Adsorption and photocatalytic activity of graphene-based nanomaterials

Abstract

The world population is growing at an alarming rate and this has resulted in the intensification of industrial activities causing contamination of air, soil and aquatic ecosystems. The demand for pure drinking water is growing with population and people still have insufficient access to it. In most African rural communities, ground water is the primary source for drinking purpose. However, if beyond permissible level, F - is a toxicant chemical in ground water sources with tremendous health disorder. Thus, this work presents green a synthesis method of graphene based nanomaterials, characterizations of the synthesized nanomaterials, and its application in nano adsorption of F and photocatalysis of methylene blue (MB) for environmental protection. The synthesized products were characterized by GC-MS, UV-Vis, XRD, FT-IR, DTA, TGA, HR-TEM and EDAX. Solvent extraction method was used to obtain the phytochemical compounds from Vernonia Amygdalina (VA) plant leaves. The UV-Vis analysis of VA leaf extract revealed different peaks in water, methanol and ethanol extracts. The GC-MS records displayed a number of complex compounds of which six of them were common to all solvents five of them were common to two solvents and the rest were unique to their corresponding extracting solvents. UV-Vis records of reduced graphene oxide (rGO) revealed major peaks at 262, 252, 297 nm and XRD records showed different peaks at 2Ɵ = 240 , 21.30 , 19.80 in methanol, ethanol and water extracts respectively. Methanol extraction of VA was the preferred method for the large scale production of graphene based materials. The fast, non-explosive two step method in the absence of ice bath were used to obtain graphene oxide (GO). rGO-Bi2S3 was synthesized using a single step refluxed method for the degradation of methylene blue (MB). The comparative degradability degree of the dye under visible light irradiation with and without catalyst was studied. The performance tests showed 99% degradation of MB with and 7% without catalyst under the same condition within 25 minutes. The durability and reusability tests for the catalyst were also studied for five cycles. The maximum loss of capacity of the catalyst within 25 minutes was 0.5%. Hence, rGO-Bi2S3 was found to be the ideal material for the degradation of MB. The adsorption of F - from ground water by GO under specific conditions such as: agitation rate of 120 rpm, contact time of 90 minutes, adsorbent dosage of 0.42 mg/L, initial F - concentration of 10 mg/L and pH of 6.8 was investigated. The results obtained showed 99.3% F - removal from NaF prepared solution and 91.6% fluoride removal from real sample. The retained properties of GO after adsorption observed by UV-Vis analysis confirmed the recyclability of the adsorbent. The results obtained also showed that the adsorption kinetics with the coefficient of determination (R2 ) for pseudo- first-order (SFO) and pseudo-second-order (SSO) were 0.96 and 0.99 respectively. Based on these results, the adsorption of F - onto GO is a pseudo-second-order kinetics type. Furthermore, according to the adsorption isotherm study, Freundlich isotherm model showed a good fit to the experiments with R 2 (0.99). The adsorption capacity of the adsorbent was found to be 301.43 mg/g. Hence, this study showed that GO is the preferred and effective adsorbent material for the removal of F - from ground water.