Cristina Domènech

Oxidative dissolution of pyritic sludge from the Aznalcóllar mine (SW Spain)

Abstract As a result of the collapse of a mine tailing dam, a large extension of the Guadiamar valley was covered with a layer of pyritic sludge. Despite the removal of most of the sludge, a small amount remained in the soil, constituting a potential risk of water contamination. The kinetics of the sludge oxidation was studied by means of laboratory flow-through experiments at different pH and oxygen pressures. The sludge is composed mainly of pyrite (76%), together with quartz, gypsum, clays, and sulphides of zinc, copper, and lead. Trace elements, such as arsenic and cadmium, also constitute a potential source of pollution. The sludge is fine grained (median of 12 μm) and exhibits a large surface (BET area of 1.4±0.2 m 2 g −1 ). The dissolution rate law of sludge obtained is r =10 −6.1(±0.3) [O 2 (aq)] 0.41(±0.04) a H+ 0.09(±0.06) g sludge m −2 s −1 (22 °C, pH=2.5–4.7). The dissolution rate law of pyrite obtained is r =10 −7.8(±0.3) [O 2 (aq)] 0.50(±0.04) a H+ 0.10(±0.08) mol m −2 s −1 (22 °C, pH=2.5–4.7). Under the same experimental conditions, sphalerite dissolved faster than pyrite but chalcopyrite dissolves at a rate similar to that of pyrite. No clear dependence on pH or oxygen pressure was observed. Only galena dissolution seemed to be promoted by proton activity. Arsenic and antimony were released consistently with sulphate, except at low pH conditions under which they were released faster, suggesting that additional sources other than pyrite such as arsenopyrite could be present in the sludge. Cobalt dissolved congruently with pyrite, but Tl and Cd seemed to be related to galena and sphalerite, respectively. A mechanism for pyrite dissolution where the rate-limiting step is the surface oxidation of sulphide to sulphate after the adsorption of O 2 onto pyrite surface is proposed.

Nitrate pollution of groundwater; all right. . ., but nothing else?

Contamination from agricultural sources and, in particular, nitrate pollution, is one of the main concerns in groundwater management. However, this type of pollution entails the entrance of other substances into the aquifer, as well as it may promote other processes. In this study, we deal with hydrochemical and isotopic analysis of groundwater samples from four distinct zones in Catalonia (NE Spain), which include 5 different aquifer types, to investigate the influence of fertilization on the overall hydrochemical composition of groundwater. Results indicate that intense fertilizer application, causing high nitrate pollution in aquifers, also homogenize the contents of the major dissolved ions (i.e.; Cl(-), SO4(2-), Ca(2+), Na(+), K(+), and Mg(2+)). Thus, when groundwater in igneous and sedimentary aquifers is compared, significant differences are observed under natural conditions for Cl(-), Na(+) and Ca(2+) (with p-values ranging from <0.001 to 0.038), and when high nitrate concentrations occur, these differences are reduced (most p-values ranged between 0.054 and 0.978). Moreover, positive linear relationships between nitrate and some ions are found indicating the magnitude of the fertilization impact on groundwater hydrochemistry (with R(2) values of 0.490, 0.609 and 0.470, for SO4(2-), Ca(2+) and Cl(-), respectively). Nevertheless, the increasing concentration of specific ions is not only attributed to agricultural pollution, but to their enhancing effect upon the biogeochemical processes that control water-rock interactions. Such results raise awareness that these processes should be evaluated in advance in order to assess an adequate groundwater resources management.

