Applied Geochemistry (v.16, #5)

At the Monteponi Pb–Zn mine located in south-western Sardinia intensive dewatering has been carried out over about 100 a. A marked increase in the salinity and Hg concentration of deep groundwater occurred as the water table level was lowered from +15 to −160 m a.s.l. over this period. Closure of the mine implied the cessation of the expensive pumping system, and prompted the assessment of the contamination risk for the shallow groundwater system supplying Iglesias town. This study shows that deep water was mixed into the shallow aquifer as the water table level rose. An increase of salinity, mainly due to Na and Cl, has been observed in shallow groundwater. The input of Cl facilitates the dissolution of Hg. Moreover, the progressive mine flooding is causing an increase of other dissolved metals, mainly due to the weathering of primary sulphides and secondary minerals present in the ore, and remobilization of metals in the mine waste left in the galleries. A stratification process will allow the saline water at depth to settle, and this is expected to occur in a relatively short time (few years). The leaching of metals represents a greater concern for the quality of shallow groundwater, and the time necessary to clean up will be much longer (probably several decades) than that expected for the stratification of the water body.

The use of adsorption data from single sorbate systems to model metal adsorption in SO4-rich waters, such as acid mine drainage, can lead to inaccurate predictions of metal speciation. The adsorption of Cu and Zn on ferrihydrite, for example, is enhanced at low pH values in the presence of SO4. This effect can only be accurately modeled using the diffuse layer model and surface complexation theory if ternary surface complexes, ≡FeOHCuSO4 or ≡FeOHZnSO4, are taken into consideration. Intrinsic adsorption constants for the formation of these ternary complexes on ferrihydrite have been derived from experimental data. When included in the model, Cu and Zn adsorption in the presence of SO4 is accurately predicted for a wide range of metal, ferrihydrite and SO4 concentrations. Adsorption of Cu and Zn onto the SO4-rich Fe oxyhydroxide, schwertmannite, could also be accurately predicted and is indistinguishable from adsorption onto ferrihydrite in the presence of high solution SO4 concentrations (e.g. 0.01 mol kg−1 SO4).

Consideration on the occurrence of the Al13 polycation in natural soil solutions and surface waters by Frédéric Gérard; Jean-Pierre Boudot; Jacques Ranger (513-529).
Equilibrium speciation calculations were performed (1) for soil solutions and streamwaters collected in central and eastern France and (2) for simulated waters at 0 and 25°C, to assess the highest concentration of Al13 that could be reached in waters in the absence of complexing ligands other than OH. A comprehensive and updated set of aqueous Al species, including polymeric hydroxyaluminosilicates (HAS), and their corresponding thermodynamic formation constants, were used. Results suggest that the concentration of the Al13 polycation in natural waters has been largely overestimated in some past studies using equilibrium models to calculate Al speciation, owing to oversimplification (many Al ligands not considered) and the unrecognised temperature dependence of some formation constants. The Al13 concentration in mildly acidic natural waters may not exceed a few μmol l−1 at AlT on the order of 10−4 mol l−1 and should be less than 1 μmol l−1 at AlT=10−5 mol l−1. Monomeric Al–Si species may not significantly interfere with the formation of Al13, but the formation of both HAS polymers (proto-imogolite precursors) and organo-Al complexes have a marked detrimental effect on the Al13 concentration. The maximum concentration of Al13 decreased upon increasing temperature from 0 to 25°C. In contrast, the pH range wherein Al13 may occur increases slightly with temperature and the most acidic pH value above which Al13 may be formed has been underestimated. At T=25°C, the Al13 polycation may be a significant Al species (4 to 5% of AlT) at pH < 4.5 if AlT > 10−4 mol l−1. The results of this study and the use of HAS polymers to calculate Al speciation in moderately natural acidic soil solutions were in better accordance with soil mineralogy. This research suggests strongly that Al13 should be negligible in natural soil and surface waters and may not control either Al3+ activity or Al-trihydroxide formation through polymerisation/depolymerisation steps. Also, from a biological point of view, the toxicity of Al13 to plants and aquatic organisms in natural conditions may be considered to be very low.

