Rice consumption is the major pathway for human methylmercury (MeHg) exposure in inland China, especially in mercury (Hg) contaminated regions. MeHg production, a microbially driven process, depends on both the chemical speciation of inorganic divalent mercury, Hg(II), that determines Hg bioavailability for methylation. Studies have shown that Hg(II) speciation in contaminated paddy soils is mostly controlled by natural organic matter and sulfide levels, which are typically thought to limit Hg mobility and bioavailability. Yet, high levels of MeHg are found in rice, calling for reconsideration of the nature of Hg species bioavailable to methylators in paddy soils. Here, we conducted incubation experiments using a multi-isotope tracer technique including ¹⁹⁸Hg(NO₃)₂, natural organic matter bond Hg(II) (NOM-¹⁹⁹Hg(II)), ferrous sulfide sorbed Hg(II) (≡FeS-²⁰⁰Hg(II)), and nanoparticulate mercuric sulfide (nano-²⁰²HgS), to investigate the relative importance of geochemically diverse yet relevant Hg(II) species on Hg methylation in paddy soils across a Hg concentration gradient. We show that methylation rates for all Hg(II) species tested decreased with increasing Hg concentrations, and that methylation rates using NOM-¹⁹⁹Hg(II) and nano-²⁰²HgS as substrates were similar or greater than rates obtained using the labile ¹⁹⁸Hg(NO₃)₂ substrate. ≡FeS-²⁰⁰Hg(II) yielded the lowest methylation rate in all sites, and thus the formation of FeS is likely a sink for labile ¹⁹⁸Hg(NO₃)₂ in sulfide-rich paddy soils. Moreover, the variability in the methylation data for a given site (1 to 5-fold variation depending on the Hg species) was smaller than what was observed across the Hg concentration gradient (10³–10⁴ fold variation between sites). These findings emphasize that at broad spatial scales, site-specific characteristics, such as microbial community structure, need to be taken into consideration, alongside the nature of the Hg substrate available for methylation, to determine net MeHg production. This study highlights the importance of developing site-specific strategies for remediating Hg pollution.

This study demonstrated that nitrogen-doped carbon materials (NCMs) could effectively catalyze the chlorine elimination process in hexachloroethane (HCA) declorination in sulfide-containing environments for the first time. The kₒbₛ values of HCA dechlorination by sulfide in the presence of 10 mg/L NCMs were higher than that of no mediator at pH 7.3 by one or two orders of magnitude. The catalytic capabilities of NCMs on HCA dechlorination were evident in common ranges of natural pH (5.3–8.9) and it could be accelerated by the increase of pH but be suppressed by the presence of dissolved humic acid. Moreover, NCMs exhibited much better catalytic capability on HCA dechlorination compared to the carbon materials, mainly owing to the combined contributions of pyridine N, including enhanced nucleophilic attack to HCA molecule by generating newborn C–S–S and activation of HCA molecule by elongating C–Cl bonds. The functions of pyridine N in micron-sized NCMs with mesopores were better than in nano-sized NCMs on HCA dechlorination. These findings displayed the potential of NCMs, when released into sulfide-containing environments, may significantly increase the dechlorination of chlorinated aliphatic hydrocarbons.

The sulfate pollution in water environment gains more and more concerns in recent years. The discharge of domestic, municipal, and industrial wastewaters increases the riverine sulfate concentrations, which may cause local health and ecological problems. To better understand the sources of sulfate, this study collected water samples in a typical agricultural watershed in East Thailand. The source apportionment of sulfide was conducted by using stable isotopes and receptor models. The δ³⁴SSO₄ value of river water varied from 1.2‰ to 16.4‰, with a median value of 8.9‰. The hydrochemical data indicated that the chemical compositions of Mun river water were affected by the anthropogenic inputs and natural processes such as halite dissolution, carbonate, and silicate weathering. The positive matrix factorization (PMF) model was not suitable to trace source of riverine sulfate, because the meaning of the extracted factors seems to be vague. Based on the elemental ratio and isotopic composition, the inverse model yielded the relative contribution of sulfide oxidation (approximately 46.5%), anthropogenic input (approximately 41.5%), and gypsum dissolution (approximately 12%) to sulfate in Mun river water. This study indicates that the selection of models for source apportionment should be careful. The large contribution of anthropogenic inputs calls an urgent concern of the Thai government to establish effective management strategies in the Mun River basin.

