Study On The Interaction Of Copper And Lead With Humic Acid-Dissimilated Iron Reduction Process

Nowadays,the problem of heavy metal pollutions is becoming more and more serious in wetland soil,and the migration and transformation rules of heavy metals in soil are attracting more and more attentions of human.Microbial mediated iron reduction is the main form of Fe（Ⅲ）reduction in soil,which affects the transference of heavy metals,and is an important means of bioremediation.Humic acid is an important component of soil organic matter and an electron transport substance in the process of dissimilated iron reduction.However,most of the current studies focused on the inhibitory effects of heavy metal ions on the dissimilated iron reduction process,and there were few studies on the transference of heavy metals in the dissimilated iron reduction process,especially under the multiple combination of humic acid-dissimilated iron reduction,which had great practical significance for the remediation of heavy metal pollutions in wetland soil.Therefore,in this paper,from the point of view of two systems:single colony pure culture and soil mud culture,the effects of electron transport substances on Fe（Ⅲ）reduction were compared based on the analysis of humic acids elements from different sources and the basic properties of UV and FTIR spectras.Taking heavy metals Cu and Pb as research objects,the effects of different concentrations of Cu²⁺and Pb²⁺on the reduction process of Fe（Ⅲ）/humic acid-Fe（Ⅲ）were analyzed,and discussed with microbial logistics equation.At the same time,the improved BCR method was used to divide Cu and Pb into weak acid extractable state,reducible state,oxidizable state and residue state,so as to explore the influences of electron transfer substances on the reduction process of dissimilated iron,the influences of heavy metals Cu and Pb on humic acid-dissimilated iron reduction process and the migration and transformation of Cu and Pb in this process.Thus this research further deepened the understanding of the interaction between heavy metals and humic acid-dissimilated iron reduction process.The main results were as follows:（1）Elliott Soil Humic Acid from soil and Pahokee Peat Humic Acid from peat wetland had similar element composition and structure,but Elliott Soil Humic Acid had higher humification degree and more quinone groups than Pahokee Peat Humic Acid.Electron transport substances（Humic acid and AQDS）could promote Fe（Ⅲ）reduction,which was reflected in the increase of Fe（Ⅲ）reduction rate and Fe（Ⅱ）production,and the promotion degree was enhanced with the increase of added concentration.Heavy metals Cu and Pb inhibited the reduction of Fe（Ⅲ）.With the increase of the concentration of Cu²⁺and Pb²⁺,the inhibition degree became stronger,and even the reduction of Fe（Ⅲ）did not occur,but 0.5 mmol·L^-1Pb²⁺promoted the reduction.Heavy metals Cu and Pb inhibited the reduction of humic acid-Fe（Ⅲ）,and the inhibition increased with the increase of Cu²⁺concentration.When the concentration of Pb²⁺was higher than 1.0 mmol·L^-1,Pb²⁺began to inhibit the reduction of Fe（Ⅲ）.The addition of humic acid weakened the inhibition of Cu and Pb.（2）Humic acid could transfer electrons between microorganisms and Fe（Ⅲ）only if the soil contained sufficient carbon sources.With the glucose concentration increased as exogenous carbon source,the reduction effect of Fe（Ⅲ）increased first and then decreased.With the humic acid and AQDS concentration increased,the reduction effects of Fe（Ⅲ）increased first and then decreased.In the process of soil Fe（Ⅲ）reduction,Cu inhibited the reduction of Fe（Ⅲ）,and the effect increased with the increasing concentration of Cu²⁺,which showed that the reduction rate of Fe（Ⅲ）and the production of Fe（Ⅱ）decreased.However,Pb could promote the reduction of Fe（Ⅲ）,and the promotion degree increased with the increase of Pb²⁺concentration.In the process of soil humic acid-Fe（Ⅲ）reduction,the inhibition degree of Cu to Fe（Ⅲ）reduction increased with the increase of Cu²⁺concentration.The addition of 100 and800 mg·kg^-1Pb²⁺inhibited Fe（Ⅲ）reduction,while 400 mg·kg^-1Pb²⁺still promoted Fe（Ⅲ）reduction.In accordance with the pure culture system,the addition of humic acid enhanced the reduction process of Fe（Ⅲ）,weakened the inhibition of Cu,but the enhancement of Pb group was not as good as the control group.（3）The forms of Cu in wetland soil were mainly reducible state and oxidizable state,and the forms of Pb were mainly weak acid extractable state and reducible state with high mobility.With the reduction process of Fe（Ⅲ）,the migration and bioavailability of Cu and Pb in the tested soils were increased,which showed that the content of acid-extracted state was increased.In soil with 50 and 200 mg·kg^-1Cu²⁺added,Cu was mainly transformed from reducible state to oxidizable state,and in soil with 800 mg·kg^-1Cu²⁺added,Cu was activated and transformed to the weak acid extractable state with lower stability.Pb could be activated by adding 100 mg·kg^-1and 800 mg·kg^-1Pb²⁺,which mainly showed the transformation from residue state and oxidizable state to reducible state and weak acid extractable state.With the reduction process of humic acid-Fe（Ⅲ）,the stability of Cu and Pb in the tested soil,the soil with different concentrations of Cu²⁺and the soil with different concentrations of Pb²⁺were improved,and the bioavailability of Cu and Pb was decreased,which was manifested as the content of oxidizable state and residue increased.