Preparation of Iron doped Zeolite-coated porous clay ceramic membrane (Fe/ZSM−5) for heavy metal filtration: Electrochemical study of the rejection mechanism

This study investigates the fabrication and performance of Fe/ZSM−5 zeolite-coated porous ceramic membranes (mZSM) for the removal of heavy metal (HM) ions from synthetic wastewater. The membrane was synthesized via a hydrothermal process, utilizing a clay support doped with 3 wt. % activated carbon (AC) to enhance porosity. The effectiveness of the Fe/ZSM−5-coated membrane in rejecting Pb²⁺, Cd²⁺, Hg²⁺, and Cu²⁺ ions was systematically evaluated. SEM reveals coffin-like or intergrown Fe/ZSM−5 crystal structures. XRD highlights MFI framework reflections at 7–9° and 23–25° 2θ, corresponding to the (101), (020), (501), (151), and (303) planes. FTIR displays characteristic ZSM-5 bands at 401 cm⁻¹, 590 cm⁻¹, 679 cm⁻¹, 781 cm⁻¹, and 1101 cm⁻¹. BET analysis shows a smaller average pore radius of 1.704 nm, a higher surface area of 251 m²/g, and a larger pore volume of 0.214 cm³/g. The mZSM membrane demonstrated superior HM removal efficiencies in the order of Cu²⁺ (77.6 %) < Hg²⁺ (85.6 %) < Cd²⁺ (95.1 %) < Pb²⁺ (98.8 %), outperforming the uncoated clay support. Water flux and permeation tests showed that the initial pure water flux of the clay support was 199 L·m⁻²·h⁻¹ (LMH), stabilizing at 144 LMH after 90 min, while the Fe/ZSM−5-coated membrane exhibited a significantly lower flux of 62.2 LMH due to reduced pore size and enhanced ion-exchange interactions. Electrochemical studies using Differential Pulse Voltammetry (DPV) confirmed that electrostatic repulsion played a dominant role in HM rejection rather than direct adsorption. However, the order of metal accumulation on the electrode surface (Cd²⁺ = 221 < Pb²⁺ = 447 < Cu²⁺ = 654 μA < Hg²⁺ = 667) differed from the filtration order. This discrepancy suggests that only the preconditioning potential can elucidate the fouling mechanism of HM at the mZSM membrane surface. These findings highlight the potential of Fe/ZSM−5-modified membranes for industrial wastewater treatment, offering high rejection rates and sustainable performance.