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Researchers develop all lignocellulose hydrogel water purification system

Scientists at the Royal Institute of Technology (KTH) in Sweden, in collaboration with Politecnico di Torino in Italy have engineered a more sustainable technique for producing hydrogel composites, a type of material that is widely being studied for the removal of heavy metals, dyes, and other pollutants in wastewater streams. The research team has developed a hydrogel derived from plant cellulose and spiked with small carbon dots produced in a microwave oven.

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

Professor Minna Hakkarainen who leads the Division of Polymer Technology at KTH, says 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. These all-lignocellulose hydrogels offer a promising, sustainable solution to help ensure access to clean water, said Professor Minna Hakkarainen.

Hydrogels combine absorption with adsorption

“The hydrogel composites can be made from 100 percent lignocellulose, or plant matter – the most abundant bioresource on Earth,” she says. 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 synthesized from biomass with the help of microwave heat. The hydrogel composites are then cured with ultraviolet (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.

Professor Hakkarainen says the new process also reinforces the stability of the hydrogel composites so that they can outlast ordinary hydrogels for repeated cycles of water purification.

The results have been described in a paper Photocurable “all-lignocellulose” derived hydrogel nanocomposites for adsorption of cationic contaminantsthat has been published in the journal Sustainable Material and Technologies.

Low-temperature biomass carbonization

Graphene oxide has become a favored 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. Our route is based on common bio-based raw materials and significantly milder processes with less impact on the environment said Professor Hakkarainen.

Graphene is derived from graphite, a crystalline form of carbon that most people would recognize as the “lead” in pencils. In oxidized form, it can be used in hydrogels but the oxidation process requires harsh chemicals and conditions.

Synthesizing graphene from biomass often requires high temperatures. By contrast, the researchers at KTH found a way to carbonize biomass at much lower temperatures.

They have reduced sodium lignosulfonate, a byproduct of wood pulping, into carbon flakes by heating it in water in a microwave oven. The water is brought to 240 degrees C, and it is 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. It works as well, if not better than hydrogel systems currently in use, ended Professor Minna Hakkarainen.

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