Electrolytic oxidation of elemental mercury

Abstract

The present application relates generally to processes, systems, and compositions for disposal of liquid elemental mercury (Hg0) collected during processes such as crude and natural gas processing. In one embodiment, the application pertains to a process for converting Hg0 to HgS. The process comprises electrolytically oxidizing an aqueous composition comprising Hg0 to form dissolved Hg2+ in the presence of a composition comprising acetylacetone; sodium perchlorate; and perchloric acid. The formed dissolved Hg2+ is reacted with Na2S to form HgS. The formed HgS may be separated using a solid-liquid separation process.

Description

ELECTROLYTIC OXIDATION OF ELEMENTAL MERCURYCROSS REFERENCE TO RELATED APPLICATION

[0001] This application claims priority to U. S. Provisional Application Serial No. 63 / 759,723, filed on February 18, 2025, the entire disclosure of which is incorporated herein by reference.FIELD OF THE INVENTION

[0002] The present application relates generally to processes, systems, and compositions for disposal of elemental mercury (Hg°) collected during processes such as, for example, crude and natural gas processing.BACKGROUND AND SUMMARY

[0003] What is needed are improved processes, systems, and compositions for processes, systems, and compositions for disposal of elemental mercury (Hg°) collected during processes such as crude and natural gas processing. Ideally, such improved processes, systems, and compositions would be cost-efficient and energy efficient. It would further be beneficial if such processes were rapid and did not require high temperatures and / or pressures. It would further be beneficial if such processes yielded stable particles and / or did not lead to incomplete reaction and / or entrainment of Hg° in products that may be difficult to separate. Advantageously, the instant application meets one or more up to all of the aforementioned needs.

[0004] The present application relates generally to processes, systems, and compositions for disposal of elemental mercury (Hg°). In one embodiment, the application pertains to a process for converting Hg° to dissolved Hg2+. The process comprises providing an aqueous composition comprising Hg° and then electrolytically oxidizing at least a portion of the Hg° under conditions to form dissolved Hg2+in the presence of a suitable electrolyte composition comprising a suitabledisproportionation-promoting agent. The suitable electrolyte facilitates the forming of dissolved Hg2+without substantial formation of Hg+1species or substantial precipitate formation.

[0005] In another embodiment the application pertains to a process for converting Hg° to HgS. The process comprises electrolytically oxidizing an aqueous composition comprising Hg° to form dissolved Hg2+in the presence of a composition comprising acetylacetone; sodium perchlorate; and perchloric acid. The formed dissolved Hg2+is then reacted with sulfide ions derived from, e.g. Na2S or K2S, to form HgS(s). The formed HgS is then separated using a solid-liquid separation process.

[0006] In another embodiment the application pertains to an electrolyte composition comprising water; perchloric acid; sodium perchlorate; and acetylacetone.BRIEF DESCRIPTION OF THE DRAWINGS

[0007] So that the way the above recited features, advantages, and objects of the present invention are attained and can be understood in detail, a more particular description of the invention, briefly summarized above, may be had by reference to the embodiments thereof which are illustrated in the appended drawings. It is to be noted, however, that the appended drawings only illustrate preferred embodiments of this invention and are therefore not to be considered limiting of its scope, for the invention may admit to other equally effective embodiments that vary only in detail. In the drawings:

[0008] Figure 1 depicts an experimental setup for Hg° oxidation by selective electrolytic anode oxidation

[0009] Figure 2 depicts a pH speciation diagram of different Hg(II) species in an aqueous system.

[0010] Figure 3 depicts change of dissolved Hg2+concentration in electrolytes with time in an anode oxidation example with 3V applied potential with 0.1 M HClO4, 0.1 M NaClO4, and 1 M acetylacetone.

[0011] Figure 4 depicts oxidation curves of elemental Hg° anode oxidation experiments at different initial H+concentration (denoted by -log[H+]) and ionic strength (denoted by I) with maximum dissolved Hg2+concentrations and oxidation rates noted in each graph but note that the relative slopes among the curves do not reflect the actual differences among the oxidation rates due to different time scales.

[0012] Figure 5 depicts trend of (a) Hg oxidation rate with initial pH and log I and (b) maximum Hg2+concentration with final pH and log I wherein I represents the ionic strength.

[0013] Figure 6 depicts reduction potential vs SHE (Standard Hydrogen Electrode) of MRR and HER in the range of interest wherein the original values are obtained from Atkins Physical Chemistry, 8thedition,1and then processed with the Nernst Equation to obtain the relationship with pH and log [Hg2+],

[0014] Figure 7(a) depicts (a) Powder XRD data of the black precipitates formed after adding Na2S to the electrolytes.

[0015] Figure 7(b) depicts an SEM picture of the black precipitates.

[0016] Figure 7(c) depicts an SEM picture of the black precipitates.

[0017] Figure 7(d) depicts EDS mapping of the black precipitates, where red and yellow dots represent Hg and S, respectively.

[0018] Figure 8 depicts a representative flow diagram of the process.

