Reactive Transport of Chromium in Water Circuits around the Mine Area of Libiola (Sestri Levante, Genoa)

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3 Marina Accornero Reactive Transport of Chromium in Water Circuits around the Mine Area of Libiola (Sestri Levante, Genoa)

4 Copyright MMV ARACNE editrice S.r.l. via Raffaele Garofalo, 133 a/b Roma (06) fax (06) ISBN I diritti di traduzione, di memorizzazione elettronica, di riproduzione e di adattamento anche parziale, con qualsiasi mezzo, sono riservati per tutti i Paesi. Non sono assolutamente consentite le fotocopie senza il permesso scritto dell Editore. I edizione: luglio 2005

5 Introduction I. Obectives II. Results and conclusions II. 1 The use of synthetic cationic and anionic exchangers II. 2 The aqueous complexes of Cr3+ and CrO42- ions II. 3 Mixing of acid mine waters and stream waters and related precipitation and sorption processes II. 4 Chromium distribution II. 5 Practical implications Main Abbreviations in this Work Speciation of Aqueous Chromium Preliminary remarks Comparison of thermodynamic data and database extention Aqueous ions and chromium species involved in hydrolysis reactions Polymeric species of trivalent chromium Chromium-sulphate complexes: nature of the interactions in the aqueous phase and association constants Metal-chromate complexes: estimation of dissociation costants Final remarks on the database used for speciation calculations.....

6 2. Analytical Methods for the Determination of Chromium in Natural Waters and Related Problems Limitations of analytical methods Characteristics and functioning of synthetic ion exchangers se of synthetic ion exchangers as a tool of separation and preconcentration for the chemical and isotopic analysis of chromium Cr(III)-Cr(VI) separation by means of cation exchange, H+-form resins: a quantitative study using a synthetic solution Recovery of Cr(VI) by use of Cl form anion exchange resins: batch and column tests on synthetic solutions Additional factors potentially affecting the Cr(III)-Cr(VI) separation Chromium in the groundwaters from the ligurian ophiolitic basins: a critical review of previous analytical results Solid Phases Possibly Limiting Cr (III) and Cr (VI) Solubilities in AMD-Polluted Water Circuits Preliminary considerations Solubility and thermodynamic stability of solid phases associated to AMD Fe(III) compounds eochemical behavior of chromium: natural processes affecting element mobility and solid phases limiting its solubility Incorporation of Cr(III) into solid precipitates Reduction of Cr(VI) to Cr(III) Direct sequestration of chromate Completion of the thermodynamic database The Gromolo Catchment and the Waters Discharges from the Libiola Mine Geological frameork of the study area Lithological features of the Gromolo basin Genesis of acid aters discharging from the Libiola mine

7 5. Sampling and Analysis of Waters in the Gromolo Basin Waters types and sampling sites Field measurements and sampling operations Water analysis Analysis of the anions Determination of carbonate alkalinity and TDIC computation Analysis of major cations and trace elements Fe(II) determination in mine waters Determination of Cr(III) and Cr(VI): pre-treatement of samples and analysis Chromium in the waters of the romolo catchment: interpretation of analytical results Mine waters Stream waters and groundwaters Tables Table I Results of chromium analysis Table II Analytical results for mine waters Table III Analytical results for stream waters and groundwaters Chemical Classification of Waters Anionic and cationic triangular plots Square diagram of Langelier-Ludig Triangular cross-section of the L-L pyramid: salinity diagram Magnesium-sulphate correlation plot Bicarbonate-magnesium-silica triangular plot Geochemical Modeling of Mixing, Precipitation of Solid Phases, and Sorption: the Fate of Chromium and Other Pollutants in Surface Waters Geochemical modeling of mixing and precipitation of secondary solid phases

