Effects Of Modifier-plant Remediation On Heavy Metal Geochemistry Of Lead And Zinc Residues And Its Mechanism

The Shuicheng-Weining-Hezhang ore belt in northwestern Guizhou is the main producing area of Guizhou non-ferrous metal lead-zinc mine.During the historical period,a large number of waste zinc smelting residues produced by the large-scale zinc smelting activities in this area spread over mountain gullies and river channels,resulting in a series of environmental pollution problems,which seriously affected the local life and production.In recent years,scholars at home and abroad have carried out a lot of research work in mine ecological restoration,among which phytoremediation is regarded as the most economic and ecological management method.However,there are still some controversies about the influence of phytoporemediation on the characteristics of heavy metal migration and transformation,which is reflected in the inconsistent views on whether the influence of root exudate in the rhizosphere microdomain of remediation on the bioavailability of heavy metals in the geochemical unstable sulfur-bearing tailings and contaminated soilispromotedorinhibited.However,thebioavailability,geochemical characteristics and environmental risks of heavy metals in alkaline zinc smelting residues with stable geochemical properties and their mechanisms of action are still unclear.Therefore,this study to improver and pioneer herb joint repair method of zinc smelting slag as the research object,and set up seven processing group,four of them have no plants on the control of the treatment group:waste residue（F）,waste residue+low grade phosphate rock powder（FL）,waste residue+renewal（FZ）,waste residue+modifier（low grade phosphate rock powder mixed with renewal）（FG）,and processing of the three plants exists groups:slag+modifier+Lolium perenne（FGH）,waste residue+modifier+Trifolium repens（FGS）,waste residue+modifier+Lolium perenne+Trifolium repens（FGT）;The drench solutions of the above 7 treatment groups were collected periodically（5 times in total）,which were denoted as T1,T2,T3,T4 and T5 respectively.The geochemical characteristics and influencing mechanism of heavy metals in the system of"waste residues-leachate-plants"were systematically studied.The research results are as follows:（1）the physicochemical properties and heavy metal characteristics of waste residue leachate were significantly affected by the modifier-plant remediation.And no plants treatment group（F,FL,FZ,FG）,compared with existing processing plants group（FGH,FGS,FGT）residual leaching filtrate pH overall declined with the increase of leaching time,and the processing of plant groups with no plants processing pH in T5 there were significant differences（p?0.05）.The electrical conductivity（EC）of leachate of waste residues in the plant treatment group was all higher than that in the non-plant treatment group（except for the FZ group T1 and T2）,indicating that plants participated in promoting the dissolution of soluble minerals in waste residues.The oxidation-reduction potential（ORP）of leachate in the treatment group with plants was lower than that in the treatment group without plants at T4-T5.The main anions in the leachate were SO₄^2-,followed by NO₃^-and Cl^-,F^-was the least.The total leaching amount of F^-and Cl^-decreased with the increase of leaching time,while NO₃^-and SO₄^2-both increased first and then decreased with the increase of leaching time,and plant participation promoted the leaching of Cl^-and NO₃^-from waste residue.The total leaching rate of 4 heavy metal ions in the leaching filtrate was Zn>Pb>Cd>Cu,indicating that Zn was more easily leached than the other 3.On the whole,the leaching amount of 4 heavy metals decreased with the increase of leaching time.Plants participate in the later stage to increase the leaching amount of Cu and reduce the leaching amount of Pb and Zn,while Cd increases or decreases with time.The correlation indicated that the change of physicochemical properties of waste residue leaching solution after the combination of modifier and plant was the important reason affecting the leaching of heavy metals.（2）The influence of modification-plant combined remediation on the bioavailability and occurrence of heavy metals in waste residues varies with the types of heavy metals.In the treatment groups（FGH,FGS,FGT）,the proportion of Cu reducible state and oxidizable state increased and decreased respectively.The acid soluble state of Pb increased,but the reduced state decreased,indicating that both the modifier and the plant combined remediation increased the bioavailability of Cu and Pb in the waste residue.The effective state contents of Zn and Cd in the waste residues of FGH,FGS and FGT treatment groups all decreased to different degrees in the FG treatment group.Compared with FG treatment group,the proportion of acid soluble Zn in waste residues of FGH,FGS and FGT treatment groups decreased respectively,while the proportion of residue state increased respectively.Compared with FG treatment group,the proportion of acid-soluble Cd in waste residues of FGH,FGS and FGT treatment groups increased respectively,while that of reducible Cd decreased respectively.It indicates that the modifier-plant combined repair reduces Zn and increases the bioavailability of Cd.