The biogeochemical cycling of sulfur in soil is closely associated with the mobility and bioavailability of heavy metals; however the influence of sulfur on the behavior of metal-based nanoparticles has not yet been studied. The influence of S fertilizer (S⁰ and Na₂SO₄) applied in paddy soils on CuO NPs behavior in soil pore water was explored in the present study. Synchrotron-based techniques were applied to investigate the migration and speciation transformation of CuO NPs in soil pore water colloids. The application of sulfur fertilizer increased the zeta potential of soil colloids from the rice rhizosphere region and reduced the size of the colloids. Sulfur fertilization decreased the concentration of Cu in soil pore water in the rice rhizosphere region. S⁰ fertilizer reduced the Cu concentration in soil colloids (by 55.8%–73.5%), while Na₂SO₄ increased the Cu concentration in soil colloids (by 173.8%–265.1%). Sulfur fertilization changed the spatial distribution of Fe³⁺ and Cu²⁺ in colloids, making these ions more likely to be aggregated on the edges of soil colloids. Speciation transformation of CuO NPs happened during the process of migration. The main Cu speciation in the soil colloids were CuO NPs, Cu-Cysteine, Cu₂S and Cu-Citrate. Sulfur fertilization increased the proportion of Cu₂S (by 40.5%) in soil pore water colloids from the rice rhizosphere region, while the proportion of CuO NPs was reduced (by 18.4%). Sulfur fertilization changed the morphology and elementary composition of colloids in soil pore water, thus influencing the migration of CuO NPs in the soil column through soil colloids.

Eutrophic freshwater lake ecosystems are receiving increasing public attention due to a global increase in large-scale harmful cyanobacterial blooms in surface waters. However, the contribution of phytodetritus accumulation in benthic sediments post-bloom remains unclear. In this study, field investigations were performed using microsensors to evaluate benthic phytodetritus mats by measuring TOC/TN ratios, pigments, biodegradable compounds and odorants as descriptive parameters. Results show that the massive amount of phytodetritus trapped by aquatic plants gradually evolved into benthic cyanobacterial detritus mats, which were characterized as anoxic, reductive and low pH. It was confirmed that the occurrence of odorants is more serious in the detritus mats due to decay and decomposition of the accumulated phytodetritus. The mean odorant content in the vegetated zones was 3–52 times higher than that in the unvegetated zones. The dominant odorants were dimethyl trisulfide (DMTS), β-ionone and β-cyclocitral, with mean contents of 52.38 ng·(g·dw)-1, 162.20 ng·(g·dw)-1 and 307.51 ng·(g·dw)-1, respectively, in the sediment. In addition, odorant production appears to be associated with the distribution of biodegradable compounds in the sediment. This is supported by the marked correlation observed between biodegradable compounds and odorants. Multiple regression analysis showed that biodegradable compounds can be used as indicators to predict odorant content in the sediment. It is noteworthy that the odorant trend in the water column and sediment is symmetrical, indicating a risk of diffusion from the sediment to the water column. This study helps to clarifying the contributions of benthic cyanobacterial detritus mats to odorant production in shallow eutrophic lakes. The information provided herein may also be useful for future management of aquatic ecosystems.

