Ferrate removal of metal contaminants in the environmental remediation field: synchrotron radiation studies applied to the Hg contaminated waters from Abbadia San Salvatore, Amiata area

In the past decades, high-valent iron compounds, commonly called ferrates (containing Fe(IV-V-VI)), have drawn lot of interests as sustainable chemical treatments for drinking water/wastewaters/industrial effluents remediation. Due to their properties such as high oxidizing power over the entire pH, selective reactivity and non-toxic decomposition by-products with high coagulant performances, ferrate(VI) have been proposed as multi-purpose water treatment chemicals/coagulants/disinfectants. While a large literature has been produced about the elements-bearing acid mining drainage elements and the Hg release currently represents an ongoing environmental challenge, almost no studies have been published regarding the mercury removal from water using ferrate(VI), except for two inconsistent papers examining the removal of a wide range of metal ions from lab prepared solutions (Bartzatt et al., 1992; Murmann & Robinson, 1974). This study investigates the Hg-removal from water by ferrate(VI) in comparison with more traditional (e.g. FeCl3, “Fe(III)”) and modern methods (e.g. nanoscale iron based reductants, nZVIs and bimetallic Fe(0)+Ag(0) NPs), while starting to elucidate its removal mechanism using synchrotron radiation techniques (X-ray Absorption Spectroscopy). Particularly, for the first time, natural contaminated waters were tested, using samples collected from the Abbadia San Salvatore former Hg-mining area (ASSM) with starting Hg content of ~ 167 and 202 ppb. Together with two solid reagents containing Fe(VI) (Mixfer and solid ferrate “SFe(VI)”), a liquid ferrate synthesized with an original procedure was also tested (“LFe(VI”). The synthesis was developed at the Department of Earth Science of the University of Florence and achieved yields comparable to the best liquid ferrate syntheses ever published. From tests using 500 ppb and 1 ppm Hg(II) laboratory-prepared solutions, LFe(VI) presented ~ 91 % Hg removal, in agreement with the preliminary studies published and comparable with the Fe(III) results. Because of the much more complex nature of ASSM contaminated water, such efficiencies have not been replicated in terms of absolute values, due to the presence inside of the natural-water precipitate and in the suspected suspended particulate of β-HgS particles, as highlighted by SEM and XAS observations. Due to Fe(VI) oxidative power, HgS was dissolved by the application of the ferrates(VI) products, increasing the Hg(II) content in the water but efficiently removing and collecting mercury from sources normally difficult to reach, as well as impacting on the factors favorable to methyl-mercury formation (e.g. reducing conditions). The nZVIs tested did not show results comparable with the Hg removal from literature, probably due to a strong passivation layer affecting their surfaces. Moreover, due to their reductive properties (Hg(II) adsorbed and reduced to Hg(0)), mitigating the risk posed by formation of volatile Hg(0) has to be considered prior to their application. The bimetallic Fe+Ag NPs showed the highest efficiencies under the ASSM natural conditions and the formation of an Hg-Ag amalgam (adsorption Hg(II) -> reduction to Hg(0) -> amalgamation). Hg amalgam formation provides benefits in respect to mechanical strength and stability, likely reducing the tendency of Hg(0) to volatilize. While being far from perfect and presenting high degrees of improvements, this study succeeded in creating a solid platform for testing environmental remediation procedures with natural matrices. Possible future developments regarding this project involve repeating the removal tests varying more the boundary conditions, the combination of different reagents, e.g. KClO + nZVIs and KClO + Fe+Ag NPs, in order to evaluate the possible synergistic effects, and the use of instruments not available to the Department of Earth Sciences, such as an XPS microscope or a HR-TEM.