Risk Levels Of Heavy Metals Research Articles

Heavy metal dynamics in riverine mangrove systems: A case study on content, migration, and enrichment in surface sediments, pore water, and plants in Zhanjiang, China

Mangroves serve a crucial role as metal accumulators in tropical and subtropical marine ecosystems, particularly in riverine mangroves, which frequently interact with terrestrial sources. In this study, we focused on the Gaoqiao and Jiuzhou Rivers within the Zhanjiang mangrove forest in Guangdong, China, and collected leaves and surface sediments from the dominant mangrove plant, Aegiceras corniculatum, near the riverbanks. We focused on seven heavy metals (Cr, Cu, Zn, As, Cd, Pb, and Hg) in mangrove leaves, surface sediments, and pore water due to their environmental significance and frequent occurrence in mangrove ecosystems. We employed multivariate statistical methods and pollution indicators to assess the potential sources and risk levels of heavy metals in these sediments. Our results reveal that the concentrations of the seven heavy metals in the sediments of the Gaoqiao and Jiuzhou Rivers varied significantly, ranging from 0.03 mg/kg to 100.00 mg/kg. Cd posed the highest ecological risk, followed by Hg and As. The comprehensive potential ecological risk in the Gaoqiao River was lower than that in the Jiuzhou River, likely due to the distribution of industrial enterprises (such as printing and cement plants) in the upper reaches of the Jiuzhou River. Additionally, the heavy metal content in the leaves and pore water of A. corniculatum ranged from 0.01 to 51.58 mg/kg and 0.001 to 133.70 μg/L, respectively. There was a significant correlation between the heavy metal concentrations in the leaves and the pore water of A. corniculatum. Notably, the leaves of A. corniculatum exhibited a remarkable Hg-enrichment capability, highlighting its potential as a mercury accumulator. Most heavy metals in A. corniculatum leaves, pore water, and sediment were concentrated in the middle and upper reaches of the river, primarily due to anthropogenic terrestrial inputs from residential production activities upstream. Consequently, heavy metal pollution in riverine mangroves is primarily associated with human activities such as aquaculture, agricultural planting, and industrial production.

Preparation of eco-friendly and high-strength ceramsite by granite scraps, granite fine mud, and phosphogypsum: Response surface methodology optimization, environmental safety assessment

Granite scraps (GS), Granite fine mud (GFM), and phosphogypsum (PG) are solid wastes containing harmful substances produced during the processing of granite and the production of phosphate fertilizer. Their resourceful and harmless utilization holds great significance in reducing environmental pollution. This study explores the preparation of eco-friendly and high-strength ceramsite using GS as the primary material, GFM as the binder, and PG as the regulator. The performance of ceramsite was studied by conducting single-factor experiments to examine the impact of the GS, GFM, and PG mass ratios. The Box-Behnken response surface methodology was utilized to optimize the effect of sintering conditions on the strength of ceramsite. The results suggested that the properties of the ceramsite are affected by the material ratios. The sintering temperature and keeping time notably influence the strength of ceramsite. Under optimal preparation conditions, the ceramsite achieved a bulk density of 1085.40kg/m3, apparent density of 2253.63kg/m3, 1-h water absorption of 0.13%, hydrochloric acid soluble rate of 0.016%, and compressive strength of 34.52MPa. Importantly, high-temperature sintering plays an essential role in fixing heavy metals and reducing the ecological risk level of heavy metals in ceramsite, which ensures excellent environmental performance and application prospects for ceramsite.

Impact of elevated CO2 on microbial communities and functions in riparian sediments: Role of pollution levels in modulating effects

