Preparation And Characterization Of Biochar Composites And Its Adsorption Behavior And Mechanism Investigation Of Heavy Metal Ions

Biochar is usually generated from incomplete pyrolysis of biomass,which is considered cost-effective and favorable for heavy metals removal.Despite of the attractiveness of biochar,one limitation on the wide application is that it is difficult to recycle from the treated wastewater.The study investigated the influence of pyrolysis condition on the adsorption performance of biochar,which was prepared using cocoanut husks as raw material.The calcium alginate/biochar microsphere?CA/BC7?and calcium alginate@Fe₃O₄/biochar magnetic microsphere?MM?were prepared using calcium alginate?CA?encapsulated biochar?BC7-10-3?or BC7-10-3 and Fe₃O₄.The magnetic biochar?MBC?was synthesized according to the co-precipitation method.Zero-valent iron biochar composite?ZBC8-3?was prepared by carbothermic method successfully.The characteristics and mechanism of heavy metal ion adsorption on BC7-10-3?BC7?,CA/BC7,MM,MBC and ZBC8-3 were discussed based on characterization of the materials.The main purpose of this paper is to improve the adsorption capacity and efficiency of biochar,which have great value in heavy metal sewage treatment.The main results are summarized as follow:?1?The adsorption capacity of biochar?BC7-10-3,BC6-10-3 and BC5-10-3?for Cu?II?and Pb?II?increased along with the increasing pyrolysis temperature.The reason is that biochar had huge surface area and decomposed to generate aromatic nucleus under higher temperature,which may form conjugated?bond with heavy metal.The isothermal thermodynamic datas for Cu?II?adsorption of the BC7 and CA/BC7 conformed to the Langmuir model.The maximum adsorption capacities of the BC7 and CA/BC7 from the Langmuir equation were 20.28 mg/g and 35.46 mg/g at pH=5.The adsorptions of Cu?II?by the BC7 and CA/BC7 were in good agreement with pseudo-first-order model,which suggested the rate limiting step was governed by diffusion.The main reason is BC7 and CA/BC7 are porous structure,so it takes more time for heavy metals to diffuse into the inside of the materials.The mechanism for Cu?II?adsorption by BC7 was mainly to form Cu?II?-O-Si and Cu?II?-Complex.The isothermal thermodynamic datas for Pb?II?adsorption of the BC7 and CA/BC7 conformed to the Langmuir model.The maximum adsorption capacities of the BC7 and CA/BC7 from the Langmuir equation were 93.20 mg/g and 155.04 mg/g at pH=5.The adsorption of Pb?II?by the BC7 was in good agreement with pseudo-second-orderequationindicatingthatthechemisorptionwasthe rate-controlling step.The reason is the pore structure of biochar?BC7?was hid by Pb₃?CO₃?₂?OH?₂ and PbCO₃,which generated through the reactions of Pb?II?and carbonate,so the Pb?II?adsorption mainly occured on the surface of biochar.The adsorption of Pb?II?by the CA/BC7 was in good agreement with pseudo-first-order model,which suggested the rate limiting step was governed by diffusion.The carbonate on BC7 surface dissolved in aqueous solution during the preparation and cleaning of CA/BC7,so there was no deposit produced in the process of Pb?II?adsorption by CA/BC7.The nature of Cu?II?and Pb?II?adsorption by CA/BC7 took place through the ion exchange between Ca?II?and Cu?II?or Pb?II?as well as the formation of coordination complex.The functional groups?e.g.C=C,Si-O,O-H,COO^-?played a key role in the adsorption.The results showed the adsorption mechanism of CA/BC7 mainly was chemisorption.?2?The isothermal thermodynamic datas for Cu?II?adsorption of the MBC and MM conformed to the Langmuir model.The maximum adsorption capacitys of the MBC and MM from the Langmuir equation were 13.87 mg/g and 40.42 mg/g at pH=5.The adsorptions of Cu?II?by the MBC and MM