8 7.1.1 Sequence of precipitation Compositional variations in the solid mixtures of hydroxides and jarosites Electrostatic models of solutes adsorption onto the hydrous ferric oxides (HFO) Characteristics and surface charge of HFO Electrostatic sorption models or surface complexation models Application of the Diffuse Layer Model to model solute adsorption on ferrihydrite upon mixing of the Libiola mine waters with surface waters Results and discussion Distribution of major and trace elements: comparisonof analytical data with the results of mixingadsorption geochemical modeling Behavior of major elements: sulphate correlation plots The fate of chromium Distribution of other transition metals: Cu, Zn, Ni, and Mn Tables Table IV Concentrations of dissolved and adsorbed ions computed by the MINTEQA2 simulation Table V Computed percentages of adsorption on ferrihydrite and distribution coefficients for metals and oxyanions Table VI Computed concentration of surface complexes on ferrihydrite as a function of ph The Treatment of Libiola Acid Mine Waters: Preliminary Considerations se of chemical treatments to limit the environmental impact of AMW on the neighbouring ater bodies The liming hypothesis: experimental evidence and theoretical simulation Hydrogeological Characterization of the Alluvial Deposits of the Sestri Levante Plain Collection of stratigraphic data and related information

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10 Introduction This thesis is mainly aimed at understanding the speciation and the fate of dissolved chromium into the waters, chiefly acid, discharging from the derelict Libiola Fe-Cu mine, as well as in other stream waters and groundwaters of the area. Mining activities, protracted for approximately one century ( ), enhanced enormously the release of different metals, including chromium, from rocks to mine waters and, subsequently, to the neighbouring water bodies. The presence of chromium-rich ophiolitic rocks determines the attainment of comparatively high concentrations of this element in waters. I. Objectives. In detail, the main objectives of this work are: The study of the redox state of chromium and its aqueous speciation in mine waters as well as in surface waters and groundwaters variably impacted by Acid Mine Drainage(AMD). The study of the main processes of natural attenuation of chromium concentrations in the water bodies surrounding the mine site, that is Fe(II)-driven reduction of hexavalent chromium, mixing between mine waters and stream waters, precipitation of iron hydroxides, and sorption of cations and anions on these newly formed solids. The hydrological characterization of the alluvial deposits and the construction of a numerical model for the underlying bedrock, as preliminary steps to model water flow and transport of pollutants. The evaluation of the extent of hexavalent chromium contamination, particularly for the groundwaters utilized as drinking waters. To achieve these objectives, the following activities were carried out: (i) Several sampling campaigns of the mine waters, surface waters, and groundwaters of the investigated area, which were subjected to suitable field and laboratory analysis. Special care was devoted to the 11

11 12 use of cationic and anionic exchange resins which should allow the separation of the two main forms of dissolved chromium, i.e., Cr(III) and Cr(VI), at least based on an initially optimistic point of view. (ii) An extensive work of collection and critical review of thermodynamic data referring to the formation of Cr(III) and Cr(VI) aqueous complexes and to the dissolution of solid phases possibly limiting chromium solubility in AMD-polluted circuits. (iii) The simulation of the geochemical processes of interest by means of suitable software codes, namely (a) EQ3/6, version 7.2b (Wolery, 1992; Wolery and Daveler, 1992) for speciation-saturation calculations and simulation of mixing between acid mine waters and surface waters and (b) MINTEQA2 (Allison et al., 1991; Hydrogeologic, 1998) in order to reproduce the sorption of several cations and anions on the iron hydroxides precipitating from polluted surface waters; (iv) The 3D-reconstruction of the alluvial deposits in the terminal sectors of the Gromolo and Petronio basins, through the collection, codification, computerization and geo-statistical treatment of the available stratigraphic data for wells, drillings, and piezometers. II. II.1 Results and Conclusions. The use of synthetic cationic and anionic exchangers. In order to study chromium transport in the aqueous phase, it is essential to determine its redox state in the different types of sampled waters, since mobility and dispersion of chromium are chiefly linked to its oxidation state. With the aim of separating the two chromium redox forms before analysis, all sampled waters were passed through a column charged with the synthetic cationic exchanger Dowex 50WX8, that retains only the positively charged ionic species due to exchange reactions with the resin functional groups, while an equivalent amount of H + ions are released to the aqueous solution. Several preliminary experiments were performed with the specific purpose to evaluate both the efficiency of the resin for Cr(III) - Cr(VI)

