| In the process of gypsum stacking, treating and utilization, Hg2+in gypsum tends to be released to the environment and thus causes secondary mercury pollution. For instance, Hg2+in gypsum could be leached into water, soil and groundwater by stacking and rain erosion. Further, Hg2+can be deoxidized to Hg0by a small amount of S(IV) in gypsum, leading to atmospheric pollution. Therefore, without any treatment of Hg2+in gypsum, it could cause great damage to the environment and human health. In this study, sodium dithiocarbamate (DTCR) was utilized to precipitate Hg2+in gypsum and prevent Hg2+leaching in the coexistence of other trace metals.Samples of FGD gypsum were collected from five coal-fired power plants, namely, XS, QQ, CX, BL and BS. Metal content in gypsum was analyzed by ICP and the results showed that1) the order of metal content from the same kind of gypsum (XS, QQ and BS) was Al> Fe> Mg;2) the order of trace metal content in each five gypsum was Zn> Pb> Ni> Cu> Cd. In the five gypsum samples, sample from XS had the highest content of Fe, Ni and Cu while Zn, Pb and Cd were found to be the highest in the sample from BL. All the trace metals detected were found to be the lowest in the sample from CX.According to the heavy metal content analyzed above and their interference order on Hg2+stabilization, Cu, Ni, Pb and Zn were selected to investigate their effects on the precipitation ability of DTCR over Hg2+in gypsum leachate under different pH values. The results indicated that Cu2+had the biggest inhibitory effect on Hg2+precipitation by DTCR during gypsum leaching and the interference order of four trace metals on Hg2+precipitation by DTCR was Cu2+> Ni2+> Pb2+≈Zn2+. The interference order of pH value on Hg2+precipitation was pH3.0> pH4.5> pH7.0. Hg2+was found to be higher in leachate of lower pH value but the effect of pH value differed in different trace metals. Cu2+could inhibit DTCR precipitation over Hg2+under any pH value. However, at a pH of3.0, Hg2+in leachate was higher when Ni2+, Pb2+and Zn2+increased and thus these three trace metlas promoted Hg2+precipitation by DTCR. When pH value of leachate was4.5, both Pb2+and Zn2+had neither inhibition nor promotion on Hg2+precipitation, however, when Ni2+concentration increased to2.0μg/g, Hg2+concentration increased in leachate but afterwards decreased. When the pH of leachate became7.0, Hg2+in leachate was stable along with the increasing concentration of the three trace metals.On the basis of experiments above, the simulated gypsum contaminated by Hg2+was conducted to avoid deoxidization of Hg2+by factors such as S(IV) in gypsum. Trace metals and pH were investigated to test their effects on Hg2+leaching and stabilization in stimulated gypsum. XRD showed that there was no significant difference in physical and chemical properties between the simulated gypsum and the new synthetic one. Thus, the contaminated gypsum was utilized in the following experiments. Results illustrated that all the four trace metals had inhibitory effect on the Hg2+precipitation by DTCR at a pH of4.5, leading to an increase of Hg2+in gypsum leachate. When concentration of four coexisting trace metals increased, the Hg2+concentration increased in the leachate at a pH of4.5and7.0while it remained stable at pH of3.0. This might be due to the interaction of four trace metals on the DTCR precipitation over Hg2+. |
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