The catalyst (3 mg CNx) was added to 2 mL of 1 mol L -1 KOH solution containing 25 mg mL -1 ethylene glycol and 1.6 μL H2PtCl6 solution (8% wt.) for in situ photodeposition of metallic Pt upon light exposure in a 7.9 mL pyrex vial. In the case of HGM/CNx composites, an amount was added such that the final mass of CNx in the experiment was 3 mg. In the case of turbid waste experiments, CNx|Pt and HGM/CNx|Pt already loaded with metallic Pt by chemical reduction were used, and thus no H2PtCl6 solution was added to the PR solution. A stir bar was added and the vial was sealed with a septum. This suspension was purged with N2/CH4 gas (98/2% v/v) for 10 min, then irradiated by a solar light simulator (Newport Oriel, 100 mW cm -2 )