(2016): Formation and Significance of Laurionite in Geothermal Brine
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Originally published as: Regenspurg, S., Driba, D. L., Zorn, C. (2016): Formation and significance of laurionite in geothermal brine. ‐ Environmental Earth Sciences, 75, 865 DOI: http://doi.org/10.1007/s12665‐016‐5668‐4 1 Title page: 2 Authors: Simona Regenspurg1*, Dejene Legesse Driba1, Carolin Zorn1 3 Title: Formation and significance of laurionite in geothermal brine 4 Affiliations: 5 1 Helmholtz Centre Potsdam 6 GFZ German Research Centre for Geosciences 7 Section 4.1 Reservoir Technologies 8 International Centre for Geothermal Research (ICGR) 9 Telegrafenberg, D‐14473 Potsdam, Germany 10 11 * corresponding author: email: regens@gfz‐potsdam.de; phone:+49 (0)331 288 1437 12 13 14 15 Abstract 16 The metal lead (Pb) is an environmentally significant, frequently occurring element in 17 geothermal brines. It was found to precipitate from saline fluids predominantly as laurionite 18 (PbOHCl) forming one of the most dominant scaling minerals identified at the geothermal site 19 Groß Schönebeck (Germany). In this study, its formation conditions and relevance for the 20 environment are investigated and discussed. To simulate laurionite precipitation, various 21 amounts of base (NaOH) were added to solutions containing 3 M NaCl, 1 M CaCl2, and 10 mM 22 Pb(NO3)2 at 25 °C, 80 °C, and 133 °C. The formed precipitates were analyzed by X‐ray 23 diffraction and scanning electron microscopy. In nearly all experiments laurionite had formed 24 as the predominant/ only mineral. Experimental results showed the same trends for 25 equilibrium calculations performed with the code PhreeqC and the database “geodat”. The 26 results indicate that the formation of laurionite from geothermal, Pb‐bearing saline brine is 27 very probable, with the pH value as the main controlling parameter for its formation. It can be 28 assumed that laurionite plays an important role in these systems not only by removing Pb 29 from the brine but also by buffering the pH value in the fluid. Since scales removed from 30 geothermal wells often contain significant amounts of the radioactive 210Pb isotope, it can be 31 assumed that the mineral laurionite acts as scavenger for this isotope and is therefore of 32 environmental relevance. Adequate disposal and handling of the formed scales need to be 33 considered. 34 35 Keywords 36 Laurionite, scaling, lead, geothermal, chloride, pH control 37 1 1 1 Introduction 2 Saline, geothermal fluids from deep sedimentary basins or from crystalline rocks are often 3 enriched in toxic and/or radioactive metals such as copper (Cu), arsenic (As), lead (Pb), or 4 radium (Ra) (Kharaka & Hanor 2004; Regenspurg et al., 2010; 2014). These elements 5 accumulate in the formation water due to their long term interactions between water and the 6 host rock as well as due to the formation of aqueous complexes between the metals and 7 ligands such as chloride complexes that prevent the metals from precipitation (Frape et al. 8 2004, Kharaka & Hanor 2004). Lead often occurs in very high concentrations in geothermal 9 fluids for example with up to 95 to 96 ppm (as Pb2+) in brines from the Salton Sea and Soultz 10 geothermal sites, respectively (McKibben et al. 1987; Scheiber et al. 2013). It is of special 11 relevance in these systems not only because it is very toxic, but also because deep formation 12 fluids often carry elevated amounts of the natural radioactive isotope 210Pb (an intermediate 13 decay product of the uranium‐238 series) with a half‐life of 22.2 years (Takahashi et al. 1987). 14 In presence of sulphate, sulphide, carbonate, or oxide, Pb2+ forms some hardly soluble salts 15 (e.g. PbSO4, PbS, PbCO3, PbO) thereby removing the Pb from the aqueous phase. When 16 producing geothermal brine for heat or electricity production the physicochemical conditions 17 of the brine often change resulting in a potential oversaturation of those salts. Mineral 18 precipitations (“scaling”) generally represent a problem for geothermal power plant operation 19 because they are clogging pipes thereby reducing the fluid flow through the system or they 20 are coating the heat exchanger and thus reducing the efficiency of the geothermal plant. If 21 these precipitates form within the pores of a reservoir, it might even results in reservoir 22 damage and in complete failure of the system. In addition, some of these solid residues 23 collected from filters and pipes are often enriched with natural occurring radioactive isotopes 24 such 210Pb, 226Ra, 228Ra, 232Th, and 238U and have to be deposed adequately (Degering & Köhler, 25 2011). 