Selenatreduktion durch sulfidiertes nullwertiges Eisen
Zusammenfassung der Projektergebnisse
Sulfidation of zero-valent iron has gained increasing interest in recent years because of the obvious benefits in water treatment and remediation processes. The effectiveness of this process has been studied for a series of compounds, but until the time of submission of the proposal (September 2018), no experience has been obtained in regard to the removal of Se oxyanions. In 2019 two important articles were published that substantially expanded the state of knowledge about removal of Se(VI) by sZVI in the presence of dissolved oxygen. Following the observations made in these articles we have adapted our work programme. In particular, our research objectives were directed towards the understanding of the role of surface sulfur species forming upon sulfidation for the removal of Se(VI). These species may comprise mackinawite (FeS), surface bound polysulfide species as detected in our group during sulfidation of ferric oxyhydroxides (FeS x) as well as also pyrite and elemental sulfur. Selenate was used as a probe compound to study this research question. We applied X-ray Photoelectron Spectroscopy, X-ray Absorption Spectroscopy, and XRD. In addition, we performed wet chemical analysis by sequential extraction of the reduced sulfur species as Acid Volatile Sulfur (AVS) and Chromium Reducible Sulfur (CRS). We performed initial-rate batch experiments in the presence of dissolved oxygen in which the Se(VI) concentrations (5 and 50 mg/L) and the initial S/Fe molar ratios of the ZVI particles were varied (0, 0.1 and 0.6). We used commercially available ZVI particles and sulfidized the particles following a recipe described in the literature. Removal of Se(VI) was observed both, in experiments with non sulfidized ZVI and sulfidized ZVI. A clear enhancement of the removal rate by sulfidation could be observed with almost 100 % efficiency after 10 h at the low S/Fe ratio and at low Se concentration (5 mg/L). Surprisingly, the rate decreased at the higher S/Fe ratio. The lowest rate and incomplete removal was observed without sulfidation. The pattern was similar at the higher initial Se concentration (50 mg/L), albeit the efficiency was lower in all three experimental settings with no major difference between the degree of sulfidation. XPS and XANES probing of the particles surfaces after reaction with a Se loading of 50 mg/L provides clear evidence that sZVI particles reduce Se(VI) in the water to Se(0) species. Between 79.5 – 95.6 % of the Se were recovered as Se(0) and 3.7 – 19.4 % as Se(IV). XPS analysis clearly indicates the increase in effectiveness of Se(VI) reduction with increase in S content on ZVI and suggests that FeSx species are actively consumed during the reduction of Se(VI) into Se (0). In the presence of Se(VI), a distinct decrease in AVS (FeS and surface bound polysulphides) contents relative to values in the absence of Se(VI) is obvious with the Total Reduced Inorganic Sulfur (TRIS = AVS + CRS) content remaining constant. Such relationship suggests involvement of AVS species in the reduction of Se(VI) and an internal conversion of the TRIS pool during the reaction. We therefore propose a reduction of Se(VI) to Se(0) by the FeSx or FeS species, which themselves become oxidized to S(0). In our study, we were able to experimentally demonstrate for the first time substantial contribution of reactive species forming upon sulfidation (FeSx species) to the reduction of the target contaminant selenate. Our findings have severe implications about the effectiveness of the sulfidation process for contaminant transformation in the subsurface. Sulfidation does not only enhance the overall efficiency of electron transfer by ZVI particles. Moreover, sulfidation generates highly reactive FeSx species that will contribute to the overall degradation velocity of contaminants.
