Phytoremediation On Biogeochemical Processes Of Heavy Metals In Indigenous Zinc Smelting Waste Slag

In the historical period,large-scale indigenous zinc smelting activities were conducted in the northwest of Guizhou.After decades and even hundreds of years of continuous accumulation,large amounts of bare zinc smelting waste slag rich in heavy metals were left behind.The high content of heavy metals remaining in the zinc smelting waste slag can be easily transported to the surrounding water bodies,soils,plants,and other environmental medias by means of hydraulic transportation and wind diffusion,which seriously threatens the water quality safety,food safety of agricultural products,and ecological environment safety in the surrounding and downstream regions,and eventually enter the human body through the food chain or direct absorption,increasing the risk of human health exposure.Therefore,in order to effectively control the continuous spread of heavy metal pollution in waste slag through severe slag erosion,rainwater leaching,and dust,the implementation of vegetation reconstruction or ecological restoration of waste slag sites is not only the key work to control the spread of heavy metal pollution,but also the core means to improve the quality of the local ecological environment.However,there are two views on the effect of phytoremediation on the migration and transformation characteristics of heavy metals in waste slag.The view of activation is that the organic acids secreted by the roots of revegetated plants increase the risk of heavy metals leaching;another view of phytostabilization is that the development of a self-sustaining ecosystem can achieve in situ phytostabilization of heavy metals by regulating the biogeochemical processes of heavy metals at the waste slag-plant interface.Most studies on the risk of heavy metals leaching by phytoremediation are based on indoor simulated revegetation of herbaceous,and mainly consider the effect of the dominant role of single environmental factor such as plant root exudates on the bioavailability of heavy metals.However,the synergistic effects of plant root exudates,plant litter,and heterotrophic microbial communities are rarely considered.The synergistic effects on the bioavailability of heavy metals are namely that the improvement of micro-environment conditions at the interface of waste slag-plant roots and the soil formation processes in waste slag influencing the biogeochemical processes of heavy metals.Especially,there are few reports on the effects of revegetation of woody plants on the biogeochemical processes of heavy metals of waste slag in the rhizosphere micro-domain of plants.In this study,the waste slag site of the demonstration project of“in-situ control of heavy metal pollution releasing from waste slag site and soil-free rapid ecological restoration of waste slag site”conducted by our research group in Weining County,northwest Guizhou Province,was seclted as the research area,and the four dominant plants?Broussonetia papyrifera L.,Robinia pseudoacacia L.,Cryptomeria fortunei L.,and Arundo donax L.?established in the waste slag site for 5 years were investigated.We investigated the effects of four dominant plants on the physicochemical and microbiological characteristics,the occurrence fractions and migration and transformation of heavy metals,the formation of aggregates and particulate organic matter?POM?and their effects on the migration and transformation of heavy metals in the waste slag.This study aims to reveal the effects of revegetation of four dominant plants on the biogeochemical processes of heavy metals in indigenous zinc smelting waste slag and its mechanisms,and to provide a scientific basis for further large-scale application of this direct revegetation technology to remediate the waste slag in similar metal smelting areas in our country,especially in the indigenous zinc smelting region in northwest Guizhou.The results obtained in this study are as follows:?1?Revegetation of four dominant plants significantly improved the physico-chemical properties and promoted the accumulation of nutrients in the zinc smelting waste slag.The characteristics of the bare slag without plants are mainly neutral alkali,containing a higher percentage of sand particles,the physical structure and water holding capacity are poor,and the available nutrient content is extremely low.Compared with the bare waste slag,revegetation of four dominant plants decreased the pH values and redox potential?Eh?,and significantly increased the electrical conductivity?EC?and the percentage of clay and silt particles of the waste