Optimisation of caffeine and antidepressants extraction from sediments and sewage sludge using experimental designs

This study proposes a microwave-assisted method for the simultaneous extraction of highly prescribed antidepressants (citalopram, venlafaxine, fluoxetine, sertraline and amitriptyline) and caffeine from sediments and sewage sludge for subsequent HPLC-PDA analysis. Because the sludge and sediment matrices have high contents of organic material, they strongly interact with the analytes and hinder extraction. Thus, a carefully optimised analytical methodology is required for quantitative extraction. A simplex-centroid design was applied to optimise the solvent composition, and a three-factor central composite rotational design was used to optimise the extraction protocol with regards to pH, amount of solvent and processing time. Samples (in triplicates) were fortified with a standard mixed solution of all the analytes and extracted according to the experimental design in each study. The extraction steps included: 30s vortexing, 20s microwave heating at 10W, cooling to room temperature (25 degrees C) in an ultrasonic bath for 60s, 2min centrifugation at 2000 rpm, and filtration. Analysis of variance and lack-of-fit tests were used to assess the significance of data fitting at 95% confidence. The desirability function was the optimisation tool used to obtain the ideal extraction conditions. As a result, a binary mixture of methanol and acetonitrile in 45:55 and 53:47 (v/v) ratios was indicated as the optimum solvent composition for the simultaneous extraction of all the target drugs from the sludge and sediment, respectively. The optimised extraction conditions were: 3 extraction cycles with 4mL of solvent at pH 3 for sewage sludge extraction and 4 extraction cycles with 3mL of solvent at pH 11 for sediment extraction. Further, low recoveries were obtained for extractions from sediment as compared to sludge indicating strong interaction of antidepressants and caffeine with the acidic organic components of sediments. It was found that the optimisation of pH of the extraction phase was crucial for the efficient extraction of the analytes from these environmental matrices.