[0019] Figure 9 depicts a representative embodiment of a system employing the process.DETAILED DESCRIPTION

[0020] Certain terms are used throughout the following description and claims to refer to particular system components. As one skilled in the art will appreciate, different companies may refer to a component by different names.Definitions

[0021] The terms "comprise" (as well as forms, derivatives, or variations thereof, such as "comprising" and "comprises") and "include" (as well as forms, derivatives, or variations thereof, such as "including" and "includes") are inclusive (i.e., open-ended) and do not exclude additional elements or steps. For example, the terms "comprises" and / or "comprising," when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and / or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and / or groups thereof. Accordingly, these terms are intended to not only cover the recited element(s) or step(s) but may also include other elements or steps not expressly recited. Furthermore, as used herein, the use of the terms "a" or "an" when used in conjunction with an element may mean "one," but it is also consistent with the meaning of "one or more," "at least one," and "one or more than one." Therefore, an element preceded by "a" or "an" does not, without more constraints, preclude the existence of additional identical elements.

[0022] The use of the term "about" applies to all numeric values, whether or not explicitly indicated. This term generally refers to a range of numbers that one of ordinary skill in the artwould consider as a reasonable amount of deviation to the recited numeric values (i.e., having the equivalent function or result). For example, this term can be construed as including a deviation of ±10 percent of the given numeric value provided such a deviation does not alter the end function or result of the value. Therefore, a value of about 1% can be construed to be a range from 0.9% to 1.1%. Furthermore, a range may be construed to include the start and the end of the range. For example, a range of 10% to 20% (i.e., range of 10%-20%) includes 10% and also includes 20%, and includes percentages in between 10% and 20%, unless explicitly stated otherwise herein. Similarly, a range of between 10% and 20% (i.e., range between 10% - 20%) includes 10% and also includes 20%, and includes percentages in between 10% and 20%, unless explicitly stated otherwise herein.

[0023] The term "if may be construed to mean "when" or "upon" or "in response to determining" or "in accordance with a determination" or "in response to detecting," that a stated condition precedent is true, depending on the context. Similarly, the phrase "if it is determined [that a stated condition precedent is true]" or "if [a stated condition precedent is true]" or "when [a stated condition precedent is true]" may be construed to mean "upon determining" or "in response to determining" or "in accordance with a determination" or "upon detecting" or "in response to detecting" that the stated condition precedent is true, depending on the context.

[0024] It is understood that when combinations, subsets, groups, etc. of elements are disclosed (e g., combinations of components in a composition, or combinations of steps in a method), that while specific reference of each of the various individual and collective combinations and permutations of these elements may not be explicitly disclosed, each is specifically contemplated and described herein. By way of example, if an item is described herein as including a componentof type A, a component of type B, a component of type C, or any combination thereof, it is understood that this phrase describes all of the various individual and collective combinations and permutations of these components. For example, in some embodiments, the item described by this phrase could include only a component of type A. In some embodiments, the item described by this phrase could include only a component of type B. In some embodiments, the item described by this phrase could include only a component of type C. In some embodiments, the item described by this phrase could include a component of type A and a component of type B. In some embodiments, the item described by this phrase could include a component of type A and a component of type C. In some embodiments, the item described by this phrase could include a component of type B and a component of type C. In some embodiments, the item described by this phrase could include a component of type A, a component of type B, and a component of type C. In some embodiments, the item described by this phrase could include two or more components of type A (e.g., Al and A2). In some embodiments, the item described by this phrase could include two or more components of type B (e.g., Bl and B2). In some embodiments, the item described by this phrase could include two or more components of type C (e.g., Cl and C2). In some embodiments, the item described by this phrase could include two or more of a first component (e g., two or more components of type A (Al and A2)), optionally one or more of a second component (e.g., optionally one or more components of type B), and optionally one or more of a third component (e.g., optionally one or more components of type C). In some embodiments, the item described by this phrase could include two or more of a first component (e.g., two or more components of type B (Bl and B2)), optionally one or more of a second component (e.g., optionally one or more components of type A), and optionally one or more of a third component (e g., optionally one or more components of type C). In some embodiments, the item describedby this phrase could include two or more of a first component (e.g., two or more components of type C (Cl and C2)), optionally one or more of a second component (e.g., optionally one or more components of type A), and optionally one or more of a third component (e.g., optionally one or more components of type B).

[0025] This written description uses examples to disclose the invention, including the best mode, and also to enable any person skilled in the art to make and use the invention. The patentable scope is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they have elements that do not differ from the literal language of the claims, or if they include equivalent elements with insubstantial differences from the literal language of the claims.

[0026] Unless defined otherwise, all technical and scientific terms used herein have the same meanings as commonly understood by one of skill in the art to which the disclosed invention belongs. All citations referred herein are expressly incorporated by reference.General Processes, Compositions, and Systems

[0027] Liquid Hg° is a metallic conductor which can act as an anode and be oxidized when an external potential is applied. Electrochemical methods as described herein are usually more controllable and have more engineerable kinetics than other methods. The processes described here advantageously often do not produce substantial amounts of insoluble HgO, Hg2Cl2, or other precipitates instead of soluble Hg2+products that may be further converted to products such as HgS.

