Alkaline thermal activation of persulfate for efficient thiocyanate degradation: Mechanistic insights and application to cyanide-laden wastewater.

Efficient degradation of thiocyanate (SCN-) under alkaline conditions is essential to suppress the volatilization of toxic hydrogen cyanide (HCN). However, conventional catalyst-activated persulfate (PDS) advanced oxidation processes often exhibit limited efficiency in such alkaline media. This study presents a catalyst-free, thermally activated PDS system that effectively overcomes this limitation. At pH 12 and 60 °C, the system achieved complete SCN- removal within 720 min, with a reaction rate constant (5.66 × 10-3 min-1) 2.2 times higher than that at pH 2. Critically, the accumulation of the toxic intermediate cyanide (CN-) was effectively controlled, with its final concentration maintained below 0.01 mg L-1. Mechanistic studies confirmed the established pH-dependent radical transition: sulfate radicals (SO4•-) dominated under acidic conditions, while hydroxyl radicals (•OH) prevailed under alkaline conditions, accounting for 90.97% of the contribution. The superior performance under alkaline conditions is attributed to the higher reaction rate constant between •OH and SCN-, coupled with the system's inherent buffering capacity. The degradation pathway involved the sequential conversion of SCN- to CN-, cyanate (CNO-), and ultimately to NH4+, NO2-, and NO3-, with a balanced total nitrogen. When applied to real gold mining wastewater containing both SCN- and refractory iron-cyanide complexes, a hybrid "UV pretreatment + Heat/PDS" process achieved simultaneous removal of SCN- and total cyanide (to below 0.01 mg L-1), reducing the electrical energy per order (EE/O) by 74.6% compared to UV/PDS alone. This work validates an efficient and safe strategy for SCN- remediation by leveraging a homogeneous, thermally activated PDS system under essential alkaline conditions, offering a practical solution for the treatment of complex cyanide-containing wastewater.