Effect of pH on ammonium adsorption by natural zeolite clinoptilolite

Clinoptilolite, a zeolite mineral with a high cation exchange capacity and surface area, has ion-exchange properties that can be utilized to adsorb NH 4+, protecting it from losses during composting of N-rich animal manures. A mmonium adsorption by the natural zeolite clinoptilolite was studied to asc ertain 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 La ngmuir, Freundlich, and Temkin isotherms. All models described the NH4+ ads orption data successfully (r(2)greater than or equal to 0.939). The one-sur face 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 cap acity (X-m) of the zeolite increased linearly with pH (r(2)=0.994), and was estimated to be 9,660 mg N kg(-1) at pH 4, 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 minim izing N-losses via NH3 volatilization during composting of N-rich animal ma nures.