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Mercury Removal from Concentrated Sulfuric Acid by Electrochemical Alloy Formation on Platinum
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Mercury Removal from Concentrated Sulfuric Acid by Electrochemical Alloy Formation on Platinum
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  • Vera Roth
    Vera Roth
    Department of Physics, Chemical Physics, Chalmers University of Technology, Gothenburg SE-41296, Sweden
    More by Vera Roth
    Orcidhttps://orcid.org/0000-0002-1914-2020
  • Julia Järlebark
    Julia Järlebark
    Department of Physics, Chemical Physics, Chalmers University of Technology, Gothenburg SE-41296, Sweden
    Stena Center, Atium, Gothenburg SE-41292, Sweden
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  • Alexander Ahrnens
    Alexander Ahrnens
    Stena Center, Atium, Gothenburg SE-41292, Sweden
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  • Jens Nyberg
    Jens Nyberg
    Boliden Kokkola, Kokkola FI-67101, Finland
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  • Justin Salminen
    Justin Salminen
    Boliden Kokkola, Kokkola FI-67101, Finland
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  • Teodora Retegan Vollmer
    Teodora Retegan Vollmer
    Department of Chemistry and Chemical Engineering, Nuclear Chemistry and Industrial Materials Recycling, Chalmers University of Technology, Gothenburg SE-41296, Sweden
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  • Björn Wickman*
    Björn Wickman
    Department of Physics, Chemical Physics, Chalmers University of Technology, Gothenburg SE-41296, Sweden
    *Email: [email protected]
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    Orcidhttps://orcid.org/0000-0001-7119-9529
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ACS ES&T Engineering

Cite this: ACS EST Engg. 2023, 3, 6, 823–830
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https://doi.org/10.1021/acsestengg.2c00417
Published April 12, 2023

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Abstract

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Mercury is a highly toxic heavy metal, and improved removal processes are required in a range of industrial applications to limit the environmental impacts. At present, no viable removal methods exist commercially for mercury removal of aqueous solutions at high acidic conditions, such as concentrated sulfuric acid. Herein, we show that electrochemical mercury removal based on electrochemical alloy formation on platinum, forming PtHg4, can be used to remove mercury from concentrated sulfuric acid. Thin platinum film electrodes and porous electrodes with supported platinum are used to remove more than 90% of mercury from concentrated acid from a zinc smelter with an initial mercury concentration of 0.3–0.9 mg/kg, achieving high-quality acid (<0.08 mg/kg) within 80 h. The removal process is carried out in 50 mL laboratory-scale experiments and scaled up to a 20 L pilot reactor with retained removal efficiency, highlighting excellent scalability of the method. In addition, the removal efficiency and stability of different electrode substrate materials are studied to ensure high-quality acid and a long lifetime of the electrodes in harsh chemical conditions, offering a potential method for future large-scale mercury decontamination of sulfuric acid.

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Introduction

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Sulfuric acid (93–98%) is one of the most widely used chemicals in the world with an annual production of more than 200 million tonnes per year, with applications in, e.g., mining, fertilizer, and paper industries. Sulfuric acid is mainly produced from sulfur dioxide gas via a so-called contact process, where sulfur dioxide is made by burning of fossil fuels or it can exist as a by-product from zinc and copper smelting processes. (1−3) When sulfuric acid is produced by the metal producing industry, it can be highly contaminated as the ore can contain varying amounts of metals such as chromium, nickel, copper, zinc, lead, manganese, cadmium, arsenic, and mercury. (4) Using the method of roasting in smelting, the mercury from the ore is volatilized as mercury vapor and it exits with the off gases and enters the product acid. (5,6) This mercury contamination can to some extent be mitigated using gas phase cleaning methods. However, these methods can be expensive and are often not sufficient to completely remove mercury from sulfuric acid below the required limits. (7,8) For commercial applications, sulfuric acid is considered to be of technical-quality grade with regard to mercury when its content is below 0.30 mg/kg and high-purity grade when the content is below 0.08 mg/kg. (4) Depending on the application of sulfuric acid, the remaining mercury can end up in the produced product and/or in the waste streams from the processes and can, thus, be spread to the environment and increase the risk of human exposure. (5)
Mercury is a heavy metal with high toxicity and bioaccumulation and is widely dispersed in the environment, posing a serious threat to all living organisms. (9−11) Mercury can be emitted to the environment through natural sources, such as volcanic eruptions and rock weathering. However, emissions through anthropogenic sources, such as mining and smelting plants, are substantially higher than natural ones and it is estimated that 1,130,000 tonnes of mercury was released into the environment through anthropogenic sources between the years 1850–2010. (12) In 2015, the global annual emissions from anthropogenic sources were estimated to be 2220 tonnes, with 300 tonnes emitted from nonferrous metal production. (13) During the last decades, efforts have been made to reduce the emissions of mercury; however, it persists in the environment as mercury released years ago is still circulating in the environment today. After its initial release into the environment, surfaces such as land and water can repeatedly re-emit mercury into the atmosphere, where mercury can be transported vast distances. As a result, mercury does not only have an impact on a local scale, but it is a global concern, affecting millions of people’s lives and their environment around the world. (14−16)
Current established methods for mercury decontamination of aqueous solutions are far from ideal. These methods include adsorption, precipitation, coagulation, electrodialysis, flocculation, flotation, ion exchange, and membrane filtration, each with their own advantages and disadvantages. Depending on the application, the main drawbacks can include low selectivity for mercury, poor efficiency at very low mercury concentrations, and cost efficiency. (17−20) Some methods are also highly pH-dependent, e.g., for effective sulfide precipitation, a pH of 7–9 is needed, and for methods involving adsorption, the optimum pH ranges between 4–5. (21) In addition, the materials used for most adsorption methods, as well as ion-exchange and membrane separation methods, can degrade when used to treat concentrated acids. Thus far, no commercial solution has been presented for removing mercury from concentrated acids, such as sulfuric acid. (20)
We have recently introduced an electrochemical mercury removal technique based on electrochemical alloy formation which was shown to be able to remove mercury from aqueous streams with pH between 0–6.6. (22) In this study, we continue further development of this removal method by examining for the first time its potential to remove mercury from concentrated sulfuric acid, both from synthetic samples and authentic industrial acid produced from a zinc smelter.

Experimental Section

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Fabrication of Working Electrodes

For laboratory-scale experiments, electrodes made of polished fused silica glass (Mark Optics Inc.) (15 × 30 × 0.5 mm), stainless-steel plate 316L (SS316L) (15 × 25 × 1 mm) (Harald Pihl), SS316L 45 PPI foam (15 × 25 × 5 mm) (Alantum, Fraunhofer IFAM), and a Duocel reticulated vitreous carbon (RVC) 60 PPI foam (15 × 40 × 5 mm) (ERG Aerospace) were used. The glass electrodes were coated with 100 nm of platinum with 3 nm titanium adhesion layer using physical vapor deposition (Lesker PVD 225 e-beam evaporator, base pressure ∼10–7 mbar). A pattern of titanium and platinum was created on the glass electrode using deposition masks consisting of a 15 × 15 mm square at the bottom of the substrate with a 1 mm connector strip to the top, where the contact was made. Both the SS316L plate and the RVC foam were coated with 200 nm of platinum with 2 nm of titanium as the adhesion layer using a PVD sputter system (Nordiko 2000 sputter). In addition, for a control experiment, an RVC foam without platinum coating was used. The SS316L foam was coated with approximately 1–2 μm of platinum with 2 nm of gold as the adhesion layer using a high-temperature electrolysis process in a molten salt solution in a commercial process performed by the company Umicore. A thin SS316L wire (0.5 mm) was attached to the top part of the SS316L and RVC foams for electric contact, while for the glass and SS316L plate electrodes, contact was established by using a polymer-covered copper wire attached with copper-conductive tape and sealed with hot glue, as described earlier. (22) For pilot experiments, an SS316L foam material (210 × 270 × 3 mm) (Alantum, Fraunhofer IFAM) was coated with platinum by Umicore in the same process as described above. Photographs and further information about the electrodes used can be seen in the Supporting Information (SI).

