Under water

Natural gel solution effectively removes pollutants from water

Image credit: Sime Basioli | Unsplash

Researchers in Sweden have developed a more eco-friendly way to remove heavy metals, dyes and other pollutants from water.

Researchers from KTH Royal Institute of Technology, in Stockholm, Sweden, in collaboration with Politecnico di Torino, engineered a more sustainable technique for producing hydrogel composites, a type of material that is widely studied for wastewater decontamination.

The solution was found in filtering wastewater with a gel material taken from plant cellulose and spiked with small carbon dots produced in a microwave oven.

Minna Hakkarainen, who leads the Division of Polymer Technology at KTH Royal Institute of Technology, said that the hydrogels remove contaminants such as heavy metal ions, dyes and other common pollutants.

“The total amount of water on Earth doesn’t change with time, but demand does,” she said. “These all-lignocellulose hydrogels offer a promising, sustainable solution to help ensure access to clean water.”

The hydrogel composites can be made from 100 per cent lignocellulose, or plant matter, described by Hakkarainen as the most abundant bioresource on Earth. One ingredient is cellulose gum (carboxymethyl cellulose, or CMC), a thickener and emulsion derived commonly from wood pulp or cotton processing byproducts and used in various food products, including ice cream.

Added to the hydrogel are graphene oxide-like carbon dots synthesised from biomass with the help of microwave heat. The hydrogel composites are then cured with UV light, a mild process that takes place in water at room temperature.

Hydrogels consist of a network of polymer chains that not only absorb water, but also collect molecules and ions by means of electrostatic interactions – a process known as adsorption. According to Hakkarainen, the new process also reinforces the stability of the hydrogel composites so that they can outlast ordinary hydrogels for repeated cycles of water purification.

Before and after. An all-lignocellulose hydrogel before (left) and after it has adsorbed methylene blue from an aqueous solution.

Image credit: Giuseppe Melilli

Graphene oxide has become a favoured additive to this mix, because of its high adsorption capacity, but the environmental cost of graphene-oxide production is high.

“Graphene oxide is a great adsorbent, but the production process is harsh,” Hakkarainen said. "Our route is based on common bio-based raw materials and significantly milder processes with less impact on the environment.”

Graphene is derived from graphite, a crystalline form of carbon. In oxidised form it can be used in hydrogels, but the oxidation process requires harsh chemicals and conditions. Synthesising graphene from biomass often requires temperatures of up to 1,300°C.

By contrast, the researchers at KTH found a way to carbonise biomass at much lower temperatures. They reduced sodium lignosulfate, a byproduct from wood pulping, into carbon flakes by heating it in water in a microwave oven. The water is brought to 240°C and then kept at that temperature for two hours.

Ultimately, after a process of oxidation, they produced carbon dots of about 10 to 80 nanometers in diameter which are then mixed with the methacrylated CMC and treated with UV-light to form the hydrogel.

“This is a simple, sustainable system,” Hakkarainen said. “It works as well, if not better, than hydrogel systems currently in use.”

The research was published in Sustainable Materials and Technologies, Volume 27, April 2021.

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