Activated Carbon: Heavy Metal Removal
In the second in a five part series on the applications of activated carbon, the use of activated carbon to sequester heavy metals is examined. Heavy metals, for example cadmium and lead, are notoriously difficult to remove from solution and there are serious consequences if these make their way into water courses, drinking water or the sea. Activated carbon is part of the solution.
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.
Granulated Activated Carbon Filters In Biological Settings
Heavy metals tend not to play well with biological systems (excluding those directly involved, of course) and therefore their removal is key.
Using biological systems to enhance removal with GAC has been shown when using yeast. This combination GAC-biosorbent has been shown as effective at removing cadmium, copper and zinc from solution. When using the GAC-yeast-alginate combined heterogeneous filter, some 90% of metal ions are removed from solution(6). Further bio-adsoprtion processes with granular activated carbon have been shown suitable for the removal of various heavy metals from primary treated sewage. A GAC dose of 5 g L-1 was responsible for 54% and 96% of contaminant removal by adsorption and bio-adsorption respectively(7).
Lead and nickel ions are able to be removed from aqueous solution and runoff via the use of a sequencing batch reactor employing bio-sludge and granular activated carbon. This compound system is in many ways similar to a fixed bed reactor, albeit one that differs in its heterogeneity and operating temperature. Interestingly, adsorption of contaminants was increased with a greater hydraulic retention time. I.e. the longer spent in the filter, the better the removal. This is to be expected, up to the total capacity of the filter. In excess of 800 and 750 mg of nickel and lead respectively are readily removed per gram of activated carbon - biosludge composite. Metal ions were subsequently recovered from the compound filter by treatment with dilute nitric acid.(8).
Granular activated carbon may be sourced from non-commercial means, and such activated carbon that has been prepared from apricot stone is effective across a range of heavy metals. Research has shown though that GAC from non-commercial sources suffers from a lack of homogeneity and a propensity to require a more defined pH range in which to work effectively(9).
Modifications To The Traditional Activated Carbon Filter Set Up
As extensively discussed above, the fact that activated carbon possesses a huge surface area relative to its volume is key in its ability to adsorb heavy metals from solution. The ability of a chemist or engineer to carry out surface modification on granular activated carbon - that is, change the surface properties by means of a chemical reaction or physical treatment - to further enhance the already established properties represents an exciting development. Factors such as larger surface area, greater levels of chemical inertness and even stronger mechanical strength are all common properties sought(10) for wastewater treatment.
Examples include where researchers sought to enhance the acidity at the surface of the activated carbon. By using a process of nitric acid acidification, preceded by de-ashing and followed by treating at 1,273 K, they noted that adsorption of heavy metals including cadmium was much stronger whilst the adsorption of aromatics was poorer(11). Other such performance enhancements have been noted with granular activated carbon that has been modified by microwave treatment, steam activation and ultrasound treatment. Each of these has been shown as effective in the laboratory setting at enhancing the removal profile for certain heavy metals and their compounds(12).
A prime example of the simpler end of surface modification is the treatment of a granular activated carbon material with citric acid. Researchers found that a modest doping increased active carbon surface area by 34% and increased its copper adsorption capacity to almost 15 mg Cu per g(13), some 140% better than the unmodified carbon. Similarly, the removal of heavy metal ions from aqueous solution has been accelerated when using sodium sulfide-treated granular activated carbon as the adsorbent for the removal of mercury, lead and nickel(14). Interestingly too, this study showed that adsorption in this case was not temperature dependent at all. Acid and base treatment is a popular choice, with pretreatment of GAC with 1.0 M nitric acid and potassium hydroxide showing enhanced adsorption across a wide range of heavy metals, especially cadmium and copper(15).
Whilst surface modification of granular activated carbon is becoming popular, it should be noted that it adds extra cost and complexity. Furthermore, it brings up the problem of how the chemicals used to do the treatment should themselves be disposed of. A greater study into costs versus benefits should be considered(16).
- Granular activated carbon is an excellent material for the filtration of a wide variety of materials and contaminants from water
- Some of the most pernicious pollutants which occur from activities such as mining include heavy metals. Iron and manganese residues are easily removed by GAC filtration
- Much more toxic (thus problematic) heavy metals pose more of a hazard and are also removed from standing or running water by GAC filtration
- Combining GAC filtration with biological systems like yeast can provide an even more effective filtration pathway for some of the most difficult contaminants to isolate
- GAC is easily modified at the surface, for example by acidification, which may aid in modulating the surface properties of GAC to have even broader use cases.
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