Treatment of limonene-polluted air streams by sequential GAC adsorption and electrochemically H2O2-based oxidation: Challenges and perspectives

Abstract

The sequenced granular activated carbon (GAC)–oxidation process stands out as an advanced treatment that combines the adsorption ability of GAC to capture contaminants with a subsequent oxidation step to degrade or transform these pre-adsorbed into less toxic or added-value products. With this two-step treatment it is possible to extend GAC’s service life and enhances process sustainability. However, research gap remains regarding its application for non-polar gaseous pollutants and the use of electrogenerated oxidants in the oxidation step. To address this, a case study was conducted using limonene (LIM), a non-polar, hydrophobic volatile organic compound common found in indoor air and industrial products. Here, LIM-polluted gaseous streams were treated using a fixed-bed setup, followed by oxidation with electrochemically produced hydrogen peroxide (H₂O₂). Initial tests showed that LIM in aqueous solutions was not efficiently degraded by H₂O₂ alone, but radical oxidation, promoted by H₂O₂ activation by UVC light or O₃, improved reactivity. On GAC, H₂O₂ unexpectedly proved effective due to interactions with free radicals, though UVC irradiation provided minimal additional improvement. In contrast, the addition of ferrocene (Fc), a promising heterogeneous catalyst, significantly enhanced degradation, achieving removal efficiencies up to 35 % in neutral media. This improvement was attributed the Fc-mediated activation within GAC. Eight different intermediates were identified, supporting an oxidation mechanism that varies between technologies. Overall, characterization revealed minimal structural changes in GAC post-treatment. BET analysis showed slight surface area variations for UVC and Fc/H₂O₂ processes, but a decrease in 69 % for H₂O₂ alone, and a 24.4 % increase for UVC/H₂O₂.