The Importance of Measuring Ultraviolet Fluence Accurately: A Review of Microcystin-LR Removal by Direct Photolysis

Many ultraviolet (UV) experiments are conducted without ensuring that fluence is measured accurately, which may significantly affect the interpretation of the findings. This is especially evident for microcystin-LR, a common cyanobacterial toxin. Reports on the effectiveness of direct photolysis of microcystin-LR by 254 nm UV light are highly inconsistent in the literature, with pseudo-first-order rate coefficients reported from 10(-2) to 10(-5) cm(2)/mJ. It is demonstrated systematically that most of these studies did not follow proper fluence calculation procedures. The findings of improper studies were eliminated or, if possible, corrected using standard protocols. After accounting for these deficiencies, we find the average rate coefficient is 3.1 +/- 0.82 x 10(-3) cm(2)/mJ and is used to determine a new quantum yield at 254 nm (0.048 mol/Einstein). These findings are used to recharacterize the photolysis of microcystin-LR whereby it is shown that in ultrapure water 84% of the microcystin-LR can be transformed into nontoxic products by a fluence of 600 mJ/cm(2), typical of UV advanced oxidation. Thus, direct photolysis may be an important part of a multibarrier treatment approach.