12 Introduction. 13 separation and the resistance of the organic molecules (styrene divinylbenzene) forming its matrix to Cr(VI), which is known to be a rather strong oxidizing agent. In these experiments, the total mass of chromium was quantitatively recovered from synthetic solutions containing 10 ppm of both Cr(III) and Cr(VI), but the initial 1:1 ratio between the two redox forms was altered during the migration through the column, independent on the ph of the initial solution. These results were ascribed to partial reduction of hexavalent chromium (involving on the average 36% of it) coupled with oxidation of the reduced carbon of the styrene divinylbenzene copolymer. On the contrary, this reduction process did not take place during the separations carried out for groundwater and stream water samples of the Gromolo catchment, probably because the concentrations of Cr(VI) (< to ppm) are markedly lower than those adopted in the experiment. It may be worth noting that the partial reduction of hexavalent Cr, which occurs during the separation step, at least at relatively high concentrations, have important practical implications, since it brings about an alteration of the isotopic ratios of the two Cr redox forms. The anionic resin Dowex 1X8 was also tested. The main obstacle to its direct use is represented by the strong retention of hexavalent chromium on the resin, which prevents its recovery through eluition with either hydrochloric acid or concentrated saline solutions. Experimental tests carried out during this thesis confirmed, instead, that use of 2M HNO 3 brings about the complete reduction of Cr(VI) to Cr(III) and the virtually complete (97% of total) recovery of the Cr(VI) initially retained by resin, by using a volume of acid twice the volume of the initial solution. Smaller amounts of acid could be utilized adopting a continuous-cyclic elution mechanism. Summing up, the use of cationic and anionic exchangers as a tool for the separation and preconcentration of chromium forms and sample preparation for isotopic analysis is certainly not immediate. This mainly depends on the high reactivity of hexavalent chromium. These difficulties are not clearly underscored in the geochemical literature. For instance, some authors have recently submitted to the U.S. Environmental Protection Agency (EPA) an analytical technique

13 14 based on the use of ion exchange resins, as a standard method for the determination of the redox state of chromium, without any word of caution concerning the difficulties discussed above. Further possible problems on the use of ion exchange resins are caused by complexation effects. On the one hand, in aqueous solutions poor of complexing agents (i.e., anions for Cr 3+ ion and cations for CrO 4 2- ion), dissolved trivalent chromium is mostly present in form of positively charged species (apart from Cr(OH) 3 and Cr(OH) 4 - under relatively high ph values) and hexavalent chromium in form of oxyanions. On the other hand, in acid mine waters, which have a complex chemical matrix and, in particular, high sulphate contents, neutral and/or negatively charged Cr(III)-sulfate complexes do originate, such as Cr(OH)SO 4, Cr 2 (OH) 2 (SO 4 ) 2 and, especially the di-sulphate Cr(SO 4 ) 2 - complex, which is poorly known in the existing literature. Of course, all these Cr(III)-species are not retained by the cationic exchange resin. Besides, the stable persistence of hexavalent chromium in such solutions is precluded, due to the high Fe(II) contents of acid mine waters, as evidenced by literature data on the kinetics of Cr(VI) reduction driven by ferrous iron. Consequently, it can be hypothesised that separations carried out on these waters by means of ion exchange resins reproduce the ratio between cationic and anionic species of trivalent chromium. II.2 The aqueous complexes of Cr 3+ and CrO 4 2- ions. Analytical results confirmed that treatment of acid mine waters with the cationic exchanger Dowex 50WX8 brings about the separation of the cationic species of trivalent chromium from its anionic species. Based on these findings, it was obtained an experimental value for the overall dissociation constant of the Cr(SO 4 ) 2 - complex, which turned out to be as log b 2. In this way it was reduced the uncertainty on this dissociation constant, which was initially estimated through different empirical relationships, obtaining log b 2 values ranging from and