26 The Pb mineral laurionite (PbOHCl) has recently been found to occur at some geothermal sites 27 (e.g. Groß Buchholz and Groß Schönebeck) located in the North German Basin (Hesshaus et 28 al. 2013; Regenspurg et al. 2015). At the Groß Schönebeck in situ geothermal research 29 platform fluid production had to be terminated in 2013 due to decreasing production rates 30 that possibly were linked to the simultaneous formation of massive scaling that accumulated 31 in the production well, clogging it for nearly 200 m (Blöcher et al., 2015). Laurionite was 32 detected as one of the four most dominant minerals (together with native copper, magnetite, 2 1 and barite) in the material that filled the well (Regenspurg et al. 2015). Laurionite can thus be 2 considered as significant in those geothermal systems, although it is unclear at which chemo‐ 3 physical conditions it actually forms. 4 The white or colorless orthorhombic hydroxy halogenide laurionite (Pb(OH)Cl) with clear 5 cleavability at {101}, typically forms tabular, prismatic, or acicular crystals (Venetopoulos & 6 Rentzeperis 1975). It is dimorph with the monoclinic paralaurionite that also crystallizes as 7 prismatic and transparent shapes (Merlino et al. 1993). Both occur rarely in nature (Merlino 8 et al. 1993) but can be found as formation product from Pb‐bearing industrial waste or as 9 secondary mineral in the oxidation zone of Pb‐containing ore deposits (Russell, 1928; Palaehe, 10 1934). The very first description of laurionite was from several antique mining slag deposits in 11 the area of Laurion (Attica, Greece; Köchlin 1887). These slag residues have been dumped into 12 the sea about 2000 years ago, allowing the enclosed Pb to react with the chloride of sea water 13 (Angel & Scharizer 1932). Laurionite, however also frequently forms as part of solid waste in 14 lead smelters (Ioannidis et al. 2006). Artificially, laurionite has been produced by reaction of 15 lead and sodium chloride and was used as make up already by the ancient Egypt (Walter et 16 al., 1999; Hess, 1999) or as white pigment for medieval paintings (Hoyer 2010). 17 Edward et al. (1992) investigated in detail the mineral formation of the PbO‐H2O‐HCl system 18 at 292.2 K. At these conditions a range of lead minerals can form depending on the pH value 19 and chloride content of the solution: cottunite (PbCl2), penfieldite (Pb2Cl3OH), fiedlerite 20 (Pb3Cl4(OH)2), laurionite (PbOHCl), paralaurionite (PbOHCl), mendipite (Pb3O2Cl2), blixite 21 (Pb2Cl(OH)3, and litharge (PbO) have been observed in carbonate‐free systems. As shown in 22 the stability diagram with increasing pH and increasing salinity, the laurionite becomes more 23 stable (Fig. 1, Edward et al., 1992). However, chloride activities used for these calculations 24 (e.g. Fig. 1) were still clearly below concentrations of many geothermal brines especially of 25 those hosted in sedimentary basins (Kharaka & Hanor 2004), such as measured at the 26 geothermal site of Groß Schönebeck (about 5 M chloride; Regenspurg et al. 2010). 27 Consequently, most databases used for geochemical equilibrium calculations, for example 28 those included in the code PhreeqC (Parkhurst & Apello 2013), do not contain laurionite and 29 its equilibrium constants as mineral species. Solely the MINTEQA2 version 4 and WATEQ4F 30 database (Peterson et al. 1987; Allison et al. 1990;U EPA, 1998, Ball & Nordstrom, 2001) 31 consider laurionite and gives a solubility constant of log K = 0.623 for PbOHCl + H+ = Pb2+ + Cl‐ 32 + H2O. The llnl database considers paralaurionite with a log K = 0.2035 (Wolery 1992). 3 1 However, in both the MINTEQA2 and in the WATEQ4F database, enthalpies are not included 2 for laurionite and thus they should not be used for calculations at higher temperatures. 3 Moreover, the activity coefficient calculations included in these databases either apply the 4 extended (Wateq) Debye‐Hückel attempt which is valid up to 0.7 mol/Kg or the Davies 5 equation which goes up to 0.5 mol/kg. For solutions of higher salinities (like in many 6 geothermal brines) the Pitzer equations should be used. The Geodat thermodynamic database 7 has been recently developed especially for applications at geothermal conditions (Moog & 8 Cannepin 2014). It can be implemented in other geochemical codes such as PhreeqC and 9 calculates metal speciation explicitly for highly saline solutions. The database includes various 10 lead chloride species as a function of temperature and ionic strength based on the stability 11 constants from Luo & Millero (2007). It allows determining lead chloride speciation with Pitzer 12 activity coefficients for ionic strengths from 0 to 6 M and for up to 300 °C although no T‐ 13 dependent solubility data for laurionite are available. 14 Due to limited data availability and restricted potential to perform geochemical equilibrium 15 modeling, the aim of this experimental study was to investigate if and at which geothermal‐ 16 similar conditions laurionite can form and if the application of equilibrium modeling by using 17 the Geodat database shows similar trends as observed by the experimental data. The 18 prediction of potential laurionite formation in a geothermal system is of high relevance for 19 geothermal well operation in order to develop methods of avoiding Pb scaling and thus 20 reducing the enrichment of radioactive 210Pb in the above ground installation or in the solid 21 waste (e.g.