| Turning biomass wastes into biochar as soil contaminant immobilization materials brings the“win-win”effect:waste reuse and environmental remediation.Due to the abundant surface functional group and conjugated aromatic structure in biochar,biochar had high redox reactivity.It can be involved in soil relevant redox reactions and thus immobilize soil pollutant during soil application.However,studies about electron donating and mediating capability of biochar and its underlying electron transfer mechanisms during the heavy metals immobilization（e.g.,Cr（Ⅵ）reduction）were lack.In this study,electron transfer mechanisms of biochar during soil Cr（Ⅵ）reduction were studied and the participation as well as interaction mechanisms of soil main components in this electron transfer process were evaluated.The main conclusions of this thesis are as follows:（1）All biochar derived from peanut shell effectively reduced Cr（Ⅵ）into Cr（III）,and the reduction capability decreased as the pyrolysis temperature increased（400-600°C）,and then increased（600-800°C）.The electron donating moieties transformed from the–OH functional groups at lower pyrolysis temperature（<600°C）to the functional groups associated with conjugated structure at higher temperature（>600°C）.Biochar could mediate the reduction of Cr（Ⅵ）by lactate,with the reduction rates constant of Cr（Ⅵ）increased up to 16.3 and 345 times that by either biochar or lactate alone,respectively.The mediation capability of biochar increased with elevated pyrolysis-temperature due to the alternation of the electron mediating moiety from functional group to the conjugated clusters.（2）Biochar can mediate the electrons from soil organic acids（low molecular weight organic acids and humic acids）for Cr（Ⅵ）reduction.Redox reactions and sorption process occurred during the interaction between biochar and organic acids,which determined the Cr（Ⅵ）reduction process.The redox reactions were more important to the electron transfer between biochar produced at 400℃（BC400）and organic acids due to the repeated cycle of reduction-oxidation of surface functional groups.The reduction rate of Cr（Ⅵ）by organic acids mediated by BC400 was 1.10-7.09×10-3 h-1,among which tartaric acid had the best reduction efficiency due to its highest reducing capability.For biochar produced at 700℃（BC700）,the sorption process of organic acids was the key factor to the direct electron mediating process through the conjugated structure of biochar.The reduction rate of Cr（Ⅵ）by organic acids mediated by BC700was significantly higher and ranged 7.40-864×10-3 h-1,with the oxalic acid having the best reduction efficiency due to its highest sorption capacity by BC700.Compared to the small molecular weight organic acids,biochar show lower mediating capacity with humic acids.（3）Iron minerals inhibited the reduction of Cr（Ⅵ）by biochar,with the decrease of Cr（Ⅵ）reduction rate constant obtained by pseudo first-order reaction model from 2.18-2.47×10-2 h-1 to 0.71-1.51×10-2 h-1.The decrease of reduction rate constant was because:（1）the loss of electron donating moieties in biochar;and（2）inhibition of electron transfer between biochar and Cr（Ⅵ）due to surface coverage by biochar-Fe complex.Soluble Fe3+and amorphous iron mineral had more obvious negative effect on the Cr（Ⅵ）reduction.However,the coexistence of iron minerals with lactate enhanced the reduction of Cr（Ⅵ）by biochar,with the rate constant increasing from2.58-3.05×10-2 h-1 to 2.91-27.2×10-2 h-1.The positive effect was also attributed to two reasons:（1）lactate can decrease the surface Fe-coverage of biochar through chelating process;（2）electron from lactate can be mediated by Fe（II）as well as the biochar and thus enhancing the Cr（Ⅵ）reduction.（4）Aging process decrease the Cr reducing capacity of biochar as the reduction rate decreased from 2.18-2.47×10-2 h-1 to 0.67-1.95×10-2 h-1.The aging process with mineral can decrease the proportion of reductive-C-OH on biochar surface from 26.77-43.74%to 6.94-24.14%,which caused the decrease of electron donating capacity and Cr（Ⅵ）rate constant.Iron showed greatest decrease of-C-OH due to the oxidization reaction with biochar during the aging process and thus caused the lowest reduction rate of Cr（Ⅵ）.By contrast,the manganese and kaolinite had less decrease of reductive–C-O and Cr（Ⅵ）reduction rate due to the reduction process as well as physical coverage by mineral.Biochar-mineral complex was formed on the biochar during the aging process which can protect the biochar from oxidization during aging.The mediating capacity of biochar also altered with the mediated reduction rate changed from 3.91-5.82×10-3 h-1 to 0.33-22.3×10-3 h-1.Iron and manganese increased the mediating capacity due to their own redox activity,while aluminum,calcium,and kaolinite showed neutral or negative effect on the mediating process because of the surface coverage and complexation.（5）Biochar can also effectively reduce and immobilize the Cr（Ⅵ）in static incubation system and artificial Cr（Ⅵ）polluted soil.In the static biochar-Cr（Ⅵ）-soil components incubation system,biochar can fix about28.5-82.4%Cr（Ⅵ）.In the artificial Cr（Ⅵ）polluted soil,the addition of biochar can also reduce the extractable Cr（Ⅵ）by 6.6-50.5%.In the simulated soil incubation system,chromium may be sorbed or exist as Cr2O3,Cr OOH,Cr（OH）3,and FexCr2-xO3on the surface of biochar and iron mineral.Similar to the electron transfer mechanisms which we evaluated in the solution system,organic acids and iron minerals can participate in the electron transfer path during the Cr（Ⅵ）reduction by biochar and thus changed the Cr（Ⅵ）removal rate.Iron alone inhibited the electron transfer process,while it will facilitate the reduction of Cr（Ⅵ）when it co-existed with the organic acids.Different composition in soil affect reducing capacity and the electron transfer route of Cr（Ⅵ）reduction by biochar.Our results indicated that pyrolysis temperature affects the formation of functional groups and conjugated carbon structure of biochars which have a distinguishable influence on the electron donating and mediating ability for Cr（Ⅵ）reduction.Besides,our results systemically revealed the differing effects and electron transfer mechanism of various soil redox-active components including organic acids and soil minerals for Cr（Ⅵ）reduction by biochar.The results obtained from this study can help to establish the linkage among biochar,pollutants,and soil components in soil electron network,which better explains the multifunctional roles of biochar during the redox processes such as Cr（Ⅵ）reduction in soil and provides theory basis for the practical soil Cr（Ⅵ）immobilization with biochar. |
PDF Full Text Request