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Humic acid and L-cysteine-codecorated magnetic Fe 3 O 4 nanoparticles (HA/LC-MNPs) were synthesized using a coprecipitation method. Humic acid fractions abundant with carboxyl and hydroxyl groups can be selectively coated on the surface of MNPs during synthesis. HA/LC-MNPs with abundant heteroatoms (N, S, and O) show excellent removal capacity, great selectivity, and also fast trapping of Hg 2+ in a wide pH range. The adsorption capacity of HA/LC-MNPs for Hg 2+ can reach 206.5 mg/g, and the chemisorption was attributed to the major adsorption form. In competitive adsorption, HA/LC-MNPs preferentially adsorbed Hg 2+ with an affinity order of Hg 2+ > > Pb 2+ > Cu 2+ ≫ Zn 2+ > Cd 2+ . In total, 93.91% of Hg 2+ can be quickly captured in the presence of a 6000 times higher concentration of competing metal ions (Pb 2+ , Cu 2+ , Cd 2+ , and Zn 2+ ) within 30 min. The adsorption mechanism was analyzed using X-ray photoelectron spectroscopy (XPS). It suggested that the HA/LC-MNPs enhanced the adsorption capacity of Hg 2+ because of the complexing abilities of the multiple thiol, amino, and carboxyl groups in sorbents with Hg 2+ , the ion exchange ability of the carboxyl group, and the negative charge surface. All in all, HA/LC-MNPs are a potentially useful and economic material for the selective removal of Hg 2+ from polluted water.