Determination of Hg-FA complexion ratio

As an important component of DOM in most natural waters, including pore-waters, FA has lower molecular weight and better solubility property than the other fractions HA and can be dissolved in both acidic and basic aqueous systems (168). Several FA isolates have been obtained and found to enhance the dissolution of cinnabar (59, 63). Therefore, FA was chosen to represent natural DOM in this study to investigate the role of Hg-DOM complexation in cinnabar dissolution. In particular, the study focused on the re-adsorption of Hg-FA on cinnabar. Similar to the previous experiments on the adsorption of Hg-

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cysteine on cinnabar, titration was conducted to determine a proper ratio between Hg(II) (HgNO3) and FA in order to prepare Hg-FA complexes with minimal free Hg and FA present in the resulting mixture. Ether free Hg or FA has an impact on the adsorption of Hg-FA by decreasing or increasing released Hg from cinnabar dissolution (63, 152). These experiments were performed by adding decreasing amounts of HgNO3 to a fixed concentration of FA to produce Hg-FA complexes with a range of Hg:FA mass ratios (1:2, 1:5, 1:10, 1:50, and 1:100). The range of Hg:FA mass ratios was determined by a preliminary estimate. The elemental sulfur content of the Waskish FA was known as 0.29% (w/w, IHSS). However, it was reported that not all S could bind with Hg. The reactive center (RSH) was assumed roughly accounting for 0.15% of DOC on a mass basis by Skyllberg (169). Since the FA used in this work contains 53.63% of DOC, RSH equals 0.08% of FA on the basis of Skyllberg’s estimation. Reduced S fraction (moL/moL of DOM) was calculated as 0.21% for two DOM fractions isolated from Everglades (170). Therefore, if 0.0038 mol (1 g) of HgNO3 was used to bind with FA, a similar number of moles of RSH which is 0.125 g should be used to form Hg-FA complex. Since the mass of RSH is 0.21 % of FA, the FA needed should be around 60 g. The estimated mass ratio of 1:60 just falls in the range used in this work. The titration end point for this Hg-FA complexation experiment was free Hg that was not bound by FA (which in some cases was called “reactive” Hg in previous studies). One of the key analytical steps was to distinguish

Hg-DOM from other forms of Hg present in the solution. Previous studies have confirmed that the Hg(II) complexes containing inorganic ligands (e.g. chloride or hydroxide) and LMW organic ligands (e.g. cysteine) are generally considered reducible by SnCl2 completely (155), whereas the Hg-FA complexed formed through complexation of Hg with

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Waskish FA could not be reduced, just like Hg-DOM complexes of a variety of other DOM(155). The Hg which could be reduced by SnCl2 was labeled reactive Hg and used to represent unbound Hg in my work.

20 40 60 80 100 1:2 1:5 1:10 1:50 1:100 Hg binding%

Hg:FA (mass ratio)

1 hour

24 hour 72 hour

Figure 3.5 The percentage of Hg bound with FA with different mass ratios of Hg to FA at 1, 24

and 72 hours.

Previous study indicated that greater than 85% Hg can bind with dissolved fraction of natural organic matter (NOM) in the presence of excess amounts of NOM after 24 hours (155). To determine a proper binding ratio between Hg and FA, unbound and bound Hg were detected after 1, 24, and 72 hours in this work. The binding percentages for samples with different ratios of Hg and FA after 1, 24, and 72 hours were shown in Fig. 3.5. The binding percentage of Hg increased from 1 to 24 hours and no significant differences in Hg binding percentages were observed between 24 and 72 hours (One way ANOVA, T test, P>0.05). Therefore, the complexation reaction reached equilibrium in 24 hours. Largest Hg binding percentage by FA occurred at a Hg:FA ratio of 1:100 under the experimental conditions. Further increases in FA concentration were not tested. However, even at the Hg:FA ratio of 1:100, 30 - 40% of free Hg existed in the solutions. It’s possible

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that lower mass ratio of Hg:FA (< 1:100) should be applied to make a complex with minimum unbound Hg because of the uniqueness of FA isolated in this work. While, lower Hg and FA ration may decrease the percentage of bound Hg and bring more unbound FA. The extra FA could enhance cinnabar dissolution to make the adsorption underestimated. Therefore, a proper binding ratio of Hg and FA couldn’t be determined in this experimental condition.

3.4.5 The role of Hg-FA complexes in cinnabar dissolution

Without preparation of a Hg-FA complex with minimum amount of free Hg and FA, the role of complexation between Hg and FA in cinnabar dissolution was investigated by adding FA solution directly to cinnabar suspension rather than performing a thermodynamic adsorption experiment by spiking Hg-FA to cinnabar. Reactive Hg was detected by directly reduced by SnCl2 and total Hg was detected using additional pre- oxidation process involving UV and BrCl treatment. Then the concentrations of complexes were calculated by subtraction of reactive Hg from total Hg and the percentage of bound Hg could be estimated. The effect of re-adsorption in cinnabar dissolution was expected to be evaluated by comparing the increasing extent of unbound Hg and complexed Hg with the increasing of FA. The results of cinnabar dissolution in the presence of FA are shown in Fig. 3.6. Less Hg was detected in solution including both unbound Hg and complexed Hg with the increase in the amounts of FA added. After 1 mg/L of FA was spiked into the cinnabar suspension, around 43 ppb of Hg was released from cinnabar including 23 ppb of unbound Hg and 20 ppb of complexed Hg. Decreasing amounts of all these Hg fractions were observed with higher concentrations of spiked FA. When 20 mg/L of FA was spiked, only 5.3 ppb of total Hg was released from cinnabar and 2 ppb of this Hg was in the bound

