Use of modified clays for removal of phosphorus from aqueous solutions

Abstract

Phosphorus (P) removal from aqueous solutions was investigated using modified bentonite, calcite, kaolinite, and zeolite with FeCl3, CaCl2, and NaCl. The maximum sorption capacity of P was obtained by modified adsorbents with Fe(3+) ions (Fe-adsorbents). The results showed that P sorption capacity by Fe-adsorbents (bentonite (1.31 mg g(-1)), calcite (1.97 mg g(-1)), kaolinite (1.31 mg g(-1)), and zeolite (1.58 mg g(-1))) was improved by ∼467, 107, 409, and 427 %, respectively, compared to unmodified adsorbents (bentonite (0.28 mg g(-1)), calcite (1.82 mg g(-1)), kaolinite (0.32 mg g(-1)), and zeolite (0.37 mg g(-1))). Sorption isotherms were well described by the Freundlich model. Desorption experiments showed that the desorption capacity was in order of unmodified adsorbents > modified adsorbents with Na(+) ions (Na-adsorbents) > modified adsorbents with Ca(2+) ions (Ca adsorbents) > Fe-adsorbents. Effect of pH and ion strength was also investigated. At different pH, changes in the ionic strength had little effect on the adsorption. Results showed that double-layer model (DLM) could model P adsorption onto modified adsorbents over a wide range of pH and varying ionic strength. According to the scanning electron microscopy (SEM) images and saturation indices (SIs), high P removal by adsorbents was partly due to the P precipitation.

Keywords: Adsorption; Desorption; Modified adsorbents; Phosphorus removal; Thermodynamic parameters.