Freeze–dried agarose gels: A cheap, simple and recyclable adsorbent for the purification of methylene blue from industrial wastewater

• Agarose is a natural polysaccharide derived from a sustainable source (seaweeds).
• As efficient, but recyclable and cheaper than commercial activated carbon.
• Hydrogel formulation removes dependence on particle/mesh size or need for filtering.
• Freeze-drying simplifies packaging, shipment and long-term storage requirements.
• Conceptually simple and can have rapid industrial translation and uptake.

Dye colourants are being produced yearly on the kilotonne-scale and a significant percentage end up as highly-polluting industrial effluents. Freeze–dried agarose gels are demonstrated here to be efficient adsorbents for the removal of methylene blue, an important industrial dye. A hydrogel adsorbent offers advantages over powdered formulations, which can be difficult to handle. Freeze-drying further allows the adsorbent to be packaged, transported and stored in a dry format, thus conferring cost savings. Parameters such as the volume or concentration of agarose or dye, exposure time, pH and gel/water contact area influenced adsorption capacity and kinetics. Salt inhibited adsorption in a dose-dependent manner and this was exploited for the recycling of adsorbent and dye. Langmuir and Freundlich isotherms were also applied to model the adsorption process. The freeze–dried agarose gel achieved an adsorption capacity of 10.4 ± 0.2 mg/g, which was comparable to commercial activated carbon assessed under similar conditions. Additionally, unlike most activated carbon, agarose is derived from a renewable source. Since agarose is cheaply available commercially, this method can enjoy rapid industrial translation.

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