Effect of pH on ammonium adsorption by natural Zeolite clinoptilolite

Abstract

Clinoptilolite, a zeolite mineral with a high cation exchange capacity and surface area, has ion‐exchange properties that can be utilized to adsorb NH₄ ⁺, protecting it from losses during composting of N‐rich animal manures. Ammonium adsorption by the natural zeolite clinoptilolite was studied to ascertain the effectiveness of the zeolite as an NH₄ ⁺ adsorbent at pH 4, 5, 6, and 7. The NH₄ ⁺ adsorption data were fitted to the one‐ and two‐surface Langmuir, Freundlich, and Temkin isotherms. All models described the NH₄ ⁺adsorption data successfully (r²≥0.939). The one‐surface Langmuir, Freundlich, and Temkin were converted to pH‐dependent forms. The amount of NH₄ ⁺ adsorbed increased as pH and initial NH₄ ⁺concentration increased. From the one‐surface Langmuir isotherm, the NH₄ ⁺adsorption capacity (X_m) of the zeolite increased linearly with pH (r²=0.994), and was estimated to be 9,660 mg N kg^‐1 at pH4, 11,220 mg N kg^‐1 at pH 5, 12,720 mg N kg^‐1 at pH 6, and 13,830 mg N kg^‐1 at pH 7. The adsorption of higher amounts of NH₄ ⁺with increasing pH and initial NH₄ ⁺concentration is an important characteristic of the zeolite that can be beneficial to minimizing N‐losses via NH₃volatilization during composting of N‐rich animal manures.