Performance Differences of Hexavalent Chromium Adsorbents Caused by Graphene Oxide Drying Process

Abstract

In this study, the influence of drying conditions on amine (−nH 3) functionalization of graphene oxide (GO) was evaluated, and the hexavalent chromium (Cr(VI)) adsorption efficiency of the prepared materials was compared. 3-[2-(2-aminoehtylamino) ethylamino]propyl-trimethoxysilane (3N) was used for amine functionalization. The synthesized materials were analyzed by SEM, BET, TGA, XPS, and EA. TGA results showed that the solution-GO (SGO) was functionalized by more 3N molecules than freezedried GO (FDGO) and oven-dried GO (ODGO). Additionally, XPS analysis also showed that the ratio of N/C and Si/C was relatively high in SGO than FDGO and ODGO. The maximum adsorption capacity of SGO, FDGO, and ODGO for Cr(VI) was 258.48, 212.46, and 173.45 mg g −1 , respectively. These results indicate that it is better to use SGO without drying processes for efficient amine functionalization and Cr(VI) removal. However, when the drying process is required, freeze-drying is better than oven-drying. The active development in various industrial sectors, such as textile, metal finishing, leather tanning, stainless steel, and electroplating 1-7 contribute to the discharge of contaminated wastewater containing inorganic (heavy metals, phosphates, nitrates, sulfates, etc.), organic (phenols, dyes, pesticides, pharmaceutical compounds, etc.), and biological (bacteria, viruses, etc.) compounds. Among these, heavy metals such as chromium (Cr), arsenic (As), copper (Cu), lead (Pb), cadmium (Cd), and mercury (Hg) are typical toxic substances that exist as cations and anions in water 8. Chromium exists in two stable oxidation states, which are the hexavalent chromium (Cr(VI)) and trivalent chromium (Cr(III)) variants 9. While Cr(III) is minimally toxic, Cr(VI) is highly toxic and causes significant environmental damage 10. Further, Cr(VI) can cause carcinogenesis and mutations in humans. For these reasons, the World Health Organization (WHO) regulates the concentration of Cr(VI) in drinking water to less than 50 ng•L −1 11. To comply with this rigorous and necessary regulation, the application of water treatment techniques that can remove low concentrations of Cr(VI) is required. Various techniques, such as, redox, ion-exchange, adsorption, membrane filtration, and the like 12-16 are applied according to the purpose. However, the removal of low concentrations of Cr(VI) with most of these wastewater treatment systems is highly challenging. Among these approaches, adsorption is more effective for Cr(VI) removal since adsorbents are eco-friendly and not generating by-products 17. Therefore, we conducted further research on advancing adsorption technology for the removal of Cr(VI) from wastewater. For the first time, in this study, we developed an adsorbent based on graphene oxide (GO) and evaluated its Cr(VI)-adsorption performance. GO has been used in various fields such as electronic, biological, and physical applications 18 owing to its several advantages, such as straightforward synthesis, good solution stability, and the large surface-area-...