Congo red adsorption in water using a novel nano zero-valent iron intercalated Zn–Al–CO3 layered double hydroxide: adsorption isotherm, kinetics and thermodynamics

Abstract

Annually, thousands of dyes are produced and sold under different names. The majority of these dyes are toxic and their wide usage poses a major risk to the environment and humans. This research employed the co-precipitation method to synthesise Zn–Al–CO3 and 1–5% nano zero-valent iron (nZVI)/Zn–Al–CO3 layered double hydroxide (LDH) adsorbents to remove Congo red (CR) from water. The LDH’s composition, structural features and morphology were studied using X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), Zetasizer/potential (ZP) and scanning electron microscopy (SEM) coupled with electron dispersive X-ray (EDX). Batch adsorption studies revealed that 5% nZVI/Zn–Al–CO3 LDH was most efficient in CR removal as compared to the other LDHs. Also, an initial CR concentration of 60 mg/L, LDH dosage of 0.2 g, pH of 6, temperature of 313 K and contact time of 80 min were the optimal operation conditions which ensured the maximum removal of CR by the 5% nZVI/Zn–Al–CO3 LDH. Kinetic and isotherm studies revealed that pseudo-second-order (PSO) kinetic and Redlich-Peterson models had the best fit compared to other kinetic and isotherm models used. The Langmuir maximum adsorption capacity (73.83 mg/g) at the optimal operation conditions of this study was greater as compared to other adsorbents used in CR removal. Finally, thermodynamic studies disclosed that ΔG◦ values were negative at all temperatures, ΔH◦ was 93.09 kJ/mol and ΔS◦ was 0.32 kJ/mol indicating that CR removal by 5% nZVI/Zn–Al–CO3 LDH was spontaneous, endothermic, and had high disorderliness respectively.