Destruction of microcystins (cyanotoxins) by UV-254 nm-based direct photolysis and advanced oxidation processes (AOPs): influence of variable amino acids on the degradation kinetics and reaction mechanisms.
Hepatotoxic microcystins (MCs) are the most frequently detected group of cyanobacterial toxins. This study investigated the degradation of common MC variants in water, MC-LR, MC-RR, MC-YR and MC-LA, by UV-254 nm-based processes, UV only, UV/H2O2, UV/S2O8(2-) and UV/HSO5(-). Limited direct photolysis of MCs was observed, while the addition of an oxidant significantly improved the degradation efficiency with an order of UV/S2O8(2-) > UV/HSO5(-) > UV/H2O2 at the same initial molar concentration of the oxidant. The removal of MC-LR by UV/H2O2 appeared to be faster than another cyanotoxin, cylindrospermopsin, at either the same initial molar concentration or the same initial organic carbon concentration of the toxin. It suggested a faster reaction of MC-LR with hydroxyl radical, which was further supported by the determined second-order rate constant of MCs with hydroxyl radical. Both isomerization and photohydration byproducts were observed in UV only process for all four MCs; while in UV/H2O2, hydroxylation and diene-Adda double bond cleavage byproducts were detected. The presence of a tyrosine in the structure of MC-YR significantly promoted the formation of monohydroxylation byproduct m/z 1061; while the presence of a second arginine in MC-RR led to the elimination of a guanidine group and the absence of double bond cleavage byproducts. It was therefore demonstrated in this study that the variable amino acids in the structure of MCs influenced not only the degradation kinetics but also the preferable reaction mechanisms.
Environmental Engineering and Science Program, University of Cincinnati, Cincinnati, OH 45221-0012, United States; Nireas-International Water Research Centre, University of Cyprus, P.O. Box 20537, 1678 Nicosia, Cyprus.
Xuexiang
X
He
Office of Research and Development, U.S. Environmental Protection Agency, Cincinnati, OH 45268, United States.
Armah A
AA
de la Cruz
Institute of Advanced Materials, Physicochemical Processes, Nanotechnology and Microsystems, National Center for Scientific Research "Demokritos", Patriarchou Grigoriou & Neapoleos, 15310 Agia Paraskevi, Athens, Greece.
Anastasia
A
Hiskia
Water Quality Department, Athens Water Supply and Sewerage Company (EYDAP SA), Oropou 156, 11146 Galatsi, Athens, Greece.
Triantafyllos
T
Kaloudis
Department of Chemistry and Biochemistry, Florida International University, Miami, FL 33199, United States.
Kevin
K
O'Shea
Environmental Engineering and Science Program, University of Cincinnati, Cincinnati, OH 45221-0012, United States; Nireas-International Water Research Centre, University of Cyprus, P.O. Box 20537, 1678 Nicosia, Cyprus. Electronic address: dionysios.d.dionysiou@uc.edu.
Dionysios D
DD
Dionysiou
D000596:Amino Acids / Q000737:chemistry; D006861:Hydrogen Peroxide / Q000737:chemistry*; D017665:Hydroxyl Radical; D007700:Kinetics; D052998:Microcystins / Q000737:chemistry* / Q000528:radiation effects; D010084:Oxidation-Reduction; D010782:Photolysis; D013431:Sulfates / Q000737:chemistry*; D014466:Ultraviolet Rays; D014874:Water Pollutants, Chemical / Q000737:chemistry / Q000528:radiation effects; D018508:Water Purification / Q000379:methods*
D000596:Amino Acids; D052998:Microcystins; D013431:Sulfates; D014874:Water Pollutants, Chemical; D017665:Hydroxyl Radical; D006861:Hydrogen Peroxide
Hydrogen peroxide; Mechanism; Microcystins; Second-order rate constant; UV-254 nm; Variable amino acids