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upstart writes in with an IRC submission for RandomFactor:

Noble metals dissolved without aqua regia[*]:

A common way of extracting noble metals from other materials is by dissolving that material in solution. However, dissolving noble metals remains a big challenge due to their low reactivity, and effective recycling of noble metals requires high dissolution rates and controllable selectivity.

'Noble metals can be dissolved using different hydrometallurgical methods. Unfortunately, all of them have drawbacks,' explains Serhiy Cherevko, who studies noble metal dissolution on a fundamental level at the Helmholtz Institute Erlangen-Nürnberg for Renewable Energy but was not involved in this research. 'The most common approach is to use hydrochloric acid as a complexing agent and nitric acid, chlorine, or hydrogen peroxide as an oxidant.'

A mixture of hydrochloric acid and nitric acid is also known as aqua regia, from the Latin for royal water, and is frequently used to dissolve gold and platinum in industry. However, it is a potentially dangerous mixture and there are environmental concerns related to its use. Because of this, there is a significant drive to find alternatives.

[...] Binnemans' team prepared a very concentrated mixture of hydrated AlCl3 and Al(NO3)3 to use as their solvent. They tested it on metal wires and spent automotive catalysts.

In the case of palladium, nitrate ions in the solution oxidised the metal. A chloropalladate (II) complex between the oxidised metal and chloride anions formed and was stable at the low pH. In a matter of hours, the palladium had fully dissolved in solution, and the palladium metal was then recovered via reductive precipitation using ascorbic acid.

'Platinum can also be dissolved but it just requires longer,' notes Riaño. 'Rhodium remains practically intact, but this is also the situation when you use aqua regia; it's just too inert and you require higher temperatures and pressures to digest it.'

While the aluminium salt solutions cannot be reused, they can be disposed of safely and are much less noxious than aqua regia. However, further work is needed to optimise yields and achieve high purities of the recovered metals.

[*] Aqua regia on Wikipedia.

Journal Reference:
Proton magnetic resonance chemical shifts and the hydrogen bond in concentrated aqueous electrolyte solutions, (DOI: 10.1021/j100634a011)

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