Evaluation of olive stone biochar as valuable and inexpensive agro‐waste adsorbent for the adsorption and removal of inorganic mercury from Nile tilapia aquaculture systems

Abstract

The study's objective was to evaluate using olive stone biochar (OSB), an inexpensive agro-waste product, to adsorb and remove inorganic mercury from Nile tilapia (Oreochromis niloticus) aquaculture systems. First, the OSB's adsorption capacity was evaluated by testing different concentrations: 1, 2 and 3 g L−1 for 0, 24, 48, 72 and 120 h. The concentration of Hg decreased with increasing OSB concentration up to 48 h, with the highest Hg adsorption rate observed at 2 g OSB L−1. An experimental study assessed the OSB's impact on Nile tilapia immune status and growth efficiency. The experiment used 180 Nile tilapia divided into four treatment groups (CONT, OSB, Hg and Hg +OSB), with three replicates per treatment. Fish were fed a normal basal diet twice daily for 60 d. The CONT group served as a control without any treatment. The OSB group was exposed to 2 g OSB L−1 of water, the Hg group was exposed to 0.084 mg Hg L−1, and the Hg +OSB group was exposed to 0.084 mg Hg L−1 with 2 g OSB L−1. Hg exposure caused liver and renal dysfunction and reduced growth performance, haematological indices, total antioxidant capacity, superoxide dismutase and non-specific immune parameters. Adding OSB to the pond water reduced these effects. In conclusion, adding 2 g OSB L−1 to aquarium water and changing it every 48 h mitigates the immunosuppressive effects of sub-chronic mercury toxicity and lowers mercury residues in fish muscle.