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 sites