Modified zeolite as a means of purifying irrigation water – Study
PhD Thesis – Author: Dionysiou, Ourania-Nina S., Supervisor: Misopolinos, Nikolaos, Aristotle University of Thessaloniki, School of Agriculture, 2010.
This study investigated certain conditions that influence the adsorption of B and P on surface-modified natural zeolite (clinoptilolite), with hexadecyl-trimethyl-ammonium bromide (C19H42BrN or HDTMA–Br), with a view to its use as a decontamination agent for irrigation water and aquatic systems. Specifically, the following were investigated: (a) the effect of washing the zeolite with HCl prior to its modification, and (b) the effect of increasing the pH on the adsorption of B by the zeolite.
Subsequently, the following were investigated: c) the effect of the ‘external solution/adsorbent’ ratio, (d) the effect of the equilibration time, (e) the effect of temperature, (f) the effect of pH, and (g) the effect of the presence of NO₃⁻ on the adsorption of P onto the zeolite. Finally, the following was investigated: (h) the rate of P release from the zeolite. The results showed that the modified natural zeolite has the capacity to adsorb B, which was reduced when preceded by washing with HCl, and increased when preceded by washing with HCl and the pH was adjusted to the alkaline range (pH ~ 9.5).
In all cases, however, the amount of adsorbed B, expressed as a percentage of the initially added amount, was much lower than those reported in the literature for other anions. Furthermore, the concentration of B in the equilibrium solution exceeded the critical limit of 3 mg L1 (for use of water for irrigation), even when the concentration of B in the external solution was 5 mg L1. With regard to P, the experiments showed that P adsorption on the modified natural zeolite increased as the ‘external solution/adsorbent’ ratio decreased. It was found that the equilibrium time required for P adsorption to be complete was ≤ 1 h and did not vary with the concentration of P in the external solution. It was found that the P adsorption reaction was more accurately described by the Langmuir equation than by the Freundlich equation, and that it was exothermic.
Furthermore, as the pH increased from the acidic to the alkaline range, P adsorption increased. In addition, there was considerable evidence that the modified natural clinoptilolite exhibited greater adsorption affinity for divalent phosphate anions than for monovalent ones. Where the external solution contained both phosphate and nitrate anions, as long as the concentration of nitrate anions ranged from 0 to 25 mg L⁻¹, the adsorption of P was not reduced by the presence of nitrate anions. However, when the concentration of nitrate anions doubled, the adsorption of P decreased.
It was also observed that the modified natural zeolite has the capacity to adsorb large quantities of P, which it releases with difficulty and at a slow rate. The largest proportion of P was desorbed during the first 24 hours, in less than 15 minutes. Thereafter, P desorption stabilised at very low levels and continued steadily up to 240 h, with concentrations of desorbed P in the equilibrium solution ranging from 0.1 – 0.3 mg L⁻¹. It was also found that the desorption of P does not depend on time or on its initial concentration in the material. Finally, by combining the results from the experiments concerning the adsorption and desorption of P, general conclusions can be drawn regarding the mechanism of adsorption of phosphate anions on modified natural clinoptilolite. It appears that adsorption is due to electrostatic forces, which arise between the positively charged surface of the zeolite and the negative charge of the ion present in the external solution (anion exchange mechanism). The results of the above experiments demonstrate that the use of natural clinoptilolite modified with HDTMA–Br as an adsorbent can be considered a good method for the decontaminationwater and aquatic systems with high concentrations of P.
Under certain conditions, the material can be considered safe in terms of the environmental impacts of its use for the removal of excess P from irrigation water and other P-enriched water systems. Finally, there is a possibility that, if the adsorbed substance consists solely of phosphate anions, the zeolite can be incorporated into the soil to meet the plants’ P requirements.