The electrochemical regeneration of granular activated carbons: A review
Introduction
The large surface area, microporous structure, and increased surface reactivity associated with granular activated carbon (GAC) [1,2] makes it one of the most powerful adsorbent materials used in purification processes. In the present day, industries such as oil and gas, pharmaceutical, and waste water treatment are now incorporating increasing amounts of GAC into their manufacturing processes. With increasing amounts of legislation being placed on environmental protection, product quality, and liquid and gas emissions, the global demand for activated carbon continues to increase nearly 10% annually from its global consumption of 4.28 million tons in 2012 [3].
Although proven to be highly effective, the leading shortcoming of GAC is the limited lifetime for which it can perform. Upon reaching its adsorptive capacity, it is no longer effective and is most often sent for disposal and replaced with fresh material. However, the continuous material change out that results is not financially viable, and the disposal of exhausted material into landfill leads to the possibility of toxic contaminants leaching into the environment [4]. A more economical and environmentally friendly option is to regenerate the material such that it can be used for several cycles of adsorption and regeneration.
The regeneration of GAC aims to restore the original adsorptive capacity of the GAC by removing contaminants that have accumulated onto the carbon surface, while damaging the carbon as little as possible [5]. Several regenerative techniques are extensively covered in the literature and currently applied in industrial applications; this includes thermal [6], chemical [7,8], and microbial methods [9]. However, these methods are met with limitations such as high energy consumption and carbon attrition [10], pore blockage and oftentimes the requirement for secondary treatments [11], and slow regeneration rates [12] respectively.
In an attempt to overcome these restrictions, attention is being turned towards a relatively newer technology, electrochemical regeneration, at which saturated GAC is placed between two oppositely charged electrodes. Upon application of an electric current, both desorption and degradation processes of the adsorbed contaminants are observed. Narbaitz and Cen [13] were one of the first to study this technique, utilizing a 1-litre electrochemical reactor to regenerate 1.2 g of phenol saturated GAC. Following 5 h of treatment, the GAC was 95% regenerated, with consecutive treatments resulting in only a 2% loss in adsorptive capacity per cycle due to minimal carbon losses. As these results exceeded the outcomes of other regenerative methods, in terms of adsorptive capability and prolonged use, it gave merit to conduct further research in the field.
Despite the recent application of electrochemical technology, it has had limited discussion within the literature. Thus, a review has been conducted that aims to present 1) the theory of electrochemical regeneration, 2) what research has been done related to the electrochemical treatment of activated carbon, and 3) identify limitations and future engineering prospects associated with the technology.
Section snippets
Regenerative mechanism
The application of an electric current across and exhausted bed of granular activated carbon results in two main regenerative processes. First, enhanced desorption from the GAC surface commences, Fig. 1(A)–(C), and results in an adsorbent free of contaminant species. Second, electrochemical reactions occurring at the electrodes and polarized GAC particles stimulate the degradation of contaminant species, completely removing them from the system, Fig. 1(D). An ideal electrochemical regenerative
Operational considerations
Although the electrochemical regeneration of GAC is known to be a functional treatment, research to date has greatly varied in reactor configuration and operating parameters, with research groups utilizing different types of GAC and model contaminants. Although this makes it difficult to gain an understanding of the underlying mechanism, kinetics, and possibility of scale up, the following section aims to encapsulate and compare how different reactor configurations used during the
Industrial application
The large-scale implementation for the electrochemical regeneration of GAC has not yet been implemented, largely due to a lack of understanding and optimisation of the treatment process. However, a technology operating on a similar principle has recently been developed by Arvia Technology Ltd., whereby contaminant adsorption and electrochemical destruction occur within a single cell. The Arvia Organics Destruction Cell™ (ODC), depicted in Fig. 7, passes contaminated wastewater through a bed of
Conclusion
The literature demonstrates that electrochemical treatments are an effective method for the regeneration of exhausted GAC, with high regeneration efficiencies being obtained within a few hours. Even so, the development and application of this technology has progressed slowly since its initial exploration over two decades ago. Research to date has predominantly focused on variations in reactor configurations and operating parameters, with research groups utilizing different types of GAC and
Declarations of interest
None.
Acknowledgments
The authors acknowledge the research financial support provided from the Australian Antarctic Science Project 4029 and The Particulate Fluids Processing Centre.
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