Electrolyte Preparation

Concentrated sulfuric acid from a zinc smelting plant in Kokkola, Finland, was provided by Boliden, and used as received for experiments both at the laboratory scale and pilot. At laboratory-scale (50 mL) experiments, three different batches of acid with initial mercury concentrations of approximately 0.3, 0.8, and 0.9 mg/kg were used. For the pilot-scale experiments (20 L), the acid batch had an initial mercury concentration of 0.8 mg/kg. Synthetic concentrated sulfuric acid (97%, Suprapur, Merck) with an initial mercury concentration of 0.8 mg/kg was used at the laboratory scale for experiments using the RVC foam electrode without platinum. In addition, synthetic acid with an initial mercury concentration of 5.5 mg/kg was used for electrode analysis.

Analytical Techniques

The concentration of all metals in solution was analyzed using inductively coupled plasma mass spectrometry (ICP-MS; iCAP-Qc, Thermo Fischer). X-ray diffraction (Siemens Diffraktometer D5000) was used to investigate the platinum glass electrode before and after electrochemical treatment.

Electrochemical Experiments

Laboratory Scale: 50 mL Cell

All experiments at the laboratory scale were performed using a three-electrode system with 50 mL of concentrated sulfuric acid as the electrolyte. Fused silica glass plate, SS316L plate and foam, as well as RVC foam-supported Pt electrodes were used as the working electrode (WE), with a Pt wire or a diamond electrode (DIACHEM, Condias) as the counter electrode (CE), and a Ag/AgCl (SI Analytics, B3610+) electrode as the reference electrode (RE). A potentiostat (Gamry Reference 600) was used to apply a constant potential of +0.18 V vs the reversible hydrogen electrode (RHE). This potential of +0.18 V vs RHE was chosen to provide a large overpotential for electrochemical alloy formation, (22−24) while avoiding hydrogen evolution reaction (HER). With HER, gas bubbles could cover parts of the electrode surface and hinder further mercury reduction, as less surface area is available on the electrode. The experiments were performed at a temperature of 20 °C and with active stirring using a glass-covered magnetic stirrer. Samples were taken from the electrolyte before, during, and after the experiments for ICP-MS analysis of Hg, Fe, and Pt contents. Each time, a sample of 100 μL was removed from the electrolyte and diluted with 1 M hydrochloric acid (prepared from Suprapur 36%, Supelco HCl, and Milli-Q water, >18 MΩ/cm) with 2 μg/L bismuth (Bi Internal Standard in 2% HNO3, 10 mg/L, VHG Labs) to a concentration with a dilution factor of 50 in the interval of 0–33 μg Hg/L, for ICP-MS analysis. A schematic illustration and photographs of the laboratory cell can be seen in the SI.

Pilot Scale: 20 L Cell

All pilot-scale experiments were performed with a two-electrode setup in a 30 L SS316L reactor (Denios) containing 20 L of concentrated sulfuric acid as the electrolyte. Two SS316L foams were used as the cathode with two diamond-coated Si electrodes (200 mm × 20 mm × 6 mm) (DIACHEM Condias) as the anode. A bench top power supply (EA Elektro-Automatik) was used to control the potential or current applied to the electrodes. Two separate pilot experiments were performed: one with a three-electrode setup using the potential control and one with a two-electrode setup using the current control. For simplification, the cathode and the anode are referred to as the working electrode (WE) and the counter electrode (CE), respectively, both in the two-electrode setup and the three-electrode setup. A Ag/AgCl reference electrode (RE) was used during the setup of the potential control experiment to achieve the same potential of the WE as in the laboratory-scale experiments. The voltage between WE and CE was adjusted during the first couple of hours of the experiment but was kept constant at −2.98 V from 2.5 h forward. During the current control experiment, a constant current of −0.7 A was applied to the WE to achieve roughly the same current density as in the laboratory-scale experiments. Experiments were performed at temperatures varying in the range 13–16 °C and with a propeller stirrer (IKA EUROSTAR 40 digital, 3-bladed 316L) at 500 rpm. Samples were taken from the electrolyte before, during, and after the experiment for ICP-MS analysis of the Hg, Fe, and Pt contents. Each time, three samples of 100 μL were removed from the electrolyte and diluted to a concentration suitable for ICP-MS analysis, as described above. A schematic illustration and photographs of the experimental setup can be seen in the SI.

Results and Discussion

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Mercury Removal and Alloy Formation in Concentrated Sulfuric Acid in a 50 mL Laboratory Cell

It has previously been shown that mercury containing aqueous solutions can be efficiently decontaminated using a method based on an electrochemically induced formation of an alloy between mercury in solution and a platinum cathode electrode, in the thermodynamically favored phase PtHg4. (25) The mechanism between platinum and mercury during electrochemical alloy formation has been discussed in the literature previously. (22−27) It can be described as a multi-step process, where divalent mercury ions are reduced to elemental mercury on an electrode’s surface when sufficient potential is applied to form an alloy with platinum. The overall chemical reaction for the formation of PtHg4 is
Pt(s)+4Hg2++8e−→PtHg4(s)
This technique has the potential to offer several important advantages over established methods, e.g., it has been shown that the alloying process is not affected by pH (in the interval 0–6.6). The method has high efficiency, extremely high selectivity for mercury, and is able to remove mercury in solutions with initial mercury concentrations down to 50 μg/L. The process is reversible, and thus, the electrodes and platinum can be regenerated and reused. (22) However, the technique has not been applied to concentrated acids before, where it could be expected that high acidity might make the mercury ions more stable in the acid than in the alloy.
To evaluate the possibility of using electrochemical alloy formation to remove mercury from highly acidic streams, concentrated sulfuric acid with an initial mercury concentration of approximately 0.30 mg/kg was decontaminated using a 15 × 30 mm active area of a platinum-coated fused silica glass (Pt/Glass plate) electrode with an applied potential of +0.18 V vs RHE, and the results are shown in Figure 1a. It can be observed that more than 80% of the mercury content was successfully removed from the acid within 140 h as the mercury concentration was reduced from the initial 0.30 mg/kg to a final concentration of 0.06 mg/kg. This means that the process was able to decrease the mercury concentration well below technical quality and even below high purity.

Figure 1

Figure 1. Mercury removal at the laboratory scale (50 mL) using (a) a Pt/glass electrode (15 × 30 mm) and (b) an SS316L plate (15 × 25 mm), as well as two different foam materials: (c) SS316L foam (15 × 25 mm), where technical quality (0.30 mg/kg) of the acid is reached within 20 h and targeted high-purity quality (0.08 mg/kg) within 80 h. (d) RVC foam (15 × 40 mm) and RVC foam without platinum coating. Solid lines represent the time where the potential (0.18 V vs RHE) was applied to the working electrode (0–118 and 0–123 h for RVC and Pt/RVC foam, respectively) and dashed lines correspond to the time when the potential was removed from the electrode (118–123 and 133–134 h for RVC and Pt/RVC foam, respectively). For Pt/RVC foam, technical quality (0.30 mg/kg) of the acid is reached within 20 h, and high-purity quality (0.08 mg/kg) is not quite achieved. The curve fittings serve as a visual aid without representing the best fit of the data spread.

