Introduction
Heavy metals are in many cases toxic to plant and aquatic life. Runoff from mining practices, poorly managed waste sites and underregulated industrial activity can leave water courses and the resultant soils incapable of being used. It is imperative that heavy metals are removed as soon as possible - to prevent illness and environmental damage. The use of an activated carbon filter is one of the most effective, reliable and resilient methods for immobilising heavy metals. Other sources of heavy metals in water include weathering from buildings (particularly copper and iron), vehicular emissions and from plumbing installations. The primary method of filtration is size exclusion by means of adsorption. Many heavy metals are well suited to GAC filtration owing to their complimentary electronic properties (such as electronegativity) and atomic size.
Adding A Carbon Filter
Activated carbon filters can be used in several ways. In line filtration as part of an industrial treatment strategy or as a post-treatment method where stagnant or otherwise waste water is passed through an activated carbon filter prior to other processing. It is important to note that other processing is typically required to ensure the suitability of the water to return to rivers or the sea. This is done in the traditional way just like how domestic sewage is treated.
Industrial wastes are the prime sources of heavy metal contamination. Chromium(iii) ions are a common feature in the wastewater arising from tanneries. Because of the prevalence of their locations in the developing world, some water treatment may have historically been poor. Two types of commercial granular activated carbon were shown in research to remove 98.6 and 93% of chromium present in wastewater(1). This high success rate is directly attributed to surface area.
Iron and manganese presence in groundwater presents similar problems to chromium. Such groundwater is often reddish in appearance due to oxidation. When the groundwater was treated with granular activated carbon, the water became almost completely clear in as little as six hours at room temperature, with the GAC reported as removing as much as 3.60 and 2.55 mg of iron and manganese respectively per gram of carbon(2). Fe and Mn are especially suited to removal via GAC owing to their electronegativities and atomic radii.
More hazardous materials include cadmium and lead. Simulation studies into their removal have suggested that using GAC is effective when deployed in a fixed bed column (intended to replicate in line constant filtration), with near complete removal(3). Applying these ideas in the real world, researchers were able to show that GAC can immobilise - and therefore remove - cadmium and lead, in addition to copper and chromium, from river sediment. Researchers noted that the ideal pore size for this real world example was in the region of 0.075 - 0.18 mm. Like other research, the authors hypothesised that a combination of high degree of porosity and compatible electronic properties were instrumental in the effective removal(4).
When deposited onto other carbon types such as carbon nanotubes and carbon-encapsulated magnetic nanoparticles, GAC can also be effective at removing cobalt ions from solution. Authors note, however, that the grafting onto other carbon types is beneficial for GAC, but the addition of GAC to nanotubes, say, would not increase the adsorption behaviour of those nanotubes(5). Notwithstanding this, the authors clearly state that particle size of the contaminant is a key arbiter in whether or not it may be removed by filtration.
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