The presence and persistency of pharmaceuticals and personal care products (PPCPs) in the environment attracted great attention recently. Among them, antibiotics and pain-killers accounted for a large quantity. Although many works were devoted to the investigation of their removal in wastewater treatment processes, most of the PPCPs studied were of cationic nature. The net repulsive interactions between anionic PPCPs and negatively charged sorbents make them difficult to be removed in wastewater treatment. In this study, 2:1 clay minerals illite and montmorillonite (MMT) were modified with different amounts of cationic surfactant cetyltrimethylammoium bromide (CTAB). The types and sites of interactions between the surfactant-modified clays and the anionic drug diclofenac (DC) were investigated. The uptake of DC on the modified clays was controlled by the CTAB loading level and its surface configuration on the clays. The adsorption sites of DC were limited to the external surfaces of modified illite due to its non-swelling nature. On the contrary, both the external and interlayer sites were available for the adsorption of DC on modified MMT. A CTAB bilayer formation resulted in significant increase in DC adsorption due to the formation of extensive admicelles. FTIR results showed participation of the benzene ring, N-H, and CH2CH3 for the interactions between DC and modified MMT, suggesting that partitioning of DC into the admicelles of CTAB played a significant role in DC uptake. The results could extend the application of surfactant-modified clays for the removal of anionic PPCPs in the wastewater treatment processes. (C) 2016 Elsevier B.V. All rights reserved.