04-Pennisi 37-40

Atti Soc. tosc. Sci. nat., Mem., Serie A., 117-119 (2012) pagg. 37-40, figg. 2, tab. 1; doi: 10.2424/ASTSN.M.2012.26

MADDALENA PENNISI (*), PAOLO ORLANDI (*) (**)

CHEMICAL AND MINERALOGIC CHARACTERISTICS OF BOROGYPSUM PRODUCED BY ACID DISSOLUTION OF COLEMANITE AT LARDERELLO (ITALY)

Abstract - Chemical and mineralogic characteristics of borogypsum Ores are found in Anatolia (Turkey) and California produced by acid dissolution of colemanite at Larderello (Italy). Large (USA), where the mineral is stratabound in Tertiary amount of borogypsum, the by-product of the acid boric industrial extraction from colemanite, were produced at Larderello industry rocks (Helvaci, 1977); colemanite also occurs in the and discharged into the environment during the last century, before salars of Argentina and Bolivia (Helvaci & Alonso, the regulations for environmental concern were established. In this 2000). Colemanite was considered a secondary miner- study we investigate the chemical and mineralogical characteristics al formed after original deposition of borax (Helvaci, of the waste, to define geochemical tracers of this source for environmental application. 1977), until a primary origin was recognized by Hel- vaci and Alonso (2000). Key words - Boric acid production, colemanite, borogypsum, envi- For industrial application, boric acid is generally pro- ronmental pollution, Larderello (Tuscany, Italy). duced by dissolving colemanite (50.81 % B2O3) in a sulfuric acid solution, according to the following reaction: Riassunto - Caratteristiche chimiche e mineralogiche del borogesso prodotto per dissoluzione dell’acido dalla collemanite a Larderello (Ita- 2CaB O (OH) . H O + 2H SO + 6H O = 2CaSO . lia). Grandi quantità di borogesso, scarto industriale del processo di 3 4 3 2 2 4 2 4 produzione dell’acido borico dalla colemanite, sono state riversate nel- 2H2O + 6H3BO3 l’ambiente dall’industria chimica di Larderello, prima dell’emanazione della legislazione a favore della tutela ambientale. In questo studio ven- After reaction, boric acid is crystallized from the gono presentate le caratteristiche chimiche e mineralogiche del boro- solution (Kurtbas et al., 2006), while the so called gesso, allo scopo di definire dei traccianti geochimici da applicare nelle ricerche ambientali indirizzate al riconoscimento di questo inquinante. borogypsum (which consists mainly of gypsum crystals and boron compounds) precipitates as industrial Parole chiave - Produzione acido borico, colemanite, borogesso, by-product. inquinamento ambientale, Larderello (Toscana). Data on the X-ray diffraction pattern and the chemical composition of the waste produced by the boric acid INTRODUCTION industry were reported by Elbeyli & Piskin (2004) for samples from Eti Holding Bandirma Borax and Boric After boric acid (known with the name of «sedative Acid Palnts, in Turkey. The X-ray diffraction pattern salt of Homberg») was identified as a natural phase is dominated by gypsum. The chemical analyses indi- (sassolite) at Monterotondo by Höefer (1977), the cate that the boron content of the solid waste is gener- production of boric acid from the boron-rich geother- ally in the order of several units wt%, and varies on mal fluids of Larderello was envisaged by Mascagni in the basis of the relative yield of each industrial extrac- 1812 and the first tons of boric acid finally produced tion procedure. in 1819 (Cappelo & Culivicchi, 2010). In the mid ’50, Large amounts of borogypsum waste have been dis- the industrial production of boric acid moved world- charged by the boric acid industry into the environment, wide towards extractive processes from boron miner- mainly in weastern Turkey, were about 62% of the als. The dissolution mechanisms and kinetics of boron world’s boron ores are found (Elbeyli and Piskin, 2004). minerals were extensively investigated (Kurtbas et al., In Tuscany, the Larderello area represents a site were 2006 and references therein), and colemanite became significant discharges have also occurred during the one of the most important minerals used around the 20th century. In this area, during the ’60s the boric world for boric acid production. acid production from geothermal fluids was converted

