Characterizing graphene oxide waste stream and assessing its impact on anaerobic co-digestion with municipal sludge.

This study evaluated anaerobic co-digestion as a promising strategy for managing organic-contaminated waste streams generated from nanomaterial synthesis. The novel approach enabled precise quantification of organic content, efficient biomethane recovery, and a sustainable redirection of ethanol-contaminated graphene oxide (GO) dispersions. The proposed method achieved high accuracy (93-97%) in detecting organic content in ethanol-contaminated GO dispersions, significantly outperforming the conventional total chemical oxygen demand (tCOD) method, which only reached 75-77% accuracy. Additionally, co-digestion of trace ethanol content in GO dispersions with municipal sludge substantially enhanced methane production kinetics, resulting in a 17.6% increase in specific methane yield (per tCOD added) and a 284% increase in total methane production. Parallel anaerobic digestion (AD) experiments using conductive GO nanosheets (without ethanol) revealed the synergistic impact of GO nanosheets and trace ethanol content as a key mechanism driving these improvements. Furthermore, the study provided evidence of the biological reduction of GO and its magnetite-decorated counterpart, magnetic GO, as indicated by a shift in the I/I ratio from 1.06 to 0.77 and a G-band shift from 1606 cm⁻-1565 cm⁻. This reduction decreased the stability of nanosheets in the AD liquid phase, promoting their partitioning into the solid phase. This process facilitates the adsorption of the GO phase within the digestate and allows for the slow release of micronutrients when used as soil amendments.