Activated Carbon: Biological Material
It is widely known that the outcome of certain biological materials entering or persisting in the water supply: illness. Made easier to remove via the use of activated carbon filtration, this third article in a series of activated carbon for water purification, looks at just that. Clean potable water supplies are essential for life - it’s that simple. Additionally, using activated carbon as a support for biological material may be useful.
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
Because of the lack of need of redox chemistry for the secondary removal of lead (i.e. lead that has been derived from scrap sources in the most part), pyrite has no part to play here unless the lead is melted down and added into a conventional (primary) lead isolation process. Lead, like many other metals, is infinitely recyclable providing impurities can be removed.
Chlorination roasting is a method to extract high value metals from tailings. By roasting tailings materials (from lead, gold and silver mining, for example) and then subjecting this material to a chlorine rich atmosphere, high value metals readily form their chloride salts in situ, which are then easily separable and dechlorinated later. Pyrite has been proposed as a material to enhance the conversion of the chloride source (calcium chloride) into elemental chlorine(5), as per the following formula:
CaCl2 + FeS2 + O2 → CaSO4 + Fe2O3 + Cl2
Chlorine gas may then react with any metal present (such as lead, gold, etc., that is in low concentration and widely dispersed through the tailings) to form the chloride of that metal:
M + Cl2 → MCl2 (or other chlorides of the metal, where ‘M’ is a metal)
The chlorine would eventually be produced by simple thermal decomposition of the calcium chloride, but pyrite enhances this. The use of pyrite also has broader applications in the clean up of various metal tailings, including cyanide(6). Overall, the advantage of being able to use easy to handle and relatively inexpensive pyrite to enhance the calcium chloride process pays dividends.
Activated Carbon As A Host: Using Biological Materials To Purify 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.
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.
- 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
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