Characteristics and diversity of microbial communities in lead-zinc tailings under heavy metal stress in north-west China.

High-throughput 16S rRNA and 18S rRNA sequencing were performed to study the changes of soil microbial diversity and community structure under different heavy metal pollution levels in Chengxian lead-zinc mining area, Gansu Province. In this study, we characterized the main physicochemical properties, multiple heavy metal pollution, and microbial community structure of the soil in the tailings. The results show that the soil near the tailings pond was alkaline, barren and the heavy metals were seriously polluted. The microbial diversity and richness of S1 and S2 sites were significantly lower than that of CK2 site (P < 0·05), indicating that the heavy metal pollution could change the physicochemical properties and microbial community structure in soil. Among 97 identified core operating taxa of fungal communities, Ascomycota, Teguta and Basidiomycota were dominant at the phylum level, while among 1523 identified core operating taxa of bacterial communities, Actinomycota was dominant at the phylum level. In addition, the redundancy analysis and Spearman correlation analysis showed that the physicochemical properties and the heavy metal concentration had significant effects on the composition and distribution of soil microbial community. The basic characteristics of soil physicochemical properties, multiple heavy metal pollution and microbial community structure in the tailings were revealed, hoping to provide a basis for ecological rehabilitation of tailings by revealing the variance rule of microbial community diversity in the future.

Sustainable and efficient stabilization/solidification of Pb, Cr, and Cd in lead-zinc tailings by using highly reactive pozzolanic solid waste.

Lead-zinc tailings (LZTs) are industrial by-products containing a large number of heavy metals that seriously harm the ecological environment and human health. This study was performed to propose a sustainable and efficient method for immobilizing Pb, Cr, and Cd in LZTs by using solid waste. To better assess the immobilization performance and mechanism, the leaching toxicity, fraction distribution, unconfined compressive strength, environmental risk assessment, and hydration products were explored. The LZTs were mixed and molded with different constituents of ground granulated blast furnace slag (GGBFS) and rice husk ashes (RHAs) at different curing temperatures. Results suggest that ≥99% of the Pb, Cr, and Cd were immobilized mainly in the form of residual fractions in the LZTs. The amounts of Pb, Cr, and Cd in the bioavailable fractions notably decreased by approximately 99.83%, 99.58%, and 97.05%, respectively. After stabilization/solidification (S/S) disposal, Pb, Cr, and Cd showed low to even no risk. The RHAs were effective to stabilize Pb, and GGBFS was effective to stabilize Cr. However, both materials showed almost equal effects to Cd. Ettringite, C-S-H gel, and portlandite were the main hydration products to immobilize Pb, Cr, and Cd, and these hydration products provided a source of strength. Honey-comb or reticular network C-S-H gel possessed higher specific surface area, higher pore volume, and bigger pore size than the other materials. The proposed method could explain the sustainability and efficiency of the S/S of Pb, Cr, and Cd in LZTs by using RHAs. This study opens up new perspectives for disposing heavy metal by using accessible agricultural solid waste (i.e., RHAs) in rural areas, and the solidified block shows certain economic benefits.

Effects of peat on plant growth and lead and zinc phytostabilization from lead-zinc mine tailing in southern China: Screening plant species resisting and accumulating metals.

In order to investigate the toxicity-resistance of eighteen Chinese native plants in lead (Pb)-zinc (Zn) mine tailings, we categorized their resistance to Pb and Zn, and tested their potential for phytoremediation effectiveness of Pb and Zn. Fourteen woody plant species belonging to 12 families, and 4 herbaceous species belonging to 4 families, were grown in pots with mixtures of 100% tailing +0% peat (CK), 90% tailing +10% peat (A1), and 80% tailing + 20% peat (A2), respectively. Plant height and biomass, chlorophyll content, and Pb and Zn contents of non-rhizosphere spoil mixtures and plant tissues were measured. Fifteen of the plants grew in all three spoil mixtures. Both A1 and A2 had higher plant height and biomass increment and chlorophyll contents than CK. The content of Pb and Zn in plant shoots and roots was CK > A1 > A2. The value of BCF less than 0.1, compared to 1, was a more precise classification basis for plants excluding metals. Screening for Pb and Zn resistant plants and their bioremediation potential produced the following candidate species: Sapium sebiferum, Salix matsudana, Hibiscus cannabinus, Corchorus capsularis, Ricinus communis, and Populus nigra. These species were highly Pb and Zn tolerant species, with notable growth characteristics and capacities to bioaccumulate Pb and Zn from the mine tailings. Compared to CK, the removal of Pb and Zn from non-rhizosphere spoil increased by an average of 9.64% and 9.6%, respectively in A1, but decreased in A2. The results indicated candidate species and 10% peat addition in the tailing were significant in phytoremediation of Pb and Zn regarding environmental safety.

