Mercury speciation in a tropical soil association; Consequence of gold mining on Hg distribution in French Guiana

Mercury (Hg) speciation was compared in French Guiana pristine soils and in Hg-contaminated soils impacted by former (~ 1950's) gold mining activities which used Hg for gold amalgamation. Four selective extractions were performed on soil samples to assess the fraction of Hg present as Hg(II) and bond to organic matter (extracted by NH4OH and KOH), to amorphous iron oxides (ascorbate) and to soil components other than refractory minerals (HCl/HNO3). In addition, pyrolysis was used to quantify the content of elemental Hg in contaminated soils. X-ray diffraction (XRD) and X-ray fluorescence micro-mapping (µXRF) were used in combination to selective extractions to assess the nature of targeted components, the possible overlaps between the different extraction procedures and the spatial correlation between Si, K, Fe, Au and Hg. In soil profiles from pristine toposequences, Hg concentrations (0.01–0.49 µg·g− 1) decreased with increasing depth in soil matrix. Hg concentrations also decreased from ferralsols to acrisols and further to gleysols. In pristine soil matrix, Hg was mainly associated to the clay-size fraction (< 2 µm) which was mainly constituted of amorphous and crystalline Fe oxides (Al-substituted goethite and hematite), gibbsite and fine organic matter (OM), whose relative abundances vary along the soil association. Total Hg concentration was positively correlated with sulfurs and organic carbon suggesting the association of Hg with OM sulfur-bearing functional groups. Gleysols were depleted in Hg because of the prevailing reducing conditions that lead to the dissolution of iron oxides. In the same soil profiles, Hg concentrations in ferruginous nodules, which make up most of the soil coarse fraction (> 2 mm), were similar to those reported in the pristine soil matrix. These nodules mainly contained Al-substituted hematite and goethite and were especially abundant upslope in ferralsols and acrisols. Gold-mined gleysols were strongly disorganized by former activities as neither the original structure nor the texture was preserved. Soil granulometry was dominated by gravels, sands and silts. Hg concentrations (0.09–9.22 µg·g− 1) largely exceeded those in pristine soils. µXRF allowed the identification of Au-amalgamated Hg and of elemental Hg droplets. Pyrolysis confirmed Hg to be mainly present in its elemental form in contaminated soils. Selective extractions showed additional minor contributions of Hg(II) associated to OM, and to Al or Fe oxides. The combination of selective extractions with XRD and µXRF data showed that extraction efficiency is strongly dependent on the soil type, and that this efficiency needs to be determined on a soil-by-soil basis for Hg speciation studies. KOH extraction was especially delicate as crystalline and amorphous oxides were extracted together with organic matter.