Clofibric acid is the metabolite and active principle of blood lipid regulators, it represents the class of acidic pharmaceuticals, and is one of the most persistent drug residues detected in the aquatic environment worldwide. This interdisciplinary work evaluates the effect of solution pH and water hardness in clofibric acid adsorption onto commercial activated carbons. Kinetic and equilibrium assays revealed that the highest clofibric acid removal efficiencies (>70%) were attained at pH 3, and that at pH 8 water hardness degree plays a fundamental role in the adsorption process. In hard water at pH 8 the removal efficiency values increased by 22 or 46% points depending on the carbon sample. Adsorbents' textural properties also affect the adsorption process since for the microporous sample (CP) the increase of water hardness has a great influence in kinetic and equilibrium data, while for the micro + mesoporous carbon (VP) the variation of the water hardness promoted less significant changes. At pH 3 the increase of water hardness leads to changes in the adsorption mechanism of clofibric acid onto CP carbon signaled by a transition from an S-type to an L-type curve. At pH 8 the change from deionized water to hard water doubles the maximum adsorption capacity of sample CP (101.7 mg g(-1) vs 211.9 mg g(-1), respectively). The adsorption enhancement, with water hardness under alkaline conditions, was reasoned in terms of calcium complexation with clofibrate anion exposed by molecular modeling and conductivity studies. Ca2+ complexation by other acidic organic compounds may also occur, and should be considered, since it can play a fundamental role in improved design of water treatment processes employing activated carbons. (C) 2015 Elsevier B.V. All rights reserved.