The influences of arbuscular mycorrhizal fungus on phytostabilization of lead/zinc tailings using four plant species.

A greenhouse experiment was conducted to investigate the effects of the arbuscular mycorrhizal fungus Funneliformis mosseae on three parameters: Pb, Zn, Cu and Cd accumulation, translocation and plant growth in perennial ryegrass (Lolium perenne), tall fescue (Festuca arundinacea), showy stonecrop (Hylotelephium spectabile) and Purple Heart (Tradescantia pallida). The purpose of this work is to enhance site-specific phytostabilization of lead/zinc mine tailings using native plant species. The results showed that mycorrhizal fungi inoculation significantly increased plant biomass of F. arundinacea, H. spectabile and T. pallida. The Pb, Zn, Cu and Cd concentrations in roots were higher than those in shoots both with and without mycorrhizae, with the exception of the Zn concentration in H. spectabile. Mycorrhizae generally increased metal concentrations in roots and decreased metal concentrations in shoots of L. perenne and F. arundinacea. In addition, it was found that the majority of the bioconcentration and translocation factors were lower than 1 and mycorrhizal fungi inoculation further reduced these values. These results suggest that appropriate plant species inoculated with mycorrhiza might be a potential approach to revegetating mine tailing sites and that H. spectabile is an appropriate plant for phytostabilization of Pb/Zn tailings in northern China due to its higher biomass production and lower metal accumulation in shoots.

Optimization of Cementitious Material with Thermal-Activated Lead-Zinc Tailings Based on Response Surface Methodology.

The accumulation of lead-zinc tailings will cause a series of problems, including geological disasters and environmental pollution. Efficient secondary utilization of lead-zinc tailings is crucial. In this study, the activity of lead-zinc tailings was stimulated by thermal activation. The optimal thermal activation parameters are a thermal activation temperature of 900 °C and a holding time of 30 min. Based on the response surface methodology, the effect of raw materials content on cementitious material strength was analyzed, and the relational model between cementitious material strength and experimental variables was established. The results show that the sensitivity order of cementitious material strength at 28 days curing age is sand/cement ratio > water/cement ratio > fly ash content > tailing content. According to the relational model, the optimal materials ratio is as follows: tailing/fly ash/cement = 28.99%:14.58%:56.43%, and the sand/binder ratio and water/binder ratio are 1:1 and 0.47, respectively. The corresponding cost is CNY 290.965 per ton, which is the lowest. The strength of cementitious material with these parameters can reach 20 MPa, which meets the requirements of "Technical specification for application of solid waste cementitious material (T/CECS 689-2020)".

Analysis of unconfined compressive strength and environmental impact of MICP-treated lead-zinc tailings sand instead of sand as embankment material.

Tailings can be used as embankment materials instead of sand. However, they contain large amounts of heavy metal pollutants, which can lead to groundwater pollution. In this study, (lead-zinc) Pb-Zn tailings with five particle sizes and Sporosarcina pasteurii were used as test materials. Combined with the unconfined compressive strength (UCS) and leaching of heavy metal pollutants from Pb-Zn tailings, the feasibility of applying microbial induced carbonate precipitation (MICP)-treated Pb-Zn tailings to embankment materials was analysed from the perspective of strength and environmental performance. The results showed that the UCS and carbonate content of the specimens made of Pb-Zn tailings treated using MICP decreased with a decrease in the number of Pb-Zn tailing particles. The pH value of the leaching solution after MICP treatment of Pb-Zn tailings sand was stable at 7.83-8.03, and the fixation rate of metal ions was 90.28 %-100 %. FTIR, X-ray diffraction, scanning electron microscopy, and energy-dispersive spectroscopy tests showed that after the Pb-Zn tailings with particle sizes less than 100 mesh were treated using MICP, the number of carbonate crystals, crystal uniformity, and crystal overlap on the surface of the sample were considerably higher than those of the tailings with particle sizes greater than 250 mesh. The compressive strength and environmental performance of Pb-Zn tailings with particle sizes less than 100 mesh treated using MICP are good, and they are more suitable for embankment materials.

