Efficient DR-23 adsorption by ZIF-67-acetate and sulfate in single and mixed dye matrices

Abstract

Abstract Toxicants such as Direct Red-23 (DR-23) require advanced wastewater treatment techniques to mitigate environmental pollution. Herein, two variants of ZIF-67 (acetate and sulfate) were fabricated and investigated for the advanced adsorption of DR-23. Using different counterions for the cobalt metal source led to the development of crystals with different morphologies. ZIF-67-acetate and ZIF-67-sulfate were fully characterized (XRD, FTIR, FESEM) before use as adsorbents. Both ZIF variants demonstrated superb efficacy in preliminary tests, achieving ~ 99% removal. Further experiments showed > 95% removal performance of ZIF-67-acetate over a wide pH range < 9, with a sharp drop at pH 10. The kinetic models were fitted to the data under optimal and non-optimal treatment conditions. At pH 6.5 and pH 9, the calculated adsorption capacities of 248.79 mg/g and 168.73 mg/g, respectively, closely matched the experimental values, while the adsorption rate constant (k 1 ) was 2.1 times lower at the higher pH. Adsorbent capacity further increased by DR-23 concentration (50–200 mg/L) in the range of 50–200 mg/L, despite an 18% drop in removal percentage. The equilibrium study revealed a maximum adsorption capacity (qmax) of 414.33 mg/g, indicating a multilayer, favorable adsorption phenomenon. Interestingly, ZIF-67-acetate showed strong affinity for DR-23 in mixed dye anionic-anionic (DR-23 and Tartrazine) and anionic-cationic (DR-23 and malachite green-MG) systems. Furthermore, in a real matrix, the adsorbent maintained a high removal efficiency of > 90%. The negative ΔG° values of ~ 5-5.9 kJ/mol and positive ΔH° were indicative of the spontaneous and endothermic nature of DR-23 uptake. A good reusability was observed for 67-acetate which is indicative of its promising applicability in real treatment systems. Mechanistically, DR-23 adsorption onto ZIF-67 variants was governed by electrostatic attraction, π–π stacking, coordination interactions, and hydrogen bonding, resulting in a spontaneous, endothermic, and efficient process for the synthetic and real water matrices.