Reduced graphene oxide supported on polyurethane foams for metal recovery from e-waste

The growth of global population and digitalization is rising the global demand of materials. In the last decade, critical raw materials (CRMs) have received much attention, due to their importance in strategic sectors such as the transition to climate change, aerospace, and safety. Waste from electric and electronic equipment (WEEE) represents an enormous resource due to their wide metal composition. However, nowadays, the majority of waste materials is wrongly discarded or misplaced, producing a huge loss of valuable metals. WEEE can be treated in different ways to recover the contained critical raw materials, spanning from pyrometallurgical treatments to hydrometallurgical ones. The adsorption step, used into hydrometallurgical plants after various leaching steps, is a versatile technique because of its high recovery efficiency, short extraction time, high enrichment factor, simplicity, and low cost. The adsorption process makes also use of a wide variety of sorbents with different characteristics, such as silica- and carbon-based, green, and polymeric materials. Graphene based materials have shown a great potential in this field of application, also owing to their high specific surface area. In this work, we exploit the adsorption behaviour of a 3D object composed by reduced graphene oxide (rGO) supported on commercial polyurethane (PU) flexible foams of 20 PPI. PU foams have been dip coated with a rGO aqueous dispersion, obtained after a mild reduction of commercial GO with L-ascorbic acid, performed under magnetic stirring at room temperature and pressure. The excess of dispersion on the foam has been removed by the use of compressed air; then, the coating has been consolidated in the oven at a temperature of 40 °C for 1 hour. The foams have been used to remove and recover different metals that can be found in e-waste. Lanthanum, copper, and silver were selected as representative of e-waste critical (and strategic) raw materials. Adsorption was carried out on simulated mono- and multi- ionic solutions at different initial concentrations, from 1 mg/L to 1 g/L. The volume of the contacted solution was 100 mL. The experiments were performed under magnetic stirring at room temperature and pressure. The solution was analysed via ICP-OES (Inductively Coupled Plasma – Optical Emission Spectroscopy) before and after the adsorption experiment, to determine its composition. Preliminary results revealed a higher affinity of the rGO-coated foams for silver ions, being able to reach a capture efficiency of 59 wt.%. Moreover, SEM analysis performed at high magnifications, ranging from 5 kX to 25 kX, revealed the presence of silver clusters on the surface of rGO.