Regeneration of odorous sulphur compound-exhausted activated carbons using wet peroxide oxidation: The impact of chemical surface characteristics
Graphical Abstract
Introduction
Odorous pollutants such as volatile organic sulphur compounds (VOSCs) are emitted by a wide range of industrial facilities, including pulp and paper mills and wastewater treatment plants. The presence of VOSCs at very low concentration can cause annoyances in the vicinity of industrial areas. Hence, the low odor threshold of VOSCs (Smet et al., 1998, Suffet et al., 2004, Barczak et al., 2022) entails the challenge to find new control techniques with higher elimination yields.
Adsorption process using activated carbons (ACs) has been the most common used technique to remove odorous compounds given its high efficiency in the retention of volatile organic compounds, including VOSCs (Brasquet and Le Cloirec, 1997, Jalilvand et al., 2020, Li et al., 2021, Vega et al., 2013, Vega et al., 2015); and its affordable operational maintenance (Estrada et al., 2012). However, the low odor threshold of VOSCs shortens the real–life of the adsorbers. Therefore, exhausted ACs must be either replaced with virgin material or regenerated in order to restart their adsorptive capacities. The high acquisition cost of ACs together with the production of hazardous waste has led to the regeneration of spent ACs to become an economic alternative to reduce operating costs (Sheintuch and Matatov-Meytal, 1999).
AC regeneration based on desorption methods (thermal or extraction) has been regularly applied (Boulinguiez and Le Cloirec, 2010, Li et al., 2021, Sunarso and Ismadji, 2009). However, desorption methods often require high pressures and temperatures, raising the regeneration cost. Moreover, a partial loss of the adsorbent material takes place during this process. In order to overcome such drawbacks, oxidation methods (chemical, electrochemical or microbiological) have been developed (Aktas and Ceçen, 2007, Mustafa et al., 2021, Valdés and Zaror, 2006, Zhang, 2002). In particular, the use of chemical regeneration techniques allows achieving a complete or partial mineralization of adsorbed organic compounds, recovering the adsorption capacity of the carbonaceous matrix (Ince and Apikyan, 2000, Faria et al., 2005, Ding et al., 2020).
Among oxidative regeneration methods, Advanced Oxidation Processes (AOPs) have emerged as highly efficient treatments to regenerate ACs. AOPs are based on the generation of free radicals, especially hydroxyl radials (•OH), using oxidants such as hydrogen peroxide or ozone, to initiate a chain of radical oxidation reactions (Glaze and Kang, 1989, Santos et al., 2020). Within all of AOPs, catalytic wet peroxide oxidation (CWPO) appears as an economic alternative to eliminate adsorbed VOSC and to recover adsorptive capacity of ACs, due to its operational simplicity under mild conditions that contribute to lower operational costs (Garcia-Mora et al., 2021, Ribeiro et al., 2016, Zazo et al., 2006). Few AC regeneration studies have been focused on the removal of odorous VOSCs. Regeneration methods including thermal desorption (Boulinguiez and Le Cloirec, 2010, Giraudet et al., 2014, Li et al., 2021), extraction (Cui and Turn, 2009), non-thermal plasma (Chen et al., 2013) and electrochemical (Conti-Ramsden et al., 2012) have been commonly used. However, there is a lack of knowledge about the use of CWPO to regenerate VOSC-exhausted ACs.
This work aims to investigate the technical feasibility to apply wet peroxide oxidation process to regenerate sulphur compound-saturated ACs and to understand the role of surface composition of ACs during the adsorption-regeneration cycles. The effect of chemical surface composition of microporous materials during the oxidative regeneration of exhausted ACs using wet hydrogen peroxide and the implications of AC chemical surface functionalities in the formation of radicals and in the oxidation of adsorbed nuisance odor pollutants during different operating cycles, is addressed here using temperature programmed desorption coupled to mass spectroscopy (TPD-MS) and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). In particular, in this study in order to understand the influence of chemical surface features of ACs, two commercial ACs are chemically modified using nitric and hydrofluoric acid to selectively introduce a higher content of oxygen-containing functional surface groups and to reduce the amount of mineral matter, respectively. Thus, six AC samples with different content of surface functionalities are assessed. TPD-MS and DRIFTS analyses allowed identifying the chemical surface groups involved in the oxidative regeneration of VOSC-exhausted ACs. Dimethyl sulphide (DMS) was used here as a target nuisance odor pollutant, representative of VOSCs emitted by a great variety of industries. As a result a surface reaction mechanism is proposed and recommendations are given for the design of new carbon materials and their application in a novel process for the removal of nuisance odor pollutants using catalytic wet peroxide oxidation during different operating cycles.
Section snippets
Materials
Two commercial ACs were used as parent materials in this study and were supplied by Desotec, Tarragona, Spain (Airpel 1DS1) and Calgon Carbon Corporation, Pittsburgh, PA, USA (Filtrasorb 300). As-received materials were ground and sieved to obtain a particle size between 300 and 425 µm. All AC samples were washed with ultra-pure water and were subsequently dried overnight at 105 °C and stored in desiccators before being used. Sample Airpel 1DS1 is denoted as D; whereas sample Filtrasorb 300 is
Characterization of ACs
Textural and chemical surface properties of the studied ACs are summarized in Table 1. Figs. S1 and S2, included as supporting information, display nitrogen adsorption-desorption data and pore-size distribution curves of each AC sample, respectively. All AC samples exhibit a Type II isotherm due to multilayer physisorption with a hysteresis loop, according to the IUPAC classification. Results evidence the presence of micropores with an average pore radius of 22.8 Å in all ACs. As-received AC
Conclusions
Chemical modification of the surface of as-received ACs brought by the treatment with nitric and hydrofluoric acid allowed to identify both adsorption and regeneration mechanism of DMS-exhausted activated carbons using wet peroxide oxidation. Experimental evidences show that the use of ACs with high content of basic surface groups and high amounts of mineral matter increases the generation of radicals, favoring the oxidative regeneration of adsorbed DMS. However, during the cycling regeneration
Declaration Of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This study was financially supported by the Government of Chile trough CONICYT FONDECYT/Postdoctorade program (Grant N° 3150118) and CONICYT FONDEQUIP (Grant N° EQM150103) to which the authors are grateful.
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