Graphene oxide-iron oxide nanocomposites for dye contaminated wastewater remediation

Abstract

The development of active and stable heterogeneous Fenton-like catalysts have emerged as an alternative to overcome the practical limitations related to the homogeneous Fenton catalyst, where various iron species and/or iron oxides are immobilised within the structure of different catalyst supports. Clay, alumina, zeolite and carbonaceous materials such as activated carbon and carbon nanotubes have been used as catalyst supports of choice by the scientific community. However, there is a knowledge gap associated with using high aspect ratio 2D (dimension) graphene oxide (GO) as an alternative catalyst support. Of particular interest, it is postulated that the structure and functionalities of GO as a support confers to the resultant catalyst overall catalytic activity beyond the conventional Fenton catalysts. In this thesis, the structural and physicochemical properties of resultant catalyst with their corresponding catalytic activity were systematically investigated. To this end, the synergistic interaction between GO and immobilised iron oxide nanoparticles (Fe3O4 NPs) was proposed for an oxidative degradation of synthetic dye acid orange 7 (AO7), which is a major water pollutant from textile production. The GO‒Fe3O4 nanocomposites were initially synthesised through a facile one-pot method by co-precipitating irons salts onto GO sheets in a basic solution. The formation of GO‒Fe3O4 was postulated as follows: (i) Fe3+/ Fe2+ ions are adsorbed and coordinated by the carboxyl groups (C=O) of GO sheets, (ii) hydrolysed ions form nanoclusters on GO sheets when NaOH is introduced, (iii) followed by condensation of the nanoclusters to form Fe3O4 nuclei, and (iv) further nucleation and growth of Fe3O4 crystallites on GO sheets were due to the redox reaction as the pH is increased to 10. The incorporation of GO led to an enhancement on the catalytic activity of the nanocomposites with 76% AO7 removal over the control catalysts of Fe3O4 NPs and GO sheets, which corresponds to 48 and 22%, respectively. Further improvements on the catalytic activity of GO‒Fe3O4 were performed by modifying the synthesis through pre-hydrolysing iron salts prior to GO addition at pH 4 with various GO loadings. The key finding of this new method is the formation of two sets of different mesoporous structure. At low GO loadings ≤10 wt%, GO–Fe3O4 nanocomposites resulted in high surface area up to 409 m2 g-1, in tandem with high 92‒98% degradation of AO7. By contrast, GO loadings >10 wt% led to reduced surface area and lower GO‒Fe3O4 activity (60%). The presence of strong interfacial interactions (Fe–O–C bonds) in the nanocomposites contributed to the superior degradation of AO7, in tandem with structural-morphological features. The operational conditions of heterogeneous Fenton-like reaction were evaluated and modelled as a function of nanocomposites dosage, pH, temperature, oxidant and dye concentrations. Best results showed a fast 80% degradation in ~20 min, whilst ~98% of AO7 was successfully removed after 180 min of reaction time. Optimal conditions were determined for nanocomposites (GO(5wt%)‒Fe3O4) at the catalyst dosage of 0.2 g L-1, initial pH of 3 and 22 mM of H2O2 concentration at 298 K. The kinetics for the oxidative degradation of AO7 was found to be a pseudo-first-order reaction following the Langmuir-Hinshelwood mechanism. A noteworthy finding was the high activity (>98%) and recyclability of GO‒Fe3O4 over 7 cycles whilst the Fe3O4 NPs exhibited a severe loss of activity (~0%) at the 5th cycle. It was found that the ratio of Fe3+/Fe2+ for GO‒Fe3O4 remained almost constant over the 7 cycles, contrary to Fe3O4 NPs which underwent a significant Fe2+ decrease. The synergistic effect of GO in the GO‒Fe3O4 nanocomposite was able to accelerate the ≡Fe3+/≡Fe2+ redox cycles for the fast reduction of ≡Fe3+ to ≡Fe2+ which is actively participating in the decomposition of adsorbed H2O2 into HO• radicals during catalysis. The X-ray photoelectron spectroscopy (XPS) analysis of spent GO‒Fe3O4 showed that the sp2 carbon domains (C=C) slightly decreased after every cycle, thus suggesting some degree of oxidation of the carbon basal plane. It is therefore postulated that the unusual stability in the GO‒Fe3O4 is attributed to a donor-acceptor mechanism of the nanocomposites, in which the electrons donated from the oxidation of the GO are used for the regeneration of ≡Fe2+ and thus maintaining the Fe3+/Fe2+ ratio. Finally, the partial substitution of zinc (Zn) into Fe3O4 in the presence of GO was investigated, in view of the UV photocatalyst properties of zinc oxides. A slight change on the physicochemical properties of GO–Fe3-xZnxO4 lead to an increase in photocatalytic activity, where x=0.2 gave the higher degradation of AO7 in the UV-assisted Fenton-like reactions at 60 min reaction. It was found that the activity of the catalyst without GO always gave lower values ~30% of AO7 removal. Therefore, GO has proven again its beneficial use as an active component in the case of UV-assisted Fenton-like reaction as well.