Photo-degradation of alfalfa saponins by UV–visible multi-wavelength irradiation

Abstract

A photochemical treatment with UV multi-wavelength irradiation was applied to investigate the degradation of the saponins mixture obtained from alfalfa leaves. The photo-degradation was studied in an aqueous solution at a pH range of 4–7 and temperature range of 20–80 °C. The samples were irradiated for 80 min using a mid-pressure mercury lamp with emission wavelengths between 250 and 740 nm. The dependence of the absorbed radiation on the concentration of alfalfa saponins was evaluated, and a linear dependence was found for the range of concentrations experimented throughout the photo-degradation. The alfalfa saponins mixture absorbed radiation between 470 and 610 nm with a maximum absorption peak at 543 nm. This means that any lamp emitting within this wavelength range could produce some degree of photo-degradation. A reaction mechanism was proposed, matching the experimental pseudo-first-order kinetic model that best fitted the evolution of the saponins mixture concentration. The photo-degradation rate was found to increase with increasing temperature and decreasing pH value, causing a reduction of 80% in the concentration of the alfalfa saponins mixture after 80 min at pH 4 and 80 °C.