Photochemical Formation Of Hydroxyl Radical Research Articles

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 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 (RHO) 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 RHO values than the HMW counterparts. For IHSS standards, the RHO 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 RHO 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 RHO 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.

Temperature dependence of the photochemical formation of hydroxyl radical from dissolved organic matter.

The temperature dependence of the photochemical production of the hydroxyl radical (•OH) from dissolved organic matter (DOM) was investigated by measuring the apparent temperature dependence of the quantum yield (Φa) for this process. Temperature dependent Φa values were analyzed using the Arrhenius equation. Apparent activation energies obtained for DOM isolates purchased from the International Humic Substances Society ranged from 16 to 34 kJ mol(-1). Addition of 40 units mL(-1) catalase, used to hinder the hydrogen peroxide (H2O2)-dependent pathway to •OH, did not impact the observed activation energy. However, an increase in activation energy was observed in lower molecular weight DOM obtained by size fractionation. We also measured the temperature dependence of p-benzoquionone photolysis as a model compound for DOM and observed no temperature dependence (slope p = 0.41) for the formation of phenol from oxidation of benzene (the •OH probe used), but a value of about 10 kJ mol(-1) for p-benzoquinone loss, which is consistent with formation of a quinone-water exciplex. These data provide insight into DOM photochemistry as well as provide parameters useful for modeling steady state •OH concentrations in natural systems.

Correction to Photochemical Formation of Hydroxyl Radical from Effluent Organic Matter: Role of Composition

Correction to Photochemical Formation of Hydroxyl Radical from Effluent Organic Matter: Role of Composition

Photochemical Formation of Hydroxyl Radical from Effluent Organic Matter: Role of Composition

The photochemical formation of hydroxyl radical (HO(•)) from effluent organic matter (EfOM) depends upon the chemical properties of this heterogeneous mixture. In this study, two EfOM samples collected from wastewater treatment plants (WWTP A and B) were fractionated by both hydrophobicity (bulk and non-humic) and apparent molecular weight (AMW). The apparent quantum yield for HO(•) formation (ΦHO(•)) and the maximum fluorescence quantum yield (ΦF) were subsequently measured for each subfraction. The formation rates of HO(•) (considering only the hydrogen-peroxide-independent pathways) for the bulk waters were 4.8 × 10(-10) and 9.6 × 10(-11) M s(-1) for WWTP A and B, respectively. For the AMW fractions, the values of ΦHO(•) increased as the AMW of the material decreased. For the WWTP A sample, the ΦHO(•) increased from 2.54 × 10(-4) (bulk water) to 6.29 × 10(-4) for the <1 kDa fraction, and for the WWTP B sample, the value of ΦHO(•) increased from 6.50 × 10(-5) for bulk water to 3.45 × 10(-4) for the <1 kDa fraction. In the case of fluorescence, the values of ΦF ranged from 2.37 × 10(-4) (bulk water) to 3.48 × 10(-4) (<1 kDa fraction) for WWTP A and 3.19 × 10(-4) (bulk water) to 5.75 × 10(-4) (<1 kDa fraction) for WWTP B. There was a linear correlation between ΦHO(•) and ΦF, suggesting that different photophysical processes occur in the chemical components of the fractions. Understanding the formation of HO(•) from EfOM is essential for understanding wastewater-impacted aquatic systems because these results influence the photochemical degradation and mineralization of trace organic contaminants.

Photochemical Formation of Hydroxyl Radical from Effluent Organic Matter

The photochemical formation of hydroxyl radical (HO•) from effluent organic matter (EfOM) was evaluated using three bulk wastewater samples collected at different treatment facilities under simulated sunlight. For the samples studied, the formation rates of HO•(R(HO•)) were obtained from the formation rate of phenol following the hydroxylation of benzene. The values of R(HO•) ranged from 2.3 to 3.8 × 10(-10) M s(-1) for the samples studied. The formation rate of HO• from nitrate photolysis (R(NO3)(HO•)) was determined to be 3.0 × 10(-7) M(HO)• M(NO3)(-1) s(-1). The HO• production rate from EfOM (R(EfOM)(HO•)) ranged from 0.76 to 1.3 × 10(-10) M s(-1). For the wastewater samples studied, R(EfOM)(HO•) varied from 1.5 to 2.4 × 10(-7) M(HO)• M(C)(-1) (s-1) on molarcarbon basis, which was close to HO• production from nitrate photolysis. The apparent quantum yield for the formation of HO• from nitrate (Φ(NO3-HO•)(a)) was determined as 0.010 ± 0.001 for the wavelength range 290-400 nm in ultrapure water. The apparent quantum yield for HO• formation in EfOM (Φ(EfOM-HO•)(a)) ranged from 6.1 to 9.8 × 10(-5), compared to 2.99 to 4.56 × 10(-5) for organic matter (OM) isolates. The results indicate that wastewater effluents could produce significant concentrations of HO•, as shown by potential higher nitrate levels and relatively higher quantum yields of HO• formation from EfOM.

