The adsorption of phytate onto an Fe–Al–La trimetal composite adsorbent: kinetics, isotherms, mechanism and implication

Abstract

Phytate is the most abundant organic phosphorus (P) in the environment and is also an important bioavailable P source for algal blooms in some lakes. A novel Fe–Al–La (FAL) tri-metal composite adsorbent was developed by the coprecipitation method. The maximum adsorption capacity was 2.09 μmol m⁻² at 298 K and an initial pH of 4.0, and it could keep high adsorption capacity when the pH varied from 4.0 to 9.0. The dominant process for the adsorption of phytate by the FAL adsorbent was surface chemical reactions mainly by monolayer coverage. The adsorption was best described by Langmuir isotherms, and its kinetics by a pseudo-second-order kinetic equation. Thermodynamic parameters indicated that adsorption of phytate by the FAL adsorbent was a spontaneous and endothermic process. The adsorption capacity decreased with pH variation from 3.2 to 11.0, especially when pH > 9.0. The sequence of strength of competition of coexisting anions with phytate was CO₃²⁻ > SO₄²⁻ > NO₃⁻ > Cl⁻. Dissolved organic matter (DOM) also competed for adsorption sites with phytate on the surface of the FAL adsorbent. Fourier transform infrared (FT-IR) spectroscopic and X-ray photoelectron spectroscopic (XPS) analyses showed that phytate had been adsorbed onto the surface of the FAL adsorbent and that Fe, Al and La all participated in adsorption. The prepared FAL adsorbent exhibited potential for removing both phytate and other phosphate species during treatment of wastewaters including those from pig and poultry manures. The FAL adsorbent could also be a potential agent for immobilization of both phytate and phosphate in overlying water and lake sediments. This study also indicated that eutrophication of lakes would increase the potential of phytate to be a bioavailable form of P in blooming of algae.