**Background:** Hydrophobic organic chemicals (HOCs), particularly persistent organic pollutants (POPs) like hexachlorobenzene (HCB), pose long-term environmental risks due to their persistence, toxicity, and bioaccumulation potential. While sorption to organic matter is well-studied, the role of mineral surfaces—which dominate soil and sediment matrices—remains poorly understood. This study hypothesized that clay minerals have a higher potential to adsorb HOCs than previously recognized, and that this potential depends on clay mineral type and exchangeable cations.
**Methods:** The authors conducted miniaturized batch equilibrium sorption experiments following OECD Test Guideline 106. Twelve phyllosilicate minerals were tested, including kaolinites (KGa-2, KGa-1b), smectites (STx-1b, SAz-2, SWy-3, Bavarian bentonite, NAu-1, NAu-2), hectorite (SHCa-1), illite (IMt-2), vermiculite, and chlorite (CCa-2). Additionally, the Ca-montmorillonite STx-1b was fractionated (<2 µm) and homoionically exchanged with nine cations: Li+, Na+, K+, Rb+, Cs+ (alkali metals) and Mg2+, Ca2+, Sr2+, Ba2+ (alkaline earth metals). HCB was applied at 1–5 µg L−1, and mineral masses ranged from 10–400 mg in 10 mL of 0.01 mol L−1 MCl solution. After 24 h equilibration, HCB concentrations were measured by headspace solid-phase microextraction (HS-SPME) coupled with GC–MS. Adsorption coefficients (Kd) were derived from linear regression of adsorbed versus aqueous concentrations. DFT calculations (PBE-D3 functional, VASP suite) were performed on single-layer montmorillonite models with compensating cations to compute adsorption energies (Eads) and geometric parameters. Additional molecular calculations on HCB–cation complexes were performed using Turbomole with COSMO implicit solvent.
**Key Results:** HCB adsorption to native clay minerals varied by over two orders of magnitude (log Kd 0.9–3.3). The highest adsorption was observed for chlorite (log Kd = 3.32, Kd = 2,076), followed by Bavarian bentonite (log Kd = 2.82), hectorite (log Kd = 2.75), and vermiculite (log Kd = 2.75). The lowest adsorption was for Ca-montmorillonite STx-1b (log Kd = 0.85) and kaolinites (log Kd ≈ 1.0). Adsorption did not correlate with specific surface area (R2 < 0.10) or cation exchange capacity (R2 < 0.15). For cation-exchanged montmorillonite, alkali metal cations showed a wide range of log Kd values: Na-Mnt (1.28), Li-Mnt (1.59), K-Mnt (1.61), Rb-Mnt (2.50), and Cs-Mnt (3.75). In contrast, alkaline earth metal cations showed uniformly low adsorption: Mg-Mnt (1.35), Ca-Mnt (1.33), Sr-Mnt (1.38), Ba-Mnt (1.31). DFT calculations showed that the strongest adsorption energies were for Ca-Mnt (−96.2 kJ mol−1) and Na-Mnt (−75.9 kJ mol−1), while Cs-Mnt had the weakest calculated Eads (−23.8 kJ mol−1) despite having the highest experimental log Kd. Gas-phase HCB–cation interaction energies were much larger for divalent cations (e.g., Mg2+: −461.5 kJ mol−1) than monovalent cations (e.g., Cs+: −38.7 kJ mol−1), but COSMO solvent calculations showed that hydration strongly destabilized all HCB–cation complexes, with most interaction energies becoming positive in solution. The authors found that the interplay between ionic radius (IR) and hydration enthalpy (Hhyd) governs adsorption, with the relationship between log Kd and the composite parameter Hhyd/ρ showing comparable trends for both cation groups.
**Clinical Implications:** This study has no direct clinical implications as it is an environmental chemistry investigation. However, understanding the environmental fate of persistent organic pollutants like HCB is relevant to public health, as HCB accumulates in the food chain and is associated with adverse health effects. The finding that clay minerals can strongly adsorb HCB (log Kd up to 3.3) suggests that mineral-rich soils and sediments with low organic carbon content may still act as significant sinks for HCB, potentially reducing its bioavailability and transport in groundwater. The study also provides mechanistic insights that could inform remediation strategies, such as the use of cation-modified clays for pollutant immobilization.