Introduction
Granular activated carbon, GAC, is widely known for its filtration abilities, based around its high porosity, excellent surface chemistry and highly resilient nature. GAC is effective at the removal of industrial waste and organic pollutants from water, oftentimes being deployed as an in-line filtration solution, so as to ensure safe levels of discharge into local water courses, rivers and/or the sea(1). As an initial example of the power of GAC, the removal of volatile organics from a mixed and unregulated industrial gas outflow is possible. Almost complete VOC removal from a 5 mg L-1 contaminated gas can be observed with a modest deployment of GAC in a fixed bed filter.
Using A Granular Activated Carbon Filter
As with all deployments of GAC filters, the efficacy of the system revolves around several measurable properties of the GAC itself. These are surface area, pore volume, packing density, and pore size distribution. In general, the greater the surface area, the better the adsorption. Additionally, pore volumes are effectively tailorable to the substance that seeks to be filtered. Relatively larger particle sizes, for example, are especially useful for methane adsorption(2). It is true that different variations on the GAC render it more or less suitable for different contaminants.
As with gases, vapourised organics are able to be cleaned from air by GAC. Toluene vapours have been shown to be readily captured at rates up to 90% - an excellent result for such a volatile material(3). More impressive is that the filter in this research was only 3 cm deep and operated at a relatively low flow rate of just 60 mL min-1. As with methane, adsorption proceeds via physisorption, as expected. The effect of particle size distribution here is key, as researchers found that GACs with higher proportions of microporous structures were better adsorbents of toluene and other similar compounds. They stated that the greater immobilisation of the toluene was due to the greater share of high energy adsorption sites dispersed through the more microporous GAC, relative to less microporous or mesoporous GAC(4). GACs that have been either ‘filled up’ with adsorbed volatile organics or that have been doped with the same in the laboratory form what is called a ‘heel’. This can be thought of as the porous sites being blocked. When this is the case, filtration capacity is reduced. In samples with high oxygen content, it was shown that both chemisorption and physisorption played a role in heel formation; whereas with GACs with lower oxygen contents, only physisorption occurred(5). This informs the design of future GAC filters.
A further advantage of GAC filters is their thermal stability. One study looked at the thermal stability and decomposition of perfluoroalkyl substances on GAC. These substances are the same as those used on non-stick cookware, and so themselves have a high resistance to strong temperatures. Encouragingly, in the attempted thermal removal of the perfluoroalkyl compounds from the GAC at high temperature, no degradation of the GAC was noticed(6).
Co-pollutants are also tackled by GAC. One study probed the use of GAC to remove both phenol and cadmium waste residues from a downstream water course. Performing as expected according to surface chemistry principles, in excess of 280 mg g-1 of phenol was removed from water, alongside over 80% of the cadmium present(7). In general, adsorption of various volatile organic compounds to GAC tends to behave with data in accordance with the Langmuir isotherm. Furthermore, kinetic restrictions (ie. the ability for an adsorbent particle to move) are more pronounced if the contaminant and pore size are poorly matched, as would be expected(8), calculated by example with dibenzofuran, a common industrial pollutant.
Overall, these show the broad scope for GAC in the industrial waste residue clean up process.
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