The impact of elevated CO2 levels on microorganisms is a focal point in studying the environmental effects of global climate change. A growing number of studies have demonstrated the importance of the direct effects of elevated CO2 on microorganisms, which are confounded by indirect effects that are not easily identified. Riparian zones have become key factor in identifying the environmental effects of global climate change because of their special location. However, the direct effects of elevated CO2 levels on microbial activity and function in riparian zone sediments remain unclear. In this study, three riparian sediments with different pollution risk levels of heavy metals and nutrients were selected to explore the direct response of microbial communities and functions to elevated CO2 excluding plants. The results showed that the short-term effects of elevated CO2 did not change the diversity of the bacterial and fungal communities, but altered the composition of their communities. Additionally, differences were observed in the responses of microbial functions to elevated CO2 levels among the three regions. Elevated CO2 promoted the activities of nitrification and denitrification enzymes and led to significant increases in N2O release in the three sediments, with the greatest increase of 76.09 % observed in the Yuyangshan Bay (YYS). Microbial carbon metabolism was promoted by elevated CO2 in YYS but was significantly inhibited by elevated CO2 in Gonghu Bay and Meiliang Bay. Moreover, TOC, TN, and Pb contents were identified as key factors contributing to the different microbial responses to elevated CO2 in sediments with different heavy metal and nutrient pollution. In conclusion, this study provides in-depth insights into the responses of bacteria and fungi in polluted riparian sediments to elevated CO2, which helps elucidate the complex interactions between microbial activity and environmental stressors.

Pollution characteristics, distribution and risk level of heavy metals in sediments of the Yangtze River estuary

Pollution characteristics, distribution, risk and sources of 7 heavy metals in sediments of Yangtze River Estuary were investigated. Total concentration ranges of As, Cr, Cu, Cd, Pb, Zn and Ni were [0, 16.5], [1.48, 51.3], [2.66, 318], [0, 0.99], [35.6, 992], [8, 91.3] and [1.88, 108] mg/kg, respectively. Based on the potential ecological risk index and Geoaccumulation index, it was determined that Pb is the most polluted heavy metal. According to class I standard of “Marine sediment quality” of China, mean baseline levels multiples were Pb (8.34) > Cu (0.57) > Cr (0.37) > Zn (0.355) > Ni (0.352) > As (0.28) > Cd (0.00). The study also found the heavy metal content of Pb is the most serious, but most of the Pb content comes from the residual state, which has minimal impact on the environment. The East Nanhui Shoal was identified as the most polluted sub-area in terms of Pb pollution, followed by other specific locations. Considering the pollution level and transport costs, the study concluded that dredge soils of the Yangtze River Estuary Deepwater Channel are not suitable for the restoration of East Hengsha Shoal.

Solidification of municipal solid waste incineration fly ash with alkali-activated technology

Alkali-activation is effective municipal solid waste incineration fly ash (MSWIFA) solidification/stabilization (S/S) technology. Percolation and migration of heavy metals in MSWIFA S/S matrix is a complicated and slow process. Here, several alkali-activated MSWIFA samples are selected to comparatively investigate the long-term leaching behavior and environmental availability of Pb, Zn and Cd when exposed in different erosion environment. Acid environment posed the more serious destroy to MSWIFA S/S matrices. RAC demonstrated that potential risk level of heavy metals is higher in acid rain environment, and Cd, Zn showed the prominent risk. When soaked in acid rain solution, the surface of alkali-activated MSWIFA S/S matrices was cracked seriously and a large number of hardened slurry peeled off. However, more stable structural properties and lower heavy metal leachability can be found in alkali-activated MSWIFA/aluminosilicate. The immobilization efficiency of Pb, Zn and Cd were all above 99.0%. Microstructure and morphology results indicated that there is new phase Friedel's salts generated and much more amorphous substance such as C-(A)-S-H gel with incorporation of aluminosilicate, which all contributed much to the formation of compact and stable microstructure, then significantly facilitated the encapsulation of heavy metal. These findings will provide theoretical basis and new insight for resource utilization and security landfill of MSWIFA.

Distribution Characteristics, Ecological Risks, and Source Identification of Heavy Metals in Cultivated Land Under Non-grain Production

