Heavy metal coprecipitation with hydrozincite [Zn(5)(CO(3))(2)(OH)(6)] from mine waters caused by photosynthetic microorganisms

Abstract

An iron-poor stream of nearly neutral pH polluted by mine tailings has been investigated for a natural phenomenon responsible for the polishing of heavy metals in mine wastewaters. A white mineralized mat, which was determined to be hydrozincite [Zn(5)(CO(3))(2)(OH)(6)] by X-ray diffraction analysis, was observed in the stream sediments mainly in spring. The precipitate shows a total organic matter residue of 10% dry weight and contains high concentrations of Pb, Cd, Ni, Cu, and other metals. Scanning electron microscopy analysis suggests that hydrozincite is mainly of biological origin. Dormant photosynthetic microorganisms have been retrieved from 1-year-old dry hydrozincite. The autofluorescent microorganisms were imaged by a scanning confocal laser microscope. A photosynthetic filamentous bacterium, classified as Scytonema sp. strain ING-1, was found associated with microalga Chlorella sp. strain SA1. This microbial community is responsible for the natural polishing of heavy metals in the water stream by coprecipitation with hydrozincite.

FIG. 1

Schematic map of the sampling area, with tailings distribution (hatched areas). Samples 1 to 11 (●) are from the Rio Naracauli stream; samples A to C were collected in the tributaries before the inflows.

FIG. 2

XRD spectra of some selected precipitates in Rio Naracauli, with relative intensity peaks of hydrozincite (H), calcite (Cal), and quartz (Qz).

FIG. 3

Photos of white precipitate obtained by SEM. (A) Formation of tubing network of hydrozincite produced by microorganisms. Bar, 400 μm. (B) Detail of a tubing section giving a better view of the biological origin of hydrozincite. Bar, 4 μm. (C) (Left) Further details with naked parts of a sheath. Bar, 10 μm. (Right) Close-up detail of the sheath on a framed rectangle. (D) EDX elemental analysis of a sheath encrustation revealing high Zn concentrations.

FIG. 4

(A) Photograph of a piece of old, dried, consolidated hydrozincite with dark spots, with cyanobacterium Scytonema sp. in a dormant state. Bar, 0.5 cm. (B) The same piece of hydrozincite after a period of incubation in BG-11 medium (diluted 1:10) under dim light, clearly showing growth of Scytonema sp., which was entrapped in the hydrozincite matrix and which forms blue-green filaments. Bar, 0.5 cm. (C) In the transmission mode the Scytonema sp. clearly grows out of the hydrozincite tubing. A thick-wall heterocyst (short arrow) is clearly visible. Bar, 12, μm. (D) The same image by SCLM shows single autofluorescent cells of a Scytonema sp. Autofluorescence is due to chlorophyll a encapsulated in the sheath. Where the heterocyst occurs (short arrow), no autofluorescence appears, since the photosynthetic apparatus is degenerated. Moreover, hydrozincite emits fluorescence (long arrow). Bar, 12 μm. (F) Coculture of Scytonema sp. and the microalga Chlorella sp. The cyanobacterium shows filaments of different diameters. Bar, 12 μm. (G) The same image taken by SCLM (the sheath of the Scytonema sp. becomes empty and wide with aging and autofluorescence [arrow]). Bar, 12 μm.