[0028] The processes here employ substantial selectivity of the oxidation process to usually formmostly dissolved Hg2+. Selecting electrolytes as described herein facilitate producing Hg2+without substantial formation of a precipitate. It has been discovered that Hg2+may form a precipitate with, e.g. Cl’, PO43’, SO42-, OH-23, and therefore the instant processes may not employ substantial amounts of an electrolyte with such anions. Similarly, in some embodiments substantial amounts of NO3' may not be suitable as an electrolyte since it could act as oxidant and be consumed in acidic environments. Depending upon other reactants and conditions, Cl' may also form a precipitate.

[0029] It has also been discovered that formation of Hg(I) should be reduced and / or avoided. Typically, both Hg(I) and Hg(II) may be oxidation products of Hg°. Hg(I) often exists as a Hg22+dimer in solution. Hg(I) may undergo a disproportionation reaction at elevated pH or when ligands are added (Eq. 1 below), with, for example, Hg° microdroplets forming as the product. Hg(I) in the product stream may be undesirable because any sulfides introduced to stabilize Hg2+after the oxidation process, and sulfide will disproportionate Hg(I), forming a mixture of Hg° and HgS. Moreover, Hg(I) could react with chloride to for a precipitate on the Hg° electrode surface and slow or stop the reaction. Therefore, the instant application contemplates employing one or more disproportionation - promoting agents to facilitate a disproportionation of an amount of Hg22+generated._, OH“, S2-, NH3, AA

[0030] Hg2+- Hg°(l) + Hg2+(1)

[0031] Advantageously, the processes and compositions here facilitate the use of an aqueous based electrolytic oxidation system that may selectively oxidizes liquid Hg° into Hg2+. Thus, in some embodiments due to the insoluble oxidation product issue described, a perchlorate (ClO4-) orsimilar performing anion may be selected as the electrolyte anion. In this manner insoluble Hg(II) product formation may be substantially avoided.

[0032] Mercury(II) perchlorate is highly soluble in water, which often prevents the formation of an oxidation film on the anode and at the same time provides a relatively high electrolyzing rate and high or near maximum Hg2+concentration. To assist with the Hg(I) selectivity issue discussed above, acetylacetone or a similar functioning compound may be employed as a disproportionation-promoting agent. Such compounds may exist at many different pH, have a high solubility in water, and / or resist substantial oxidation or reduction in the cell.

[0033] In some embodiments, a perchlorate-acetylacetone system is employed. A chemical reaction without and with consideration of further sulfurization is shown in Eqs. 2 and 3, respectively.„, ElectricityHg°(l) + 2H+> Hg2++ H2(g) (Electrolytic cell) (2)HC104, NaClO4, AAHg°(l) + 2H++ S2“ HgS + H2(g) (Whole System) (3)

[0034] The operating parameters (pH and / or ionic strength) that affect the Hg° anode oxidation rate and the maximum Hg2+concentration formed may vary. Generally, for both quantities, a higher value may be desired to maximize reactor efficiency. Maximum reaction rates and / or capacity may vary depending upon reactants and / or conditions. As shown in the specific examples, maximum reaction rates and capacity may be at about 5 M ionic strength when employing, for example, 5 M HClO4. Using the present disclosure a person of ordinary skill may employ mass transfer theory perhaps increase and / or maximize the relationship between adesirable high Hg2+concentration and a pH employed. In the examples below the composition and morphology of precipitated HgS after adding sulfide into the solution was investigated with both powder X-Ray Diffraction (Powder XRD) and Scanning Electron Microscopy (SEM). Based on the specific examples single cell anode oxidation may exhibit a relatively high oxidation rate along with in some cases repeatability and / or stability.Components and Steps

[0035] The processes, components employed, and systems generally relate to converting Hg° to dissolved Hg2+. The dissolved Hg2+may be employed in any useful manner. In some embodiments at least a portion up to all of the Hg2+may be converted to a solid such as HgS which may then be separated, if desired, and put to a beneficial use such as a pigment in paints, rubbers, plastics, batteries, etc. In other embodiments, the solution of dissolved Hg2+could be injected into a liquid waste or wastewater disposal well for permanent disposal and isolation.

[0036] Generally, an aqueous composition comprising Hg° is provided for electrolysis, e.g., electrolytically oxidizing Hg° under conditions to form dissolved Hg2+. The conditions employed may vary depending upon the concentrations and components of the electrolysis system employed. Typically, a suitable electrolyte composition is employed that does not result in substantial precipitates being formed.

[0037] Suitable electrolyte compositions may vary but in some embodiments comprise an electrolyte composition with a pH of from about 0 to about 9 and / or a composition with an ionic strength of from about 0.5M to about 10M. Suitable electrolyte compositions may comprise an alkali metal salt of a strong acid such as sodium perchlorate along with, if desired, a strong acidsuch as perchloric acid. Such combinations have been found to have a suitable pH and ionic strength to accomplish the oxidation as shown in the examples herein.

