Inhibitory effects of Ca²⁺ on ammonium exchange by zeolite in the long-term exchange and NaClO–NaCl regeneration process

The inhibitory effects of calcium ion (Ca²⁺) on ammonium (NH₄⁺) exchange by zeolite were investigated in the long-term exchange and sodium hypochlorite - sodium chloride (NaClO–NaCl) regeneration process, and alleviation measure was developed and validated in this study. The batch experiments indicated that NH₄⁺ removal efficiency, exchange kinetics and equilibrium isotherms were significantly dependent on the coexisting Ca²⁺. The exchange capacity decreased from 0.58 to 0.40 mg g⁻¹ by increasing initial Ca²⁺ concentration from 0 to 100 mg L⁻¹. The inhibitory effect of Ca²⁺ on NH₄⁺ exchange efficiency was fitted to the competitive inhibition Monod model with half-saturation rate constant of 134.7 mg L⁻¹. Ca²⁺ addition reduced the NH₄⁺ removal rate and lengthened the exchange equilibrium time of zeolite. Periodic precipitation of Ca²⁺ in the form of calcium carbonate from the used regenerant maintained the removal efficiency of NH₄⁺ commendably by alleviating inhibition effect of Ca²⁺ and extended the working life of zeolite. The major chemical compositions of natural and regenerated zeolite were basically unchanged. Compared to Bohart-Adams model and Thomas model, the Dose-Response model could predict the breakthrough curve well, and the fitted parameter further confirmed that NaClO–NaCl regeneration with periodic Ca²⁺ removal is an effective method to maintain efficient NH₄⁺ from wastewater by zeolite.