To develop effective strategies for removing phenolic compounds, in-situ graphene oxide/manganese oxide (GO/MnOX, X = 2, 3/4) composites were fabricated via a sustainable method by synchronously utilizing manganese ions and acidic liquid waste during synthesis of GO. Comparatively, traditionally ex-situ GO/MnO2 composites were synthesized to verify the difference between in-situ and ex-situ synthesis. Based on the schematic and mass flow analysis, in-situ synthesis exhibited better atom economy and less waste emission than ex-situ synthesis. Then, the results of batch experiments exhibited that GO/MnOX composites possessed higher removal efficiency and wider pH range for p-cresol (p-CR) and p-tert-butylphenol (p-TBP) than GO/MnO2 composites. The maximum removal capacities of GO/MnOX composites were 107.68 mg/g for p-CR and 135.41 mg/g for p-TBP. And GO/MnOX composites could retain high removal efficiency for p-TBP (>90%) after five recycles. For in-situ GO/MnOX composites, GO sheets not only promoted the adsorption of phenolic compounds and by-products, but also enhanced the oxidation capacity of MnOX via an electron transfer interaction. Especially, the detection of ring cleavage products indicated further oxidation for p-TBP. Possessing an eco-friendly fabrication strategy and augmented adsorption-oxidation capability, in-situ GO/MnOX composites are expected to be applied in the removal of phenolic compounds.
Phenolic compounds have given rise to emerging concerns regarding human health and ecosystems due to their widespread use in the pharmaceutical, petroleum and petrochemical, pesticide, plastic, and paper industries (Danquah et al., 2018). Consisting of a hydroxyl group bonded directly to a benzene ring, phenolic compounds possess a stable conjugated system, leading to biological accumulation and poor biodegradability (Ou et al., 2018). Thus, an urgent need exists to develop an efficient method for removing phenolic compounds (Villar da Gama et al., 2018; Zhong et al., 2018). Various methods, including chemical oxidation (Jiang et al., 2015), adsorption (Villar da Gama et al., 2018), and membrane separation (Zagklis and Paraskeva, 2015), have been reported to remove phenolic compounds. Among them, oxidation constitutes an effective and attractive method due to the potential to disrupt the structural stability of pollutants (Wang et al., 2015). Manganese oxides, ubiquitous in natural soil and synthetic materials, are traditional and cost-effective adsorbents and oxidants in wastewater treatment. Especially, manganese oxides are able to oxidize various aromatic Lewis bases, such as phenolic and aniline compounds (Grebel et al., 2016; Wang et al., 2018b). Previous studies demonstrated that phenolic compounds are susceptible to oxidation by manganese dioxide (MnO2) through sequential one electrontransfer reactions, forming a series of intermediate products (Abdullah et al., 2017; Remucal and Ginder-Vogel, 2014). However, the intermediate products, such as phenolic multimer and benzoquinones, would be released back into the aqueous solution.