Layered double hydroxides anchored with fulvic acid as a novel composite adsorbent for the simultaneous extraction of phosphate and ammonia nitrogen from liquid digestate

Simultaneously extracting phosphate (HPO 4 2- ) and ammonia nitrogen (NH 4 + ) nutrients from the wastewater is of great importance but still poses a challenge as HPO 4 2- and NH 4 + hold opposite charge. In this study, a novel multifunctional composite material (LDH-FA) was prepared through a straightforward flocculation reaction between positively charged layered double hydroxide (LDH) nuclei with negative fulvic acid (FA) macromolecule to synergistic extraction of HPO 4 2- and NH 4 + from liquid digestate. The results show that LDH-FA can capture HPO 4 2- by anion exchange of LDH while simultaneously adsorb NH 4 + through electrostatic attraction of FA. The Zeta potential of LDH-FA is reduced after HPO 4 2- adsorption which further promotes the NH 4 + adsorption, exhibiting a synergistic adsorption performance for HPO 4 2- and NH 4 + . The adsorption capacity of LDH-FA for P can reach 13.7 mg/g, while simultaneously achieving a N adsorption capacity of 279.4 mg/g in liquid digestate. Coexisting ions such as HCO 3 - , SO 4 2- , K + do not impede the adsorption; however, sufficient HPO 4 2- content is crucial for the synergistic adsorption. Through six consecutive adsorption cycles with the supplementary addition of HPO 4 2- after each adsorption, the cumulative removal rates for P and N can reach 99.1 % and 91.3 %, respectively, with average adsorption capacities of 12.1 mg/g and 116.7 mg/g.