From rice-husk waste to selective BTEX adsorbents: modified MWCNTs reveal a co-adsorption swing effect and improve field monitoring

Abstract

In this study, we report for the first time the application of rice husk-derived multi-walled carbon nanotubes (MWCNT) and their three modifications by NaOCl, H₂SO₄/HNO₃, and H₂O₂ treatment for the adsorption of gaseous BTEX mixtures (benzene (B), toluene (T), ethylbenzene (E), and xylene (X)). The adsorption capacity followed the order of pristine MWCNT < H₂SO₄/HNO₃-MWCNT < H₂O₂-MWCNT < NaOCl-MWCNT, while the preferential uptake of BTEX increased in the sequence B < T < E < X, independent of their initial concentrations, with particularly high selectivity for xylene. Notably, the adsorption efficiency of individual BTEX components was lower than that observed for the mixture. More unexpectedly, stronger adsorbed molecules (e.g., xylene and ethylbenzene), which are typically difficult to desorb under single-gas adsorption, became readily desorbable in the mixture, whereas the opposite trend was observed for weakly bound species such as benzene and toluene. These findings support the DFT-predicted “co-adsorption effect” hypothesis governing inter-species swing adsorption between aromatic hydrocarbons. A field application using NIOSH method 1501 further demonstrated that the developed adsorbents outperformed commercial activated carbon in monitoring BTEX concentrations at a fuel station, especially for xylene. These field observations provide preliminary evidence that substituting rice husk-derived MWCNTs for commercial AC could improve the reliability of the NIOSH method 1501 in determining aromatic hydrocarbons.