Fatty acids promote fulvic acid intercalation by montmorillonite

Organic matter (OM) adsorbed to the interlamellar surfaces of swelling clay minerals is particularly stable, with residence times as long as 2000 to 10,000 years. Intercalation of OM by smectitic clay minerals may therefore increase the stable C reservoir in both soils and sediments. However, the precise mechanisms of adsorption and intercalation of the myriad organic constituents by swelling clay minerals in soils and sediments remain unresolved. Model adsorption experiments with montmorillonite (Mt), a ubiquitous expanding layer silicate, showed that environmentally relevant fatty acids possessing long-chain alkyl C facilitate the adsorption and subsequent intercalation of fulvic acid (FA). Palmityl palmitate, comprising two C16:0 chains, promotes greater FA adsorption than does stearic acid, the latter possessing a single C18:0 chain. Furthermore, maximum FA interlayer adsorption was observed where the fatty acid was adsorbed to Mt prior to introduction of the humic material; the alkyl C chains of the fatty acid evidently serving as attachment points to enhance uptake and optimize placement of FA at the margins of the interlayer space. These data reveal a previously unknown mechanism through which humic materials become intercalated by expanding layer silicates, thus contributing to the stable OM pool in soils and sediments.