Three SiO 2 -based materials were prepared via covalent immobilization of carboxyl groups (COOH), phenolic groups (GA), or humic acid on an SiO 2 surface. Their sorbing properties were evaluated for removal of heavy metals (Pb 2+ , Cd 2+ , Cu 2+ , Zn 2+ , and Mg 2+ ) from aqueous solution. The data show a significant improvement for metal uptake, compared to unmodified silica that can be attributed to the adsorption of metals to the deprotonated form of functional groups (COOH, GA, HA). The metal-uptake capacity of the SiO 2 –HA material was 10 times higher that those of the other two materials. The present data provide direct experimental proof that HA can be viewed and modeled as a combination of –COO and R–OH functional groups.