The activity of a biological photosynthetic community promotes the seasonal precipitation of hydrozincite, Zn5(CO3)2(OH)6, from heavy-metal contaminated waters of the Rio Naracauli stream, Sardinia. The precipitation removes from waters not only zinc, but also other heavy metals, such as Cd, Cu, Pb. The phenomenon has remarkable environmental implications, and may have remediation applications. In this study, we investigate the nature of Cd binding to hydrozincite by release tests in deionized water, backed by X-ray absorption spectroscopy (XAS) spectra collected at the Cd K-edge, and by synchrotron-based anomalous X-ray diffraction (AXRD) spectra. Release tests indicate that Cd is weakly bound to hydrozincite, being released to a significantly higher rate than Zn. The absence of a residual corresponding to a crystalline phase in the anomalous diffraction pattern indicates that, up to bulk concentration of 2 wt%, Cd occurs in hydrozincite in an essentially disordered environment. The analysis of the weak signal of the second shell in the extended X-ray absorption fine structure (EXAFS) spectra suggests a local environment similar to cadmium carbonate, but distinct from otavite. We conclude that Cd is bound to hydrozincite as a disordered amorphous surface precipitate. The loose nature of the binding suggests a limited potential of hydrozincite both as a control on the mobility of cadmium in natural waters, and as a remediation tool for contaminated effluents.

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