In this study, three kinds of monocarboxylic acids, formic, acetic, and propionic acids, are first applied to modified microalgae residuals (M1-RD, M2-RD, and M3-RD, respectively) after lipid extraction, aiming to enhance the adsorption capacity and selective binding ability for Hg(II) ions in simulated wastewater. The effect of pH, temperature, and initial Hg(II) concentration was investigated to identify the optimum adsorption conditions. Batch adsorption tests showed that the maximum adsorption efficiency obtained was 96.7% for Ml-RD, 91.1% for M2-RD, and 84.4% for M3-RD at pH 4.05 compared to 48.5 and 57.6% for raw and residual microalgae at pH 5.01, respectively. The adsorption capacity of raw, residual, and modified microalgae increased with the increase of the temperature. Langmuir and Freundlich isotherm model tests showed that the maximum equilibrium adsorption capacity among three modified adsorbents reached 63 +/- 3 mg/g for M1-RD, in contrast to 17 +/- 1 and 25 +/- 2 mg/g for raw and residual microalgae, respectively. The characterization of the modified sample by Fourier transform infrared spectroscopy, Brunauer-Emmett-Teller, and scanning electron microscopy showed that the fibrous structure of the algae was decomposed by carboxyl groups in organic acids involved in microalgae, resulting in a larger surface area and more binding sites. In consideration of ion interference in the actual process, a kind of actual desulfurization wastewater from a 1000 MW coal-fired power plant (Guangdong, China) was introduced in the simulated adsorption system. Verification tests in desulfurization wastewater showed that up to 96% of Hg(II) ions were removed, probably as a result of co-precipitation of mercury and other co-existing ions.