Separation and recovery of Pd and Fe as nanosized metal sulphides by combining solvent extraction with biological strategies based on the use of sulphate-reducing bacteria

Platinum group metals (PGMs) are highly demanding metals in current high-techs. However, their supply is limited due to their scarcity in natural resources and expensive mining. Solvent extraction in combination with microbial communities were employed for the recovery of palladium from aqueous media. Accordingly, extraction of Pd(II) by N,N'-dimethyl-N,N'-dicyclohexylthiodiglycolamide (DMDCHTDGA) in toluene and subsequent stripping by acidic thiourea were carried out, followed by bio-recovery employing biogenic sulphide generated from sulphate-reducing bacteria. The results revealed an excellent extraction performance of DMD-CHTDGA towards Pd(II) from metal complex mixtures in 2,4 and 6 M HCl. However, Fe(III) was co-extracted from 4 and 6 M HCl, being completely removed with deionized water during scrubbing. 94-99% of extracted Pd (II) were stripped using acidic thiourea. Over 99% of stripped Pd(II) were recovered using two solutions containing biogenic sulphide: an effluent from a bioremediation process of acid mine drainage treatment, and a supernatant solution from Postgate B nutrient medium. 78-99% of scrubbed Fe(III) were bio-recovered. Analysis of all the precipitates collected during palladium bio-recovery showed that they were composed by Pd and S, which is consistent with the synthesis of nanosized PdS. The elemental analysis of the iron precipitate also suggests the presence of iron sulphide. Henceforth, the method engaged is environmentally sustainable, safe and cheap, thus attractive to be employed, to separate and recover both Pd and Fe from waste materials.