Binding of Cu, Co, and Cs to fluorescent components of natural organic matter (NOM) from three contrasting sites

Aqueous natural organic matter (NOM) contains different types of functional groups (carboxylic, phenolic, sulfidic, etc.), and hence could change the speciation of metals in environmental systems. This work is a proof-of-concept study on the interaction of three metals (Cu, Co, and Cs) with NOM using fluorescence spectroscopy. The specific aim was to determine the conditional stability constants for these three metals with NOM optical components, obtained from the quenching of fluorescence signals. Three contrasting water types were sampled in Northern Ontario: a pristine source (Cross Rd.), an urban-impacted source (Junction Creek), and an industrially impacted creek (Copper Cliff creek). In this investigation, Cu²⁺ was used as a benchmark, whereas Co²⁺ and Cs⁺ analyses were novel applications of this technique. Humic-like (H-like; terrestrial and microbial), fulvic-like (F-like), and protein-like (P-like) fluorescence components were found in various proportions at the three sampling sites. For these samples, the fluorescence signals of the H-, F-, and P-like components were quenched upon additions of Cu²⁺. The computed conditional stability constants (as log K^c) ranged from 4.46 to 6.06. In contrast, K^c values with Co²⁺ were measurable only for the two H-like components of the pristine sample (log K^c 3.02-4.05). Cesium (Cs⁺) induced quenching only for the P-like component at the industrial-impacted site (log K^c 4.82-5.03). While this study corroborates earlier reports that Cu²⁺-NOM interactions can be measured by fluorescence, we are showing for the first time a direct chemical interaction of Co²⁺ and Cs⁺ with specific NOM components, as reported by fluorescence quenching.