Red mud as a magnesium carrier for enhanced N and P recovery from wastewater by the struvite method

The struvite method has been widely used to recover N and P from wastewater. However, the drawbacks of alkali consumption and small crystallization are not negligible. Therefore, alkaline porous carrier materials are greatly desired to enhance struvite crystalline precipitation and maintain suitable pH values. In this study, using red mud (RM) as a carrier, MgO was loaded onto RM by co-precipitation method to prepare an N and P recovery material (MgO–RM). A response surface methodology based on the Box–Behnken design was used to explore the effects of the factors on N and P recovery. The multi-objective optimization of the recovery process was carried out using the desirability function approach to achieve an economically feasible recovery. Characterizations, including SEM-EDS, BET, FTIR, XRD, and XPS, were carried out to explore the recovery mechanism. The results demonstrated that the nano-sized MgO was well deposited on the RM surface, resulting in a larger specific surface area and greater reactivity of the MgO–RM. Using RM as a carrier significantly increased the struvite crystal size, and the MgO–RM could maintain the pH value of the solution in a suitable range for struvite growth. Under optimal conditions (dosage = 3.5 g/L, N/P = 1.8, and pH value = 3.4), the maximum N and P recovery capacity by MgO–RM was 57.23 mg/g and 128.05 mg/g. The recovery process may involve coupled reactions between physical adsorption, ion exchange, coordination exchange, and chemical precipitation. Struvite produced by chemical precipitation is the main recovery mechanism.