Production of 34S-labeled gypsum (Ca34SO4.2H2O)

Produção de gesso (Ca34SO4.2H2O), marcado com 34S

NOTE

Production of ³⁴S-labeled gypsum (Ca³⁴SO₄.2H₂O)

Produção de gesso (Ca³⁴SO₄.2H₂O), marcado com ³⁴S

Alexssandra Luiza Rodrigues Molina Rossete^I; José Albertino Bendassolli^I,*; Everaldo Máximo^I; Carlos Roberto Sant Ana Filho^I; Raquel de Fátima de Ignoto^II

^IUSP/CENA - Lab. de Isótopos Estáveis, C.P. 96 - 13400-970 - Piracicaba, SP - Brasil

^IIUniversidade do Grande ABC - UniABC, AV. Industrial, 3.330 - Campus Universitário - 09080-511 - Santo André, SP - Brasil

ABSTRACT

Agricultural gypsum (CaSO₄.2H₂O) stands out as an effective source of calcium and sulfur, and to control aluminum saturation in the soil. Labeled as ³⁴S it can elucidate important aspects of the sulfur cycle. Ca³⁴SO₄.2H₂O was obtained by chemical reaction between Ca(OH)₂ and H₂³⁴SO_4, performed under slow agitation. The acid was produced by ion exchange chromatography using the Dowex 50WX8 cation exchange resin and a Na₂³⁴SO₄ eluting solution. After precipitation, the precipitate was separated and dried in a ventilated oven at 60ºC. From 2.2 L H₂SO₄ 0.2 mol L^-1 and 33.6 g Ca(OH)₂, 73.7 ± 0.6 g Ca³⁴SO₄.2H₂O were produced on average in the tests, representing a mean yield of 94.6 ± 0.8%, with 98% purity. The ³⁴SO₂ gas was obtained from Ca³⁴SO₄.2H₂O in the presence of NaPO₃ in a high vacuum line and was used for the isotopic determination of S in an ATLAS-MAT model CH-4 mass spectrometer.

Key words: sulfur 34, stable isotope, isotopic determination

RESUMO

O gesso agrícola (CaSO₄.2H₂O) destaca-se como fonte eficiente de cálcio e enxofre e na redução da saturação de alumínio no solo. O ³⁴S como traçador isotópico pode elucidar aspectos importantes no ciclo do enxofre. Para tanto o Ca³⁴SO₄.2H₂O foi obtido por reação química entre o Ca(OH)₂ e solução de H₂³⁴SO_4, realizada sob agitação lenta. O ácido foi produzido por cromatografia de troca iônica, utilizando resina catiônica Dowex 50WX8 e solução eluente de Na₂³⁴SO₄. Após a precipitação foi separado o precipitado e realizada a secagem em estufa ventilada à temperatura de 60ºC. Nos testes, a partir de 2,2 L de H₂SO₄ 0,2 mol L^-1 e 33,6 g de Ca(OH)₂, foram produzidos em média 73,7 ± 0,6 g de Ca³⁴SO₄.2H₂O representando um rendimento médio de 94,6 ± 0,8%, com pureza de 98%. A partir do Ca³⁴SO₄.2H₂O na presença de NaPO₃, em linha de alto vácuo, obteve-se o gás ³⁴SO₂ utilizado para a determinação isotópica do S no espectrômetro de massas ATLAS-MAT modelo CH-4.

Palavras-chave: enxofre 34, isótopo estável, determinação isotópica

INTRODUCTION

Gypsum is widely used in agriculture as an important nutrient input to obtain productivity increase. This compound has a two-fold function, as an effective source of calcium and sulfur, and as an aluminum saturation reducer in deeper soil layers (Vitti & Malavolta, 1985; Paolinelli et al., 1990). Gypsum is used notably in the recovery of soils with excess of sodium in order to improve root environment of acid subsoils, and is based upon the ion exchange reaction of calcium with sodium in the soil (Raij, 1988; Malavolta, 1979).

Sulfur presents four stable isotopes: ³²S, ³³S, ³⁴S, and ³⁶S, with natural abundances of 95.02; 0.75; 4.21; and 0.02 (atom %), respectively (Krouse et al., 1996). Most studies performed to date using S as a tracer have employed the ³⁵S radioisotope; this has been very useful in studies involving the dynamics of this nutrient (Lal & Dravid, 1990; Arora et al., 1990; Sharma & Kamath, 1991; Bansal & Motiramani, 1993; Patnaik & Santhe, 1993; Fitzgorald et al., 1999). However, compounds labeled with the ³⁴S stable isotope present some advantages over the radioisotope, including the following: they are not radioactive; the experiments are not limited by time; there is no exposition to radiation; and no safety measures against radiation are required.

At present, the international tendency of using non-radioactive techniques whenever possible in isotopic tracer experiments must not be forgotten, especially in field work (Zhao et al., 2001). The first studies using the ³⁴S isotopic tracer were developed by Hamilton et al. (1991) and Awonaike et al. (1993), and in Brazil by Trivelin et al. (2002).