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In order to confirm that the alloy formed in concentrated sulfuric acid is in the phase PtHg4, as previously shown in diluted acids, (22) an additional removal experiment was performed at the laboratory scale with a Pt/Glass plate electrode. The electrode was characterized with X-ray diffraction (XRD) before and after electrochemical mercury removal in concentrated acid, and the XRD pattern obtained can be seen in the SI. To obtain clear and visible XRD patterns, the 100 nm platinum film was saturated to approximately 25% of its theoretical PtHg4 saturation limit using sulfuric acid with an initial mercury concentration of 5.5 mg/kg. The results show the presence of PtHg4 after electrochemical mercury removal from concentrated acid.
For practical applications, a thin platinum film on glass is not a viable electrode design as all current needs to be conducted laterally in the thin platinum film and, thus, the electrode is very sensitive to scratches and wear. For industrial applications, it would be desirable to use a conductive substrate with strong adhesion to platinum and with a relatively low price. Potential support materials for platinum include high-quality corrosion-resistant stainless steel, such as SS316L and Reticulated Vitreous Carbon (RVC), both shown to have excellent corrosion resistance in concentrated sulfuric acid at room temperature. (28−30)
To assess the possibility of using SS316L as a support material, a 15 × 25 mm platinum-coated SS316L plate (Pt/SS316L plate) electrode was used to decontaminate concentrated sulfuric acid with an initial mercury concentration of approximately 0.80 mg/kg, and the results are shown in Figure 1b. The Pt/SS316L plate was observed to be stable in the concentrated acid, and the process was able to remove around 60% of mercury in 120 h, reducing the initial concentration from 0.76 mg/kg to a final concentration of 0.32 mg/kg. Thus, it can be concluded that platinum-coated SS316L can be used as the working electrode in the mercury removal process. However, in this experiment, with an initial mercury concentration of 0.80 mg/kg and an electrode active area of 15 × 25 mm, it was not possible to reach technical or high purity quality within 120 h. Had the experiment been conducted for a longer time, or if a larger active surface area electrode had been used, it is most likely that the mercury concentration would have been reduced below both technical- and high-purity quality. (18,22) For practical applications, short process times are desired and, thus, the most viable path is to use electrodes with a much higher surface area than the flat glass and steel plate electrodes. The platinum needs to be deposited onto a high surface area support to increase the rate of mercury removal and minimize the amount of platinum required, as precious metals such as platinum come at a very high cost. It can be noted that the mercury removal rate from the plate electrodes Pt/SS316L, presented in Figure 1b, is very low, approximately 0.3 μg/cm2 h for an initial concentration of 0.8 mg Hg/kg. This estimation of the removal rate is based on the difference between the first two data points (Figure 1b), after one hour of electrochemical treatment and divided by the area of the electrode used.
Foam materials are a type of material that offers a very high surface area, and by using a 15 × 25 mm platinum-coated SS316L foam (Pt/SS316L foam) electrode for the removal experiment, over 90% of the mercury content was removed within 120 h. The mercury concentration was reduced from an initial 0.80 mg/kg to a final concentration of 0.05 mg/kg, and technical quality was achieved within 20 h and high purity within 80 h, as shown in Figure 1c. Comparing the removal rates between the platinum-coated SS316L plate and foam (Figure 1b,c), it is clear that the rate is enhanced around 10 times for the foam, due to its large surface area. Thus, it can be stated that a platinum-coated foam electrode can be used to remove mercury well below the technical- and high-purity target levels from concentrated sulfuric acid at a rate suitable for industrial processes.
However, in these experiments, it was found that the SS316L foam was unstable in the concentrated acid during mercury removal, as iron, to a large extent, and platinum, to a minor extent, was found to leach from the electrode during the experiment, as discussed further below. As an alternative support material, a 15 × 40 mm platinum-coated RVC foam (Pt/RVC foam) electrode was used instead of the SS316L foam to decontaminate concentrated sulfuric acid with an initial mercury concentration of approximately 0.9 mg/kg. Using an RVC foam material, which is a metal-free carbon material, instead of the SS316L foam or other metal electrode materials, will prevent potential metal ion contamination of the acid from the cathode electrode. The results are shown in Figure 1d, where around 90% of the mercury content was removed from the acid within 120 h to yield a final concentration of 0.10 mg/kg. Technical quality of the acid was reached within 20 h. In this experiment, the mercury concentration appears to level out and not decrease below 0.10 mg/kg, a value just above the targeted high-purity quality. Up to 20 h, the mercury removal for the RVC foam is very similar to that for the SS316L foam, and it is somewhat unexpected that the mercury concentration does not continue to decrease as much for the RVC foam after 20 h. A possible explanation could be that when platinum is deposited on a surface of a porous substrate, the platinum film thickness will vary from virtually zero up to the nominal thickness of the deposited material. During the removal process, the part of the foam with the thinnest platinum layers will get saturated, leading to a less platinum surface area on the surface being able to absorb mercury, which could lead to lower removal rates. In addition, the deposition for the RVC foam was performed from only one side of the foam material; thus, the porous foam has limited coverage and the available platinum surface is likely much smaller for the RVC foam than for the SS316L foam. With an improved coating technique, it is likely that the Pt/RVC foam electrodes would enable a mercury concentration well below that of the targeted high-purity quality.
In order to show the mercury removal process without platinum on the working electrode, an additional experiment was performed with a pure RVC foam electrode without platinum coating in 50 mL of synthetic concentrated sulfuric acid with an initial concentration of 0.8 mg/kg, and the result can be seen in Figure 1d together with the platinum-coated RVC. It is clear that with an applied potential, the mercury concentration in the acid is reduced also on the electrode without platinum, but at a much lower rate compared to the Pt/RVC foam electrode. This is most likely a result of regular mercury ion reduction to metallic mercury in the porous RVC structure and trapping of mercury in the porous structure. However, the mercury that is plated or attracted to the electrode is not stable in the absence of platinum, and when the potential is turned off, the mercury concentration rapidly increases to the original concentration (the dashed line in Figure 1d). This is in contrast to the case with a platinum-coated RVC where the mercury removal is much faster, and mercury is stable on the electrode also after the potential is removed. In this case, the mercury concentration remains at the lowest value also after turning off the potential. It is clear that with platinum on the electrode, it is possible to form a stable alloy also in highly acidic conditions as well as decrease the mercury concentration of the acid.
While these results are very promising, they reflect mercury removal from concentrated sulfuric acid via electrochemical alloy formation at the laboratory scale (50 mL). To be used in real applications, the method needs to be scaled up to treat hundreds or even thousands of tonnes of concentrated sulfuric acid within given time constraints.

