Development of Graphene-Based Photocatalysts for Remediation of Hexavalent Chromium

Chromium is used extensively in several industries, as a result of which different Cr species are released into the aqueous ecosystems as effluents. Though Cr appears in various forms, Cr (VI) and Cr (III) species are regarded as the most stable forms in aqueous medium. Cr (VI) is extremely toxic, mutagenic, and teratogenic. On the other hand, Cr (III) toxicity is around 300 fold less than that of its oxidized form and is considered as a micronutrient for living beings. Thus, it is very much significant to reduce Cr (VI) to Cr (III) for passivizing its toxicity. Among the approaches applied so far, semiconductor-based photocatalytic Cr (VI) reduction is recognized as a sustainable technology since it can be operated in the presence of inexhaustible solar energy. Nevertheless, several solar light-responsive semiconductors have been used for this purpose; their efficiency lies far from expectation, basically due to their low surface area, narrow response in the visible light region, and poor ability for charge separation. To eliminate these bottlenecks, the semiconductors are usually coupled with materials of exceptional electron transport potential and enormous surface area. Graphene is a 2D material with exceptionally high surface area, prominent electron acceptance features, ultrafast electron transport affinity, and excellent ability to widen the visible light absorption window. Coupling graphene with semiconductor photocatalyst leads to enhanced photocatalytic activity. Several graphene-based composites have been fabricated as well as different strategies have been adopted for efficient Cr (VI) photoreduction. In this chapter, we have briefly highlighted the basic principles of photocatalytic Cr (VI) reduction over graphene-based photocatalysts after introducing sources of Cr (VI) contamination as well as its toxicity, the importance of Cr (VI) photoreduction, and merits of graphene-based photocatalysts. Then, the factors affecting photocatalytic Cr (VI) reduction were discussed. The design principles and photoreduction efficiency for graphene-based binary composites were described. The ternary composites’ construction for improved photoreduction of Cr (VI) was then manifested. After highlighting the concluding remarks, the emerging challenges and future research directions were addressed.