Molecular weight-dependent heterogeneities in photochemical formation of hydroxyl radical from dissolved organic matters with different sources

Dissolved organic matter (DOM) is ubiquitous in aquatic ecosystem and characterized by a wide range of molecularweight (MW) distribution. In this study, a total of nine bulkDOMsamples, including five InternationalHumic Substances Society (IHSS) standards and four naturally collected samples, were fractionated into low MW (LMW-, <1 kDa) and highMW(HMW-, 1 kDa similar to 0.45 mu m) fractions, withMW-dependent heterogeneities in photochemical formation of hydroxyl radical (HO center dot) was investigated. The formation rate of HO center dot (R-HO center dot) for the bulk sampleswere 4.60-7.27 x 10(-12) M/s/mg-C/L for IHSS standards and 4.63-7.66 x 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 center dot values than the HMW counterparts. For IHSS standards, the R-HO center dot decreased from 4.68-8.46 x 10(-12) M/s/mg-C/L for LMW fraction to 3.67-6.66 x 10(-12) M/s/mg-C/L for HMW fraction, and for naturally collected samples, the value of R-HO center dot decreased from 5.21-12.04 x 10(-12) M/s/mg-C/L for LMW fraction to 3.25-6.49 x 10(-12) M/s/mg-C/L for HMWcounterpart. A positive correlation between the net R-HO center dot and the normalized intensities of fluorescent peak [Em/Ex: (400-500)/(230-250) nm] was found, showing that the HO center dot formation was strongly coupled to the abundance of humic-like substances. The results indicate that aquatic DOM is an important pool for HO center dot formation, and characterization of MWdistribution rather than average MWis thus required to explain the DOM-induced formation potential of HO center dot. (C) 2020 Elsevier B.V. All rights reserved.