Colemanite (CaB3O4(OH)3 H2O) is a complex inobo- into extraction from colemanite imported from West- 2- rate in which the chains of composition [B3O4(OH)3] ern Anatolia (Cappelo & Culivicchi, 2010). After a pe- extend along [100]. In these chains tetrahedra and tri- riod in which the extraction was performed by using angles, centred by boron atoms and linked through ammonium bicarbonate, the production of boric acid 4- corners to form rings of composition [B3O5(OH)3] , at Larderello moved towards the acid procedure, are present. which used locally produced (at Scarlino) sulfuric acid

(*) Istituto di Geoscienze e Georisorse, CNR, Via Moruzzi 1, 56124 Pisa, Italy. (**) Dipartimento Scienze della Terra, Via S. Maria 53, 56126, Pisa, Italy. 38

(Cappello & Culivicchi, 2010). The amount of boron becker a tiny crust of minute tabular colourless crys- in the borogypsum waste produced in Tuscany was in- tals that successive EDS-SEM and XRPD with Gan- versely correlated with the yield of B-extraction, that dolfi camera investigations revealed to be sassolite, increased in time from 60% up to 90% (Cappello & natural boric acid (Fig. 1). Culivicchi, 2010). Favored at the time by the lack of awareness for environmental protection, about 70,000 tons/year of boron-rich waste have been released in the Possera creek, left tributary of the Cecina River, that crosses the Larderello village, where the industrial activity was concentrated (Grassi & Squarci, 2004). After the regulations were established, and till the industrial process was active (up to 2002), the waste was disposed in a local landfill, where it is still confined. The unregulated dump of borogypsum contaminated shallow and groundwaters through bank infiltration. In the ’70 of 20th century, the boron content in sur- face waters of the Possera stream increased up to about 400 mg/L, and up to 10 mg/L in groundwater extracted from local wells (Grilli Cicilioni et al., 1995; Grassi et al., 2004). The contamination of the Cecina aquifer and the isotopic characteristics of boron and strontium in colemanite, borogypsum and groundwaters were discussed in Pennisi et al. (2006a), and the Fig. 1 - Dark gray pseudo-hexagonal sassolite crystals, up to 2 mm isotopic signatures of B and Sr used to constraint the in diameter, associated with white microcrystalline gypsum crystals. origin of colemanite (Emet ore). Back-scattered SEM photo. In this study we investigate the mineralogical and bulk chemistry of borogypsum with the aim to: a) highlight the B-bearing phases eventually associated to sassolite; On the basis of these data, we can state that borogyp- b) identify those elements that are associated to boron sum consists of a white microcrystalline powder (and calcium and sulfur), and that could eventually formed by the aggregation of finely interwoven tiny trace borogypsum in the environment. acicular crystals of gypsum, and including minutes mi- caceous crystals of sassolite (Fig. 2).

EXPERIMENTAL DATA Chemical characteristics of borogypsum Mineralogical data The bulk chemical characterization of borogypsum was The X-ray powder pattern of borogypsum revealed on- performed by means of X-ray fluorescence method us- ly the presence of gypsum. A weak peak at 5.9 Å (that ing about 3 grams of materials previously heated at represents the most intense diffraction peak in the X- ray powder diffraction pattern of sassolite) suggested the presence of traces of sassolite (boric acid). Microprobe EDS-SEM analyses of the heavy fraction of borogypsum, obtained by means of a saturated solution of sodium politungstate (D = 2.9 g/cm3) revealed the presence of not negligible amounts of celestite and realgar as accessory phases.

Experimental procedure To verify the occurrence of any residual soluble boron-bearing phases within the mass of borogypsum, 200 grams of this material were powdered, put into a becker with 500 ml of cold water, and stirred for 2 minutes. Once the insoluble part settled, the solution was filtered and evaporate up to complete dryness. At Fig. 2 - borogypsum: aggregate of pale gray microcrystalline tiny the end of evaporation we observed at the bottom of acicular crystals of gypsum associated with sporadic minute white fragments of celestite and realgar. Back-scattered SEM photo. 39

200°C for five hours. Major elements were analysed senical borogypsum waste». The authors suggested using Cu Ka radiation while a Mo Ka radiation was that the arsenic in realgar and orpiment occurring in used for trace elements. Boron was analysed by the ore could partialy dissolve during the leaching

Prompt Gamma Neutron Activation (PGNA) at Act- process (with concentrate H2SO4) and be removed labs, Canada. as insoluble Ca/Fe arsenate. Aydin et al. (2003) evi- The chemical analysis of borogypsum produced at denced that over 99% of As present in the feed re-

