Interfacial anion engineering of nickel-iron layered double hydroxide/ nickel foam for capture and carbonate-induced release of metal-cyanide complexes

Abstract

Interfacial engineering of layered double hydroxides (LDHs) provides a tunable pathway for controlling anion-exchange reactivity; however, the role of interlayer energetics in adsorption mechanisms remains insufficiently understood. Here, nickel–iron LDHs (NiFe-LDHs) were directly electrodeposited onto three-dimensional nickel foam to construct a binder-free interfacial platform for the removal of tetracyanonickelate (Ni(CN)42−), a stable coordination complex prevalent in electroplating wastewater. By tailoring interlayer anions (SO42−, Cl−, NO3−), we reveal that adsorption behavior is governed not by surface area but by anion-dependent energetic accessibility of the interlayer region. Among the prepared adsorbents, NiFe-NO3− exhibited the highest uptake capacity (qₑ = 14,072.84 mg/g), facilitated by weak host–guest affinity and enhanced gallery charge compensation. Kinetic and diffusion analyses confirmed a transition in adsorption pathway from film-diffusion limitation (NiFe-SO42−) to interlayer-driven anion exchange (NiFe-NO3−). The adsorption selectivity of NiFe-NO3− was further validated in simulated electroplating wastewater containing competitive ions (CN−, SO42−, NO3−, Cu2+, Zn2+). A carbonate-triggered relaxation strategy enabled controlled displacement of (>99 %) intercalated Ni(CN)42−, yielding a stabilized NiFe-CO32− phase that resists re-adsorption and enables safe releasing. This study establishes interlayer energetic control as a rational framework for designing advanced anion-exchange materials for selective removal of strongly coordinated toxic anions.