Remediation of mercury-polluted soil in a mining area using nanoscale zero-valent iron

Asturias, located in northern Spain, is a region with a rich history of mining and metallurgical activities intertwined with coal and metals. The Hg-mining district situated in the centre of this region was the Spain’s second-largest Hg manufacturer from 60s to 70s. Several areas were affected by the presence of Hg, specifically in the “El Terronal” mine, which stands out as one of the most affected areas, primarily due to mining and subsequent pyrometallurgical processing.

A pilot-scale remediation using nanoscale zero-valent iron (nZVI) was conducted on a soil plot with a Hg concentration of 1500 mg/kg in “El Terronal”. The nZVI proved to be an exceptional amendment for Hg immobilization, revealing an 86% reduction in Hg mobility within 72 hours of applying the nZVI, sustained over a period of 32 months. Nevertheless, the long-term effects and mechanisms of Hg remediation remain not fully understood. In light of this, the present work focused on elucidating Hg speciation changes 6 years after the application of nZVI in a Hg-polluted soil under field conditions.

Soil samples were taken from both the treated plot and an adjacent untreated plot before nZVI application and after 6 years. Initially, a simplified USEPA Method 3200, involving sequential extraction, was used to evaluate the Hg mobility. Subsequently, the identification of Hg species was conducted using a mercury temperature programmed desorption (HgTPD) device. Based on preliminary results, the reference database for mercury compounds in this work were HgO, cinnabar (HgS), metacinnabar (HgS), HgCl₂, HgSO₄, corderoite (Hg₃S₂Cl₂), Hg complexed to humic acid, and Hg adsorbed to goethite. Furthermore, X-ray absorption spectroscopy was employed to complement the study of mercury speciation. In this regard, soils and patterns were prepared as pellets from finely ground and homogenized powder. The Hg L_III-edge spectra XANES were collected in transmission and fluorescence modes at 70-80 K on CLAESS beamline (ALBA synchrotron).

The concentration of Hg in the mobile fraction was below detection limit in both samples, treated and untreated soils. However, a decrease on the semi-mobile fraction was found from 57% to 39% in the treated soil with nZVI respect to the untreated soil. This result is in accordance with the Hg immobilization reported at the beginning of the field remediation. With respect to HgTPD, the Hg species identified corresponds to Hg bound to S. On the other hand, the data treatment by linear combination fits of the Hg L_III-edge spectra XANES demonstrated that the Hg solid speciation in the treated and untreated soil samples is similar, mainly dominated by cinnabar, metacinnabar, and Hg adsorbed to goethite. This result, may indicate that the Hg mobile is associated to a minor phase, not quantifiable, or to a Hg immobilization from all phases without preference.

In summary, the mobility of Hg is linked to only a minor fraction of the total concentration of these elements in the soil, with this metal being predominantly associated with cinnabar, metacinnabar and goethite. Consequently, the complete observation of the effects of immobilization processes on Hg speciation was hindered by resolution limitations.