Effects Of Root Exudates On The Availability And Migration Characteristics Of Heavy Metals In Indigenous Zinc Smelting Slag

Lead-zinc mineral resources in Guizhou Province are abundant,and its production accounts for about 10% of the total lead and zinc production in the country.It is mainly distributed in the counties such as Shuicheng,Weining and Hezhang of western Guizhou.The method of smelting zinc in the western Guizhou Province has a long history,but its process is simple,resulting in many heavy metals remaining in the residue.The large-scale land-based zinc smelting activity in this area left behind a large area of waste slag yard and contaminated soil,which has potential ecological risk for the surrounding ecological environment.Phytoremediation technology is a relatively cheap green governance technology that is both safe and economical.In recent years,many scholars around the world have conducted extensive research on the effects of plant root exudates on heavy metals and have obtained a series of research results.However,there are still controversies regarding the effects of root exudates on the effectiveness and migration characteristics of heavy metals.In this study,we used a bare earth smelting residue that had been stored for many years in Houchang Town,Weining County,western Guizhou as the research object,and simulated the exogenous addition of root exudates,including oxalic acid,tartaric acid,valine,glycine and a combination the mixture of the above four substances,is also artificial root exudate(ARE),were used in the indigenous zinc smelting slag,The effects of different root exudates components on the availability and migration of heavy metals in indigenous zinc smelting slag were studied,by changing the physicochemical and microbiological characteristics of the waste residue matrix and promoting the agglomeration of waste residue.The results showed:(1)Compared with the control,the root exudates of different concentrations of single and mixed components can promote the dissolution of the indigenous zinc smelting slag as a whole.The specific performance is that the pH value of the slag is decreased,the electrical conductivity(EC)is increased,and the effect on the pH of theindigenous zinc smelting slag matrix is shown as follows: glycine > ARE > tartaric acid > oxalic acid ? valine,EC changes in indigenous zinc smelting slag treated with high concentration of root exudates showed ARE > glycine > oxalic acid > valine >tartaric acid.(2)The effects of mixed root exudates treatment on the available Cu,Pb,Zn,and Cd in the indigenous zinc smelting slag were larger than those of the single component.With the increase of the added concentration,the content of available Cu,Zn,and Cd in the treated slag was ARE > glycine > valine > oxalic acid,tartaric acid.Compared with the control,the amino acid and ARE treatment can significantly increase the content of Cu,Zn,and Cd in the effective state of the indigenous zinc smelting slag,the activation effect of different treatment in the slag is ARE >glycine > valine,and the activation intensity increases with the concentration.The treatment of oxalic acid and tartaric acid had no significant effect on the available Zn and Cd of the indigenous zinc smelting slag,but could inhibit the activation of Cu in the indigenous zinc smelting slag.Compared with the control,the organic acid had a certain activation effect on the active Pb of the indigenous zinc smelting slag,and the amino acid and ARE treatment could inhibit the activity of the active Pb.The content of available Pb in the residue after various treatment was oxalic acid > tartaric acid >the control > valine > ARE and glycine.(3)The addition of root exudates in each fraction affected the occurrence of the residual slag Cu,Pb,Zn,and Cd.Treatments with different concentrations of tartaric acid,proline,glycine,and ARE reduced the content of soluble Cu and Zn,reduced Pb,and oxidizable Cd in waste slag,increased residual Cu,Pb,Cd,and oxidizable Cu,Pb,Zn content.The oxalic acid treatment decreased the content of acid soluble,oxidizable and residual Zn and oxidizable Cd in waste slag,and increased the contents of acid-soluble Cu,reducible Zn and residual Cd.(4)The addition of root exudates can significantly change the distribution of waste agglomerates.The main manifestations are that root exudates of different concentrations reduce the proportion of agglomerates with particle sizes of 1 ~ 2 mm and <0.25 mm in the indigenous zinc smelting slag,and increaseThe proportion ofagglomerates of 0.5 ~ 1 mm and 0.25 ~ 0.5 mm.Compared with the control,the treatment of root exudates at different concentrations of single and mixed fractions resulted in the migration of heavy metals in each agglomerate.The heavy metal Cu in the waste slag is relatively enriched in agglomerates with a particle size of <0.25 mm,Zn is mainly enriched in microaggregates(<0.25 mm),and Cd is deficient in agglomerates of 1 ~ 2 mm,but at 0.5 ~ 1 mm,0.25 ~ 0.5 mm,and <0.25 mm aggregates were enriched.Pb in the indigenous zinc smelting slag under the influence of root exudates was mainly enriched in aggregates of 0.5 ~ 1 mm and 0.25 ~ 0.5 mm,while the aggregates of <0.25 mm showed a deficit state.