Carbon quantum dots (CQDs) nanofiltration membranes towards efficient biogas slurry valorization

In this article, novel nanofiltration (NF) membranes with highly selective surface and fast water and ion transport channels were fabricated for the valorization of biogas slurry. Unlike conventional thin film composite (TFC) membranes, the novel NF membranes compose hydrophilic carbon quantum dots (CQDs) as the interlayer. The carboxyl groups from CQDs react with the polyethyleneimine (PEI), leading to the lower cross-linking degree in the interfacial polymerization layer of NF membranes. Besides, the presence of the hydrophilic CQDs can create fast water and ion transport channels in the NF membranes. The water permeability of membranes is therefore enhanced significantly from 3.6 to 9.7 LMH·bar⁻¹. The NF membranes were used to concentrate the amino acids from biogas slurry; it achieved rejection rates above 90% towards model amino acids such as lysine, glutamic acid and leucine, due to its highly positive electrical properties, while allowing most of the unwanted salt ions to permeate through. The excellent selectivity between amino acids and salt ions demonstrates the great potential application of the NF membranes in biogas slurry valorization.