New trends involving the use of ultrasound energy for the extraction of humic substances from marine sediments Original Research Article

A new and fast method involving the use of ultrasound energy and moderate temperature has been developed to extract humic substances from marine sediments. The extractive process has been optimised by using a Plackett–Burman design as a multivariate optimisation approach. Temperature and the number of repeated extractions were the most significant variables (P=95%) affecting the extraction of both fulvic and humic acids from marine sediments. These two variables have optimum values of 65 °C and three repeated extractions. The combination of ultrasound energy and moderate temperature was also studied to optimise an acid treatment prior the sodium hydroxide extraction. Results from the Plackett–Burman design have revealed that all factors, except the ultrasound power for humic acids extraction, and the temperature and number of repeated acid treatments for fulvic acids extraction, were statistically significant (P=95%). Therefore, some of these factors were optimised using a central composite designs. The optimised acid treatment consisted in one sonication step at room temperature and ultrasound power at 17 kHz, with 10 ml of 6.0 M hydrochloric acid for 15 min. The developed method has been found to be precise (relative standard deviations of 8.7% for HA and 11.4% for FA, for 11 determinations) and its results were comparable (in terms of humic and fulvic acids concentrations and in terms of elemental (C, H and N) composition) to those obtained after applying methods based on mechanical stirring. The method was applied to marine sediments from the Rı́a de Arousa estuary.