This study aimed to synthesize a new hybrid polymethyl methacrylate (PMMA) nanocomposites as catalysts in water treatment with high efficiency. We prepared three different materials, PMMA alone, then TiSO4/PMMA, and CaSO4/PMMA. After that, we made a screening experiment and found that TiSO4/PMMA was effective for Ca+2 removal and CaSO4/PMMA was effective for Mg+2 removal, and we made the optimization process according to this principle. The pH, contact time, dose, initial pollutant concentration, and temperature were performed to obtain the optimum parameters that achieve the highest Ca+2, Mg+2 removal efficiency as main cations causing hardness. The nanocomposite samples were characterized using different techniques. The CaSO4 nanoparticles were distributed uniformly across the CaSO4/PMMA matrix but TiSO4/PMMA has the parallel plate's morphology. Accordingly, the structures of the hybrids have been postulated. The maximum elimination value was 87.5% for Ca+2 and 65% for Mg+2. Adsorption isotherm and kinetics were demonstrated to set the nature of the reaction. Langmuir adsorption isotherm models were fitted with the experimental data for Mg+2 and Ca+2. The data of the pseudo-second-order kinetic model adequately fit the experimental values for the adsorption of Mg+2 and Ca+2. The positive values of ΔGo for Ca+2 (1.86 to 5.8 KJ/mol ) at all temperatures, and ΔGo for Mg+2 (1.5 KJ/mol) at 25oC indicated spontaneous adsorption process while the negative values for Mg+2 at 40oC and 60oC indicated a non-spontaneous adsorption process.