[0038] If desired, a suitable disproportionation-promoting agent may be employed with the electrolyte composition. Such agents may be useful as a catalyst in facilitating the Hg22+to Hg0reaction while avoiding Hg(I) precipitates or at least keeping precipitates minimal. Suitable disproportionation-promoting agents may vary depending upon on the other components, the equipment, and desired results. In some embodiments the disproportionation-promoting agent comprises acetylacetone.

[0039] The concentration of disproportionation-promoting agent in an electrolyte composition may vary depending upon the specific components. In some embodiments wherein the electrolyte composition comprises sodium perchlorate and perchloric acid while the disproportionation-promoting agent comprises acetylacetone, the ratio of disproportionation-promoting agent to electrolyte may be from about 1% to about 10%. That is, suitable electrolyte compositions may comprise, for example, water; perchloric acid; sodium perchlorate; acetylacetone.

[0040] Once formed Hg2+is typically already dissolved in an aqueous solution. This aqueous solution may be reacted, if desired, with a suitable sulfide under conditions to form HgS. Suitable sulfides may comprise, for example, an alkali metal sulfide such as Na2S or K2S. The HgS formed is solid and thus may be separated in any convenient solid-liquid separation such as centrifuging, flocculation, filtration, or any combination thereof.Examples

[0041] The following materials were employed in the examples: Mercury (99.9995% trace metalsbasis), sodium perchlorate monohydrate (97+%), perchlorate acids (70%), and acetylacetone (99+%). Platinum sheet electrodes (10 mm x 10 mm) and mesh electrodes with 99.99% purity were employed.

[0042] Anode oxidation examples were conducted in a glass-made single-cell electrolytic cell with 50 mm diameter as shown in Figure 1. The electrolytes comprise sodium perchlorate, perchloric acids, and acetylacetone. To determine the Hg° oxidation rate at the anode, the change in Hg2+concentrations were measured in the electrolyte solution over 80 min, measuring the dissolved Hg2+concentration every five minutes. The electrolytes are composed of 0.1 M HClO4, 0.1 M NaCK and 1 M acetylacetone. A 3V potential was applied between a platinum cathode and a mercury anode. Measurements were also conducted in electrolytes with different initial sodium perchlorate and perchloric acid concentration ratios to test how pH and ionic strength affect the reaction rate and maximum dissolved Hg2+concentration achievable. Initial perchloric acid concentrations of 0.001 M, 0.01 M, 0.1 M were used. Sodium perchlorate was added to these solutions to provide ionic strengths of 1 M, 2 M, 5 M for analysis. The acetylacetone concentrations in all electrolytes was 1 M.

[0043] A platinum mesh electrode contacts liquid mercury serving as the anode, while a platinum plate electrode acted as the cathode in the examples. However, if desired a cathode of an inert metal such as Pt, carbon may be employed. The cathode may also comprise elemental Hg°. In some embodiments using Hg° in both the cathode and anode may offer some benefits in reactor design.

[0044] Liquid elemental Hg° can also serve as a cathode. Elemental mercury was poured into an electrolytic cell with a depth of 5 mm. The platinum mesh electrode contacted the metallic Hg°anode. The platinum plate electrode was suspended in the solution, acting as cathode. An adjustable DC power supply was connected to the electrodes. The examples were conducted with 3 V applied DC potential. About 25 mL of electrolyte was added into the electrolytic cell for each batch. The electrolyte was sampled every two minutes at the first ten minutes to determine an oxidation rate, and sampled three times after the mercury concentration stabilized to determine the maximum Hg2+concentration that can be achieved. A 100 μL electrolyte sample was collected at each time point for analysis of dissolved Hg(II), which sample was relatively negligible compared to the electrolyte volume.

[0045] A pH electrode (Orion™ ROSS Ultra™, ThermoFisher) was used to measure the electrolyte pH at the beginning and end of each oxidation batch experiment. A sodium perchlorate – perchloric acid solution with I = 2 M and [H+] = 0.1 M was used as the electrode filling solution to minimize the liquid junction potential. Cold Vapor Atomic Fluorescent Spectroscopy (CVAFS) (Brooks Rand Instrument) was used to measure the Hg2+concentration in the solution following EPA method 1631E. CVAFS measures the total mercury in the solution, including Hg0, Hg22+, and Hg2+. It was assumed that little to no Hg° is present in the solution as it was not observed visually and it has a low solubility (56 pg / L). The presence of Hg22+was observed as a white precipitate formed after adding 1 M NaCl solution into the electrolytes. Due to a high concentration of total mercury, the Hg22+is negligible compared to Hg2+when there were no white precipitates observed by bare eyes. That is, if no Hg° or Hg22+is observed, one may assume the total Hg concentration measured was equivalent to Hg2+concentration.

[0046] HgS was prepared by mixing the electrolytes with a 0.1 M NaOH + 0.1 M Na2S solution at a 1:1 (volume) ratio, followed by centrifugation and a water wash. Powder XRD (MalvernPanalytical Empyrean, Cu-Ko.) and SEM (FEI Quanta 600) were deployed to analyze the structure and morphology of HgS precipitates.

[0047] To selectively oxidize Hg°(l) into Hg2+, electrolytes were tested to avoid any substantial formation of oxidation film on the Hg0anode. Several electrolyte systems were inspected for their ability to form only dissolved Hg2+, i.e. to substantially avoid any solid Hg(II) precipitate from forming.

