Hydrozincite bioprecipitation: a promising tool for bioremediation of waters contaminated by harmful metals. Hydrochemical factors and morphological features of the biomineralization process

The 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

Geochemical and mineralogical datasets on waters and stream precipitates from an abandoned mining site: Montevecchio-Ingurtosu district, Rio Irvi (SW Sardinia)

Geochemical 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

Ex situ phytoremediation trial of Sardinian mine waste using a pioneer plant species

The mitigation of metals contamination is currently a crucial issue for the reclamation of mine sites. Indeed, mine wastes are often disposed in open dumps and consequently pollutants are subjected to dispersion in the surrounding areas. In this study, the potential use of Helichrysum microphyllum subsp. tyrrhenicum for phytostabilization was evaluated in ex situ conditions. Ninety specimens were randomly selected and were planted in three substrates (reference substrate, mine waste materials, and mine wastes with compost). Mineralogical compositions of substrates, rhizosphere, and roots were assessed through X-ray diffraction (XRD). Zn, Pb, and Cd concentrations of substrates, rhizosphere, soil pore waters, and plant tissues were determined. The phytostabilization potential was determined through the application of biological accumulation coefficient (BAC), biological concentration factor (BCF), and translocation factor (TF). Moreover, survival and biometric parameters were assessed on plant specimens. The polluted substrates and related rhizosphere materials were mainly composed of dolomite, quartz, pyrite, and phyllosilicate. Zn was the most abundant metal in substrates, rhizosphere, and soil pore waters. XRD analysis on roots showed the presence of amorphous cellulose and quartz and Zn was the most abundant metal in plant tissues. H. microphyllum subsp. tyrrhenicum restricts the accumulation of the metals into roots limiting their translocation in aereal parts, indicating its potential use as phytostabilizer (BCF, BAC, TF &lt; 1). Survival and growth data showed a great adaptability to different substrates, with an evident positive effect of the implementation of compost which increased the plant survival and decreased the metals uptake into roots

PRELIMINARY STUDIES ON BIOPRECIPITATION PROCESSES MEDIATED BY SULFATE REDUCING BACTERIA (SRB) AND METAL IMMOBILIZATION IN MINE IMPACTED ENVIRONMENTS.

Mining activity often leaves a critical legacy represented by huge volumes of mine wastes and residues, usually made up of highly reactive materials, which lead to the mobilization and dispersion of harmful elements in soils and waters. Although these extreme environments are adverse to the development of living organisms, it has been observed that some microorganisms are able to adapt, playing a role in metal mobility, and becoming part of the resilience of the system itself. The Iglesiente and Arburese (SW Sardinia, Italy) mine districts, now abandoned, have been exploited for centuries by mining activities aimed at Pb-Zn extraction from sulfides and non-sulfides (calamine) deposits. Here, biogeochemical barriers naturally occur as an adaptation of the ecosystem to environmental stresses. Studies, from macroscale to microscale, showed that sulfate-reducing bacteria (SRB) may influence metal mobility by mediating the precipitation of secondary authigenic metal sulfides under reducing conditions. Specifically, framboids of Zn sulfides and Fe sulfides have been observed in the sections of stream sediments core characterized by the presence of abundant organic matter, especially residues of vegetal tissues (e.g. roots and stems of Juncus acutus and Phragmites australis). Laboratory-scale experiments were performed to better understand the bioprecipitation processes. For this purpose, anaerobic batch tests were carried out using high polluted mining waters (Zn and sulfate concentrations up to 102 and 103 mg/l, respectively) inoculated with native selected sulfate-reducing bacteria from stream sediments collected in the investigated areas. Dramatic decrease (up to 100%) in Zn and sulfate was observed in solutions. Moreover, scanning electron microscopy - energy dispersive spectroscopy (SEM-EDS) analysis, performed on solids recovered at the end of the experiments, showed the presence of precipitates characterized by a tubular morphology and made up by S and Zn. SRB inocula were studied by next-generation sequencing (NGS) approach, with the aim to compare the microbial diversity of the different SRB communities and to search for indigenous novel metal-tolerant sulfidogenic microorganisms. These findings represent a valuable step forward to plan effective bioremediation strategies for reducing metal mobility and dispersion. Also, bioprecipitation mediated by SRB can have great potentialities for metal recovery and our results can help to develop biomining techniques. The authors acknowledge CESA (E58C16000080003) from RAS and RAS/FBS (F72F16003080002) grants, and the CeSAR (Centro Servizi d'Ateneo per la Ricerca) of the University of Cagliari, Italy, for SEM analysis

