The photocatalytic activity of the as-prepared FeCoNiCuZn HEA was examined for the 21 degradation of four representative pharmaceutical pollutants (viz., TC, SMX, IBP, and DCF), at 1 reactor (Lelesil Innovative Systems, India), under visible light illumination. A 450 W high-2 pressure mercury lamp, fitted with a UV cut-off filter (λ < 420 nm), was used as a visible light 3 source. In a typical experiment, 50 mg of the FeCoNiCuZn powder was suspended onto the 4 targeted PhAC (5 mg L -1 , 100 mL) solution. The suspension was then magnetically stirred 5 without visible light exposure for 60 min in order to establish the adsorption/desorption 6 equilibration. Thereafter, the reaction mixture was exposed to visible light and aliquots were 7 withdrawn at pre-fixed time intervals. The extent of photocatalytic degradation was quantified 8 using an UV/Vis spectrophotometer (UV Plus, Motras Scientific Instruments Pvt. Ltd., India) or 9 an ultra-high-performance liquid chromatography (UHPLC) system (UltiMate 3000, Thermo 10 Fisher Scientific Inc., United States). Specifically, the residual concentration of TC was estimated 11 at the maximum absorbance wavelength of 358 nm. Meanwhile, the residual amount of SMX, 12 IBP, and DCF was quantified with the aid of a diode array detector, set at a wavelength of 270 13 nm, 227 nm, and 220 nm respectively. Further, the photocatalytic degradation products were 14 identified using a LTQ XL linear ion trap liquid chromatography-mass spectrometry (LCMS) 15 system (Thermo Fisher Scientific Inc., United States). The photocatalytic degradation efficiency 16 (%) of the FeCoNiCuZn powder at any time t after visible light irradiation was calculated according to the following equation: