0.498 mg/kg and 0.351-0.765 mg/kg,respectively.The content of soil available As increased by 0.661 mg/kg at most when BC was applied,while the content of soil available As decreased gradually with the extension of culture time when FMBC was applied,and decreased by 2.429mg/kg at most.Treated in non-flooded soil for 90 d,FMBC group had the optimal passivation efficiency for Cd and As,reaching 59.85% and 32.74%,respectively.In addition,correlation analysis showed that p H value,CEC and SOM were significantly negatively correlated with available Cd（P < 0.01）,but not with available As（P > 0.01）.The results of fractions analysis showed that BC and FMBC mainly transformed the acid-extraction Cd into residue Cd,and the acid-extraction As and reducible As into residue As.