LDHs/alginate-like extracellular polymers composite from sludge: Efficient nitrate/phosphate recovery and its potential as a plant nutrient fertilizer

Abstract

Nutrients such as nitrogen (N) and phosphorus (P) reclamation from wastewater prior to discharge, specifically targeting concurrent nitrate and phosphate recovery, are vital for mitigating eutrophication and enabling resource circularity. In this study, three-dimensional spherical MgFe-layered double hydroxide functionalized with sludge-derived alginate-like exopolymers (MgFe-LDH-ALE) was elaborated for the recovery of nitrate and phosphate, with subsequent valorization of spent adsorbents as plant fertilizers. MgFe-LDH-ALE with a specific surface area of 44.54 m 2 ‧g -1 exhibited Langmuir maximum adsorption capacity of 17.51 and 34.15 mg‧g -1 for nitrate and phosphate at 25 o C, respectively. Adsorption kinetics followed a pseudo-second-order model, confirming chemisorption governed by combined intraparticle and film diffusion. Mechanism studies revealed synergistic pathways, including electrostatic attraction, interlayer anion exchange, surface complexation, and electron transfer. Post-adsorption MgFe-LDH-ALE-N/P composites, enriched with Fe/Mg/N/P, significantly enhanced lettuce growth, significantly accelerating the root length, plant height, stem diameter, leaf area, and chlorophyll a and chlorophyll b contents of lettuce. This sludge-to-resource strategy innovatively transforms waste into agricultural amendments while closing nutrient loops in wastewater treatment. • ALE was successfully componded with MgFe-LDH, realizing sludge recycling and pollutant removal. • N/P is removed by electrostatic attraction, interlayer anion exchange and surface complexation. • Post-adsorption MgFe-LDH-ALE-N/P significantly promote lettuce growth and chlorophyll production.