Bioelectrochemical treatment of untreated spent car catalyst: Effects of solids loading on platinum group metal redistribution and process limitations

Spent car catalyst (SCC) is a secondary source of platinum group metals (PGMs), but recovery commonly requires high-temperature smelting or aggressive chemical leaching. Bioelectrochemical treatment offers milder conditions, though most studies use dissolved metal salts rather than untreated catalyst solids, leaving their behaviour with real SCC poorly characterised. Cupriavidus metallidurans CH34 was operated in dual-chamber microbial fuel cells with untreated SCC at 0; 10,000; 50,000; and 100,000 ppm for 168 h, with abiotic controls at each loading. Voltage, anolyte pH, and biomass were monitored, and platinum (Pt), palladium (Pd), and rhodium (Rh) were quantified in the liquid, biomass, electrode, and residual solid fractions. At 10,000 ppm, voltage reached 0.442 ± 0.007 V compared with 0.378 ± 0.018 V without SCC. Rh was detected in the liquid (7.46 ± 0.31%) and biomass (10.29 ± 0.45%). A comparable liquid-phase fraction was measured in the corresponding abiotic reactor (5.64%), indicating that dissolution was chemical rather than microbially driven. Pt and Pd remained predominantly in the solid. At 50,000 ppm and above, biomass and voltage declined sharply, with >98% of each metal retained in the solid. Mobilisation per gram of SCC was greatest at 10,000 ppm, where pH remained highest. Overall yields were insufficient for recovery. The biological contribution was Rh capture rather than enhanced dissolution.