Perchlorate production by photodecomposition of aqueous chlorine solutions.

Aqueous chlorine solutions (defined as chlorine solutions (Cl(2,T)) containing solely or a combination of molecular chlorine (Cl(2)), hypochlorous acid (HOCl), and hypochlorite (OCl(-))) are known to produce toxic inorganic disinfection byproduct (e.g., chlorate and chlorite) through photoactivated transformations. Recent reports of perchlorate (ClO(4)(-)) production-a well-known thyroid hormone disruptor- from stored bleach solutions indicates the presence of unexplored transformation pathway(s). The evaluation of this potential ClO(4)(-) source is important given the widespread use of aqueous chlorine as a disinfectant. In this study, we perform detailed rate analysis of ClO(4)(-) generation from aqueous chlorine under varying environmental conditions including ultraviolet (UV) light sources, intensity, solution pH, and Cl(2,T) concentrations. Our results show that ClO(4)(-) is produced upon UV exposure of aqueous chlorine solutions with yields ranging from 0.09 × 10(-3) to 9.2 × 10(-3)% for all experimental conditions. The amount of ClO(4)(-) produced depends on the starting concentrations of Cl(2,T) and ClO(3)(-), UV source wavelength, and solution pH, but it is independent of light intensity. We hypothesize a mechanistic pathway derived from known reactions of Cl(2,T) photodecomposition that involves the reaction of Cl radicals with ClO(3)(-) to produce ClO(4)(-) with calculated rate coefficient (k(ClO4-)) of (4-40) × 10(5) M(-1) s(-1) and (3-250) × 10(5) M(-1) s(-1) for UV-B/C and UV-A, respectively. The measured ClO(4)(-) concentrations for both UV-B and UV-C experiments agreed well with our model (R(2) = 0.88-0.99), except under UV-A light exposure (R(2) = 0.52-0.93), suggesting the possible involvement of additional pathways at higher wavelengths. Based on our results, phototransformation of aqueous chlorine solutions at concentrations relevant to drinking water treatment would result in ClO(4)(-) concentrations (~0.1 µg L(-1)) much below the proposed drinking water limits. The importance of the hypothesized mechanism is discussed in relation to natural ClO(4)(-) formation by atmospheric transformations.

Photooxidation of chloride by oxide minerals: implications for perchlorate on Mars.

We show that highly oxidizing valence band holes, produced by ultraviolet (UV) illumination of naturally occurring semiconducting minerals, are capable of oxidizing chloride ion to perchlorate in aqueous solutions at higher rates than other known natural perchlorate production processes. Our results support an alternative to atmospheric reactions leading to the formation of high concentrations of perchlorate on Mars.

Photocyclization reactions of cyclohexa- and cyclopenta-fused pyridinium salts. Factors governing regioselectivity.

The results of studies described in this report show that irradiation of 1,2-cyclopenta-fused pyridinium perchlorate in aqueous base promotes a remarkably regioselective photocyclization reaction that results in exclusive formation of a single tricyclic allylic alcohol. Moreover, transformation of this photoproduct to a spirocyclic amido diester followed by enzymatic desymmetrization produces an enantiomerically pure monoalcohol. This chemistry comprises a highly concise sequence for the preparation of what should become a useful synthon in synthetic organic chemistry.

Determination of rate constants for the reaction of hydroxyl radicals with some purines and pyrimidines using sunlight.

Sunlight mediated hydroxyl radical production from aqueous ferric perchlorate at low pH has been investigated using deoxyribose-thiobarbituric acid assay. The rate of production of hydroxyl radical was found to be dependent on the time of irradiation. Hydroxyl radical scavengers can compete with deoxyribose for hydroxyl radicals produced in the system leading to a decreased yield of thiobarbituric acid chromogen. The second-order rate constants of the added scavengers can be determined using a simple competition kinetic method. The rate constants for the reaction of hydroxyl radical with a number of purine and pyrimidine derivatives were determined using this method. The rate constants obtained (1-7 x 10(9) dm(3) mol(-1) s(-1)) were found to be in good agreement with those reported using pulse radiolysis technique. The rate constants of dimethyluracil, xanthosine, amino and methyl substituted pyrimidines, cytidine monophosphate and uridine monophosphate were also determined by this method. It is proposed that sunlight mediated production of hydroxyl radical coupled with deoxyribose-thiobarbituric acid assay is a simple and efficient method for the determination of rate constants for the reaction of hydroxyl radical with a wide range of biomolecules.