Mineralogical attenuation for metallic remediation in a passive system for mine water treatment

Passive systems with constructed wetlands have been consistently used to treat mine water from abandoned mines. Long-term and cost-effective remediation is a crucial expectation for these water treatment facilities. To achieve that, a complex chain of physical, chemical, biological, and mineralogical mechanisms for pollutants removal must be designed to simulate natural attenuation processes. This paper aims to present geochemical and mineralogical data obtained in a recently constructed passive system (from an abandoned mine, Jales, Northern Portugal). It shows the role of different solid materials in the retention of metals and arsenic, observed during the start-up period of the treatment plant. The mineralogical study focused on two types of materials: (1) the ochre-precipitates, formed as waste products from the neutralization process, and (2) the fine-grained minerals contained in the soil of the wetlands. The ochre-precipitates demonstrated to be poorly ordered iron-rich material, which gave rise to hematite upon artificial heating. The heating experiments also provided mineralogical evidence for the presence of an associated amorphous arsenic-rich compound. Chemical analysis on the freshly ochre-precipitates revealed high concentrations of arsenic (51,867 ppm) and metals, such as zinc (1,213 ppm) and manganese (821 ppm), indicating strong enrichment factors relative to the water from which they precipitate. Mineralogical data obtained in the soil of the wetlands indicate that chlorite, illite, chlorite–vermiculite and mica–vermiculite mixedlayers, vermiculite, kaolinite and goethite are concentrated in the fine-grained fractions (<20 and <2 μm). The chemical analyses show that high levels of arsenic (up to 3%) and metals are also retained in these fractions, which may be enhanced by the low degree of order of the clay minerals as suggested by an XRD study. The obtained results suggest that, although the treatment plant has been receiving water only since 2006, future performance will be strongly dependent on these identified mineralogical pollutant hosts.Fundação para a Ciência e a Tecnologia (FCT

Investigation of physico-chemical features of soil colloidal suspensions.

The knowledge of surface properties of soil mobile colloids is of great importance to predict their role, but these properties are difficult to assess due to the heterogeneity and the complex nature of natural colloids. Physico-chemical features of natural colloids circulating in the soil solution were investigated with complementary approaches: microscopic techniques (TEM and AFM), potentiometric titrations and electrophoretic measurements. This study was realized on two colloidal suspensions isolated from soil solutions collected in situ from the plough layers of two different soils (MC and MPM). Colloidal particle properties including morphology, size distribution, specific surface area, electrophoretic mobility, and surface site concentration were compared before and after oxidation of the organic matter of the colloidal suspensions by H2O2 treatment. Statistical analysis of the geometrical features of TEM and AFM images revealed a comparable mean size of 170 70 nut for the two samples, and a mean height of 6.5 +/- 4.6 mn and 9.9 +/- 8.2 nm for the MPM and MC, respectively. From the 3D-analysis of AFM images, the specific surface areas were assessed at 203 m(2)/g and 260 m(2)/g for MC and MPM suspensions, respectively. Phyllosilicates (illite, interstratified clays and smectite) primarily present in the two samples seem to dominate the surface charge behaviour of the soil colloidal suspensions as seen by potentiometric or electrophoretic titrations. However, TEM after staining and AFM observations show also the presence of organic phases with different status according to the sample, i.e. surface coatings or interparticle bonds. The data clearly indicate the influence of organic matter on the main physico-chemical characteristics of colloids (morphology, size distribution and surface charge) related to the reactivity and the transfer of colloids in soils. (c) 2006 Elsevier B.V. All rights reserved

Microscale and Pb distribution patterns in subsurface soil horizons: an indication for metal transport dynamics

In many studies on soil pollution, authors conclude that there is no downward migration of metal elements if no evidence for enrichment can be inferred from profiles of total metal contents. We assessed possible transfer of mobile and less mobile metal pollutants to depth in subsurface horizons of a heavy metal contaminated soil, by a study of specific pedofeatures in thin sections by optical microscopy, and their corresponding Zn and Pb distribution patterns by synchrotron-based X-ray microfluorescence. In the B horizon (70 cm depth), Zn accumulation was predominantly associated with clay-iron coatings. Strong correlation was found between Zn and Fe (r = 0.94), Zn and Mn (r = 0.75), Zn and Ti (r = 0.84), and Zn and K (r = 0.88), but significant correlation was absent between Zn and Pb. In the C horizon (100 cm depth), clear Pb accumulation was observed in distinct iron coatings, with large correlation coefficients found between Pb and Fe (r = 0.94-0.75), whereas correlation between Zn and Fe was absent. Detected Zn concentrations were small and attributed to the local natural geochemical background. These results were then compared with data of the composition of gravitational soil water collected in other soils from the same study area. Thus, Zn accumulation in the B horizon was ascribed to interception of dissolved Zn2+ by negatively charged constituents of clay-iron coatings. In contrast, Pb accumulation in C horizons was related to precipitation of Pb-bearing iron colloids leading to neoformed, optically pure iron oxyhydroxide crystals and coatings. We demonstrate very localized accumulation of almost immobile Pb which occurs at greater depth than the more mobile Zn. The common, but strongly localized, occurrence of Pb-bearing iron coatings in the soil groundmass explained the absence of changes in the total Pb concentrations of the C horizon compared with the concentrations in the B horizon