Hydrozincite seasonal precipitation at Naracauli (Sardinia-Italy): hydrochemical factors and morphological features of the biomineralization process

Hydrozincite [Zn5(CO3)2(OH)6] precipitation from Naracauli waters (SW Sardinia) is promoted by a microbial community made up of a filamentous cyanobacterium (Scytonema sp.) and a microalgae (Chlorella sp.). Hydrozincite bioprecipitation is responsible for the natural removal of heavy metals, especially Zn, from the stream waters. Thus, hydrozincite could be used 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 paper aims to investigate the variables controlling the biomineralization process, and the hydrochemical factors that affect hydrozincite precipitation. Morphological analysis shows 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. According to field observations, correlated with speciation and equilibrium calculation, the maximum intensity in 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 reach the highest values. Concomitantly, Zn2+/CO32- molar ratio reaches values close to 1, indicating that kinetic processes have a role on the hydrozincite biomineralization process.