Gray Carbon in Effluent: Implications for Marine Carbon Sequestration

Approximately 1,000 km³ of wastewater is generated globally each year, positioning sewage treatment plants as significant sources of dissolved organic carbon (DOC). The carbon sequestration potential of DOC from sewage effluents, particularly its transformation into recalcitrant forms, remains underexplored.Our study used Fourier transform ion cyclotron resonance mass spectrometry to examine the molecular composition of DOC, specifically carboxyl-rich alicyclic molecules (CRAMs), in effluents from sewage treatment plants (eSTP), river discharge outlets (RDO), and estuaries. Results revealed increases in O/C ratios, molecular complexity, and CRAMs as distance from the estuary grew, underscoring estuaries’ role in enhancing organic matter recalcitrance. Dark culture experiments also indicated substantial CRAM formation, though eSTP molecular composition remained stable. Microbial carbon pump (MCP) processes contributed to DOC transformation, underscoring microbial influence.We term the recalcitrant DOC in wastewater “gray carbon,” highlighting its sequestration potential and relevance in carbon trading. Further research on gray carbon’s composition, transformations, and role in blue carbon dynamics will support policies advancing carbon management, technology development, and carbon neutrality goals.