Introduction
The idea behind using GAC as a filter for biological material is based on its porosity and surface properties. Molecules from viruses to macromolecules to polycellular organisms are able to be trapped by GAC. But using GAC with biological molecules supported on it can remove conventional pollutants; viruses can be immobilised via a GAC and metal combination.
Adding A Carbon Filter: Simple Filtration
Researchers have found that there is a strong correlation between proteins (comparatively large macromolecules) and the increased amount of macro-sized pores, ie. exactly what GAC provides. This effect of the pore-size distribution on biological GAC suggests that adsorption across the whole surface is dynamic, which opens the doors to differing amounts of residence time, leading to a variance in biodegradation(1). Biodegradation has historically been considered less important as adsorption in GAC filtration of biomolecules. Macro-sized pores have diameters between 0.2 and 10 μm.
Examples of conventional filtration pathways with biological GAC include those with bromate. Bromate is oxidised to bromide by GAC. Removal of bromate is critical in ensuring water is good enough for biological expression(2). Conventional removal with hydrogen peroxide is not as attractive a pathway. In the water treatment pathway, bacteria species Polaromonas and Hydrogenophaga are the predominant bacteria found filtered out by granular activated carbon filters(3) - amongst many others - suggesting a broad applicability of GAC as a bacteria removing filter. Problems with bacteria colonising on GAC post-filtration are discussed later.
Other biomolecule types include viruses. As early as the 1970s, research was proceeding into the adsorption of poliovirus onto activated carbon(4). This is especially important as one of the modes of spreading poliovirus is by water. If water is readily filtered of poliovirus, it cannot spread. Reducing the solution pH of waste or stagnant water to mildly acidic 2.5 to 4.5 renders GAC a much better performing adsorbent. By grinding GAC to a pore size of around 10 μm, viruses and bacteriophages are more readily removed from buffered aqueous solution. The advantage of the smaller particle size of the GAC meant that there was a smaller electrophoretic repulsive force between virus and surface, this combined with a more hydrophobic virus surface (activated carbon is also hydrophobic) meant better filtration(5).
The difference in activation modes - steam activated or chemically activated - on GAC has been investigated for the biodegradation of dissolved organic carbon (DOC). Experimentation showed little to no difference between the modes. Ozonation, which is a common pretreatment method for GAC, was also probed and was found to have no impact(6).
There are a broad array of procedures for removing bacterial material from GAC filters, many of which are similar(7). It goes without saying that GAC filters are more than capable of resisting any.
As ever with GAC filtration, the aim is to remove as much material as possible. GAC filtration facilitates microbial processes that themselves are able to remove biodegradable organic carbon - and other materials - from surface or otherwise standing water that may or may nor have been treated with ozone. The overall process ensures biological stability of the water.
Research has shown that long term deployments of GAC for water purification can benefit from maintaining a biofilm on the surface of the activated carbon. Reaching an ecological equilibrium on biological activated carbon makes for a more effective and resilient filter, providing the integrity of the biofilm can be maintained(8), in fact, GAC biofilters' lifetimes can be prolonged in this way. Maintenance of such a healthy biofilm can be ensured with a slightly elevated pH and a decreased dissolved oxygen content, ensuring excess filamentous bacteria do not form.
Building on this idea, research has looked into the effect of filter bed depth of biological GAC on overall filter efficacy, with longer filter pathways being responsible for a greater bacterial species diversity through the bulk. It was found that overall performance is not correlated to the elevated presence of such bacteria, counterintuitively(9). Researchers suggest that the increased functionality (ie. enhanced filtration performance) is a result of more even distribution of biological material.
Such behaviour is useful. The biosorption of Cr(VI) species by three bacteria supported on GAC. These bacteria are known to reduce the chromium to a lower oxidation state, which is then adsorbed by the GAC. At 50 mg L-1 chromium in aqueous solution, uptake of the metal by the biological GAC ranged between 1.96 and 3.60 mg g-1. Doubling the concentration of the chromium led to uptake ranges of 0.66 to 1.12 mg g-1 across the three bacteria types(10).
