Closing the active carbon cycle: Regeneration of spent activated carbon from a wastewater treatment facility for resource optimization

Highlights

• Microwave heating enhanced porosity restoration in spent active carbon.

• Partial gasification of spent active carbon enhanced adsorbates removal from active sites.

• Optimal regeneration was detected in microwave reactors under CO₂ atmosphere.

• Inert atmosphere promoted blockage of pores by pyrolysed adsorbates, hindering regeneration.

Abstract

This study focusses on the regeneration of spent activated carbon sourced from a wastewater treatment facility using both conventional and microwave reactors with similar reaction parameters. The active carbon was characterized by iodine and methylene blue adsorption as a measure of micro- and mesoporosity. The process parameters were adjusted to identify the influence of intensifying operating parameter on the effectiveness of the regeneration process. Optimal activity was observed in the sample regenerated at 600 °C for 2 h in CO₂. There was a 17.1 % and 141 % increase in iodine and methylene cyanine adsorption values, respectively, versus the spent activated carbon. This indicates restoration of ∼83 % and ∼90 % of the adsorptive capabilities of fresh activated carbon. The adsorption properties of regenerated carbon are enhanced in microwave reactors under CO₂ atmosphere due to the benefits of microwave heating in expediting the thermal desorption of adsorbates via devolatilization and partial oxidation with minimal disruption to the carbon’s pore structure. This demonstrates the feasibility and advantage of microwave reactors for reactivating spent active carbon and the importance of reaction parameter optimization for obtaining regenerated active carbon with enhanced quality.

Introduction

Due to its adsorptive properties, activated carbon (AC) has diverse applications in various sectors such as food/water, air/gas purification, pollution control, smelting, pharmaceuticals, and catalysts [1]. Its use involves the adsorption of impurities affecting the odor, taste, and color of gaseous and liquid compounds. The versatility of AC has inherently led to an increase in its annual demand to about 1.8 million tons in 2016 with >55 % in powdered form; the largest consumer is the water treatment industry [2].

Depending on the volume of substances being processed, active carbon can become exhausted within days or weeks. This implies a high annual demand if it is not regenerated for reuse [1,2]. There are currently three disposal methods for spent active carbon: landfill, incineration, and reactivation. Approximately 68 % of the granular active carbon becomes reactivated, whereas ∼95 % of powdered AC goes to a landfill because it is considered the least expensive disposal method. As a result, the constant use and disposal of powdered AC pose a resource and waste management challenge that must be carefully addressed. Landfilling suffers from competitive market usage, stringent policies and regulations on carbon disposal, and pollution considerations [3]. Thus, reactivation of the spent AC is a sustainable approach, but there is no cost-effective approach to reactivating carbon.

Different forms of AC can be utilized for the treatment of industrial wastewater. These approaches remove organic pollutants from wastewater. Based on their adsorption capabilities, a saturating stage is reached where the resource is either disposed of or regenerated for reuse [4]. Methods include thermal, biological, and chemical regeneration routes, but the thermal process is the only commercially viable technique [5,6]. Such thermal routes involve pyrolytic (<800 °C) and/or oxidative temperatures (∼800 °C) for the removal of volatile adsorbates and/or the decomposition of charred residues deposited from the pyrolyzed adsorbates [1]. The thermal regeneration is usually done from 700 to 900 °C and is mostly oxidative in furnaces that decompose and oxidize impurities.

The regeneration of AC is highly dependent on parameters such as temperature and duration; thus, this step requires optimization for various waste-AC streams [7]. Pyrolysis is a promising approach for the decomposition of organic adsorbates from AC, and conventional thermal systems have been researched for this purpose [2,8,9]. Pyrolysis uses less energy and minimizes pollution due to oxidation. Recently, the use of microwave reactors has been suggested for AC regeneration due to its compatibility with homogenous and rapid heating [5]. This offers benefits such as compact system sizing, rapid response, and higher efficiency during intermittent operations. Although microwave reactors for regeneration have been investigated, the various studies, their large scale application remain very limited [4,10,11].

