Isotopic Fractionation Characteristics of Speciated Mercury from Local Biomass Combustion in the Tibetan Plateau.

As the Third Pole of the world, the Tibetan Plateau (TP) is sensitive to anthropogenic influences. Biomass combustion is one of the most important anthropogenic sources of mercury (Hg) emissions in the TP. However, due to the lack of knowledge about Hg emission characteristics and activity levels in the plateau, atmospheric Hg emissions from biomass combustion in the TP are under large uncertainties. Here, based on pilot-scale experiments, we found that particle-bound mercury (PBM; mean of 83.1-87.7 ng/m3) occupied 17.93-49.31% of the total emitted Hg and the PBM δ202Hg values (average -1.65‰ to -0.77‰) were significantly higher than those of the corresponding feeding biomass. The Δ200Hg values of total gaseous mercury and PBM were more negative (-0.08‰ to -0.05‰) than other anthropogenic emissions, providing unique isotopic fingerprints for this sector. Together with the investigated local activity levels, Hg emissions from biomass combustion reached 402 ± 74 kg/a, which were dozens of times higher than previous estimates. The emissions were characterized by conspicuous spatial heterogeneity, concentrated in the northern and central TP. Specialized Hg emissions and the Hg isotope fingerprint of local biomass combustion can aid in evaluating the influence of this sector on the fragile ecosystems of the TP.

Differentiated emission control strategy based on comprehensive evaluation of multi-media pollution: Case of mercury emission control.

In order to comprehensively evaluate the environmental impact of multi-media mercury pollution under differentiated emission control strategies in China, a literature review and case studies were carried out. Increased human exposure to methylmercury was assessed through the dietary intake of residents in areas surrounding a typical coal-fired power plant and a zinc (Zn) smelter, located either on acid soil with paddy growth in southern China, or on alkaline soil with wheat growth in northern China. Combined with knowledge on speciated mercury in flue gas and the fate of mercury in the wastewater or solid waste of the typical emitters applying different air pollution control devices, a simplified model was developed by estimating the incremental daily intake of methylmercury from both local and global pollution. Results indicated that air pollution control for coal-fired power plants and Zn smelters can greatly reduce health risks from mercury pollution, mainly through a reduction in global methylmercury exposure, but could unfortunately induce local methylmercury exposure by transferring more mercury from flue gas to wastewater or solid waste, then contaminating surrounding soil, and thus increasing dietary intake via crops. Therefore, tightening air emission control is conducive to reducing the comprehensive health risk, while the environmental equity between local and global pollution control should be fully considered. Rice in the south tends to have higher bioconcentration factors than wheat in the north, implying the great importance of strengthening local pollution control in the south, especially for Zn smelters with higher contribution to local pollution.

Systematic control technologies for gaseous pollutants from non-ferrous metallurgy.

Air pollutant emissions represent a critical challenge in the green development of the non-ferrous metallurgy industry. This work studied the emission characteristics, formation mechanisms, phase transformation and separation of typical air pollutants, such as heavy metal particles, mercury, sulfur oxides and fluoride, during non-ferrous smelting. A series of purification technologies, including optimization of the furnace throat and high-temperature discharge, were developed to collaboratively control and recover fine particles from the flue gas of heavy metal smelting processes, including copper, lead and zinc. Significant improvements have been realized in wet scrubbing technology for removing mercury, fluoride and SO2 from flue gas. Gas-liquid sulfidation technology by applying H2S was invented to recycle the acid scrubbing wastewater more efficiently and in an eco-friendly manner. Based on digital technology, a source reduction method was designed for sulfur and fluoride control during the whole aluminum electrolysis process. New desulfurization technologies were developed for catalytic reduction of the sulfur content in petroleum coke at low temperature and catalytic reduction of SO2 to elemental sulfur. This work has established the technology for coupling multi-pollutant control and resource recovery from the flue gas from non-ferrous metallurgy, which provides the scientific theoretical basis and application technology for the treatment of air pollutants in the non-ferrous metallurgy industry.

Highly Resolved Inventory of Mercury Release to Water from Anthropogenic Sources in China.

This study developed an up-to-date and point-source-based inventory of mercury (Hg) releases to water in China by applying probabilistic release factors that combined industry removal efficiencies, reuse of reclaimed water, and receiving water types. In 2017, the national mercury release to water was estimated to be 50 (35-66) tons, in which 47%, 8%, 7%, and 25% were from nonferrous metal smelting, vinyl chloride monomer (VCM) production, coal-fired boilers, and domestic sewage, respectively. Approximately 95% of mercury was released to inland rivers, and the rest was discharged to lakes or coastal water. The significant sources were identified based on their mercury releases to water. The control of mercury release to water in China shall focus on zinc smelting plants, municipal sewage treatment plants, and the VCM production process. For zinc smelting plants, China can tighten the limit of mercury concentration in discharged wastewater and combine Hg-catcher device in traditional integrated treatment. For municipal sewage treatment plants and the VCM production process, promoting processes of Hg-free production can reduce mercury inputs at the source. Our study provides insights for other parties to identify the relevant sources of mercury release to water and to conduct control measures, so as to promote the global convention implementation.