（3）Interplanting of Lolium perenne and Trifolium repens increased the transport coefficient of heavy metals.There was no significant difference in the enrichment coefficient of heavy metals between Lolium perenne and Trifolium repens in single species and interplanting mode,and the effect of different planting methods on the enrichment coefficient of the same plant was lower than 0.70%.However,the transport coefficient of heavy metals in Lolium perenne and Trifolium repens intercropping mode was higher than that in the single species mode of the two plants（except ryegrass’s transport to Zn）,and in particular,the transport coefficient of heavy metals in clover intercropping mode was 4.33%⁹.02%higher than that in the single species mode.Under the interplanting mode of Lolium perenne and Trifolium repens,the transport coefficient of ryegrass was 1.1².5 times higher than that of Lolium perenne,indicating that the interplanting mode of two pioneer herbaceous plants had better repair effect than that of single species,while the repair effect of Trifolium repens on waste residue was stronger than that of Lolium perenne.（4）The microstructure and mineral composition of waste residue were significantly changed by the combination of modifier-plant.Under the repair effect of Lolium perenne single species mode（FGH）and its interplanting with Trifolium repens（FGT）,the surface characteristics of the waste residue showed that a large number of small particles bonded together to form larger particles,indicating that the plant root system had the function of wrapping and agglomerating the small particles in the waste residue.However,under the repair effect of FGS,the surface characteristics of waste residues showed that there were a large number of pores and pits,indicating that the plant roots promoted the biophysical weathering of waste residues through physics（root splitting）and chemistry（root secretion dissolution）.Compared with the treatment group without plant（FG）,the atomic percentages of O,Si,Fe,Al,Ca and Mg in the treatment group with plant（FGH and FGT）were all increased to varying degrees,indicating that the combination of modifier and plant（Lolium perenne and Trifolium repens）promoted the weathering process of zinc smelting slag into soil.The mineral composition of waste slag mainly includes quartz,gypsum,feldspar,calcite,dolomite,pyrite,magnetite and siderite and other mineral phases.（5）The physicochemical properties,nutrients and morphological characteristics of heavy metals of waste residues in vertical profile were significantly affected by the combination of modifier-plant.The pH and EC of waste residues in treatment groups with and without plants showed an increasing trend as the gradient increased.Under the repair mode of single Lolium perenne（FGH）and interplanting with Trifolium repens（FGT）,the effective nitrogen content of waste residue increased with the increase of waste residue profile depth,while under the repair mode of single Trifolium repens（FGS）,the effective nitrogen content of waste residue decreased with the increase of waste residue profile depth.The available phosphorus content of waste residue in the treatment group（FG）without plant presence decreased with the increase of waste residue profile depth,while the available phosphorus content of surface waste residue（0-16cm）in the treatment group（FGH,FGS and FGT）with plant presence was the highest,indicating that the single species and interplanting mode of Lolium perenne and Trifolium repens promoted the accumulation of phosphorus in the surface.The contents of heavy metal available state in the vertical profile of waste residues repaired by improver and plant were higher in the surface layer（0-8 cm）,the subsurface layer（8-16 cm）and the bottom layer（32-40 cm）,and lower in the middle layer（16-32 cm）.Moreover,the proportion of acid-soluble heavy metals（Cu,Pb,Zn,Cd）increased with the increase of waste residue profile depth.Under the repair mode of ryegrass single species（FGH）and clover interplanting（FGT）,the proportion of Pb and Zn acid soluble state in the waste residue with a depth of 0-16cm was higher than that of the waste residue with a depth of 16-24cm,indicating that ryegrass restoration increased the content of Pb and Zn acid soluble state.（6）Repairing plant root exudates can effectively promote the agglomeration of fine particle residues to form large particle residues and affect the bioavailability of heavy metals.Based on the distribution characteristics of ¹³C in the residue of Lolium perenne root exudates with different particle sizes,it can be seen that ¹³C in the residue of Lolium perenne root exudates with different particle sizes（d≥2、2 mm>d≥1 mm and d<0.25 mm）was 3.16‰,1.85‰and 1.47‰higher than that of the control residue,respectively.The results showed that the root exudates were mainly distributed in the residues with particle sizes of d≥2、2 mm>d≥1 mm and d<0.25mm.Combined with different particle size of residual quality of aggregate distribution characteristics,compared with the control slag,root system secretions in the d<0.25 mm、2 mm>d≥1 mm diameter for slag aggregate quality proportion were reduced 1.76%and increased by 1.62%,that perennial Lolium perenne root exudates in promoting fine of slag forming large size slag plays an important role.Compared with the control group,the addition of root exudates increased the acid soluble state of 4 kinds of heavy metals in the particle size of waste residue d≥2 mm and d<0.25 mm.In conclusion,conditioner-pioneer herb joint repair of geochemical characteristics of heavy metals in zinc smelting slag method the influence of the mechanism is mainly manifested in the following aspects:one is plant roots activity by changing the physical and chemical properties of rhizosphere waste residue of,anion and nutrient content,as well as the geometrical shape,adding Cu,Pb,Cd and reduce the biological effectiveness of zinc;Second,the root system and its secretions can promote the aggregation of fine particles to form large particles,increase the content of acid soluble state of large particles,and increase the bioavailability of heavy metals.