Ca2+ removal from water by the use of Na-palygorskite for potential water softening

Abstract

Sodium-treated palygorskite (Na-Pal) sample was investigated for the sorption of Ca2+ ions with the aim of treating water hardness. The effective modification of the mineral with Na+ was verified by XRD and FT-IR techniques. Batch kinetic experiments in standard solutions were performed proving that 30 g/L of Na-Pal were highly satisfactory as the Ca2+ removal reached the 85% for 100 mg/L Ca2+ initial concentration, which is very promising for the softening of moderate or hard waters. The Ca2+ removal found to be pH and temperature independent, with high removal rates at room temperature and common pH values of water samples (pH 4–12), rendering these circumstances ideal for the low-cost maintenance of the procedure that took place within the first 5 min. The linear form of the Langmuir isotherm model expressed better (R2 = 1) the Ca2+ sorption, which means that takes place at specific homogeneous sites of Na-Pal. Thermodynamic analysis proved the non-spontaneous (positive ΔG0), physical, and exothermic nature (ΔH0 = −10.8197 kJ/mol) of the reaction, while the kinetic models proved the chemisorption of Ca2+ by Na-Pal. HIGHLIGHTS Na-Pal is effective for Ca2+ removal and water softening.; 30 g/L Na-Pal reached 85% Ca2+ removal in moderate/hard waters.; Effective Ca2+ removal in room temperature pH 4–12 and time 5 min under no special conditions that increase cost.; Ca2+ sorption occurs at specific homogeneous sites of Na-Pal and is characterised as non-spontaneous, physical, exothermic reaction.; Simultaneous ion exchange and Ca2+ adsorption in Na-Pal structure.;

Keywords

• calcium ions
• ion exchange
• palygorskite
• sodium treatment
• water hardness
• water softening