Removal of trace mercury(II) from aqueous solution by in situ formed Mn-Fe (hydr)oxides

The efficiency and mechanism of trace mercury (Hg(II)) removal by in situ formed manganese-ferric (hydr)oxides (in situ Mn-Fe) were investigated by reacting KMnO₄ with Fe(II) in simulated solutions and natural water. In the simulated solutions, the impact of coagulant dosage, pH, and temperature on mercury removal was studied. Experimental results showed that in situ Mn-Fe more effectively removed mercury compared with polyaluminum chloride (PAC) and iron(III) chloride (FeCl₃), and that mercury existed in the form of uncharged species, namely Hg(OH)₂, HgClOH(aq), and HgCl₂(aq). Fourier transform infrared spectroscopy demonstrated that in situ Mn-Fe contained hydroxyl groups as the surface active sites, while X-ray photoelectron spectroscopy (XPS) measurements revealed that MnO₂ or MnOOH and FeOOH were the dominant species in the precipitates. XPS analysis indicated that an Hg-Mn-Fe mixture was formed in the precipitates, suggesting that mercury was removed from solutions via transfer from the liquid phase to solid phase. These results indicated that the primary mercury removal mechanisms in in situ Mn-Fe were surface complexation and flocculation-precipitation processes. Satisfactory removal efficiency of mercury was also observed following in situ Mn-Fe in natural waters.