Potential environmental risk of trace elements in fly ash and gypsum from ultra-low emission coal-fired power plants in China.

The ultra-low emission retrofitting (ULE) in China's coal-fired power plants (CFPPs) enhances removal efficiencies of trace elements, which may increase their contents in fly ash and gypsum. However, their potential environmental risks in these wastes have been scarcely evaluated. Experiments indicated that the trace elements in fly ash and gypsum accounted for approximately 92.9-98.2% of the total outputs. Most trace elements in these wastes existed mainly as mobile/leachable forms, except for the Hg in fly ash (residual form). We comprehensively evaluated the potential environmental risks of trace elements in fly ash and gypsum from ULE CFPPs in China using a modified risk assessment approach that integrates a trace element enrichment model for waste, and chemical speciation datasets. We found that nationally, trace elements in gypsum represented low levels of potential risk, even after ULE. However, the potential moderate environmental risk of fly ash has aroused attention because of trace element pollution, where Hg and Cd contributed the major risks. The relatively high risks from fly ash are mainly distributed in Guangxi, Hunan and Hebei provinces. The disposal of fly ash in these areas should be given special attention in the future.

Distribution and emissions of trace elements in coal-fired power plants after ultra-low emission retrofitting.

Various hazardous trace elements emitted from anthropogenic activities are attracting increasing public awareness. This study comprehensively explored the distribution and emissions of trace elements in coal-fired power plants (CFPPs) after ultra-low emission retrofitting by conducting field experiments, literature surveys, and model calculations. High levels of volatile Hg and semi-volatile As/Pb were mainly observed in fly ash and gypsum (96.6%-98.5%), while the proportion of non-volatile Cr in bottom ash was 9.23%. The Hg and As/Pb removal efficiencies were remarkably improved by ultra-low emission retrofitting, increasing by 5.67% and 2.08%/2.63%, respectively. However, ULE retrofitting only slightly affected (0.17%) non-volatile elements. These improvements were mainly attributed to the low-low-temperature electrostatic precipitator. Owing to the enhanced particle-capturing efficiencies, the concentrations of trace elements in the emitted gas of the tested CFPPs were low, ranging from 0.21-1.50 µg/m3, but accounted for a high proportion of the gas phase (61.8%-100%). Based on the national database of coal quality and their behaviour in CFPPs, we found that most of the concentrations of trace elements emitted from Chinese CFPPs were significantly lower than the internationally existing emission limits. However, owing to the skewed distribution characteristics of the emitted concentrations, we suggest issuing or revising the corresponding emission limits and improving the control of intense trace element pollution in China.

Flame synthesized nanoscale catalyst (CuCeWTi) with excellent Hg0 oxidation activity and hydrothermal resistance.

In view of poor hydrothermal resistance of impregnation prepared catalysts (Cu5Ce5W9Ti-I), this paper aims to enhance thermal and hydrothermal resistance of Cu/Ce based catalysts for Hg0 oxidation via flame synthesis technology. The result found that the flame synthesis method could form nanoscale Cu10Ce10W9Ti-F particles with smaller lattice size (8-25 nm), more stable carrier structure and more oxygen vacancies. The inter-doping and inter-substitution of Ce, Cu and Ti oxides created more oxygen vacancies (Ce3+) and L-sites (O2-). Furthermore, the carrier TiO2 of Cu10Ce10W9Ti-F existed in form of highly thermostable rutile rather than anatase. High Hg0 oxidation efficiency (MOE) of 83.9-99.7% at 100-450 °C proved excellent oxidation activity of Cu10Ce10W9Ti-F catalyst. Moreover, the thermal and hydrothermal treatment (700 °C) only decreased MOE by less than 5% since L-sites kept fine thermostability of Cu10Ce10W9Ti-F. The flame synthesis was proven to be a promising catalyst preparation method to enhance thermal and hydrothermal resistance.

Impact of ultra-low emission technology retrofit on the mercury emissions and cross-media transfer in coal-fired power plants.

