Adsorption of Perfluorooctanoic Acid (PFOA) by Micro/Nanoscale Layered Double Hydroxides (LDHs) under Groundwater Hydrochemical Conditions: Performance and Mechanisms

Perfluorooctanoic acid (PFOA), a representative per- and polyfluoroalkyl substance (PFAS), has attracted widespread concern because of its extreme persistence, mobility, and frequent occurrence in groundwater and other aquatic environments, where it poses potential risks to health. Adsorption is considered a practical option for PFAS removal because of its operational simplicity and relatively low cost. Among available adsorbents, layered double hydroxides (LDHs), which possess positively charged metal hydroxide layers and exchangeable interlayer anions, exhibit strong affinity for anionic contaminants and therefore show promise for PFOA removal. In this study, MgAl-LDH, NiAl-LDH, and ZnAl-LDH were synthesized via a urea hydrolysis route using Mg, Ni, and Zn as divalent metals and Al as the trivalent metal. Their adsorption performance toward PFOA at groundwater-relevant concentrations was systematically evaluated, and the governing mechanisms were explored under representative groundwater geochemical conditions. All three LDHs showed rapid adsorption kinetics, reaching more than 95% of equilibrium uptake within 30 min. The adsorption data were well fitted by the Langmuir model, with NiAl-LDH exhibiting the highest adsorption capacity (q_max=1 483.46 μg·g^-1) and affinity (lg K_d =4.56). Groundwater matrix experiments further showed that Cl^- and NO₃^- (10 mg·L^-1), natural organic matter (< 10 mg·L^-1), and coexisting PFAS exerted only minor effects on PFOA removal, whereas HCO₃^- and SO₄^2-(10 mg·L^-1) significantly inhibited adsorption. Combined with the pH-dependent zeta potential responses and chain-length-dependent removal trends, these results indicate that PFOA adsorption is mainly driven by electrostatic attraction to positively charged surface sites, with additional contributions from non-electrostatic interaction such as hydrogen bonding and pore filling. Overall, this study clarifies the performance and environmental applicability of LDHs for PFOA removal under groundwater-relevant hydrochemical conditions and provides practical guidance for adsorbent selection and remediation strategy development at PFAS-impacted sites.