This paper presents results of a study of adsorption properties of the original (I6) and carboxyl-modified (I6COOH) detonation nanodiamonds (DNDs) dispersed in aqueous solutions, with respect to dissolved ions: Cu2+, Pb2+, NO3-. Application of complementary spectroscopic methods allows comparative analysis of the adsorption activity of DNDs in aqueous solutions of metal salts. It was shown that both types of functionalized NDs readily adsorb nitrate ions and metal cations, and the adsorption efficiency of I6COOH is approx. three times higher than that of I6. Hypothetic mechanism of nitrate and copper ions adsorption on NDs surface is proposed.