Adsorption of Phosphate and Nitrate Ions on Oxidic Substrates Prepared with a Variable-Charge Lithological Material

Abstract

This work evaluates phosphate and nitrate ion adsorption from aqueous solutions on calcined adsorbent substrates of variable charge, prepared from three granulometric fractions of an oxidic lithological material. The adsorbent material was chemically characterized, and N2 gas adsorption (BET), X-ray diffraction, and DTA techniques were applied. The experimental conditions included the protonation of the beds with HCl and H2SO4 and the study of adsorption isotherms and kinetics. The lithological material was moderately acidic (pH 5) with very little solubility (electrical conductivity 0.013 dS m−1) and a low cation exchange capacity (53.67 cmol (+) kg−1). The protonation reaction was more efficient with HCl averaging 0.745 mmol versus 0.306 mmol with H2SO4. Likewise, the HCl-treated bed showed a better adsorption of PO4−3 ions (3.296 mg/100 g bed) compared to the H2SO4-treated bed (2.579 mg/100 g bed). The isotherms showed great affinity of the PO4−3 ions with the oxide surface, and the data fit satisfactorily to the Freundlich model, suggesting a specific type of adsorption, confirmed by the pseudo-second-order kinetic model. In contrast, the nitrate ions showed no affinity for the substrate (89.7 µg/100 g for the HCl-treated bed and 29.3 µg/100 g bed for the H2SO4-treated bed). Amphoteric iron and aluminum oxides of variable charges present in the lithological material studied allow for their use as adsorbent beds as an alternative technique to eliminate phosphates and other ions dissolved in natural water.