Study On The Preparation Of Reduced Modified Mineral Materials And Their Remediation Effects On Chromium And Vanadium Pollution

As a heavy metal contaminant,the Chromium（Cr） acidic group faces a significant challenge in remediating contaminated soil due to its various valence,complex geochemical processes,high migration capacity,and poor performance in long-term remediation.This study introduced reduced Fe（II）into layered double hydroxide layers to increase the reduction and stabilization ability of the Cr acidic group.Batch experiments were conducted to verify the reduction-stabilization effect and long-term performance of materials on Cr（Ⅵ）pollution,evaluate the ecological impact of the soil pollution remediation process,and explore the influence of natural components（siderophore and humic acid）in soil on Cr（Ⅵ）remediation.Finally,statistical meta-analysis and machine learning methods were employed to investigate and predict the possibility of re-oxidation of pollutants in the soil after restoration to provide reference and support for the actual remediation technology of Cr（Ⅵ）contaminated soil.The main conclusions are as follows:（1）The reduced layered double hydroxide material（U-Fe/Al-LDH）prepared by ultrasonic-assisted co-precipitation showed an apparent cross-layered structure and strong adsorption coupling reduction ability.The kinetic adsorption process was in line with the pseudo-second-order kinetic model and controlled by chemical reaction,with a theoretically maximum adsorption capacity of 118.65 mg/g.The pH value showed a limited effect on this material,and the Cr removal process by this material is a spontaneous endothermic reaction.Materials with a positive surface charge adsorb HCr O₄^-and Cr O₄^2-by electrostatic attraction and anion exchange.At the same time,Fe（II）reduces Cr（Ⅵ）to Cr（Ⅲ）,and Cr（Ⅲ）reacts with Fe（Ⅲ）and OH^-released by the material to form Cr（OH）₃ and Cr_xFe_1-x（OH）₃ precipitates.After the leaching of adsorbed saturated materials for 30 days,the static leaching amount of total Cr was only 0.0663 mg/L.The removal effect of U-Fe/Al-LDH on Cr（Ⅵ）was stable and would not cause secondary pollution.（2）Soil incubation conditions were screened and a 360d long-term incubation experiment was conducted for the remediation of Cr（Ⅵ）-contaminated soil.The total Cr（Ⅵ）content and extractable Cr（Ⅵ）concentration in the blank samples of black soil（BS）and yellow soil（YS）increased first and then stabilized,and the application of remediation materials could effectively reduce both contents without re-oxidation.After the application of remediation materials,soil pH approached neutral,soil electrical conductivity（EC）value and cation exchange capacity（CEC）value increased.In a series of long-term evaluation experiments,using remediation materials all can effectively reduce the concentration of Cr（Ⅵ）in the leaching solution.After repeated freeze-thaw cycles and dry-wet alternations,the repaired BS soil and YS soil still met the V water limit in Environmental quality standards for surface water（GB 3838-2002）（<0.10 mg/L）and met the requirement in identification standards for hazardous wastes-identification for extraction toxicity standard（GB 5085.3-2007）（<5.00 mg/L）,respectively.Through instrument characterization and grading extraction experiments,Cr and Fe in the soil samples showed an excellent association phenomenon,especially the repaired soil samples,and the restoration process changed the soil Cr from an unstable form to a more stable form,and most of Cr（Ⅵ）in the soil was converted to Cr（Ⅲ）.（3）Changes in the plant bioavailable toxicity of the remediation process were investigated by the plant germination rate experiment,which proved that the application of remediation materials had a pronounced promotion effect on the growth index,the promotion effect on the root length was more significant.Changes in human bioavailability toxicity were explored by in vitro bioavailability experiments,in both the gastric phase and intestinal phase,remediation material treatments could effectively reduce the bioavailability of Cr in soil.The diffusive gradients in thin–films（DGT）extraction results also showed that the remediation process reduced the bioavailability of pollutants,and the concentration of C_DGT had a good positive correlation with the total Cr（Ⅵ）content,the extractable Cr（Ⅵ）concentration,the exchangeable Cr content extracted by Tessier method,and the bioavailability content by vitro bioavailability experiments.Meanwhile,the enzyme activity experiment and microbial community diversity test found that the enzyme activity in the remediated soil increased significantly,and the number of microbial species was higher and more diverse.（4）The reduction ability of Cr（Ⅵ） by natural components in soil,such as siderophore（desferrioxamine B（DFOB）as an example）and humic acid（fulvic acid（FA）as an example）,was verified by binary system and ternary system experiments.The results showed that DFOB had a good reduction ability to Cr（Ⅵ）under acidic pH conditions,and the reaction process was speedy.However,in the common pH environment,DFOB and Cr（Ⅵ）will form a slight competitive adsorption,which slightly inhibits the removal of Cr（Ⅵ）by LDH.The coexistence of FA will occupy some active sites and significantly inhibit the removal of Cr（Ⅵ）by LDH.In the soil medium,the Cr（Ⅵ）leaching concentration of the DFOB100+LDH treatment was 0.0363 mg/L,significantly lower than that of the LDH treatment.FA inhibited the reduction and passivation of Cr（Ⅵ）in soil by LDH.（5）Meta-analysis showed that oxidizing agents,soil properties,aging conditions,and experimental conditions may affect the oxidation of Cr（Ⅲ）.Soil properties with clay texture,pH≥8,low CEC capacity,easily reduced Mn content and Cr（Ⅲ）content,high Eh value,and Fe content can promote the reoxidation of Cr（Ⅲ）.The optimized machine learning algorithm model was employed to predict the long-term stability of BS0.2 soil samples after LDH remediation.The results predicted that after ten years,the extractable Cr（Ⅵ）concentration in soil was still lower than the Class III limit of the Standard for groundwater quality（GB/T 14848-2017）（≤0.05 mg/L）.These further proved that the material prepared in this study has an excellent reduction-stabilization effect,environmental ecological benefit,and popularization prospect for Cr（Ⅵ）pollution.It provides technical support for the actual Cr（Ⅵ）pollution control project to achieve economic,green and accurate repair.