[0048] OH', SO42-, CT, and ClOF were tested. Only ClOF (perchlorate) did not form a substantial oxidation film on Hg° anode. When the DC current was turned on, all of the other anions formed visible and somewhat sticky oxidation films that slowed or stopped the reaction and were somewhat difficult to separate from the liquid Hg° surface.

[0049] An oxidation film appeared to form in electrolytes with OH-, indicating that the precipitate formation process may be pH dependent. Powder XRD showed the anode oxidation film formed in 0.1 M NaOH was comprised of crystalline (Pmnm) HgO. While not wishing to be bound by any particular theory, the HgO oxidation film may be formed by Hg2+transformation into Hg(OH)2 at basic pH. Hg(OH)2 may not be a stable solid phase and further transformed into HgO. A pH speciation diagram of different Hg(II) species in equilibrium with HgO is shown in Figure 2, where all thermodynamic data are from reported values. The diagram shows that the thermodynamically allowable Hg2+concentration is higher at lower pH, which means that potentially the HgO oxidation film should dissolve at low pH. Therefore, the electrolytes for electrolysis experiments here were conducted at low pH with perchloric acid / sodium perchlorate electrolytes.

[0050] Besides Hg(II), Hg° oxidation also may have Hg(I) as its oxidation product, which thedissolved form is Hg22+ions. However, Hg(I) is not desired in this system because it will potentially undergo a disproportionation reaction (Eqn. 1 above), generating Hg° that may be entrained with HgS precipitates. It was surprisingly discovered that adding acetylacetone into the electrolytes facilitated the elimination of Hg22+by promoting the disproportionation reaction in the electrolytic cell. When the reaction was fast enough, Hg22+rapidly disproportionated into Hg0and Hg2+and Hg0returned to the bulk liquid Hg° anode. Thus, there will only be Hg2+in a substantial amount of the solution.

[0051] Electrolytic anode oxidation experiments at different acetylacetone concentrations were conducted to observe the reaction phenomenon, deduce the function of acetylacetone at different concentrations, and determine an optimal acetylacetone concentration for anode oxidation experiments. A series of acetylacetone concentrations at 0.01 M, 0.1 M, 0.5 M, 1 M, and 2 M were used in 0.1 M HClO4 / O.9 M NaCICE electrolyte, and a 3 V potential was applied in all five groups. The electrolytes were sampled, observed, and analyzed qualitatively after 80 min of applied 3V potential.

[0052] Different colors were observed in electrolytes of the five different groups, which are summarized in the Table below. Saturated NaCl solutions were added into each sample dropwise, to test whether Hg(I) exists in the electrolytes. Equation (4) is a characteristic equation, where Hg22+forms a white Hg2C12 precipitate when Cl- is added (as NaCl).

[0053] Based on observation, the groups with 0.01 M and 0.1 M acetylacetone formed a white precipitate (Figure 2(b)), and groups with 0.5 M and 1 M acetylacetone formed a yellow precipitate while the group with 2M acetylacetone formed no observable precipitate. After obtaining the precipitates by centrifugation, the precipitates were dispersed in acetone. The yellow precipitateformed in 0.5 M and 1 M acetylacetone groups completely dissolved in acetone, while a white precipitate formed in 0.01 M and 0.1 M groups did not. The white precipitates were further investigated with powder XRD and the pattern was found to be well matched with crystalline Hg2C12 (P21 / m).

[0054] Hg22++ 2 Cl-→ Hg2Cl2(s) (4)

[0055] Electrolytes with the different acetylacetone concentrations were collected after 80 min of electrolysis at 3V at 0.01 M, 0.1 M, 0.5 M, 1 M, 2 M. A white precipitate formed after adding saturated NaCl into the 0.01 and 0.1 M acetylacetone experiment groups. A yellow precipitate formed after adding saturated NaCl into 1 M acetylacetone experiment group. The yellow precipitate dissolved in acetone, indicating that is it not a Hg(I) species. The summary of observed experimental phenomena at different acetylacetone concentration is shown in the table below.[Acetylacetone] 0.01 M 0.1 M 0.5 M I M 2 M Color of Yell Colorless Ven- light yellow Strong yellow Strong yellow electrolytes ’ ow Precipitates after Yes Yes Yes YesNo adding NaCl? White White Yellow Yellow Precipitatesbehavior after Remains Remains Dissolves Dissolves N / A acetone washingPrecipitateHg2Cl2(XRD) Hg2Cl2(XRD) N / A N / A N / A characterization

[0056] Figure 3 shows Mercury Oxidation Kinetics of am Perchlorate - Acetylacetone System. Specifically, Figure 3 shows change of dissolved Hg2+concentration in the electrolytes with time in the anode oxidation experiment with 3V applied potential, an electrolyte with 0.1 M HClO4, 0.1 M NaClO4, and 1 M acetylacetone.

