C. Ayora

Smectite dissolution kinetics at 80°C and pH 8.8

Abstract The kinetics of dissolution of smectite from the Cabo de Gata volcanic deposit was investigated in the present study. Assuming that the sample is composed solely of smectite, the structural formula of the treated smectite was calculated to be K0.19Na0.51Ca0.195Mg0.08(Al2.56Fe0.42Mg1.02)(Si7.77Al0.23)O20(OH)4 Two types of experiments were carried out: batch experiments to obtain equilibrium data and stirred-flow-through experiments to measure the smectite dissolution rate. All experiments were carried out at a temperature of 80°C and pH of 8.8. mAfter more than 2 yr smectite was still dissolving in the batch experiments, but at a very slow rate. The slow dissolution rate indicates that the system is reasonably close to equilibrium with respect to smectite dissolution. Therefore, the average ion activity product (5 ± 4 × 10−53), obtained from the last samples of the batch experiments, is used as a proxy for the equilibrium constant of the smectite dissolution reaction at 80°C given as Smectite+20H 2 O→0.51Na + +0.19K + +0.195Ca 2+ +1.1Mg 2+ +0.42Fe(OH) 4 − +2.79Al(OH) 4 − +7.77H 4 SiO 4 +0.08OH − In the flow-through experiments at steady state, the average Al/Si (0.33 ± 0.03) and Mg/Si (0.15 ± 0.03) ratios were in very good agreement with these molar ratios of the whole rock analysis (0.35 and 0.14, respectively). The major achievements and conclusions of the present study are as follows: For the first time we present a full stoichiometric dissolution of smectite (i.e., stoichiometric dissolution was observed for Al, Si, and Mg), and show that the obtained dissolution rate is a good measure of the smectite dissolution rate. Pretreatment of the smectite surfaces is necessary to obtain reliable and stoichiometric kinetic results. The dissolution rate of the sample reflects the dissolution rate of the montmorillonitic layers. Under the experimental conditions smectite dissolution rate is not inhibited by aluminum. The dissolution rate of smectite decreases as a function of the silicon concentration. This observation may be explained both by the effect of deviation from equilibrium on dissolution rate and by silicon inhibition, expressed as Rate=−8.1×10 −12 ·(1− exp (−6×10 −10 ·( ΔG r RT ) 6 )) and Rate=( 3.7·10 −17 C Si ) respectively. The current data set cannot be used to differentiate between these two possible reaction mechanisms.

Extraction of soluble major and trace elements from fly ash in open and closed leaching systems

Abstract This study focuses on the mobility of water-soluble major and trace elements in six Spanish fly ashes using room temperature open and closed and heated (90°C) closed leaching systems in an attempt to reduce the content of undesirable species in coal combustion by-products to increase their potential applications. The leaching trends of the trace impurities are consistent with the dissolution of small solid particles or coatings on the surface of the ash solid phases rather than with the dissolution of a homogeneous glass phase. The leaching rate of the different trace impurities can be arranged in decreasing order as B≥Mo≥Se>Li>Sr≥Cr≥As=Ba=Cd=V>Sn>Rb=Zn≥Cu=Ni=Pb>U>Co>Mn. Although several elements showed increased mobility in the room temperature open leaching system with respect to the closed extraction, the differences obtained for major impurities were not significant with the exception of the extraction ratios obtained for As and V in high lime fly ashes. The extraction ratios were higher for the heated leaching than those obtained for room temperature extraction for the elements Al, Si, K, Na, Ba, Cr, Rb, Sr and V. However, the mobility of major impurities (Ca and Fe) and heavy metals did not increase considerably in the heated extraction (generally

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.

The structure of schwertmannite, a nanocrystalline iron oxyhydroxysulfate

Abstract Schwertmannite is a poorly crystalline mineral that forms ochre rusts and precipitates in acid mine environments. Despite its ubiquity and its role as scavenger of important contaminants such as arsenic or selenium, its structure has not been yet determined. Here, a structure for schwertmannite is presented based on pair distribution function (PDF) data, X-ray diffraction (XRD) analyses, and density functional theory (DFT) calculations. We propose a structure formed by a deformed frame of iron octahedra similar to that of akaganeite. Simulations of X-ray diffraction patterns unveil the presence of long-range order associated with the position of the sulfate molecules, providing a useful way to discern two types of sulfate complexes in the structure. The simulations suggest that two sulfate molecules per unit cell are present in the structure forming one outer sphere and one inner sphere complex inside the channels formed by iron octahedra. Knowledge of the positions of the sulfates in the structure will help to better understand exchange processes with oxyanions of trace contaminants, such as arsenate, chromate, or selenate, that strongly influence their biogeochemical cycling in mining ecosystems.

Kinetics of dolomite-portlandite reaction: application to portland cement concrete

The dedolomitization reaction kinetics are studied through several long-term experiments consisting of an aqueous dispersion of fine powders of dolomite and portlandite with different alkalinity, temperature and silica content. The experimental results are reproduced through computer simulation, which allows the estimation of the apparent dissolution constant rates for dolomite. These are discussed together with other parameters influencing the kinetics, in particular the modification of the specific surface of dolomite. The parameters obtained make it possible to predict the behavior of the system beyond experimental periods. Both experimental and simulated results are discussed in connection with the expansion and cracking occurrences observed in portland mortars and concretes made with dolomitic limestone.

