Bioaccumulation and depuration of zinc in the tissues of Heterobranchus longifilis exposed to zinc oxide nano-particles

Industries use zinc oxide nano-particles (ZnO-NPs) to produce most commercial and medicinal goods, but indiscriminate discharge of their effluents into the aquatic environment may accumulate in the tissues of the fish. The ability of these metal oxides to accumulate in the tissue of fish due to their nano size could lead to a reduction in the nutritional value of the fish. Therefore, this study aimed to evaluate the rate of zinc bioaccumulation in tissues of Heterobranchus longifilis. Juveniles of H. longifilis were exposed to sub-lethal  concentrations (0.0, 6.00, 8.00, 10.00, 12.00 mg/l) of ZnO-NPs for 60 d and depurated for 30 d. At the end of each experiment, fish were sacrificed and analyzed for bioaccumulation. The increasing order of concentration of zinc was as follow: bone< muscle < blood < skin < kidney < liver < GIT < gills having 0.068mg/g to 0.263 mg/g from muscle to gills. That was above the permissible limits (0.050 mg/g) in fish tissues, except in the bone with 0.054 mg/g, which was still within the acceptable limits. Gills accumulated the highest (0.263 mg/g) content of zinc, and bone had the lowest (0.054 mg/g). After the depuration period, the content of zinc in the tissues decreased in all the tissues but still remained above the maximum permissible limits in gills, GIT, kidney, liver and skin (0.175 mg/g, 0.162 mg/g, 0.066 mg/g, 0.071 mg/g and 0.073 mg/g respectively). However, the bone, blood and muscle were able to depurate the zinc content to values (0.031 mg/g, 0.034 mg/g, and 0.044 mg/g, respectively) below the acceptable limits of Zn in fish tissues. The results indicated that zinc from ZnO-NPs bioaccumulated in H. longifilis tissues, but the fish could depurate the metal naturally. However, the ability to depurate the bioaccumulated metal is time-dependent.