Geochemical cycle of mercury associated with wet deposition and inflows in a subalpine wetland.

Subalpine wetland is a mercury (Hg) sensitive ecosystem, but there is poor understanding of Hg behavior in this typical wetland. Here, distribution and speciation of Hg in waters of a subalpine wetland (Dajiuhu) in China were investigated, and an initial model of the Hg geochemical cycle in the wetland was established based on Hg mass balance calculations. Concentrations of both total Hg (THg, 9.52 ± 6.61 ng L-1) and total methyl mercury (TMeHg, 0.34 ± 0.44 ng L-1) in the waters during the wet season were higher than in the dry season. The majority of THg was in dissolved form whereas most TMeHg was in particle form. The geochemical models suggested that, due to the wet deposition and surface runoff, the input of THg and TMeHg into the wetland in the wet season (222 and 2.74 g year-1, respectively) was higher than that in the dry season (57.9 and 1.15 g year-1, respectively). The output of THg and TMeHg from the wetland underground runoff in the wet season was estimated to be 154 and 2.51 g year-1, respectively, and in the dry season was 15.9 and 0.43 g year-1, respectively. Other losses of Hg were due to volatilization of Hg0 from the sediment water (30.5 and 12.5 g year-1 in the wet and dry seasons, respectively). The flux of the settling of particulate Hg in the wet season was higher than that in the dry season. The fluxes of Hg diffusion from the porewater were relatively low in comparison to the fluxes of inflows and wet deposition. The flux of oxidation was higher than reduction, while the flux of methylation was higher than demethylation. These results indicated that the elevated levels of THg and MeHg in the Dajiuhu wetland are a consequence of rainfall and surface runoff inputs.

Distribution patterns and sources of heavy metals in soils from an industry undeveloped city in Southern China.

The accumulations of heavy metals in urban soils are derived from natural parent materials and complex anthropogenic emission sources. This paper investigated metal contamination in urban soils at an industry undeveloped city (Haikou) in southern China, an ideal place to quantitatively assess the contribution of metals from different sources. The concentrations of most heavy metals in the urban soils of Haikou were much lower than their guideline values and that of those from other big cities in China. In contrast, the chemical speciation of metals in this study was similar to those from other cities. The spatial distributions of heavy metals and principal component analysis (PCA) revealed that basaltic parent materials, traffic emissions, and coal combustion were the main factors controlling the distribution of metals in the soils. The Pb isotope signatures of the Haikou soils were greatly different from those of the Beijing and Shanghai soils, but similar to those of the Guangzhou soils, suggesting the common sources of Pb in southern China cities. The results of ternary mixing model of Pb isotopes showed that the contributions of Pb from natural background, coal combustion and traffic emission sources were 5.3-82.4% (mean: 39.7 ± 21.1%), 0-85.7% (mean: 25.5 ± 24.6%), and 1.9-64% (mean: 34.8 ± 22.9%), respectively. This suggests that traffic emission is still the most important anthropogenic source of Pb in Haikou.

Profiles, source identification and health risks of potentially toxic metals in pyrotechnic-related road dust during Chinese New Year.

Potentially toxic metal (PTM) pollution in road dust is of great concern, however, our understanding of PTMs released by pyrotechnic displays and their adverse impacts on human health in road dust is limited. Here, we studied PTM pollution levels and Pb isotope signatures in pyrotechnic ash and road dust (aged dust and pyrotechnic-influenced dust) samples from eight cities in China during Chinese New Year and carried out a human health risk assessment. Pyrotechnic ash had higher values of Cr, Co, Ni, Cu, Zn, As, Sr and Pb but lower values of Mn and Cd than Chinese background soil. Pyrotechnic-influenced dust had significantly higher Cu and Cr values than aged dust, with enrichment of Sr, Cu, Pb, Cr and Ni in road dust after pyrotechnic displays. Both 208Pb/206Pb and Sr values were used to confirm the presence of pyrotechnic ash in road dust. A positive matrix factorization demonstrated that pyrotechnic events contributed 70.1%, 50.4%, 36.6% and 35.5% of the Sr, Cu, Cr and Pb values to these road dust, respectively. We found that non-carcinogenic and carcinogenic risks related to PTMs in road dust were at safe levels during the Chinese New Year, although both risks were elevated following pyrotechnic events. Typically, PTM pollutants related to pyrotechnic events contributed 33.99% to non-carcinogenic and 21.83% to carcinogenic risks, suggesting that more attention needs to be paid to this source of PTM pollution in China. Current results improve our understanding of PTM pollution in pyrotechnic-influenced road dust and health risks related to pyrotechnic displays in China.

