Effect And Mechanism Of Reduction-immobilization Of Hexavalent Chromium By Ball-miiling Modified Zero-valent Iron Biochar Composites

With the development of industrial processes such as chromium salt processing,electroplating,textile dyeing,and leather tanning,tons of chromium-containing wastewater and chromium-containing industrial waste residues are generated,but they are often not reasonably disposed of,resulting in massive water and soil pollution,and eventually endangering plant and animal as well as human health.In recent years,zero-valent iron biochar composites（ZVI/BC）have been widely used in the treatment of Cr（VI）pollution.With the advantages of low cost and environmental friendliness,carbothermal reduction is one of the common methods for the preparation of ZVI/BC.However,the non-uniform disperse of ZVI on the surface of biochar and limited electron transfer ability are the unavoidable problems of ZVI/BC prepared by the carbothermal reduction method.Based on this,in this thesis,well dispersed ZVI/BC was prepared by ball milling method.The effects of ball milling modification on the physicochemical properties of ZVI/BC were compared in detail by various characterization tools.The remediation effect of ZVI/BC and BM-ZVI/BC on Cr（VI）for Cr（VI）-contaminated water and soil were compared.The mechanism of Cr（VI）removal from the water phase and remediation of soil by BM-ZVI/BC was investigated by batch experiments,characterization,and theoretical calculations.The main conclusions of this research are as follows:（1）The ZVI/BC prepared by carbothermal reduction method was modified by high-energy ball milling.Through a series of morphological and structural characterization and electrochemical experiments,ball milling could reduce the particle size,increase the specific surface area by 2.8 times,enhance the exposure and dispersibility of Fe on the material surface,increase the content of oxygen-containing functional groups from 0.68 mmol g^-1to0.95 mmol g^-1,reduce the charge transfer resistance from 41.2Ωto 36.2Ωand decrease the corrosion potential by 0.23 V,resulting in BM-ZVI/BC with a better Cr（VI）reduction ability.（2）The study of Cr（VI）removal from the aqueous phase by BM-ZVI/BC showed that the maximum Langmuir adsorption capacity was 117.7 mg g^-1at298 K,which was 2.1 times higher than ZVI/BC.The Cr（VI）removal process by BM-ZVI/BC was consistent with pseudo-second-order kinetic model,and the rate constant of BM-ZVI/BC was 7.0 times that of ZVI/BC.Both solution p H and acid anion had significant effects on the removal of Cr（VI）by BM-ZVI/BC.In addition,the acid-washing experiments showed that the contribution of adsorption to the removal of Cr（VI）by BM-ZVI/BC was26.8%,and the response sequence of oxygen-containing functional groups analyzed by two-dimensional correlation infrared spectroscopy was:phenol-OH>-COO->C-O-C.The 1,10-phenanthroline shielding experiments showed that Fe（0）was the main reducing substance with a contribution of 73.4%,followed by the surface-bound Fe（II）（21.3%）and dissolved Fe²⁺（5.24%）.Furthermore,density functional theory calculation was used to reveal the mechanism of Cr（VI）removal by BM-ZVI/BC at the molecular scale,where the synergy between phenolic-OH and Fe（0）could improve the adsorption affinity and charge transfer flux to Cr（VI）.The main mechanism of Cr（VI）removal by BM-ZVI/BC was adsorption,reduction,and precipitation.Eventually,stable Cr-Fe oxides(Fe OCr₂O₃and Cr_1.3Fe_0.7O₃)were formed.（3）In the remediation of Cr（VI）-contaminated soil,BM-ZVI/BC showed a better remediation effect and faster remediation rate compared with ZVI/BC at 3%dosage.The fixation efficiency of BM-ZVI/BC reached over 93.1%after 4 h of application,which could reduce the concentrations of Cr and Cr（VI）in the simulated waste leachate from 8.52 mg L^-1and 7.44 mg L^-1to0.59 mg L^-1and 0.37 mg L^-1,and the concentrations of Cr and Cr（VI）in the simulated gastric leachate from 2.56 mg L^-1and 1.30 mg L^-1to 1.64 mg L^-1and 0.038 mg L^-1.On the 5th day of remediation,the Cr（VI）content in the soil was reduced from 170.4 mg kg^-1to 2.98 mg kg^-1,and the reduction fixation efficiency reached 98.3%.After 110 days of BM-ZVI/BC remediation,the weak acid-extracted state of chromium in the soil was transformed into more stable reducible,oxidizable,and residual chromium.The mechanism of BM-ZVI/BC remediation of Cr（VI）-contaminated soil was reduction,precipitation,and co-precipitation,which eventually formed Cr OOH,chromium-iron oxide,knorringite,pyrope chromian,and magnesiochromite.The 16S r RNA gene sequencing showed that BM-ZVI/BC facilitated the improvement of bacterial community richness and diversity in the soil with a 24.1%increase in the Shannon index.The stability of the remediation effect was evaluated using dry-wet alternation,freeze-thaw cycle,and simulated acid rain leaching experiments.Cr in the soil was strongly immobilized by BM-ZVI/BC,which was not easily affected by acid rain leaching.In addition,the concentrations of Cr and Cr（VI）in the simulated waste leachate remained low(less than 0.10mg L^-1)after frequent dry-wet and freeze-thaw cycles,so the remediation effect of BM-ZVI/BC was less influenced by dry-wet alternation and freeze-thaw cycles.In summary,BM-ZVI/BC has an excellent remediation effect on both Cr（VI）-contaminated water and soil,and simulated extreme weather changes have less negative impact on its soil remediation effect.BM-ZVI/BC is a green and environmentally friendly environmental functional material for Cr（VI）remediation.