Peanut hull powders were chemically modified using mercaptoacetic acid to develop a convenient, lowcost, and highly efficient adsorbent for the removal of Hg(II) from aqueous solutions. A pseudo-second-order rate equation fitted the kinetic data better than a pseudo-first-order rate equation did for both the raw and the mercapto-modified peanut hull powder. The equilibrium data for both the raw and the mercapto-modified peanut hull powder fitted well with the Langmuir, Freundlich, and Temkin isotherm models. Mercapto-modification improved the adsorption capacity of Hg(II) onto peanut hull powder. Fourier-transform infrared spectra revealed that mercapto groups, together with hydroxyl and carboxyl groups, were the main function groups of Hg(II) biosorption onto chemical-modified peanut hull powder.