[show abstract][hide abstract] ABSTRACT: The occurrence of Cl2(-·) in natural waters would depend on the budget between triplet-sensitised photogeneration (which might have second-order rate constant of 10(7)-10(9) M(-1) s(-1)) and scavenging by dissolved organic matter (DOM, with possible rate constant of 10-10(3) L (mg C)(-1) s(-1)). The steady-state [Cl2(-·)] in brackish to saline waters might be in the range of 10(-14)-10(-12) M in mid-latitude summertime, coherently with data of phenol photochlorination in seawater. Steady-state [Cl2(-·)] would be enhanced by chloride (up to a plateau above 0.1 M Cl(-)) and inhibited by DOM. The radical Cl2(-·) would also be a major oxidant of nitrite to the nitrating agent (·)NO2 in brackish- and salt-water. This issue may explain the sustained formation of nitrophenols in phenol-spiked seawater and in natural brackish waters impacted by phenolic pollutants (Rhône delta, Southern France).
[show abstract][hide abstract] ABSTRACT: The present work describes in details the chemical structure of the complex Fe(III)-EDDS and the predominance of different species with respect to pH. These results were obtained with Ab initio calculations. From the photoredox process, the formation of hydroxyl radical was confirmed and HO● is the main species responsible for the degradation of the organic compound present in aqueous solution. The degradation of 4-tert-butylphenol (4-t-BP), used as a model pollutant, was investigated in different conditions. For the first time the second-order rate constant of the reaction between HO● and 4-t-BP and the formation rate of HO● (Rf•OH) from the photochemical process were evaluated. Through the degradation of 4-t-BP, the effect of Fe(III)-EDDS concentration, oxygen and pH was also investigated. The pH, which play a role on the iron cycle and on the Fe(III)-EDDS speciation, was noticed as an important parameter for the efficiency of 4-t-BP degradation. Such result could be explained taking into account the complex speciation and presence of a predominant form (FeL-) up to pH 8. These results are very useful for the use optimization of such iron complex in water treatment process.
The Journal of Physical Chemistry A 12/2013; · 2.77 Impact Factor
[show abstract][hide abstract] ABSTRACT: The sunlight filter benzophenone-4 (BP-4) is present in surface waters as two prevailing forms, the singly deprotonated (HA-) and the doubly deprotonated one (A(2-)), with pKa2 = 7.30 ± 0.14 (μ ± σ, by dissociation of the phenolic group). In freshwater environments, BP-4 would mainly undergo degradation by reaction with OH and direct photolysis. The form HA(-) has a second-order reaction rate constant with OH (kOH) of (1.87 ± 0.31)·10(10) M(-1) s(-1) and direct photolysis quantum yield Φ equal to (3.2 ± 0.6)·10(-5). The form A(2-) has (8.46 ± 0.24)·10(9) M(-1) s(-1) as the reaction rate constant with OH and (7.0 ± 1.3)·10(-5) as the photolysis quantum yield. The direct photolysis of HA(-) likely proceeds via homolytic breaking of the O-H bond of the phenolic group to give the corresponding phenoxy radical, as suggested by laser flash photolysis experiments. Photochemical modelling shows that because of more efficient direct photolysis (due to both higher sunlight absorption and higher photolysis quantum yield), the A(2-) form can be degraded up to 3 times faster than HA(-) in surface waters. An exception is represented by low-DOC (dissolved organic carbon) conditions, where the OH reaction dominates degradation and the transformation kinetics of HA(-) is faster compared to A(2-). The half-life time of BP-4 in mid-latitude summertime would be in the range of days to weeks, depending on the environmental conditions. BP-4 also reacts with Br2(-), and a rate constant [Formula: see text] was measured at pH 7.5. Model results show that reaction with Br2(-) could be a potentially important transformation pathway of BP-4 in bromide-rich (e.g. seawater) and DOM-rich environments.
[show abstract][hide abstract] ABSTRACT: The occurrence of sunscreen agents in natural environment is of scientific concern recently due to their potential risk to ecology system and human beings as endocrine disrupting chemicals (EDCs). In this work the photodegradation mechanism and pathways of sunscreen agent 2-phenylbenzimidazole-5-sulfonic acid (PBSA) were investigated under artificial solar irradiation with the goal of assessing the potential of photolysis as a transformation mechanism in aquatic environments. The quantum yield of PBSA direct photolysis in pH 6.8 buffer solution under filtered mercury lamp irradiation was determined as 2.70 × 10(-4). Laser flash photolysis (LFP) experiments confirmed the involvement of PBSA radical cation (PBSA(+)) during direct photolysis. Acidic or basic condition facilitated PBSA direct photolysis in aqueous solution. Indirect photolysis out-competes direct photolysis as a major process for PBSA attenuation only at higher level of photosensitizers (e.g., NO3(-) > 2 mM). Thus, direct photolysis is likely to be the major loss pathway responsible for the elimination of PBSA in natural sunlit surface waters, while indirect photolysis (e.g., mediated by HO) appeared to be less important due to a general low level of steady-state concentration of HO ([HO]ss) in natural surface waters. Direct photolysis pathways of PBSA includes desulfonation and benzimidazole ring cleavage, which are probably initiated by the excited triplet state ((3)PBSA(∗)) and radical cation (PBSA(+)). Conversely, hydroxylation products of PBSA and 2-phenyl-1H-benzimidazole as well as their ring opening intermediates were found in nitrate-induced PBSA photolysis, suggesting the indirect photodegradation was primarily mediated by HO and followed a different mechanism.
