Adsorption of protamine and papain proteins on saponite

Abstract

Due to the increased importance of bionanocomposites, protamine and papain proteins were adsorbed on Na⁺- and on Cs⁺-exchanged saponite from aqueous solution. Protein analysis of equilibrium solutions and thermogravimetric analyses of biocomposites were used to prepare adsorption isotherms. Based on the isotherm shape, and on the amounts of protein adsorbed and the amounts of Na⁺ and Cs⁺ released, the initial protein sorption apparently was due to ion exchange. Additional sorbed protein was weakly retained and could be removed by washing with water. From ion exchange, the average charge of the protamine adsorbed was estimated to be +13.1 to +13.5. Similar papain measurements could not be made due to partial decomposition. Quantitatively, protamine was adsorbed at levels up to 400 mg/g on Na⁺-saponite and 200 mg/g on Cs⁺-saponite. The maximum protamine adsorption was 650 to 700 mg/g for Na⁺-saponite and 350–400 mg/g for Cs⁺-saponite. Protamine was sorbed to edge surfaces and the basal spacing of the interlamellar region of saponite was 1.75 nm. Protamine displaced only 36% of the Cs⁺ in Cs⁺-saponite and expanded the interlamellar region by 36% for a basal spacing of 1.6 nm. Papain sorption to Na⁺-saponite occurred by a two-step process: (1) adsorption to saponite particle external surfaces followed, (2) by partial intercalation. Quantitatively, Papain was adsorbed up to 100 mg/g for Na⁺-and Cs⁺-saponite. Greater initial papain concentrations resulted in a 450 mg/g maximum for Na⁺-saponite, but no increase above 100 mg/g for Cs⁺-saponite. Papain apparently only sorbed to external Cs⁺-saponite surfaces that were estimated to be 33–40 m²/g. Step-wise thermal decomposition of the saponite-protein composites occurred between 300 and 800°C.