Inputs and sources of Pb and other metals in urban area in the post leaded gasoline era.

The contamination status of heavy metals in urban environment changes frequently with the industrial structure adjustment, energy conservation and emission reduction and thus requires timely investigation. Based on enrichment factor, multivariate statistical analysis and isotope fingerprinting, we assessed comprehensively the inputs and sources of heavy metals in different samples from an urban area that was less impacted by leaded gasoline exhaust. The road dust contained relatively high levels of Cr, Pb and Zn (with enrichment factor >2) that originated from both exhaust and non-exhaust traffic emissions, while the moss plants could accumulate high levels of Pb and Zn from the deposition of traffic exhaust emission. This suggest that the traffic emission is still an important source of metals in the urban area although gasoline is currently lead free. On the contrary, the occurrences of metals in the urban soils were controlled by natural sources and non-traffic anthropogenic emission. These findings revealed that different samples would receive different inputs of metals from different sources in the urban area, and the responsiveness and sensitiveness of these urban samples to metal inputs can be ranked as moss ≥ dust > soil. Taken together, our results suggested that in order to avoid generalizing and get detail source information, multi-samples and multi-measures must be adopted in the assessment of integrated urban environmental quality.

Comparison of heavy metal bioaccessibility between street dust and beach sediment: Particle size effect and environmental magnetism response.

Direct ingestion of urban sediment particles represents an important pathway of human exposure to heavy metals. The effect of particle sizes on metal bioaccessibilities in human digestive system has not been fully understood. In this study, an in-vitro simulation experiment (PBET), along with environmental magnetic measurements, is conducted on two urban sediments (street dusts and beach sediments) with different particle sizes (Φ31.1 ± 7.36 µm for street dusts and Φ134 ± 21.1 µm for beach sediments) for the purposes of assessing the particle size effect on metal bioaccessibilities in simulated gastric and intestinal tracts, and exploring the environmental magnetism response to different digestion processes. For street dusts, the bioaccessibilities of heavy metals decrease significantly from gastric (12.1 (Cu) ~ 39.9% (Pb)) to intestinal phase (0.41 (Pb) ~ 2.08% (Cd)) due to an increase in digestive juice pH. However, for beach sediments, the metal bioaccessibilities in the intestinal phase is similar to, or even higher than, those in the gastric phase. These demonstrate that clay minerals and Fe/Mn oxides concentrated in fine particles play an important role in adsorbing and fixating heavy metals in neutral intestinal tract. Compared with those of the original samples, the χfd% values of the PBET treated street dusts decrease significantly, and the decreasing extents (Δχfd%) are positively correlated with the concentrations of the PBET extracted Fe (p < 0.05). However, a reverse trend is observed for the beach sediment samples. These findings suggest that the magnetic minerals formed during the digestion process might affect the metal bioaccessibilities in gastrointestinal tract.

Effects of exogenous dissolved organic matter on the adsorption-desorption behaviors and bioavailabilities of Cd and Hg in a plant-soil system.

Dissolved organic matter (DOM) is one of the most active soil components and plays critical direct and indirect roles in heavy metal migration, transformation, bioavailability, and toxicity in soils. In this study, isothermal adsorption/desorption experiments and pot experiments were performed and samples were physically characterized to study the effects of different sources of DOM on adsorption and desorption behavior and bioavailability of Cd and Hg in a plant-soil system. The results showed that microbial DOM promoted Cd and Hg adsorption in soil and decreased Cd and Hg bioavailability to pak choi (Brassica chinensis Linn.). In contrast, straw DOM and farmyard manure DOM decreased Cd and Hg adsorption and improved Cd and Hg migration and bioavailability. These results might be explained by the different types of DOM having different molecular weights and degrees of aromaticity. Cd was more readily desorbed by the soil and was more phytoavailable than Hg. We concluded that exogenous microbial DOM can inhibit Cd and Hg migration and bioavailability in soil but straw DOM and farmyard manure DOM can activate Cd and Hg in soil and promote Cd and Hg accumulation in plants. The results could help in developing rational agricultural fertilization regimes.

Apportionment of sources of heavy metals to agricultural soils using isotope fingerprints and multivariate statistical analyses.