Fabrication of Lead-Zinc Tailings Sintered Brick and Its Effect Factors Based on an Orthogonal Experiment.

The existence of lead-zinc tailings threatens the social and ecological environment. The recycling of lead-zinc tailings is important for the all-round green transformation of economic society. In this study, the possibility of fabricating sintered ordinary bricks with lead-zinc tailings was studied based on orthogonal experimentation, and the phase composition and micromorphology of sintered products were analyzed by X-ray diffraction (XRD) and scanning electron microscope (SEM). With lead-zinc tailings as the main material, and clay and fly ash as additives, the effect of clay content, forming pressure, sintering temperature, and holding time on physical properties of sintered bricks was analyzed. The results show that clay content and sintering temperature have a major effect on compressive strength, while sintering temperature and holding time play an important role in water absorption. During sintering, mica, chlorite, and other components in lead-zinc tailings are decomposed to form albite, hematite, maghemite, and anhydrite, which play a role in the strength of bricks. The optimal process parameters were found to be a ratio of lead-zinc tailings:clay:fly ash = 6:3:1, forming pressure of 20 MPa, firing temperature of 1080 °C, and holding time of 60 min. The corresponding compressive strength and water absorption were 34.94 MPa and 16.02%, which meets the Chinese sintered ordinary bricks standard (GB/T 5101-2017).

Effects of active silicon amendment on Pb(II)/Cd(II) adsorption: Performance evaluation and mechanism.

In this study, a high-Si (Si) adsorbent (APR@Sam) was prepared by acid leaching slag (APR) from lead-zinc (Pb-Zn) tailings based on high-temperature alkali melting technology. The synthesized Si-based materials were applied to aqueous solutions contaminated with Pb and cadmium (Cd) to investigate the crucial role of active Si in sequestering heavy metals. The adsorption capacities of APR@Sam and the Si-depleted material (APR@Sam-NSi) were studied under different pH and temperature conditions. The results showed that as the pH increased from 3 to 7, the adsorption capacity increased, the active Si content in the solution increased by 63 %, and the maximum pH of the solution after adsorption was 7.12. After the removal of active Si, the Pb (II) and Cd (II) adsorption capacities of APR@Sam decreased by 45 % and 11.96 %, respectively. OH- promoted the release of Si into the solution, enhancing the material's adsorption efficiency. The reaction mechanism is mainly attributed to surface complexation guided by Si-O and Si-O-Si bonds, metal cation exchange, and bidentate coordination. The results indicated that the Si component is critical for the removal of Pb (II) and Cd (II) by APR@Sam and provide valuable insights into resource recovery strategies from leaching residues.

Geochemical properties, heavy metals and soil microbial community during revegetation process in a production Pb-Zn tailings.

Lead-zinc (Pb-Zn) tailings pose a significant environmental threat from heavy metals (HMs) contamination. Revegetation is considered as a green path for HM remediation. However, the interplay between HM transport processes and soil microbial community in Pb-Zn tailings (especially those in production) remain unclear. This study investigated the spatial distribution of HMs as well as the crucial roles of the soil microbial community (i.e., structure, richness, and diversity) during a three-year revegetation of production Pb-Zn tailings in northern Guangdong province, China. Prolonged tailings stockpiling exacerbated Pb contamination, elevating concentrations (from 10.11 to 11.53 g/kg) in long-term weathering. However, revegetation effectively alleviated Pb, reducing its concentrations of 9.81 g/kg. Through 16 S rRNA gene amplicon sequencing, the dominant genera shifted from Weissella (44%) to Thiobacillus (17%) and then to Pseudomonas (comprising 44% of the sequences) during the revegetation process. The structural equation model suggested that Pseudomonas, with its potential to transform bioavailable Pb into a more stable form, emerged as a potential Pb remediator. This study provides essential evidence of HMs contamination and microbial community dynamics during Pb-Zn tailings revegetation, contributing to the development of sustainable microbial technologies for tailings management.

Risk Assessment of Heavy Metal in Farmlands and Crops Near Pb-Zn Mine Tailing Ponds in Niujiaotang, China.

