As a major entry point of mercury (Hg) to aquatic food webs, algae play an important role in taking up and transforming Hg species in aquatic ecosystems. However, little is known how and to what extent Hg reduction, uptake, and species transformations are mediated by algal cells and their exudates, algal organic matter (AOM), under either sunlit or dark conditions. Here, using Chlorella vulgaris (CV) as one of the most prevalent freshwater model algal species, we show that solar irradiation could enhance the reduction of mercuric Hg(II) to elemental Hg(0) by both CV cells and AOM. AOM reduced more Hg(II) than algal cells themselves due to cell surface adsorption and uptake of Hg(II) inside the cells under solar irradiation. Synchrotron radiation X-ray absorption near-edge spectroscopy (SR-XANES) analyses indicate that sunlight facilitated the transformation of Hg to less bioavailable species, such as β-HgS and Hg-phytochelatins, compared to Hg(Cysteine)2-like species formed in algal cells in the dark. These findings highlight important functional roles and potential mechanisms of algae in Hg reduction and immobilization under varying lighting conditions and how these processes may modulate Hg cycling and bioavailability in the aquatic environment.
Hao Zhang,Xupeng Liu,Biao Zhou,Ziyue Chen,Jundi Cheng,Kehang Zeng,Lizhi Zhang,Hongwei Sun,Zhihui Ai
Mercury (Hg) is a toxic metal of great concern, and its reduction from Hg(II) to Hg(0) is crucial to prevent its mobilization and transformation to even more hazardous methylmercury. Fe° could reduce Hg(II), but the role of active hydrogen species (*H) remains unexplored. Herein, we prepared the phosphate-modified zero-valent iron by ball milling (P-ZVI bm ) and investigated its Hg(II) reduction and immobilization capacity compared with the ZVI bm counterpart. P-ZVI bm rapidly adsorbed and reduced Hg(II) from water at a rate 22 times faster than ZVI bm , with *H accounting for 36% of Hg(II) reduction. A greatly higher proportion of negative surface electrostatic potential on P-ZVI bm (50.38%) than ZVI bm (37.61%) facilitated the adsorption of Hg(II) cation. H 2 O adsorption on P-ZVI bm was more favorable, as demonstrated by the in situ ATR-FTIR spectra, H 2 O TPD-mass spectra, and the substantially lower adsorption energy (-0.78 eV) than ZVI bm (-0.17 eV). The energy span of H 2 O dissociation to *H also decreased from 2.80 eV on ZVI bm to 1.99 eV on P-ZVI bm . Moreover, *H adsorption energy on P-ZVI bm (1.70 eV) was lower than on ZVI bm (-1.20 eV), thereby inhibiting consumption of *H by H 2 evolution. P-ZVI bm demonstrated remarkable efficacy in removing Hg(II), Cu(II), Ni(II), Cd(II), and Pb(II) from electroplating and mining wastewater. These results highlight P-ZVI bm as a promising material for Hg(II) and other heavy metal remediation and underscore the essential role of *H in the enhanced Hg(II) reduction.
Recently, simultaneous removal of SO2, NOx and oxidized mercury in wet flue gas desulfurization (WFGD) scrubber has become a research focus. Mercury re-emission in traditional WFGD system has been widely reported due to the reduction of oxidized mercury by sulfite ions. However, in multipollutants simultaneous absorption system, the formation of a large quantity of nitrate and nitrite ions as NOx absorption might also affect the reduction of oxidized mercury in the aqueous absorbent. As such, this paper studied the effects of nitrate and nitrite ions on mercury re-emission and its related mechanism. Experimental results revealed that the nitrate ions had neglected effect on mercury re-emission while the nitrite ions could greatly change the mercury re-emission behaviors. The nitrite ions could initially improve the Hg0-emission through the decomposition of HgSO3NO2–, but with a further increase in the concentration, they would then inhibit the reduction of bivalent mercury owing to the formation of Hg-nitrite complex [Hg(NO2)x2-x]. In addition, the subsequent addition of Cl– could further suppress the Hg0 emission, where the formation of a stable Hg-SO3–NO2–Cl complex was assumed to be the main reason for such strong inhibition effect.
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