Adsorption kinetics of phosphorus on a calcium silicate hydrate based adsorbent

Abstract

Phosphorus (P) is a fundamental element for plant and animal growth; however, an excessive amount of phosphorus can cause a threat to ecological environmental safety and human health. Therefore, this study aims to synthesize an adsorbent based on calcium silicate hydrates using mixture of polonite and calcium oxide and to determine its adsorption capacity for phosphorus ions. Additionally, pseudo-first- and pseudo-second- order kinetic models were employed to understand the adsorption process.

The adsorbent based on calcium silicate hydrate was synthesized in a mixture of CaO and Polonite (CaO/SiO₂ molar ratio 1.5) under hydrothermal conditions (16 h, 200 °C). It was determined that during hydrothermal treatment two crystalline (tobermorite and α-C₂SH) and semicrystalline type calcium silicates hydrates were formed. Batch adsorption experiments were carried out at temperatures of 25, 35, and 45 °C in a thermostatic absorber by stirring 10 g of synthetic adsorbent in 1 l of KH₂PO₄ solution containing 0.2 g of P⁵⁺/L (20 mg of P⁵⁺ per gram of adsorbent) of phosphate ions. The duration of adsorption lasted up to 168 h. It was determined that adsorption capacity of synthetic adsorbent for phosphorus ions depends on the reaction duration and adsorption temperature. Synthetic adsorbent showed an extremely high adsorption capacity (>18 mg P⁵⁺/g) for phosphorus ions under all adsorption conditions. The most intensive adsorption occurred at a temperature of 35 °C as within 1 h 1 g of adsorbent adsorbed 16.6 mg of phosphorus. The equilibrium was reached after 48 h, when adsorption capacity reached 18.7 mg P⁵⁺/g. The kinetic calculations and the results of X-ray diffraction showed that chemisorption occurred during the experiments.