Crosslinked Polyethyleneimine Networks for Rapid and Selective Gold Recovery from Electronic Waste

Electronic waste, or e-waste, is one of the fastest-growing waste streams as electronic gadgets become increasingly prevalent and short-lived. E-waste contains a significant amount of precious metal gold (Au), whose retrieval offers substantial financial and environmental benefits. We have previously developed triazine-crosslinked polyethyleneimines (TCPEIs) that can capture Au(III) with high capacity, high selectivity, and fast kinetics. Herein, to better understand the underlying mechanisms, we studied the gold adsorption on crosslinked polyethyleneimines (CPEIs) with rigid crosslinkers of varying lengths and a different PEI type. Specifically, a triazine (2,4,6-trichloro triazine, TCT) and a piperazine-bridged triazine dimer (1,4-di(4,6-dichloro-1,3,5-triazine-2-yl)piperazine, DTP) were used to crosslink linear PEIs to generate CPEIs − lTCPEI and lDCPEI, respectively. lDCPEI, with a longer rigid crosslinker, consistently showed faster adsorption kinetics across all three tested initial gold concentrations than lTCPEI, and the lower the gold concentration, the more pronounced the difference in their adsorption rates. At 20 ppm, a gold concentration that is relevant to the acidic leaching solution of electronic waste, lDCPEI exhibited a gold adsorption rate constant (K2 = 0.5837 g mg−1 min−1) that is more than twice as high as that of lTCPEI (K2 = 0.2231 g mg−1 min−1). The gold recovery efficiency of lDCPEI is also significantly higher than that of lTCPEI at this concentration. The maximum adsorption capacities of the lDCPEI and lTCPEI as determined by Langmuir isotherm analysis are, however, comparable, and are 2777 and 2857 mg g−1, respectively. X-ray diffraction analysis indicated that the predominant mechanism of initial adsorption is electrostatic interactions, followed by the reduction of Au(III) ions. In the presence of competing metal ions of even higher concentrations, lDCPEI was able to recover 99% of the gold. Moreover, the desorption study showed that 99% of the adsorbed gold could be recovered within 30 s. This simple approach to e-waste recycling offers sustainable benefits for both the circular economy and environmental protection.