Synthesis of low-cost hierarchical zeolite 13X structure from abundant natural bentonite for efficient adsorption of polyphenols from olive mill wastewater

Abstract

Olive mill wastewater (OMW), rich in polyphenols, poses environmental challenges due to toxicity and biodegradation resistance. This study synthesized Zeolite 13X from natural bentonite via alkaline fusion at 550 °C with 2.0 M NaOH and hydrothermal treatment at 100 °C for 48 h. XRF, XRD, BET, FTIR, and SEM-EDS confirmed the conversion into a hierarchical, predominantly microporous framework with secondary mesoporous features. The surface area increased from 78 m²/g (raw bentonite) to 539 m²/g (Zeolite 13X), with a highly enhanced micropore volume (0.2550 cm 3 /g). Batch adsorption was optimized at pH 2 using 20 mg of adsorbent and a 90 min contact time for the total phenolic fraction in olive mill wastewater, quantified by the Folin–Ciocalteu method as gallic acid equivalents (GAE). Under these conditions, kinetic experiments yielded an equilibrium adsorption capacity of 171.00 mg GAE/g at 45 °C, while a maximum experimental adsorption capacity of 192.39 mg GAE/g was achieved during equilibrium isotherm studies. The adsorption kinetics followed a pseudo-second-order model, while the equilibrium data were best described by the Langmuir isotherm, which predicted a theoretical apparent monolayer adsorption capacity of 199 mg GAE/g. Thermodynamic analysis confirmed that the adsorption process was spontaneous and endothermic. Applying Zeolite 13X to real OMW reduced phenolic compounds by 82%, alongside improved turbidity, BOD 5 , and COD. The mechanism involves π–π stacking, hydrogen bonding, donor-acceptor complexation, and pore-filling, influenced by pH and a pH PZC of 8.85. The complementary roles of micropores and secondary mesopores facilitate the entrapment of varying polyphenol sizes within the multicomponent matrix. Regeneration studies showed efficiencies above 70% after five cycles. Furthermore, incorporating Zeolite 13X into carbon paste electrodes (CPE) created a sensitive sensing platform. The CPE/Zeolite 13X (40%) composite exhibited the lowest charge transfer resistance (R ct = 381.37 Ω) and a 7-fold enhancement in anodic peak current compared to bare CPE. These results highlight Zeolite 13X as a sustainable, cost-effective material for the simultaneous adsorptive treatment and electrochemical sensing of polyphenol in OMW.