Preparation of Al-Na modified zeolite for the removal of hardness ions from industrial wastewater

Abstract

High-hardness industrial wastewater, in which hardness ions mainly refer to Ca2+ and Mg2+, can cause scaling, equipment failure, increased energy consumption, and limited water reuse. In this study, commercial 4A zeolite was sequentially modified by Al introduction and NaOH-NaCl activation for softening actual industrial wastewater. The raw wastewater had total hardness of 664.0 mg/L as CaCO3, Ca2+ of 137.6 mg/L, Mg2+ of 77.8 mg/L, pH 6.2, alkalinity of 93 mg/L, and chemical oxygen demand of 420 mg/L. Under optimal conditions, namely NaOH/NaCl = 1:1, alkali-salt concentration of 2 mol/L, modification at 60 ℃ for 8 h, solution pH 6, adsorbent dosage of 3 g/L, contact time of 2.5 h, 20 ± 2 ℃, and 500 rpm, total hardness removal exceeded 99.9%. After five cycles of regeneration and reuse using 2mol/L NaOH NaCl, the removal rate remained above 95% Characterization confirmed surface roughening, Na enrichment, and framework preservation. Al-Na-Zeo exhibited an accessible mesoporous structure with a BET surface area of 46.87 m2 g-1 and an average pore diameter of 5.5 nm, facilitating hardness-ion transport. The removal mechanism involved Na+/Ca2+/Mg2+ exchange and surface-assisted precipitation, while kinetic and equilibrium data fitted pseudo-second-order and Freundlich models, respectively.