Green Synthesis of Thiourea-Grafted Activated Carbon for Efficient Adsorption of AuCl4−

The thiourea grafting method can effectively improve the ability of activated carbon to recover chloroauric acid (AuCl4−). Conventional grafting strategies rely on acyl halide reactions using reagents such as SOCl2 and CH2Cl2, which suffer from instability and high toxicity. Herein, we propose a green grafting strategy for thiourea by activating carboxyl groups with EDC-HCl/NHS in acetonitrile, followed by the amidation reaction to obtain thiourea-modified carbon (AC-NCS). FT-IR and XPS analyses confirm the successful grafting of thiourea onto the activated carbon surface. Compared with pristine activated carbon, the adsorption capacity of AC-NCS is 104.8 mg/g, increased by 5.76 times. Furthermore, it maintains a recovery rate of 84.2% after three cycles. XPS and FT-IR further reveal that adsorption occurs on the thiourea sulfur atoms (C=S) and protonated primary amines(-NH3+), and the recovery of chloroauric acid is achieved through a synergistic “reduction–electrostatic attraction” mechanism. This method reduces the dependence on highly toxic reagents and provides a promising approach for the efficient and green recovery of gold from secondary resources.