Extraction of available and labile fractions of mercury from contaminated soils: the role of operational parameters

One of the recognized challenges in chemical extraction of mercury from soil is the diversity of procedures that are currently available in literature and that differ in terms of the extractant solution used, soil:extractant ratio and duration of extraction. Therefore, this study focused on establishing the role of operational parameters for extraction of the available and labile fractions of mercury from soils, considering different soil:extractant ratios and the kinetics of extraction. The suitability of 1.0 mol L− 1 ammonium acetate at pH 7 and 0.1 mol L− 1 HCl as extractants for the available fraction and the extraction of the labile fraction using 0.5 mol L− 1 hydrochloric acid was investigated. No statistical differences were found between ammonium acetate and 0.1 mol L− 1 HCl; therefore both extractants can be used for estimating the available fraction. It was also observed that a soil:extractant ratio of 1.5 g of soil to 100 mL of extractant favors mercury extraction. For the available fraction an extraction of 30 min seems enough, as no further significant change was observed in the quantity of mercury extracted thereafter. For the labile fraction increase the extraction duration to at least 24 h is recommended.

The data was fitted into kinetic models, and it was observed that the two first-order reactions and the diffusion models help to understand the behavior of mercury extraction from soil, clearly showing that in all cases the rate of mercury extraction was faster in the first 10 h and declined after that period. The characteristics of the soil influenced the extraction of mercury, and it was verified that pH and particle size of the soil influenced the mercury extraction process, as results suggested that an acidic soil might have a reduced ability to strongly retain metals and soils with higher porosity showed lower rates of mercury extraction.