Mercury accumulation in soil from atmospheric deposition in temperate steppe of Inner Mongolia, China.

Mercury (Hg) is a toxic and persistent pollutant and has long-term impacts on ecological systems and human health. Coal-fired power plants (CFPPs) are the main source of anthropogenic Hg emission, and the emitted atmospheric Hg is deposited to the surrounding environments which causes soil pollution. To assess the effects of atmospheric Hg from CFPPs in China on the temperate steppe, Hg contents in the topsoil and subsoil were analyzed for samples collected from 80 sites in central Inner Mongolia during 2012-2015. The average content of Hg in topsoil and subsoil were 14.9 ± 10.4 µg kg-1 and 8.9 ± 5.8 µg kg-1, respectively. The principal components analysis (PCA) indicated that the soil organic matter content and atmospheric deposition were the main factors determining soil Hg content in Inner Mongolia. We used the power plant impact factor (PPIF) to evaluate the impacts of the surrounding CFPPs. The PPIF results showed the most positive correlation with Hg content in topsoil at more than 400 km distances, indicating that the contribution of the long-range transport of Hg emitted from CFPPs is regional in scale. Considering the potential of Hg accumulation in soil, long-term and regional measurements of soil Hg and stricter emission-limit standards for power plants should be implemented to control soil Hg pollution in China.

Sulfur-modified rice husk biochar: A green method for the remediation of mercury contaminated soil.

Mercury (Hg) contamination of surface soils has increased by ~86Giga grams due to anthropogenic activities. There is an urgent need to find new, effective and preferably 'green' remediation technologies to protect human health and the environment. Sulfur-modification of sorbents can greatly enhance Hg sorption capacity - by forming low solubility HgS (cinnabar). However, S-modified sorbents are not considered suitable for soil remediation due to the economic cost and secondary environmental impacts of sorbents such as granulated activated carbon (GAC), and the toxicity of S-modifiers such as thiol compounds. It was previously found that if biochar is used as an alternative to GAC then the overall environmental impact can be significantly reduced. However, due to a lack of experimental evidence, the practicality of S-modified biochar remains uncertain. The present study was undertaken to provide a proof-of-concept for the 'green' remediation of Hg contaminated soils with rice husk biochar modified with non-toxic elemental S. It was found that the S modification process increased the biochar S content from 0.2% to 13.04% via surface deposition or volume pore filling. This increased the biochar's Hg2+ adsorptive capacity (Qmax) by ~73%, to 67.11mg/g. To assess the performance of S-modified rice husk biochar for soil remediation it was applied to a high 1000mg/kg Hg2+ contaminated soil. Treatment dosages of 1%, 2% and 5% (dry wt.) were found to reduce freely available Hg in TCLP (toxicity characterization leaching procedure) leachates by 95.4%, 97.4% and 99.3%, respectively, compared to untreated soil. In comparison, unmodified rice husk biochar reduced Hg concentrations by 94.9%, 94.9% and 95.2% when applied at the same treatment dosage rates, respectively. This study has revealed that S-modified rice husk biochar has potential to stabilize Hg as a 'green' method for the remediation of contaminated soils.

Is surface water acidification a serious regional issue in China?

Acidic deposition used to cause severe surface water acidification in Europe and the North America, but no damage of surface water acidification was reported under continuous heavy acid deposition in China. We present the first detailed study on regional surface water acidification in southern and northeastern China based on a survey of 255 forested headwater streams, which were investigated during 2010-2015 to explore the status and mechanism of surface water acidification. South China has a subtropical climate with high acid deposition, whereas northeast China is located in a cold temperature zone with relatively low acid deposition. High pH and acid neutralizing capacity (ANC) were observed for almost all of the streams, indicating that surface water acidification may not be a serious regional issue in China both at present and in the future. In northeast China, where soil types are similar in low weathering rate to those in Northern Europe and North America, the ANC of surface waters was lower than that in South China, indicating a higher acid-sensitivity of the surface water. In comparison, the high pH and ANC of streams in southwest China resulted from high soil weathering and atmospheric calcium (Ca) deposition, despite elevated atmospheric acid inputs due to the high sulfur (S) and nitrogen (N) deposition. Comparison of stream nitrate (NO3-) and sulfate (SO42-) concentrations with modeled N and S deposition showed significant N and possible S sinks in the catchments in south China, probably due to denitrification of NO3- in groundwater discharge zone and SO42- adsorption by acid soils respectively, which may also buffer the acidifying effect of S and N depositions. It seems that considerable NO3- leaching to stream waters does not occur in China unless N deposition is higher than 1.8keq·ha-1·yr-1 (2.50g N·m-2·yr-1) (or greater values in certain regions).

[Deposition of Sulfur, Nitrogen and Mercury in Two Typical Forest Ecosystems in Southern China].

Bulk precipitation collectors and canopy throughfall collectors were applied to measure the deposition of sulfur, nitrogen, and mercury at two forest ecology stations, Qianyanzhou (QYZ) in Jiangxi Province and Huitong (HT) in Hunan Province, from December 2013 to November 2014. During the observation period, the volume weighted average concentrations of SO42--S, NO3--N, NH4+-N, and Hg at QYZ station were 1.89 mg·L-1, 0.957 mg·L-1, 0.401 mg·L-1, and 12.5 ng·L-1 in the bulk precipitation, respectively, and 2.39 mg·L-1, 1.18 mg·L-1, 0.897 mg·L-1, and 22.2 ng·L-1 in the throughfall, respectively. The concentrations of these components increased by different proportions for the bulk precipitation compared to the throughfall. At HT station, the average concentrations of SO42--S, NO3--N, NH4+-N, and Hg in the throughfall were 2.93 mg·L-1, 1.60 mg·L-1, 0.502 mg·L-1, and 22.0 ng·L-1, respectively. In addition, atmospheric deposition fluxes based on the throughfall were 3.56 g·(m2·a)-1 for sulfur, 3.02 g·(m2·a)-1 for nitrogen, and 30.6 µg·(m2·a)-1 for mercury at QYZ station, while the corresponding fluxes were 6.18 g·(m2·a)-1, 4.48 g·(m2·a)-1, and 37.3 µg·(m2·a)-1 at HT station. The deposition rates of all three elements were the highest in summer. The contribution of dry deposition to the total deposition of mercury was similar to that of wet deposition, while wet deposition contributed more to the total deposition of sulfur and nitrogen than did dry deposition. Unlike sulfur and nitrogen depositions, which were mainly derived from anthropogenic sources, mercury deposition might have a greater contribution from natural sources, especially at HT station.