Remediation Of Coconut Fiber Biochar On Environmental Contamination Of Lead And Its Immobilization Mechanism

1.Soil pollution has drawn worldwide attention in the recent decades,especially the pollution of heavy metals in China.The survey reports of 2018 in China showed that there was 16.1%sampling sites exceeding the environmental quality standards of heavy metals(GB15618-1995)in agricultural soils.The contents of lead(Pb),cadmium(Cd),and arsenic(As)were highest in Southwest China and middle Yangtze River regions,while mercury(Hg)and chromium(Cr)were highest in Southwest China,East Coast of China and middle Yellow River regions.Nonferrous metal smelting,nonferrous metal mining,and Chemical manufacturing industry and chemical materials and chemical products manufacturing are three major industrial sources of wastewater for Pb in China.The main Pb emission regions of industrial wastewater are Hunan,Inner Mongolia and Jiangxi provinces in China.The techniques of in-situ or ex-situ,on-site or off-site,and biological,physical and chemical are often used in combination with each other for more economical and efficient remediation of a contaminated site.Biochar,a special organic material derived from pyrolysis of biomass has gained attention as a soil amendment for immobilization of heavy metals in soils.The Langmuir-qm values of most non-modified biochars ranging from 2.25 to 175.4 mg g-1.Moreover,many advanced technics e.g.,electron probe micro-analysis(EPMA),X-ray absorption fine structure(XAFS),micro-X-ray fluorescence(μ-XRF)spectromicroscopies,scanning electron microscopy combined with energy dispersive X-ray spectroscopy(SEM-EDS),Fourier transformation infra-red spectroscopy(FTIR),X-ray photoelectron spectroscopy(XPS)and X-ray diffraction(XRD)are combined used to reveal the immobilization mechanism of heavy metals to biochar.2.Unraveling sorption mechanisms of lead(Pb)to silicon(Si)-rich biochar at molecular scale in aqueous solution are essential for the effective application of the biochars to the remediation of Pb and other metal(loid)s pollution in the environment.Thus,this study investigated the contributions of phytoliths and other compounds to the Pb sorption on Si-rich coconut fiber biochar(CFB500)and the corresponding sorption mechanisms using spectroscopic techniques,including the μ-XRF,XAFS,SEM-EDS,and XRD.The μ-XRF and XAFS results showed that K,Ca,Cu,Mn,and Fe were released and significantly related to Pb in Pb-loaded CFB500;four major Pb species were formed with similar structure to Pb carboxylate(e.g.,Pb(C22H3O2)2),Pb3(PO4)2,PbSiO3,and PbCO3,accounted for 43.8%,33.2%,12.0%,and 7.3%,respectively.The precipitation of Pb2+ with CO32-and OH-resulted in the formation of Pb3(CO3)2(OH)2 precipitate on the surface of Pb-loaded CFB500.On phytoliths in CFB500,Pb2+ ions were mainly sorbed on the sites of silicate with a structure similar to PbSiO3.The contribution of binding sites for Pb2+ sorption was ascribed to the outer-wall of carbon skeleton of CFB500,which was stronger than that provided by the mineral oxide aggregate and phytoliths on CFB500.Organic carbon functional groups,inorganic carbonates,silicates and phosphates on CFB500 mostly dominated the sorption sites for Pb2+.The conceptual model of Pb sorption mechanisms on CFB500 was divided into the following three parts:(1)cation exchange-Pb exchanged with Na+,K+,Ca2+,Mg2+ and other cations in mineral oxides,inorganic compounds and organic carbon skeleton of CFB500;(2)complexation with functional group-surface complexation with free carboxyl and hydroxyl functional groups,resulting in the formation of C-O-Pb-O-C(or C-O-Pb)chain or ring structures;(3)other sorption processes-physical sorption,inner sphere complexation with the free hydroxyl of phytoliths carbon skeleton(mainly owe to silicates),phosphates and mineral oxides,and other surface precipitation.Our results suggest that CFB500 was a promising material for the remediation of Pb-contaminated aqueous environments(e.g.,wastewater).3.In this study,we applied coconut fiber-derived biochar(CFB)to a lead(Pb)-contaminated soil(at 600 mg kg-1)at 2%and 4%(w/w),in order to explore the efficacy and mechanism of CFB to immobilize Pb in soil during a 150-day incubation experiment.Our approach integrated various techniques namely micro-X-ray fluorescence,sequential extraction,X-ray absorption fine structure,scanning electron microscopy combined with energy dispersive X-ray spectroscopy to evaluate the Pb immobilization.Results indicated that the distribution of Pb in the studied soil was significantly affected by CFB application.The Pb content in organic matter bound fraction of the studied soil increased by 29.5%and 33.5%amended with 2%and 4%CFB,respectively,compared to control soil after 150-day of incubation.Lead-loaded humic acid(HA)and Pb3(PO4)2 were higher in the biochar-amended soil(2%CFB)as compared with the control soil.The result of FTIR showed that the vibration of carboxylate ion of SoilPb(ACFB)was obviously stronger than in SoilPb,and suggested that carboxyl groups on CFB could offer many carboxyl sorption sites for Pb to form more carboxylate ion in Pb contaminated soil.The CFB particles possibly offer more binding sites of PO43-and carboxylic functional groups than the binding sites of-FeO(OH),SiO32-,-Al2O3 and organic functional groups provided by the original soil particles alone(no biochar)for Pb.Thus,biochar-amended soils have higher capacity to immobilize Pb than the control soil.Overall,this study highlighted that CFB can be a potential candidate to immobilize Pb for the restoration and remediation of Pb-contaminated soils.4.In order to explore the influence of CFB on Pb bioavailability and accumulation in rice and its mechanism in contaminated tropical paddy soils with two kinds of parent materials(granite and basalt)under different Pb stress levels,the pot experiment of rice was conducted.The data of experiment was measured by combined used of chemical methods and spectroscopy techonics(e.g.,XRD,μ-XRF,XAFS,SEM-EDS,FTIR and XPS).Results showed that the contents of bioavailable Pb in studied soils,the concentrations of Pb in rice roots,rice shoots and brown rice all decreased in both of granite and basalt soils with CFB amendment under different Pb stress levels than in the control soils,respectively.The CFB amendment could significantly decrease the content of Fe/Mn plaques on rice roots.At the seedling period,the iron and manganese(Fe/Mn)plaques on rice roots could protect the rice plants from absorbing more Pb.The Pb2+ irons could be transported easily from clean-roots(without Fe/Mn plaques)to shoots of rice plants when bioavailable Pb in studied soils crossed the Fe/Mn plaques barrier on rice roots.The relative contents of Pb(PO4)2 and Pb-loaded HA in granite and basalt soils amended with CFB were increased by 7.1 and 6.1%and increased by 5.2 and 7.6%under Pb stress level of 5000 mg kg-1 compared to the control soils,respectively.The major immobilization mechanisms of Pb in studied soils amended with CFB might attribute to the increased binding sites of phosphate,some carbon containing functional groups and carboxylic groups.The CFB amendment might increase the risk of Pb uptake by rice plants in contaminated studied soils since the Fe/Mn plqaues barrier for Pb decreased in CFB-amended granite and basalt soils in comparation to the control soils,especially under the Pb contamination level no less than 2500 mg kg-1.Overall,the risk of Pb uptake by crops must be considered when applying biochar to Pb contaminated soils.