J. de Pablo

EXPERIMENTAL STUDY AND MODELING OF THE U(VI)-FE(OH)3 SURFACE PRECIPITATION/COPRECIPITATION EQUILIBRIA

Abstract The kinetics and thermodynamics of the coprecipitation of U(VI) with Fe(III) oxyhydroxides have been studied under controlled laboratory conditions, by means of chemical solution and spectroscopic techniques. The monitoring of uranium concentration after the simultaneous precipitation of Fe(III) and U(VI) from a homogeneous solution indicates that a fast and quantitative sorption of uranium on the Fe(III) oxyhydroxide surface occurs. This is followed by the gradual release of a part of the uranium from the solid phase which can be attributed to a structural rearrangement of the solid surface. Equilibrium uranium concentrations are attained after fifteen days of contact. Several surface complexation models are tested to model the behavior of the system and a simple thermodynamic approach has been proposed in order to describe this post-adsorption phenomena. A conditional solubility constant for the uranium limiting phase is defined as K SO ∗ = K SO · χ , where KSO is the solubility constant for the individual solid phase and χ stands for the molar fraction of the uranium in the bulk of the solid. Treatment of data obtained at different U(VI) to Fe(III) initial concentration ratios and at fixed pH value has allowed us to propose the surface precipitation of schoepite on Fe(III) oxyhydroxide as the uranium solubility limiting phase. It has been shown that the behavior of the system can be equally described by both surface complexation and conditional solubility approaches. However, the surface complexation modelling requires the knowledge of the surface properties of the solid. The much simpler coprecipitation approach is only based on mass balances and stoichiometric relationships, which can be a useful predictive tool for the trace metal concentrations in natural systems.

Treatment of acid mine drainage by sulphate-reducing bacteria using permeable reactive barriers: A review from laboratory to full-scale experiments

Acid mine drainage in-situbioremediation has in the last decades drawnthe attention in the field of environmentalbiotechnology. The most recent treatmenttechnique are the permeable reactive barriersusing sulphate-reducing bacteria. This viewdescribes the basis of many of the currentapproaches to use sulphate-reducing bacteria inacid mine drainage treatment, from laboratoryto full-scale realisations, and the limitationsencountered when applied to full scaleapplications.

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.

Identification of organophosphorus insecticides and their hydrolysis products by liquid chromatography in combination with UV and thermospray-mass spectrometric detection

Liquid chromatography with ultraviolet detection (LC-UV) and positive and negative ion mode (PI and NI, respectively) thermospray LC-mass spectrometry (LC-MS) were used for the analysis of the organophosphorus pesticides azinphos-methyl, diazinon and parathion-methyl and their corresponding breakdown products obtained after basic hydrolysis (pH 7-11). LC analysis was performed in the reversed-phase mode using methanol-water (80:20) or methanol-water (70:30) + 0.1 M ammonium acetate for LC-UV or LC-MS, respectively. By employing NI thermospray LC-MS the identification of p-nitrophenol, showing the [2M-H]- ion as the base peak, was feasible and confirmed the LC-UV chromatogram at 220 nm. When the PI mode was used, [M + NH4]+ and [M + H]+ ions were obtained as base peaks for azinphos-methyl and diazinon, respectively. The degradation rates varied from diazinon, which showed no degradation during a period of ten days, to azinphos-methyl and parathion-methyl, for which degradation occurred rapidly when the pH was increased from 7 to 11.

Kinetics of corrosion and dissolution of uranium dioxide as a function of pH

A continuous flow-through reactor with a thin layer of solid particles (size ranging from 100 to 300 μm) was used to obtain a deeper knowledge on the mechanism of dissolution of UO2 under oxidizing conditions. Using this methodology the dissolution rate of uranium dioxide was determined at three different oxygen partial pressures (5, 21, and 100% in nitrogen) and as a function of pH (between 3 and 12) in a noncomplexing medium. From the results of these experiments the following rate equation was derived: In addition, XPS characterizations were performed to determine the U(IV)/U(VI) ratio on the solid surface at different experimental times and conditions. These results showed that at acidic conditions (pH below 6.7) the final solid surface presents a stoichiometry close to UO2, while at alkaline conditions the final solid surface average composition is close to UO2.25. This information was integrated with the results of the leaching experiments to present a model for the mechanism of dissolution of uranium dioxide under the experimental conditions.