The impact of the Aznalcóllar mine tailing spill on groundwater

As a consequence of a mine tailing dam collapse on the 25th April 1998, more than 4000 ha of the Guadiamar riverflat and farmlands were flooded by 4 hm3 of sulphide slurry. A number of open wells (12 of the 47 analyzed) were also flooded and the water was contaminated. Before the spill, the groundwater in the aquifers was of calcium-carbonate and calcium-sulphate type, with moderate mineralisation and near neutral pH. With the exception of some of the wells close to the mine, this groundwater had a low concentration of the metals associated with the Aznalcóllar mine. After the flood the following metals had anomalous concentrations in well water: Zn, Mn, Pb, Co, Cd and Tl. Of these, Zn seems to be the best tracer of the contamination, owing to its high concentrations. During the 5 months following the spill, water from the unflooded wells did not show an increase in metal concentration. Apart from some exceptions in August, the metal concentration in the affected wells showed a progressive decrease reaching levels closer to those in the wells free from contamination. Nevertheless, in the following dry seasons the draw-down of the water level may lead to exposure and weathering of sulphides in the wells, which could cause an increase in pollution. Therefore, thorough cleaning of all highly contaminated wells is strongly recommended.

Sludge weathering and mobility of contaminants in soil affected by the Aznalcollar tailing dam spill (SW Spain)

As a result of the collapse of the Aznalcollar mine tailings dam, a large extension of the Guadiamar valley in SW Spain was covered with a layer of pyrite sludge. Although most of the sludge was removed, a small amount remains in the soil, constituting a potential source of water pollution. A column experiment was carried out in order to determine the rate of sludge oxidation in the soil, and the existence of metal retention processes. The column was filled with a mixture of sludge and a sandy soil common in the region. At different time intervals, the column was leached with water and the resulting solution analysed. The pH of the water dropped to values around 2 after 260 days and then remained constant due to the buffering role of silicate dissolution. The concentration of Zn, Cd and Co in the leachates matched the expected values from flow-through experiments at atmospheric oxygen pressure. This indicates that oxygen diffusion in the pores was complete. Moreover, no efficient processes existed for retaining Zn, Cd and Co, which formed soluble salts and were entirely incorporated into the infiltrating water. During the first 2 months of the experiment, when pH was higher than 4.5, the concentrations of Fe and Al were very low. Saturation indices showed that the solution was in equilibrium with amorphous Fe(OH)3 and Al(OH)3. Subsequently, at lower pH values, jarosite [(Na,K)Fe3(SO4)2(OH)6] formed. This solid phase was identified by X-ray diffraction (XRD) and Scanning Electron Microscopy (SEM). Jarosite was also responsible for the depletion of As, Pb, Sb and probably Tl in the water resulting from infiltration.

Long-term geochemical evolution of the near field repository: Insights from reactive transport modelling and experimental evidences

The KBS-3 underground nuclear waste repository concept designed by the Swedish Nuclear Fuel and Waste Management Co. (SKB) includes a bentonite buffer barrier surrounding the copper canisters and the iron insert where spent nuclear fuel will be placed. Bentonite is also part of the backfill material used to seal the access and deposition tunnels of the repository. The bentonite barrier has three main safety functions: to ensure the physical stability of the canister, to retard the intrusion of groundwater to the canisters, and in case of canister failure, to retard the migration of radionuclides to the geosphere. Laboratory experiments (< 10 years long) have provided evidence of the control exerted by accessory minerals and clay surfaces on the pore water chemistry. The evolution of the pore water chemistry will be a primordial factor on the long-term stability of the bentonite barrier, which is a key issue in the safety assessments of the KBS-3 concept. In this work we aim to study the long-term geochemical evolution of bentonite and its pore water in the evolving geochemical environment due to climate change. In order to do this, reactive transport simulations are used to predict the interaction between groundwater and bentonite which is simulated following two different pathways: (1) groundwater flow through the backfill in the deposition tunnels, eventually reaching the top of the deposition hole, and (2) direct connection between groundwater and bentonite rings through fractures in the granite crosscutting the deposition hole. The influence of changes in climate has been tested using three different waters interacting with the bentonite: present-day groundwater, water derived from ice melting, and deep-seated brine. Two commercial bentonites have been considered as buffer material, MX-80 and Deponit CA-N, and one natural clay (Friedland type) for the backfill. They show differences in the composition of the exchangeable cations and in the accessory mineral content. Results from the simulations indicate that pore water chemistry is controlled by the equilibrium with the accessory minerals, especially carbonates. pH is buffered by precipitation/dissolution of calcite and dolomite, when present. The equilibrium of these minerals is deeply influenced by gypsum dissolution and cation exchange reactions in the smectite interlayer. If carbonate minerals are initially absent in bentonite, pH is then controlled by surface acidity reactions in the hydroxyl groups at the edge sites of the clay fraction, although its buffering capacity is not as strong as the equilibrium with carbonate minerals. The redox capacity of the bentonite pore water system is mainly controlled by Fe(II)-bearing minerals (pyrite and siderite). Changes in the groundwater composition lead to variations in the cation exchange occupancy, and dissolution-precipitation of carbonate minerals and gypsum. The most significant changes in the evolution of the system are predicted when ice-melting water, which is highly diluted and alkaline, enters into the system. In this case, the dissolution of carbonate minerals is enhanced, increasing pH in the bentonite pore water. Moreover, a rapid change in the population of exchange sites in the smectite is expected due to the replacement of Na for Ca.

