Mechanism Study Of Mercury Removal And Stabilization Of Mechanochemical Modified Fly Ash

Heavy metal mercury is the fourth largest pollutants in coal combustion after dust,SO_x,and NO_x.It has neurotoxicity,atmospheric mobility,bioaccumulation,and hidden potential,which pose serious harm to the natural environment and human health,attracting widespread concern around the world.As the largest coal producer and consumer,China is facing a more severe mercury pollution situation than other countries.Activated carbon injection mercury removal technology is currently the most mature and competitive technology for mercury removal from coal-fired flue gas.However,there are many disadvantages such as high operating cost,poor adaptability,and affects the quality of fly ash.In addition,it also has the risk of secondary mercury pollution in the mercury removal product.Therefore,developing a new type of mercury removal adsorbent with a wide range of sources and low price to replace activated carbon,and achieving the stabilization of mercury in the injection mercury removal product,has great practical significance for the large-scale application and sustainable development of coal-fired flue gas mercury removal technology.In this paper,a high-efficiency mercury removal adsorbent was prepared by mechanochemical bromine modification from coal-fired fly ash.Following the methodology of“experiment to theory,macrocosm to microcosm”,through the combination of experimental exploration,physicochemical characterization analysis,and quantitative calculation simulation,the mechanism of improving the mercury removal performance of fly ash by mechanochemical method and the mercury removal mechanism of the fly ash-based adsorbent were systematically studied.Then,taking the mercury-saturated fly ash-based adsorbent as the research object,the stabilization mechanism of mercury by the mechanochemical method was also studied.The purpose is to provide basic data and theoretical reference for the development of mechanochemical modified mercury removal fly ash and mechanochemical stabilization of mercury removal products.First,Mechanochemical activated fly ash（FA-MC）and mechanochemical brominated fly ash（FA-MC-Br）were prepared by modifying coal-fired fly ash on an omnidirectional planetary ball mill.The impregnated brominated fly ash（FA-I-Br）was prepared under the same amount of modifier.The mercury removal ability was evaluated on a fixed bed test bench,showing that the mercury removal performance of FA-MC-Br was significantly higher than that of FA-MC and FA-I-Br,which indicated that the mechanochemical bromine modification is a cost-effective and suitable preparation process of mercury removal adsorbents for large-scale industrial applications.The physicochemical characterization results show that the mechanochemical activation can reduce the particle size of fly ash and enhance the effective contact area of heterogeneous reaction with mercury,thereby improving the mercury removal ability of fly ash.On this basis,mechanochemical bromine modification improves the surface activity by increasing the amorphous phase structure of fly ash and increases the active sites such as C-Br,Fe³⁺,oxygen-containing functional groups with weakly adsorbed oxygen,thus the efficient improvement of fly ash mercury removal performance is realized.The removal of mercury by FA-MC-Br reflects adsorption and oxidation,among them,the contribution of adsorption is less than 10%,and the heterogeneous oxidation of mercury is dominated（>90%）.The temperature-programmed desorption results show that Hg Br₂ and Hg O are the main adsorption species of mercury in FA-MC-Br.Among them,Hg Br₂ is generated by the oxidation reaction of C-Br in FA-MC-Br with gaseous Hg⁰,which follows the Eley-Rideal mechanism.Hg O is generated by the oxidation reaction of Fe³⁺or weakly adsorbed oxygen with gaseous Hg⁰.The generated Hg Br₂ and Hg O are desorbed into the gas phase,and the active sites are exposed again to continue oxidizing Hg⁰,the cycle is repeated to achieve the removal of mercury.Secondly,to explore the mechanism of improving mercury removal performance by mechanochemical bromine modification,the activated carbon,iron oxide,titanium oxide,and aluminosilicate were selected to simulate the main components of fly ash,and were subjected to mechanochemical activation and bromine modification respectively.The mercury removal performance before and after modification of the simulated components was analyzed by fixed bed test bench.After mechanochemical activation,the mercury removal performance of the four samples is not significantly improved.While the mechanochemical bromine modification can significantly enhance the mercury removal ability of activated carbon,and also has a certain promotion effect on iron oxide,but not on titanium oxide and aluminosilicate.Combined with the physicochemical characterization results,it was found that the pore structure,particle size distribution,and surface crystal phase were poorly correlated with their mercury removal performance.The efficient mercury removal ability of the mechanochemical bromine-modified fly ash is