The simultaneous adsorption of nitrate and phosphate by an organic-modified aluminum-manganese bimetal oxide: Adsorption properties and mechanisms

A novel organic-modified aluminum‑manganese bimetal oxide (OABO) adsorbent was fabricated to adsorb nitrate and phosphate from wastewater and reduce eutrophication risk. Several techniques were applied to investigate the surface characteristics of OABO. The characterization results showed that OABO has multiple functional groups, including hydroxyl groups, impregnated sulfate ions, grafted amine groups and coordinated chloride ions. The adsorption properties of nitrate and phosphate were studied via kinetic and isotherm adsorption experiments in both single-component and bi-component systems. The experimental results demonstrated that the nitrate and phosphate adsorption equilibria could be reached within 2 min and 8 h, respectively, while the Langmuir maximum adsorption amounts of NO₃⁻-N and PO₄³⁻-P were determined to be 19.45 mg/g and 33.16 mg/g at T = 25 °C in the bi-component system. Moreover, nitrate adsorption decreased with increasing pH, while phosphate adsorption was barely affected by pH changes. The adsorption characteristics of nitrate and phosphate in the bi-component system were similar to those in the single-component system, indicating weak competition between the two pollutants. A spectroscopic analysis was conducted on samples after adsorption to reveal the different adsorption mechanisms of the two pollutants. Nitrate adsorption involved ligand exchange with coordinated chloride ions and electrostatic interactions with grafted amine groups. In contrast, the phosphate adsorption mechanisms mainly included surface complexation and ion exchange with sulfate. Furthermore, this adsorbent could be regenerated and reused in five consecutive cycles with a proper eluent. Therefore, OABO has great potential for the co-removal of nitrate and phosphate from contaminated water.