14 Introduction. 15 The knowledge of the stability of the di-sulphate complex of Cr(III) resulted to be a fundamental step for unraveling the speciation of Cr(III) in mine waters, where sulphate concentration in some cases exceeds ppm. It turned out that the Cr(III) speciation is, in fact, described completely by the two sulphate-complexes Cr(SO 4 ) 2 - and CrSO 4 + and by the non-complexed ion Cr 3+. Due to the introduction of high concentrations of metals into the surface waters and groundwaters of the study area, where Cr(VI) resulted to be the prevailing Cr form, it is important to know the stability of metal-chromate complexes, to reconstruct Cr(VI) speciation. Again, very limited data are available in the geochemical literature. Therefore, the association constants for the chromate complexes of Cu, Zn, Mn, Ca, Mg and Al were derived through linear correlations between the standard molar Gibbs free energies of formation of the metal cations and those of the corresponding chromate complexes. Among these, the CuCrO 4 complex resulted to be very stable, in agreement with the findings by Pettine et al. (1998) on the different rates of Cr(VI) reduction when the Cu 2+ ion is either absent (low reduction rate) or present (high reduction rate due to the formation of the CuCrO 4 complex). Dissolved Cr(VI) resulted to be the dominantly present as CuCrO 4 in the polluted stream waters samples GR-5, GR-6, and GR-7. Contrasting thermodynamic data are available in the existing literature for the products of the progressive hydrolysis of Cr 3+ ion. Although these discrepancies affect the predominance limits among these Cr(III) aqueous species, they have negligible consequences on the computed speciation for surface waters, since in the great majority of samples Cr(III) is not detectable, being limited by the incorporation into precipitating solid phases, chiefly ferrihydrite. II.3 Mixing of acid mine waters and stream waters and related precipitation and sorption processes. As already recalled, the fate of chromium and other pollutants in surface waters was simulated by modeling, in separate steps, the precipitation/coprecipitation effect (by means of the software package

15 16 EQ3/6) and the sorption on ferrihydrite (by means of the code MINTEQA2), which is the most important authigenic phases. Theoretical results were then compared with analytical data to highlight the main processes governing the distribution of dissolved constituents. In the mixing simulation between the Gromolo River waters collected upstream of the mine area, with 5 ppb of Cr(VI), and a mine water, with a Cr(III) content of about 1000 ppb, the precipitation of the most typical solid phases associated to AMD-polluted water circuits, such as the Libiola area, was allowed. These phases are (in order of appearance) a solid mixture of jarosites, schwertmannite, alunite, basaluminite, and a solid mixture of hydroxides, with a ferrihydrite molar fraction close to 95%. In the second step, the total concentration of relevant solutes and the moles of precipitating ferrihydrite computed by the EQ6 mixing experiment were plugged, as input data, in the code MINTEQA2, version 4.0 (Allison et al., 1991; Hydrogeologic, 1998a, b) to model the sorption of relevant cations (H +, Cr 3+, Cu 2+, Zn 2+, Mn 2+, Ni 2+, Ca 2+, and Mg ) and anions (CrO 4 and SO 2-4 ) on ferrihydrite, adopting the electrostatic diffuse layer model (Dzombak and Morel, 1990). This software redistributes the total amount of a given element, not incorporated in solid phases, between the true dissolved fraction and the fraction complexed on the surface sites of ferrihydrite, considering the values of the intrinsic constants assigned to the different surface complexation reactions and ph, ionic strenght, and concentrations of dissolved substances in the aqueous solution. The results of this model were also utilized to compute the distribution coefficients (K d ) for the different solutes, which are needed to describe the reactive transport of pollutants into porous media. The comparison between the theoretical concentrations of relevant solutes and available analytical data (obtained in this study or in previous work) suggests that the concentration of major elements in surface waters is chiefly controlled by: (i) attainement of the solubility products of specific phases: Fe is mainly incorporated in ferrihydrite and subordinately in schwertmannite and jarosite; Al is chiefly sequestered into