The purpose of this study was to explore the distribution characteristics and potential ecological risks of soil heavy metal pollution of cultivated land under non-grain production. Taking a typical area around Hangzhou Bay as an example, 254 topsoil samples (0-20 cm) of cultivated land were collected, and the content of eight soil heavy metals in four different cultivated land use types, including grain, seedlings, vegetables, and fruits, was analyzed. The ecological risk was assessed by the Nemerow pollution index and the potential ecological risk index, and the PMF model was used to identify the source of soil heavy metals in the study area. The results showed that the average contents of As, Cr, Cd, Cu, Hg, Ni, and Zn were all higher than the soil background value, except for Pb, but were lower than the national risk control standard for soil contamination of agricultural land. Non-grain production had a significant impact on the accumulation of heavy metals in soil. The content of heavy metals in nurseries and orchards was relatively high, followed by vegetable fields, and the lowest in grain fields. The Nemerow index showed that the cultivated land in the study area was in a light pollution level as a whole, and the single-factor pollution risks of Hg, Cd, and As were relatively high. The potential ecological risk levels of heavy metals in different cultivated land use types were:nurseries>orchards>vegetable fields>grain fields. The PMF results showed that the main sources of soil heavy metals in the study area were mixed sources of industrial emissions (36.8%), natural parent material sources (28.4%), atmospheric deposition sources (21.4%), and agricultural activity sources (13.4%). In conclusion, the increase in the application of chemical fertilizers and pesticides was the direct reason for the increase in soil heavy metal content caused by non-grain production of the cultivated land, whereas the industrial and mining emissions and atmospheric deposition accelerated the increase in soil heavy metal content in the study area.

Current Situation of Agricultural Soil Pollution in Jiangsu Province: A Meta-Analysis

In recent years, heavy metal contamination in agricultural soils in Jiangsu Province has attracted more and more attention. However, most studies have been characterized by their small scale, few samples, and short-term monitoring. The overall status and temporal accumulation characteristics of heavy metals have not been fully reflected. Therefore, this paper attempted to provide a comprehensive analysis of the current status of heavy metals and provide accurate information for soil pollution management in Jiangsu Province. This paper collected available data in the literature (1993–2021) on heavy metal-polluted agricultural soils in Jiangsu Province. Based on these available data, the weighted mean values of each heavy metal were obtained by meta-analysis. Then, the ecological risks in soils were evaluated and spatiotemporal variations in the accumulation of heavy metals were explored. In addition, suggestions for pollution prevention and control were made by predicting future heavy metal concentrations. The results showed that Cd and Hg were the major polluting elements in Jiangsu Province. The spatial enrichment of heavy metals followed the pattern of southern > northern > central. Heavy metal concentrations in Nanjing, Suzhou, and Xuzhou should be paid special attention. The ecological risk level of heavy metals in agricultural soils in Jiangsu Province was high, predominantly contributed by Hg and Cd. The accumulation of most heavy metals gradually decreased after 2010, while the opposite happened with Cd. Jiangsu Province should continue to take active pollution control measures in order to maintain the decreasing trend of heavy metal concentrations in farmland soils. This study could provide a scientific and theoretical basis for the development of pollution control and soil remediation measures.

Structural and functional characteristics of soil microbial communities in response to different ecological risk levels of heavy metals.

The potential ecological risk index (RI) is the most commonly used method to assess heavy metals (HMs) contamination in soils. However, studies have focused on the response of soil microorganisms to different concentrations, whereas little is known about the responses of the microbial community structures and functions to HMs at different RI levels. Here, we conducted soil microcosms with low (L), medium (M) and high (H) RI levels, depending on the Pb and Cd concentrations, were conducted. The original soil was used as the control (CK). High-throughput sequencing, qPCR, and Biolog plate approaches were applied to investigate the microbial community structures, abundance, diversity, metabolic capacity, functional genes, and community assembly processes. The abundance and alpha diversity indices for the bacteria at different RI levels were significantly lower than those of the CK. Meanwhile, the abundance and ACE index for the fungi increased significantly with RI levels. Acidobacteria, Basidiomycota and Planctomycetes were enriched as the RI level increased. Keystone taxa and co-occurrence pattern analysis showed that rare taxa play a vital role in the stability and function of the microbial community at different RI levels. Network analysis indicates that not only did the complexity and vulnerability of microbial community decrease as risk levels increased, but that the lowest number of keystone taxa was found at the H level. However, the microbial community showed enhanced intraspecific cooperation to adapt to the HMs stress. The Biolog plate data suggested that the average well color development (AWCD) reduced significantly with RI levels in bacteria, whereas the fungal AWCD was dramatically reduced only at the H level. The functional diversity indices and gene abundance for the microorganisms at the H level were significantly lower than those the CK. In addition, microbial community assembly tended to be more stochastic with an increase in RI levels. Our results provide new insight into the ecological impacts of HMs on the soil microbiome at different risk levels, and will aid in future risk assessments for Pb and Cd contamination.