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form. These results indicated that FA inhibits rather than enhances cinnabar dissolution and the inhibition effect increases at higher concentrations of FA. A possible pathway that could account for the inhibitory effects of FA on cinnabar dissolution is proposed. The adsorption of FA could lead to the coating of active surface sites on cinnabar to inhibit cinnabar dissolution. Upon mixing of the FA solution with cinnabar suspension, rapid adsorption of FA onto cinnabar surface happens as proposed by Ravichandran and Waples (59, 63). Concentrations of DOM decreased following the reaction with cinnabar in both works. In Ravichandran’s work, about 15% of DOM (10.6 mg C/L) was observed decreasing after spiked into 2 g/L of cinnabar. In Waples’s work, an adsorption isotherm was fitted by the Langmuir model and the values of qm and k were determined to be 0.14 mg C/m2 and 0.14 L/mg C using 2 to 16 mg C/L of DOM spiked in 10 g/L of HgS at pH 6 (59). They also obtained an estimated amount of DOM on HgS that ranged from 0.03 to 0.84 mg C/m2 when 10 mg C/L of DOM was spiked in 2 to 80 g/L of cinnabar. The adsorption of DOM on cinnabar has been further confirmed via an electrophoretic mobility experiment (63). In Ravichandran’s work, the negative potential of cinnabar surface increased from -35 to -55 mV at pH 6 indicating that the adsorption of humic substances on cinnabar (63). Previous work has reported that the adsorption of DOM enhances cinnabar dissolution by forming complexes with Hg on cinnabar surface and then releasing to the solution. In this work, the adsorption of FA on cinnabar plays another role by covering the dissolution sites and inhibiting cinnabar dissolution. The opposite reports in the literature could be the result of differences in DOM structures from various sources. The variety of DOM sources makes understanding the interaction between ether Hg or HgS and DOM complicated (59).

69 0 5 10 15 20 25 0 10 20 30 40 50 Free Hg Hg and FA complex Total Hg H g d eted cted (  g /L )

Fulvic acid spiked (mg/L)

Figure 3.6 Concentrations of released Hg as unbound Hg, bound Hg, and total Hg from cinnabar

dissolution at 24h in the presence of 1, 2.5, 5, 10, and 20mg/L of FA.

The inhibition effect may be related to the ratios between DOM and HgS used. As in this work, 1-20 mg DOM/L or 0.54-10.8 mg C/L of FA and 0.1 g/L of cinnabar were mixed. While in previous studies, around 10 mg C/L of DOM was used with 10 g/L of cinnabar. The ratio of DOM to cinnabar was 0.0054-0.108 in this work, much higher than that previously used which was 0.001 (59). However, other studies with ratios as high as 0.05 showed DOM enhanced cinnabar dissolution. When the concentration of cinnabar is fixed, the concentration of dissolved Hg does not increase linearly with the concentrations of DOM, as observed in Ravichandran’s work (63). This observation could be caused by the inhibition of the adsorption of DOM on cinnabar dissolution. Therefore, the ratio of DOM to HgS could be a factor affecting the role of DOM in cinnabar dissolution, among other factors such as the composition and properties of DOM.

3.5 Conclusions

In this work, the roles of thiol-containing organic ligands and re-adsorption of released Hg were investigated. The results indicate that the roles of small molecules and complex

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DOM fractions in cinnabar dissolution are different. In the presence of a small molecule L-cysteine, re-adsorption of Hg-Cys plays an important role on cinnabar dissolution. As proposed in Fig. 3.1, the complexation of Hg-Cys decreases the concentrations of dissolved Hg and forces the dissolution reaction to move to the right direction, which is one role L- cysteine plays in enhancing cinnabar dissolution. Meanwhile, the presence of L-cysteine decreases the re-adsorption of dissolved Hg via complexation, as Hg-Cys shows lower adsorption capacity than that of unbound dissolved Hg on cinnabar surface. For the role of DOM in cinnabar dissolution, the Waskish FA used in this work does not enhance but inhibits cinnabar dissolution, possibly through coating the dissolution sites on cinnabar surface. The inhibitory effect of FA on cinnabar dissolution observed here, in contrary to previously reported enhancing effect, suggest that caution should be exercised when evaluating the role of DOM in cinnabar dissolution, as the interaction of DOM with cinnabar is rather complicated depending on the varieties of DOM structures and compositions, the ratio between DOM and cinnabar, and probably other experiment conditions.

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Chapter 4. Geochemical Modeling of Mercury Speciation in Surface Water and