China has applied the ultra-low emission technology in coal-fired power plants to control traditional air pollutants and to reduce Hg emissions synergically. In this study, we applied field experiment, model calculation, and literature review to evaluate the Hg control effect of ultra-low emission technology and the potential cross-media effect comprehensively. The dominant ultra-low emission technology significantly improves the atmospheric Hg removal efficiency from 75% to 87%. Such improvement mainly comes from the effect of dust removal devices. Based on the calculated distribution characteristic of Hg content of wastes, we find out that the improvement of Hg control effect of air pollution control devices significantly increase the Hg content of fly ash, which rises from 0.16 mg/kg to 0.33 mg/kg. However, the Hg content of gypsum decreases from 0.75 mg/kg to 0.51 mg/kg. Whether or not to carry out ultra-low emission retrofits, Hg contents of wastes from coal-fired power plants are overall lower than the limit of 25 mg/kg which is intended to be set as the limit for Hg-containing wastes. However, the embodied more than two hundreds of tons Hg in these wastes still require policies to guide the disposal of these wastes.

Promoting SO2 Resistance of a CeO2(5)-WO3(9)/TiO2 Catalyst for Hg0 Oxidation via Adjusting the Basicity and Acidity Sites Using a CuO Doping Method.

The competition between SO2 and elemental mercury (Hg0) for active sites was an important factor for suppressing the Hg0 oxidation properties of catalysts. There were obvious differences in properties of basicity and acidity between SO2 and Hg0. Raising the SO2 resistance via adjusting the basicity and acidity sites of catalysts was promising for reducing the competition between SO2 and Hg0. This study aimed to form multiple active sites with different basicities via Cu, Fe, Mn, and Sn doping. The results indicated that Cu doping had the best modification performance. Five percent CuO doping could significantly improve the SO2 resistance of CuO(5)-CeO2(5)-WO3(9)/TiO2 and increase the mercury oxidation efficiency (MOE) from 54.7 to 85.5% in the condition (6% O2, 100 ppm NO, 100 ppm NH3, and 100 ppm SO2). CO2 temperature-programmed desorption analysis showed that CuO(5)-CeO2(5)-WO3(9)/TiO2 exhibited weak basic sites (CeO2), medium-strong basic sites (Cu-O-Ce), and strong basic sites (CuO). Therefore, the CuO in the Ce-O-Cu structure was prioritized for the reaction with acid gas SO2 and protected CeO2 from SO2 poisoning. This study prepared a highly SO2-resistant catalyst for Hg0 oxidation. This research and development will be conducive for use in Hg0 oxidation in actual coal-fired flue gases.

Behavior of Sulfur Oxides in Nonferrous Metal Smelters and Implications on Future Control and Emission Estimation.

This study jointly conducted a field investigation and thermodynamic model simulation in three nonferrous metal smelters (NFMS) to identify sulfur oxides (SOX) formation, transformation, and emissions in the flue gas. Most of SOX was released as sulfur dioxides (SO2) at the outlet of the furnace with the molar proportion of sulfur trioxides (SO3) of 1.0-4.1%. The formation of SO3 in smelters depended on temperature, material composition, and flue gas components. These factors were relatively certain once the production was designed. During the use of air pollution control devices (APCDs), SO3 removal alternated with its formation in the APCDs of the smelting/roasting step. Deep clean measures could not ensure standard emissions of SO3 in all smelters. Under the strict production and emission requirements, we recommended the combined effort of production design, parameter optimization, and deep clean measures to control SO3 pollution. In addition, recognizing the underestimation of the national inventory (10% at the most) due to the lack of emissions from NFMS, we suggested the attention on SOX emissions from sectors using high-sulfur raw materials in the pyroprocess. Besides, the higher potential of SO3 on secondary particle formation highlighted the distinction of SO3 and SO2 emissions in inventories for better evaluation of their environmental impact.

A Review on Adsorption Technologies for Mercury Emission Control.

This study summarized existing adsorption technologies for the removal of elemental mercury in the flue gas. Both carriers (e.g., active carbon (AC), pyrolyzed char, inorganic adsorbents and fly ash) and various modification methods (pore structure improvement, oxygen-containing functional groups addition and new active reagents impregnation) were compared to shed light on the development of future adsorption technology. AC and char possibly performed more mercury adsorption capacity (MAC) compared with fly ash and inorganic adsorbents since carbon atom existence was easier to form the active halogen groups (C-X) and oxygen containing groups. Though both pore structure improvement and chemical group formation improved the MAC of adsorbents, the chemical modification methods (oxygen-containing functional groups addition and new active reagents impregnation) were more effective. The impregnation of halogen, sulfur and metal chloride could distinctly form lots of active sites on the adsorbents and developed high effective mercury adsorbents. In the future, the adsorption researches possibly focus on SO2 and H2O resistance of adsorbents, separable adsorbents, low-cost chemical modification methods, and utilization potential of fly ash.