Removal of Phosphate from Aqueous Solutions Using Chemically Synthesized Vaterite Polymorph of Porous Calcium Carbonate Nanoparticles under Optimized Conditions

Abstract

Eutrophication is one of the most adverse impacts of nutrient contamination of water bodies where the phosphate is considered to be the primary limiting factor. The vaterite polymorph of porous calcium carbonate nanoparticles (VPCCNPs) were synthesized and used to remove orthophosphate ions in water. In this study, the VPCCNPs were synthesized chemically, using calcium acetate and sodium bicarbonate in a water-ethylene glycol media, at a temperature of 100°C, in a reaction time of 24 hours. Synthesized nanoparticles were characterized by X-ray diffractometry to confirm that the crystalline phase of calcium carbonate formed is spherical vaterite polymorph. Scanning electron microscopy coupled with energy dispersive X-ray analysis further confirms the spherical shape of the vaterite nanoparticles and the presence of only calcium, carbon, and oxygen thus showing high purity of the synthesized calcium carbonate nanoparticles. The dynamic laser light scattering-based particle size analysis (DLS) shows the average particle size to be 25.5 nm. The Fourier transform infrared spectroscopy was used to find functional groups before and after the adsorption of phosphate by vaterite nanoparticles. The phosphate removal efficiency of synthesized nanoparticles was tested with different concentrations of phosphate solutions (2–80 mg/L), pH levels (5–12), adsorbent dosages (0.025–0.250 g), and contact times (5–120 min). Ion chromatography was used to analyse the phosphate concentrations in water samples. The maximum phosphate removal percentage of 100% was obtained with 50 mL of 2 mg/L phosphate solution and 0.15 g of the synthesized nanoparticle. Adsorption data were well fitted with the Langmuir adsorption isotherm model and the pseudo-second-order kinetic model with R2 of 0.99 and 0.98 (rate constant -0.083 g g-1 min-1), respectively. The presence of F−, NO3−, and SO42− has no effect on phosphate adsorption since 100% phosphate removal is obtained in the presence of these ions. Furthermore, the particle shows a 100% removal of orthophosphate ions available in eutrophic water regardless of the presence of many other ions in natural water bodies. The study presents a viable option for removing excess phosphate in natural water to desirable levels as a means for controlling eutrophication.