Photochemical Formation of Hydroxyl Radicals in Tissue Extracts of the Coral Galaxea fascicularis

Various stresses induce the formation of reactive oxygen species (ROS) in biological cells. In addition to stress-induced ROS, we studied the photochemical formation of hydroxyl radicals (˙OH), the most potent ROS, in coral tissues using phosphate buffer-extracted solutions and a simulated sunlight irradiation system. ˙OH formation was seen in extracts of both coral host and endosymbiont zooxanthellae. This study is the first to report quantitative measurements of ˙OH photoformation in coral tissue extracts. Our results indicated that whether or not coral bleaching occurred, coral tissues and symbiotic zooxanthellae have the potential to photochemically produce ˙OH under sunlight. However, no significant difference was found in the protein content-normalized formation rates of ˙OH between corals incubated under different temperatures and irradiance conditions. ˙OH formation rates were reduced by 40% by reducing the UV radiation in the illumination. It was indicated that UV radiation strongly affected ˙OH formation in coral tissue and zooxanthellae, in addition to its formation through photoinhibition processes.

Photochemical Formation of Hydroxyl Radicals in Red Soil-Polluted Seawater on the North of Okinawa Island, Japan

Land development has caused runoff of red soil into the ocean on the north side of Okinawa Island, Japan. In an attempt to clarify the impacts of this “red soil pollution” on the oxidizing power of seawater, we studied the formation of hydroxyl radical (•OH), the most potent oxidant in the environment, in red soil-polluted waters using a 313-nm monochromatic light. •OH was photochemically formed in the red soil-polluted water samples, and the formation rates of •OH decreased as salinity increased, i.e., as red soil-polluted river water gets mixed with seawater. The photo-formation rates of •OH showed good correlations with dissolved Fe concentrations (R 2 = 0.96) and [NO2 −] + [NO3 −] concentrations (R 2 = 0.87), while a negative and weak correlation was found with dissolved organic carbon concentrations (R = −0.78). Theoretical calculation showed that direct photolysis of NO3 −, Fe(OH)2+, and hydrogen peroxide all together accounted for less than 10% of the observed •OH formation in the red soil-polluted waters. Comparison between filtered and unfiltered samples showed that red soil particles were not the main sources of •OH, and the photolysis of NO2 − could account for at most 78% of the observed •OH formation rates. We found that the Fenton’s reaction (a reaction between Fe(II) and H2O2) could possibly account for the observed formation of •OH in the red soil-polluted waters.

Photochemical formation of hydroxyl radicals catalyzed by montmorillonite

In this work, the photooxidation of benzene and the formation of phenol in aqueous suspensions of the iron-rich montmorillonite under irradiation of a 250 W metal halide lamp ( λ ⩾ 365 nm) were investigated. We confirmed that hydroxyl radicals ( OH) were produced by illuminating montmorillonite and was responsible for the photooxidation of benzene in aqueous suspensions of montmorillonite. Low pH value facilitated the formation of hydroxyl radicals in the pH range of 2.0–10.0. The OH concentration increased with increasing the concentration of montmorillonite in aqueous solutions in the range of 0–20.0 g l −1. Higher concentration like 25.0 g l −1 montmorillonite inhibited the OH production. Iron, predominantly free iron in the clays, is believed to be one of the most important factors determining OH formation. Structural irons in montmorillonite have contributions to OH formation, especially in the presence of carboxylate ions. The formation of OH from montmorillonite under irradiation of near UV and visible light indicates that clays might play important role not only in transfer through adsorption but also in transformation through oxidation of organic compounds on the surface of clay particles in air, water, soil or even top sediments.

Sources and sinks of hydroxyl radical in sea‐salt particles

We have examined the photochemical formation of hydroxyl radical (•OH) in aqueous extracts of supermicron sea‐salt particulate matter (SS PM) collected from the coast of northern California. All extracts formed •OH during illumination, indicating that this process is widespread in sea‐salt particles. Scaling extract results to SS PM conditions reveals that in situ rates of •OH photoformation are typically 100–1000 μM hr−1 in midlatitude sea‐salt particles (summer, midday, 88% relative humidity). These rates are comparable to calculated rates of partitioning of gas phase •OH to the particles and are 3–4 orders of magnitude greater than •OH photoformation rates in surface seawater. Photolysis of nitrate was a dominant source of •OH in the particle extracts, accounting for an average of 59 ± 25% of its formation. The other sources of •OH have not been identified, but photolysis of organic compounds derived from seawater is likely important. The lifetimes of •OH in the sea‐salt particles are of the order of 10−9–10−8 s and are primarily controlled by reactions with unidentified, but probably organic, compounds. Bromide and chloride are also significant sinks of •OH, typically accounting for approximately 25% of its loss. The rapid formation and destruction of •OH in sea‐salt particles likely significantly affects the chemistry of halides, organic compounds, and other reduced species in SS PM. In turn, these particle reactions probably alter the budgets of gases such as ozone and volatile organic compounds in the marine boundary layer.

