Evaluation of the interactions of arsenic (As), boron (B), and lead (Pb) from geothermal production wells with agricultural soils.

Geothermal energy is a low-pollution energy source. However, air, soil, and water near geothermal plants may be affected by their operation. One of the largest geothermal energy sources in the world, Cerro Prieto, has a capacity of 720 MW and is located in northwest Mexico near an agricultural area. The abstracted geothermal fluids, which are enriched with arsenic (As), boron (B), lead (Pb), cadmium (Cd), and other heavy metals, are either reinjected into the aquifer or sent to an evaporation pond located in the geothermal plant. Because spills have occurred in other geothermal zones, it is important to evaluate the effect of those contaminants on the soils of the surrounding area and their possible infiltration into shallow groundwater. To that aim, soils (one chromic Vertisol and two calcic Regosols) from three sites near the Cerro Prieto Geothermal Plant were sampled to evaluate their behavior regarding As, Pb, and B retention. Batch experiments were carried out using the soils as the sorbent and geothermal water from three geothermal production wells as the sorbate. Raw water concentrations in each well were as follows: As: 0.2442 mg/L, 0.2774 mg/L, and mg/L; B: 18.409 mg/L, 13.5075 mg/L, and 16.646 mg/L; and Pb: 0.22 mg/L, 0.13 mg/L, and 0.26 mg/L. The physicochemical characteristics of the soils were determined and compared to the experimental results. A good adjustment of the chromic Vertisol sample to Freundlich isotherms was observed for As (r2 > 0.9), followed by Pb (r2 = 0.61), and B (r2 > 0.5). As retention also showed a good adjustment to the Langmuir model (r2 > 0.9). The retention followed the order Pb >As ≫B in one of the two calcic Regosols, while the other only retained Pb ≫ As. Cationic exchange capacity; clay minerals; carbonate; organic matter; and iron, aluminum, and manganese amorphous and crystalline oxides influenced the soils' retention capacities. Irrigation with geothermal water could not imply a toxicity risk to plants grown in the chromic Vertisol soil due to its high Pb and As sorption capacity. Pb concentration could not be a toxicity issue in the calcic Regosols for the same reason, but As and B could be. B would be a hazard to vegetables and water due to its low or lack of retention in the three soils and also for its possible infiltration into shallow groundwater used for irrigation in the area. This study highlights the importance of maintaining adequate operation and control of the disposal of geothermal fluids in geothermal plants.

Remediation by means of EDTA of an agricultural calcareous soil polluted with Pb.

The dispersion of mine tailings affects ecosystems due to their high content of potentially toxic elements. Environmental risk increases when the soil impacted by tailings is used for agriculture; this use may result in health impacts. This study analyzes the feasibility of remediating a calcareous soil (used for maize cultivation) polluted with lead in the semiarid zone of Zimapán, México, by using EDTA as an extractant. Total geoavailable and bioaccessible concentrations in the gastric and intestinal phases were determined to evaluate lead availability and health risk. The soil was then washed with EDTA, and the geochemical fractionation (interchangeable, carbonates, Fe/Mn oxy-hydroxides, organic matter-sulfides, and residual) and impact on the mesophile bacteria and fungi/yeast populations were analyzed. The results showed total Pb concentrations up to 647 ± 3.50 mg/kg, a 46% bioaccessible fraction (297 ± 9.90 mg/kg) in the gastric phase and a 12.2% (80 ± 5 mg/kg) bioaccessible fraction in the intestinal phase, indicating a health and environmental risk. Meanwhile, the geochemical fractionation before washing showed a Pb fraction mainly consisting of Fe/Mn oxy-hydroxides (69.6%); this reducible fraction may progressively increase its bioaccessibility. Geochemical fractionation performed in the washed soil showed differences from that determined before the treatment; however, the iron and manganese fraction, at 42.4%, accounted for most of the Pb. The soil microbiology was also modified by EDTA, with an increase in aerobic bacteria and a decrease in fungi/yeast populations. Although 44% total lead removal was achieved, corresponding to a final concentration of 363.50 ± 43.50 mg/kg (below national and USEPA standards), washing with EDTA increased the soluble and interchangeable lead concentrations. Statistical analysis indicated a significant effect (p < 0.05) of EDTA on the soil's geochemical fractionation of lead.

Evaluation of the influence of main groundwater ions on arsenic removal by limestones through column experiments.

The influence of common groundwater major ions on arsenic (As) retention by native limestones was studied through column experiments. Columns were packed with rock particles (0.5-1.41 mm) and fed with solutions containing As, and chloride (Cl), sulfate, bicarbonate or fluoride (F) in concentrations similar to those measured in one of the most As-rich wells of Mexico. Besides, other solutions were also treated containing multiples or submultiples of those anion concentrations. Physico-chemical parameters, As, and each anion concentrations were determined weekly along 4 months. After the end of the experiments rocks collected from the top of the columns were analyzed by XRF, XRD, and SEM-WDS. Concentrations of As decreased from 1.2 mg/L to values below the Mexican drinking water standard (0.025 mg/L), since the first week in the solutions containing F or Cl keeping low values afterwards, indicating that they do not interfere with As removal. However, although As strongly decreased in the solutions containing sulfate since the first week, it started to increase from the 12th week in the highest concentrated solution. Bicarbonate was the anion affecting most As retention, since, after its decrease below 0.025 mg/L in the 2nd week for both solutions (30 mg/L and 300 mg/L), it started to increase since the 7th week in the most concentrated one, but maintained a low concentration in the least concentrated solution. Saturation index calculations and XRD analyses did not evidence the formation of As minerals. However, SEM elemental maps and XRF analyses showed the presence of As on the rocks after the treatments. Results indicate that As may be retained by sorption. Sulfate and bicarbonate compete with As for sorption sites. Results showed that native limestones are an option for treating As polluted water. Experiments also indicated that bicarbonate and sulfate may interfere with As removal depending on their concentrations.

Kinetic approach for the appropriate selection of indigenous limestones for acid mine drainage treatment with passive systems.

Acid mine drainage treatments using limestones have been widely reported in the literature; however, additional studies are needed to select the most effective limestone type based on an adequate characterization and in consideration of the kinetics of the rock's reaction upon exposure to high iron concentrations. In this study, with the aim to select the most appropriate limestone to use in a passive treatment system, the regular characterization (calcium carbonate analysis, determination of specific superficial area, and porosity) was complemented with a heterogeneous kinetic analysis of limestone dissolution. The physico-chemical conditions of high acidity and a high Fe concentration were similar to those measured in leachates from the "Compañía Minera Zimapán" (CMZ) tailings impoundment located in a historical Mexican mining zone. Column experiments were carried out with the selected limestone to treat leachates from two tailing deposits; one highly weathered and un-active (CMZ) and the other still active (San Miguel Nuevo). Removal efficiencies close to 100% were reached for arsenic, iron, cadmium, and aluminum. There was also a partial removal of zinc and silica, and the pH increased close to neutrality. Electrical conductivity, sulfate levels, and oxidation reduction potential were also measured during the experiments. Concentration profiles for some elements were established. Chemical results, stoichiometric relationships between elements obtained by scanning electron microscopy-energy dispersive spectroscopy, and scanning electron microscopy-wavelength dispersive spectroscopy allowed for determining the chemical associations of the elements at the surface. The results indicated that the methodology for limestone selection to treat AMD from San Miguel Nuevo tailings was adequate; however, additional studies are required to improve the permeability and the lifetime of the system used to treat CMZ leachates.