| Black carbon(BC)is a particulate carbonaceous materials produced from the incomplete combustion of fossil fuel and biomass.BC is involved in many biogeochemical and environmental processes in urban ecosystem.In this paper,a total of 115 topsoil samples and 4 soil profiles were collected from the urban area of steel industrial city,Anshan,Northeastern China.Black carbon(BC)and total carbon(TC)concentrations in urban topsoils(0-10 cm)and vertical profiles were determined using chemical oxidation technique.The BC concentrations in urban topsoils are in the range of 1.86 to 246.46 g/kg with an average of 33.86 g/kg,accounting for on average 75.73%of the TOC.Both BC and TOC concentrations decrease sharply with soil depth,whereas BC/TC ratio shows little variation with depth.The spatial distribution of BC in urban soils reveals that the BC concentration is much higher in the northern part of the city,which is consistent with the steel production.Enrichment factors(EF)of BC in urban soils range from 0.28 to 37.23 with an average of 5.11,indicating that the urban soils studied are moderately or severely accumulated by the BC.The distribution factors(DF)of BC are the highest in 1000-500 ?m size fraction,while the lowest in 50-2 ?m fraction.The mass loading of BC in 250-50 and 50-2 ?m size fractions accounts for 76.2%of bulk soil,indicating these two size fractions responsible for BC accumulation in soils.Strong correlation is found between BC and pollution load index(PLI),indicating the possibility of similar sources of BC and heavy metals in soils.The ratio of BC/TC in soils ranges from 0.45 to 0.97,with an average of 0.75.BC sources identified qualitatively by BC/TC ratio show a mixed source of fossil fuel combustion and vehicle emissions.The average BC/TC ratio is 0.88 for steel industrial area,suggesting that the main sources of BC are coal combustion during steel production.The BC concentration and BC/TC ratio may reflect the degree of human activities and pollution sources in urban soils.The study demonstrated that BC in urban soils is an effective indicator of anthropogenic pollution.Black carbon(BC)is the key for understanding many geological and environmental processes of carbon.The chemical composition,morphology,molecular structural and stable carbon isotopic composition of BC in urban topsoils were examined using elemental analysis,scanning electron microscopy with energy-dispersive X-ray spectroscopy(SEM-EDS),Fourier transform infrared spectroscopy(FTIR),thermogravimetric analysis(TGA),X-ray diffraction(XRD),and stable carbon isotope(?13C).Elemental analysis shows that the studied BC has larger carbon content,with the average more than 70%.The H/C and O/C atomic ratios of BC are on average 0.73 and 0.18,respectively.The BC particle displays different morphology,including porous spherical,irregular porous fragmentary,and blocky shapes.The porous spherical BC particles have atomic molar O/C ratio determined by SEM-EDS ranging from 0.04 to 0.37,which is identified as characteristic BC derived from coal combustion.XRD indicates that BC exists in mainly combining with mineral phases hematite(Fe2O3),kaolinite(Al2Si2O5(OH)4),quartz(SiO2)and calcite(CaCO3).The FTIR spectra of BC particles show major bands at approximately 3400 cm-1(O-H),2920 cm-1(C=H),1600 cm-1(C=C),1230 cm-1(C=Oo),and 1070 cm-1(C=O).The stable carbon isotope(?13C)of BC ranges from-24.48‰ to-23.18‰ with the average of-23.79‰±0.39.The O/C ratio and carbon isotopic(?13C)value of studied BC suggest that they are mainly derived from fossil fuel combustion.It could conclude that BC in urban topsoil recorded the history of atmospheric pollution and could use as an indicator to distinguish the source of contamination in urban environments.The distribution,mobility,and potential environmental risks of heavy metals in various particle size fractions of BC particles were studied.The BC particles collected from 10 representative urban topsoils were separated and classified into three particle size fractions.Their heavy metal contents(Cd,Cr,Cu,Pb,Zn and Ni)of each size fraction were determined.The bioaccessibility and leachability of heavy metals in BC particles were evaluated using the toxicity characteristic leaching procedure(TCLP)and ethylenediaminetetraacetic acid(EDTA)extraction,respectively.The results indicated that the contents of five heavy metals(Cd,Cr,Cu,Pb,Zn and Ni)in the different size fractions increased with the decrease of particle size.The<50?m size fraction of BC had the highest content of heavy metals,indicating that the this fraction was polluted by heavy metals more seriously than the other size fractions.The DFs of heavy metals in the finest fraction followed the order of Zn(3.22)>Cu(2.84)>Pb(2.61)>Cr(2.19)>Cd(2.05).The enrichment factor(EF)suggested that the enrichment degree of heavy metal in BC increased with the decrease of the particle size,especially for Cd and Zn.Meanwhile,Cd and Zn bioavailability in BC was also higher than other metals in BC particles.The results suggested that fine particles of BC were the main carriers of potentially toxic metals in BC particles.The variation of heavy metals in various size fractions should be taken into account in environment assessments.In order to understand the characterization of BC from biomass combustion,21 kinds of green biomass were used to produce BCs at the different pyrolysis temperatures and times.Their physiochemical properties were characterized by element analysis(C,H,O and N),ash,yield,pH,EC,total carbon,cation exchange capacity,functional group(acid,alkali,phenol and carboxyl)and surface negative charge.Results indicated that the C content,pH,EC of BCs was increased as pyrolysis temperature increased,while the H,O,and N content,total carbon(TC),cation exchange capacity(CEC),yield and surface negative charge was decreased with pyrolysis temperature decreased.Acid function group decreased with the pyrolysis increased,and other groups(alkali,phenol and carboxyl)increased with the pyrolysis increased.The correlation analysis,principle component analysis and cluster analysis among BCs basic properties were also carried out.The results indicated that there was a high significant relationship between pH and total carbon,ash,EC,carboxyl and surface negative charge.Four principle components were extracted and the cluster analysis of BCs species was divided into four groups.Our results suggest that the physicochemical properties of BCs are largely depended on the feedstocks.The differences in physiochemical properties of BCs will affect the effect of different types of BCs in environmental implication.The detailed characterization of BCs can be used to evaluate their potential application capacity as soil amendment and adsorbent for pollutants.The BC source varies a lot in urban soil.In this study,six kinds of BC particles were used to investigate the basic properties and the inner relationship between BC and mineral particles.Their physiochemical properties were characterized by element analysis,Fourier transform infrared spectroscopy(FTIR),scanning electron microscopy(SEM)with energy-dispersive X-ray spectroscopy(EDS)and X-ray diffraction(XRD).Results indicate BC has lager carbon content,almost all the BC particles more than 50%,except WSBC sample with the low carbon content.The FTIR spectra of BC particles show major bands at approximately 3400 cm-1(O-H),1600 cm-1(C=C),1230 cm-1(C=O),and 1070 cm-1(C=O).The XRD patterns differed considerably among the six kinds of BC particles.The main crystalline phases in all BC particles were calcite(CaCO3)and quartz(SiO2),while only ASBC exists mineral phase hematite(Fe2O3)and major band C=H(2920 cm-1).The morphology and pore structure of BC particles are observed by SEM/EDS.The BC particles displays different morphology in different BCs,and the biomass BC shows regular porous,sharp edges and no spherical,while coal BC has iregualar porous fragmentary,porous spherical or blocky shapes.SEM observation indicates the contact forms of BC and minerals mainly including surface contact,surface embedded and embedded internal combination. |
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