Mobilization of platinum and palladium from exhausted catalytic converters using bio-cyanide and an ionic-liquid as mass transport carriers

Abstract

The present study investigated the resource circulation of precious metals (Pt and Pd) from exhausted catalytic-converters using a green integrative approach of bio-solvo-chemical techniques. Herein, the used biogenic cyanide as a lixiviant medium was produced via the oxidative decarboxylation of glycine during the late exponential growth phase of Chromobacterium violaceum. The advantage of microbial activity was utilized in two-fold: (i) in situ production of the bio-cyanide lixiviant and (ii) bio-destruction of residual cyanide from the Pt/Pd-depleted bleed solution. More than 95% of Pt and Pd were efficiently dissolved in bio-cyanide solution by autoclaving under the optimized condition of temperature, 150 °C; pO₂, 14 bar; and time, 120 min. Furthermore, the solvo-chemical extraction of the Pt/Pd-cyanide complex was performed using a green ionic-liquid solvent (Cyphos IL101). The parametric studies revealed that >96% Pt and Pd were extracted under the optimal conditions of ionic-liquid concentration, 0.15 mol L⁻¹; extraction pH, 10.4; and temperature, 20 °C. The extraction thermodynamics yielded negative values of enthalpy changes (i.e., , −39.8 kJ mol⁻¹ and , −34.9 kJ mol⁻¹), which indicated the exothermic characteristics of outer-sphere coordination between the ionic-liquid and Pt/Pd(CN)₄²⁻ compounds. Subsequently, an excellent selectivity on Pd and Pt stripping from the loaded organic phase was achieved by sequential contact with 2.0 mol L⁻¹ NH₄SCN and 1.5 mol L⁻¹ S(CH₂CH₂OH)₂ solutions. The tested recyclability of cyanide-bearing raffinate, regeneration and reusability of the ionic-liquid, and bio-degradation of cyanide-bleed solution drive the process towards zero-discharge of toxic substances. Significantly low-emission exploitation of precious metals (reduced from 10.8 × 10⁶ to only 1.63 t CO₂-e) from the waste materials with a sustainable process index value of 0.0145 cap per kg reveals that the process is suitable for sustainable development.