[0057] Figure 3 shows the dissolved Hg2+concentration change during mercury anode oxidation in 25 mL of electrolyte composed of 0.1 M HClO4, 0.1 M NaClO4, and 1 M acetylacetone. A 3 V external potential is applied between the platinum cathode and mercury anode. The dissolved Hg2+concentration linearly increases with a constant oxidation rate for the first 50 min of reaction, then reaches a maximum dissolved Hg2+concentration that remains stable. Both quantities are important for the system design, where the oxidation rate decides the Hg° removal rate, and the maximum Hg2+concentration decides the capacity of the reactor after which sulfides have to be added to the solution to lower the Hg2+concentration such that Hg° oxidation can resume. The addition of Na2S or K2S or other source of sulfide ions may be added to fluid that has exited the reactor and entered another mixing chamber. Using K2S instead of Na2S may lead to precipitation or KCIO4, and / or allow for removal of K+ from the electrolyte.

[0058] Examples were conducted to review pH and ionic strength and the effect on oxidation rate and substantially maximum Hg2+concentrations achievable. Nine mercury oxidation examples were conducted. A 3x3 experimental matrix included an initial HClO4concentration of 0.001 M, 0.01 M, 0.1 M and a total initial ionic strength of 1 M, 2 M, 5 M achieved with, for example, the addition of sodium perchlorate. The measured Hg oxidation curves of each group are summarized in Figure 4.

[0059] The oxidation curves of all the oxidation groups generally have two points in common. First, Hg2+concentrations generally increase linearly at the beginning of oxidation reaction. Second, the dissolved Hg2+reached a maximum concentration and stop accumulating with time, meaning the macroscopic oxidation rate was zero or near zero. However, the reaction did not stop even though the macroscopic rate is zero. There was a constant current observed during the period when the Hg2+concentration was constant. Considering Hg oxidation is typically the most likely reaction on the anode, it may be that the oxidized Hg2+ions were being reduced on cathode, leaving the overall Hg2+concentration unchanged. In addition, the mercury reduction reaction (MRR) may happen when Hg2+concentration is high, where at low Hg2+concentration the hydrogen evolution reaction (HER) is favored instead. In addition, the cathode seemed to be substantially covered with mercury droplets after about 1 hour oxidation.

[0060] Cathode (MRR): |Hg2++ e“ Hg(l) E° = 0.852 V (2)

[0061] Cathode (HER): H++ e-→ H2E⁰ = 0.000 V (3)

[0062] Anode: ½Hg2++ e-→ Hg(l) E⁰ = 0.852 V (4)

[0063] The trends for how pH and ionic strength affect the Hg2+oxidation rate and the maximum dissolved Hg2+concentrations are shown in Figure 5. The initial pH values are used for trends of oxidation rates and the final pH values are used for trends of maximum dissolved Hg2+concentration since they are measured in the initial and final stages of the examples, respectively. Planar surface fitting shows that the Hg oxidation rate is usually positively related to the initial pH and log I (log 10 value of ionic strength) of the electrolytes. The slope ofRate-loglis significantly greater than that of Rate - pH, indicating that ionic strength may have a more significant impacton reaction rate than pH. This is very likely because the ionic strength increases the electrolyte conductivity, and high conductivity pushes both the current and oxidation rate higher. The solution pH may play a role in oxidation rate. It indicates the oxidation kinetics is possibly not mediated by H+concentration, and the Hg anode oxidation process may be somewhat irrelevant to H+. The maximum Hg2+concentration may be positively related to the log I and negatively related to a final pH value.

[0064] Mass Transfer Interpretation of Maximum Hg2+Concentration

[0065] A reason why the dissolved Hg2+concentration reaches a maximum value could be because the HER is replaced by an MRR reaction when the dissolved Hg2+concentration is high enough. That is, the electric power may be moving Hg° from the anode to the cathode. The reason why HER is replaced by MRR may be thermodynamics and / or kinetics. However, the reduction potential of HER may be lower than that of MRR in the range of interest as shown in Figure 6 which depicts reduction potential vs. SHE (Standard Hydrogen Electrode) of MRR and HER in a range of potential interest. The original values are obtained from Atkins Physical Chemistry, 8thedition, and then processed with the Nernst Equation to obtain the relationship with pH and log [Hg2+].

[0066] Thermodynamics generally favors the MRR over the HER, which may be opposite to the formation of hydrogen bubbles on the cathode at the beginning of the oxidation reaction.

[0067] Therefore, the observed phenomenon may be caused by kinetic reasons. Kinetic limitations of an electrode process can be a result of reaction barrier or mass transfer limitations. The reaction barrier limitation can be caused by overcoming the reaction barrier, where anadditional applied voltage (overpotential) is employed to cross the barrier and raise a reaction rate. This overpotential can be due to, for example, a Butler-Volmer model at low overpotential. However, the reaction rates will usually not infinitely increase if the overpotential is continually increased because mass transfer will eventually limit the current and causing mass transfer loss. A 3V applied potential in this experiment is ~2.1 to 3 V higher than the equilibrium potential of the electrolytic cell (Eq. 2 - 4) above, which is much higher than the typical range the Butler-Volmer can describe (several tens to hundreds of mV). A limiting current equation may be applied which describes the maximum current being limited by mass transfer (Eq. 5).