Electrode Stability during Mercury Removal

As indicated above, the Pt/SS316L foam was found to release relatively large amounts of iron and a small amount of platinum during the removal experiment. Figure 2a shows iron and mercury concentrations in sulfuric acid during the removal process and as can be seen, iron continuously leached out of the foam and after about 60 h, the concentration of iron in the acid had increased from an initial value of about 16 mg/kg to above the technical quality limit given for iron, 30 mg/kg. At 120 h, the iron concentration had increased to approximately 48 mg/kg, which is unacceptable from a commercial point of view and indicates that these electrodes would have a limited lifetime. It can also be noted that the relatively high initial iron levels (about 16 mg/kg) in this batch of acid were higher than the iron limit for high-purity quality (15 mg/kg). In addition, platinum was found to also leach out of the SS316L foam material, where the concentration rapidly increased at the start of the removal process when the potential was applied but seemed to level off after 1–2 h and stay constant at about 0.03 mg/kg for the rest of the experiment, as can be seen in Figure 2b. It is likely that at least part of the leached platinum from the Pt/SS316L foam results from corrosion of the foam support, but it might also originate from inhomogeneities in the deposition process where some platinum might have low adhesion to the substrate. It is important to note that after the initial expel, the remaining platinum on the electrode appears to be stable, and despite the continuous leaching of iron, no additional platinum is lost during the rest of the removal process. It is difficult to identify the reasons for the low stability of the SS316L foam, and it is also unexpected since other forms of SS316L are used globally to store and handle sulfuric acid in industry. A potential explanation might be that the foam did not have the stated high-quality SS316L structure and composition throughout the material due to defects from fabrication or as a result of the high-temperature plating process.

Figure 2

Figure 2. Stability of the Pt/SS316L foam (15 × 25 mm) during mercury removal at the laboratory scale with the corresponding increase in concentration for iron (a) and platinum (b). The iron concentration rapidly increases above high-purity quality (15 mg/kg) and after about 60 h, the concentration reached above the technical limit (30 mg/kg). The platinum concentration jumps (0.05 mg/kg) when a potential is applied to the system and then remains somewhat stable throughout the 120 h. No high-purity technical limits are given for platinum. The curve fittings serve as a visual aid without representing the best fit of the data spread.

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The Pt/RVC foam material demonstrated excellent stability in the concentrated sulfuric acid. The material is a carbon-type foam, insoluble in concentrated acid, and thus, iron or other metals leaching out is not an issue in this case. Similar to the Pt/SS316L foam, platinum leached out of the Pt/RVC foam at the start of the removal process and after 120 h, the platinum content in the acid was about 0.03 mg/kg. Again, this can originate from the deposition process itself where some of the platinum might have low adhesion to the substrate. The results for the Pt/RVC foam can be seen in the SI.

Mercury Removal and Alloy Formation in Concentrated Sulfuric Acid in a 20 L Pilot Reactor

To assess the prospects of implementing the electrochemical mercury removal process at a larger scale, the laboratory-scale experiments in a 50 mL reactor, as described in the previous sections, were scaled up 400 times to a reactor able to treat 20 L of concentrated sulfuric acid, which will be referred to as a pilot reactor. The pilot experiment was performed two times and yielded similar results in both cases; one was performed using potential control and the other using current control. The results from the latter experiment are presented and discussed here in this section, while the results from the former can be seen in the SI.
Concentrated sulfuric acid with an initial mercury concentration of approximately 0.75 mg/kg was decontaminated using Pt/SS316L foam electrodes and a constant current of −0.7 A, over the span of 7 days. Figure 3a shows mercury removal with the change in mercury concentration in the 20 L pilot as well as the results at the laboratory scale using a similar Pt/SS316L foam electrode (data from Figure 1c). The decrease in mercury concentration and the corresponding increase in iron concentration during the pilot removal process are presented in Figure 3b.

Figure 3

Figure 3. Mercury removal from concentrated sulfuric acid using Pt/SS316L foam electrodes, where (a) the blue curve corresponds to mercury removal at the laboratory scale (50 mL), while the red curve represents removal in a 20 L pilot reactor. In both cases, technical quality (0.30 mg/kg) of the acid is reached within 24 h and high-purity quality (0.08 mg/kg) within 80 h. (b) Red curve corresponds to the mercury removal in a 20 L pilot reactor, while the black curve is the corresponding increase in iron concentration. After 120 h, the iron concentration in the pilot increases above technical quality (30 mg/kg). The curve fittings serve only as visual aids without representing the best fit of the data spread.

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It can be observed that mercury is successfully removed from the 20 L reactor of concentrated sulfuric acid as well. The experiment removes over 98% of the mercury content within 160 h and technical quality is reached around the 20 h mark and high-purity quality within 80 h. The rate of mercury removal in the pilot reactor is initially very similar to the rate measured at the laboratory scale; however, after 10 h, the rate of removal in the pilot becomes lower and technical quality is achieved slightly later than that at the laboratory scale.
After 60 h, the removal trend becomes similar and high-purity quality is achieved in both cases at similar times, within 80 h. There are several possible reasons for the small differences in removal behavior between the laboratory scale and pilot reactor experiments, (i) the cathode electrode area vs electrolyte volume was approximately 40% larger in the pilot experiment which should result in an increased removal rate. (ii) The temperature during the pilot experiments varied between 13–16 °C, while at the laboratory scale, it was constant at 20 °C, something that should result in a lower reaction rate and slower mercury removal in the pilot experiment. (18) (iii) In the pilot reactor experiments, two rather small anode electrodes were used in contrast to one of roughly equal size to the cathode electrode, as was used in the laboratory-scale experiments. (iv) The geometry of the reactors and the mixing of the acid were different in the two experiments, which could also impact the rate of removal. Despite these differences, it is observed that the removal process largely follows the same trend and reached mercury concentrations well below technical and high-purity quality during the experiments. Similar to the mercury removal at the laboratory scale using the Pt/SS316L foam electrode, the iron concentration in the acid increases almost linearly throughout the 7 days, increasing above the iron technical limit of 30 mg/kg after 120 h. It can be noted that the quality of the concentrated sulfuric acid was again poor to begin with in terms of iron contamination, as the iron concentration in the acid was initially above the iron high-purity limit of 15 mg/kg. To avoid increased iron contamination of the acid, the pilot experiment could have been carried out using the Pt/RVC foam material as experiments at the laboratory scale showed promising results. However, Pt/SS316L foams were more readily available of the size and quality needed for the pilot than the Pt/RVC foams. Despite Pt/SS316L foams lacking stability in the concentrated acid at the laboratory scale, mercury removal was still excellent, and thus, the material was used to evaluate the scalability of the removal method in a 20 L pilot-reactor.
The pilot experiments confirm excellent scalability of the technique, while maintaining the same efficiency. The technique’s scalability is crucial for commercialization, and these results show the technique’s potential for real applications, where it needs to be further scaled up to treat hundreds or even thousands of tonnes of sulfuric acid.

Conclusions

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Using a novel method based on electrochemical alloy formation on platinum, over 90% of the initial mercury content was removed from commercial concentrated sulfuric acid from a smelting plant, achieving high quality of the acid with respect to mercury content below important industrial limits of 0.30 and 0.08 mg/kg. The method was successfully employed at the laboratory scale, where mercury was removed from 50 mL of concentrated sulfuric acid below a technical quality limit of 0.30 mg/kg and a high-purity quality limit of 0.08 mg/kg, within 80 h. The method was scaled up 400 times from the laboratory scale to a 20 L pilot reactor and concentrated sulfuric acid was again decontaminated below technical and high-purity quality limits within 80 h. This confirmed scalability indicates that the method has great potential for industrial-scale applications, treating thousands of tonnes of concentrated sulfuric acid, which has never been demonstrated before. It can also be mentioned that the electrochemical alloy formation process is reversible, and the electrodes can be regenerated by reversing the electrical potential and releasing the mercury in a controlled way and, thus, allow reuse of the electrodes. However, to consider this novel method for applications at a larger scale, finding a suitable support material for the platinum electrode is of paramount importance to ensure high quality of the treated sulfuric acid and long-lasting electrodes that can survive several cycles of mercury removal and regeneration. A potential candidate, a stainless-steel foam (SS316L), was shown to offer efficient mercury removal; however, the material was discovered to have poor corrosion resistance to the concentrated acid and thus unsuitable for future applications. An alternative support material, a carbon foam (RVC), displayed efficient mercury removal without corrosion or degradation of the material in the concentrated acid, presenting a very promising candidate as the support material for the platinum electrode for further development at large-scale mercury decontamination.