Larderello is reported in Tab. 1. Associated to SO3 mains in the borogypsum, increasing the environmental (42.3%), CaO (30%), and B2O3 (2%), borogypsum risk by this waste. 11 contains, in decreasing order, subordinated SiO2 The B, Sr and S isotopic values (expressed as d B, 87 86 34 (5%), MgO (2.4%), Al2O3 and Sr, (0.7%), Fe2O3 Sr/ Sr and d S, respectively) of the borogypsum pro- (0.35%), K2O (0.3%), As (0.2%), and Na2O (0.1%). duced at Larderello (colemanite from Emet deposit) Ti, Ba, Zr, Ce, La, and Rb were also detected, but in measured by Pennisi et al. (2006a) are added in Table concentration lower than 100 mg/Kg. 1. Borogypsum has a negative d11B value of –13‰, a 87Sr/86Sr ratio of 0.70942, and a d34S of 1.2‰.

Tab. 1 - Chemical composition of borogypsum produced at Larde- rello. Isotopic composition of boron and strontium (Pennisi et al., 2006) and sulfur (Amadori, 2009) are included. DISCUSSION

Elbeyli et al. Alp et al. Borogypsum This sudy The borogypsum waste produced at Larderello from 2004 2009 industrial treatment of colemanite from Emet ore deposit is mainly composed by sulfur, calcium and wt % Na2O 0.11 0.1 0.06 K2O 0.21 0.79 0.04 boron, with associated strontium and arsenic, from ce- CaO 28.1 25.2 28.41 lestite and realgar. Borogypsum consists of a dense net- MgO 2.25 0.88 1.06 work of gypsum crystals in which the boric acid (not Al2O3 0.68 1.37 0.75 recovered during the industrial process of acid attack SiO2 4.91 7.74 6.99 of colemanite) is trapped in the form of minutes mica- TiO 0.03 nd 0.1 2 ceous crystals. The amount of sassolite crystals in boro- Fe2O3 0.33 0.74 0.72 SO3 40.0 35.6 40.1 gypsum is controlled by the recovery yield of the in- B2O3 2.07 7 2.8 dustrial process that differs at each productive site. As2O3 0.93 1.45 The isotopic signatures of B, and Sr significantly dif- SrO 0.23 0.66 fers from those of the local geogenic fluids (d11B = Pc 20.1 17.83 +13‰ ± 4, Pennisi et al., 2006b, d34S = 6‰ ± 0.9, mg Kg-1 Pb 7 Amadori, 2009; 87Sr/86Sr = 0.7086 ± 1; Pennisi et al., Ba 34 2006a), suggesting that these parameters could be Zr 97 sensitive tracers of borogypsum in the environment. Ce 17 11 87 86 La 13 The d B, and Sr/ Sr isotopic signature represents a Rb 29 potential tools to evidence environmental contamina- permil d11B* -13.3 tion, in those sites where - as in the Larderello area - 34 Emet colemanite was used for boric acid production. d S** 1.2 34 87Sr/86Sr* 0.70942 In contrast, the d S signature univocally reflects the isotopic signature of the H2SO4 used at each specific site for the industrial extraction.

The B2O3 component represents the residual boric High arsenic and boron content, associated to the acid (sassolite) not recovered by the industrial produc- dissolution of gypsum in water (Kuechler et al., 2004; tion, while high As and Sr are relict of the realgar and Colombani & Bert, 2007), makes borogypsum a waste celestite associated to colemanite in the Turkish ore of high environmental concern, and its storage a huge deposits (Helvaci and Alonso, 2000). The silicatic problem for the industry. However, it was shown that component has to be referred to the matrix of the ore the use of arsenical borogypsum as an additive in light- deposit. weight concrete or in Portland cement clinker increas- The comparison with the chemical data from Elbeyli es its compressive strength (Boncukcuogly et al., 2002; et al. (2004) and Alp et al., (2009) shows that borogyp- Elbey & Piskin, 2004; Kavas et al., 2005; Alp et al., sum wastes differ in terms of boron content (B2O2 = 2009). Accordingly, the addition of borogypsum in ce- 2.1, 7, and 2.8%, respectively), and confirms that the ment production could represent an useful remedia- boron-recovery is peculiar for the industrial sites. Alp tion, because of the restricted mobility shown by et al., (2009) also measured an As2O3 content of 1.45 boron and arsenic in new-formed solid phases after mg/kg in borogypsum, and accordingly define it as «ar- calcination. 40

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