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Hydrozincite bio-precipitation: a promising tool for bioremediation of waters contaminated by harmful metals. Hydrochemical factors and morphological features of the biomineralization process.
Full text linkThe Ingurtosu Pb-Zn deposit (Sardinia, Italy) was exploited for about a century until 1968. Huge amounts of tailings were abandoned, resulting in long-term harmful metal dispersion processes in both soils and waters. The maximum Zn concentration in waters from the Naracauli stream catchment area attains several hundreds of mg per litre, whereas Cd and Pb concentrations are in the order of thousands of μg per litre, despite the near neutral to slightly alkaline pH values (6.2-8.4). Zn concentration in waters is positively correlated with Pb, Cd, Ni and Co
concentrations. The strongest correlation was observed between Zn and Cd, with a constant Zn/Cd ratio (close to 100) among samples that could suggest the weathering
of a relatively uniform composition of sphalerite from tailings and mine wastes. Waters from tributaries show the lowest concentrations of contaminants. The highest
contents in harmful and toxic elements were observed in waters that drain flotation tailings and mine wastes. Metals concentrations change under different seasonal
conditions. The highest concentrations were observed under high-flow condition (October-April), probably due to the high runoff through the tailings and to aqueous
transport of these metals in association with very fine particles, i.e. <0.4 μm. Zn, Pb, Cd, Cu and Ni concentrations in waters of the Naracauli stream, the main
stream of the area, are abated by the seasonal bioprecipitation of hydrozincite, Zn5(CO3)2(OH)6. Hydrozincite precipitation is promoted by a microbial community made up of a filamentous cyanobacterium (Scytonema sp.) and a microalga (Chlorella sp.). Hydrozincite could be used in a controlled process to attenuate metal pollution in mining waters. Information on environmental conditions that promote the biomineralization process is fundamental for the development of remediation
strategies. This work aims to investigate the variables
controlling the biomineralization process, and the hydrochemical factors that affect hydrozincite precipitation. According to field observations, correlated with speciation and equilibrium calculations, the optimum condition for hydrozincite precipitation occurs in late spring of rainy years, when the hydraulic regime in the stream reaches stationary conditions, and SI values with respect to hydrozincite and pH reach the highest values, in agreement with the higher stability of the hydrozincite solid phase in contact with slightly alkaline waters. Concomitantly, Zn 2+/CO32- molar ratio reaches values close to 1, indicating that kinetic processes have a role on the
hydrozincite biomineralization process. Conversely, heavy rain events occurring in late spring appear to inhibit biomineralization, likely due to the decrease in the SI
values resulting from the dilution effect of rain water.
Results from morphological analysis show that hydrozincite morphology varies, and depends on the environmental conditions. Changes were observed between samples
collected in late spring and samples collected in summer, and among samples precipitated under different water flow conditions. Hydrozincite samples collected in
summer are characterized by globules with a larger diameter than those collected in spring, this variation can be ascribed to a difference in the production of external
mucilage sheaths by cyanobacteria colonies in response to stress conditions. Considering influence of water flow, it was observed that hydrozincite sheaths precipitated under low flow condition have more or less a constant diameter, whereas under high flow conditions sheaths become thinner at the final ends. This particular morphology is due to the influence of hydrodynamics on the structure of the biofilm
and, consequently, on biomineral shape. Diel cycles in dissolved Zn, Co, Ni and Mn were found to occur in a selected station along the Naracauli stream. The highest change in concentration was observed for Zn: the difference between the minimum (3 mg/l) and maximum (4.7 mg/l) Zn
concentrations is 1.7 mg/l (about 35%). The minimum values occurred at h 17:00 and maxima between h 02:00 and h 05:00. The timing of diel cycles in Co and Ni is very similar to that for Zn, but the ranges of Co and Ni concentrations are much smaller than Zn. Increased nocturnal concentrations could result from a combination of geochemical and biological processes. Considering the relations among temperature, pH and Zn contents, temperature and pH would seem to be the parameters that control variations in Zn concentration. Water temperature shows a well defined
diurnal cycle. Maximum water temperature was observed between the h 13:00 and h 17:00. Water temperature varies due to change in air temperature and incident solar
radiation. The pH values vary between 7.7 and 8.1; the highest values were observed during the sunny hours and the lowest during the night or early morning. The diel pH
cycle derives from photosynthesis (predominant during the day) and respiration (predominant during the night). Obtained results may be explained by adsorption-desorption reactions (onto colloids, carbonates, bacterial surfaces and biofilms) and/or different rates of mineral precipitation between the morning and the night time. Diurnal metal cycles should be taken into account to evaluate environmental conditions, potential risks and cleanup of contaminated site
The Fluminese mining district (SW Sardinia): impact of past lead-zinc exploitation on aquatic environment
No full textA hydrogeochemical survey was carried out in the Fluminese district (SW Sardinia, Italy) to investigate the
impact of past mining activities on the aquatic system. Waters in the Fluminese are near-neutral to slightly
alkaline. Mine drainages had the highest SO4 (640 mg/L), Zn (40 mg/L) and Cd (97 μg/L); other metals show a
wide range in concentrations depending on dominant mineral assemblages at each mine. Dissolved Pb in mine
waters is in the range of 1 to 53 μg/L but Pb > 10 μg/L also occurs in spring and well waters. The metal load
discharged daily into the sea under low-flow condition is estimated at about 2160 g for Zn, 21 g for Cd and 30 g
for Pb
Geochemical behaviour of rare earth elements in mining environments under non-acidic conditions
No full textThe rare earth elements (REE) were determined in water and solid samples at the abandoned mine of Ingurtosu
(Sardinia, Italy). Research was performed to evaluate the main factors that control the geochemical behaviour of REE
in near-neutral pH mining environments. REE show the highest concentrations (13-100 μg/l) in drainages from mine
tailings, especially when water sampling was carried out under high flow conditions. Positive correlations between
ΣREE and Zn and Pb were observed, possibly indicating REE sorption on Zn- and Pb-rich fine particles (<0.4 μm).
