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Mineralogical controls on arsenic mobility in the Baccu Locci stream catchment (Sardinia, Italy) affected by past mining

Published online by Cambridge University Press:  05 July 2018

F. Frau  and
C. Ardau
F. Frau*
Affiliation:
Dipartimento di Scienze della Terra, Università di Cagliari, Via Trentino, 51, I-09127, Cagliari, Italy
C. Ardau
Affiliation:
Dipartimento di Scienze della Terra, Università di Cagliari, Via Trentino, 51, I-09127, Cagliari, Italy
*
*E-mail: frauf@unica.it
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Abstract

Mineralogical-chemical techniques (XRD, SEM/EDX, WDXRF) and a sequential selective extraction procedure were applied to mine-waste materials and stream sediments from the Baccu Locci stream catchment (Sardinia, Italy) affected by serious As contamination as a consequence of past mining. Results indicate that solid-state speciation of As is mainly dominated by the presence of Fe(III) hydroxides (arsenical ferrihydrites with various Fe/As molar ratios) occurring as coatings of silicate grains, in which As is contained as sorbed or co-precipitated species. Scorodite (FeAsO4·2H2O) is common too, whereas arsenopyrite is generally subordinate but, owing to its relatively rapid oxidation, environmentally significant. Moreover, some unidentified arsenates of Ca-Fe or K-Fe were also detected. Arsenic contained in these phases is slowly, but continuously, released in relatively small amounts through three main mechanisms: (1) oxidation of residual arsenopyrite to scorodite; (2) decomposition of scorodite into a hydroxide or oxide of Fe(III); (3) desorption/release from Fe(III) hydroxides. Decomposition of the unidentified arsenates is also probable, e.g. Ca-Fe arsenate → calcite + Fe(III) hydroxide + As release. The flotation tailings are widely scattered and distributed in the middle–lower Baccu Locci stream catchment, and represent the most dangerous As-generating contamination source in the study area.

Keywords

arsenicsolid-state speciationsequential extractionsmine-waste materialsenvironmental mineralogyBaccu Locci mineSardiniaItaly
Type
Research Article
Information
Mineralogical Magazine , Volume 68 , Issue 1 , February 2004 , pp. 15 - 30
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 2004

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