“Treating Waste with Waste”: Utilizing Phosphogypsum to Synthesize Porous Calcium Silicate Hydrate for Recovering of Fe 2+ from Pickling Wastewater

Phosphogypsum is a by-product of the wet-process phosphoric acid production, and it is rich in Ca and S. Long-term storage of Phosphogypsum can cause serious pollution to the environment; therefore, promoting the sustainable utilization of Phosphogypsum is crucial. This study proposes the use of Phosphogypsum and silicic acid in a sodium hydroxide solution for the hydrothermal synthesis of porous calcium silicate hydrate adsorbent, which is used for adsorbing Fe 2+ from simulated hydrochloric acid pickling wastewater. Under the optimal synthesis conditions (37.5 g/L of NaOH, calcium/silicon ratio of 1.0, liquid/solid ratio of 15:1 mL/g, 110 °C, and 4 h), the conversion rate of SO 4 2− in Phosphogypsum is 87.41%. Porous calcium silicate hydrate exhibits excellent OH − release capability in Fe 2+ -containing pickling wastewater. The adsorption process for Fe 2+ ; is mainly chemical adsorption, involving ion exchange between Ca 2+ and Fe 2+ , as well as complexation reactions of O-Si-O group, -OH group, and Si-O group with Fe 2+ . This technology aims to provide a solution for the sustainable utilization of Phosphogypsum and the recovery of Fe 2+ from pickling wastewater, which has significant practical importance.