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Formalin (aqueous solution of formaldehyde stabilized with methanol) is one of the most used disinfectants in aquaculture. It is used to eliminate infectious agents but may be responsible for negative effects on fish and water quality. This work focuses on physical–chemical properties of formalin and on its reactions and its use in intensive aquaculture and in environment. The types of treatments and the action mode of formalin are also described. The main consequences of formalin exposure to fish are damage in gills and alterations in mucous cells. Formalin also interacts with some treatments adopted in aquaculture establishments, for example biological filter, where the nitrification process seems to be strongly affected. With respect to formalin inactivation, formaldehyde (effluent) may be diluted with water or specific treatments should be conducted to decrease the concentration levels before its discharge into the environment. The common methodologies applied to aquaculture are not completely efficient on formalin removal from water, and the advanced oxidation processes may be a good alternative.

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BDE-209 is a brominated flame retardant and a priority contaminant, which has been found in several environmental matrices, namely, in water. To date, there are no quantum yield data for BDE-209 photodegradation by sunlight in water, to allow predicting half-life times in aquatic systems. In this work, the kinetics of BDE-209 photodegradation in water was studied and the influence of different fractions of aquatic humic substances (HS) was evaluated. Aqueous solutions of BDE-209 exposed for different periods of time to simulated sunlight were analyzed by HPLC-UV after being concentrated using dispersive liquid-liquid microextraction (DLLME) or solid-phase extraction (SPE). The photodegradation of BDE-209 in aqueous solution followed pseudo-first-order kinetics. The average quantum yield obtained of 0.010 ± 0.001 (about 20-fold lower than the quantum yield determined in ethanol) allow to predict an outdoor half-life time of 3.5 h. The photodegradation percentage of BDE-209 was not significantly affected by the XAD-4 fraction of HS, but it decreased substantially in the presence of humic and fulvic acids. Light screening by the humic substances could not explain this delay, which is probably the result of the association of the compound with the hydrophobic sites of the humic material.