Supporting Information

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The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsestengg.2c00417.

  • Additional experimental details; electrode materials; methods; uncertainties; and illustrations and photographs of the experimental setup (PDF)

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Author Information

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  • Corresponding Author
  • Authors
    • Vera Roth - Department of Physics, Chemical Physics, Chalmers University of Technology, Gothenburg SE-41296, Sweden;  Orcidhttps://orcid.org/0000-0002-1914-2020
    • Julia Järlebark - Department of Physics, Chemical Physics, Chalmers University of Technology, Gothenburg SE-41296, Sweden;  Stena Center, Atium, Gothenburg SE-41292, Sweden
    • Alexander Ahrnens - Stena Center, Atium, Gothenburg SE-41292, Sweden
    • Jens Nyberg - Boliden Kokkola, Kokkola FI-67101, Finland
    • Justin Salminen - Boliden Kokkola, Kokkola FI-67101, Finland
    • Teodora Retegan Vollmer - Department of Chemistry and Chemical Engineering, Nuclear Chemistry and Industrial Materials Recycling, Chalmers University of Technology, Gothenburg SE-41296, Sweden
  • Author Contributions

    V.R., J.J., A.A., and B.W. contributed to the preparation of the electrodes and the design of the experimental process. V.R. and J.J. conducted the electrochemical experiments, and V.R. conducted the ICP-MS analysis. All authors discussed the results and wrote the manuscript.

  • Notes
    The authors declare the following competing financial interest(s): (1) a patent application (EP 17199244.9) has been filed, and B.W. is listed as one of the inventors. (2) The startup company Atium is currently developing a commercial water treatment system based on the patent, with B.W. as one of the minority owners and A.A. working in product development. (3) The industrial samples obtained from Boliden Kokkola where J.N. and J.S. are employed.

Acknowledgments

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The authors gratefully thank the financial support from the Swedish Research Council for Sustainable Development (Formas). Part of this work was performed within the Strategic Innovation Program Swedish Mining Innovation, funded by Vinnova, the Swedish research council Formas, and the Swedish Energy Agency. We thank Linnéa Strandberg for sputter deposition of platinum on the RVC material and Marcus Hedberg for X-ray diffraction characterization, performed at Myfab Chalmers and the Chalmers Material Analysis Laboratory, CMAL.

References

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  • Abstract

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    Figure 1

    Figure 1. Mercury removal at the laboratory scale (50 mL) using (a) a Pt/glass electrode (15 × 30 mm) and (b) an SS316L plate (15 × 25 mm), as well as two different foam materials: (c) SS316L foam (15 × 25 mm), where technical quality (0.30 mg/kg) of the acid is reached within 20 h and targeted high-purity quality (0.08 mg/kg) within 80 h. (d) RVC foam (15 × 40 mm) and RVC foam without platinum coating. Solid lines represent the time where the potential (0.18 V vs RHE) was applied to the working electrode (0–118 and 0–123 h for RVC and Pt/RVC foam, respectively) and dashed lines correspond to the time when the potential was removed from the electrode (118–123 and 133–134 h for RVC and Pt/RVC foam, respectively). For Pt/RVC foam, technical quality (0.30 mg/kg) of the acid is reached within 20 h, and high-purity quality (0.08 mg/kg) is not quite achieved. The curve fittings serve as a visual aid without representing the best fit of the data spread.

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    Figure 2

    Figure 2. Stability of the Pt/SS316L foam (15 × 25 mm) during mercury removal at the laboratory scale with the corresponding increase in concentration for iron (a) and platinum (b). The iron concentration rapidly increases above high-purity quality (15 mg/kg) and after about 60 h, the concentration reached above the technical limit (30 mg/kg). The platinum concentration jumps (0.05 mg/kg) when a potential is applied to the system and then remains somewhat stable throughout the 120 h. No high-purity technical limits are given for platinum. The curve fittings serve as a visual aid without representing the best fit of the data spread.

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    Figure 3

    Figure 3. Mercury removal from concentrated sulfuric acid using Pt/SS316L foam electrodes, where (a) the blue curve corresponds to mercury removal at the laboratory scale (50 mL), while the red curve represents removal in a 20 L pilot reactor. In both cases, technical quality (0.30 mg/kg) of the acid is reached within 24 h and high-purity quality (0.08 mg/kg) within 80 h. (b) Red curve corresponds to the mercury removal in a 20 L pilot reactor, while the black curve is the corresponding increase in iron concentration. After 120 h, the iron concentration in the pilot increases above technical quality (30 mg/kg). The curve fittings serve only as visual aids without representing the best fit of the data spread.

