Activated Carbon: Industrial Wastes And Organic Pollutants
Modern mining and industry has made great strides in recent years in terms of pollution minimisation - but more can be done. Contaminants in industrial runoff and mining tailings - such as volatile organic compounds, amines, oxides of metals etc. - can be better removed with the help of activated carbon filtration. The fourth in this series, this article looks at how activated carbon is a valuable tool in the arsenal to keep pollution to a minimum.
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.
Further Examples Of Industrial Pollutant Removal
There are multiple examples of GAC removing VOCs from industrial wastes. Simultaneously both VOCs such as phenol and related compounds, and heavy metals such as copper, zinc and cadmium(9). Research shows that the heavy metals are removed preferentially in the order Cu > Zn > Cd. Related compounds acenaphthylene and phenanthrene were also removed at the same time, albeit at decreasing rates as the amount of metal adsorbed increased. It is reported that the GAC’s zeta potential is modulated by the presence of the metals, which leads to the VOC adsorption decrease.
Notwithstanding this, a broad spectrum of VOCs are readily removed from wastewater by GAC. Phenol related compounds o-cresol, p-nitrophenol, m-methoxyphenol, benzoic acid and salicylic acid have all been shown to be easily removed at economically viable and scalable volumes(10). This is particularly attractive as one filter can remove so many pollutants - simultaneously - from compromised water.
Cyanide is toxic to humans, animals and aquatic species and dissolves easily in water. Thankfully, GAC filtration is an effective and robust method for its removal. Using GAC that has been gently acidified at the surface was shown to be a good method. At neutral pH, in excess of 94% of cyanide was removed from solution at a concentration of 90 mg L-1(11), in as little as 25 minutes at room temperature. The acid treatment is facile and uses a dilute acid application before filtration.
Although not highly toxic, methyl tertiary butyl ether (MTBE) needs to be removed from water courses as it is incredibly persistent within it, is fairly miscible with water, and adds a discouraging smell to the water. GAC rich fixed bed filters are effective at removing concentrations of MTBE up to 2000 μg L-1, in as little as ten minutes(12) from sitting groundwater.
One sector known for its use of harsh chemicals and - at times - poor disposal and treatment processes in the developing world. As companies in this industry work on notoriously tight margins, an effective water purification system is crucial. But that system also needs to be relatively cheap and inexpensive to maintain and regenerate. Enter GAC. Studies have shown that cyclooctadiene can be removed from textile factory industrial effluent that contains indigo dye(13). Optimal treatment pathways were identified as 125 minutes at 12.5 °C at pH 8.5 - all conditions not difficult to reach and maintain. Cyclooctadiene is highly toxic. In the dyeing of nylon (a particularly difficult fabric to dye) strongly acidic dyes are used. Because they are hard to treat, they are often discharged directly into a river or the sea. A study showed that a fixed bed GAC filter could be used to treat the effluent(14), but problems came when it was realised that multiple dyes and treatment agents are being used. The vast majority of the non-coloured effluent products were readily adsorbed, but only a minority of the highly acidic coloured dyes. More work needs to be conducted in this area.
Comparison With Other Filter Types
As with many processes in industry, that there are other ways to clean up and filter industrial runoff water should not come as a surprise. Research has shown that GAC is a superior filtering device compared to similar compounds and filtration protocols such as dried activated sludge and fly ash. In an experiment probing phenol removal from water, at a concentration of 100 g L-1, GAC was able to remove at a rate of 108 mg g-1. Dried activated sludge and fly ash were effective to only circa 80 mg g-1 and 18 mg g-1 respectively(15). These effects are attributable to significant differences in porosity. Other dried sludge studies have only observed phenol adsorption to a level of around 50 mg g-1(16) - with the sludge being less tolerant of elevated temperatures. One of the other advantages of GAC over other methods is how easy it is to regenerate, especially after having been used as a filter for volatile organics. A simple flush with a low concentration solution of surfactant followed by purging with pure water is often enough to render the filter ready for use again(17).
- Granular activated carbon is a highly effective filter medium for cleaning up a broad range of industrial wastes from water
- It is highly active against volatile organic compounds such as toluene and phenol, but also dissolved metal ions which may be present in industrial waste
- GAC has the advantage of working in a wide variety of conditions, including at high temperature, making it a good candidate for in-flow filtration in cooling towers for example
- GAC outperforms the competition across a range of pollutant types
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