Evaluation and optimisation of methylene blue removal measurement uncertainty in photodegradation studies

Abstract

The photodegradation of carcinogenic and teratogenic organic contaminants, such as active substances of medicines or preservatives of personal care products, not removed from wastewaters by conventional treatments, is a promising solution to reduce the impact of such emissions. Nevertheless, new catalysts need to be developed to make this technology feasible. Catalysts performance can be assessed from photodegradation tests using degradation markers such as methylene blue. The difference between the photodegradation kinetic constants of two catalytic alternatives is meaningful if its absolute value is larger than the expanded uncertainty of the difference, which results from the combination of relevant measurement uncertainty components, namely pollutant concentration before and after depletion and irradiation time. This work presents algorithms for estimating differences of kinetic constants of methylene blue removal, with uncertainty, implemented in a user-friendly MS Excel spreadsheet. The developed measurement models allow the optimisation and reliable comparison of kinetic constants. This tool is particularly useful to distinguish small efficiency differences of catalysts with equivalent production and use costs. If a portion of the methylene blue solution subjected to irradiation is used to calibrate the spectrometer between 0.26 mg L−1 and 1.3 mg L−1 in a wavelength interval between 550 nm and 700 nm, if the solution irradiation time is larger than 11 min and the irradiated solution is quantified after dilution with negligible uncertainty, it is possible to distinguish the difference of kinetic constants between 0.11 ms−1 and 0.56 ms−1.