were in good agreement with pseudo-second-orderequationindicatingthatthechemisorptionwasthe rate-controlling step.The isothermal thermodynamic datas for Pb?II?adsorption of the MBC and MM conformed to the Langmuir model.The maximum adsorption capacitys of the MBC and MM from the Langmuir equation were 70.62 mg/g and167.78 mg/g at pH=5.The adsorption of Pb?II?by the MBC was in good agreement with pseudo-second-order equation indicating that the chemisorption was the rate-controlling step.The adsorption of Pb?II?by the MM was in good agreement with pseudo-first-order equation,which suggested the rate limiting step was governed by diffusion.The adsorbing capacitys of the MBC for Cu?II?and Pb?II?were lower than that of biochar.The surface of MBC covered with Fe₃O₄ could impair the adsorption of heavy metals by functional groups in the biochar meanwhile make it hard for the heavy metal ions to diffuse into the inside for adsorption.The kinetic constants of MBC for Cu?II?and Pb?II?adsorption were higher than that of MM.MBC had a higher adsorption rate,which showed the adsorption of MBC for Cu?II?and Pb?II?mainly performed on its surface in some extend.At pH=2³,Fe₃O₄ on the surface of MBC decomposed severely,while Fe₃O₄ enclosed by MM slightly decomposed.Compared with MBC,MM had better adsorption capacity and chemical stability.The maximum saturation magnetization of MM was 11.75 emu/g.Thus,the MM can be separated between the liquid and solid phase by using a magnet.The thermal stability of MM was higher than that of calcium alginate.The isothermal thermodynamic datas for Co?II?adsorption of the BC7,CA and MM conformed to the Langmuir model.The maximum adsorption capacities of the BC7,CA and MM from the Langmuir equation were 10.54,12.59 and 16.23 mg/g at pH=6.The adsorption of Co?II?by the MM was in good agreement with pseudo-second-order equation indicating that the chemisorption was the rate-controlling step.The results of Co?II?adsorption by BC7,CA and MM showed MM had better adsorption capacity than BC7 and CA,and performed good collaborative advantage.The main reason was the specific surface area and mean pore size of MM were higher than that of CA under the support of biochar and Fe₃O₄,which maked for heavy metal irons to diffuse into the composite material for adsorption.The nature of Pb?II?,Cu?II?and Co?II?adsorption by MM took place through the ion exchange between Ca?II?and Pb?II?or Cu?II?,Co?II?as well as the formation of coordination complex.The functional groups?e.g.Fe-O,Si-O,O-H,COO^-?played a key role in the adsorption.?3?The reduction behavior of Fe³⁺in the preparation of zero-valent iron cocoanut biochar?ZBC8-3?by the carbothermic reduction method is that Fe³⁺was first converted into Fe₃O₄,which was subsequently decomposed into FeO,and finally reduced to Fe⁰.A minor amount of?-Fe₂O₃ was produced in the process.The isothermal thermodynamic datas for Cu?II?removal of the ZBC8-3 conformed to the Langmuir model.The maximum removal capacity of the ZBC8-3 from the Langmuir equation was 169.49 mg/g at pH=5.The removal of Cu?II?by the ZBC8-3 was in good agreement with pseudo-first-order equation,which suggested that the rate limiting step was governed by diffusion.The main reason is that Cu₂O and FeOOH were formed by redox reaction of Fe⁰ and Cu?II?,blocking the pore structure of the ZBC8-3 to some extent.As a result,diffusion of Cu?II?to the inside of the ZBC8-3was hindered in the consequent reaction.The Cu?II?removal mechanism on ZBC8-3involved the reduction of Cu?II?by Fe⁰ to produce Cu⁰ and Cu₂O,while C-O-C,Si-O,C=C and-O-H formed a complex with Cu?II?.Biochar composites were prepared in this paper had good theoretical significance and application prospects in heavy metal sewage treatment.