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      9
      Global sources and pathways of mercury in the context of human health
      Sundseth, Kyrre; Pacyna, Jozef M.; Pacyna, Elisabeth G.; Pirrone, Nicola; Thorne, Rebecca J.
      International Journal of Environmental Research and Public Health (2017), 14 (1), 105/1-105/14CODEN: IJERGQ; ISSN:1660-4601. (MDPI AG)
      This paper reviews information from the existing literature and the EU GMOS (Global Mercury Observation System) project to assess the current scientific knowledge on global mercury releases into the atm., on global atm. transport and deposition, and on the linkage between environmental contamination and potential impacts on human health. The review concludes that assessment of global sources and pathways of mercury in the context of human health is important for being able to monitor the effects from implementation of the Minamata Convention targets, although new research is needed on the improvement of emission inventory data, the chem. and phys. behavior of mercury in the atm., the improvement of monitoring network data, predictions of future emissions and speciation, and on the subsequent effects on the environment, human health, as well as the economic costs and benefits of reducing these aspects.
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    10. 10
      Ariya, P. A.; Amyot, M.; Dastoor, A.; Deeds, D.; Feinberg, A.; Kos, G.; Poulain, A.; Ryjkov, A.; Semeniuk, K.; Subir, M.; Toyota, K. Mercury Physicochemical and Biogeochemical Transformation in the Atmosphere and at Atmospheric Interfaces: A Review and Future Directions. Chem. Rev. 2015, 115, 3760– 3802,  DOI: 10.1021/cr500667e
      10
      Mercury Physicochemical and Biogeochemical Transformation in the Atmosphere and at Atmospheric Interfaces: A Review and Future Directions
      Ariya, Parisa A.; Amyot, Marc; Dastoor, Ashu; Deeds, Daniel; Feinberg, Aryeh; Kos, Gregor; Poulain, Alexandre; Ryjkov, Andrei; Semeniuk, Kirill; Subir, M.; Toyota, Kenjiro
      Chemical Reviews (Washington, DC, United States) (2015), 115 (10), 3760-3802CODEN: CHREAY; ISSN:0009-2665. (American Chemical Society)
      A review concerning Hg physicochem. properties and biogeochem. transformations in the atm. and atm. interfaces is given. Topics discussed include: introduction (planetary Hg sources, physicochem. properties of atm. Hg compds., atm. Hg speciation challenges, rapid Hg depletion events [polar, marine boundary layer, Dead Sea], heterogeneous chem. reactions involving Hg, can Hg-contg. compds. nucleate in the atm., Hg bioaccumulation, Hg modeling); anal. methods for speciation of reactive Hg species (international measurement networks, sampling atm. Hg species; detecting atm. Hg2+ and particulate Hg, detg. oxidized Hg species in water and soil); kinetics and mechanistic reactions and uncertainty sources: lab., theor./computational, field studies (lab. study understanding of reaction kinetics, are radical/non-radical initiated reactions free from side reactions and what is the effect of a third body on reaction kinetics, lab. and theor. study uncertainties, Hg gas/liq. partitioning and inaccurate Henry Const. use, heterogeneous chem., redox reactions, Hg interactions with fly ash and components, Mg interactions with S2- and dissolved org. matter and particle impact). Other topics discussed include: Hg exchange at atm./aq. interface (freshwater/marine, Hg cycling in cryosphere); Hg exchanges between atm./terrestrial environment (emissions, deposition, barren soil, vegetation); global and regional atm. Hg modeling (incorporated chem. processes; uncertainties of gaseous Hg oxidn. by OH-, O3, Br-, redn. processes, Hg2+ gas/aerosol partitioning); global ocean Hg modeling, discussion; global/regional terrestrial Hg modeling; and future directions.
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    11. 11
      Shotyk, W. Arctic Plants Take up Mercury Vapour. Nature 2017, 547, 167– 168,  DOI: 10.1038/547167a
      11
      Arctic plants take up mercury vapour
      Shotyk, William
      Nature (London, United Kingdom) (2017), 547 (7662), 167-168CODEN: NATUAS; ISSN:0028-0836. (Nature Publishing Group)
      A brief review commenting on the work of D. Obrist et al. (ibid, p201) in this issue of Nature. Trace elements are enriched in plants by natural processes, human activities or both. An anal. of mercury in Arctic tundra vegetation offers fresh insight into the uptake of trace metals from the atm. by plants.
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    12. 12
      Streets, D. G.; Horowitz, H. M.; Jacob, D. J.; Lu, Z.; Levin, L.; ter Schure, A. F. H.; Sunderland, E. M. Total Mercury Released to the Environment by Human Activities. Environ. Sci. Technol. 2017, 51, 5969– 5977,  DOI: 10.1021/acs.est.7b00451
      12
      Total Mercury Released to the Environment by Human Activities
      Streets, David G.; Horowitz, Hannah M.; Jacob, Daniel J.; Lu, Zifeng; Levin, Leonard; ter Schure, Arnout F. H.; Sunderland, Elsie M.
      Environmental Science & Technology (2017), 51 (11), 5969-5977CODEN: ESTHAG; ISSN:0013-936X. (American Chemical Society)
      We est. that a cumulative total of 1540 (1060-2800) Gg (gigagrams, 109 grams or thousand tonnes) of mercury (Hg) have been released by human activities up to 2010, 73% of which was released after 1850. Of this liberated Hg, 470 Gg were emitted directly into the atm., and 74% of the air emissions were elemental Hg. Cumulatively, about 1070 Gg were released to land and water bodies. Though annual releases of Hg have been relatively stable since 1880 at 8 ± 2 Gg, except for wartime, the distributions of those releases among source types, world regions, and environmental media have changed dramatically. Prodn. of Hg accounts for 27% of cumulative Hg releases to the environment, followed by silver prodn. (24%) and chems. manufg. (12%). North America (30%), Europe (27%), and Asia (16%) have experienced the largest releases. Biogeochem. modeling shows a 3.2-fold increase in the atm. burden relative to 1850 and a contemporary atm. reservoir of 4.57 Gg, both of which agree well with observational constraints. We find that approx. 40% (390 Gg) of the Hg discarded to land and water must be sequestered at contaminated sites to maintain consistency with recent declines in atm. Hg concns.
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    13. 13
      UN Environment. Global Mercury Assessment; Geneva, Switzerland, 2018.
      There is no corresponding record for this reference.
    14. 14
      Ericson, B.; Caravanos, J.; Chatham-Stephens, K.; Landrigan, P.; Fuller, R. Approaches to Systematic Assessment of Environmental Exposures Posed at Hazardous Waste Sites in the Developing World: The Toxic Sites Identification Program. Environ. Monit. Assess. 2013, 185, 1755– 1766,  DOI: 10.1007/s10661-012-2665-2
      14
      Approaches to systematic assessment of environmental exposures posed at hazardous waste sites in the developing world: the Toxic Sites Identification Program
      Ericson, Bret; Caravanos, Jack; Chatham-Stephens, Kevin; Landrigan, Philip; Fuller, Richard
      Environmental Monitoring and Assessment (2013), 185 (2), 1755-1766CODEN: EMASDH; ISSN:0167-6369. (Springer)
      In the developing world, environmental chem. exposures due to hazardous waste sites are poorly documented. We describe the approach taken by the Blacksmith Institute's Toxic Sites Identification Program in documenting environmental chem. exposures due to hazardous waste sites globally, identifying sites of concern and quantifying pathways, populations, and severity of exposure. A network of local environmental investigators was identified and trained to conduct hazardous waste site investigations and assessments. To date, 2,095 contaminated sites have been identified within 47 countries having an estd. population at risk of 71,500,000. Trained researchers and investigators have visited 1,400 of those sites. Heavy metals are the leading primary exposures, with water supply and ambient air being the primary routes of exposure. Even though chem. prodn. has occurred largely in the developed world to date, many hazardous waste sites in the developing world pose significant hazards to the health of large portions of the population. Further research is needed to quantify potential health and economic consequences and identify cost-effective approaches to remediation.
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    15. 15
