Efficient and selective phosphate removal via scalable alkanolamine modified La-clay: Insights into protonation gradients at the inner Helmholtz plane.

Lanthanum-modified bentonite clay shows promise for phosphorus removal but suffers from inherent electrostatic repulsion and insufficient La dispersion, impairing interfacial mass transfer and phosphate selective adsorption, especially at the low P concentrations (≤0.5 mg·L-1) with interfering anions. Herein, we presented an ethanol-assisted humus removal strategy to synthesize amino-functionalized bentonite with abundant exposed La and amino sites (NH2-La-Ben(Et)), achieving an exceptional capacity of 58.8 mg P g-1, surpassing most clay-based counterparts. Spectroscopic analyses combined with dynamics simulations revealed that protonated amines via hydrogen bonding with adjacent water molecules, generated a localized proton gradient (ΔpH=0.47) at the inner Helmholtz plane (IHP), thereby facilitating phosphate mass transfer (0.82 min-1) through enhanced electrostatic attraction (Ψ=27.80 mV), increased phosphate diffusion coefficient (4.37 × 10-7 cm2·s-1) and shortened PO43- accumulation distance (2.85 vs. 3.68 nm). Impressively, NH2-La-Ben(Et) exhibited high selectivity against competing anions and immobilized phosphate through enhanced inner-sphere complexation (62.5 %, BB-H1/BM-H1) and decreased LaPO4 precipitation (37.5 %). Long-term column-flow operation (8000 BV breakthrough) and techno-economic analysis further validated its practical effectiveness and cost-efficiency ($65.76 kg-1 P). This work provides a scalable IHP-engineering approach for efficient and selective phosphate capture, shedding light on the design of clay-based environmental absorbents and elucidating their inherent structure-activity relationships.