Lignocellulose hydrogel warms to removing pollutants

Heavy metals, dyes and other pollutants could soon be removed from water with a plant cellulose gel material spiked with carbon dots produced in a microwave.

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

This is the claim of researchers from KTH Royal Institute of Technology in Sweden, in collaboration with Politecnico di Torino, Italy, who said they have engineered a more sustainable technique for producing hydrogel composites for wastewater decontamination. The team’s results are published in Sustainable Materials and Technologies.

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“The total amount of water on Earth doesn’t change with time, but demand does,” said Minna Hakkarainen, who leads the Division of Polymer Technology at KTH. “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 – the most abundant bioresource on Earth, she said in a statement. One ingredient is cellulose gum (carboxymethyl cellulose, or CMC), a thickener and emulsion commonly derived from wood pulp or cotton processing by-products and used in various food products. Added to the hydrogel are graphene oxide-like carbon dots synthesised from biomass with microwave heat. The hydrogel composites are then photocured with UV light in water at room temperature.

Hydrogels consist of a network of polymer chains that can absorb water and also collect molecules and ions via adsorption. Hakkarainen said the new process also reinforces the stability of the hydrogel composites so that they can outlast ordinary hydrogels for repeated cycles of water purification.

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 said to be 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 that in oxidised form can be used in hydrogels. However, the oxidation process requires harsh chemicals and conditions and synthesising graphene from biomass often requires temperatures of up to 1300oC.

The researchers at KTH found a way to carbonise biomass at much lower temperatures by reducing sodium lignosulfate, a by-product from wood pulping, into carbon flakes by heating it in water in a microwave oven. The water is brought to 240oC, and it is kept at that temperature for two hours.

They produced carbon dots of about 10nm to 80nm in diameter, which are then mixed with the methacrylated CMC and treated with UV-light to form the lignocellulose hydrogel.

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