Apportioning sources of environmental pollutants is key to controlling pollution. In this study, the sources of heavy metals to 234 agricultural soils from the Jianghan Plain (JHP) (∼22454 km2) in central China were discriminated between using Cd and Pb isotope compositions and multivariate statistical analyses. Concentrations of some metals in JHP soils (0.48 ±â€¯0.2, 48.2 ±â€¯15.9, 0.12 ±â€¯0.23, 48.8 ±â€¯16.4, 36.5 ±â€¯9.8, and 96.8 ±â€¯42.2 mg kg-1 for Cd, Cu, Hg, Ni, Pb, and Zn, respectively) were higher than background concentrations in Chinese soil. The Cd isotope compositions for the JHP soils (δ114/110Cd values -0.76‰ to -0.25‰) were similar to Cd isotope compositions found for smelter dust and incinerator fly ash, indicating Cd was supplied to the JHP soils by ore smelting and/or refining processes. The Pb isotope compositions for the JHP soils (206Pb/207Pb 1.182-1.195 and 208Pb/206Pb 2.078-2.124) were between the Pb isotope compositions found for Chinese coal and natural sources, which a binary isotope mixing model indicated contributed 52% and 48%, respectively, of the Pb in JHP soils. Cluster analysis and positive matrix factorization indicated that the sources of heavy metals in JHP soils may consist of smelting and/or refining activities, coal combustion, agricultural activities, and natural sources (including Han River sediment and soil parent materials). The isotope fingerprints and multivariate statistical analyses together indicated that coal combustion and smelting and/or refining activities were the main anthropogenic sources of heavy metals polluting JHP soils.

Methylmercury production in a paddy soil and its uptake by rice plants as affected by different geochemical mercury pools.

The formation of neurotoxic methylmercury (MeHg) in paddy fields and its accumulation by rice plants is of high environmental concern. The contribution of different geochemical mercury (Hg) pools in paddy soils to MeHg production and its accumulation by rice seedlings is not well-studied up to now. Therefore, we investigated the impact of different inorganic Hg forms, including HgCl2, nano-particulated HgS (nano-HgS), Hg bound with dissolved organic matter (Hg-DOM), ß-HgS, and α-HgS, at levels of 5 mg Hg/kg soil and 50 mg Hg/kg soil, on the production of MeHg in the soil during rice growing season. Further, we studied the uptake of MeHg by the roots, stalks, leaves, and grains of rice in the tillering, panicle formation, and ripening growth stages, and compared these treatments to a non-polluted soil (control). MeHg contents in HgCl2 polluted soil were the highest, and were 13.5 times and 36.1 times higher than control in 5 and 50 mg/kg Hg treatments, respectively. MeHg contents in α-HgS, ß-HgS, nano-HgS, and Hg-DOM polluted soil were 3.9, 2.6, 2.4, and 1.7 times, and 4.4, 15.1, 6.7, and 10.9 times higher than control in 5 and 50 mg/kg Hg treatments, respectively, suggesting the mobilization and methylation of these Hg complexes. The ratio of MeHg to total Hg in the pore water (indication of methylation potential) in HgCl2 and ß-HgS treatments were higher than in Hg-DOM, α-HgS, and nano-HgS treatments. HgCl2 treatment resulted in significantly higher MeHg contents in the root, stalk, leaf, and brown rice than nano-HgS, Hg-DOM, ß-HgS, and α-HgS treatments both in 5 and 50 mg/kg Hg polluted soils. Rice grain in HgCl2 treatment showed a potential hazard to human health, as indicated by high health risk index (HRI > 1) of MeHg. Current results improve our understanding of MeHg production in soil polluted with different Hg forms, and the assessment of human health risks from consumption of MeHg-laden rice grain at Hg polluted sites with different Hg forms in soils.

Source discrimination of atmospheric metal deposition by multi-metal isotopes in the Three Gorges Reservoir region, China.

Concentrations of heavy metals, as well as isotopic compositions of mercury (Hg) and lead (Pb), in mosses (Bryum argenteum) from the Three Gorges Reservoir (TGR) region were investigated to decipher the sources of atmospheric metals in this region. Higher contents of metals (0.90 ±â€¯0.65 mg/kg of Cd, 24.6 ±â€¯27.4 mg/kg of Cu, and 36.1 ±â€¯51.1 mg/kg of Pb) in the mosses from TGR were found compared with those from pollution-free regions. Principal component analysis (PCA) grouped the moss metals into four main components which were associated with both anthropogenic and natural sources. The ratios of Pb isotopes of the mosses (1.153-1.173 for 206Pb/207Pb and 2.094-2.129 for 208Pb/206Pb) fell between those of the traffic emissions and coals. Similarly, the compositions of δ202Hg (-4.29∼-2.33‰) and Δ199Hg (within ±0.2‰) were comparable to those of the coals and coal combustion emissions from China and India. These joined results of Pb and Hg isotope data give solid evidences that the coal combustion and traffic emissions are the main causes of metal accumulation in the TGR region.