slag,respectively.Meanwhile,revegetation of four dominant plants significantly increased the moisture content and the content of available nutrients?N,P,and K?and organic matter in the rhizosphere slag.The nutrient accumulation between rhizosphere and no-rhizosphere slag of four dominant plants had significant differences.?2?Revegetation of four dominant plants significantly improved the enzyme activiti-es and the numbers and activities of three dominate microbial communities in the zinc smelting waste slag.The enzyme activities and the numbers and activities of three microorganisms,including bacteria,fungi,and actinomycetes,were extremely low in the bare waste slag without vegetation growth.Revegetation of four dominant plants significantly increased the enzyme activities,such as sucrase,alkaline phosphatase,urease,and catalase,the numbers of three dominated microbial communities and microbial activities?respiratory intensity?.Revegetation of B.papyrifera has the greatest increases in the enzyme activities?e.g.,sucrase,alkaline phosphatase,urease,and catalase?,the numbers of three microbial communities?e.g.,bacteria,fungi,and actinomycetes?,and microbial biomass,as well as revegetation of R.pseudoacacia has the greatest increases in the microbial activity?e.g.,respiration intensity?in the waste slag.The enzyme activities,microbial numbers,microbial activity and microbial biomass in the rhizosphere slag were higher than that in no-rhizosphere slag.Correlation analysis showed that the changes in the content of nutrient and moisture in the waste slag were the main factors influencing the enzyme activities and the numbers and activities of the microorganisms;and the improvement of the biochemical and microbiological characteristics of the waste slag were also the main factors influencing the accumulation of nutrients in the waste slag.?3?Revegetation of four dominant plants significantly increased the community composition and diversity of bacteria,fungi,and archaea.Study on microbial community structure diversity of indigenous zinc smelting waste slag based on high-throughput sequencing technology,the results showed that the predominant phylum of bacteria in the bare slag and rhizosphere slag was Proteobacteria,Actinobacteria,Acidobacteria,Chloroflexi,Bacteroidetes,Gemmatimonadetes,and Saccharibacteria.The predominant phylum of fungi in the bare slag and rhizosphere slag was Ascomycota,Zygomycota,Basidiomycota,and Rozellomycota.The predominant phylum of archaea in the bare slag and rhizosphere slag was Thaumarchaeota and Euryarchaeota.Compared with the bare slag without plants,revegetation of four dominant plants significantly increased the relative abundance of Proteobacteria,Bacteroidetes,Acidobacteria,Planctomycetes,Saccharibacteria,Zygomycota,Rozellomycota,and Thaumarchaeota,and increased some bacterial groups with special functions,such as the plant growth-promoting rhizobacteria?PGPR??e.g.,Flavobacterium,Streptomyces,and Arthrobacter?and nitrogen-fixing bacteria?e.g.,Bradyrhizobium?.Additionally,revegetation of four dominant plants increased the diversity index of bacterial community and the richness indices of bacteria,fungi and archaea communities in the waste slag,but reduced the diversity indices of fungi and archaea communities in the waste slag.The canonical correspondence analysis?CCA?between the environmental factors of waste slag and the composition and diversity of microbial communities indicated that the significant improvement of the rhizosphere microenvironmental conditions,including pH and the content of moisture and available nutrients,are the main factors affecting the microbial community compositions in the waste slag,as well as the main factors influencing the microbial community compositions and diversity in the rhizosphere slag of different plant species have diffenences.?4?Revegetation of four dominant plants significantly influenced the migration and transformation characteristics of heavy metals in the waste slag.The contents of heavy metals?Cu,Pb,Zn,and Cd?in the zinc smelting waste slag were Pb>Zn>Cu>Cd.After5 years of plants establishment on the waste slag,with the increase in the age of revegetation,the plant tissues?e.g.,roots,stems,and leaves?absorbed heavy metals from the waste slag to different extents,and the contents of heavy metals in plant tissues were roots>leaves>stems.However,compared with the total amount of heavy metals in the waste slag,the heavy metal elements in the four dominant plant tissues showed low accumulation characteristics,and the enrichment coefficients of heavy metals?Cu,Pb,Zn,and Cd?in the roots and stems of the four dominant plants