| Wet flue gas desulfurization (WFGD) system used for SO2 removal has been increasingly recognized to be effective in the co-removal of Hg. But there still exits two issues with this technology. Firstly, it is difficult to capture the elemental mercury (Hg0) due to the diverse properties of Hg0, including volatility and chemical stability. Thus, in order to increase the capture efficiency of Hg in flue gas, numerous studies have been conducted to oxidize Hg0 to Hg2+. Secondly, the captured Hg might undergo complex migaration and transformation in the desulfurization slurry and re-distribution among gaseous, liquid and solid phase. The re-emission of Hg0 resulted from the reduction of Hg2+ would lead to a damping of the total Hg removal efficiency, while the release of Hg from FGD gypsum would also increase the risk during its utilization. To combat the negative impacts of Hg on the environment, we propose a systhematic study to investigate Hg migaration and transformation in WFGD slurry and its release rules between the liquid and gaseous phases during the utilization of FGD gypsum, as well as to establish an optimal stabilization method of Hg in desulfurization gypsums to reduce the secondary pollution of Hg.Firstly, the distribution of captured Hg2+(L) in the simulated WFGD slurry was studied as a function of various factors. Experimental results indicated that the pH value had a strong effect on Hg2+ reduction but had negligible influence on the Hg2+ retention by the grained fraction of gypsum. Increasing in pH value resulted in higher Hg2+ reduction and over 46% of Hg2+ was found to be reduced into Hg0 at pH 5.0. Besides, increased slurry temperature also promoted the Hg2+ reduction and Hg0 re-emission as well as the Hg2+ retention efficiency by gypsum. Other factors including S(IV), Cl- and NO3- concentrationwere found to inhibit Hg2+ transformation into Hg0 but slightly promoted the Hg2+ retained in gypsum. Moreover, the addition of sodium sulfide (Na2S),2,4,6-trimercaptotiazine, trisodium salt nonahydrate (TMT), and sodium dithiocarbamate (DTCR) prevented the Hg2+ reduction and precipitation out as insoluble HgS, Hg3(TMT)2 and Hg(DTCR)2 on gypsum.Secondly, the chemical composition as well as the Hg speciation was studied in this paper. XRD analysis on bulk FGD gypsum showed that calcium sulfate (CaSO4) was the predominant crystalline phase. Other less intense peaks appeared to be those for silica, ferric oxide and some other minor minerals, such as calcium sulfite and calcite. The Hg content in four samples varied widely and it showed a significant correlation between Hg and sulfite contents in gypsum with a correlation coefficient (R2) of 0.925. Sequential Chemical Extraciton (SCE) result indicated that Hg was mainy distributed in the strong complex phase, ranging from 60% to 80%, while residual Hg accounts for about 30% of the total Hg. Water soluble mercury in Sample SX accounted for 30% of the total extract, which might be attributed to the relatively high chlorine content in coal. Arsenic and iron were predominantly partitioned in the Fe-associated fraction and residual fraction compared to the other labile fractions, while Se was mainly existed in Fe-oxide fractions and carbonate fractions.Thirdly, the release of Hg from FGD gypsum under acidic conditions and heat treatment was studied. Experimental results revealed that the acid rain type had a negligible effect on Hg leaching, while L/S ratio and initial pH had obvious effect on Hg leaching from gypsum. The Hg extraction quantity increased when the L/S ration increased. Meanwhile, the Hg extraction rate generally increased with decreasing pH value, which suggested that the environmental risk of FGD gypsum increased during the multipurpose utilization processes. The release of Hg from gypsum exhibited biphasic kinetics. The experimental data was in good agreement with MM, which was indicated by the high values for the coefficient of determination. Besides, during the leaching process, some of the Hg in leachate would be reduced to Hg0 and re-emitted to the atmosphere, the Hg reduction kinetics followed the pseudo-first-order kinetic model well. Moreover, the rapid release of Hg was related to the ration of water soluble Hg at some extent (R2=0.818), which signified of more attentions for its stabilization. During heat treatment process, the Hg release increased with temperature. The Hg release kinetics at 400 ℃ exhibited the pseudo-first order kinetic model well with the correlation coefficient R2 ranging from 0.976 to 0.998. Although the total Hg content in FGD gypsum decreased after treated at 200 ℃, its concentration in leachate was slightly increased, which might be resulted from the transformation of strong complex Hg to water soluble and diluted acid extractable Hg.At last, a chitosan stabilized maganetic FeS nanoparticles was synthesized and used in the stabilization of Hg in FGD gypsum. SEM and TEM results showed that the particles are nearly spherical in shape and uniform in size around the diameter of 20 nm. The specific surface area of the nanoparticles is 21.34 m2/g revealed by the BET test. Kinetic experiment was conducted to study the sorption rate of Hg by the nanoparticles. Results showed that the sorption displayed a rapid initial rate within the first 120 min and then slowed down till equilibrium. The date was fitted both the pseudo-first-order kinetic model and pseudo-second-kinetic model with the coefficient R2 of 0.991 and 1, respectively. While the coefficient of the internal particle diffusion model was only 0.223, which indicated that the rate limiting step appeared to be the sorption process rather than diffusion. The classical Langmuir model and Freundlich model were used to interpret the sorption isotherms first. Result revealed that the data fitted the Freundlich model (R2=0.994) better than Langmuir model (0.887). The adsorption coefficient Kf was reached to 84.89±1.37 (mg/g)/(mg/L)n, which indicated that the nanoparticles possessed good sorption capacity for Hg. Besides, the sorption was nonlinear indicated by the n of 0.155±0.006. The saturated adsorption capacity (Qmax,L) calculated by Langmuir was 131.95±9.85 mg/g. To be more mechanistically sound, a dual-Mode was proposed to fit the sorption data. Results showed that the Hg removal by nanoparticles contains adsorption and precipitation. Meanwhile, although the relative contributions of adsorption vs precipitation decreased with the increased Hg2+ concentrations, the whole removal process was still dominated by adsoption. Higher mercury uptake was observed when the intial pH value increased from 6.0 to 10.0, whereas significant capacity loss was observed at pH 4.0. High Cl- concentration would inhibit Hg uptake. The findings would provide theoretical basis for Hg co-removal by WFGD system. |
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