To accurately evaluate the pollution and risk of heavy metals in crops and farmlands near mines, we determined the contents of Cr, Ni, Cu, As, Cd, Pb, and Zn in 10 farmland soil sampling sites and six crops (pak choi, rice, spring onion, radish, Chinese cabbage, Chrysanthemum coronarium) in an area near the Niujiaotang Pb-Zn mine in Duyun City, China. Four evaluation methods were compared, including the potential ecological hazard index, Nemeiro comprehensive pollution assessment, risk assessment code, and the ratio of secondary phase to primary phase methods. The average concentration of As, Cd, Pb, and Zn exceeded the soil environmental background levels in Niujiaotang and Guizhou Province. Cd exceeded the standard substantially, and Zn pollution accumulation was the most evident. Heavy metal contamination of crops was in the order pak choi > Chinese cabbage > spring onion > paddy > radish > Chrysanthemum coronarium, whereas heavy metal concentration in crops were in the order Zn > As > Cr > Cd > Ni > Pb. The levels of all heavy metals except Cu exceeded Chinese food hygiene standards. Carcinogenic and non-carcinogenic chemicals in crops present significant risks to adults and children. Risk evaluation considering the morphological contents of heavy metals rather than their total concentration was more accurate for environmental quality assessment of agricultural soils. Samples should be collected at different times to study the spatial and temporal distribution, and further studies on the migration transformation of heavy metals between the tailings pond-soil-crop should be conducted.

Stabilization of Pb(II) in wastewater and tailings by commercial bacteria through microbially induced phosphate precipitation (MIPP).

Microbially induced phosphate precipitation (MIPP) is an effective and eco-friendly method for Pb(II) stabilization. The phosphate-solubilizing microorganisms (PSM) for MIPP are commonly isolated from Pb(II)-contaminated sites through a series of intricate and time-consuming enrichment and purification processes. This research used ready-made commercial bacteria to develop a simple MIPP process. Bacillus subtilis (BS, CCTCC AB 98002) was selected from two commercial PSM strains owing to more effective Pb(II) removal. Compared to the most isolated microorganisms, BS released more than twice as much inorganic phosphorus (Pi) as well as had a high-level Pb(II) tolerance. BS could remove >99% of Pb(II) from 500 mg/L Pb(II)-containing water at the optimal 0.05 M sodium glycerophosphate (SGP), pH 7-9, and ≤0.03 M MgCl2, outperforming most isolated microorganisms. In addition, BS could mitigate the contamination risk of the lead­zinc tailings, by reducing the readily leachable Pb(II) concentration from 0.81 mg/L (over the regulatory limit of 0.1 mg/L) to 0.00042 mg/L. The unstable Pb(II) in the solution and tailings was ultimately stabilized to Pb5(PO4)3Cl after the SGP phosphorlysis and phosphate precipitation processes. In conclusion, commercial BS is a superior alternative to isolated microorganisms for MIPP on Pb(II) stabilization. The simple-processed and high-effective BS-based MIPP provides the MIPP method a new insight for widespread implementation in the remediation of heavy metals-containing wastewater, soil, and waste.

Solidification/stabilization of heavy metals and its efficiency in lead-zinc tailings using different chemical agents.

Lead-zinc tailings are generated during the mining process which is considered as hazardous solid waste due to its high heavy metal content and leachability in the natural state. At present, the most effective technology for disposing heavy metals in solid wastes is the solidification/stabilization (S/S) technique. In terms of S/S technology, chemical stabilization is one of the most potential and practical method. This paper aims to investigate the S/S property of four typical chemical agents (Na2S, NaH2PO4, TMT and Na2EDTA) on the heavy metals in lead-zinc tailings. The results reveal that the heavy metals lead and zinc in tailings are stabilized more effectively by using chelating agents TMT than by using inorganic chemical agents Na2S and NaH2PO4. When the dosage of TMT reaches 4%, the leaching concentration of lead and zinc is 0.18 and 14.60 mg/L according to toxicity characteristic leaching procedure (TCLP), and the stabilization efficiency of lead and zinc is 99.31% and 80.92%, respectively, while the leaching concentration of lead and zinc just drops to 0.41 and 16.00 mg/L with addition of 10% NaH2PO4. Furthermore, the leaching concentration of heavy metal lead in tailings treated by 4% Na2EDTA increases to 53.44 mg/L which far exceeds the standard of pollution control. Therefore, considering stabilization efficiency and dosage, TMT is the preferred agent for solidifying heavy metals in lead-zinc tailings.

