Diatomite and manganese-oxide-modified-diatomite (Mn-diatomite) were tested as adsorbents for Pb2+, Cu2+, and Cd2+ removal from water. Impregnating the surface of diatomite with 0.38 g of manganese oxide per gram diatomite shows an increase of 2.4-fold in the surface area of the modified diatomite. The adsorption capacities were 99, 51, and 26 mg/g Mn-diatomite for Pb2+, Cu2+, and Cd2+, respectively, obtained at solution pH 4, while values of 24, 21, and 16 mg/g diatomite were obtained at the same conditions and for the same metals. The obtained adsorption kinetics experimental data display that 95% of the original Pb2+ concentrated was adsorbed by Mn-diatomite within 10 min, while kinetic data for diatomite showed a lower rate for Pb2+ uptake. The filtration quality of diatomite was significantly enhanced with the surface modification by manganese oxide.