at around 0.1.Compared with the bare slag,the total amount of Cu and Pb in the rhizosphere slag significantly decreased and the total amount of Zn and Cd in the rhizosphere slag slightly increased,respectively.Revegetation of four dominant plants increased the availability of Pb and reduced the availability of Cu,Zn,and Cd in the rhizosphere slag to varying degrees,except for the increase of Zn and Cd availability in the rhizosphere slag of A.donax.Revegetation of four dominant plants reduced the percentage of acid-soluble Cu,Pb,Zn,and Cd in the rhizosphere slag,and increased the percentage of reducible,oxidizable,and residual fractions of Cu,Pb,Zn,and Cd to varying degrees.?5?The improvement of the micro-environmental conditions of waste slag in the presence of four dominant plants significantly influenced the migration and transformation characteristics of heavy metals in the waste slag.The analysises of correlation and canonical correspondence showed that the contents of oxalic acid and dissolved organic carbon?DOC?in the rhizosphere slag were significantly positively and negatively correlated with DTPA-Pb and DTPA-Cu in waste slag,respectively.The increase in nutrient content and the abundance and diversity of the microbial community in the waste slag reduced the availabilities of Cu,Pb,Zn,and Cd in the waste slag,but the increase of oxalic acid content and microbial diversity in the rhizosphere slag increased the availabilities of Pb in the waste slag.Revegetation of four dominant plants significantly reduced the bioavailabilities and changed the migration and transformation characteristics of heavy metals?Cu,Pb,Zn,and Cd?by the comprehensive processes,including improve the rhizosphere microenvironmental conditions?e.g.,enhancing nutrient and organic matter accumulation,microbial community abundance and diversity increase,organic acids and dissolved carbon release?,plants uptake,and changing the occurrence fractions of heavy metals from acid-soluble fraction to reducible,oxidizable and residual fractions.?6?Revegetation of four dominant plants significantly changed the mineral composition,morphology and microelement composition of the waste slag.The morphological characteristics and microelement composition of the waste slag were characterized based on scanning electron microscopy and energy dispersive X-ray spectroscopy?SEM-EDS?,and the results showed that the surface of bare waste slag without plants is relatively smooth and flat.However,the surface of rhizosphere slag showed loose and porous,which is caused by the interpenetration effects of a large number of dense plant roots.Compared with the bare slag,the proportion of Zn element in the micro-area of rhizosphere slag was significantly reduced,and the relative proportions of Si,Al,and Fe elements increased to varying degrees.The X-ray diffractometer?XRD?was used to semi-quantitative analyze the mineral composition of the bare and rhizosphere slag,the results showed that some of the minerals in the rhizosphere slag had a significant change compared to the bare slag,e.g.,the relative contents of primary minerals such as quartz,cristobalite,hornblende,and feldspar decreased,while the relative contents of secondary minerals such as illite,gypsum,and calcite increased in the rhizosphere slag.The secondary minerals formed by the plants involved in the waste slag have a positive effect on reducing the availability of heavy metals in the waste slag.?7?Revegetation of four dominant plants significantly changed the composition of aggregates and their distribution of heavy metals in the waste slag.The four dominant plants growth promote the biological weathering of large-size waste slag to small-size waste slag,which is showed that significantly reduced the mass percentage of large-size waste slag?1⁵ mm?,and significantly increased the mass percentage of small-size waste slag?0.5¹ mm,0.25⁰.5 mm,<0.25 mm?,and there is obvious agglomeration in the small-size slag.The heavy metals in the rhizosphere slags and bare slags were mainly distributed in the small particle size slag,and the heavy metal elements in the large particle size slag aggregates and small particle size slag aggregates were the statuses of deficient?enrichment factor is less than 1?and enrichment?enrichment factor is greater than 1?,respectively,except for Zn element in the slag aggregate with particle size of 2⁵ mm was enrichment status.The mass load of heavy metals in the fine particle size of waste slag?