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Among all homogeneous catalysts, cobalt ions show the highest catalytic performance for the activation of peroxymonosulfate (PMS). Herein, we report a Co2+/PMS/H 3 BO 3 system that can effectively generate reactive oxygen species (ROS) with ultra–low Co2+ dosage (5 μg/L). Co2+/PMS/H 3 BO 3 system showed ultrafast reactivity and wide applicability for various pollutants. Sulfamethoxazole (SMX, 2 mg/L) could be completely removed within 5 min, and the corresponding k obs reached up to 1.1239 min−1. The introduction of H 3 BO 3 significantly promoted the generation of ROS. The turnover frequency (TOF) calculated through dividing k obs by the cobalt ions concentration is as high as 224.78 min−1, which is much higher than most of the current research. Through a series of theoretical and experimental analyses, the complex of H 2 BO 3 −–MS (HSO 5 B(OH) 3 –) was inferred to be the key substance that led to the excellent performance of the system. This work provides new insights into the Co2+/PMS system in the presence of borate buffer. [Display omitted] • Trace cobalt(II) (5 μg/L) can efficiently activate PMS in the presence of boric acid. • The corresponding k obs value reached up to 1.1239 min−1 in Co2+/PMS/H 3 BO 3 system. • H 2 BO 3 −-PMS was the key substance that caused the excellent performance of the system. • Co2+/PMS/H 3 BO 3 system showed wide applicability for various pollutants. • This study provides novel insights into the Co2+/PMS system in borate buffer. [ABSTRACT FROM AUTHOR]