Jordi Cama

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.

Arsenic removal by goethite and jarosite in acidic conditions and its environmental implications

Schwertmannite (Fe(8)O(8)(OH)(5.5)(SO(4))(1.25)), jarosite (KFe(3)(SO(4))(2)(OH)(6)) and goethite (FeOOH) control natural attenuation of arsenic in acid mine drainage (AMD) impacted areas. Batch experiments were conducted to examine the sorption capacity of synthetic goethite and synthetic jarosite at highly acidic pH (1.5-2.5), at two ionic strengths (0.02-0.15 mol dm(-3), NaCl) and at sulphate concentrations in the range of 5 x 10(-3) to 2.8 x 10(-1) mol dm(-3). In the absence of competitive effects of other anions, K-jarosite presents better removal efficiency than goethite for As(V). The maximum sorption capacity is estimated to be 1.2 x 10(-4) and 7.0 x 10(-6)mol m(-2) for jarosite and goethite, respectively, under similar experimental conditions. The variation of arsenic sorbed on goethite as a function of the equilibrium arsenic concentration in solution fits a non-competitive Langmuir isotherm. In the case of K-jarosite, sorption data could not fit a Langmuir or Freundlich isotherm since sulphate-arsenate anion exchange is probably the sorption mechanism. Ionic strength and pH have little effect on the sorption capacity of goethite and jarosite in the small range of pH studied. The presence of sulphate, which is the main anion in AMD natural systems, has a negative effect on arsenic removal since sulphate competes with arsenate for surface sorption sites. Moreover, mobilization of arsenic in the transformation of schwertmannite to jarosite or goethite at pH 2-3 is proposed since the sorption capacities of goethite and K-jarosite are considerably lower than those reported for schwertmannite.

The effect of pH and temperature on kaolinite dissolution rate under acidic conditions

Abstract The main goal of this paper is to demonstrate a new rate law describing the combined effect of pH (0.5 to 4.5) and temperature (25°C to 70°C) on kaolinite dissolution rate, under far from equilibrium conditions, as a step towards establishing the full rate law of kaolinite dissolution under acidic conditions. Dissolution experiments were carried out using non-stirred flow-through reactors fully immersed in a thermostatic water bath held at a constant temperature of 25.0°C, 50.0°C or 70.0°C ± 0.1°C. Kaolinite dissolution rates were obtained based on the release of silicon and aluminum at steady state. The results show good agreement between these two estimates of kaolinite dissolution rate. Kaolinite dissolution rates range as a function of temperature and fluid composition from 8 ± 1 × 10 −15 mol m −2 s −1 (at 25°C and pH 4.5) to 1.5 ± 0.2 × 10 −11 mol m −2 s −1 (at 70°C and pH 0.5). In general, dissolution rate increases with temperature and decreases with pH. The combined effect of pH and temperature is modeled by two independent proton promoted reaction paths. The first reaction path controls the overall dissolution rate at pH ≥ 2.5, whereas the second path controls it below pH 0.5. Between pH 0.5 and 2.5 the two reaction paths influence the rate. Using this model the effects of pH and temperature on the overall dissolution rate of kaolinite under acidic condition can be described by: Rate=2·10 2 ·e −22/RT · 2·10 −10 ·e 19/RT ·a H + 1+2·10 −10 ·e 19/RT ·a H + +5·10 7 ·e −28/RT · 1.4·10 −7 ·e 10/RT ·a H + 1+1.4·10 −7 ·e 10/RT ·a H + where R is the gas constant, T is the temperature (K) and a H + is the activity of protons in solution.

Removal of cadmium, copper, nickel, cobalt and mercury from water by Apatite II™: Column experiments

Apatite II™, a biogenic hydroxyapatite, was evaluated as a reactive material for heavy metal (Cd, Cu, Co, Ni and Hg) removal in passive treatments. Apatite II™ reacts with acid water by releasing phosphates that increase the pH up to 6.5-7.5, complexing and inducing metals to precipitate as metal phosphates. The evolution of the solution concentration of calcium, phosphate and metals together with SEM-EDS and XRD examinations were used to identify the retention mechanisms. SEM observation shows low-crystalline precipitate layers composed of P, O and M. Only in the case of Hg and Co were small amounts of crystalline phases detected. Solubility data values were used to predict the measured column experiment values and to support the removal process based on the dissolution of hydroxyapatite, the formation of metal-phosphate species in solution and the precipitation of metal phosphate. Cd(5)(PO(4))(3)OH(s), Cu(2)(PO(4))OH(s), Ni(3)(PO(4))(2)(s), Co(3)(PO(4))(2)8H(2)O(s) and Hg(3)(PO(4))(2)(s) are proposed as the possible mineral phases responsible for the removal processes. The results of the column experiments show that Apatite II™ is a suitable filling for permeable reactive barriers.

