Elizabeth Hernández-Álvarez

Assessment of Atmospheric Metal Pollution in the Urban Area of Mexico City, Using Ficus benjamina as Biomonitor

Concentrations of vanadium, chromium, cobalt, nickel, copper, zinc, antimony, and lead were measured in Ficus benjamina leaves from the Mexico City urban area in order to assess their enrichment against background values. The instrumental analysis was performed using inductively coupled plasma mass spectrometry and the analytical method was tested using two certified reference materials from the National Institute of Standards and Technology (1547 Peach Leaves and 1573a Tomato Leaves). Enrichment factors were calculated, i.e., total to background concentration ratio, for each metal. Low enrichments of vanadium, cobalt, nickel, and copper (≈2), and mild enrichments of chromium and zinc (4.4, 4.5 respectively) were found in the entire area; oppositely, high enrichments were assessed for antimony (28.6) and lead (17.2). However, results indicate that metal concentrations strongly depend on the specific urban sub-area. Increments of metals were attributed to natural, vehicular, and industrial sources.

Metal accumulation by plant species growing on a mine contaminated site in Mexico

The present work aims to assess the efficiency of heavy metal accumulation of native species growing in contaminated soils in the mining district of Taxco, Mexico. Soil and tailing sampling was conducted in three study sites: La Concha, El Fraile, and a control site. The study localities present diverse metal concentrations with significant differences in their proportion in the geochemical fractions. Results show that species Cupressus lindleyi and Juniperus deppeana accumulate Zn and Mn in anomalous concentrations at La Concha, where Zn is present in soluble fractions. Manganese, despite not being present mostly in the soluble fraction in soils and tailings, seems to have been increased in the soluble fraction after the plant growth. In contrast, samples of the same species taken at El Fraile and in the control site, where Zn and Mn are mainly contained in the residual fraction, do not show an anomalous enrichment. Other analyzed species growing under the same contamination conditions in La Concha (Jacaranda mimosifolia and Psidium guajava) do not show anomalous concentrations. These facts confirm the Zn and Mn accumulation capacity of C. lindleyi and Ju. deppeana, which depends on their accumulation ability and on the concentration of these elements in the soluble fraction.

Fractionation and mobility of thallium in areas impacted by mining-metallurgical activities: Identification of a water-soluble Tl(I) fraction

Mining and metallurgy generate residues that may contain thallium (Tl), a highly toxic metal, for which it is currently not feasible to determine its geochemical speciation through X-ray absorption spectroscopy due to a combination of very low contents and the interference of accompanying high arsenic contents. Therefore, fractionation studies in residues and soils are required to analyze the mobility and bioavailability of this metal, which in turn provide information to infer its speciation. For this purpose, in this work a modification of the BCR procedure was applied to residues and contaminated soils from three mining zones of Mexico and two mining zones of Spain, spanning samples with acidic to alkaline pH values. The Tl extraction procedure consisted of the following fractions: (1) water-extractable, (2) easily exchangeable and associated to carbonates, associated to (3) poorly-crystalline and (4) crystalline Fe and Mn oxyhydroxides, and (5) associated to organic matter and sulfides; and finally a residual fraction as associated to refractory primary and other secondary minerals. The extracted contents were analyzed by Inductively-Coupled Plasma with Mass Spectrometry. Surprisingly, water-soluble, in Tl(I) oxidation state, was detected in most areas, regardless of the pH, a fact that has not been reported before in these environments, and alerts to potential health risks not previously identified. Most of the samples from a metallurgy area showed high levels of Tl in non-residual fractions and a strong correlation was obtained between extracted Mn and Tl in the third fraction, suggesting its association to poorly crystalline manganese oxides. In the majority of samples from purely mining environments, most of the Tl was found in the residual fraction, most probably bound to alumino-silicate minerals. The remaining Tl fractions were extracted mainly associated to the reducible mineral fractions, and in one case also in the oxidizable fraction (presumably associated to sulfides). Capsule: Soluble Tl(I) was found in all soil samples contaminated with either mining or metallurgical wastes. Additionally, in those affected by metallurgical wastes a very strong Tl-Mn correlation was found.

Development and validation of an analytical method for the determination of lead isotopic composition using ICP-QMS

This work reports a method for the precise and accurate determination of Pb isotope composition in soils and geological matrices by ICP-QMS. Three reference materials (AGV-2, SRM 2709 and JSO-1) were repeatedly measured, using ICP-QMS instruments in order to assess the quality of this analytical procedure. Mass discrimination was evaluated for Pb/Pb with Pb isotope reference material NIST SRM 981, and the correction applied to the above mentioned reference materials to achieve good accuracy of the analytical methodology. An internal standardization using 205Tl was employed to avoid instrumental drift. The measured isotope compositions of all analyzed reference material were comparable with the reported (JSO-1 and SRM 2709) and certified values (AGV-2). The accuracy results agreed in average within 0.002% for 207Pb/204Pb and 0.825% for 207Pb/206Pb. These data corroborate that ICP-QMS results to be reliable in terms of accuracy and precision, for the determination of the Pb isotopic compositions in soils and geological matrices.

Increase of the Environmental Pt Concentration in the Metropolitan Area of Mexico City Associated to the Use of Automobile Catalytic Converters

This work shows the results of de investigations concerning Pt concentration in the environment of Mexico City, product of the impact of the use of catalytic converters in the urban area. This research was carried out by assessing Pt concentration in dust samples and PM2.5 collected in the metropolitan area of Mexico City. Dust samples were divided in “road dust” and “urban dust” according to vehicle traffic exposure. Pt concentration in road dust samples, exposed to high traffic density, range from 37.7 to 231 µg kg−1 (mean 112 µg kg−1), whereas Pt concentration found in urban dust, collected in areas with low traffic density, varied between 26.4 and 86.6 µg kg−1 (mean 57 µg kg−1). PM2.5 samples were collected in five sites of the urban area during three different seasons (dry-warm, rainy, dry-cold). Results do not show significant spatial or temporal variation. Pt concentration ranges from 1 to 79 pg m−3. The comparison of Pt concentration in dust and PM2.5 samples presented here, with data reported for other urban areas, as well as with data previously reported for PM10 collected in 1991 and 2003 for the metropolitan area of Mexico City, allows to conclude a significant increase of Pt in this urban environment.