(5)Under the action of root exudates,the effectiveness of heavy metals in various particle size aggregates of the indigenous zinc smelting slag is mainly affected by the particle size and the type of element.Specifically,after the addition of different concentrations of root exudates,the distribution of available Cu,Zn,and Cd contents in the aggregates of various size fractions changed significantly with decreasing particle size,and the distribution of available Pb was reversed.Different concentrations of root exudates can increase the content of available Cu,Zn,and Cd in the aggregates with diameters of 1 ~ 2 mm,while the effects of amino acids and ARE all significantly reduce the content of available Pb in aggregates.(6)Different concentrations of organic acids and amino acid additions have an effect on the nutrient content of the indigenous zinc smelting slag matrix.Compared with the control,the addition of amino acids and ARE at different concentrations increased the contents of total nutrients(total nitrogen,available nitrogen,organic matter,and available potassium),and the effects on total nitrogen and available nitrogen were ARE > glycine > valine.The addition of different concentrations of organic acids respectively increased and decreased the content of available nitrogen and available potassium in the indigenous zinc smelting slag.Both mono-component and mixed-component root exudates reduced the available phosphorus content.(7)Treatment with different concentrations of amino acids and ARE significantly increased the activeness of hydrolitic enzyme(urease,phosphatase,sucrase)and microbial population in the indigenous zinc smelting slag.Oxalic acidand tartaric acid had little effect on the activeness of hydrolitic enzyme,and to a certain extent inhibited microbial reproduction.The single component oxalic acid,tartaric acid,valine and glycine increased the catalase activity of the indigenous zinc smelting slag in different degrees,while the root exudates of the mixed component had inhibitory effects on catalase activity of the slag.Under the effects of root exudates with different concentrations of single and mixed components,the number of microorganisms in the three major groups of indigenous zinc smelting slag was ranked as actinomycetes > bacteria > fungi.High concentration of glycine and ARE treatment can effectively increase the strength of indigenous zinc smelting slag respiration.In general,the addition of root exudates has a significant improvement on the physicochemical and microbial environment of the indigenous zinc smelting slag.(8)Under the action of root exudates,the bioavailability of Cu,Pb,Zn,and Cd in the indigenous zinc smelting slag matrix is closely related to the activity of microorganisms,nutrients,and enzyme activity in the indigenous zinc smelting slag,there is also a close relationship between nutrients,microorganisms,and enzyme activity,forming a cycle,it indirectly acts on heavy metals.Among them,the available states of Cu,Zn,and Cd were significantly or extremely significantly positively correlated with pH,EC,nutrient,microbial activity,and hydrolase enzyme activity.In general,the active Pb was significantly or extremely significantly negatively correlated with microbial activity,nutrients,and hydrolase enzyme activity,and was significantly positively correlated with catalase.The bioavailability of Cu,Pb,Zn,and Cd in the indigenous zinc smelting slag matrix is closely related to their morphologies,and there are complex interactions between the bioavailability and occurrence patterns of different heavy metals.In summary,the root exudates of Perennial ryegrass have certain effects on the bioavailability and migration of heavy metals in indigenous zinc smelting slag.The addition of amino acids and organic acids both increase the bioavailability of heavy metals Cu,Zn,and Cd,and further The leaching of heavy metals was increased.The leaching of the root exudates of the mixed components was stronger than that of the single component root exudates.The leaching of the amino acids in the root exudatesof the single component was stronger than that of the organic acids,and the root exudation of the amino acids-containing components was also found.The substance shows a fixed effect on heavy metal Pb.Excessive heavy metals can affect the normal growth of plants.Different plants produce different types and contents of root exudates under different environmental conditions,and the mechanism of action for different heavy metals is different.Therefore,the process of using phytoremediation methods for the smelting of zinc slag waste yards Among them,reasonable screening and collocation of plants are needed according to specific conditions,which constitutes a slag field ecosystem with natural succession of vegetation,plants,animals,microorganisms,and other biological species that are gradually becoming stable.