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.

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.

Oxytetracycline (OTC) is one of the most used antibiotics in aquaculture. This raises concerns due to its effects in human and animal health, the environmental contamination and the consequences arising therefrom. This review focuses on the OTC application in intensive aquaculture. It compiles and complements the dispersed information, presenting a comprehensive study about the OTC effects, from its administration to farmed fish, until its release into the surrounding aquatic systems. Several topics about OTC are presented and discussed: physicalchemical properties, interaction with ions, application to farmed fish, effects in water quality and possible treatments, biological activity and toxicity of OTC byproducts, environmental fate. Based on the information gathered, one concludes that OTC establishes strong complexes with Ca 2+ and Mg 2+ , which may have implications on its biological activity. OTC acts as inhibitor of protein synthesis and is poorly metabolized or unmetabolized by fish. Its common recommended daily dose is about 75 mg kg À1 but depends on the administration conditions and on target fish species. Regarding OTC removal from water within a semiclosed aquaculture's water circuit, ozonation and photodegradation are not completely efficient methods as none of them promotes total mineralization. However, photodegradation can be applied as a complementary method prior to ozonation, allowing to reduce costs. OTC by-products do not appear to retain its biological activity, but toxicity studies mimicking environmental conditions were not found. In open circuits (cages), water column where OTC is administered not suffer a specific treatment and the antibiotic is easily spread by the surrounding environments, affecting their inhabitants. OTC is released in the aquatic system in freely dissolved form and/or associated with organic or inorganic compounds, being subjected to natural photolysis, hydrolysis and/or biodegradation.

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This review highlights the recent findings about biotransformation in different organisms (dinoflagellates, bivalves, humans) and presents a critical revision on the latest analogues, M-toxins. A “toxicological traffic light” is also proposed.

A solar photocatalytic process, using TiO2–rGO as photocatalyst, is proposed to degrade the formalin disinfectant. This process can treat aquaculture freshwater before its discharge or recirculation, promoting sustainable water.

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