Synthesis of high ion exchange zeolites from coal fly ash

This study focuses on the synthesis at a pilot plant scale of zeolitic material obtained from the coal fly ashes of the Teruel and Narcea power plants in Spain. After the optimisation of the synthesis parameters at laboratory scale, the Teruel and Narcea fly ashes were selected as low and high glass fly ashes. The pilot plant scale experiments were carried out in a 10 m3 reactor of Clariant SA (Barcelona, Spain). The results allowed obtaining 1.1 and 2.2 tonnes of zeolitic material with 40 and 55% of NaP1 content, in two single batch experiments of 24 and 8 hours, for Teruel and Narcea fly ashes, respectively. The cation exchange capacities (CEC) of the final product reached 2.0 and 2.7 meq g-1 for Teruel and Narcea zeolitic material, respectively, which are very close to the usual values reached by the high quality natural zeolitic products. Finally, with the aim of testing possible applications of the commercial NaP1-IQE and pilot plant NaP1-Narcea zeolitic products in water decontamination, efficiency for metal uptake from waste waters from electroplating baths was investigated

The deviation-from-equilibrium effect on dissolution rate and on apparent variations in activation energy

Abstract Dissolution rates of minerals depend on several factors. Among them, the factor measuring the deviation of the solution from equilibrium has not been sufficiently investigated. The dissolution rate of kaolinite as it would be “measured” by experiments has been calculated from an expression deduced from experiments. The calculations show that the “measured” dissolution rate may deviate significantly from that corresponding to a far-from-equilibrium solution. The deviation increases as temperature increases, the ratio between flow rate and reactive surface decreases, and pH tends to neutral. This deviation leads to dramatic changes in activation energy as deduced from a regression of a limited number of experiments. Activation energy values tend to decrease apparently as pH tends to neutral, exactly as suggested in the literature for some silicates. Therefore, the deviation-from-equilibrium factor may significantly affect measured dissolution rates, and must be taken into account in obtaining the activation energy values of dissolution/precipitation reactions.

CHEPROO: A Fortran 90 object-oriented module to solve chemical processes in Earth Science models

Accurate prediction of contaminant migration in surface and ground water bodies, including interaction with aquifer and hyporheic zone materials requires reactive transport modeling. The increasing complexity and the procedure-oriented type of programming often used in reactive transport hinder codes reuse and transportability. We present a Fortran 90 module using object-oriented concepts that simulates complex hydrobiogeochemical processes (CHEPROO, CHEmical PRocesses Object-Oriented). CHEPROO consists of a general structure with two classes. The Nodal Chemistry class accounts for the description of local chemistry and geochemical state variables. As such, it provides many functions related to basic operations (evaporation, mixing, etc.) and can easily grow on this direction (extreme dry conditions, biochemical state variables, etc.). The Chemical System class includes kinetic and thermodynamic models that describe reactions between and within phases. As such, it can grow in the direction of increasingly complex chemical systems (solid solutions, microorganisms as individual phases, etc.), without loss in the handling of simple problems. These two classes are overlaid by CHEPROO, a general structure designed for interaction with other codes. CHEPROO can be used as a geochemical tool for the modeling of complex processes such as biodegradation or evaporation at high salinities. However, many functions CHEPROO are devoted to coupling a broad range of chemical processes to other phenomena (flow, transport, mechanical). We have shown that reactive transport (based on either DSA or SIA approaches) could be easily implemented into existing conservative transport code with a minimal number of changes.

Biogenic hydroxyapatite (Apatite II™) dissolution kinetics and metal removal from acid mine drainage.

Apatite II™ is a biogenic hydroxyapatite (expressed as Ca(5)(PO(4))OH) derived from fish bone. Using grains of Apatite II™ with a fraction size between 250 and 500 μm, batch and flow-through experiments were carried out to (1) determine the solubility constant for the dissolution reaction Ca(5)(PO(4))(3)(OH) ⇔ 5Ca(2+) + 3PO(4)(3-) + OH(-), (2) obtain steady-state dissolution rates over the pH range between 2.22 and 7.14, and (3) study the Apatite II™s mechanisms to remove Pb(2+), Zn(2+), Mn(2+), and Cu(2+) from metal polluted water as it dissolves. The logK(S) value obtained was -50.8±0.82 at 25 °C. Far-from-equilibrium fish-bone hydroxyapatite dissolution rates decrease by increasing pH. Assuming that the dissolution reaction is controlled by fast adsorption of a proton on a specific surface site that dominates through the pH range studied, probably ≡PO(-), followed by a slow hydrolysis step, the dissolution rate dependence is expressed in mol m(-2) s(-1) as where Rate(25 °C) = -8.9 × 10(-10) × [9.96 × 10(5) × a(H+)]/[1 + 9.96 × 10(5) × a(H+)] where a(H+) is the proton activity in solution. Removal of Pb(2+), Zn(2+), Mn(2+) and Cu(2+) was by formation of phosphate-metal compounds on the Apatite II™ substrate, whereas removal of Cd(2+) was by surface adsorption. Increase in pH enhanced the removal of aqueous heavy metals. Using the kinetic parameters obtained (e.g., dissolution rate and pH-rate dependence law), reactive transport simulations reproduced the experimental variation of pH and concentrations of Ca, P and toxic divalent metal in a column experiment filled with Apatite II™ that was designed to simulate the Apatite II™-metal polluted water interaction.