Realistic and sustainable phosphate adsorption using intercalant-engineered exfoliated serpentinite: mechanistic insights and real-water validation

Abstract

This study investigates phosphate adsorption using exfoliated serpentinite produced by intercalation with potassium nitrate (KN/SP), urea (U/SP), and potassium acetate (KC/SP), with emphasis on performance, mechanism, and realistic applicability. All modified adsorbents showed strongly improved phosphate uptake, reaching maximum capacities of 127.85 mg g^-1 (KN/SP), 154.18 mg g^-1 (U/SP), and 183.54 mg g^-1 (KC/SP), confirming the advantage of acetate-assisted exfoliation. Kinetic behavior followed the pseudo-first-order model, indicating rapid, surface-controlled adsorption. Equilibrium was best fitted by the Langmuir isotherm, supporting monolayer adsorption on energetically uniform sites. Statistical physics modeling provided steric and energetic descriptors, revealing high densities of accessible sites (Nm = 39.22-63.51 mg g^-1), with KC/SP showing the highest site density. The number of adsorbed species per site (n ≈ 4) suggests multisite/multidocking adsorption via cooperative interactions with multiple surface functionalities. Adsorption energies (<25 kJ mol^-1) are consistent with a physisorption-dominated mechanism (electrostatic attraction, ion-dipole interactions, and hydrogen bonding), in line with the observed fast uptake and suggesting that regeneration and long-term reusability may be feasible; these latter aspects are inferred from adsorption energetics rather than directly demonstrated by extended regeneration tests in this work. Matrix effects were evaluated using competing anions (SO₄ ^2-, NO₃ ^-, HCO₃ ^-) and coexisting metals (Pb²⁺, Cu²⁺, Cd²⁺, Zn²⁺), showing only limited suppression. Validation in authentic Lake Qarun water confirmed robust phosphate removal, particularly for KC/SP, highlighting its potential as a scalable adsorbent for complex waters.