RESEARCH ARTICLE Colloids as a sink for certain pharmaceuticals in the aquatic environment Khalid Maskaoui & John L. Zhou Received: 7 May 2009 / Accepted: 1 December 2009 / Published online: 19 December 2009 # Springer-Verlag 2009 Abstract Background, aim, and scope The occurrence and fate of pharmaceuticals in the aquatic environment is recognized as one of the emerging issues in environmental chemistry and as a matter of public concern. Existing data tend to focus on the concentrations of pharmaceuticals in the aqueous phase, with limited studies on their concentrations in particulate phase such as sediments. Furthermore, current water quality monitoring does not differentiate between soluble and colloidal phases in water samples, hindering our under- standing of the bioavailability and bioaccumulation of pharmaceuticals in aquatic organisms. In this study, an investigation was conducted into the concentrations and phase association (soluble, colloidal, suspended particulate matter or SPM) of selected pharmaceuticals (propranolol, sulfamethoxazole, meberverine, thioridazine, carbamaze- pine, tamoxifen, indomethacine, diclofenac, and meclofe- namic acid) in river water, effluents from sewage treatment works (STW), and groundwater in the UK. Materials and methods The occurrence and phase asso- ciation of selected pharmaceuticals propranolol, sulfame- thoxazole, meberverine, thioridazine, carbamazepine, tamoxifen, indomethacine, diclofenac, and meclofenamic acid in contrasting aquatic environments (river, sewage effluent, and groundwater) were studied. Colloids were isolated by cross-flow ultrafiltration (CFUF). Water samples were extracted by solid-phase extraction (SPE), while SPM was extracted by microwave. All sample extracts were analyzed by liquid chromatography-tandem mass spectrometry (LC-MS/MS) in the multiple reaction monitoring. Results and discussion Five compounds propranolol, sulfa- methoxazole, carbamazepine, indomethacine, and diclofe- nac were detected in all samples, with carbamazepine showing the highest concentrations in all phases. The highest concentrations of these compounds were detected in STW effluents, confirming STW as a key source of these compounds in the aquatic environments. The calculation of partition coefficients of pharmaceuticals between SPM and filtrate (observed partition coefficients, K obs p , K obs oc ), between SPM and soluble phase (intrinsic partition coefficients, K int p , K int oc ), and between colloids and soluble phase (K coc ) showed that intrinsic partition coefficients K int p ; K int oc   are between 25% and 96%, and between 18% and 82% higher than relevant observed partition coefficients values, and are much less variable. Secondly, K coc values are 3–4 orders of magnitude greater than K int oc values, indicating that aquatic colloids are substantially more powerful sorbents for accumulating pharmaceuticals than sediments. Furthermore, mass balance calculations of pharmaceutical concentrations demonstrate that between 23% and 70% of propranolol, 17–62% of sulfamethoxazole, 7–58% of carbamazepine, 19–84% of indomethacine, and 9–74% of diclofenac are present in the colloidal phase. Conclusions The results provide direct evidence that sorption to colloids provides an important sink for the pharmaceuticals in the aquatic environment. Such strong Responsible editor: Thomas Braunbeck K. Maskaoui : J. L. Zhou School of Life Sciences, University of Sussex, Falmer, Brighton BN1 9QG, UK J. L. Zhou (*) School of Life Sciences, University of Sussex, Falmer, Brighton BN1 9QG, UK e-mail: j.zhou@sussex.ac.uk Environ Sci Pollut Res (2010) 17:898–907 DOI 10.1007/s11356-009-0279-1