      Hu, Z.; Kurien, U.; Murwira, K.; Ghoshdastidar, A.; Nepotchatykh, O.; Ariya, P. A. Development of a Green Technology for Mercury Recycling from Spent Compact Fluorescent Lamps Using Iron Oxides Nanoparticles and Electrochemistry. ACS Sustainable Chem. Eng. 2016, 4, 2150– 2157,  DOI: 10.1021/acssuschemeng.5b01612
      15
      Development of a Green Technology for Mercury Recycling from Spent Compact Fluorescent Lamps Using Iron Oxides Nanoparticles and Electrochemistry
      Hu, Zhenzhong; Kurien, Uday; Murwira, Kuzivakwashe; Ghoshdastidar, Avik; Nepotchatykh, Oleg; Ariya, Parisa A.
      ACS Sustainable Chemistry & Engineering (2016), 4 (4), 2150-2157CODEN: ASCECG; ISSN:2168-0485. (American Chemical Society)
      The widespread use of energy efficient Hg contg. lamps and impending regulations on the control of Hg emissions has necessitated the development of green Hg control technologies such as nanosorbent capture and electrolysis regeneration. We describe a 2-step green technique to remove and recycle Hg from spent compact fluorescent lamps (CFLs). The 1st element included the assessment of capture efficiencies of Hg vapor on magnetite (Fe3O4) and maghemite (γ-Fe2O3), naturally abundant and ubiquitous components of atm. dust particles. Around 60 μg of Hg vapor can be removed ≤90% by 1.0 g of magnetite nanoparticles, within a time scale of minutes. The 2nd step included the development of an electrochem. system for the Hg recycling and regeneration of used nanoparticles. Under optimized conditions, ≤85% of Hg was recovered as elemental Hg. Postelectrolysis regenerated Fe oxide nanoparticles were used in several sorption-electrolysis cycles without loss of the adsorption capacity, morphol., and surface area. The low energy usage for electrolysis can be supplied by the solar panels. The implications of our results within the context of green technol. are discussed.
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    16. 16
      Driscoll, C. T.; Mason, R. P.; Chan, H. M.; Jacob, D. J.; Pirrone, N. Mercury as a Global Pollutant: Sources, Pathways, and Effects. Environ. Sci. Technol. 2013, 47, 4967– 4983,  DOI: 10.1021/es305071v
      16
      Mercury as a Global Pollutant: Sources, Pathways, and Effects
      Driscoll, Charles T.; Mason, Robert P.; Chan, Hing Man; Jacob, Daniel J.; Pirrone, Nicola
      Environmental Science & Technology (2013), 47 (10), 4967-4983CODEN: ESTHAG; ISSN:0013-936X. (American Chemical Society)
      A review is given. Hg is a global pollutant that affects human and ecosystem health. We synthesize understanding of sources, atm.-land-ocean Hg dynamics and health effects, and consider the implications of Hg-control policies. Primary anthropogenic Hg emissions greatly exceed natural geogenic sources, resulting in increases in Hg reservoirs and subsequent secondary Hg emissions that facilitate its global distribution. The ultimate fate of emitted Hg is primarily recalcitrant soil pools and deep ocean waters and sediments. Transfers of Hg emissions to largely unavailable reservoirs occur over the time scale of centuries, and are primarily mediated through atm. exchanges of wet/dry deposition and evasion from vegetation, soil org. matter and ocean surfaces. A key link between inorg. Hg inputs and exposure of humans and wildlife is the net prodn. of methylmercury, which occurs mainly in reducing zones in freshwater, terrestrial, and coastal environments, and the subsurface ocean. Elevated human exposure to methylmercury primarily results from consumption of estuarine and marine fish. Developing fetuses are most at risk from this neurotoxin but health effects of highly exposed populations and wildlife are also a concern. Integration of Hg science with national and international policy efforts is needed to target efforts and evaluate efficacy.
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    17. 17
      Wang, L.; Hou, D.; Cao, Y.; Ok, Y. S.; Tack, F. M. G.; Rinklebe, J.; O’Connor, D. Remediation of Mercury Contaminated Soil, Water, and Air: A Review of Emerging Materials and Innovative Technologies. Environ. Int. 2020, 134, 105281  DOI: 10.1016/j.envint.2019.105281
      17
      Remediation of mercury contaminated soil, water, and air: A review of emerging materials and innovative technologies
      Wang, Liuwei; Hou, Deyi; Cao, Yining; Ok, Yong Sik; Tack, Filip M. G.; Rinklebe, Jorg; O'Connor, David
      Environment International (2020), 134 (), 105281CODEN: ENVIDV; ISSN:0160-4120. (Elsevier Ltd.)
      Mercury contamination in soil, water and air is assocd. with potential toxicity to humans and ecosystems. Industrial activities such as coal combustion have led to increased mercury (Hg) concns. in different environmental media. This review critically evaluates recent developments in technol. approaches for the remediation of Hg contaminated soil, water and air, with a focus on emerging materials and innovative technologies. Extensive research on various nanomaterials, such as carbon nanotubes (CNTs), nanosheets and magnetic nanocomposites, for mercury removal are investigated. This paper also examines other emerging materials and their characteristics, including graphene, biochar, metal org. frameworks (MOFs), covalent org. frameworks (COFs), layered double hydroxides (LDHs) as well as other materials such as clay minerals and manganese oxides. Based on approaches including adsorption/desorption, oxidn./redn. and stabilization/containment, the performances of innovative technologies with the aid of these materials were examd. In addn., technologies involving organisms, such as phytoremediation, algae-based mercury removal, microbial redn. and constructed wetlands, were also reviewed, and the role of organisms, esp. microorganisms, in these techniques are illustrated.
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    18. 18
      Bengtsson, M. K. O.; Tunsu, C.; Wickman, B. Decontamination of Mercury-Containing Aqueous Streams by Electrochemical Alloy Formation on Copper. Ind. Eng. Chem. Res. 2019, 58, 9166– 9172,  DOI: 10.1021/acs.iecr.9b01513
      18
      Decontamination of Mercury-Containing Aqueous Streams by Electrochemical Alloy Formation on Copper
      Bengtsson, Mattias K. O.; Tunsu, Cristian; Wickman, Bjoern
      Industrial & Engineering Chemistry Research (2019), 58 (21), 9166-9172CODEN: IECRED; ISSN:0888-5885. (American Chemical Society)
      Hg in aq. streams poses severe health and environmental concerns and requires improved techniques for decontamination. One such technique is electrochem. alloy formation on Pt, which can effectively decontaminate Hg-contg. aq. streams at concns. relevant for both industrial and natural waters. This study examines the viability of Cu as an alternative to Pt. Hg removal is faster on Cu and works both with and without an applied cathodic potential. Without it, however, Cu dissoln. becomes a problem. Cu dissoln. is preventable in neutral pH and in H2SO4 solns. under potential control, and dissolved Cu ions can be plated back onto the electrode. In the presence of nitrate or chloride anions, Cu electrodes degrade rapidly even under potential control. Thus, there are practical restrictions for Hg decontamination via electrochem. alloy formation on Cu, but it can be applied to solns. where Cu is stable under potential control.
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    19. 19
      Crini, G.; Lichtfouse, E. Advantages and Disadvantages of Techniques Used for Wastewater Treatment. Environ. Chem. Lett. 2019, 17, 145– 155,  DOI: 10.1007/s10311-018-0785-9
      19
      Advantages and disadvantages of techniques used for wastewater treatment
      Crini, Gregorio; Lichtfouse, Eric
      Environmental Chemistry Letters (2019), 17 (1), 145-155CODEN: ECLNBJ; ISSN:1610-3653. (Springer)
      During the last 30 years, environmental issues about the chem. and biol. contaminations of water have become a major concern for society, public authorities and the industry. Most domestic and industrial activities produce wastewaters contg. undesirable toxic contaminants. In this context, a const. effort must be made to protect water resources. Current wastewater treatment methods involve a combination of phys., chem. and biol. processes, and operations to remove insol. particles and sol. contaminants from effluents. This article provides an overview of methods for wastewater treatment, and describes the advantages and disadvantages of available technologies.
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    20. 20
