TiO2-rGO nanocomposite as an efficient catalyst to photodegrade formalin in aquaculture's waters, under solar light

Formalin or hydrate of formaldehyde, a common disinfectant used in aquaculture, is not able to absorb sunlight, therefore its photodegradation must be catalyzed. To take advantage of sunlight for its photodegradation, TiO2-reduced graphene oxide (TiO2-rGO) nanocomposites with different GO contents (0.25-3.0 wt%) were synthesized and characterized by scanning electron microscopy (SEM), UV-vis and ATR-FTIR spectroscopies. Additionally, their specific surface areas (S-BET), pore volume and size were determined. The photocatalysts efficiency for formaldehyde (FM) degradation under simulated sunlight was assessed in aqueous solution. Among the nanocomposites considered, TiO2-rGO 0.5 wt% was the most efficient, achieving 93.8 +/- 4.6% of FM photodegradation in 90 min. The same irradiation conditions were applied in natural water matrices (aquaculture's freshwater and saltwater) to evaluate the influence of natural water constituents on photocatalysis. The photodegradation percentages obtained were 63.7 +/- 0.6% and 16.0 +/- 1.4% in aquaculture's freshwater and saltwater, respectively. This lower efficiency to photodegrade FM in natural aqueous matrices is attributed to the inhibition of photocatalytic oxidation of organic compounds caused by dissolved organic matter (DOM) and inorganic ions existing in the aquatic environment. Nevertheless, the results obtained for photocatalytic efficiency in natural waters are promising, suggesting a great potential of application in green photocatalysis to remove FM from contaminated freshwater.