16 Introduction. 17 basaluminite and alunite; K is temporarily incorporated in jarosite and alunite and is restituted to the aqueous solution upon their dissolution (ii) acid water stream water mixing (Mg, Na, Cl, HCO 3, SiO 2 ) (iii) mixing combined with leaching of solid phases (Ca and ph), although the sorption on ferrihydrite is not negligible for some major constituents, like SO 4, Ca and Mg. The distribution of traces is mainly governed by sorption on ferrihydrite and secondarily by coprecipitation, excepting for trivalent chromium which is significantly incorporated into the hydroxide solid mixture. The removal of Cr(III), Cu, Zn, Ni, Mn, and Cr(VI) from the aqueous solution occurs in narrow ph ranges, as proven by the steep sorption curves and it is pratically complete below the ph of equivalence for + - FeOH and FeO on ferrihydrite, which lies between 7.3 and 2 7.4, for the examinated aqueous system. II.4 Chromium distribution. In the surface waters and in groundwaters sampled upstream of the mine area, Cr(VI) is the stable form of chromium, whereas acid mine waters represent localized sources of Cr(III). The ph increase, caused by addition of bicarbonate-rich waters to acid-sulphate waters, drastically reduce Cr(III) mobility (see above). Moreover, Cr(VI) contributed by surface waters is reduced to Cr(III), until complete oxidation of Fe(II) (which is introduced in the system by acid mine waters) to Fe(III), driven by reduction of dissolved O 2. Consequently, Cr(VI) results to be stable only in mixtures with high fractions of surface water. The distribution of Cr(VI) is not influenced by incorporation into precipitating solid phases, whereas sorption on ferrihydrite is particularly efficient to remove it. Interestingly, the sorption curve of Cr(VI) is rather peculiar in that it mimics the sorption curves of cations and it is the mirror image of those of anions. In fact, chromate sorption is affected by both: (i) the great stability of the CuCrO 4 neutral complex (see above) and (ii) the competitive effects with sulphate, whose association with the ferrihydrite surface modifies strongly the total charge of the sorbent.

17 18 II.5 Practical implications. The natural attenuation of Libiola AMD deriving from dilution with the Gromolo creek water and related processes of coprecipitation and sorption on ferrihydrite is not sufficient for mitigating the environmental impact of AMD on this aqueous system. Although these processes decrease the contents of several dissolved metals below the maximum acceptable concentrations set by the Italian regulation, the sorbed metals can be released and re-introduced in the aqueous system. Therefore, suitable remediation techniques have to be selected, tested, and implemented to mitigate the environmental threat caused by AMD. At present, different strategies can be followed to reduce the acidity and the concentrations of potentially harmful elements and species (PHES), including the neutralization of acid mine waters through reaction with alkaline solutions or application of solid carbonates (liming) and the dilution of acid mine waters with low-salinity waters (Evangelou, 1995). In any case, the precipitating Fe(III) oxyhydroxides (ferrihydrite and schwertmannite) and Al hydroxisulfates (e.g., basaluminite) have to be separated and these solids have to be properly disposed. As a practical implication of this investigation, let us take into account the liming hypothesis, whose experimental testing indicates that the ph of the aqueous solution should stabilise at ~ 5.5. Comparing this ph value with the sorption edges for Cr 3+, Cu 2+, Zn 2+, Ni 2+, Mn 2+, and CrO 4 2-, it is evident that only Cr 3+ and Cu 2+ are expected to be eliminated effectively from the aqueous solution through sorption on ferrihydrite (and coprecipitation for what concerns trivalent Cr) at ph of 5.5, whereas the same does not applies to the other constituents. Therefore, liming does not seem a suitable methodology, at least under the conditions of our experiments.