The biomass of Cystoseira indica (RB) was chemically modified by cross-linking it with epichlorohydrin (CB1, CB2), and the same was oxidized by potassium permanganate (CB3) which was later employed for the treatment of Cu(II) and Ni(II) from effluents. The results indicated that biosorption equilibriums were rapidly established in about 30 and 75 min for Cu(II) and Ni(II), respectively. The metal biosorption was strictly pH dependent, and maximum removal of metals was observed at pH 6.0. The biosorption data dovetail the Langmuir isotherm, and the process obeyed pseudo-second-order kinetics. An intraparticle diffusion based Weber-Morris model was applied to evaluate rate-limiting steps, and the results suggested that film diffusion controlled the overall biosorption process. Fourier transform infrared analysis revealed that amino, hydroxyl, carboxyl, ether, and hydroxyl functional groups were involved in the metal binding and the sorption process was dominated by complexation as well as ion exchange. The loaded biosorbent was regenerated using HCl and used repeatedly over five cycles with little loss of uptake capacity beyond the second cycle.