Synthesis of composites based on activated carbon and TiO 2 doped with Co and Fe from residues for the photocatalytic degradation of Cibacron yellow F-4G dye

Abstract

Dyes present in industrial wastewater discharged from the textile industries are hard to remove in natural environments due to their high solubility, stability and low biodegradability in aqueous media. Advanced oxidation processes could offer a costeffective and desirable method to degrade dyes in effluents and wastewaters and subsequently convert these compounds to harmless species. The aim of the present research is to synthesize novel composites with photocatalytic properties to be used in the degradation of the textile dye Cibacron Yellow F-4G (CYF-4G) in an efficient and sustainable way. These novel composites are based on activated carbon (AC) and titanium (IV) oxide (TiO2) doped with Fe and Co (M-TiO2), synthesized from waste streams of the Cuban agricultural and metallurgical industries demonstrating a circular economy approach. The ACs were prepared from agricultural waste: rice husk (RH), coffee husk (CH) and peanut hull (PH); TiO2 was electrolytically synthesized using titanium scrap as anode, subsequently doped with Co and Fe in the following proportions: Co-TiO2 (0.5- 99.5) and Fe-TiO2 (1.0-99.0). A total of four M-TiO2/AC composites were prepared: 5- 95% and 10-90% for each metal. All synthesized materials were characterized structurally, morphologically and chemically. Adsorption studies were performed for ACs under the following conditions: 20 mg adsorbent/30 mL of dye solution,120 rpm, 24 hours agitation time, from 50 to 350 mg/L of dye, pH = 2 and T: 25°C and for PHACbased composites: 1.2 g/L of adsorbent dosage, 120 rpm and 4 h agitation time, from 50 to 350 mg/L of dye pH = 2, and T: 25°C. The following experimental conditions were used for photodegradation: 0.08g/L of catalyst dosage, 120 rpm and 2 h agitation time, 100 mg/L of dye pH = 2, and T: 25°C. The kinetics of the photodegradation processes under visible radiation were studied using UV-visible spectroscopy. It is concluded that the synthesized TiO2 revealed the presence of the anatase phase with a size of 26 nm and a purity of 99 %, regardless it was synthesized from the anodization of titanium scrap, this methodology showed an efficiency of 92 %. The PHAC showed the best chemical, textural and morphological properties for its possible use as adsorbent. Therefore, PHAC was selected as the carbonaceous support for the synthesis of the photocatalyst composites. The synthesized Fe-TiO2 or Co-TiO2 photocatalysts were efficiently deposited on PHAC in the expected ratio and in a homogeneous way. The experimental data of adsorption equilibrium studies showed that the Langmuir-Freundlich isotherm best models the adsorption behavior of CYF-4G on all ACs and composites based on PHAC and was confirmed analytically by comparing their corresponding R2 and RSME values. The adsorption kinetic process follows the Pseudo-Second Order kinetic model and reaches equilibrium at 4 hours in a multistage process with a rapid adsorption of the dye on the external surface and followed by a dye diffusion towards the intraparticles. The kinetic process of the photocatalytic degradation of the CYF-4G dye on the studied composites using visible light was well described by the Langmuir-Hinshelwood model. A decrease in the synergistic effect values (RC ), rate constants (k2 and kLH) and an increase in the half-life t1/2 were found for the CYF-4G dye molecule when composites with a higher percentage of Fe-TiO2 or Co-TiO2 (ACFeTi10 and ACCoTi10) were used. The highest RC values of 4.15 and 4.09 were found between adsorption and photocatalytic degradation for the composites doped with 5% of Fe-TiO2 or Co-TiO2 catalysts on PHAC support. ACFeTi5 composite showed the highest synergistic effect on dye degradation with 100% degradation in 90 min. A synergistic effect of adsorption and photocatalysis was found for Co-TiO2/AC and Fe-TiO2/AC composites. Results pointed to a very efficient removal and degradation of the dye and an in-situ regeneration of the composites in a sustainable way. From theoretical calculations, in combination with the experimental determinations of the UV-visible study, a scheme of photocatalytic pathways of CYF-4G dye was proposed. The photodegradation of the CYF-4G dye molecule could occur through the cleavage of the azo double bond with the subsequent cleavage of the sulfonate group from the ring and the subsequent formation of hydroxylated compounds. Finally, these compounds, due to further reactions with the (•OH) radicals, could degrade into smaller molecules and finally in the mineralization of the dye.