[0068] = nFAkiti^

[0069] The ki is the mass transfer coefficient of species i, a is the activity of species i, F is Faraday constant, A is the surface area, and ii,i is the limiting current. For limiting currents of H+and Hg2+, their ratio is thereforeil, H+ / il, Hg2+= k(H+)a(H+) / 2k(Hg2+)a(Hg2+)(6)

[0071] The mass transfer coefficient is related to the ion diffusion coefficients. Assuming the electrode-water interface applies the surface renewal theory as Eq. 7. Given DH+= 9.31 × 10-9m2 / s, DHg2+= 0.913 × 10-9m2 / s,27we can derive the ratio between the mass transfer coefficients as Eq. 8

[0072]

[0073] 3.16 (8)fcHg2 +

[0074] When it Hg2+ = 100 it H+, the HER rate is negligible compared to MRR. Then we get

[0075] = 0.0063 (8)aHg2+

[0076] It means that when the activity of H+is around 0.0063 times of the activity of Hg2+, theMRR will replace the HER. This value is close to the — — value of the oxidation group at - aHg2+log[H+] = 3 and I = 2 M, which is —— = 0.0052 (assuming ang2+ = 0.1 [Hg2+] at I = 1 M). This “Hg2+model predicts that the maximum Hg2+concentration decreases with pH.

[0077] HgS Generation from the System

[0078] Powder XRD and SEM data of black precipitates (Figure 7) formed after adding Na2S were obtained and analyzed. The powder XRD data of the precipitates matches with XRD data of Cubic HgS (P-HgS). No other main peak was observed, which means the precipitate is likely substantially pure P-HgS with powder XRD grade. This is consistent with the HgS being formed at low temperature and pressure, which is the 0-HgS form of HgS. SEM pictures indicated that HgS particles are mainly polycrystalline. Energy Dispersive Spectrometry (EDS) mapping shows Hg and S existing. No obvious mercury droplets are observed among HgS particles, indicating Hg22ions are well eliminated, and the particles may be disposed.

[0079] Specifically Figure 7 shows:7a) Powder XRD data of the black precipitates after adding Na2S to the electrolytes 7(b)(c) SEM pictures of the black precipitates 7(d) EDS mapping of the black precipitates, where red and yellow dots represent Hg and S, respectively.

[0080] In sum, the examples demonstrate a new electrolytic method to rapidly oxidize elemental mercury into Hg2+ions in aqueous solution using an electrolyte system comprised of sodium perchlorate / perchloric acid electrolyte that may help to prevent the formation of insoluble oxidation products on the Hg anode. Acetylacetone may be employed to selectively oxidized Hg° into Hg2+without substantial (or any) Hg(I) formation. The electrolytic system showed an oxidation rate of over 20 mg min-1cm’2Hg at 3 V. Thus, for a reactor with 1 m2Hg surface area, there can be more than 12 kg of liquid Hg° oxidized in an hour. The surprising and unexpected high oxidation rate may make the system appropriate for industrial applications, such as waste liquid mercury stabilization in, for example, the petroleum industry. Trends of reaction rate and maximum dissolved Hg2+concentration versus pH and ionic strength were established. The solutions after oxidation may be readily stabilized withNa2S and / or K2S or other forms of sulfide to form pure 0-HgS precipitates that can be removed from the electrolyte which can then be reused in the process. Mass transfer theory described was discovered to explain the dissolved Hg2concentration maximum and / or predict a positive relation between H+activity and a maximum dissolved Hg2+concentration.

[0081] Figure 8 depicts a potential flow diagram while Figure 9 depicts a specific possible embodiment of a process described above.

[0082] Overall: The overall process may be batch or continuous and employ any useful Solidliquid separation as described below.

[0083] Solid-liquid separation: Solid-liquid separation as used in the disclosed process may be performed using a variety of techniques. By way of non-limiting example, such separationtechniques may include filtration, centrifugation, sedimentation (e g., in a cargo hold, in an oil storage tank), or flotation.Embodiments

[0084] 1. A process for converting Hg° to dissolved Hg2+comprising:providing an aqueous composition comprising Hg°; electrolytically oxidizing Hg° under conditions to form dissolved Hg2in the presence of a suitable electrolyte composition comprising a suitable disproportionation-promoting agent;wherein the suitable electrolyte facilitates the forming of dissolved Hg2+without substantial precipitate formation in solution or on the electrode.

[0085] 2. The process of claim 1 wherein the suitable electrolyte composition comprises an electrolyte composition with a pH of from about 0 to about 9.

[0086] 3 The process of claim 1 wherein the suitable electrolyte composition comprises a composition with an ionic strength of from about 0.5 to about 10.

[0087] 4. The process of claim 1 wherein the suitable electrolyte composition comprises an alkali metal salt of a strong acid.

[0088] 5. The process of claim 4 wherein the alkali metal salt of a strong acid comprises sodium perchlorate.

[0089] 6 The process of claim 1 wherein the suitable electrolyte composition comprises perchloric acid.

[0090] 7 The process of claim 1 wherein the suitable electrolyte composition comprises sodium perchlorate and perchloric acid.