The Tharsis mine is presently abandoned, but the past intense exploitation has left large dumps and other sulphide-rich mining wastes in the area generating acid mine drainages (AMD). The main goal of this work is to study the effect of hydrogeochemical processes, hydrological regime and the waste typology on the physicochemical parameters and dissolved concentrations of pollutants in a deeply AMD-affected zone. Extreme leachates are produced in the area, reaching even negative pH and concentrations of up to 2.2 g/L of As and 194 g/L of Fe. The results of the comparison of ore grades of sulphide deposits with dissolved concentrations in waters shows that Pb is the least mobile element in dissolution probably due to the precipitation of Pb secondary minerals and/or its coprecipitation on Fe oxyhydroxysulphates. Arsenic, Cr, and V are also coprecipitated with Fe minerals. Seasonal patterns in metal contents were identified: elements coming from the host rocks, such as Al, Mn and Ni, show their maximum values in the dry period, when dilution with freshwater is lower and the interaction of water-rock processes and evaporation is higher. On the other hand, As, Cr, Fe, Pb and V show minimum concentrations in the dry period due to intense Fe oxyhydroxysulphate precipitation. In this sense, large sulphide rich waste heaps would be a temporal sink of these elements (i.e. Pb, As, Cr and V) in the dry period, and a significant source upon intense rainfalls.

Sulfate (SO₄²⁻) contamination in groundwater and surface water is an environmental problem of widespread concern. In this study, we combined stable isotope analyses of SO₄²⁻ (δ³⁴S and δ¹⁸O) and water (δ²H and δ¹⁸O) with a Bayesian mixing model (SIAR), for the first time, to identify sources and transformation of SO₄²⁻ in an area of northern China with multiple potential sources of pollution. The overall values of δ³⁴S and δ¹⁸O-SO₄²⁻ ranged from 1.3‰ to 16.3‰ and −3.8‰–8.8‰ in groundwater, and from −1.1‰ to 9.3‰ and 2.7‰–9.2‰ in surface waters, respectively. Analyses of SO₄²⁻ isotopes and water chemistry indicated that SO₄²⁻ in groundwater and surface water mainly originated from mixing of oxidation of sulfate, sewage, chemical fertilizers, dissolution of evaporite and precipitation. There was no significant correlation between δ³⁴S and δ¹⁸O and SO₄²⁻ concentration in groundwater, indicating that bacterial sulfate reduction did not affect the SO₄²⁻ isotopic composition. SIAR model showed the main sources of SO₄²⁻ in groundwater and surface water comprised oxidation of sulfide minerals and sewage. In groundwater, oxidation of sulfide minerals and sewage accounted for 37.5–44.5% and 35.5–42.7% of SO₄²⁻, respectively. In regard to surface waters, the contribution of oxidation of sulfide minerals to SO₄²⁻ was higher in the wet season (31.8 ± 9.9%) than in the intermediate (22.4 ± 7.8%) and dry (20.9 ± 8.2%) seasons, but the contribution proportion of sewage was slightly lower in the wet season (19.9 ± 8.5%) than in the intermediate (23.8 ± 8.7%) and dry (24.2 ± 8.5%) seasons. This study indicates that it is necessary for local government to improve the treatment infrastructure for domestic sewage and optimize methods of agricultural fertilization and irrigation to prevent SO₄²⁻ contamination of groundwater and surface water.

Pesticides applied to paddy fields may pose considerable danger to non-target aquatic organisms and further threaten human health. Flufiprole is a pesticide used in rice fields; considering the widespread existence of rice-fish-farming ecosystems, the acute toxicities of flufiprole enantiomers and its six metabolites (fipronil, flufiprole sulfide, flufiprole sulfone, detrifluoromethylsulfinyl flufiprole, desulfinyl flufiprole, and flufiprole amide) to four common aquatic organisms in rice fields including Misgurnus anguillicaudatus (pond loach), Carassius gibelio (Prussian carp), Pelophylax nigromaculatus (black-spotted frog), and Daphnia magna (water flea) were investigated. Genotoxicity, pathological changes and the effects on the antioxidant system of M. anguillicaudatus were also evaluated after exposure. The LC₅₀ (EC₅₀) values showed that fipronil and desulfinyl flufiprole were the most toxic compounds and were approximately about six times as toxic as flufiprole. No enantioselective toxicity was observed between the two enantiomers. The activity of antioxidant defense enzymes and the content of malondialdehyde (MDA) in the liver and gills of M. anguillicaudatus were significantly increased by the chemicals in most cases. In addition, fipronil and desulfinyl flufiprole were found to induce an increase in the micronucleus rate in M. anguillicaudatus. Histopathological analysis showed that the liver of M. anguillicaudatus was not significantly affected by flufiprole. Our study demonstrated a potential negative effect on flufiprole-treated aquatic organisms. As an alternative to fipronil, the environmental risk of flufiprole and its metabolites to non-target organisms in rice fields cannot be ignored.