Contamination Assessment and Source Apportionment of Soil Heavy Metals in Typical Villages and Towns in a Nonferrous Metal Mining City

To investigate the soil contamination degree and potential ecological risk level of heavy metals in villages and towns in Tongling City, we collected 67 surface soil samples (including surface dusts and river sediments) from the typical districts, namely Shun'an Town, Zhongming Town, and Yi'an Economic Development Zone, and measured the contents of heavy metals including Cu, Zn, Pb, Cr, Cd, As, and Ni. Then, spatial distribution characteristics of heavy metals were analyzed, and their contamination degree and potential ecological risk were assessed. Finally, source apportionment of soil heavy metals was conducted using factor analysis. The results showed that the soil pH was weakly acidic in the study area, and the average contents of Cu, Zn, Cr, Cd, As, and Ni were 4.94, 2.89, 2.07, 0.94, 7.97, 4.03, and 2.02 times their soil background values in Tongling City, respectively. In general, the contents of soil heavy metals in the western part were higher than those in the eastern part across the studied area. According to the Nemerow pollution index, Cu, Cd, As, and Pb reached pollution levels; Zn, and Ni approached moderate pollution levels; and Cr belonged to the no pollution degree category. The Nemerow comprehensive pollution index of different land types was arranged in the order of river bed>town district>industrial land>vegetable land>agricultural land>mountain forest>village. On the whole, the contamination degree of soil heavy metals in the study area reached severe pollution levels. The order of potential ecological risk coefficients of soil heavy metals was Cd>As>Cu>Pb>Ni>Zn>Cr, in which Cd belonged to the extremely high risk level, Cu and As belonged to the medium risk level, and the others were all low risk levels. The potential ecological risk levels corresponding to different land types were as follows:river bed>town distribution>industrial land>vegetable land>agricultural land>village>mountain forest. The industrial land, vegetable land, and town district generally reached a very high risk level, and the agricultural land reached a high risk, whereas both village and mountain forest land showed a medium risk. Principal component analysis showed that Cu, Zn, Pb, Cd, and As in the study area were derived from local metal mining pollution; Cr was from both the geological background and metal mining pollution; and Ni mainly came from fossil fuel combustion.

Spatial Distribution and Source Apportionment of Heavy Metals in the Topsoil of Weifang City, East China

The distribution of heavy metals in topsoil can have a significant impact on human health. A total of 1, 556 samples were collected from the topsoil of Weifang, China. Various indices, including the pollution index (PI), the Nemerow integrated pollution index (PN), and the potential ecological risk index (RI), were used to analyze the heavy metal pollution levels. The sources of heavy metals were analyzed using the positive matrix factorization (PMF) model. The results are as follows: (1) the ecological risk level of Cu, Pb, Zn, Ni, Cr, and As in the study area is relatively safe, but the ecological risk level of Cd and Hg is relatively high, leading to an increase in the ecological risk level of heavy metals in the study area and (2) the PMF results show that there are six main sources of the eight heavy metals. Cr and Ni come from soil parent material and nonferrous metal industrial activities; As is closely related to fossil fuel (coal)-related industrial activities; Cu originated from diesel fuel-related transportation and agricultural machinery activities; Pb is derived from gasoline transportation activities; Hg is closely related to the application of pesticides in agricultural production; Cd and Zn originated from organic fertilizers used in agricultural activities.

Accumulation, chemical speciation and ecological risks of heavy metals on expanded polystyrene microplastics in seawater

