Common rhodium salts in aqueous solutions: solubility and recovery technologies
Rhodium is rare in ordinary natural waters, and its aqueous chemistry is strongly affected by ligands, pH and chloride concentration. Rhodium-bearing process streams are more commonly associated with platinum-group-metal refining and recycling, rhodium plating, or catalyst manufacture. In these streams, the dissolved forms are usually coordination complexes rather than free Rh3+ ions.
Rhodium compounds in water and process solutions
The table summarizes representative compounds and process forms. “Solubility” refers to the named compound or formulated solution, not necessarily to the species present after it dissolves. Hydration state, acidity and other ions can change the observed behavior.
| Compound or aqueous form | Where it may be encountered | Solubility and interpretation |
|---|---|---|
| Rhodium(III) chloride trihydrate, RhCl3·3H2O | Laboratory standards and chloride-based refining or recovery solutions | Water-soluble according to the ILO/WHO chemical safety card. In chloride media it forms a distribution of aquo and chloro complexes; the formula RhCl3 does not mean intact RhCl3 molecules remain in solution. |
| Anhydrous rhodium(III) chloride, RhCl3 | Solid precursor; should not be confused with hydrated rhodium chloride | Reported as insoluble or poorly soluble in water. Product specifications and hydration state matter when interpreting the term “rhodium chloride.” |
| Hexachlororhodate(III) salts, M3[RhCl6] (M = Na, K, NH4) | Acidic, chloride-rich leach liquors and isolated intermediates in platinum-group-metal refining | These contain the complex anion [RhCl6]3−. Solubility and precipitation depend on the counter-ion and the chloride, acidity, temperature and concentration of the solution. A single value for all such salts is not appropriate. |
| Rhodium sulfate or sulfate-based plating solution | Electroplating baths and their rinse waters | Commercial baths are formulated acidic aqueous solutions. Their behavior depends on the full bath formulation; a nominal sulfate formula alone does not give a universal water-solubility value. |
| Rhodium phosphate-based plating solution | Decorative and technical rhodium electroplating | Supplied as a concentrated aqueous plating product used in an acidic bath. If the concentrate is diluted incorrectly, hydrolysis and formation of basic rhodium solids can occur. |
| Rhodium acetate and other organometallic catalyst species | Catalyst manufacture, chemical synthesis and catalyst-containing process effluents | These are ligand-bound compounds, not simple inorganic salts. Solubility and separation depend on the ligand, solvent and process chemistry. |
Published solubility data are not directly comparable across these hydrated solids, complex salts and proprietary plating solutions. For process design, determine the actual feed composition and test the selected chemistry at representative acidity, chloride concentration, temperature and competing-metal levels.
Does rhodium recovery use ion exchange and membranes?
Ion exchange is an established option investigated and applied in platinum-group-metal hydrometallurgical circuits, but rhodium is not captured equally well by every resin. In acidic chloride liquors, rhodium occurs as anionic chloro complexes that may interact with anion-exchange or specialized chelating resins. Published tests on spent automotive-catalyst leach solutions found that resin performance differed by metal and resin type; rhodium loading and subsequent elution could be difficult. Resin selection therefore needs to be confirmed with the actual liquor. See Lenntech’s ion-exchange systems.
Membrane separation has been demonstrated for rhodium recovery in research, but it should not be described as a routine, widely deployed rhodium-refining technology. Polymer inclusion and supported liquid membranes have been tested to transport or separate Rh(III) from chloride solutions and spent-catalyst leachates. These studies show technical potential at laboratory scale; scale-up, membrane lifetime, selectivity in real mixed liquors and economics remain important questions.
Pressure-driven membranes such as reverse osmosis may be evaluated to recover water or concentrate a compatible dissolved-metal stream, but they do not selectively recover rhodium by themselves. Conventional microfiltration or ultrafiltration can capture rhodium-bearing particles or precipitates; they do not remove dissolved rhodium ions unless those ions are first converted to a retained solid or complex.
Other recovery and separation routes
Depending on the solution, alternatives or complementary steps include selective precipitation, adsorption, solvent extraction and solids separation. A 2026 study reported sulfide precipitation from a specific acidic acetic-acid manufacturing wastewater; its conditions and recovery results are specific to that matrix and should not be generalized to all rhodium streams. A treatment train may combine leaching, separation, recovery of rhodium from a concentrated fraction, and water or residuals treatment.
Before selecting a route, characterize total and dissolved rhodium, suspended solids, pH, chloride and other major ligands, plus competing platinum-group and base metals. Bench testing should include recovery, selectivity, elution or product purity, residuals handling and water quality.
Sources and further reading
- Royal Society of Chemistry: Rhodium
- ILO/WHO: International Chemical Safety Card 0746, Rhodium trichloride trihydrate
- Cobelo-García (2013): Kinetics and speciation of Rh(III) with humic substances
- Nikoloski, Ang and Li (2015): Recovery of Pt, Pd and Rh from acidic chloride leach solutions using ion-exchange resins
- Yoshida et al. (2020): Transport of Rh(III) from chloride media across a polymer inclusion membrane
- Fajar, Hanada and Goto (2021): Polymer inclusion membranes for recovery of PGMs from spent automotive catalysts
- Hu et al. (2026): Rhodium recovery from acidic acetic-acid manufacturing wastewater by sulfide precipitation
- Johnson Matthey: Rhodium phosphate plating solution