Sorption of methyltestosterone onto sediments and soils

Abstract

A common practice by fish farmers to change the sex of Nile tilapia from female to male is to use a male hormone, methyltestosterone (MT) to induce the sex of the tilapia. MT is an anabolic steroid with a solubility of about 3.4 mg/L. MT is fed to Tilapia at a rate of 0.06 mg per 1 kg of normal food during the first 30 days after hatching. Due to the properties of MT (high Kow and low water solubility), MT is preferably sorbed onto sediments of the fish pond. Therefore it is possible for MT to contaminate the environment and impact other aquatic organisms. However, there are not many studies on the sorption of MT onto soils and sediment. This research studies the sorption of MT onto five different types of soils and a sediment and the effect of pH and salinity on the sorption of MT using batch sorption experiments. The sorption coefficients (Kd) of MT onto sand, laterite soil, garden soil-1, garden soil-2, garden soil-3 and sediment were 4.6, 1.2, 49.4, 119.1, 122.8, and 168.8 L/kg, respectively. Sorption of MT onto these five soils were found to be positively correlated with the organic contents. Sorption of MT onto sand and garden soil-1 were not affected by pH and salinity concentration. However, the sorption of MT onto sediment were significantly affected by pH and salinity. The Kd value for the sediment increased from 299.0 to 433.1 L/kg when the pH decreased from 6.9 to 5.4 and increased from 231.50 to 495.20 L/kg when the salinity increased from 0.5 to 20 g/L. For materials with very low organic carbon content (< 0.01%), it is possible that surface area may play a role in sorption of MT as shown by the slightly higher sorption of MT onto sand as compared to laterite soils.