Effects Of Nanoscale Zero-valent Iron-loaded Biochar On Soil Carbon Sequestration During Remediation Of Chromium Contamination

The biogeochemical behaviors of Cr（VI）in terrestrial and aquatic ecosystems and how to mitigate its potential threats to animals,plants,and human health have been a long-standing area of research focus.Using chemical reduction to convert Cr（VI）to harmfulless and stable Cr（III）has been considered as an effective alternative to remediate Cr（VI）contamination.Nanoscale zero-valent iron（n ZVI）has an excellent performance in the remediation of Cr（VI）pollution due to its strong reducing property,large specific surface area and high adsorption capacity.However,pure n ZVI has obvious defects during its practical application,such as instability and easy agglomeration and oxidation,which make reduce its low efficiency.Recently,using biochar as the carrier to synthesize highly stable and active nanoscale zero-valent iron-loaded biochar（n ZVI-BC）can effectively enhance the remediation performance of Cr（VI）.Natural organic matter（NOM）exists widely in natural environments.Currently,the effect of NOM on the remediation of n ZVI-BC to Cr（VI）and the interaction processes of NOM and Cr（VI）adsorption on n ZVI-BC have not been well discussed.In addition,in the soil environment,the mineralization and sequestration of soil organic carbon（SOC）will be affected greatly after n ZVI-BC addition caused by the interactions among soil organic matter,n ZVI-BC,Cr（VI）,and redox products（Fe（II）,Fe（III）and Cr（III））.However,this aspect has not been reported in previous studies.In sum,the clarification of these issues mentioned above will better reveal the process mechanism of Cr（VI）remediation by n ZVI-BC,and has important scientific significance for a more comprehensive assessment of the environmental benefits of n ZVI-BC in the remediation of heavy metal pollution.In response to the above research deficiencies,the main research contents and the results obtained in this paper are as follows:（1）Wetland waste reed straw was used as biomass raw material to prepare biochar through lab-scale pyrolysis.Subsequently,the n ZVI-BC materials with different iron-carbon ratios（Fe:C）were prepared by liquid phase reduction method.The ionic valence,morphology,structure and functional groups of n ZVI-BC were characterized using various analytical methods such as Brunauer–Emmett–Teller（BET）,scanning electron microscopy（SEM）and X-ray diffraction（XRD）to investigate the effect of Fe:C variation on the surface properties of n ZVI-BC.The n ZVI-BC with a Fe:C ratio of 3:1 was determined as the best sorption material based on its highest Cr（VI）removal capacity(105 mg g^-1).By conducting a batch adsorption experiment under different Cr（VI）concentration gradients,the adsorption process of Cr（VI）by n ZVI-BC was found to be better fitted by the pseudo-secondary kinetic model and the Langmuir isothermal adsorption model,indicating that the adsorption process was mainly influenced by chemisorption and dominated by monomolecular layer adsorption.（2）The removal/adsorption of Cr（VI）and HA on n ZVI-BC3 and the mechanism were further investigated systematically by constructing batch adsorption experiments of Cr（VI）and NOM（with humic acid（HA）as the representative substance）,combined with the characterization analysis of surface morphology,mineral components and functional groups of n ZVI-BC before and after adsorption.The results showed that the removal capacities of Cr（VI）or HA by n ZVI-BC3 decreased significantly with the increases of HA or Cr（VI）concentrations,correspondingly.This confirmed that a strong competitive adsorption between Cr（VI）and HA on n ZVI-BC3 existed.The enhanced electrostatic repulsion and Fe OOH formation on the n ZVI-BC3 surface after adsorption of HA were considered as the main mechanisms to inhibit the reduction and adsorption of Cr（VI）.The formation of Fe（III）-Cr（III）hydroxide on the surface of n ZVI-BC3 occupying the active sites was the main mechanism of HA adsorption on n ZVI-BC3 inhibited by Cr（VI）.（3）The n ZVI-BC3 can significantly reduce the concentration of DTPA-extractable Cr（VI）in soil,improve the stability of Cr and reduce its ecological risk.Soil DTPA-extractable Cr（VI）concentration was significantly reduced from 147 mg kg^-1 in the control（CK）to 86.8 mg kg^-1 in the low addition treatment group（n ZVI-BC3（L））and 20.4 mg kg^-1 in the high addition treatment group（n ZVI-BC3（H））after 90 days.The n ZVI-BC3addition reduced the ecological risk of Cr in soil by reducing the proportion of Cr in exchangeable fraction（EX）and increasing the proportion of Cr in Fe-Mn oxide fraction（OX）and residual fraction（RS）.The main mechanisms are:i)the adsorption of Cr（VI）onto the surface of n ZVI-BC3 through surface pores,electrostatic gravitational force and adsorption of oxygen-containing functional groups.ii)The rapid redox reaction between n ZVI and Cr（VI）generated iron oxide minerals with large specific surface area and more adsorption sites,which can provide more adsorption sites for Cr（VI）.Meanwhile,the addition of n ZVI-BC3 promoted the microbial growth,which facilitates the biological reduction of Cr（VI）.iii)The redox products generated in the previous reaction steps combined together by co-precipitation to produce Cr_xFe_（1-x）（OH）₃ or Cr_xFe_（1-x）OOH.In sum,the addition of n ZVI-BC3 promoted the conversion of Cr in EX fraction to OX and RS fractions,which played an important role in the stabilization of Cr.（4）The effect of n ZVI-BC3 on SOC mineralization during the remediation of soil Cr（VI）contamination was also investigated.The results confirmed that the n ZVI-BC3 had a significant promoting effect on soil CO₂ emission in the early stage of incubation（first30 days）,but a remarkable inhibiting effect on soil CO₂ emission in the later stage of incubation.On the whole,the addition of n ZVI-BC3 demonstrated a positive reduction of the accumulated soil CO₂ emission.The intrinsic mechanism of inhibiting SOC mineralization during the remediation of soil Cr（VI）contamination by n ZVI-BC3 were mainly:i)Fe（0）and Cr（VI）rapidly reacted by redox to form Fe（II）/Fe（III）and Cr（III）.Meanwhile,iron oxide minerals with large specific surface areas and more adsorption sites may be formed in the Fe redox reaction,which can provide more adsorption sites for Cr（VI）.ii)Adsorption or co-precipitation reactions may occurred with SOC during the oxidation of Fe（II）/Fe（III）to weak crystalline iron oxides,which preserved OC in crystalline iron oxides to form Fe-OC-Cr complexes.iii)The formation of the Fe-OC-Cr complex improved the ability of SOC to resist biotic and abiotic degradation,which reduces the amount of microbially available carbon sources and consequently inhibited microbial and enzymatic activities,thus promoting soil carbon sequestration.The results of this study showed that n ZVI-BC was an efficient material for Cr（VI）remediation,and it can simultaneously enhance the stability of SOC and improve soil carbon pool storage.The results of this research can provide important scientific and theoretical support for a more comprehensive assessment of the environmental benefits of n ZVI-BC in the remediation of heavy metal contamination.