Comparison of Phytoremediation Potential of Nerium indicum with Inorganic Modifier Calcium Carbonate and Organic Modifier Mushroom Residue to Lead-Zinc Tailings.

At present, the application of phytoremediation technology in the ecological remediation of heavy metal tailings is receiving more and more attention. In this study, the physiological and biochemical response and tolerance mechanism of woody plant Nerium indicum to Pb and Zn under different proportions of inorganic modifier calcium carbonate (C1: 5%, C2: 10%, C3: 15%) and organic modifier mushroom residue (M1: 10%, M2: 20%, M3: 30%) was compared. The results showed that the pH value has a trend of C group > M group > CK group and organic matter has a trend of M group > CK group > C group. Phosphatase activity and catalase activity has a trend of M group > C group > CK group, but catalase was more vulnerable to the calcium carbonate concentration. Both modifiers can promote the transformation of Pb, Zn, Cu, and Cd in tailings to more stable organic bound and residual states. However, the stabilization effect of mushroom residue is better, and its stability is Pb, Zn > Cd, Cu. Both modifiers can increase the biomass of Nerium indicum and the modification effect of mushroom residue is better than calcium carbonate. Pb/Zn content and accumulation in Nerium indicum organs showed root > stem > leaf in all groups. Compared with the CK group, the enrichment coefficient of Pb/Zn in C1 and M1 groups decreased, while the translocation factor of Pb/Zn in C1 and M1 groups increased. With the increase in modifier concentration, the enrichment coefficient increases about 1.75~52.94%, but the translocation factor decreases rapidly (20.01~64.46%). Clearly, both the calcium carbonate and mushroom residue amendment could promote the growth ability of Nerium indicum in lead−zinc tailings and strengthen the phytoremediation potential.

Analysis of Influencing Factors of Cementitious Material Properties of Lead-Zinc Tailings Based on Orthogonal Tests.

At present, the treatment of tailings is mostly carried out in the form of stacking in tailings ponds, resulting in a huge waste of mineral resources and a major threat to the environment and ecology. Using tailings instead of a part of the cement to make cementitious materials is an effective way to reduce the accumulation of tailings. In this paper, lead-zinc tailings-based cementitious materials were prepared by using lead-zinc tailings, fly ash, and ordinary Portland cement, and the effects of four factors on the mechanical properties of lead-zinc tailings, as well as fly ash content, cement content, and water-binder ratio were studied by orthogonal experiments. The corresponding relationship between the factors and the properties of cementitious materials was determined, and the optimization and prediction of the raw material ratio of lead-zinc tailings-based cementitious materials were realized. The test showed the ratio of raw materials to be at the lowest price ratio. Synchronously the ratio that meets the minimum strength requirements was predicted. When the proportion of fly ash:lead and zinc tailings:cement = 30:40:30 and the water-binder ratio was 0.4, the predicted compressive strength of the prepared cementitious material achieved 22.281 MPa, which meets the strength requirements, while the total content of lead-zinc tailings and fly ash was the highest at this time.

Preparation and adsorption characteristics for heavy metals of active silicon adsorbent from leaching residue of lead-zinc tailings.

To comprehensively reuse the leaching residue obtained from lead-zinc tailings, an active silicon adsorbent (ASA) was prepared from leaching residue and studied as an adsorbent for copper(II), lead(II), zinc(II), and cadmium(II) in this paper. The ASA was prepared by roasting the leaching residue with either a Na2CO3/residue ratio of 0.6:1 at 700 °C for 1 h or a CaCO3/residue ratio of 0.8:1 at 800 °C for 1 h. Under these conditions, the available SiO2 content of the ASA was more than 20%. The adsorption behaviors of the metal ions onto the ASA were investigated and the Langmuir, Freundlich, and Dubinin-Radushkevich isotherm models were used to analyze the adsorption isotherm. The result showed that the maximum adsorption capacities of copper(II), lead(II), cadmium(II), and zinc(II) calculated by the Langmuir model were 3.40, 2.83, 0.66, and 0.62 mmol g-1, respectively. The FT-IR spectra of the ASA and the mean free adsorption energies indicated that ion exchange was the mechanism of copper(II), lead(II), and cadmium(II) adsorption and that chemical reaction was the mechanism of zinc(II) adsorption. These results provide a method for reusing the leaching residue obtained from lead-zinc tailings and show that the ASA is an effective adsorbent for heavy metal pollution remediation.