Experimental study and modeling of uranium (VI) transport through ferrous olivine rock columns

The Lovasjärvi intrusion (SE-Finland) contents a high percentage of ferrous olivine (> 65%). This material has been suggested as a redox-active backfill-additive in deep nuclear waste repositories, due to the large Fe(II) proportion in its mineral composition. In order to understand the processes involved in the redox buffering capacity of this material the transport of uranium (VI) through olivine columns was studied. The results showed considerable retardation factor for the U(VI), particularly in carbonate-free media. The experimental data were simulated by means of reactive transport modeling. The best agreement between the experimental and calculated data was obtained considering that the interaction of U(VI) with the olivine surface occurred at two different types of sorption sites. One type accounts for the sorption capacity of the olivine mineral, and a second type accounts for the sorption on amorphous Fe(OH)3(s) formed at expenses of the oxidative dissolution of olivine.

Characterizing sources and natural attenuation of nitrate contamination in the Baix Ter aquifer system (NE Spain) using a multi-isotope approach

Nitrate pollution is a widespread issue affecting global water resources with significant economic and health effects. Knowledge of both the corresponding pollution sources and of processes naturally attenuating them is thus of crucial importance in assessing water management policies and the impact of anthropogenic activities. In this study, an approach combining hydrodynamic, hydrochemical and multi-isotope systematics (8 isotopes) is used to characterize the sources of nitrate pollution and potential natural attenuation processes in a polluted basin of NE Spain. δ2H and δ18O isotopes were used to further characterize the sources of recharge of the aquifers. Results show that NO3- is not homogeneously distributed and presents a large range of concentrations, from no NO3- to up to 480mgL-1. δ15N and δ18O of dissolved NO3- identified manure as the main source of nitrate, although sewage and mineral fertilizers can also be isotopically detected using boron isotopes (δ11B) and δ34S and δ18O of dissolved sulphate, respectively. The multi-isotope approach proved that natural denitrification is occurring, especially in near-river environments or in areas hydrologically related to fault zones. δ34S and δ18O indicated that denitrification is not driven by pyrite oxidation but rather by the oxidation of organic matter. This could not be confirmed by the study of δ13CHCO3 that was buffered by the entanglement of other processes and sources.

Modelling the oxidation of sulphides in an unsaturated soil

Abstract Within the framework of a study on the impact of a mine-tailing spill at Aznalćollar, SW Spain, we investigated the oxidation of pyrite and other sulphides by means of two column experiments and reactive transport modelling. The columns were filled with pyritic sludge mixed up with a sandy and a clayey soil, respectively. The columns were located outdoors for 15 months and leached 10 times. Prior to simulating reactive transport, a flow model permitted a detailed description of the behaviour of the column at a daily time-scale. The most important parameter extracted was the hydraulic saturation. This parameter controlled the amount of O2 that could diffuse into the soil, which, in its turn, affected the rate of pyrite oxidation. The sandy and clayey columns behaved very differently. In the sandy column, pH dropped due to the oxidation of pyrite. As a result, silicate minerals dissolved, providing Na and/or K that precipitate together with Fe and SO4 as jarosite. The high concentration of Zn in the leachates was consistent with the concentrations predicted from sphalerite oxidation. The low As and Pb concentrations, however, were explained by their coprecipitation in the jarosite. In the clayey column, the dissolution of dolomite kept the pH high, impeding the dissolution of silicate minerals and precipitating amorphous Fe(OH)3 in the place of jarosite. The model also permitted rate laws proposed in the literature for pyrite oxidation to be discussed. We found that the oxidation of pyrite by Fe3+ was not faster than by O2, contrary reports in the literature. Finally, the model was used to predict the behaviour of other soil types and other sludge contents. According to the predictions the dissolution of jarosite was very important to maintaining the pH at a value of approximately 2, even for gravels or low reactive sand.

