A significantly high amount of organic leaching has been observed in the treatment and recovery of heavy metals by raw biosorbents. In this study, acid, base, calcium, formaldehyde, and glutaraldehyde are used for the modification of locally derived raw seaweed (RSW), Sargassum sp., so that the modified seaweeds (MSW) have less organic leaching while the metal biosorption capacity is maintained. It is determined that 0.2% of formaldehyde is the best, in regard to chemical modifications. The organic content of the filtrated water samples is only 3.84 mg/L total organic carbon (TOC), 80% less than that when the RSW is used. The metal biosorption capacity is greatly improved, while the uptake kinetics is similar to that of the RSW. The metal biosorption follows a descending sequence: lead > copper > zinc ≈ cadmium > nickel. Higher pH causes higher metal biosorption. Ion exchange has an important role in the metal uptake. A surface diffusion model well describes the biosorption kinetics. It is determined that 0.2 M hydrochloric acid (HCl) is the best, in regard to metal desorption. Approximately 90% of metal ions can be eluted from the metal-loaded MSW, which requires ∼20 min to complete. A five-cycle operation of metal sorption and desorption confirms that the MSW is much better than the RSW. The Fourier transform infrared (FT-IR) analysis demonstrates that the hydroxyl; amino, and carboxyl functional groups in the MSW provide the major biosorption sites for the metal binding. Scanning electron microscopy (SEM) analysis shows a strong coordination cross-linkage between the copper ions and the organic functional groups of the biomass.