Recently, laboratories in the USA and Europe started to produce ³⁴S highly-enriched compounds, however, at prohibited prices. In Brazil, the first studies targeted to sulfur isotope separation (especially ³⁴S) were initiated by Bendassolli et al. (1997). They used the exchange reaction between a H₂SO₃ solution (SO_{2 aq}) and bisulfite anions (HSO₃^-) adsorbed to anion resins of the quaternary ammonium type. Obtaining enriched aqueous ³⁴SO₂ allowed the production of several compounds labeled with this isotope, including: Na₂³⁴SO₄; (¹⁵NH₄)₂³⁴ SO₄; K₂³⁴SO₄; Ca³⁴SO₄.2H₂O; and H₂³⁴SO₄, among others (Maximo et al., 2000; Rossete, 2002). With such products available, the production of ³⁴S (Ca³⁴SO₄.2H₂O) isotope-labeled gypsum was possible, here made through the reaction between H₂³⁴SO₄ and Ca(OH)₂ which is the objective of this note.

MATERIAL

Equipment

ATLAS-MAT, model CH4 mass spectrometer (120º radius of tube curvature; electronic impact ionization; molecular-flow admission system; simple, Faraday-cup ion collector; and mass scan analysis system).

Pyrex glass and quartz Vacuum line; model 2M8 EDWARDS mechanical vacuum pump; model E050 EDWARDS high vacuum diffusion pump; AGD EDWARDS active vacuum gauge; APG-M EDWARDS Pirani vacuum sensor filament.

Reagents

The following reagents, all of analytical grade, were used: sulfuric and hydrochloric acid; barium and calcium chloride; barium and calcium sulfate; calcium and sodium hydroxide; and monobasic sodium phosphate. Solutions were prepared with deionized water obtained by ion exchange (5-10 mWcm resistively).

Other material used: Dowex 50WX8 cation resin with the specifications: acid type polystyrene-divinilbenzene with 8% of DVB; sulfonic functional group; mesh 100-200.

METHODS

Production of H₂³⁴SO₄ in Dowex 50W-X8 cation resin columns

H₂³⁴SO₄ was obtained by cation exchange chromatography, using an acrylic column 130 cm height and 2.1 cm diameter (system 1), and a second acrylic column of 50 cm height and 1.5 cm diameter (system 2), filled with cation resin.

The active sites of the resin were saturated with the R-H⁺ form by admitting 1.0 mol L^-1 H₂SO₄ solution at the top of the column at a flow rate of 1 to 2 cm³ cm^-2 min ^-1. The resin saturation status and the volume of H₂SO₄ solution to saturate the resin with the R-H⁺ form were determined by titration with a NaOH 0.1 mol L^-1 solution in collected effluent volumes. Next, the excess acid remaining in the interstitial pores of the resin was eliminated with deionized water and later decompaction with water in a backwash process.

In the next step, a Na₂³⁴SO₄ 30 g L^-1 eluting solution of enrichment of 5.85 ± 0.01%, was admitted with substitution of H⁺ by Na⁺ ions, producing H₂³⁴SO₄ in the eluted solution. In the H⁺ elution step, the eluted volume was collected in 100 ml (system 1) or 25 ml batches (system 2); the H⁺ concentration in each fraction was determined by titration with 0.1 mol L^-1 NaOH tritisol. This procedure allowed the determination of the H₂³⁴SO₄ mass that could be obtained in each column system.

The sodium sulfate solution (Na₂³⁴SO₄) enriched at 5.85 ± 0.01 atoms % of ³⁴S was produced at the USP/CENA Stable Isotope Laboratory.

Production of ³⁴S-labeled Ca³⁴SO₄ .2H₂O

The gypsum (Ca³⁴SO₄.2H₂O) was obtained from the stoichiometric ratio of the chemical reaction between Ca(OH)₂ and the addition of the ³⁴S-labeled H₂³⁴SO₄ solution.

In order to perform the Ca³⁴SO₄.2H₂O production tests by the chemical reaction between H₂³⁴SO₄ and Ca(OH)₂, a H₂SO₄ solution with natural isotopic abundance was used (4.22 atoms % of ³⁴S ) but with a concentration of the acid similar to that obtained by ion exchange chromatography. The nomenclature of the isotopic reagent labeling had the objective of identifying the labeling source in the process to obtain Ca³⁴SO₄.2H₂O; however, when the same physicochemical parameters are maintained, the process for the production of labeled Ca³⁴SO₄.2H₂O must be reproduced when ³⁴S-enriched compounds are used.

The reaction was performed under slow agitation. Because of the low solubility of gypsum (Ca³⁴SO₄.2H₂O) in water (2.5 g L^-1), precipitation begins after the solubility product is reached (Ksp = 1.0.10^-5). The gypsum formation reaction can be observed in equation (1).

where: (aq) and (s) indicate the aqueous and solid phases, respectively.

After precipitation, the liquid (supernatant) and the solid (precipitate) phases were separated. The solid phase was dried in a ventilated oven at 50ºC and the gravimetric quantification of the Ca³⁴SO₄.2H₂O mass was made.

A small fraction of precipitate was solubilized in 50 mL deionized water, and the S-SO₄^2- concentration was determined by the turbidimetric method (Raij et al., 2001; Malavolta et al., 1997).

