Carboxymethyl cellulose/ microfibrillated cellulose hydrogel beads for phosphate recovery: Templating effects of in situ-grown CaCO3 nanoparticles from CO2

Carboxymethyl cellulose (CMC) hydrogel is a promising adsorbent. However, its weak chemical interaction with phosphate restricts the adsorption capacity and selectivity for nutrient recovery. In this study, CaCO3 nanoparticles were grown with microfibrillated cellulose using CO2 before crosslinking CMC into hydrogel beads to improve phosphate adsorption through chemical interactions. Scanning electron microscope showed the rough surface with wrinkles and the slightly porous surface after removing CaCO3. Fourier-transform infrared spectroscopy confirmed calcite formation in CMC hydrogel and the removal through chelation based on the calcite peak at 852 cm−1. However, the calcite crystallinity was absent in the X-ray diffraction analysis due to the hindrance of the amorphous hydrogel. The phosphate adsorption capacity was increased to 109.87 mg/g by growing CaCO3 nanoparticles instead of blending. After removing CaCO3 nanoparticles, the phosphate adsorption capacity was further raised to 132.21 mg/g due to the increment of active sites and surface area. The adsorption was stable between pH 3 and pH 7, and it was only significantly affected by HCO3- due to alkalinity changes. The selective phosphate adsorption was driven by chemical interaction, as proven by the pseudo-second-order kinetic model and Langmuir isotherm. After 6 cycles, the adsorption capacity was maintained more than 80 %.