4d-1: Technologies and approaches for mitigating mercury emissions

Mercury represents a serious and persistent threat to public health due to its persistence and bioamplification throughout the food chain. Numerous methods, notably the use of activated carbon sorbents, have been employed to remove mercury from water. Current limitations include the inability to remove mercury below 10 part per trillion (ppt), need of water pretreatment systems, and reversible interactions with mercury, which could represent a secondary pollution. There is an urgent need for rapid technologies that can remove mercury from water below its environmental levels in a cost-effective fashion. In this work, a store-bought polyurethane sponge was used as a 3D matrix to grow selenium nanomaterials (NanoSe) on the surface and throughout the bulk of the sponge fibers (International Patent Application # PCT/US2016/056850). The NanoSe sponge was shown to remove mercury to levels that are undetectable by state of the art analytical methods (detection limit < 0.2 p.p.t) in less than 5 seconds. Furthermore, the sponge does not capture water nutrients, removes other heavy metal pollutants, and shows no cytotoxic effect on human cells, while exhibiting strong antimicrobial properties that prevent biofouling. Finally, the high affinity of Hg for Se results in irreversible sequestration and detoxification of mercury by the sponge, allowing the NanoSe sponge to meet the US-EPA requirements for leachability and landfill disposal.

Catalytic oxidation of Hg0 to Hg2+ by upstream SCR catalyst and removed by downstream air pollution control devices (APCDs) was supposed as an effective way to eliminate Hg0 emissions. The Hg0 oxidation efficiency was affected by many components in flue gas. In the presence of NH3, Hg0 oxidation was inhibited significantly over most of catalysts. It was believed that competitive adsorption of Hg0 and NH3 on the catalyst surface was the main reason for inhibition effect. Although plenty of catalysts, such as CeO2/TiO2, Fe2O3/TiO2, MnOx-CeO2/TiO2 and V2O5-WO3/TiO2, had been investigated for Hg0 oxidation, the mechanism of effect of NH3 on adsorption of Hg0 had been hardly reported. In this work, the competitive adsorption of Hg0 and NH3 on different catalysts was investigated to explore excellent catalyst for Hg0 oxidation in the presence of NH3. A novel reaction mechanism based on the modification of surface acidity was proposed to develop industrial catalyst for Hg0 oxidation.

The CuO/TiO2, CeO2/TiO2, Fe2O3/TiO2 and V2O5-WO3/TiO2 catalysts were prepared by ultrasonic assisted impregnation method. The XRD result indicated that CuO, CeO2, Fe2O3 and V2O5 was well-dispersed on the surface of TiO2. The adsorption performance of Hg0 was investigated in the absence/ presence of 100ppm NH3. The Hg0-TPD results indicated that the CuO/TiO2 catalyst showed the best Hg0 adsorption capacity, followed by V2O5-WO3/TiO2, CeO2/TiO2 and Fe2O3/TiO2 catalysts. In addition, the peaks of Hg0-TPD of all catalysts were mainly attributed to weak adsorption. In the presence of NH3, the adsorption performance of Hg0 was inhibited over all of the catalysts. However, CuO/TiO2 catalyst exhibited a superior performance on Hg0 adsorption compared to other catalysts. CuO/TiO2 catalyst showed the smallest BET surface area of 33.59 m2/g. XPS results displayed that the ratio of chemisorbed oxygen on CuO/TiO2 had no obvious difference compared with other catalysts. NH3-TPD spectra indicated that abundant weak acid sites appeared on CuO/TiO2 catalyst, while weak and medium-strong acid sites co-existedon V2O5-WO3/TiO2 catalysts. The weak acid sites strength per specific surface area were 983mol/m2 and 477mol/m2 over CuO/TiO2 and V2O5-WO3/TiO2 catalysts. The competitive adsorption of Hg0 and NH3 was mainly existed in the weak acid sites. It indicated that the appearance of abundant weak acid sites of CuO/TiO2 catalyst improved the adsorption performance of Hg0 in the presence of NH3. Modification of surface acidity is an effective way to develop excellent Hg0 adsorption/ oxidation materials in the presence of NH3.

The global evolution of environmental legislation (e.g. Minamata Convention on Mercury) creates a demand for a safe disposal of metallic mercury. Especially the chlor alkali industry that is currently switching its mercury based plants to mercury free technologies and is therefore looking for disposal solutions, but also the mining industry as well as the oil gas industry. The transformation of mercury a hazardous and toxic substance for both humans and the environment into mercury sulphide (HgS, cinnabar) before sending it to designated landfill sites is considered to be the safest way to dispose mercury. Mercury sulphide is not only the most stable and insoluble mercury compound but also non-toxic, it is therefore the most obvious solution to transform mercury, which is no longer needed or considered to be a waste, into this compound before it is disposed of.

The presented wet chemical process to transform mercury into mercury sulphide sets new milestones in terms of safety, conversion rate and process efficiency. The process employs a stabilisation reagent, which is mixed with pure liquid mercury. Active sulphur within the stabilisation mixture reacts with the metallic mercury to form HgS (cinnabar), resulting in a mercury-sulphide cake with less than 5 % water. A conversion rate of 99.999% of the mercury to mercury-sulphide is guaranteed. Since the process is a wet process it is not prone to gaseous mercury emissions, improving operational safety. The process has been operational since 2016 and has a capacity of approx. 1.000 t/year.

Taking into account the sensitive issues surrounding the safe disposal of mercury, traceability is a key requirement for the whole process of mercury stabilization. The complete traceability chain that has beenimplemented aroundthe stabilization process including external inspections, sampling, analysis and mass balancing is introduced and shown on practical examples.

Activated Carbon is extremely effective and feasible at removing unwanted contaminants that may pose health threats such as mercury and it has been widely accepted for the removal of mercury (Hg) from coal-fired power plant flue gas and industrial effluent. Activated carbons surface chemistry can play a key role when removing Hg, therefore the characterization of activated carbons surface is essential to understand help simplify and better recognize the chemical reactions occurring on the surface of the activated carbon. This work examines the relationship between Hg removal and the concentration of the various types of oxygen functional groups on the surface of commercially available activated carbons synthesized from various raw materials. It will primarily study the enhancement of the carboxylic, lactonic and phenolic oxygen functional groups through post activation treatments on activated carbons exhibiting lignocellulosic and graphitic structures. Traditionally Boehm titrations have been used to determine these three acidic functional groups on the activated carbon surface based on the concept of acid-base neutralization analysis. This qualitative and quantitative technique has been extensively used on activated carbons of different origins to investigate and detect surface oxygen functional groups. In this work activated carbons were acid treated at varying concentrations which were then characterized for physical and chemical properties. The physical characterizations included surface area, pore volume, average pore size, and density. The chemical characterization techniques implemented to study the amphoteric nature of activated carbon were Boehm titrations and Fourier Transform Infrared (FTIR) Spectroscopy. Activated carbons with different concentrations of functional groups were compared for Hg adsorption and treated activated carbons showed greater removal than the virgin activated carbon. Carboxylic functional group concentration was associated with Hg uptake contradictory to other theories that suggest that an active site where electron sharing occurs is responsible for the oxidation and adsorption mechanisms of Hg.