Aquatic indirect photochemical transformations of natural peptidic thiols: impact of thiol properties, solution pH, solution salinity and metal ions

Abstract

Natural peptidic thiols play numerous important roles in aquatic systems. While thiols are known to be susceptible to sensitized photoreaction, the photochemical transformation of thiols in surface waters remains largely unknown. This study systematically assessed the photochemical transformation of naturally occurring thiols, including arginylcysteine (RC), γ-glutamylcysteine (γEC), glutathione (GSH), and phytochelatin (PC) in solutions containing dissolved organic matter (DOM). The results show that all thiols underwent rapid indirect photochemical transformation. The transformation rates of thiols were highly pH-dependent and increased with increasing solution pH. γEC and GSH show lower transformation rates than free Cys, which was ascribed to their higher thiol pK values. In comparison, PC and RC show much higher transformation rates than γEC and GSH, due to more reactive thiol groups contained in the PC molecule and sorption of RC to DOM macromolecules, respectively. While all investigated pathways contributed to thiol transformation, hydroxyl radical-mediated oxidation dominated at low solution pH and singlet oxygen-mediated oxidation dominated at high solution pH in the DOM-sensitized phototransformations of γEC, GSH, and PC. Furthermore, the effects of metal complexation and solution salinity on thiol transformation rates were examined. Thiol reactivity was not affected by Fe and Ag, slightly enhanced in the presence of Zn, Cd and Hg, and significantly enhanced by Cu. Additionally, enhanced thiol transformation rates were observed in solutions with high salinity.

Halogen Radicals Contribute to the Halogenation and Degradation of Chemical Additives Used in Hydraulic Fracturing

“…The mechanism proposed herein indicates that knowledge of halogen radical formation and reactivity in advanced oxidative treatments − and sunlight photochemistry − in saline water applies to the halogenation of chemical additives in hydraulic fracturing fluids. A remaining barrier to applying knowledge on halogen radicals more directly to this emerging field is that reaction rate constants used in these studies, as well as those cited herein, are largely determined at lower temperatures than those relevant to hydraulic fracturing.…”

“…Consequently, the conversion of hydroxyl or sulfate radicals to halogen radicals decreases the degradation rate of certain organic compounds − and increases the degradation rate of others ,, in advanced oxidative treatment of brines and other saline waters. In sunlit seawater, the presence of halogen radicals accelerates the transformations of certain pollutants and biogeochemically relevant compounds. − In addition to altering reaction rates, halogen radicals also alter reaction mechanisms by enabling halogenation both directly and indirectly. Directly, halogen radicals add to the structure of certain aromatic and olefinic molecules. , Indirectly, halogen radicals react via termination reactions to generate other halogenating agents (i.e., hypohalous acid), − which react with organics through substitution − or addition …”

Electrochemical and Photoelectrochemical Water Oxidation for Hydrogen Peroxide Production

“… standard H 2 O 2 strips, [27] titration using potassium permanganate (KMnO 4 ), [27] colorimetric change from Fe 2+ to Fe 3+ , [36, 37] colorimetric determination using titanium(IV) sulfate, [38] the N , N ‐diethyl‐1,4‐phenylene‐diamine‐peroxidase (DPD‐POD) method, [33] titration using cerium sulfate (Ce(SO 4 ) 2 ), [39] high‐performance liquid chromatography (HPLC) [40] …”

Electrochemical and Photoelectrochemical Water Oxidation for Hydrogen Peroxide Production

“… standard H 2 O 2 strips, [27] titration using potassium permanganate (KMnO 4 ), [27] colorimetric change from Fe 2+ to Fe 3+ , [36, 37] colorimetric determination using titanium(IV) sulfate, [38] the N , N ‐diethyl‐1,4‐phenylene‐diamine‐peroxidase (DPD‐POD) method, [33] titration using cerium sulfate (Ce(SO 4 ) 2 ), [39] high‐performance liquid chromatography (HPLC) [40] …”