MW Regeneration of AC has been investigated with several research papers published in recent years [[11], [12], [13], [14]]. While most of these studies concluded that MW regeneration of spent AC was an efficient and economical process, majority of the research was limited to thermal regeneration in nitrogen or steam atmosphere with temperatures ranging from 300 to 950 °C and heating time of 3–90 min. The methodology adopted were adsorption isotherms, TGA, FTIR, SEMs, adsorption tests and gas chromatographs due to the specific application of the spent AC materials. However, most of these works do not provide vital comparison of MW heating to conventional heating systems such as appropriate comparison of performance at multiple temperatures and reaction time.

A few studies have investigated the use of microwave assisted regeneration of AC to aid the removal of pollutants and heavy metals for the spent AC, particularly sulphur and mercury. Such studies [15,16] have compared the use of conventional systems and microwave systems and their observations include the faster rate, lower reaction duration, increased porosity and higher mercury removal capacity for the regeneration using MW systems. Likewise [12], reported observed higher desulfurization from MW regeneration. Previous works of Ania et al. [17, 18**prev 19] have also employed MW regeneration for treating AC polluted with organic contaminants and pharmaceutical pollutants with reports of better performance of MW heating in comparison to conventional systems due to the higher rate of adsorption and adsorption capacity during intended usage. Another study by [19] focussed on the use of ultrasound and microwave technologies to eliminate organic contaminants. These investigations involved varying reaction time and power with excellent MB adsorption of 100–200 mg/g. Similarly, Qu et al. [20] used the MW for the regeneration of AC spent with zinc acetate and obtained product with exceptional adsorption capacity. Furthermore In addition, Mao et al. [13] reported that MW regeneration was 6 times faster than conventional heating systems for achieving similar capabilities. Furthermore, the preservation of the textural and porous properties of MW were also investigated by [10] which indicated that the heating mechanism of MW preserved the carbon matrix, allowing the same AC to be used and regenerated many times. All of these results were also corroborated by the works of Zhang et al. [21] on the regeneration of spent AC catalyst which revealed superiority of regenerated AC properties.

Remarkably, a study done by [22] on the use of steam reactivation reported iodine and methylene blue adsorption of 1103 mg/g and 380 mg/g which were superior properties to that of the fresh AC. Conversely, works of Caliskan [11] found the opposing trend with lower regeneration efficiency from MW systems due to enhanced cracking of adsorbates in the carbon matrix. In more recent works [23], the regeneration of spent AC catalyst from adhesive production was evaluated using N₂, CO₂ and ultrasonic spray conditions with focus on the yield, BET surface area and methylene adsorption capability. Their results showed optimal regeneration conditions with BET surface area of 1263 m²/g and methylene blue adsorption of 175–199 mg/g.

The characteristics of activated carbon differ with iodine and methylene blue adsorption capacity of 1100–2200 mg/g, 300–700 mg/g depending on the feedstock and condition of its production. Hence, the characteristics of the regenerated AC would be influenced by this. This was highlighted in the study published by [24] revealed that the regeneration process is influenced drastically by the properties of the adsorbents and also those of the adsorbates (properties and quantities). The results of their work indicated that the adsorbates properties greatly influenced the inclination towards MW or conventional heating system for better adsorptive capacity after regeneration. Overall, regeneration process would need to be tailored to the specifics of the adsorbates. This indicates the need on case-by-case evaluation for mass regeneration of AC by industrial users,

In this study, regeneration of spent activated carbon used in a waste water treatment facility was investigated to help adapt and optimise the process by comparison of both conventional and microwave heating reactors under similar operating conditions of moderate temperatures and in inert and partially oxidative atmospheres. In addition, the reaction mechanism was discussed and the influence of intensifying the process parameters was done to optimize the regeneration process. This would optimize the adsorptive properties of the regenerated AC and can determine the best of the two heating mechanisms.