Expanded polystyrene (EPS) microplastics (MPs) have become a major component of plastic debris both on land and in marine systems globally. However, the accumulation and speciation distribution of heavy metals on EPS MPs and their potential hazards to marine organisms are still unknown. In this study, a 12-month field experiment investigating EPS exposure to seawater was conducted, and concentrations and speciation of six heavy metals (Cu, Zn, Cd, Pb, As, and Ni) on the EPS MPs were studied along with an evaluation of their ecological risks. Results showed that the accumulation of heavy metals on the EPS MPs increased over the exposure time, which conformed to a pseudo first-order kinetic equation. Speciation analysis suggested that Cu, Zn, Cd, and Ni principally distributed in the acid soluble fraction, while Pb and As primarily existed in the reducible fraction. There were significant correlations among the reducible fraction, the oxidizable fraction, the residual fraction, and the carbonyl index (p < 0.05). Compared with heavy metals in sediments, the migration and bioavailability of heavy metals on the aged EPS MPs were higher. By comprehensively considering the accumulation amount, chemical speciation and toxicity, the risk levels of heavy metals on the EPS MPs were in the decreasing orders of Cd > Pb > As > Ni > Cu > Zn. The ecological risk of heavy metals on the EPS MPs increased over the exposure time, and Cd, Pb, As, and Ni were the primary contributing factors. Our study results will be helpful for better understanding the ecological risk level of heavy metals on MPs, and will provide a reference for further research on the ecological toxicity of MPs.

Ecological Risk Assessment of Heavy Metals Contamination in Lower Klang River

Estuarine and coastal environments are known to be major sinks for heavy metals. This ecosystem and its sustainability should be secured using the ecological as indicators. This study is conducted to quantify the heavy metal concentration and to assess the potential ecological risk levels of heavy metals in the sediments from Klang River Estuary. Three typical heavy metals such as cadmium (Cd), lead (Pb) and zinc (Zn) were identified. The surface sediment samples were collected from three sampling stations. The concentration of heavy metals in the sediments were arrange in a decreasing sequence of Pb > Zn > Cd. A range of pollution indicators based on single pollution indices such as Contamination Factor, Contamination Degree, Pollution Load Index and Geoaccumulation Index were considered for ecological risk analysis. In conclusion, the ecological risk levels of heavy metals in the sediments from Lower Klang River were low and unpolluted. However, an overall assessment regarding to the heavy metals’ concentrations, spatial distribution and their potential sources need to be monitored for a better understanding of the water-sediment interaction.

Spatial characteristics of ecological and health risks of toxic heavy metal pollution from road dust in the Black Sea coast of Turkey

The purpose of this study was to investigate the toxic heavy metal (Cd, Cr, Cu, Ni, Pb, Zn) concentrations of dust samples from the Black Sea coastal road in the north of Turkey. The Black Sea coastal road was represented by 50 sampling sites. The highest average concentrations were identified in Ordu for Cd (0.4 mg kg−1), Cu (132 mg kg−1), and Zn (457 mg kg−1), Samsun for Cr (59 mg kg−1) and Ni (46 mg kg−1), and Artvin for Pb (59 mg kg−1). The analysis of the environmental pollution risk levels of heavy metals was evaluated with the geo-accumulation index (Igeo), pollution assessment indexes (CF and NIPI) and pollution load indexes (PLI and PLIzone). The mean values of Igeo were found in the following decreasing order: Zn (1.2) > Cu (0.5) > Cd (0.2) > Pb (−0.4) > Cr (−0.6) = Ni (−0.6). According to pollution assessment indexes (CF and NIPI), and pollution load indexes (PLI and PLIzone), there were relatively high levels of environmental pollution in Samsun, Ordu, and Artvin provinces, compared to other provinces in the study area. Furthermore, it was identified that the significant contribution of the monomial potential ecological risk (Ei) originated from Cu in Rize (57.6%), Artvin (46.6%), Giresun (40.2%), Pb in Trabzon (31.1%), and Cd in Ordu (71.8%) and Samsun (62.6%). The mean potential ecological risk (PER) values of Ordu and Samsun were categorized as considerable risk. The health risks of toxic heavy metals in road dust for adults and children were evaluated in two different categories, as non-carcinogenic (hazard index, HI), and carcinogenic health risks (total cancer risk, TCR), considering the three possible exposure pathways. In the study area, the mean of HI and TCR for children and adults were found in the negligible non-carcinogenic health risk (HI < 1) and the lower cancer risk range (10−6 <TCR ≤ 10−4), respectively.

Pollution characteristics and risk assessment of heavy metals in sediments of Wolong Lake

In this paper, the heavy metal content in the surface sediment of 20 sampling points in Wolong Lake of Shenyang was analyzed. The accumulation degree and potential risk of heavy metal pollution in the sediment were evaluated by the geological accumulation index and potential ecological risk index. The results showed that the order of the six heavy metals accumulation index was Cd> Zn > Ni > Cr> Cu > Pb. According to the relevant evaluation standards, the main heavy metal pollutant in the sediment was Cd, and the risk order of ecological hazards was Cd> Ni > Cu > Zn = Pb> Cr. The evaluation of potential ecological risk index showed that the risk level of heavy metals in the sediment of Wolong Lake was low.

