Assessment of arsenic occurrence in different mining environments by the development and application of suitable analytical methodologies

Resumen

Arsenic is widespread in some regions of Asturias (Northern Spain) as a consequence of the abandonment of the facilities of some cinnabar mines after their closures and the currently open-air state of their corresponding spoil heaps. Natural weathering or variations in physicochemical parameters at the disposal site can alter the stability of waste materials, mobilizing mercury, arsenic and related heavy metals to the surrounding soils and sediments, provoking their solubilisation into water and the possible assimilation by the autochthon plants. The current Doctoral Thesis intends to evaluate the environmental impact provoked by arsenic in some cinnabar mining areas through the assessment of its potential mobility and availability in soils and sediments, in their surrounding surface waters and in some autochthon plants growing in those sites. Extremely high total arsenic concentrations were found in the studied soils, sediments and waste samples from the considered mine sites. Arsenic mobility is largely dependent on its associations with solid phases. With the aim of adequately evaluate arsenic mobility in soils and sediments, a novel sequential extraction procedure (SEP) has been developed and optimized for the study of arsenic fractionation in environmental solid samples. It intends to sequentially extract the most important solid phases scavenging arsenic, with special emphasis on the most labile fractions. The selectivity of the selected extracting agents was assured through experiments with natural and synthetic matrices of As-bearing minerals and arsenic-spiked materials. The protocol was proven to be applicable to variety of samples with different arsenic pollution sources. The proposed SEP was advantageously compared with other protocols currently applied to study arsenic and heavy metals mobility, since it offers a more appropriate arsenic distribution pattern. The application of this protocol to sediments and waste samples from the studied mining areas evidenced that the mobility of arsenic was dependent on the mineralogical and physicochemical characteristics of the site and on the extent of mining operations. The surrounding surface waters of the studied mine sites exhibited high arsenic concentrations. In water, arsenic speciation determines arsenic mobility, since As(III) is more mobile than As(V). The study of arsenic speciation in surrounding waters of those mines was performed by using the hyphenated technique HPLC-HG-AFS. As(V) was practically the only species found in all the tested waters. Different preservation procedures for arsenic speciation were tested, and no differences were found among the samples stored without any additive and those preserved with HCl or EDTA. An assessment of the factors controlling arsenic solubilisation in each mine site was performed based on physicochemical characterization and statistical studies of waters. Results showed that arsenic mobilization was largely in accordance with the mineralogy of each area. An investigation about possible seasonal variations of arsenic and other parameters influencing its mobility showed that they were not significantly dependent on the rainfall regime. Plants growing in the studied mine sites presented high arsenic accumulation in their tissues. For those autochthon plants the study of arsenic speciation was performed by HPLC-ICP-MS. For the extraction of arsenic species (As (III), As(V), MMA, DMA) from the plant tissues a novel extraction methodology was developed and optimized. It achieved excellent extraction efficiencies, even for plants containing extremely high arsenic contents, with proven no interconversion of species. As(V) was the predominant species, and roots were found responsible for accumulating the most arsenic. The extent of arsenic accumulation by the plants was proven to be dependent on both the amount of bioavailable arsenic in their corresponding soils, and the plant species in question.