Fractionation processes were observed normalizing REE in waters with respect to REE in mining-related residues.
The Naracauli waters show an enrichment in HREE, probably due to the high stability of dissolved HREE(CO3)2-
complexes in the studied waters. REE fractionation was not observed during hydrozincite bioprecipitation
Geochemical and mineralogical datasets on waters and stream precipitates from an abandoned mining site: Montevecchio-Ingurtosu district, Rio Irvi (SW Sardinia)
Full text linkGeochemical modelling data and Powder X-Ray Diffraction data on samples collected along Rio Irvi (Montevecchio-Ingurtosu mining district, SW Sardinia, Italy) are reported in this paper. The data show the results of data processing to calculate water chemical speciation of ions and saturation indices of relevant mineral phases. These data are related with the research article: De Giudici G. et al (2018), Application of hydrologic-tracer techniques to the Casargiu adit and Rio Irvi (SW-Sardinia, Italy): Using enhanced natural attenuation to reduce extreme metal loads, Applied Geochemistry, vol.96, 42–54. The comparison of the calculated saturation indices of relevant Fe-bearing phases with the PXRD data of samples collected along the stream confirm the quality of the SI dataset and the good correlation between the calculations and the observed data. The comparison of this dataset with others can help to deeper understand and quantify the impact of past and current mining activity on water bodies, contributing to implement the scientific background for the application of remediation actions
Hydrozincite biomineralization at Naracauli (Sardinia): mineralogical investigation from macroscale to nanoscale
No full textGeochemistry of rare earth elements in water and solid materials at abandoned mines in SW Sardinia (Italy)
No full textThis study investigates the behavior of rare earth elements (REE) in near-neutral waters that drain abandoned mines in SW Sardinia (Italy). The REE, together with other major and minor components, were determined in waters (146 samples) and solid materials (13 samples). Research was performed to evaluate the main factors that control the geochemical behavior of REE in non-acidic mining environments. Zinc, cadmium and lead are the main contaminants in the study area, either in the processing residues or in the waters that drain the mine site. Contamination of stream waters extends several km downstream of the mine, till the Mediterranean Sea.
The mine tailing (MT) materials are the main REE source, with peak concentrations of 300 mg/kg ΣREE. PAAS-normalized patterns of MT are characterized by light REE (LREE) enrichment with respect to heavy REE (HREE). The waters flowing out of the MT heaps are near-neutral (6.2 to 7.0 pH), with Zn-sulfate dominant composition, and a mean concentration of 53 μg/l ΣREE. Drainages from MT and an adit flow in the Naracauli stream headwaters (mean 1.4 μg/l ΣREE). Concentrations of REE in the Naracauli stream decrease dramatically about 400 m downstream of source. The REE decrease matches Fe and Zn decrease in waters, probably because the REE are sorbed on freshly precipitated solid phases.
Sorption processes, and/or co-precipitation with secondary phases, appear to control the REE geochemistry in the studied waters under near-neutral conditions. Also, despite the non-acidic environment, it is worth to observe that small changes in pH seem to affect the mobility of REE at Naracauli.
The PAAS-normalized REE patterns in the waters generally reflect the PAAS-normalized REE patterns in the solid materials with which the water interacts, either MT or secondary phases such as Fe-hydroxide and bio-hydrozincite. As it is expected in waters under oxidizing conditions, patterns with negative Ce anomalies are developed due to the poor solubility of Ce+ 4 species, and its consequent scavenging in solid phases. However, there is an exception: Ce shows a poor affinity for the bio-hydrozincite mineral, probably reflecting the role of bacteria in the precipitation of the Naracauli hydrozincite
Synchrotron microscopic and spectroscopic techniques to reveal the fate of Zn in pioneer plants from abandoned mining sites
Full text linkHydrozincite seasonal precipitation at Naracauli (Sardinia-Italy): hydrochemical factors and morphological features of the biomineralization process
No full textHydrozincite biomineralization occurs seasonally in the Naracauli creek (SW Sardinia).
It depends on several factors, such as the rainfall intensity, hydraulic flow, metal concentration and biological activity. In this work, it is shown that the optimum conditions for hydrozincite precipitation are
the attainment of solution saturation and Zn2+/CO3-2 molar ratio close to 1. Microscopic analysis of the
morphological features of hydrozincite biomineralization shows that these are dependent on the hydraulic flow. Besides these hydrochemical factors, the biomineralization process also depends on other environmental factors, specifically rainfall intensity
Chemical data on environmental matrices from an abandoned mining site
Full text linkThis article contains analytical data on chemical composition of waters and solid samples (mining wastes and biominerals) collected in an abandoned mining area, and they are related with the research article “Geochemistry of rare earth elements in water and solid materials at abandoned mines in SW Sardinia (Italy)” (Medas et al., 2013).
Specifically, we present physicochemical data (temperature, electrical conductivity, pH, and redox potential), major components and the main contaminants (Zn, Mn, Cd, Ni, Cu, Pb) detected in stream waters and drainages from mine wastes. Waters were monitored from 2009 to 2011 during different seasonal conditions to give an insight into metal dispersion under different hydrological conditions
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