[0091] 8. The process of claim 1 wherein the disproportionation-promoting agent rapidly disproportionates Hg22+to form Hg° and Hg2+.

[0092] 9. The process of claim 1 wherein the disproportionation-promoting agent comprises acetylacetone.

[0093] 10. The process of claim 1 which further comprises reacting the formed dissolved Hg2+with a sulfide to form HgS.

[0094] 11. The process of claim 10 wherein the sulfide comprises an alkali metal sulfide.

[0095] 12. The process of claim 11 wherein the alkali metal sulfide comprises Na2S or another sulfide or thiol to form HgS.

[0096] 13. The process of claim 10 wherein the process further comprises separating the formed HgS.

[0097] 14. The process of claim 13 wherein the separating comprises centrifuging.

[0098] 15. The process of claim 13 wherein the separating comprises filtration and / or centrifuging and flocculation.

[0099] 16. A process for converting Hg° to HgS comprising:electrolytically oxidizing an aqueous composition comprising Hg° to form dissolved Hg2+in the presence of a composition comprising acetylacetone; sodium perchlorate; and perchloric acid;reacting the formed dissolved Hg2+with NazS and / orKzS to form HgS; andseparating the formed HgS using a solid-liquid separation process.

[0100] 17. An electrolyte composition comprising:water;perchloric acid;sodium perchlorate; andacetylacetone.

[0101] 18. The composition of embodiment 17 which further comprises mercury.

[0102] 19. The composition of embodiment 17 which further comprises sodium sulfide.

[0103] 20. The composition of embodiment 17 which further comprises HgS.

[0104] 21. A system comprising:an electrolytic cell for oxidizing an aqueous composition comprising Hg° to form dissolved Hg2+; anda reactor operably linked to the electrolytic cell wherein the reactor reacts the formed Hg2+from the electrolytic cell with Na2S to form HgS; anda continuous centrifuge operably linked to the reactor to separate the formed HgS.

[0105] It will be understood that various modifications may be made to the embodiments disclosed herein. Therefore, the above description should not be construed as limiting, but merely as exemplifications of example embodiments. For example, the functions described above and implemented as the best mode for operating the present invention are for illustration purposes only. Other arrangements and methods may be implemented by those skilled in the art without departing from the scope and spirit of this invention. Moreover, those skilled in the art will envision other modifications within the scope and spirit of the claims appended hereto.

Claims

CLAIMS1. A process for converting Hg° to dissolved Hg2+comprising:providing an aqueous composition comprising Hg°;electrolytically oxidizing Hg° under conditions to form dissolved Hg2+in the presence of a suitable electrolyte composition comprising a suitable disproportionation-promoting agent; wherein the suitable electrolyte facilitates the forming of dissolved Hg21without substantial precipitate formation.

2. The process of claim 1 wherein the suitable electrolyte composition comprises an electrolyte composition with a pH of from about 0 to about 9.

3. The process of claim 1 wherein the suitable electrolyte composition comprises a composition with an ionic strength of from about 0.5 to about 10.

4. The process of claim 1 wherein the suitable electrolyte composition comprises an alkali metal salt of a strong acid.

5. The process of claim 4 wherein the alkali metal salt of a strong acid comprises sodium perchlorate.

6. The process of claim 1 wherein the suitable electrolyte composition comprises perchloric acid.

7. The process of claim 1 wherein the suitable electrolyte composition comprises sodium perchlorate and perchloric acid.

8. The process of claim 1 wherein the disproportionation-promoting agent rapidly disproportionates Hg22+to form Hg° and Hg2+.

9. The process of claim 1 wherein the disproportionation-promoting agent comprises acetylacetone.

10. The process of claim 1 which further comprises reacting the formed dissolved Hg2+with a sulfide to form HgS.

11. The process of claim 10 wherein the sulfide comprises an alkali metal sulfide.

12. The process of claim 11 wherein the alkali metal sulfide comprises Na2S.

13. The process of claim 10 wherein the process further comprises separating the formed HgS.

14. The process of claim 13 wherein the separating comprises centrifuging.

15. The process of claim 13 wherein the separating comprises centrifuging and flocculation.

16. A process for converting Hg° to HgS comprising:electrolytically oxidizing an aqueous composition comprising Hg° to form dissolved Hg2+in the presence of a composition comprising acetylacetone; sodium perchlorate; and perchloric acid;reacting the formed dissolved Hg2+with Na2S to form HgS; andseparating the formed HgS using a solid-liquid separation process.

17. An electrolyte composition comprising:water;perchloric acid;sodium perchlorate; andacetylacetone.

18. The composition of claim 17 which further comprises mercury.

19. The composition of claim 17 which further comprises sodium sulfide.

20. The composition of claim 17 which further comprises HgS.

21. A system comprising:an electrolytic cell for oxidizing an aqueous composition comprising Hg° to form dissolved Hg2+; anda reactor operably linked to the electrolytic cell wherein the reactor reacts the formed Hg2+from the electrolytic cell with Na S to form HgS; anda continuous centrifuge operably linked to the reactor to separate the formed HgS.

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