Further building on the idea of bacterial GACs, such an example has been developed using the bacteria Phragmitis communis, which is able to effect the degradation of 4-chlorophenol. When an aqueous solution of 4-chlorophenol at 100 mg L-1 was fed onto the GAC-P. communis column, around a quarter was immediately available for biodegradation whilst the rest was adsorbed onto the GAC(11). Similar tactics can be applied to other water systems seeking to remove other chlorinated organics.
Overall, the dual nature of the supported bacteria and the granular activated carbon offers synergistic benefits in ensuring cleaner water.
Activated Carbon As A Host: Using Metals And Other Materials To Immobilise Viruses
One niche - but highly relevant - application for activated carbon in water filtration is the ability to immobilise water borne viruses in concert with metals such as gold and silver, sometimes in nanoparticle form. Leaning on GAC’s excellent porosity and surface chemistry, it is readily enhanced and this enables the development and tailoring of outcomes.
Activated carbon that has been modified with nanoparticles of silver and copper oxide has been shown as effective at removing viruses from water(12). Suspensions of the T4 bacteriophage were passed through the filter, with a sample of GAC that had been doped with 0.5% by weight silver and 1.0% copper oxide being responsible for a 5.53 log reduction in the T4 bacteriophage in the water. Silver and copper content of the resulting filtrant was well below the safe limits for drinking water. Therefore, this method is suitable for water purification. Similarly, the adsorption of molecular iodine onto GAC was effective for the immobilisation of E. coli and an avian influenza virus(13). Results were compared to slaked lime, widely used as an antibacterial in agriculture, which the GAC easily outperformed. Finally, the adsorption of more contemporary viruses including SARS-CoV2 - responsible for the recent global pandemic - has been investigated. It was found that the combination of macro, micro and meso pores in GAC are sufficiently porous to immobilise the SARS-CoV2 pathogen(14). The authors of the study therefore give credence to the idea of activated carbon-containing face masks as effective for reducing transmission.
Drawbacks
As with all methods, there are some slight drawbacks to using granular activated carbon with biological materials. The first is that if post-filtration treatment doesn’t take place - or is completed inefficiently - then the very pathogens that we are trying to filter out may grow and persist on the surface of the GAC. Yersinia enterocolitica, Salmonella typhimurium and Escherichia coli are all able to colonise and grow on sterile GAC(15). This emphasises the need to ensure adequate backwashing and/or regeneration strategies for the filter. Scanning electron microscopy studies have shown that in some cases, even with disinfection with a dilute chlorine solution (2 mg L-1), GAC may be colonised by bacteria that grow in cracks and crevices(16). Therefore, part of the very nature of GAC - porosity - means that it needs to be comprehensively treated post use. Authors offer this as a hypothesis on why bacteria may persist in filters that have been left in stagnant water at length.
Summary
- Granular activated carbon - and biological activated granular carbon - are useful tools in ensuring the stability and biological purity of water
- Simple GAC filtration may be applicable to the removal of bacteria from water under certain conditions, relying on GAC’s highly porous nature to do so
- Depending on the type of GAC, it can be used to support bacteria or other molecular life to use as a filter to remove, for example, chromium from solution
- Viruses, including poliovirus, are able to be immobilised by GAC, ensuring safe potable water. Similar effects are observed with bacteriophages
- Crucial for GAC in biological settings is the ability to wash/regenerate the filter else risk a pathogen build up, which may render the filter inactive
References
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3 B. Wullings et al., J. Appl. Microbiol., 2009, 107, 1457
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6 A. K. Camper et al., J. Microbiol. Methods, 1985, 3, 187
7 M. Sholz and R. J. Martin, Water Res., 1997, 31, 2959
8 N. Boon et al., Water Res., 2011, 45, 6355
9 C. Quintiles et al., J. Hazard. Mater., 2008, 153, 799
10 P. M. L. Castro et al., Appl. Microbiol. Biotech., 1999, 52, 722
11 M. F. Silva et al., Environ. Tech., 2017, 38, 2058
12 K. Otsuki et al., J. Carbon Res., 2021, 7, 86
13 A. K. Azad et al., J. Eng. Tech. Sci., 2021, 4, 210404
14 G. A. McFeters et al., Appl. Environ. Microbiol., 1985, 50, 1378
15 G. A. McFeters et al., Appl. Environ. Microbiol., 1984, 48, 918
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