Molecular weight-dependent heterogeneities in photochemical formation of hydroxyl radical from dissolved organic...

Dissolved organic matter (DOM) is ubiquitous in aquatic ecosystem and characterized by a wide range of molecular weight (MW) distribution. In this study, a total of nine bulk DOM samples, including five International Humic Substances Society (IHSS) standards and four naturally collected samples, were fractionated into low MW (LMW-, <1 kDa) and high MW (HMW-, 1 kDa~0.45 μm) fractions, with MW-dependent heterogeneities in photochemical formation of hydroxyl radical (HO) was investigated. The formation rate of HO (R^HO) for the bulk samples were 4.60-7.27 × 10^-12 M/s/mg-C/L for IHSS standards and 4.63-7.66 × 10^-12 M/s/mg-C/L for naturally collected samples. Regardless of sample types, the LMW fraction was found to exhibit generally higher R^HO values than the HMW counterparts. For IHSS standards, the R^HO decreased from 4.68-8.46 × 10^-12 M/s/mg-C/L for LMW fraction to 3.67-6.66 × 10^-12 M/s/mg-C/L for HMW fraction, and for naturally collected samples, the value of R^HO decreased from 5.21-12.04 × 10^-12 M/s/mg-C/L for LMW fraction to 3.25-6.49 × 10^-12 M/s/mg-C/L for HMW counterpart. A positive correlation between the net R^HO and the normalized intensities of fluorescent peak [Em/Ex: (400-500)/(230-250) nm] was found, showing that the HO formation was strongly coupled to the abundance of humic-like substances. The results indicate that aquatic DOM is an important pool for HO formation, and characterization of MW distribution rather than average MW is thus required to explain the DOM-induced formation potential of HO.