Oxytetracycline (OTC) is an FDA-approved antibiotic that is commonly used in aquaculture as a prophylactic treatment or to treat fish bacterial diseases. OTC can be administered to fish in 3 ways: medicated feed, bath treatment, and injection. Regardless of the treatment method, 70 to 80% of OTC may not be metabolized by fish and will end up in the water system. Accumulation of OTC in aquaculture systems may lower the nitrification performance of biofiltration as well as increase the risk of antimicrobial resistance in cultured fish. OTC may also pose a persistent contamination issue that can impact the health of other animals and humans when it is released into the environment. Studies have shown that adsorption is the most efficient and cost-effective method to remove OTC. In this study, Moringa oleifera (MO) husk, a byproduct from Moringa oil and protein production, was tested as a low-cost biosorbent for the OTC removal.
The biosorbent was prepared by grinding the MO husk to 300 to 500 µm in diameter, followed by chemical and heat treatments. The changes in the surface morphology of the treated MO husk were evaluated by scanning electron microscopy (SEM). The concentration of the OTC was measured by detecting the absorbance at 355 nm using a UV–VIS spectrophotometer. The OTC removal study was done by mixing the appropriate amount of MO biosorbent with 25 mL of 20 mg/L OTC in a 50 mL tube wrapped in aluminum foil for 3 hours. The pH of the solution was adjusted using HCl and NaOH to the targeted value. The effect of the biosorbent dosage (0.4 to 2.0 g/L) and solution pH (5, 7, and 9) on the adsorption was tested by comparing the adsorption capacity under different conditions.
The results showed that the adsorption capacity decreased with the increase dosage while the removal rate increased (Fig.1a). The adsorption was significantly higher at the initial pH of 5 than at pH of 7 and 9 (Fig. 1b). Nonetheless, the equilibrium pH was consistently lower than the initial pH. This indicated that ion exchange might be the primary adsorption mechanism. Based on these results, we plan to investigate the effect of interfering ions commonly found in aquaculture wastewater as well as the kinetics and the isotherms of the adsorption to further understand its mechanism. Lastly, the performance of the biosorbent on the removal of OTC from the actual aquaculture discharge water will be assessed.