Adsorption mechanisms of high-levels of ammonium onto natural and NaCl-modified zeolites

Zeolites, microporous aluminosilicate minerals, have high affinity to ammonium in water. This study explored the mechanisms of ammonium adsorption onto natural zeolite and NaCl-modified counterpart under different ammonium levels (10–4000 mg-N/L) and initial pH 3.4–11.1. Ion exchange dominated the ammonium adsorption process near neutral pH, with the order of exchange selectivity following Na+ > Ca2+ > K+ > Mg2+. At high ammonium levels, Ca2+ exceeded Na+ as the dominant ions for ammonium adsorption. Conversely, in strong alkaline solutions, molecular adsorption suppressed ion exchange in ammonium adsorption. NaCl modification effectively increased ammonium adsorption capacity by increasing the Na contents in zeolite and by modifying the surface morphology to enhance film mass transfer rate. The modified zeolite presents a potential adsorbent to reduce ammonium concentration in landfill leachates, livestock wastewaters or effluents from anaerobic digestion tanks of livestock manure.

► Exchange order of cations in zeolites is disclosed under different ammonium levels. ► Contribution of molecular adsorption is assessed under different pH conditions. ► Film diffusion is enhanced on zeolite after NaCl modification.