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Novel and highly-selective method for retrieving mercury

Mercury is a highly toxic and bio-accumulative pollutant, yet with important industrial applications. This article explores an innovative mercury recovery process that may make a difference for recyclers everywhere.

Published in the latest issue of Recycling Technology / Reading time: 6 min.

Mercury-containing products are still common in certain areas of the world and are still part of recycling schemes. A notable example is the fluorescent lamp. At the beginning of the current decade, analysis of 15 types of such lamps revealed total mercury contents of between 1.6 and 27 mg per lamp. Around 40% of the investigated samples failed to pass the requirements set by European legislation of 5 mg per compact fluorescent lamp.

Exploiting mercury’s properties

Modern-day technologies to remove mercury and other toxic heavy metals from (wet) waste streams is of great importance. The ideal method would not consume chemicals, would be effective at low and high concentrations as well as in a wide pH range, and would be highly selective, easily regenerable and energy-efficient.

A process offering many of these advantages is electrochemically-driven alloy formation, which exploits the ability of mercury to form amalgams (alloys) with other metals.

The main benefits are:

  • No additional chemicals needed.
  • Effective at very low and/or high concentrations of ionic mercury in solution.
  • Unaffected by pH.
  • High chemical stability of noble metal-based electrodes.
  • High selectivity for mercury in the presence of other cations and anions.
  • Electrodes can be easily regenerated for reuse.
  • Energy-efficient process.
  • Product formation occurs on the electrode and no further separation from the feed is needed, such as via filtration.
  • Possibility of retrieving mercury in various forms: ions in solution (electrochemical regeneration) or liquid mercury/solid compounds (thermal regeneration or further treatment of the stream after electrochemical regeneration).
  • Applicable to a large variety of streams, such as landfill leachates, discharges from dental practices, condensates from waste incineration, industrial effluents and acid purification.

Mercury can form amalgams with various metals, noble metals included, and this property has been extensively exploited in the mining and production of gold and silver. The mercury-platinum system has been of interest for electrochemical studies, catalysis and applications in electrical devices (switches and contacts).

Until recently, however, there have been no reports on the use of platinum-mercury alloy formation under potential control for decontamination. In ambient conditions, the interactions between platinum and mercury are minimal, and amalgamation is impractical. When applying a negative potential to a platinum surface, the interactions between the two metals increase immensely and alloying occurs.

The thermodynamically-favoured compound to form is PtHg4, which has good chemical and thermal stability (up to 900°C is needed for complete decomposition) and great adhesion to the platinum substrate.

High saturation capacity

While other metals – including gold – can be used for amalgamation, platinum offers a high saturation capacity. Since each platinum atom can bind up to four mercury atoms (as opposed to gold, which binds 12 times less mercury as Au3Hg), the theoretical removal capabilities are >88 g of mercury per cm3 available platinum.

The process, schematised in Figure 1 (shown above), is reversible and the alloy can be broken into its constituents electrochemically or, if preferred, via thermal decomposition. This makes it possible to recover and reuse the platinum for further decontamination. The mercury retrieved during decontamination is liberated in a very small volume of solution, making it easier to handle it, to use it in selected applications or to dispose of it. With thermal treatment of loaded electrodes, mercury is recovered in metallic form.

Figure 2 shows what happens on the atomic level during decontamination. The ionic mercury species present in solution (either Hg22+ or Hg2+, shown in lighter purple) are reduced on the surface of a platinum electrode to form elemental mercury, Hg0 (darker purple). This requires electrons and can occur according to, for example, the equilibrium reaction Hg2+ + 2 electrons = Hg0. Subsequently, elemental mercury interacts with the platinum atoms at the surface (silver colour) to form a layer of alloy.

Upon formation of the first layers of PtHg4, mercury needs to penetrate this film to grow the alloy further. The process continues even after several layers of alloy are formed, albeit with reduced speed, and it is possible to grow relatively thick alloy films. Given that electrons are needed to facilitate reduction of mercury ions and alloying, there is a need for an electrical current – but not a high one. A decontamination unit can function using a rechargeable battery powered by, for example, solar cells.

Successful at laboratory scale

Tests performed in a wide pH range, from very acidic (pH 0) to neutral, revealed that the process is not hindered by the pH and that alloy formation was as effective across this pH range. This is an advantage for treating acidic industrial effluents and leach solutions generated during hydrometallurgical recycling, but also contaminated natural waters.

Oxidising feeds such as nitric acid solutions containing mercury are particularly problematic to treat using thiol resins owing to the oxidation of these active groups, which render such adsorbents ineffective; however, this is not an issue when using the electrochemical alloy route.

The method was successfully applied at the laboratory scale to reduce the mercury levels in contaminated water streams to values far below the limits for safe drinking water set by the World Health Organization (6 μg inorganic mercury per litre).

Owing to the unique property of mercury to form amalgams, and the possibility to control the potential at which this reaction occurs, the method is highly selective for retrieval of mercury even in the presence of other anions and cations, and at high salinity. This is an important advantage for treating chemically-complex streams, which usually limit the effectiveness of precipitation and ion exchange.

For effective and fast retrieval, the preference is for large surface area working electrodes, such as porous structures. Retrieval is positively affected by increases in temperature, which significantly speeds up the kinetics and the diffusion of mercury ions in the formed alloy.

Copper suited to some applications

Copper is a significantly cheaper substitute for platinum, and it was tested and shown to work for certain applications. While capable of electrochemically retrieving mercury and at a faster rate compared to platinum, copper has less binding capacity for mercury as Cu7Hg6.

Compared to platinum, the chemical stability of copper electrodes in acidic media is lower; under potential control, however, the stability of copper towards dissolution is greatly enhanced. There are practical restrictions for decontamination, but the technology is applicable to streams where copper is stable under potential control.

Examples include streams with neutral pH, sulfuric acid solutions and other streams absent of nitrate or chloride anions. The use of potential allows for dissolved copper ions to be plated back onto the electrode.

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