Synthesis and characterization of a novel Mg–Al hydrotalcite-loaded kaolin clay and its adsorption properties for phosphate in aqueous solution

Abstract

The mesoporous modified kaolin clay (MKC) was synthesized by loading Mg–Al hydrotalcite onto kaolin clay through coprecipitation method and applied for adsorption of phosphate from aqueous solution. Several techniques, including Brunauer–Emmett–Teller (BET), thermal analysis (TG–DTA), and Fourier transform infrared spectroscopy (FTIR) were employed to characterize the adsorbents. The effects of adsorbent dosage, solution pH, initial phosphate concentration, contact time, temperature, and coexistent anions on phosphate adsorption have been investigated. MKC exhibited a strong uptake affinity to phosphate in a wide pH range of 2.5–9.5, with the maximum adsorptive removal of 98.03%, at adsorbent dosage of 0.2g/50mL, pH 7.5, and initial phosphate concentration 25mgL−1. The adsorption kinetics followed the pseudo-second-order kinetic model. The Langmuir isothermal model well described the adsorption isotherm data, showing a maximum adsorption capacity for phosphate up to 11.92mgg−1 at 298K. The obtained thermodynamic parameters revealed that the adsorption of phosphate onto MKC was an exothermic and spontaneous process. Coexistent chloride, nitrate, and sulfate ions displayed an adverse effect on phosphate adsorption following the order of SO42−>NO3−>Cl−. A mechanism of adsorption that involved (i) electrostatic attraction of hydroxyl groups of the adsorbent with negatively charged phosphate ions, and (ii) anion exchange of NO3− ions that were associated with the surface or interlayer of the adsorbent with anionic phosphate ions in solution, was proposed.