Mining, construction wastewater filtered with fibres

Materials World magazine
,
26 Nov 2019

Customisable metal sorbent fibres help improve industrial effluent treatment.

Eliminating micropollutants from wastewater and air applies to a variety of applications, including in the construction and mining sectors, and is important for minimising risks to the environment from industrial processes. However, some systems may become clogged, reducing their performance. Industrial manufacturer, Ajelis, France, has developed fibre materials designed to remove metals.

The material, called Metalicapt, is made of polymer microfibres with their surfaces modified with the chemical groups acid, basic or chelating for different applications. Depending on the secondary treatment of the product, the filtration process works through ion-exchange between the material and the contaminated water. Its fibrous nature allows it to be moulded into a range of end products that can be adapted to filter various wastewater types and to target specific pollutants. The laws of diffusion in condensed matter means it produces better results than existing materials.

Polymer treatment

Polymers, commonly used for making ion-exchange resins, form the basis of the sorbent, but they are processed in a unique way. The acidic, basic and chelating organic groups contained in the fibres form strong bonds with metallic cations, for example carboxylates and phosphates. These groups are either already present in the polymers, or are introduced.

Each polymer group is for different applications. An acidic polymer is used to remove basic pollutants, for example those that are nitrogen-based. Due to its acidic nature, the same polymer reacts with basic solutions, gaining a net negative charge, which makes it useful for removing cationic pollutants. Some negatively charged groups, including R-COO- and R-PO32-, are able to form strong bonds with metallic cations, making them ideal for removing trace metals from water. The performance in chelating groups is boosted if one organic group inside the polymer is able to form multiple bonds with the targeted metallic cations.

In some cases, cage molecules are integrated into fibres to make them selective for different target metals. Such molecules make it possible to offer the targeted metallic cation an environment that matches its requirements, in terms of size and number of surrounding groups. For example, a rare earth cation of a given size in solution usually prefers to be surrounded by eight neighbouring groups – water molecules, if these cations are in aqueous solutions – in a cube-like geometry, with the cation in its centre. Some of the cage molecules developed were designed to include a hollow, cube-shaped central area of a well-defined geometry to selectively extract lanthanide cations.

Once the efficiency of these cage molecules is demonstrated, they are equipped with anchoring groups able to react with complementary groups of the polymer. A mixed solution of the functionalised cage molecules and polymer is thus processed as fibres. These are then cured to clip the molecules inside the polymeric matrix, resulting in a 3D network that immobilise the cage molecules.

The fibres can be made selective for a given metal. The metal-specific ones are made hydrophilic, so the processed solutions can reach the whole material. Under these conditions, the high added value metal solution is flowed through the fibrous network, resulting in the spontaneous concentration of this metal in the fibres.

A strong reaction

The diametre of the fibres influences the product’s performance. By analogy to microelectronics, where performance is related to dimension reduction of the transistors in the same proportion, the smaller fibre diameters compared to ion-exchange resins diameter acts in the same way. The laws of diffusion in condensed matter show an increasing diffusion time as the square of the distance. As such, microfibres with a diameter between 20–60 micrometres allow the diffusion of the micropollutants inside them to be at least 100 times faster than in the case of ion-exchange resins of a 10-times greater diameter.

Reaching the total depth of the sorbent material is an indispensable condition to have a good depolluting capacity. It prompts the question, what would be the performance of a material more than 10 times thinner? This is what is realised by creating nanofibres with a maximum diametre of a few micrometres via force spinning – the manufacturing technology based on a high-speed centrifugation of polymer solution. Based on the principle of candyfloss manufacture, this technology requires special polymer solutions with a precise viscosity and a particular composition.

The successful, high-scale production of very small diametre fibres by force spinning needs a delicate equilibrium to be found between two opposite parameters. These are:

  • The need to make the polymer solution flow through the small holes of the spinning head, requiring a low viscosity for the solutions to be processed, and
  • A sufficiently high viscosity to prevent the formation of droplets instead of fibres, due to the surface tension of the solvent.

An extensive optimisation is needed for each kind of polymer to be processed, as the solvents are different, so as the solubility of these macromolecules. These features enable the fibres to reach better thresholds for lowering micropollutants compared to conventional materials.

Filtering the waste

Wastewater in industries, including surface treatment, chemical, mining, nuclear, building and construction sectors, can be filtered with the product, which can also be used to purify high-value metals including lithium and gold from liquid waste, and mining.

The fibre form can lead to different final textile products. This feature differentiates from other techniques based on granular sorbents such as ion-exchange resin beads, where one charged specie present in the polymeric fibres is replaced by another one of the same sign from the solution to be processed.

Granular materials are not adapted to capture heavy metals released by vehicles on roads or parkings, and may allow such toxins to disperse in runoff water, which will infiltrate the soil and pollute groundwater or rivers downstream. In suburban areas, waterbodies such as ponds or lakes are largely contaminated when situated near urban roads or major highways. Applying the depolluting textile under or in ditches bordering the roads would resolve this.

The situation is also found with agricultural areas that use fertilisers, phytosanitary compounds or pesticides such as glyphosate – a carcinogen. For example, the French Bouillie Bordelaise is a chemical porridge made of copper salts and lime solution. This mixture is an effective remedy to protect vineyards and fruit gardens against diseases that reduce the harvest. In fact, Bordeaux soils in wine-growing areas are heavily contaminated with copper that can be detected at up to 1g/kg in some areas. Application of the textile in the blanket format, called Geocapt, adapted for copper elimination, would help filter out this copper.

The textile can be applied in vertical applications, for example vertical agriculture and urban design. The manufacturer is also working on creating clothing for protecting people from inhaling air pollutants and toxic gases or toxic liquid projections. Ajelis plans to introduce its thin nanofibres into air treatment devices, including for filtering toxic and harmful gases.

Fibre regeneration

Ajelis fibres can be regenerated through using the same process for ion-exchange resins using acid or base solutions. The service life of fibres depends on factors such as the chemical composition of effluent, the waste stream to be treated, number of regeneration cycles, type of material or other conditions it is exposed to. In some cases, it is not desirable to regenerate the used fibres, so they are returned to Ajelis or sent to solid waste treatment companies for disposal. In case of precious metals recycling, for example in gold cyanide recovery from surface treatment waste, the fibre can be incinerated by sending it to a metal refiner to obtain the gold nuggets.

The recovery of precious metals by recycling waste electrical and electronic equipment, the recycling of strategic metals, the decontamination of nuclear liquid-waste effluents, the purification of lithium streams in the South American salt flats are application domains Ajelis is actively working on.