Chemical composition and photochemical formation of hydroxyl radicals in aqueous extracts of aerosol particles collected in Okinawa, Japan

We investigated the chemical composition and photochemical formation of hydroxyl (OH) radicals in the water-soluble fractions (WSF) of aerosol particles collected in Okinawa, Japan. Bulk aerosol samples were collected for 2–7 days at a time by a high-volume air sampler over a 3-month period. Major ions present in the WSF solutions were SO 4 2−, Na +, and Cl −. Sulfate ion concentrations were much higher when Yellow Sand events occurred. The mass-based Cl −/Na + ratio found in the WSF solutions averaged 49.7%, much lower than the ratio in seawater, indicating that chlorine was lost from the aerosol particles. A negative correlation ( R=−0.67) was found between the Cl −/Na + ratio and the concentration of non-sea-salt–SO 4 2−. We confirmed the photochemical formation of OH radicals in the study samples using illumination experiments at 313 nm. The apparent quantum yields of OH radical photoformation, based on the total absorbance at 313 nm, ranged from ND to 0.0017, with a mean±1 SD of 0.0010±0.0005. Hydroxyl radical photoformation rates from nitrate and nitrite photolyses, estimated based on nitrate and nitrite ion concentrations and our illumination conditions, averaged 32±24% and <10%, respectively, of the total formation rates. Hydroxyl radical photoformation rates were strongly correlated with total dissolved iron concentrations ( R=0.88). A correlation also existed between OH radical photoformation rates and dissolved organic carbon concentrations ( R=0.69).

PH Dependent Photoformation of Hydroxyl Radical and Absorbance of Aqueous-Phase N(III) (HNO2 and NO2-)

Ultraviolet−visible (UV−Vis) absorption spectra of aqueous-phase N(III) (HNO2 and NO2-) were studied for the typical pH ranges observed in the atmospheric waters. The molar absorptivity of HNO2 is larger than that of NO2- in UV-A regions. The N(III) molar absorptivity at a specific pH can be determined based on the molar absorptivities and acid−base equilibrium of HNO2 and NO2- (pKa = 3.27). Photochemical formation of hydroxyl radicals (OH radicals) was also studied in aqueous solutions of N(III) for pH values between 1.9 and 6.2 (seven pH values). The OH radical photoformation rate constants showed a distinctive pH dependence, approximately 10-fold higher at pH = 1.9 than at pH = 6.2. The pH-dependent OH radical photoformation also followed the speciation pattern of HNO2 and NO2-. Hydroxyl radical photoformation rate constants (±SE) were estimated to be (3.1 ± 0.08) × 10-4 s-1 for HNO2 and (3.2 ± 0.61) × 10-5 s-1 for NO2- for vernal equinox solar noon conditions at 34° N (Higashi-Hiroshima), using the le...

Photochemical Formation of Hydroxyl Radical by Constituents of Natural Waters

A new method is employed to determine the rates of photochemical hydroxyl radical (OH) formation in aqueous solutions and in natural waters under both aerobic and anaerobic conditions. Quantum yields for OH formation from the photolysis of nitrate and nitrite obtained by this method are in good agreement with previous measurements. Photolysis of Suwannee River fulvic acid (SRFA) solutions produced the hydroxyl radical under anaerobic conditions in proportion to the SRFA concentration. Under aerobic conditions, the quantum yields for OH formation were slightly higher and exhibited a different wavelength dependence than those obtained under anaerobic conditions. Experiments employing catalase indicate that Fenton chemistry can account for at most 50% of the total signal under aerobic conditions for SRFA irradiated at 310 and 320 nm. These results indicate the presence of a dioxygen-independent pathway of hydroxyl radical production that cannot be assigned to nitrate/nitrite photolysis or to Fenton chemistry...

Aqueous-phase photochemical formation of hydroxyl radical in authentic cloudwaters and fogwaters

Aqueous-phase photochemical reactions initiated by simulated sunlight and 313-nm light form hydroxyl radical ( . OH) in authentic cloudwater and fogwater samples from various locations in the U.S. The quantum yields for . OH formation at 313 nm and the photoformation rates of . OH under sunlight conditions were quantified by the formation of phenol from the . OH-mediated oxidation of added benzene. Based on this technique, the measured photoformation rates of . OH several cloudwaters and fog-waters ranged from 0.32 to 3.0 μM h −1 under midday equinox sunlight conditions. These rates of aqueous-phase . OH photoformation are of the same magnitude as previously published estimated rates of . OH incorporation from gas-to-drop partitioning of . OH and from the aqueous-phase decomposition of O 3 transfered from the gas to the drop