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Retention of toxic pollutants by nanomagnetite aggregates

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Novel insights into magnetite’s water cleansing properties

Using pioneering techniques, REPONANO investigated magnetite’s abilities to decontaminate not only drinking water, but also acidic and phosphate-rich wastewater.

The United Nations made access to clean water a human right in 2010, and Goal Six of the Sustainable Development Goals is to 'ensure access to water and sanitation for all’. REPONANO’s water remediation results contribute to ensuring the success of these milestones. The team behind the REPONANO project has tested a new way of cleansing water of contaminants using the iron-oxide mineral magnetite. "In REPONANO, as well as improving magnetite remediation processes generally, we wanted to miniaturise the process. So we developed a microfluidic set up, taking advantage of new spectroscopic developments to visualise how polymer-nanomagnetite aggregates remove contaminants,” says researcher Evgenia-Maria Papaslioti of the Marie Skłodowska-Curie Actions-funded project. Magnetite can decontaminate drinking and waste-water in three ways: firstly by changing the composition of water: magnetite causes toxic particles to dissipate before falling to the bottom of the water for extraction. Nanomagnetite particles can also be used in microfilters, preventing the passage of contaminants. Lastly, magnetite’s oxygen-bonding properties can form what are called iron (hydr)oxide aggregates which bind to contaminant particles, removing them from the water. This method has typically been studied in large scale flow-through or batch systems which have shown great potential for removing toxic heavy metals, like selenium, lead and arsenic.

Largescale batch testing

REPONANO conducted large-scale batch experiments to test the efficiency of magnetite in immobilising toxic elements like arsenic, antimony, and uranium in highly acidic and phosphate-rich wastewaters. Helping to promote ‘green’ processes, the magnetite used had been recycled from the steel industry, thanks to a process patented by HYMAG’IN. “In our novel three-step technique, we first showed that it is possible to use zero-valent iron to decrease acidity. Then we removed the phosphate by using it to form the mineral ‘vivianite’, before adding the magnetite to immobilise the contaminants," adds Papaslioti.

Microfluidics testing

As hydrodynamics are best investigated at the microscale, the team collaborated with the Institute of Physics of Rennes to develop a microfluidic polymer device to study water decontamination by tiny magnetite particles (nanomagnetite). Already considered effective decontaminates, the team wanted to explore whether polymeric coatings could boost the decontamination capacity of nanomagnetite. So they created cylindrically-shaped aggregates made of a hydrogel (polyethylene glycol) mixed with chitosan-coated magnetite nanoparticles. “Nanomagnetite is already used as a filter material by some waste water treatment plants. However, its small size means it tends to go through filters or to plug them,” explains Laurent Charlet, project coordinator. “By increasing their size and changing their shape, our hybrid materials could stop this, while also making it easier to filter them out.” Flow-through experiments were used to test the effectiveness of these polymer-nanomagnetite aggregates at immobilising antimony. To map the antimony distribution and amount sorbed to the aggregates, as well as get its oxidation state (which drives antimony immobilisation and removal from the water), synchrotron X-ray absorption spectroscopy and X-ray micro-fluorescence were used for the first time in such systems. A 3D computer model was then developed to simulate and validate how antimony was diffused and distributed during these experiments. Identifying the specific zones where Antimony reacts with the nanomagnetite offers insights into the optimal shape and composition of the aggregates. “This holds out promise for not only more effective ways of observing natural systems but also for new water treatments based on microscopic devices, like microfluidics,” says Papaslioti. The team intend to conduct more sorption experiments with a wider range of contaminants and conditions, varying concentrations of contaminants/nanomagnetite, pH, flow rates and experimenting with aggregate composition and shape.

Keywords

REPONANO, aggregate, magnetite, water, remediation, contaminant, fertiliser, microfluidic, microfilters, Antimony, sorb

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