RSM-optimized Mg/Al-LDH biochar composite for enhanced phosphorus removal: Insights into interlayer structure evolution and adsorption mechanism

High-concentration phosphorus in pesticide tailwater threatens ecosystem balance due to its toxicity and persistence. While abundant nanosheet structures in LDH(layered double hydroxides)-modified biomass enhance wastewater anion adsorption, optimizing LDH structures to maximize phosphate adsorption and elucidating the underlying microscopic mechanisms require further research. This study employed batch adsorption experiments combined with Response Surface Methodology (RSM) to optimize the synthesis conditions of Mg/Al-LDH modified biochar (MABC). The mechanisms underlying its efficient phosphorus adsorption were systematically investigated through XRD, XPS, FTIR, and SEM characterizations. Results show that MABC6 optimized by RSM (Mg/Al molar ratio 4:1, biochar dosage 10 g·100 mL-1, roasting temperature 450 °C) exhibited high crystallinity, large specific surface area, abundant surface functional groups, and maximum layer spacing, demonstrating optimal adsorption performance (54.932 mg g-1). Chemical adsorption, multilayer adsorption, electrostatic attraction, ion exchange, inner-sphere and outer-sphere surface complexation, and ligand exchange are the main mechanisms of the MABC adsorption process. The optimized MABC6, owing to its increased surface functional groups and expanded layer spacing, promotes multilayer adsorption and ligand exchange while strengthening ion exchange and ligand exchange. This enhances the effective binding between phosphate and adsorption sites. Additionally, soil column experiments indicated that phosphorus-enriched MABC6 (MABC6-P) achieved a cumulative phosphate release rate of 17.59% within 30 days, representing a 50.21% relative increase compared to the raw biochar(BC), highlighting its promising potential for slow-release fertilizer applications. In summary, this study optimized the LDHs-biochar crystalline structure via RSM to expand layer spacing, thereby enhancing adsorption capacity and extending its application in aquatic environmental remediation.