Extraction of mercury water-soluble fraction from soils: an optimization study

The procedure for extraction of water-soluble mercury species from soil was studied and optimized. Aspects studied included the soil:water ratio, time of extraction, separation technique (centrifugation vs. filtration) and
analytical technique used to analyze the extract (pyrolysis-atomic absorption spectrometry vs. atomic fluorescence spectrometry). Results indicated that the process of extraction is not influenced by the soil:water ratio in the range studied (1.5:100 to 20:100). The kinetic study performed showed that it takes 24 h for extraction to reach equilibrium, and that the mercury removal reaction takes place in two stages, a faster one (0 < t < 6 h), followed by a slower stage (t > 6 h). Hence, a two first-order reaction model was tested and proved to fit the experimental data. The particle size distribution seemed to have an influence on this process. Results also showed that filtration is preferable to centrifugation, as it avoids the presence of colloidal material in the leachate. Concerning the analytical technique used for quantification, atomic fluorescence spectrometry offers a lower limit of quantification; therefore it is more appropriate due to the low mercury concentrations often found in this fraction. The conclusions of this study contribute to the refinement of an important step of sequential extraction procedures and soil toxicity assessment methods, and, ultimately, constitute a helpful tool for the prediction of long-term risks to the environment.