Assessment of heavy metals pollution in sediment of Citarum River, Indonesia

Heavy metals have been reported to accumulate in sediment of Citarum River. The measurement of total heavy metals may not be able to provide information about the exact dimension of pollution, thus the determination of different fractions assumed great importance. This study was performed to determine chemical fractions of heavy metals (Cu, Ni, Cr, Pb and Cd) in sediment collected at 8 locations from Citarum River. The sequential extraction procedure was used to extract heavy metals in water-soluble, acid-soluble, MnO occluded, organically bound, FeO occluded and residual fraction in sediment. Bioavailability and potential ecological risk level of heavy metals were evaluated based on bioavailability factor (BF) and risk assessment code (RAC) method. The results showed that Cu, Ni, Cr were mostly in residual form, indicate those from geological sources. Cu had low bioavailability and no risk in all sediment samples of Citarum River. Ni and Cr each was found to have risk at 2 locations. Pb and Cd were found dominantly in non-residual fraction, suggest those from anthropogenic sources. BF and RAC analysis of Pb and Cd suggest that there is a potential risk to the aquatic environment.

Risk Assessment and Source Analysis of Heavy Metals in the River of a Typical Bay Watershed

Based on the survey data of eight kinds of heavy metals in the Xiangshan Bay watershed during the flood season, the pollution status of heavy metals in water was analyzed. The potential ecological risks of heavy metals were evaluated based on the single factor pollution index (Pi), the Nemero comprehensive pollution index (Pn), and the heavy metal pollution index (HPI). Based on the correlation between land-use types and heavy metal concentrations, the sources of heavy metals were analyzed. The results show that the average concentration of heavy metals is in the order Fe > Mn > Zn > Cu > As > Cr > Pb > Cd. All heavy metal concentrations, except those of Mn and Fe, conform to the class I surface water environment quality standard. The risk level of heavy metals is low, and there is no pollution, but some contamination. Agricultural non-point source pollution is the main factor in the contamination of heavy metals. Overall, the heavy metals in the region have not reached the risk threshold. To reasonably avoid the risk of heavy metal pollution in the urban development process, it is important to strengthen the supervision of pollution sources based on agricultural non-point sources.

Heavy metal pollution and comprehensive ecological risk assessment of surface soil in diffe-rent functional areas of Shenzhen, China

To explore the impact of urban functional zones on surface soil metal pollution, we analyzed the concentrations and pollution status of eight heavy metals (i.e., Mn, Ni, Cr, Pb, Cd, Zn, Cu, and As) in surface soils from various urban functional zones of Shenzhen City. Nemero index and potential ecological hazard indices were used to quantify the ecological risk levels of different heavy metals in different urban functional zones. Moreover, we spatially classified ecological risk zones for soil heavy metal pollution. Results showed that: 1) concentrations of Cd, Zn, Cu, and As in the surface soil were heavily impacted by human activities, while those of Mn, Ni, Cr, and Pb were not. High-intensity urbanization and industrialization with the constraints of physical environment were identified as the determining factors for regional and functional variation of heavy metal pollution. 2) Pb, Cd, Zn, and Cu had high pollution risks in the surface soils, with Pb having the highest risk, which necessitated further management and control of heavy metals. The overall level of heavy metal pollution risk in the surface soils of Shenzhen was higher than that of other cities in China. 3) Nemero index and potential ecological hazard indices had different emphases in terms of assessing the risk level of heavy metals. Those indices showed different performances in the risk assessment of single heavy metal, overall ecological risks in the different urban functional zones, and ecological risk classification of heavy metal pollution in the urban surface soils. We suggest that those indices should be combined to provide more holistic information about urban heavy metal pollution.

