In recent years, persulfate activation systems have received increasing attention due to their high oxidation reactivity when removing environmental pollutants. Pyrite, the most common metal sulfide on Earth's surface, can supply abundant Fe2+ for persulfate activation. The role of the generated reactive oxygen species (ROS) in persulfate-pyrite systems however, is not fully understood. In this study, batch experiments were used to investigate p-chloroaniline (PCA) degradation by a pyrite-persulfate system. The effects of pyrite dosage, pH, temperature, air conditions (aerobic vs. anaerobic) and pyrite particle size on PCA degradation were examined. Radical detection was conducted using electron paramagnetic resonance (EPR) methods. Results from the EPR spectra indicated that PCA degradation was achieved by sulfate radical and hydroxyl radical oxidation. Aerobic conditions were more beneficial to PCA degradation than anaerobic conditions due to the generated superoxide radicals (O-2(center dot-)) that activated the per sulfate to produce more sulfate radicals (SO4 center dot-) PCA degradation also increased with higher pyrite doses and under acidic conditions (pH 3.0 and 5.0). PCA was removed completely at pH 3.0 after 60 min. Temperature increase from 10 to 50 degrees C significantly promoted PCA degradation. These findings provide new understanding of the mechanism involved in pyrite activation of persulfate which can be used to improve PCA degradation by pyrite-persulfate systems. (C) 2016 Published by Elsevier B.V.