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Yes. Research has demonstrated ways to dissolve certain noble metals without aqua regia, including concentrated acidic salt hydrates and organic-solvent systems. But there is no universal substitute: results depend on the metal, the starting material, and the conditions. Some tested metals dissolved slowly or not at all, and at least one alternative mixture was reported to be highly corrosive and to produce noxious fumes.
Why there is no single aqua regia replacement
“Noble metals” covers several elements, and a method that dissolves one does not necessarily dissolve another. Results also change with the feedstock: a pure metal wire, a uniform gold disk, and a spent automotive catalyst are different test materials. A reported dissolution result for one cannot be assumed to predict recovery from ore or mixed electronic scrap.
Dissolution is only one part of recycling. A process must also address selectivity, separation from other materials, recovery of the dissolved metal, and the hazards and controls required at each stage. The studies below are laboratory demonstrations or controlled comparisons, not evidence that every route is commercially established or suitable for household use.
What concentrated acidic salt hydrates dissolved
A 2020 study of highly concentrated hydrated aluminum chloride and aluminum nitrate solutions examined gold and platinum-group metals. In the reported wire tests, palladium dissolved fully in 4.5 hours, gold in roughly one day, and platinum in roughly two days; rhodium wire did not dissolve. These are outcomes for the tested wires and conditions, not general processing times for scrap. The findings were reported in the Royal Society of Chemistry paper “Dissolution of noble metals in highly concentrated acidic salt solutions,” with the wire results summarized by ChemistryViews.
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The same ChemistryViews summary reports that 95% of the palladium in a spent automotive catalyst was leached in 15 minutes at 80 °C. That catalyst result is not directly comparable to the wire tests: the feedstock differs, as do the reported test conditions. It demonstrates why substrate matters, rather than establishing that catalysts always process faster than pure palladium.
This route should not be treated as harmless because it avoids aqua regia. ChemistryViews describes the solution as highly corrosive and says it generates nitrosyl chloride, chlorine gas, and other noxious fumes. Its reported relative comparison with aqua regia does not make it safe for casual or home use.
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How gold-leaching alternatives compared in a controlled test
A 2019 Scientific Reports study compared reagents using uniform 99.99% gold disks. At 25 °C, its average dissolution-rate ranking was as follows. The order is specific to that study’s gold-disk tests; it is not a universal ranking for ores, e-waste, or industrial process designs.
| Reagent tested | Relative average rate in the study | Reported qualification |
|---|---|---|
| Aqua regia | Fastest | Reference in the study’s 25 °C comparison |
| Iodine | Second | Among the faster alternatives tested |
| Bromine | Third | Among the faster alternatives tested |
| Cyanide | Fourth | Rate ranking applies to the tested gold disks |
| Thiourea | Fifth | Surface precipitation occurred, although dissolution continued |
| Thiosulfate | Sixth | Performance was hindered by formation of a sulfide passive layer |
| Methanesulfonic acid | Slowest | Did not extract gold in these tests |
The paper reports rates of up to approximately 1,000 mg·h⁻¹·cm⁻² for the fast-dissolving halogen and aqua-regia cases in its comparison. That upper figure is tied to those tested cases, not to every reagent or operating condition. The authors also caution that studies using different materials and experimental conditions are often not directly comparable.
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What an organic-solvent system adds
A 2022 ChemSusChem study examined triphenylphosphine dichloride or oxalyl chloride with hydrogen peroxide in organic solvents. Under the studied conditions, it reported near-quantitative dissolution of metallic gold, palladium, and copper within minutes at room temperature.
Platinum behaved differently: it dissolved more slowly with the chlorinating oxidant alone, and adding hydrogen peroxide inhibited its dissolution. The study also describes subsequent transfer and separation chemistry, reinforcing that dissolving a metal does not by itself produce separated, purified metal. These are study-specific results, not a guarantee for mixed or contaminated feedstocks.
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What the wider recycling landscape does—and doesn’t—show
A 2023 review in Angewandte Chemie International Edition surveys multiple approaches to noble-metal recycling, including iodine/iodide, thiosulfate, thiourea, ammonia-based dissolution, and organic-solvent systems. That range shows there are several research directions, not one reagent that has been established here as the best choice for every metal and feedstock.
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- Feedstock: results on wires or uniform disks do not establish performance on ores, catalysts, or mixed electronic scrap.
- Recovery: dissolution results do not alone establish selectivity, downstream separation, or the yield of purified metal.
- Process hazards: avoiding aqua regia does not establish that another reagent system is benign.
- Evidence: the cited studies establish specific laboratory results; they do not establish a consumer-ready method or a universal commercial process.
How to interpret claims of an “alternative”
Ask what metal was tested, what the starting material was, and under what conditions. Then check whether the reported result is dissolution, leaching from a complex feedstock, or recovery of purified metal. Without those distinctions, a fast result on one substrate can be mistaken for a general recycling solution. The evidence here supports several alternatives for particular cases, but not a drop-in replacement that is proven safer, faster, or more practical across noble metals.
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