[Carbon Metabolism Characteristics of the Karst Soil Microbial Community for Pb-Zn Mine Tailings].

BIOLOG and 18S rRNA PCR-DGGE methods were used to estimate the characteristics of carbon source metabolism of the soil microbial community as well as the relationship between soil fungi and soil organic carbon in different karst land use types (corn field, citrus field, and paddy field) contaminated by Pb-Zn tailings at Sidi Village, Yangshuo, Guangxi Zhuang Autonomous Region, SW China. It was found that the concentrations of Pb, Zn, Cu, and Cd were highest in the paddy field, followed by the corn field, citrus field, and control group (dry field). In addition, the geo-accumulation index indicated that the heavy metal pollution at this area was caused by Pb and Cd and that Cd was the key environmental risk factor. With the Pb-Zn tailings, there was low microbial biomass carbon, biological entropy, and microbial carbon source metabolism in the soil samples. The highest soil organic carbon and heavy metal concentrations were found in the paddy field, followed by the corn field and citrus field. According to the DDGE results, it was found that Pycnoporus sp. ZW02.30 was found in control group soils, Fusarium solani and Fusarium oxysporum were found in the corn field and citrus field, and Penicillium decumbens was found in citrus field. These were involved in the degradation of sugars, such as starch, cellulose, hemicellulose, and lignin. However, the fungi were not found in the paddy field. Based on the low functional diversity of the soil microbial community and biological entropy as well as the high soil organic carbon concentration in the paddy field, it was concluded that soil microbial carbohydrate metabolism and the mineralization rate of the soil organic carbon was controlled by soil microorganisms, especially fungi, in different land use soils in the karst area contaminated by Pb-Zn tailings.

[Subcellular Distribution and Chemical Forms of Heavy Metals in Three Types of Compositae Plants from Lead-Zinc Tailings Area].

Field investigation on the content of heavy metals in soils and three types of widely distributing compositae plants(Artemisia lavandulaefolia, Ageratum conyzoides L., Crassocephalum crepidioides) in lead-zinc tailings farmland of Yangshuo, Guangxi Zhuang Autonomous Region was carried out, and the differential centrifugation technique and sequential chemical extraction method were used to study the subcellular distribution and chemical forms of heavy metals in these plants. The results indicated that the soil in the tailings farmland was highly contaminated by Cd, Pb and Zn, and their concentrations were 37.7, 5.7 and 8.9 times higher than their respective values of national standard for soil environment quality(GradeⅡ). The contents of Cd, Pb and Zn in the analyzed plants exceeded the normal ranges. Ageratum conyzoides L. and Crassocephalum crepidioides showed strong capability in tolerance, accumulation and transport of Cd, and they could be used as pioneer plants for Cd-phytoremediation in study area and some related areas. In addition, Cd concentrations in the stem and leave of Crassocephalum crepidioides were 159.6 mg·kg-1 and 219.5 mg·kg-1, respectively, which exceeded the threshold of Cd hyperaccumulator. Thus, Crassocephalum crepidioides can be regarded as a Cd-hyperaccumulator. Majority of total Cd, Pb and Zn were found in soluble fraction and cell wall in the plants, while only a small quantity were distributed in organelles. As for chemical forms distribution, Cd, Zn and Pb predominated NaCl-, HAC-, and HCl-extractable forms both in roots and shoots of the plants. Therefore, cell wall binding, vacuolar compartmentalization and distribution mainly in lower active chemical forms were supposed to be the main mechanisms for tolerance to heavy metals in the study plants. A difference of Cd subcellular distribution and its chemical forms in the three Compositae plants was observed. Compared with the shoot of Artemisia lavandulaefolia, more Cd was located in the vacuolus and cellular soluble parts in Ageratum conyzoides L. and Crassocephalum crepidioides. Moreover, the proportion of active chemical Cd in the shoots was lower than that in the roots. These results showed that the subcellular distribution and chemical forms of Cd were related to the plant species,and also indicated Ageratum conyzoides L. and Crassocephalum crepidioides were likely to possess a higher tolerance and accumulation of Cd.