Adsorption Of Hg(0) On Suspended Particulate Matter And Its Effect On Hg Cycling In Water

Mercury (Hg) and its compounds, strong biological toxic substances, can damage brain, kidney, lung and the whole nervous system, once entered into body. Elemental Hg (Hg(0)) accounted for over90%of total atmoshpheric Hg. Owing to its stability, Hg(0) would transfer with atmosphere to remote area, which make Hg to be a global pollutant. Hg(0) in atmosphere is mainly from natural and anthropogenic processes, and Hg(0) emission of environmental surface is a major natural pathway for Hg(0) entering into atmoshphere. Emission of Hg(0) from water plays an important role in Hg cycling, as water area contributes almost70%of global environmental surface area. However, water body is a complexed system, including different types of suspended particulate metter (SPM) and dissolved organic matter (DOM). Previous studies has figured out that SPM and DOM influenced the migration, transformation, and bioavaiblity of inorganic Hg (Hg(Ⅱ)) and methyl mercury (MeHg). However, the effect of SPM and DOM on Hg(0) is still unclear. It was previously indicated that soil mineral or organic particles and sediment can adsorb Hg(0), which can control Hg(0) entering into air and water. Dissolved Hg(0) in surface water was also proven to be negative correlated with the SPM concentrations, suggesting that SPM may affect the Hg(0) distribution in water. In addition, the reduced DOM in water can bind Hg(0) by thio group, resulting in low Hg(0) recovery. From what dicussed above, we can speculate that part of Hg(0) in water was binded with SPM (SPM-Hg(0)) and DOM(DOM-Hg(0)), while it is difficult to accurately estimate this part Hg(0) by traditional method which can only measure the dissolved Hg(0). Thus, the aim of this research is firstly to verify the existing of SPM-Hg(0) and DOM-Hg(0), and then establish a method to quantify this part Hg(0). Due to the copmlexed constituents of SPM, the adsorption characteristics between SPM and Hg(0) is also needed to be studied, from which we can know the main adsoption mechanisms and the partition coefficient of Hg(0) between liquid and solid phases. Finally, a dynamic emission model of Hg(0) in natural water (including the formation, diffusion and emission) was established to discuss the influence of SPM on Hg(0) cycling.The experiment of verifying the exsiting of SPM-Hg(0) and DOM-Hg(0) was conducted by spiking istope, the results showed that the recoveries of Hg(0) spiked into lake water were91.5±4.8%,47.5±6.2%and25.0±3.2%, respectively, after incubation in dark for0,1and4hours. However, there was no significant difference for the recovery of Hg(0) spiked into filtered lake water after4hours with that at0hour, suggesting that SPM in water influenced the recovery of Hg(0). Further thermal desorption experiments indicated that the species of decreased Hg(0) did not change, suggesting that SPM can adsorp Hg(0). Similarly, Hg(0) standard solution was spiked into DOM solutions with different concentrations, but the recovery of Hg(0) did not show an obvious decrease after incubation for4hours in dark, indicating that the chosen DOM may not affect the Hg(0) quantification. Thus, when establish the method to measure the levels of Hg(0) binding with SPM and DOM, the DOM-Hg(0) can be ignored. According to the advantage of isotope dilution (the change of Hg species can be neglected, as spiked isotope diluent of Hg could equilibrate with endogenous Hg), the combination of isotope dilution and ICP-MS was used for determination of SPM-Hg(0). Spiked Hg(0) isotope diluent in natural water equilibrated with endogenous Hg(0) after incubation4hours in dark with Hg(0) recovery of14.7±1.9%. Then the spiked and endogenous Hg(0) in natural water were purged, trapped and determined, from which total Hg(0) concentration could be known. Purging water sample immediately once Hg(0) isotope diluent was spiked could get the concentration of dissolved Hg(0). The difference of total and dissolved Hg(0) was considered as SPM-Hg(0) concentration.The results of Hg(0) adsorption on SPM indicated that particle sizes had an effect on Hg(0) adsorption, and the adsorption capacity increased with the decreasing particle size. There was a ’sorbent concentrations’effect, that is, unit adsoprion quantity decreased with the increasing sorbent. All selected typical particles including Fe3O4Al2O3and particulate organic matter (POM) can adsorb Hg(0). The adsoption capacity of Fe3O4was higher than Al2O3, due to their different structures. And the partition coefficient (Kd) values for Hg(0) distribution between liquid phase and different particles were estimated with average Kd value between liquid phase and SPM of (5.2±2.8) x104L kg-1and log(Kd) of4.66±0.20, which was comparable with the Kd of Hg(Ⅱ). This adsorption was likely the result of physisorption by Van der Walls. Additionally, DOM was found to have the ability to bond with Hg(0) by weak binding force, which had not influenced Hg(0) quantification. When Hg(0) was spiked into the mixed solutions of DOM and two metal oxides, the adsorption ability to Hg(0) was stronger than the single DOM and metal oxide. Thus, we speculated that SPM could affect the migration, transformation and fate of Hg(0). On the base of knowledge of quantitative method of SPM-Hg(0) and adsorption characteristic between SPM and Hg(0), the influence of SPM on Hg(0) dynamic emission process was conducted by establishing an emission model (including Hg(0) production, diffusion and emission). The result showed that light was found to be an important factor in Hg(0) production process, while the microbes could be ignored during this process. The oxidation and reduction rate constants of Hg were estamited by the contribution of different light (UVA, UVB and Visible). The annual Hg(0) production amount in the whole Everglades was31.89kg, when original Hg(0) accounted for10%of total Hg in Everglades water. The produced Hg(0) could further distribute in the liquid and solid phases during the diffusion process, and the Hg(0) in solid phase increased with SPM concentrations, resultIing in decreased Hg(0) to atmosphere. Thus, SPM was proved to have a large influence on Hg(0) diffusion and emission.This study verified the exsiting of SPM-Hg(0) and DOM-Hg(0) and established a method to quantify the total Hg(0) and SPM-Hg(0) by the combination of isotope dilution and ICP-MS. The SPM, two typical metal oxides (Al2O3and Fe3O4) and POM can adsorp Hg(0), and the partition coefficient of Hg(0) in liquid phases and SPM was compable with Hg(Ⅱ). However, the binding force between DOM and Hg(0) is weak without affecting the measurement of Hg(0). The major role of SPM in Hg(0) cycling was confirmed from establied dynamic emission model of Hg(0). This research expands an important field for subsequent work on the role of SPM-Hg(0) in the biogeochmical cycling of Hg.