Surface protonation data of kaolinite—reevaluation based on dissolution experiments

The aim of the present study is to compare available surface titration curves of kaolinite, to explain the differences between them, and to constrain their interpretation based on predictions of surface protonation that emerged from dissolution experiments. Comparison of six surface titration curves obtained at 25 degrees C reveals significant discrepancies, both in the shape of the curves and in the pH of the point of zero net proton charge (pH(PZNPC)). Based on an analysis of the different sites available for adsorption on kaolinite surfaces we conclude that different kaolinite samples are expected to have similar pH(PZNPC). Therefore, the major reason for the differences in the observed surface protonation is related to the different ways in which the pH(PZNPC) was determined. To compare the titration curves, some of the curves were recalculated so that the proton surface concentrations of all the titration curves would be zero around pH 5. As a result, we obtained a good agreement between the titration curves. A prediction of the molar fraction of protonated sites was retrieved from modeling of kaolinite dissolution reaction and was compared to the protonation data obtained from surface titration. The model successfully predicts the surface protonation data of most of the surface titrations.

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.

Microcosm experiments to control anaerobic redox conditions when studying the fate of organic micropollutants in aquifer material.

The natural processes occurring in subsurface environments have proven to effectively remove a number of organic pollutants from water. The predominant redox conditions revealed to be one of the controlling factors. However, in the case of organic micropollutants the knowledge on this potential redox-dependent behavior is still limited. Motivated by managed aquifer recharge practices microcosm experiments involving aquifer material, settings potentially feasible in field applications, and organic micropollutants at environmental concentrations were carried out. Different anaerobic redox conditions were promoted and sustained in each set of microcosms by adding adequate quantities of electron donors and acceptors. Whereas denitrification and sulfate-reducing conditions are easily achieved and maintained, Fe- and Mn-reduction are strongly constrained by the slower dissolution of the solid phases commonly present in aquifers. The thorough description and numerical modeling of the evolution of the experiments, including major and trace solutes and dissolution/precipitation of solid phases, have been proven necessary to the understanding of the processes and closing the mass balance. As an example of micropollutant results, the ubiquitous beta-blocker atenolol is completely removed in the experiments, the removal occurring faster under more advanced redox conditions. This suggests that aquifers constitute a potentially efficient alternative water treatment for atenolol, especially if adequate redox conditions are promoted during recharge and long enough residence times are ensured.

The use of Apatite II™ to remove divalent metal ions zinc(II), lead(II), manganese(II) and iron(II) from water in passive treatment systems: column experiments.

The conventional passive treatments for remediation of acid mine drainage using calcite are not totally efficient in the removal of certain heavy metal ions. Although pH increases to 6-7 and promotes the precipitation of trivalent and some divalent metals as hydroxides and carbonates, the remaining concentrations of some divalent metals ions do not fulfill the environmental regulations. In this study, Apatite II™, a biogenic hydroxyapatite, is used as an alternative reactive material to remove Zn(II), Pb(II), Mn(II) and Fe(II). Apatite II™ reacted with acid water releasing phosphate and increasing pH up to 6.5-7, inducing metals to precipitate mainly as metal-phosphates: zinc precipitated as hopeite, Zn(3)(PO(4))(2)·4H(2)O, lead as pyromorfite, Pb(5)(PO(4))(3)OH, manganese as metaswitzerite, Mn(3)(PO(4))(2)·4H(2)O and iron as vivianite, Fe(3)(PO(4))(2)·8H(2)O. Thus, metal concentrations from 30 to 75 mg L(-1) in the inflowing water were depleted to values below 0.10 mg L(-1). Apatite II™ dissolution is sufficiently fast to treat flows as high as 50 m/a. For reactive grain size of 0.5-3mm, the treatment system ends due to coating of the grains by precipitates, especially when iron and manganese are present in the solution.

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.

Kinetics of chalcopyrite dissolution at pH 3

The management of mine wastes and the quantitative forecast of Acid Rock Drainage ( ARD ) generation have aroused widespread interest. In order to be coupled with gas and water flux models, chalcopyrite dissolution kinetics at pH 3, at temperatures from 25 to 70 °C and at variable dissolved oxygen concentrations was studied in flow-through experiments. In the range of conditions under study, the rate of chalcopyrite dissolution is independent of the concentration of dissolved oxygen. This enables chalcopyrite to dissolve throughout the whole waste piles. The rise in temperature from 25 to 70 °C produces an increase of one order of magnitude in the chalcopyrite dissolution rate. As a result of this study, the following expression for chalcopyrite dissolution rate was obtained: \[\mathit{R_{chalcopyrite}}\ =\ 10^{{-}5.52{\pm}0.07}\mathit{e}^{\frac{{-}32{\pm}5}{\mathit{RT}}}\] where R chalcopyrite is the chalcopyrite dissolution rate (molm −2 s −1 ), R is the gas constant (kJmol −1 uK −1 ) and T is the temperature (K). In line with earlier studies carried out under different conditions, iron was released to solution preferentially over copper in all the experiments carried out. XPS examination of the samples showed that reacted surfaces were formed by phases enriched in sulfur and copper (relative to iron) compared with the initial, pristine chalcopyrite surface. However, these surface layers allow the progress of dissolution with out passivating the chalcopyrite surface. According to the apparent activation energy obtained (32±5 kJ mol − 1 ) and to the lack of rate variation with stirring, the chalcopyrite dissolution rate seems to be controlled by surface reactions and is independent of the non-stoichiometric surface layer.