Improvement of Thermally Activated Biochar with Eggshell for Phosphate Recovery and its Effectiveness as Fertilizer after Phosphorus Adsorption

Abstract

We investigated recovering P from wastewater using a convenient procedure and the readily available natural materials, eggshell and bamboo. Bamboo has not been tested for P recovery previously. Bamboo char is a source of carbon and eggshell contains CaCO3 that is a source of metal cations that have high affinity for phosphate. Therefore, the combination of pure biochar with calcium may be a good adsorbent to recover P as fertilizer. Different calcination temperatures (600, 700, and 800 °C) and mixing ratios of bamboo and eggshell powder were investigated to find effective adsorbents. To investigate the feasibility of calcined bamboo and eggshell for P removal, we also examined the pH, adsorbent dose, initial concentration, adsorption time, adsorption temperature, and presence of different ions and organic acids. The kinetics study showed that the experimental data sets were better fitted by a pseudo second-order model rather than a pseudo first-order model, indicating that chemisorption occurred. The Langmuir isotherm model estimated maximum adsorption capacities of 95.14 and 98.40 mg/g for 1:1 and 2:1 bamboo:eggshell mixtures. Monolayer adsorption occurred on a homogenous surface and was non-spontaneous at 298 K and exothermic for the 1:1 and 2:1 mixtures, and the calculated Langmuir separation factor indicated favorable conditions for P adsorption. Fourier transform infrared spectrometry showed the formation of a band at 1021 cm-1 on the 1:1 mixture, which indicated the presence of phosphate after adsorption. A desorption experiment showed that P desorption was lower in water than in neutral ammonium citrate. Finally, we demonstrated that P-loaded eggshell-modified bamboo char was an effective slow-release fertilizer for Japanese mustard spinach cultivation.

Keywords: adsorption, natural adsorbent, phosphorus removal, calcined eggshell, desorption, fertilizer, plant growth