Geomaterials can be cost-effective sorbents for use in water treatment. In this study, a heavily-weathered Tertiary soil from Xinzhou, China was used as a sorbent for defluoridation of high-fluoride drinking water. The soil is composed of quartz, feldspar, illite and goethite, with an Fe oxide content of 6.75%. Batch and column experiments were done to characterize the F removal properties and to develop an optimal activation and regeneration procedure. The soil can be regenerated following a simple base-acid rinsing procedure. This can be performed in situ, i.e., by passing the rinsing solutions directly through the treatment column. The same regeneration procedure can be used to activate the pristine soil. Fluoride sorption is described by a Freundlich isotherm model and the bulk of the uptake occurs within 1.5 h. Iron oxide coatings on soil particles and perhaps ≡FeOH surface groups at particle edges of illite grains are likely responsible for the soil's F-sorption property. As collected in the field, the soil has a low permeability and is thus unsuitable for direct use in a flow-through column. Heat-treatment at 400–500°C for 2 h, however, produces a granular and permeable sorbent. Although the soil's sorption capacity (150 μg/g ) is about a quarter of the low end range of values reported for commercially-available activated alumina, the sorption for F is specific. A batch sorption experiment in the presence of Cl, SO4 2− and HCO3 shows little or no competition from these other anions.

The Isonzo river mouth has been the source of Hg in the Gulf of Trieste (northern Adriatic sea) since the sixteenth century, making this shallow basin one of the most contaminated marine areas in the length of time and amount of metal accumulated. The occurrence and behaviour of total Hg (range 0.064–30.38 μg g−1; average 5.04 μg g−1; median 3.10 μg g−1, n=80) and related size fractions in sediments of this coastal area were investigated in detail. The relationship between total Hg and the fine silt-clay (< 16 μm) fraction has provided information on the hydrological and mineralogical fractionation process affecting this element, when compared to other heavy metals associated with fluvial inputs. Mercury contents are very high along the littoral zone of the northern (Italian) sector where this metal is present in detrital form (cinnabar) in sandy-silty sediments near the river mouth and the surrounding beaches. Within the sediments belonging to the Gulf area, Hg is bound either to fine particles or adsorbed onto the surface of clay minerals and/or partially complexed by colloids and organic matter. Recent accumulation of Hg in a 70 cm long 210Pb dated core, collected in the central part of the Gulf, was also compared to other heavy metals (Fe, Cr, Cu, Mn, Ni and Zn). A preliminary estimate of Hg enrichment shows that the first 50 cm of sediment in the central sector of the Gulf of Trieste are noticeably contaminated, reaching a maximum of up to 25-fold above the proposed natural regional background of 0.17 μg g−1. The vertical trend is well correlated to historical data of Hg extraction activity at the Idrija mine.

Structure and stability of the Fe(II)–Fe(III) green rust “fougerite” mineral and its potential for reducing pollutants in soil solutions by Jean-Marie R. Génin; Philippe Refait; Guilhem Bourrié; Mustapha Abdelmoula; Fabienne Trolard (559-570).
Fe(II)–Fe(III) layered double hydroxysalt green rusts, GRs, are very reactive compounds with the general formula, [FeII (1−x) FeIII x (OH)2]x+·[(x/n) A n·(m/n) H2O] x, where x is the ratio FeIII/Fetot, and reflects the structure in which brucite-like layers alternate with interlayers of anions An− and water molecules. Two types of crystal structure for GRs, GR1 and GR2, represented by the hydroxychloride GR1(Cl) and the hydroxysulphate GR2(SO4 2−) are distinguished by X-ray diffraction due to different stacking. By analogy with GR1(Cl) the structure of the fougerite GR mineral, [FeII (1−x) FeIII x (OH)2]x+·[x OH·(1−x) H2O]x-  Fe(OH)(2+x)·(1−x) H2O, is proposed displaying interlayers made of OH ions and water molecules (in situ deprotonation of water molecules is necessary for explaining the flexibility of its composition). The space group of mineral GR1(OH) would be R3̄m, with lattice parameters a≅0.32 and c≅2.25 nm. Stability conditions and the E h-pH diagram of Fe(OH)(2+x) (the water molecules are omitted) are determined from hydromorphic soil solution equilibria with GR mineral in Brittany (France). Computed Gibbs free energies of formation from soil solution/mineral equilibrium fit well with a regular solid solution model: μ°[Fe(OH)(2+x)]=(1−x) μ°[Fe(OH)2]+x μ°[Fe(OH)3]+RT [(1−x) ln (1−x)+x ln x]+A0 x (1−x), where μ°[Fe(OH)2]=−492.5 kJ mol−1, μ°[Fe(OH)3]=−641 kJ mol−1 and A 0=−243.9 kJ mol−1 at the average temperature of 9±1°C. The upper limit of occurrence of GR mineral at x=2/3, i.e. Fe3(OH)8, is explained by its unstability vs. α-FeOOH and/or magnetite; Fe(OH)3 is thus a hypothetical compound with a GR structure which cannot be observed. These thermodynamic data and E h-pH diagrams of Fe(OH)(2+x) can be used most importantly to predict the possibility that GR minerals reduce some anions in contaminated soils. The cases of NO3 , Se(VI) or Cr(VI) are fully illustrated.