Carbon and Chlorine Isotope Fractionation Patterns Associated with Different Engineered Chloroform Transformation Reactions

To use compound-specific isotope analysis for confidently assessing organic contaminant attenuation in the environment, isotope fractionation patterns associated with different transformation mechanisms must first be explored in laboratory experiments. To deliver this information for the common groundwater contaminant chloroform (CF), this study investigated for the first time both carbon and chlorine isotope fractionation for three different engineered reactions: oxidative C-H bond cleavage using heat-activated persulfate, transformation under alkaline conditions (pH ∼ 12) and reductive C-Cl bond cleavage by cast zerovalent iron, Fe(0). Carbon and chlorine isotope fractionation values were -8 ± 1‰ and -0.44 ± 0.06‰ for oxidation, -57 ± 5‰ and -4.4 ± 0.4‰ for alkaline hydrolysis (pH 11.84 ± 0.03), and -33 ± 11‰ and -3 ± 1‰ for dechlorination, respectively. Carbon and chlorine apparent kinetic isotope effects (AKIEs) were in general agreement with expected mechanisms (C-H bond cleavage in oxidation by persulfate, C-Cl bond cleavage in Fe(0)-mediated reductive dechlorination and E1CB elimination mechanism during alkaline hydrolysis) where a secondary AKIECl (1.00045 ± 0.00004) was observed for oxidation. The different dual carbon-chlorine (Δδ13C vs Δδ37Cl) isotope patterns for oxidation by thermally activated persulfate and alkaline hydrolysis (17 ± 2 and 13.0 ± 0.8, respectively) vs reductive dechlorination by Fe(0) (8 ± 2) establish a base to identify and quantify these CF degradation mechanisms in the field.

Compound-Specific Chlorine Isotope Analysis of Tetrachloromethane and Trichloromethane by Gas Chromatography-Isotope Ratio Mass Spectrometry vs Gas Chromatography-Quadrupole Mass Spectrometry: Method Development and Evaluation of Precision and Trueness

Compound-specific chlorine isotope analysis of tetrachloromethane (CCl4) and trichloromethane (CHCl3) was explored by both, gas chromatography-isotope ratio mass spectrometry (GC-IRMS) and GC-quadrupole MS (GC-qMS), where GC-qMS was validated in an interlaboratory comparison between Munich and Neuchâtel with the same type of commercial GC-qMS instrument. GC-IRMS measurements analyzed CCl isotopologue ions, whereas GC-qMS analyzed the isotopologue ions CCl3, CCl2, CCl (of CCl4) and CHCl3, CHCl2, CHCl (of CHCl3), respectively. Lowest amount dependence (good linearity) was obtained (i) in H-containing fragment ions where interference of 35Cl- to 37Cl-containing ions was avoided; (ii) with tuning parameters favoring one predominant rather than multiple fragment ions in the mass spectra. Optimized GC-qMS parameters (dwell time 70 ms, 2 most abundant ions) resulted in standard deviations of 0.2‰ (CHCl3) and 0.4‰ (CCl4), which are only about twice as large as 0.1‰ and 0.2‰ for GC-IRMS. To compare also the trueness of both methods and laboratories, samples from CCl4 and CHCl3 degradation experiments were analyzed and calibrated against isotopically different reference standards for both CCl4 and CHCl3 (two of each). Excellent agreement confirms that true results can be obtained by both methods provided that a consistent set of isotopically characterized reference materials is used.