Transformation of microcystin-LR and olefinic compounds by ferrate(VI): Oxidative cleavage of olefinic double bonds as the primary reaction pathway.

The presence of toxic microcystins in algal-impacted surface waters is a concern for drinking water quality management. In this study, the potential of ferrate(VI) to eliminate microcystins during drinking water treatment was assessed by investigating reaction kinetics, reaction sites, transformation products, and toxicity changes for the oxidation of microcystin-LR (MC-LR) as a representative microsystin. The investigations also included several substructural model compounds of MC-LR, such as cinnamic acid and sorbic acid, to elucidate the major transformation products and pathways of MC-LR and olefinic compounds. Second-order rate constants were determined in the pH range 6-10.4 for the reaction of ferrate(VI) with MC-LR and the model compounds. The kinetic data revealed that the olefinic double bonds in the Adda and Mdha residues of MC-LR were the primary ferrate(VI) reaction sites, while the phenyl or guanidine moiety was not the reaction site. This finding was supported by detection and identification of the MC-LR transformation products of double bond cleavage, with high peak abundance in the liquid chromatography-mass spectrometry. Furthermore, the reaction of ferrate(VI) with cinnamic and sorbic acids formed the corresponding aldehydes and organic acids with near complete carbon mass balance, indicating the oxidative cleavage of the double bonds as the primary reaction pathway. A quantitative protein phosphatase 2A (PP2A) binding assay for ferrate(VI)-treated MC-LR solutions showed that the MC-LR transformation products exhibited negligible PP2A binding activity compared to that of the parent MC-LR. Oxidation experiments in a filtered river water matrix spiked with MC-LR demonstrated the efficient elimination of MC-LR during water treatment with ferrate(VI).

Transformation of ranitidine during water chlorination and ozonation: Moiety-specific reaction kinetics and elimination efficiency of NDMA formation potential.

Ranitidine can produce high yields of N-nitrosodimethylamine (NDMA) upon chloramination and its presence in water resources is a concern for water utilities using chloramine disinfection. This study assessed the efficiency of water chlorination and ozonation in transforming ranitidine and eliminating its NDMA formation potential (NDMA-FP) by determining moiety-specific reaction kinetics, stoichiometric factors, and elimination levels in real water matrices. Despite the fact that chlorine reacts rapidly with the acetamidine and thioether moieties of ranitidine (k>10(8)M(-1)s(-1) at pH 7), the NDMA-FP decreases significantly only when chlorine reacts with the less reactive tertiary amine (k=3×10(3)M(-1)s(-1) at pH 7) or furan moiety (k=81M(-1)s(-1) at pH 7). Ozone reacts rapidly with all four moieties of ranitidine (k=1.5×10(5)-1.6×10(6)M(-1)s(-1) at pH 7) and its reaction with the tertiary amine or furan moiety leads to complete elimination of the NDMA-FP. Treatments of ranitidine-spiked real water samples have shown that ozonation can efficiently deactivate ranitidine in water and wastewater treatment, while chlorination can be efficient for water containing low concentration of ammonia. This result can be applied to the other structurally similar, potent NDMA precursors.