Citrate-enhanced leaching of arsenic from coal with simulated flue gas under UV irradiation

Simulated flue gas containing SO2 and O-2 was passed into the coal slurry with added citrate in a 1 L stirred reactor to leach arsenic from coal. The effects of main factors on the leaching and oxidation of arsenic were investigated, and the enhanced reaction mechanism was analyzed. Results of experiments under dark and UV irradiation conditions showed that citrate and radicals generated under UV irradiation could effectively enhance the leaching of coal pyrite and arsenic. For 1 mM citrate- and UV-enhanced leaching process, the leaching ratio of coal arsenic at 150 min was 3.6 times higher than that of iron. The leaching ratios of both iron and arsenic increased with decreasing pH. Under UV irradiation most of As(III) leached in solution could be oxidized. Results of experiments with free radical quenchers demonstrated that HO center dot and SO4 center dot- were the two main radicals oxidizing As(III) leached in solution. Based on FTIR and XPS characterization and leaching results, it was deduced that the added citrate could increase the total soluble iron concentration, inhibit both the re-adsorption of leached arsenic and the generation of passive Fe solid products, and enhance the formation of free radicals.