Raman spectroscopy of the system iron(III)-sulfuric acid-water: an approach to Tinto River's (Spain) hydrogeochemistry

Map of impact by acid mine drainage in the river network of The Iberian Pyrite Belt (Sw Spain)

The Iberian Pyrite Belt (IPB), in the southwest of Europe, is characterized by high levels of contamination by acid mine drainage (AMD) in a large extent of its river network. In this scenario, it is necessary to characterize the degree of pollution of the mining leachates in the AMD-generating sources as well as of the main receiving watercourses. A map of impact of each basin was developed, based on the model proposed by Grande (2011) and the European Directive 98/83/EC that defines the quality standards for drinking water. The results indicate that practically all the mining leachates exceeded the maximum concentrations established by Directive 98/83/CE for Fe and Cd, almost 90% exceeded the limit for Mn and 82% for Al. Likewise, Fe, Cd, and Mn caused 'extremely high' degradation in most sampled leachates. Similarly, these metals, in addition to Pb, produced more pollution in watercourses located downstream of exploitations.

Raman spectroscopy of efflorescent sulfate salts from Iron Mountain Mine Superfund Site, California

The Iron Mountain Mine Superfund Site near Redding, California, is a massive sulfide ore deposit that was mined for iron, silver, gold, copper, zinc, and pyrite intermittently for nearly 100 years. As a result, both water and air reached the sulfide deposits deep within the mountain, producing acid mine drainage consisting of sulfuric acid and heavy metals from the ore. Particularly, the drainage water from the Richmond Mine at Iron Mountain is among the most acidic waters naturally found on Earth. The mineralogy at Iron Mountain can serve as a proxy for understanding sulfate formation on Mars. Selected sulfate efflorescent salts from Iron Mountain, formed from extremely acidic waters via drainage from sulfide mining, have been characterized by means of Raman spectroscopy. Gypsum, ferricopiapite, copiapite, melanterite, coquimbite, and voltaite are found within the samples. This work has implications for Mars mineralogical and geochemical investigations as well as for terrestrial environmental investigations related to acid mine drainage contamination.

Iron speciation in natural hyperacid water investigated by Mössbauer spectroscopy

We have demonstrated the usefulness of the archetypical solid state-technique of Mössbauer spectroscopy to non-invasive studies of the redox and coordination chemistry of iron in a natural hyperacid solution from Iron Mountain, CA. Suitable fast cooling conditions were used to prepare a glass from the natural solution. Measurements of spectra at low temperatures (6 and 15K) and in large external magnetic fields (6T) of the glassy, frozen solution revealed a behaviour characteristic of slow paramagnetic relaxation of ferric ions. Analyses of the spectral components are in good agreement with dominance of hexaaquairon(III) ions.

A Raman spectral study of stream waters and efflorescent salts in Rio Tinto, Spain

Acidic waters and sulfate-rich precipitates are found in mine tailings such as Rio Tinto (Huelva, SW, Spain). In this work we have characterized the chemical constituents of stream water and have identified some efflorescent salts and precipitates by means of Raman spectroscopy. Variable amounts of sulfate and bisulfate are found in the aqueous samples, suggesting changes in the acidity of the solutions. An estimation of the sulfate/water relative abundance is also given. Solid samples are readily identified as gypsum and as mixtures of hydrated hydroxysulfates belonging to the copiapite group. These results are consistent with previous works reporting the mineralogy and water composition of acid mine drainage-related sites, and proves the importance of Raman spectroscopy as a tool for accurate and noninvasive analyses of acid waters and associated geochemistry.