Water and wastewater treatment by recyclable agar-graphene oxide biocomposite hydrogel in batch and fixed-bed adsorption column : bench experiments and modeling for the selective removal of organic

Abstract

Due to recent advances in developing technologies for the production and processing of carbon nanomaterials, the viable application of these materials in continuous and industrial processes has become realistic. Because of their properties, graphene-based materials (GBM) have proved to be promising as adsorbents for organic contaminants in water. Thus, it is the aim of this work to develop composites using graphene oxide (GO) and the biopolymer agar, for applications in the continuous treatment of water and industrial wastewaters. Hydrogel biocomposite samples using different proportions of graphene oxide and agar biopolymer (agar-GO) were synthesized to treat water and industrial textile wastewater through fixed-bed adsorption. The hydrogels prepared using GO and agar were characterized, and measurements revealed evidence of GO interaction with the polymer that constitutes the matrix, resulting in a 3D material, with visually disordered morphology. After characterization, preliminary batch adsorption experiments were conducted to evaluate the preliminary kinetics and pH effect in the adsorption process for the anionic dyes Acid Orange 7 (AO7) and Reactive Black 5 (RB5); the cationic dyes Nile Blue A (NB), Methylene Blue (MB), Malachite Green (MG), Basic Fuchsin (BF), and Safranin-O (SO); and the anti-malaria drug Chloroquine diphosphate (CQ). Among the compounds evaluated, the hydrogel showed satisfactory results for the removal of cationic dyes and the drug. Therefore, adsorption equilibrium isotherms were obtained and fitted to Freundlich, Langmuir and Sips models, and kinetic data were adjusted to Driving Force models, and Fick’s Diffusion equation. Fixed-bed experiments were carried out for the hydrogel produced with 80% of agar and 20% of GO, varying the feed flowrates in the column, and the breakthrough curves were obtained in each case. Experimental adsorption capacities values for the fixed-bed test, on a dry basis, were 226.46 mg.g−1 (NB), 79.51 mg.g−1 (MB), 58.25 mg.g−1 (MG), 38.11 mg.g−1 (BF), 100.00 mg.g-1 (SO), and 63.00 mg.g-1 (CQ). The column packed with the agar-GO hydrogel was tested for the treatment of both synthetic and real textile wastewater. Color and total organic carbon (TOC) were evaluated as response parameters, and there was an indication of the adsorbent selectivity for the separation of the cationic dyes present in the synthetic textile wastewater. The effluent obtained after treatment using the hydrogel indicated a decrease in the phytotoxicity of the sample, compared to the initial dye solution, for the three plant species studied (Cucumis sativus, Lepidium sativum, and Eruca sativa). Overall, agar-GO proved to be an interesting viable alternative, since a small amount of material was used to continuously treat synthetic textile wastewater, being most of the composite biodegradable. Moreover, the material exhibited remarkable regenerative capacity - in each case, after regeneration, the breakthrough curves were superimposed - proving its effectiveness for applications in wastewater treatment.