Solid surface evolution model to predict uranium release from unirradiated UO2 and nuclear spent fuel dissolution under oxidizing conditions

The dissolution of UO2 under oxidizing conditions has been studied in the last years in different waste disposal conditions. These studies have indicated the importance of the solid surface evolution during leaching experiments. In this work, a mathematical model based on X-ray photoelectron spectroscopy determinations of the solid surface was developed. This model allows the uranium release under oxidizing conditions at acidic pH or in carbonate medium to be predicted. At alkaline pH without carbonate, the formation of a UO2.33 surface layer and its equilibrium with the uranium concentration in solution could be responsible for the disagreement observed between the model and the experimental data. This model has been also applied to uranium release from spent nuclear fuel dissolution experiments carried out in granitic groundwater.

Effect of H2O2, NaClO and Fe on the dissolution of unirradiated UO2 in NaCl 5 mol kg−1. Comparison with spent fuel dissolution experiments

Copyright (c) 1996 Elsevier Science B.V. All rights reserved. The effect of H 2 O 2 , NaClO and Fe on the dissolution of unirradiated UO 2 (sr in NaCl 5 mol kg −1 has been studied at neutral to alkaline pH. Dissolution rates have been determined as a function of oxidant concentration. A general equation to correlate both parameters has been obtained: log r=(−8.0p0.2r+log[Ox 0.93& plusmn; 0.07 . The values obtained have been compared to those given for spent fuel under the same experimental conditions. The effect of iron is similar in both unirradiated UO 2 and spent fuel with a final uranium concentration around 5×10 −8 mol kg −1 which corresponds to the solubility value of UO 2 (fr under reducing conditions.

Denitrification in presence of acetate and glucose for bioremediation of nitrate‐contaminated groundwater

With the current increasing interest in aquifer denitrification, recent attention has been given to cost‐effective in‐situ treatments such as Enhanced In‐Situ Biological Denitrification (EISBD), which intends to stimulate the indigenous bacterial activity by injecting an external organic substrate and/or nutrients to the aquifer matrix. Within this context, laboratory batch assays have been conducted to develop a strategy for in‐situ denitrification of a nitrate‐contaminated aquifer in Argentona, Catalonia (Spain). The assays were run under aerobic and anaerobic conditions at a temperature of 17°C to better simulate the conditions of the aquifer. Acetate and glucose were added to assess their potential to promote heterotrophic denitrifying bacteria activity. Overall, the results revealed that indigenous micro‐organisms had the potential of reducing nitrate under appropriate conditions. Nitrate removal was complete and faster under anaerobic conditions, though high nitrate removals were also attained under initial aerobic conditions when a readily organic compound was amended at a sufficient dosage. The results also revealed that a significant amount of the available organic carbon was consumed by processes other than denitrification, namely aerobic oxidation and other microbial oxidation processes. To sum up, the results of this study demonstrated that addition of organic compounds into the groundwater is a promising method for in‐situ bioremediation of nitrate in the Argentona aquifer. This approach could potentially be applied to a number of situations in which nitrate concentration is elevated and where indigenous micro‐organisms with potential to reduce nitrate are present within the aquifer material.

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.

Liquid-liquid and solid-liquid extraction of gold by trioctylmethylammonium chloride (TOMAC1) dissolved in toluene and impregnated on amberlite XAD-2 resin

Abstract The extraction of Au(III) with the extractant trioctylmethylammonium chloride (TOMAC1) dissolved in toluene and the same extractant impregnated in a polymeric resin XAD-2 has been studied. The results of gold extraction with both systems were compared. They showed similar gold extraction behaviour (extracted species), although greater amounts of extractant were needed when impregnated resins were used. Liquid-liquid and solid-liquid procedures can be used to extract gold selectively from solutions containing zinc and copper.