The conversion of natural saltmarshes to shrimp aquaculture ponds can potentially influence the biogeochemical cycling of nutrients in coastal wetlands, but its impact on the dynamics of sediment dissimilatory nitrate (NO3−) reduction remains poorly understood. In this study, three sediment NO3− reduction processes including denitrification (DNF), anaerobic ammonium oxidation (ANAMMOX), and dissimilatory NO3− reduction to ammonium (DNRA) were examined simultaneously in a natural saltmarsh and two shrimp culture ponds (5- and 18-year-old) in July and November, using nitrogen (N) isotope-tracing experiments. Our results showed that sediment potential DNF, ANAMMOX and DNRA rates were generally higher in the shrimp culture ponds than the natural saltmarsh in the two seasons. The rates of all three processes generally increased with the age of shrimp ponds, with the magnitude of increase being less pronounced for DNF and ANAMMOX than DNRA. The contribution of DNRA to total NO3− reduction increased significantly following saltmarsh conversion to shrimp ponds, suggesting that DNRA became an increasingly important biogeochemical process under shrimp culture. DNRA competed with DNF and limited reactive N loss to some extent after natural saltmarshes converted to shrimp culture ponds. The results of redundancy analysis revealed that the availability of substrates and sulfides in sediments, rather than the bacteria gene abundance, were the most important factor influencing the NO3− reduction processes. Overall, our findings highlighted that shrimp-aquaculture reclamation may aggravate nitrogen loading in coastal wetlands by promoting the production of bioavailable ammonium.

Metal enrichment of road dust is well characterized but available data on the bioaccessibility of metals in particle size fractions relevant to human respiratory health remain limited. The study goal was to investigate the bioaccessibility of platinum group elements (PGE), which are used as catalysts in automotive exhaust converters, in the inhalable fraction of road dust. Street sweepings were provided by the City of Toronto, Canada, collected as part of its Clean Roads to Clean Air program.The particle size relevance of road dust for inhalation exposures was confirmed using a laser diffraction particle size analyzer (mean Dx(50): 9.42 μm). Total PGE were determined in both bulk and inhalable fractions using nickel sulfide (NiS) fire-assay and instrumental neutron-activation analysis (INAA). PGE lung solubility was examined for the inhalable fraction using Gamble’s extraction. Sample digests were co-precipitated with Te-Sn, to pre-concentrate and isolate PGE, prior to their measurement using inductively coupled plasma mass spectrometry (ICP-MS).Total PGE concentrations were enriched in the inhalable fraction of road sweepings. Geomean concentrations in the inhalable fraction were: palladium (Pd) (152 μg/kg), platinum (Pt) (55 μg/kg), rhodium (Rh) (21 μg/kg) and iridium (Ir) (0.23 μg/kg). Osmium (Os) concentrations were below the limit of detection (LOD). Bioaccessible PGEs (n = 16) using Gamble’s solution were below LOD for Ir and ruthenium (Ru). For the remainder, the geomean % bioaccessibility was highest for platinum (16%), followed by rhodium (14%) and palladium (3.4%). This study provides evidence that PGE in road dust are bioaccessible in the human lung.