The influence of legacy contamination on the transport and bioaccumulation of mercury within the Mobile River Basin

Achieving smartphone-based colorimetric assay for Hg2+ with a bimetallic site strategy based on Hg2+-triggered oxidase-like catalytic activity of NSC/Co6Ni3S8 nanocomposite

Modulation of the nanozyme's catalytic activity is crucial for its real applications in detecting target analytes. Herein, we fabricated the nanocomposite (NSC/Co6Ni3S8) of N, S co-doped carbon and Co6Ni3S8 by a facile sol-gel approach. Compared to NSC/Ni9S8, NSC/Co6Ni3S8 with bimetallic active sites displayed better enzyme-mimetic activity. This nanocomposite could catalyze O2 to form ·O2- and oxidize colorless 3, 3', 5, 5'-tetramethylbenzidine (TMB) into blue oxTMB. The other two free radicals (h+ and ·OH) played minor roles during the catalytic reaction. Hg2+ could integrate with S2- to form HgS and the surface charges of O2 were transferred to Hg2+ to promote O2 adsorption. DFT theoretical calculations highlight that the main reasons for the enhancing effect of Hg2+ on color development results from electron transfer and increased adsorption energy of O2 molecules onto the surface of NSC/Co6Ni3S8. By employing the oxidase-like activity of NSC/Co6Ni3S8 and Hg2+-triggered promoting effect, a colorimetric sensing platform was established for Hg2+ assay with a linear range of 10-200 μg/L and detection limit of 3 μg/L. Through integration with a smartphone-based APP "Thing Identify" software, a visual colorimetric assay for Hg2+ was constructed with a detection limit of 5 μg/L. Compared to the data detected by the mercury vapor meter, the relative recoveries of 92.4-108.1% evidenced the higher accuracy of this smartphone-based visual detection. Overall, the NSC/Co6Ni3S8-based colorimetric assay is convenient, rapid, and visual, and can be applied for routine monitoring of Hg2+ in real-world waters under outdoor conditions.

Soil Mercury Pollution Changes Soil Arbuscular Mycorrhizal Fungal Community Composition

Remediation of mercury (Hg)-contaminated soil by mycorrhizal technology has drawn increasing attention because of its environmental friendliness. However, the lack of systematic investigations on arbuscular mycorrhizal fungi (AMF) community composition in Hg-polluted soil is an obstacle for AMF biotechnological applications. In this study, the AMF communities within rhizosphere soils from seven sites from three typical Hg mining areas were sequenced using an Illumina MiSeq platform. A total of 297 AMF operational taxonomic units (OTUs) were detected in the Hg mining area, of which Glomeraceae was the dominant family (66.96%, 175 OTUs). AMF diversity was significantly associated with soil total Hg content and water content in the Hg mining area. Soil total Hg showed a negative correlation with AMF richness and diversity. In addition, the soil properties including total nitrogen, available nitrogen, total potassium, total phosphorus, available phosphorus, and pH also affected AMF diversity. Paraglomeraceae was found to be negatively correlated to Hg stress. The wide distribution of Glomeraceae in Hg-contaminated soil makes it a potential candidate for mycorrhizal remediation.

Using mercury stable isotope fractionation to identify the contribution of historical mercury mining sources present in downstream water, sediment and fish

Ecosystems downstream of mercury (Hg) contaminated sites can be impacted by both localized releases as well as Hg deposited to the watershed from atmospheric transport. Identifying the source of Hg in water, sediment, and fish downstream of contaminated sites is important for determining the effectiveness of source-control remediation actions. This study uses measurements of Hg stable isotopes in soil, sediment, water, and fish to differentiate between Hg from an abandoned Hg mine from non-mine-related sources. The study site is located within the Willamette River watershed (Oregon, United States), which includes free-flowing river segments and a reservoir downstream of the mine. The concentrations of total-Hg (THg) in the reservoir fish were 4-fold higher than those further downstream (>90 km) from the mine site in free-flowing sections of the river. Mercury stable isotope fractionation analysis showed that the mine tailings (δ²⁰²Hg: -0.36‰ ± 0.03‰) had a distinctive isotopic composition compared to background soils (δ²⁰²Hg: -2.30‰ ± 0.25‰). Similar differences in isotopic composition were observed between stream water that flowed through the tailings (particulate bound δ²⁰²Hg: -0.58‰; dissolved: -0.91‰) versus a background stream (particle-bound δ²⁰²Hg: -2.36‰; dissolved: -2.09‰). Within the reservoir sediment, the Hg isotopic composition indicated that the proportion of the Hg related to mine-release increased with THg concentrations. However, in the fish samples the opposite trend was observed-the degree of mine-related Hg was lower in fish with the higher THg concentrations. While sediment concentrations clearly show the influence of the mine, the relationship in fish is more complicated due to differences in methylmercury (MeHg) formation and the foraging behavior of different fish species. The fish tissue δ¹³C and Δ¹⁹⁹Hg values indicate that there is a higher influence of mine-sourced Hg in fish feeding in a more sediment-based food web and less so in planktonic and littoral-based food webs. Identifying the relative proportion of Hg from local contaminated site can help inform remediation decisions, especially when the relationship between total Hg concentrations and sources do not show similar covariation between abiotic and biotic media.