Several studies have hypothesized that quinone moieties on the carbon material's surface influence nitrate (NO3-) adsorption, while no confirmatory studies have been conducted. Biomass, the precursor of carbonaceous adsorbents, contains several types of quinone moieties, including anthraquinone (AQ), benzoquinone (BQ), and naphthoquinone (NQ). In our study, redox-reactive quinone moieties' role was elucidated by introducing BQ (AC-BQ), NQ (AC-NQ), and AQ (AC-AQ) on activated carbons' (AC) surface. The quinone moieties were deposited on coconut-shell-based activated carbon (AC) using simple and spontaneous physisorption methods. Confirmation of quinone moieties on AC was achieved using FTIR, XPS, and cyclic voltammetry (CV). BET, SEM, and elemental composition were other characterizations performed to understand the effect of AC's physicochemical properties on NO3-adsorption.
Modification of AC using AQ, BQ, and NQ treatment was successful, and the presence of respective quinones was confirmed through FTIR, XPS, and CV. However, for AC-BQ samples, an unconfirmed peak overlapped/interfered with the NO3-peak. Thus, AC-BQ results for NO3-  are not discussed here. Unmodified AC showed no surface quinone content, and after modification using NQ and AQ, surface quinone content increased to 2.17 and 1.71 %, respectively, for modified carbons. BET surface area of AC-NQ (178 ± 53 m2 g-1) and AC-AQ (520 ± 183 m2 g-1) reduced by 82 and 47 %, respectively, compared to unmodified AC (984 ± 22 m2 g-1). A simultaneous reduction in NO3- adsorption capacities of 53 and 21 % was observed in AC-NQ (0.95 ± 0.02 mg g-1) and AC-AQ (1.58 ± 0.07 mg g-1) in comparison to unmodified AC (2.01 ± 0.08 mg g-1). Hence, we propose that AQ and NQ have interacted with AC surface via π-π interactions, thereby blocking active sites for NO3- adsorption. The study confirmed that quinone-modified AC had reduced NO3-adsorption capacities because of changes in the surface physicochemical properties. The effect of NQ and AQ moieties on the surface of AC on NO3-adsorption was elucidated for the first time through this study.

Activated carbon surface modification using quinone containing compounds and its influence on chemisorption of nitrate in aqueous media

ACS Spring Meeting 2021

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