mainly attributed to the C-Br functional groups on the unburned carbon.Its mechanochemical loading mechanism is as follows:Na Br crystals are broken and transformed into more reactive Br radicals by the mechanochemical process,and the structure of unburned carbon is also destroyed to a large extent,generating a large number of unsaturated carbons and oxygen-containing functional groups,which are more likely to react with Br radicals and finally load in the form of C-Br.Based on density functional theory,two carbon models,Zigzag and Armchair,were established to simulate the microstructure of unburned carbon in fly ash,and carbon defects were constructed to simulate the microscopic mechanochemistry.Different degrees of carbon defects represent different degrees of mechanochemical effect.The mechanochemical interaction can construct abundant defect structures on Zigzag and Armchair carbon models.The Zigzag carbon model has a total of 9 defect configurations,and the Armchair carbon model has more diverse defect structures,with a total of 17 defect configurations.With the increase of defects,the higher the defect energy required for the formation of the corresponding defect configuration,the worse the structural stability,and the more difficult it is to generate under the action of mechanochemistry.The mechanochemical bromine activation mechanism of unburned carbon was analyzed by comparatively analyzing the bonding characteristics of Br radicals on the intact and each defect structure.Since the defect structure can enhance the binding energy of Br and surface carbon atoms,and can also improve the C-Br Mayor bond level,thus,mechanochemical bromine modification achieves efficient Br loading by increasing the surface defects of unburned carbon.On this basis,the mercury removal mechanism of unburnt carbon after mechanochemical activation/bromination modification was analyzed by comparing the adsorption and oxidation characteristics of mercury on different carbon models.The defect structure generated by mechanochemical activation can enhance the electronic interaction between Hg and surface carbon atoms to a certain extent,thereby enhancing the adsorption performance of mercury on unburned carbon,but the improvement effect is limited.The mechanochemical bromine modification constructs sufficient defect structures and C-Br functional groups on unburned carbon.By reducing the adsorption energy and oxidation energy barrier of mercury,the heat released during the adsorption process is sufficient to overcome its oxidation energy barrier,so that both adsorption and oxidation processes on unburned carbon can proceed spontaneously,which greatly improves the mercury removal efficiency of fly ash.At last,selecting an omnidirectional planetary ball mill as the mechanochemical stabilization device,the mercury stabilization performance was evaluated through toxicity characteristic leaching procedure.The stability of mercury in fly ash after mechanochemical stabilization and mechanochemical sulfur stabilization were explored.The effect of mechanochemical stabilization process parameters on mercury stabilization was investigated by changing the ball milling time and the S/Hg molar ratio.Combined with the physicochemical characterization results of fly ash before and after stabilization,it was found that mechanochemical stabilization of mercury in SMF（Simulated Mercury-saturated Fly ash）is improved by increasing the surface defects and oxygen-containing functional groups（C-O）of unburned carbon,and increased with the increase of ball milling time.However,the risk of re-release during stabilization also increased correspondingly.Under the mechanochemical sulfur stabilization,the stability of mercury in SMF is proportional to the ball milling time and the S/Hg molar ratio.The promoting effect of ball milling time on mercury stability is better than that of S/Hg molar ratio It can also effectively prevent the secondary release of mercury during mechanochemical stabilization.The effect of fly ash composition on mercury stabilization suggested that unburned carbon and calcium carbonate in fly ash contribute to the stabilization of mercury,where the mechanochemical stabilization process increases the defect structure and oxygen-containing functional group on unburned carbon,enhancing the desorption energy of Hg Br₂,thereby improving the stability of mercury.The calcium carbonate reduces the leaching characteristics of mercury by providing an alkaline environment.The DFT results show that the decomposition of Hg Br₂ requires a large amount of energy.In addition,compared with other S_n,the reaction between S₄ and Hg not only has the lowest reaction energy（-285.10 k J/mol）but also has the lowest average reaction energy barrier（107.58 k J/mol）.Based on the thermodynamics and kinetics,（HgS）₄ can be generated spontaneously.Therefore,the catalytic mechanism of the mechanochemical method for the reaction of Hg Br₂ and S is as follows:Hg Br₂ and S₈ in SMF are mechanochemically broken and decomposed into Hg and S₄,and then S₄ and Hg spontaneously react to generate（HgS）₄,thereby achieving the stability of mercury in SMF.