Influence of Ammonium on the Adsorption and Desorption of Heavy Metals in Natural Zeolites

Abstract

Natural zeolites have gained attention as low-cost adsorbents for the removal of heavy metals (HMs) from wastewater. However, their performance can be compromised by the presence of competing cations such as ammonium (NH4+). This study investigated the competitive adsorption and desorption dynamics of NH4+ and six HMs (Cd, Cr, Cu, Ni, Pb, and Zn) on two natural zeolites. Batch and column experiments using synthetic wastewater were conducted to evaluate the effects of different NH4+ concentrations, pH, and particle size on HM removal efficiency and desorption effects. Results showed that increasing NH4+ concentrations significantly reduce HM adsorption, with total capacity decreasing by ~45% at 100 mg/L NH4-N in kinetic tests. Adsorption isotherms of the HM mixture for both zeolite types followed a clear sigmoidal trend, which was captured well by the Hill model (R2 = 0.99), with loading rates up to 56.14 mg/g. Pb consistently exhibited the highest affinity for zeolites, while Cd, Cr, Ni, and Zn were most affected by NH4+ competition in the column tests. Desorption tests confirmed that NH4+ rapidly re-mobilises adsorbed metals, in particular Cd, Cu, and Zn. Slightly acidic to neutral pH conditions were optimal for minimising HM remobilisation. These findings underscore the need to consider competitive interactions and operational conditions when applying natural zeolites for HM removal, especially in ammonium-rich environments such constructed wetlands, soil filters, or other decentralised applications.