Advanced oxidation processes offer practical and cost effective solutions for the treatment of poorly biodegradable industrial wastewaters. Here, column experiments were performed to understand the role of a complexing agent, citrate, on Fenton-treatment of an actual pharmaceutical wastewater with pyrite as the catalyst under dynamic flow conditions. Our results suggest that the pharmaceutical wastewater treatment with Fenton reaction using pyrite as the catalyst was mainly regulated by the extent of Fe dissolution from pyrite, which, in turn, resulted in formation of hydroxyl radicals in solution. The Fenton treatment efficiency was much lower in the absence of citrate compared to citric acid containing systems due to clogging of column pores with oxidized Fe species. On the other hand, the addition of citrate to wastewater significantly improved Fenton process efficacy, and prolonged the lifecycle of pyrite-packed columns depending on solution pH. Low pH values were favorable for better Fenton efficiency in systems containing citrate due to combined effect of proton and ligand promoted dissolution and mobilization of oxidized Fe species. (C) 2019 Elsevier Ltd. All rights reserved.