[show abstract][hide abstract] ABSTRACT: In this paper we investigated the degradation of the rivastigmine drug induced by hydroxyl radical in synthetic and natural waters focusing on both reactivity and photoproducts identification. The hydroxyl radical formation rate was quantified by using terephthalic acid as trapping molecule and it was related with the rivastigmine degradation rate. The second order rate constant between hydroxyl radical and rivastigmine was estimated to be ∼5.8 × 10(9) M(-1) s(-1). Irradiation of rivastigmine in three natural waters (rain, lake and river) and comparison with degradation rates observed in synthetic solutions using nitrite, nitrate and hydrogen peroxide suggest that, in addition to hydroxyl radical, also nitroderived radicals (NO/NO2) are responsible for the pollutant degradation in natural media. In fact, the evaluated degradation rates in three natural waters are greatly higher than those estimated considering only the reactivity with photogenerated hydroxyl radical. Using nitrites and nitrates as photochemical OH source, the rivastigmine degradation cannot be described considering only the hydroxyl radical reactivity suggesting that NO and NO2 radicals could play a key role during indirect degradation. Moreover main degradation products have been identified by means of HPLC-MS. Hydroxylation of the aromatic ring as well as carbamate and amino chain oxidation were suggested as main reaction mechanisms, but also nitroderived compounds were characterized. Finally polychromatic irradiations of three rivastigmine doped natural waters (rain, river and lake) underlined the role of the indirect degradation that needs to be considered when direct degradation of selected pollutants is negligible under environmental-like conditions.
[show abstract][hide abstract] ABSTRACT: The nitroderivatives of polycyclic aromatic hydrocarbons are potentially important photosensitisers in the atmospheric condensed phase. Here we show that the triplet state of 1-nitronaphthalene (31NN*) is able to directly react with phenol, causing its transformation upon irradiation of 1NN in aqueous solution. Additional but less important processes of phenol degradation are reactions with OH and 1O2, both photogenerated by irradiated 1NN. Dihydroxybiphenyls and phenoxyphenols were detected as main phenol transformation intermediates, likely formed by dimerisation of phenoxy radicals that would be produced upon phenol oxidation by 31NN*. Very interestingly, irradiation with 1NN shifted the fluorescence peaks of phenol (Ex/Em = 220–230/280–320 nm and 250–275/280–320 nm, with Ex/Em = excitation and emission wavelengths) to a region that overlaps with “M-like” fulvic substances (Ex/Em = 250–300/330–400 nm). Moreover, at longer irradiation times a further peak appeared (Ex/Em = 300–450/400–450 nm), which is in the region of HULIS fluorescence. Irradiated material was also able to photoproduce 1O2, thus showing photosensitisation properties. Therefore, compounds with fluorescence properties that closely resemble those of HULIS (they would be identified as HULIS by fluorescence if present in environmental samples) can be formed upon triplet-sensitised transformation of phenol by 1NN.
[show abstract][hide abstract] ABSTRACT: The present work describes, for the first time, the use of a new and strong complexing agent, Ethylenediamine-N,N'-disuccinic acid (EDDS) in the homogeneous Fenton process. The effect of H2O2 concentration, Fe(III)-EDDS concentration, pH value and oxygen concentration on the homogeneous Fenton degradation of Bisphenol A (BPA) used as a model pollutant, was investigated. Surprisingly, the performance of BPA oxidation in an EDDS-driven Fenton reaction was found to be much higher at near neutral or basic pH than at acidic pH. Inhibition and probe studies were conducted to ascertain the role of several radicals (e.g.•OH, HO2•/O2•-) on BPA degradation. This unexpected effect of pH on Fenton reaction efficiency could be due to the formation of HO2• or O2•- radicals and to the presence of different forms of the complex Fe(III)-EDDS as a function of pH. Indeed, the reduction of Fe(III)-EDDS to Fe(II)-EDDS is a crucial step that governs the formation of hydroxyl radical, mainly responsible for BPA degradation. In addition to its ability to maintain iron in soluble form, EDDS acts as a superoxide radical-promoting agent, enhancing the generation of Fe(II) (the rate limiting step) and therefore the production of •OH radicals. These results are very promising because they offer an important new treatment option at higher range of pH values and more particularly at pHs encountered in natural conditions.