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Adsorption of Hg2+ and methylmercury (MeHg) to multi-walled carbon nanotubes (MWCNTs) modified, respectively, with hydroxyl, amine and carboxyl groups was studied. The effect of various factors like the initial pH, natural organic matter (NOM), Cl- and adsorbent dose on the sorption efficiency were evaluated. It was found that amine-modified MWCNTs showed a strong adsorption capacity to Hg2+ and MeHg, and the removal efficiency could reach up to 92% (0.5 g/L MWCNTs, and 100 μg/L Hg2+ and MeHg) which is independent of pH. NOM had complex effects on the adsorption of Hg2+ and MeHg to MWCNTs. Cl- inhibited the adsorption of Hg2+ and MeHg to MWCNTs. The adsorption of Hg2+ and MeHg was found to be inhomogeneous and homogeneous chemisorption, respectively. Our results suggested that MWCNTs modified with different functional groups can efficiently adsorb both Hg2+ and MeHg in aqueous environment.