Natural attenuation can lead to environmental resilience in mine environment

Four streams flowing in the Iglesiente and Arburese mine districts (SW Sardinia, Italy), exploited for zinc (Zn) and lead (Pb) extraction from sulphides and secondary non-sulphide mineralization (calamine ores), have been studied combining investigations from the macroscale (hydrologic tracer techniques) to the microscale (X-ray powder diffraction, scanning electron microscopy, X-ray absorption spectroscopy). In the investigated area, concerns arise from release of metals to water during weathering of ore minerals and mine-waste. Specifically, Zn is observed at extremely high concentrations (10s of mg/L or more) in waters in some of the investigated catchments. The results from synoptic sampling campaigns showed marked differences of Zn loads, from 6.3 kg/day (Rio San Giorgio) to 2000 kg/day (Rio Irvi). Moreover, natural attenuation of metals was found to occur i) through precipitation of Fe compounds (Fe oxy/hydroxides and “green rust”), ii) by means of the authigenic formation of metal sulphides promoted by microbial sulphate reduction, iii) by metal intake in roots and stems of plants (Phragmites australis and Juncus acutus) and by immobilization in the rhizosphere, and iv) by cyanobacterial biomineralization processes that lead to formation of Zn-rich phases (hydrozincite and amorphous Zn-silicate). The biologically mediated natural processes that lead to significant abatement and/or reduction of metal loads, are the response of environmental systems to perturbations caused from mine activities, and can be considered part of the resilience of the system itself. The aim of this study is to understand the effect of these processes on the evolution of the studied systems towards more stable and, likely, resilient conditions, e.g. by limiting metal mobility and favouring the improvement of the overall quality of water. The understanding of how ecosystems adapt and respond to contamination, and which chemical and physical factors control these natural biogeochemical barriers, can help to plan effective remediation actions

Assessment of origin and fate of contaminants along mining-affected Rio Montevecchio (SW Sardinia, Italy): A hydrologic-tracer and environmental mineralogy study

Hydrologic tracer techniques were applied to Rio Montevecchio (SW Sardinia, Italy), a stream affected by mine drainage, allowing the calculation of discharge and contaminant loads. Discharge along the stream showed a constant increase throughout the 2.7 km-long study reach, up to 13.6 l/s at the last synoptic point. Calculated loads of mine-related constituents were large, reaching values of 1780 kg/day for, 340 kg/day for Zn, 47 kg/day for Fe, and 50 kg/day for Mn. The difference of the cumulative instream metal loads between the first and the last synoptic sampling points indicated gains of 421 kg/day for Zn, 2080 kg/day for, 56 kg/day for Mn, and 50 kg/day for Fe. The source areas critical for contaminants loading were almost all concentrated in the first 800 meters of the stream, with the exception of Pb, whose loading occurs evenly along the whole study reach. Precipitation of secondary minerals along the streambed was responsible for a very high attenuation of Al and Fe loads (66% and 77%) and affected also and Zn loads, though less effectively. Rio Montevecchio has the second highest metal load among the rivers investigated with tracer techniques in SW Sardinia. In comparison with Rio Irvi, which has one order of magnitude higher metal loads, natural attenuation processes limit the loads in Rio Montevecchio. Results are useful to clarify the hydrogeochemical paths involved in the release and attenuation of pollutants, improving our understanding of stream responses to contamination and aiding development of site-specific remediation actions

Hydrozincite bio-precipitation: a promising tool for bioremediation of waters contaminated by harmful metals. Hydrochemical factors and morphological features of the biomineralization process.

The 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

A 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