Each year, approximately 5000 tonnes of dyes (about 50 times the weight of a blue whale) are discharged into industrial wastewater. These dyes block light from reaching aquatic plants and animals, cause unpleasant odours and pose risks to human health. Congo red, for instance, is notably persistent in the environment. Another dye, methyl orange, is difficult to remove using standard biological or chemical treatments and has been linked to cancer and DNA damage. Researchers in Egypt and the US are devising a new method to clean up such contamination.
One of the well-established routes for cleaning wastewater from dyes is using nanoparticles (of similar size to a small virus) with high surface area, which gives the dye molecules plenty of places to adsorb onto the nanoparticle surface. It’s especially useful if the nanoparticle has magnetic properties – then it’s possible to remove nanoparticles with adsorbed dyes from water using a magnet. The nanoparticles can then be washed and reused, for up to four cycles, enabling the same material to be used again and again instead of being thrown away.
Ideas for breakfast
Lead researcher Hebatullah Hassan Farghal from The American University in Cairo and her team have synthesized magnetite nanoparticles (particles made from a naturally magnetic form of iron oxide) directly in dye-polluted water. As the particles form, they pull the dye molecules out of the water and onto their own surfaces – meaning that the water is left with less dye in it. The researchers then used those same “dirty” nanoparticles to capture two entirely different dyes afterwards. This route, described in RSC Advances, skips the usual “make-wash-dry-treat” sequence. Instead, it offers a more efficient, simpler and greener way to create a working wastewater adsorbent.
Farghal’s previous research work had been on magnetic adsorbents. “The idea for this paper came exactly one or two days after my PhD defence,” she explains. “While I was having breakfast, I realized adsorbent synthesis requires time and cost…I then went to the lab and started my experiments to find that it was successful.”
Catch and catch again
First, Farghal loaded the nanoparticles with two dyes – congo red (CR) and bromocresol green (BCG) – as they formed in the wastewater. More than 90% of both dyes stuck to the particles and the loaded particles grew slightly larger (about 34 nm, compared with 26 nm for plain magnetite). Later, the same “dirty” particles went on to adsorb almost 80% of methylene blue (MB) from a single-dye system after about 3 h, dropping to just over 50% when a second dye, methyl orange (MO), was mixed in.
To reuse the particles after adsorption of MB, the researchers rinsed them in alcohol, which pulled the trapped blue dye molecules back off. They repeated this four times without any drop in performance, and none of the originally loaded dyes leaking out in the process. They note that MB removal worked best at high pH, where the nanoparticles become negatively charged (while MB molecules are positively charged). This electrostatic attraction gave the main mechanism an extra boost.
Simulations second the results
There are several different mechanisms that can happen when a dye molecule attaches to the surface of a nanoparticle. The two most common types are chemisorption, where the molecule forms new chemical bonds with the surface, and physisorption, a weaker attraction more like static cling – no permanent bond is formed, and the molecule can be pulled off relatively easily.
Several lab measurements, including infrared spectroscopy of the particle surface, already hinted that physisorption is the main mechanism responsible for binding CR, BCG and MB molecules to nanoparticles. This weak binding is also why the ethanol wash could strip the dye back off without damaging the particle.
To confirm this idea, the team also ran a computational method called density functional theory (DFT) on a single dye molecule sitting on a magnetite surface. All dyes bound with similar strength. The simulations also showed electrons flowing in opposite directions depending on the dye’s charge – from the negatively charged CR and BCG toward the surface, and from the surface toward the positively charged MB – the same charge attraction seen in the lab.
The researchers point out that DFT and experimental outcomes cannot be compared quantitatively, only qualitatively, since the first one examines a perfect molecule on an ideal surface and the second one concerns real-world conditions. They note that the DFT analysis was carried out by Ahmed A Abokifa and Mohamed S Mohamed from the University of Illinois Chicago, with Mayyada El-Sayed supervising the wider study.
Outside of the lab
In standard tap water or saline (3% NaCl), the story gets more complicated. The researchers increased the adsorbent dose by roughly 15 times – from 0.67 to 10 g/l – to try to compensate for real-world conditions. This worked in saline, but removal in tap water was only about half of what it was in distilled water, likely because calcium, magnesium and other substances naturally present in tap water compete with the dyes for space on the particle’s surface.
“Though the dose significantly increased in tap water and saline, this dose is still applied in the literature,” Farghal says, adding that her future investigation will try to close that gap and reduce costs.
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Still, one thing works in the material’s favour: making the particles is cheap. Producing 1 kg costs around $3730 – roughly eight times cheaper than other nanomaterials sometimes used for the same purpose, such as cobalt ferrite (which costs roughly $30,100 per kilogram).
Farghal says her next research will involve “circular economy approaches that will be outside the box.” Circular economy is an approach in which waste from one process becomes a resource for the next – that waste doesn’t have to be wasted. This study is itself an example of that thinking: dye-polluted water became the raw material for a new, useful adsorbent.