      Hua, K.; Xu, X.; Luo, Z.; Fang, D.; Bao, R.; Yi, J. Effective Removal of Mercury Ions in Aqueous Solutions: A Review. Curr. Nanosci. 2020, 16, 363– 375,  DOI: 10.2174/1573413715666190112110659
      20
      Effective Removal of Mercury Ions in Aqueous Solutions: A Review
      Hua, Kang; Xu, Xueliu; Luo, Zhiping; Fang, Dong; Bao, Rui; Yi, Jianhong
      Current Nanoscience (2020), 16 (3), 363-375CODEN: CNUAAB; ISSN:1573-4137. (Bentham Science Publishers Ltd.)
      A review. In order to control mercury pollution, scientists have put great efforts in the past decades. Methods: Pptn., adsorption, membrane sepn., biol. treatment and ion exchange are reviewed as a remover for mercury removal. For each material type, we not only reported on the removal mechanism, but also discussed the best areas for it. The correlation method and step-to-step focusing method have been used for refs. Results: For better mercury removal, the ways above are compared together. The mechanisms of removing mercury in different ways are summarized in this paper. Conclusion: With the exploration and application of research, people have mastered a variety of mature technologies for the treatment of mercury-contg. wastewater. Using inexpensive adsorbents is a cost-effective method for treating low concns. of heavy metal wastewater. Ion exchange with a fast removal rate has been widely used in the field of heavy metal removal from wastewater. The biol. treatment method can effectively treat low-concn. mercurycontaining wastewater. However, there is still a need to develop novel mercury removers with high capacity, fast removal rate, and low removal limit. Nanomaterials with a high sp. surface area on substrate with synergistic effects, such as high adsorption and ion exchange, are the future research points.
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    21. 21
      Patterson, J.; Barth, E.; Stein, L. Aqueous Mercury Treatment ; 1997.
      There is no corresponding record for this reference.
    22. 22
      Tunsu, C.; Wickman, B. Effective Removal of Mercury from Aqueous Streams via Electrochemical Alloy Formation on Platinum. Nat. Commun. 2018, 9, 4876,  DOI: 10.1038/s41467-018-07300-z
      22
      Effective removal of mercury from aqueous streams via electrochemical alloy formation on platinum
      Tunsu Cristian; Wickman Bjorn
      Nature communications (2018), 9 (1), 4876 ISSN:.
      Retrieval of mercury from aqueous streams has significant environmental and societal importance due to its very high toxicity and mobility. We present here a method to retrieve mercury from aqueous feeds via electrochemical alloy formation on thin platinum films. This application is a green and effective alternative to traditional chemical decontamination techniques. Under applied potential, mercury ions in solution form a stable PtHg4 alloy with platinum on the cathode. A 100 nanometres platinum film was fully converted to a 750 nanometres thick layer of PtHg4. The overall removal capacity is very high, > 88 g mercury per cm(3). The electrodes can easily be regenerated after use. Efficient and selective decontamination is possible in a wide pH range, allowing processing of industrial, municipal, and natural waters. The method is suited for both high and low concentrations of mercury and can reduce mercury levels far below the limits allowed in drinking water.
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    23. 23
      Wu, H. L.; Yau, S.; Zei, M. S. Crystalline Alloys Produced by Mercury Electrodeposition on Pt(1 1 1) Electrode at Room Temperature. Electrochim. Acta 2008, 53, 5961– 5967,  DOI: 10.1016/j.electacta.2008.03.063
      23
      Crystalline alloys produced by mercury electrodeposition on Pt(111) electrode at room temperature
      Wu, Hang Liang; Yau, Shuehlin; Zei, Mau Scheng
      Electrochimica Acta (2008), 53 (20), 5961-5967CODEN: ELCAAV; ISSN:0013-4686. (Elsevier B.V.)
      In situ scanning tunneling microscopy (STM) and RHEED were used to characterize Hg film electrodeposited onto a Pt(111) electrode at room temp. Depending on the amt. of Hg deposit, 2 different growth modes were obsd. At low Hg coverage, cryst. (0001)Hg adlayer accompanied by 30°-rotated (111)-Pt patches was found on Pt(111). Deposition of multilayer Hg resulted in layered PtHg2 and PtHg4 amalgams, which grew epitaxially by aligning their (201) and (110) planes, resp., parallel to the Pt(111) substrate. The preference of these epitaxial relations for the electrochem. formed Pt-Hg intermetallic compds. on Pt(111) could result from minimization of the surface energy.
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    24. 24
      Yoshida, Z. Structure of Mercury Layer Deposited on Platinum and Hydrogen-Evolution Reaction at the Mercury-Coated Platinum Electrode. Bull. Chem. Soc. Jpn. 1981, 54, 556– 561,  DOI: 10.1246/bcsj.54.556
      24
      Structure of a mercury layer deposited on platinum and hydrogen-evolution reaction at the mercury-coated platinum electrode
      Yoshida, Zenko
      Bulletin of the Chemical Society of Japan (1981), 54 (2), 556-61CODEN: BCSJA8; ISSN:0009-2673.
      The thermal evapn. of Hg was investigated to ascertain the structure of Hg electrodeposited on Pt. At 1st, Pt2Hg is formed on Pt, and the H overpotential at the surface is identical with that at the Hg-free Pt. With an increase in the amt. of Hg to more than the amt. corresponding to PtHg, the H overpotential increases. When a large amt. of Hg is deposited, the layer is composed of 3 Hg compds. (Pt2Hg, Pt2Hg2, and PtHg4), metallic Hg, and adatom Hg. Even when a sufficiently large amt. of Hg is deposited, e.g., several thousand at. layers, the H overpotential is much less than that at a hanging Hg drop electrode. From the general relations between the H evolution at a metal electrode and the work function of the metal, the change in the H overpotential with the increase in the amt. of Hg is attributable to the work-function change.
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    25. 25
      Guminski, C. The Hg-Pt (Mercury-Platinum) System. Bull. Alloy Phase Diagrams 1990, 11, 26– 32,  DOI: 10.1007/BF02841581
      25
      The Hg-Pt (mercury-platinum) system
      Guminski, C.
      Bulletin of Alloy Phase Diagrams (1990), 11 (1), 26-32CODEN: BAPDDW; ISSN:0197-0216.
      The Hg-Pt equil. phase diagram is assessed based on literature data. Metastable phases and crystallog. and thermodn. properties of equil. phases are reviewed.
      >> More from SciFinder ®
      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADyaK3cXitl2itL4%253D&md5=b351ae7533ea6ab8b2b22e2f84f7ee71
    26. 26
      Robbins, G. D.; Enke, C. G. Investigation of the Compound Formed at a Platinum-Mercury Interface. J. Electroanal. Chem. Interfacial Electrochem. 1969, 23, 343,  DOI: 10.1016/S0022-0728(69)80229-9
      26
      Compound formed at a platinum-mercury interface
      Robbins, Gordon Daniel; Enke, Christie G.
      Journal of Electroanalytical Chemistry and Interfacial Electrochemistry (1969), 23 (3), 343-9CODEN: JEIEBC; ISSN:0022-0728.
      The compd. formed at the Pt-Hg interface in Hg-coated Pt electrode was identified as PtHg4 by x-ray anal. The Hg was stripped from the electrode by using a const. current source until an abrupt potential change assocd. with the appearance of gray color occurred. Electrochem. studies of the cell Hg|Hg2Cl2|0.1N KCl|Hg2Cl2|PtHg4 gave an equil. potential ΔE0 = 87 mV, corresponding to a standard free energy of formation of PtHg4 of -8.0 kcal/mole at 25°.
      >> More from SciFinder ®
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    27. 27
      Martins, M. E.; Salvarezza, R. C.; Arvia, A. J. The Electrodeposition of Mercury from Aqueous Hg22+ Ion-Containing Acid Solutions on Smooth and Columnar-Structured Platinum Electrodes. Electrochim. Acta 1998, 43, 549– 561,  DOI: 10.1016/S0013-4686(97)00129-1
      There is no corresponding record for this reference.
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      Roger, F.. The Performance of Stainless Steels in Concentrated Sulphuric Acid; Stainless Steel World, 2009.
      There is no corresponding record for this reference.
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      Gunnar, W. Metal Foams for Electrodes; Fraunhofer IFAM: Dresden, 2022.
      There is no corresponding record for this reference.
    30. 30
      ERG Aerospace. Carbon Foam. RVC Foam. Open Cell Foam, Reticulated Foam. https://ergaerospace.com/ (accessed June 2022).
      There is no corresponding record for this reference.

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