ReviewReview of modifications of activated carbon for enhancing contaminant uptakes from aqueous solutions
Introduction
Activated carbon (AC) has been proven to be an effective adsorbent for the removal of a wide variety of organic and inorganic pollutants dissolved in aqueous media, or from gaseous environment. It is a widely used adsorbent in the treatment of wastewaters due to its exceptionally high surface areas which range from 500 to 1500 m2 g−1, well-developed internal microporosity structure as well as the presence of a wide spectrum of surface functional groups [1]. As an inert porous carrier material, it is capable of distributing chemicals on its large hydrophobic internal surface, thus making them accessible to reactants [2]. While the effectiveness of ACs to act as adsorbents for a wide range of contaminants is well noted [3], [4] more and more research on AC modification are gaining prominence due to the need to enable ACs to develop affinity for certain contaminants to cater for their removal from varying types of wastewater in the industries.
It is, therefore, essential to understand the various factors that influence the adsorption capacity of AC prior to their modification so that it can be tailored to their specific physical and chemical attributes to enhance their affinities toward metal, inorganic and/or organic species present in aqueous solutions. These factors include specific surface area, pore-size distribution, pore volume and presence of surface functional groups. Generally, the adsorption capacity increases with specific surface area due to the availability of adsorption site while pore size, and micropore distribution are closely related to the composition of the AC, the type of raw material used, the degree of activation during production stage and the frequency of regeneration [5]. Recent research have placed emphasis on modifying these physical and chemical attributes and this paper aims to review and summarize the various AC modification techniques and their effects on adsorption of chemical species from aqueous solutions. In order to facilitate a more focused discussion, only studies on modification of AC in granular or powdered form were reviewed. Also, it should be noted that the term modification in this paper represents treatment applied to as-received commercial AC or carbonaceous materials after activation. Studies on activation of carbonized material that incorporated modifying chemicals are not presented. Based on extensive literature reviews, the authors have categorized the techniques into three broad groups, namely, modification of chemical, physical and biological characteristics which are further subdivided into their pertinent treatment techniques (Fig. 1). These techniques were intended to significantly alter a characteristic and not two or more characteristics concurrently, even though in actual fact, all treatment effected to AC to modify a single characteristic may cause unintended minor alteration in other characteristics. Table 1 lists and compares the advantages and disadvantages of existing modification techniques with regards to technical aspects which are further elucidated in the following sections. While these characteristics are reviewed separately as reflected by numerous AC modification research, it should be noted that there were also research with the direct intention of significantly modifying two or more characteristic and that the techniques reviewed are not intended to be exhaustive. For information pertaining to detailed experimental methodology and conditions, readers are referred to the full articles listed in the references.
Section snippets
Modification of chemical characteristics
It is an established fact that the AC surface can display acidic, basic and/or neutral characteristics depending on the presence of surface functional groups [6]. As such, modification of chemical characteristics in this paper is taken to mean treatment to modify the inherent surface functional groups of AC. It has been widely recognized that chemical species removal by AC adsorption is due predominantly to the surface complex formation between the species and the surface functional groups.
Definition
It is generally accepted that the adsorptive capacity of activated carbon to remove hazardous substances can be substantially boosted by impregnation with suitable chemicals [47]. The term “impregnation”, however, is rather vaguely defined by researchers. Henning and Schafer [48] defined impregnation as fine distribution of chemicals on the internal surface of AC. However, there are researchers [49], [50] that associate AC treatment with acids or bases to increase acidic/basic surface
Modification of physical characteristics
All AC modification techniques sometimes constitute at minor changes in its physical characteristics (as exemplified by acidic treatment). However, there are research specifically aimed at significantly modifying the physical characteristics such as BET surface area and pore volume so that magnitude of these characteristics can be increased to enhance contaminant removal, especially in the case of organic adsorptions. It should be noted that the majority of these studies utilized thermal
Mechanism
It is of common knowledge that AC is more effective in removing organics than metal species from aqueous solutions. However, the specific mechanism by which the adsorption of many organic compounds takes place on AC remains ambiguous [69]. As such, numerous researchers [69], [70], [71], [72] have attempted to modify AC either by chemical or thermal treatment to provide an avenue where this mechanism can be determined experimentally. There are several factors that play significant roles in
Bioadsorption
Both natural and residual waters contain microorganisms that can be adsorbed on AC during water treatment, especially sewage treatment due to its rough and fissured surface on which microorganisms can settle and colonize easily [91]. Adsorption of microorganism, intentionally or unintentionally, onto AC has been the subject of research for the past several decades. Most research on bioadsorption on AC are directed at removal of readily biodegradable compounds that have low molecular weight [92]
Concluding remarks
In this review, a wide spectrum of AC modification techniques along with their advantages and disadvantages have been presented. Modification of AC via acidic treatment was by far, the most studied technique, perhaps attributed to its simplicity, availability of oxidizing agents (mostly as aqueous solutions) and the established nature of the technique that has been used for past several decades. Generally, oxidative treatment of AC is very favorable for enhancing uptakes of metal ions while
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