Photooxidation-Induced Changes in Optical, Electrochemical, and Photochemical Properties of Humic Substances

Abstract

Two aquatic fulvic acids and one soil humic acid were irradiated to examine the resulting changes in the redox and photochemical properties of the humic substances (HS), the relationship between these changes, and their relationship to changes in the optical properties. For all HS, irradiation caused photooxidation, as shown by decreasing electron donating capacities. Photooxidation was accompanied by decreases in specific UV absorbance and increases in the E2/E3 ratio (254 nm absorbance divided by that at 365 nm). In contrast, photooxidation had little effect on the samples' electron accepting capacities. The coupled changes in optical and redox properties for the different HS suggest that phenols are an important determinant of aquatic HS optical properties and that quinones may play a more important role in soil HS. Apparent quantum yields of H2O2, ·OH, and triplet HS decreased with photooxidation, thus demonstrating selective destruction of HS photosensitizing chromophores. In contrast, singlet oxygen ((1)O2) quantum yields increased, which is ascribed to either decreased (1)O2 quenching within the HS microenvironment or the presence of a pool of photostable sensitizers. The photochemical properties show clear trends with SUVA and E2/E3, but the trends differ substantially between aquatic and soil HS. Importantly, photooxidation produces a relationship between the (1)O2 quantum yield and E2/E3 that differs distinctly from that observed with untreated HS. This finding suggests that there may be watershed-specific correlations between HS chemical and optical properties that reflect the dominant processes controlling the HS character.

Triplet Photochemistry of Effluent and Natural Organic Matter in Whole Water and Isolates from Effluent-Receiving Rivers

“…56,57 Recently, quantum yields of 1 O 2 have been shown to correlate negatively to EDC for several OM samples undergoing photooxidation. 67 Such a relationship was also observed here between EDC and both f TMP and Φ 1O2 for isolated EfOM and DOM, and a similar trend was observed with reference fulvic and humic acids ( Figure 5). These trends are consistent with the current model for the structural basis of DOM photochemistry, i.e., that electron donating moieties participate in charge transfer complexes 29,69,71 and that the formation of 3 DOM* is impaired in DOM that has a high abundance of these complexes.…”

“…A positive correlation between Φ 1O2 and E 2 /E 3 for organic matter samples was reported previously 25,29,30,67,69 and attributed to the efficiency of 3 DOM* formation from OM precursor moieties. It is hypothesized that intermolecular complexes between electron-donating and accepting moieties in OM result in long-wave absorption (low E 2 /E 3 ) that is inefficient at producing triplet state photochemistry.…”

“…It is hypothesized that intermolecular complexes between electron-donating and accepting moieties in OM result in long-wave absorption (low E 2 /E 3 ) that is inefficient at producing triplet state photochemistry. 67 Although correlations between f TMP and E 2 /E 3 ratios have not been examined in the literature, a positive correlation should exist because of the fact that 1 O 2 is produced by 3 DOM*. Indeed, both f TMP and Φ 1O2 correlated positively with E 2 /E 3 for our complete sample set (Figure 4).…”

Photobleaching-induced changes in photosensitizing properties of dissolved organic matter

“…Previous studies have also investigated changes in DOM characteristics during irradiation (Del Vecchio and Blough, 2002;White et al, 2003;Brinkmann et al, 2003;Loiselle et al, 2012;Sharpless et al, 2014). Understanding the impact of these changes on the photochemical properties of DOMs will help to predict a Values for 20 mg C/L are presented before correction factor calculation.…”

Aquatic photooxidation of phosphite in the presence of ferric and oxalate ions

“…Furthermore, 2-propanol also shuts down other downstream reactions such as productions of singlet oxygen ( 1 O 2 ), which is initiated by Å OH [30]. Some researchers used NaN 3 or FFA as a quencher of 1 O 2 in photoreaction [31][32][33]. But some researchers also thought FFA or NaN 3 could react with other radicals like ÁOH [34,35].…”