Initial evaluation of high‐pressure ceramic nanofiltration for dissolved organic carbon isolation from surface waters across the land‐to‐ocean continuum

Marine dissolved organic carbon (DOC) is one of the largest reservoirs of fixed carbon on Earth, and its cycling contributes to ocean productivity and carbon storage. Despite its central role, efforts to characterize DOC reactivity and cycling in aquatic systems have been hampered by low recovery during isolation. The most widely applied recovery methods, solid‐phase extraction and ultrafiltration, independently capture less than half of seawater DOC. Here we investigate ceramic nanofiltration as a novel method to isolate DOC from surface waters across the land‐to‐ocean continuum. A bench‐scale prototype system employing a 200 Da pore size ceramic nanomembrane consistently retained > 82% of organic probe molecules (181–376 Da) and > 88% of bulk DOC from diverse surface waters. Salt permeation of the nanomembrane was variable (34–70%) across all surface waters, but highest in seawater (63–70%). Coastal surface seawater was size fractionated using a set of ceramic nanomembranes with pore sizes ranging from 200 to 2000 Da. Radiocarbon analysis of the size fractions revealed that an intermediate size class (i.e., 200–450 Da) is notably older than both smaller and larger size classes and bulk DOC, thereby challenging the size‐reactivity continuum paradigm within low molecular weight coastal DOC (i.e., < 2000 Da). Together, these results suggest that ceramic nanofiltration may have the potential to effectively isolate DOC and remove salts, thus enabling new experimental insights into the cycling of DOC. If scaled, this technology could be applied to greatly expand our understanding of the role of DOC as a key intermediate in the ocean carbon cycle.