<0.25 mm and 0.25¹ mm?were the highest.The proportion of acid-soluble heavy metals?e.g.,Cu,Pb,Zn,Cd?in the different particle sizes aggregates of rhizosphere slag were overall lower than that of the bare slag,and the proportion of acid-soluble heavy metals?Cu,Pb,Zn,Cd?in large particle size aggregates were higher than that of fine particle size aggregates.With the particle size of the waste slag decreases,the proportion of the reducible,oxidizable,and residual fractions of Cu,Pb,Zn,and Cd increases.The formation of aggregates in the waste slag by the four dominant plants plays an important role in reducing the bioavailability of heavy metals in the waste slag,the processes included that the heavy metals in the waste slag were adsorbed and retained on the surface of the formed highly weathered fine-grained waste slag?organic-mineral colloidal particles?,and the spatial heterogeneity of the heavy metal forms in the aggregates of the waste slag was changed.?8?Revegetation of four dominant plants significantly changed the composition of particulate organic matter?POM?and its distribution of heavy metals.The organic matter input into the waste slag was decomposed to different particle size POM,and the mass percentage of fine particle sizes POM?0.05¹ mm?in the waste slag were higher than that of the coarse particle sizes POM?>1 mm?.The content,enrichment factor and mass load of heavy metals in POM gradually increase with the decrease of POM particle sizes,and the content,enrichment factor and mass load of heavy metals in fine particle size POM?0.05¹ mm?are generally higher than that of the coarse particles POM?>1 mm?.The content,enrichment factor and mass load of heavy metals in POM with a particle size of 0.05⁰.25 mm were the highest.The mass loadings of Cu,Pb,Zn and Cd in different particle sizes POM were different,which is represented as Cu>Cd>Zn>Pb.The characterization of morphology and functional groups structure of POM with different particle sizes based on scanning electron microscopy and Fourier transform infrared spectroscopy indicated that the surface of the coarse particle size POM is smoother and smoother,and the surface of the fine particle size POM is rougher and has a larger specific surface area.The main functional groups in POM were-COOH,-OH in carbohydrates?e.g.,cellulose,hemicellulose,and monosaccharides,etc.?,and carboxylic acids,phenols,alcohols,and fatty acids,etc.,-CH₃ and-CH₂ in aliphatics,C=O in aldehydes and ketones,C=C in aryl groups,C=O and C-OH in carboxyl groups,or Si-O.The enrichment of heavy metals in the different particle sizes POM plays an important role in reducing the bioavailability of Cu,Pb,Zn and Cd in the waste slag.The fine particle sizes POM had the greatest effect on the enrichment of heavy metals in the waste slag,this is due to the fine particle sizes POM characterized by a large specific surface area,a relatively high content of organic carbon,more active sites?e.g.,carboxyl,hydroxyl,phenolic hydroxyl,etc.?,and some intermediate decomposition products with higher biological activity.In conclusion,revegetation of four dominant plants significantly influenced the biogeochemical processes of heavy metals in the waste slag by improved the physicochemical,biochemical,and microbiological properties of the waste slag and uptake by plants.These processes consist of two parts,on the one hand,revegetation of four dominant plants reduced the bioavailability of heavy metals by the synergistic effects of the root activities?e.g.,root exudates and rhizosphere microbial activity?and the returned of plant litter significantly improved the rhizosphere microenvironment conditions?e.g.,the increase in the accumulation of nutrients and organic matter and the diversity of heterotrophic microbial community in the waste slag?,changed the occurrence forms of heavy metals,and uptake by plants.On the other hand,the combined actions of root exudates-heterotrophic microorganism-plant litter promote the weathering of waste slag to different particle sizes aggregates and POMs,and improve the aggregate structures of the slag.The formed fine particle sizes slag aggregates and POMs and secondary minerals during the processes of waste slag bio-weathering into the soil with the combined effects of plants and microbes,reduced the bioavailability of Cu,Pb,Zn and Cd in the waste slag by the effects of adsorption,complexation,complexation,and precipitation,and change the spatial differentiation of the occurrence forms of heavy metals in the aggregates.These combined effects significantly affect the biogeochemical processes of heavy metals in revegetated waste slag and finally achieve the phytostabilization of heavy metals.The results of this study can provide reference for the phytoremediation of waste slag with similar properties.