Evaluation of chemical speciation and environmental risk levels of heavy metals during varied acid corrosion conditions for raw and solidified/stabilized MSWI fly ash

In this work, the leaching pattern, chemical speciation, and environmental risks of various heavy metals (Pb, Zn, Cu, Cd, Cr, and Ni) were investigated synchronously under different acid corrosion conditions through end-point pH leaching experiments. The heavy metals were present in raw, stabilized (phosphoric acid; chelating agent), and solidified (Portland cement) municipal solid waste incineration (MSWI) fly ash. The results showed that the stabilization and solidification pre-treatment could effectively decrease the leaching of most heavy metals. However, phosphoric acid stabilization and Portland cement solidification increased the solubility of Ni and Pb/Cu/Cd under low end-point pH conditions, while that of Cr and Pb increased under high end-point pH conditions. Overall, the leaching pattern of heavy metals was not affected by the addition of binders/additives. The results from speciation analysis showed that the bioavailable fractions (exchangeable and carbonate-bound) were leached out from initial raw or solidified/stabilized fly ash after distilled water leaching. However, with the decrease in end-point pH levels, the bioavailable fractions increased again due to the increase in acid corrosion on metal-bearing mineral matrixes. The risk assessment results indicated that, after exposing the raw or solidified/stabilized fly ash to highly acidic conditions, not only the high-content Pb/Zn/Cu, but also some low-content Cd posed potential risks to the environment. During the leaching process, under extremely acidic conditions, the increased environmental risks posed by Pb/Zn/Cu/Cd in residual fly ash solids were greatly ascribed to the increase in bioavailable fractions, which might result in the re-leaching of some heavy metals to the environment.

Spatial variation of heavy metal contamination in the riparian sediments after two-year flow regulation in the Three Gorges Reservoir, China

Regular impoundment of the Three Gorges Reservoir (TGR) with intensified human activities in the watershed imparts a significant effect on the environmental changes in the riparian zone. In this study, six heavy metals (Cd, Cr, Cu, Ni, Pb and Zn) in the riparian sediments of the entire TGR mainstream were investigated in 2014 and 2016 to identify their contamination and risk characteristics and decipher the main factors for the variation of the metal contamination. The results showed that the concentrations of the heavy metals in the sediments did not vary significantly between 2014 and 2016, and their contamination degrees decreased in the order of Cd> > Cu ≈ Zn > Pb > Cr ≈ Ni in 2014 and Cd> > Zn > Cu ≈ Pb > Cr ≈ Ni in 2016. The potential eco-risk of Cd was extremely high in the two years, while the eco-risk of other metals was very low. The sediments showed a moderate to high contamination level, a high potential eco-risk but a low toxic risk to aquatic biota in the two years. Spatially, the contamination and risk levels of heavy metals were relatively higher in the downstream TGR region in 2014 except for the sites close to the urban areas but in the upper-middle TGR region in 2016. Increasing anthropogenic influence contributed to the high contamination and risk levels of Cd, Cu, Pb and Zn in the upper-middle region in 2016. The results indicated that the Cd contamination in the riparian sediments of the TGR was still a vital environmental issue, and the decreased sediment inputs from the upstream major tributaries, the periodic and anti-seasonal flow regulation, local geomorphological characteristics and anthropogenic activities determined the contamination distribution of heavy metals in the riparian sediments.

Pollution characteristics and assessment of sulfide tailings from the Dabaoshan Mine, China

Sulfide at mine tailing ponds poses a source of contamination to the surrounding environment. This study was designed to investigate and assess the fresh and weathered tailings pollution in the vicinity of the Dabaoshan Mine in Guangdong Province, China for in situ rehabilitation. Characterization of the fresh and weathered tailings was performed using geochemical and mineralogical analysis. Results indicated that low sulfur weathered and fresh tailing samples still presented a relatively high acid-production potential. Cu, Zn, Pb, and Cd in fresh tailing samples mainly existed in sulfides, whereas residual silicates were the dominant chemical forms of the four metals in weathered tailing samples. According to Hakanson's ecological risk index assessment, the potential risk individual coefficients of four heavy metals similarly ranked as follows: Cd > Cu > Zn > Pb. The tailing samples in the Dabaoshan Mine presented a much high potential ecological risk. The health risk assessment showed that the carcinogenic risk and non-carcinogenic risk levels of heavy metals in tailing were both lower than 10−6 and 1.0, respectively. The main exposure route of total non-carcinogenic risk from the tailing was the ingestion of tailing particles.