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Shale gas extraction is increasing worldwide in the context of the energy crisis, yet the associated pollution by this industrial activity is poorly known. Here we analyzed organic contaminants occurring in the effluent of a shale gas wastewater treatment plant from Chongqing, China, using gas chromatography coupled to time of flight mass spectrometry. The toxicological effects of typical iodoalkanes were also evaluated using human liver carcinoma cells. We identified 115 compounds belonging to the classes of alkanes, aromatic hydrocarbons, halogenated hydrocarbons, amines, nitriles, aldehydes, ketones, esters and alcohols in wastewater. Several halogenated hydrocarbons were identified for the first time in shale gas wastewater effluents, notably iodine compounds. Mechanisms of formation of iodoalkanes benzyl bromide and benzyl iodide are discussed. Halogenated N-heterocyclic compounds originate probably from fracturing chemicals. Toxicity evaluation shows that cell growth for iodoalkanes treatments reduced in a dose-dependent manner, especially for diiodoalkane, suggesting that the cytotoxicity may be related to the number of substituted iodine atoms. [ABSTRACT FROM AUTHOR]