The S-SO₄^2- concentration was obtained from the absorbance values (spectrophotometer), using a calibration curve obtained from S-Sulfate standards (5 to 20 mg L^-1). This procedure allowed the calculation of the yield of the reaction by which Ca³⁴SO₄.2H₂O was obtained, as well as its chemical purity.

The amount of soluble sulfate at the solution phase (supernatant) was also obtained by turbidimetry. The objective of this step was to quantify the soluble Ca³⁴SO₄.2H₂O concentration and then balance the mass of the reaction in relation to sulfur.

Sample preparation for the isotopic determination

A Ca³⁴SO₄.2H₂O sample of about 10.0 mg (approximately 1.8 mg S), together with sodium metaphosphate (NaPO₃) was placed in a quartz tube (Q1) of approximately 30 cm (Figure 1). The ratio between the Ca³⁴SO₄.2H₂O mass and the reagent (NaPO₃) was 1:3 (w/w) (Halas & Wolacewicz, 1981). This step has the purpose of obtaining ³⁴SO₂ gas from Ca³⁴SO₄.2H₂O to determine the ³⁴S isotopic abundance (atoms % of ³⁴S) by mass spectrometry.

The NaPO₃ was obtained burning the NaH₂PO₄.H₂O in a muffle for 2 hours at 200ºC. This step is performed to remove the structural water from the compound and to eliminate possible organic impurities. Equation (2) shows the process by which NaPO₃ is obtained.

A 2 g copper metal ring was added approximately 2 cm above the mixture (sample/reagents) using quartz wool. The metal copper is used to retain oxides formed during combustion and to convert possible SO₃ into SO₂, avoiding isotopic fractionation (Yanagisawa & Sakal, 1983; Rafter, 1957).

Next, the QT tube was connected to the high vacuum line and vacuum was established in the entire system using mechanical (MP) and diffusion pumps (DP). Traps Tr₁ and Tr₂ were later supplied with a dry ice and ethanol mixture ( 73ºC) or with liquid nitrogen (-196ºC), respectively. The Tr₁ and Tr₂ traps are designed to trap water and SO₂ vapors, respectively, formed during combustion. The high vacuum line to obtain SO₂ adapted from Bailey & Smith (1972) can be observed in Figure 1.

In the next step, the MF oven heated to 900ºC was displaced vertically up to the QT tube, where it remained for a 10-minute interval during which the Ca³⁴SO₄.2H₂O combustion occurred in the presence of reagents, forming the SO₂ gas (equation 3). Depending on the conditions (O₂ partial pressure), the SO₃ gas can be produced (equation 4). The formation of SO₃ is not desirable, since it can cause isotopic fractionation. Equation 5 shows the conversion of SO₃ to SO₂ via reduced copper.

The SO₂ gas entrapped in Tr₂ was then transferred to the sample tube (ST), by alternating the liquid N₂ ("trap") from Tr₂ into the sample tube (ST).

Later, sample tube (ST) containing the SO₂ gas was transferred from the high vacuum line and connected to the mass spectrometer admission system and the isotopic determination of S was made (atoms % of ³⁴S).

The QT tube was cleaned after combustion of the sample in the high vacuum line. Initially, the quartz wool was removed from the oxidized copper ring (CuO), and the oxidized copper was then transferred to a glass container for later reduction (Cuº) in a vacuum line using hydrogen (Bendassolli et al., 2002).

Finally, the QT tube was scraped to remove residues from the reaction, and washed with HCl 6.0 mol L^-1 so it could be reused in other combustions.

Isotopic determination of S

The mass spectrometry analysis of isotopic ratios or atoms % (abundance) of light elements, for the most part, is made on properly purified gaseous samples, and in a low- or medium-resolution-power mass spectrometer. To accomplish this, the sample preparation system involves some chemical transformations, and gases free from any atmospheric contaminant are required for their production.

For the isotopic determination of S in the SO₂ gas samples contained in the storage flasks, the mass spectrometer worked with a heated admission system (70ºC) due to the polar nature of the SO₂ and SO molecules, avoiding a memory effect between analyses.

A cryogenic trap containing dry ice and ethanol (-73ºC) was adapted on the mass spectrometer admission system, in order to retain water contained in the gas samples.

The abundance determination in atoms % of ³⁴S in SO₂ samples was performed in mass spectrometer according to Bendassolli et al., 1997.

RESULTS

The preliminary H₂³⁴SO₄ production tests were performed with natural Na₂³⁴SO₄. The labeled-H₂³⁴SO₄ production process must be reproduced, because the physicochemical parameters must be preserved when enriched ³⁴S-isotope compounds are used.

In the process of production of H₂ ³⁴SO₄ using the 2.1 cm diameter (130 cm height) acrylic column, 3.0 L of H₂SO₄ solution were required for complete saturation of the active sites of the resin to the R-H⁺ form; 2.2 L deionized water were needed to eliminate the excess acid from the resin interstitial volume. Resin decompaction was performed later with deionized water.

In system 1, containing 426 cm³ cation resin in the R-H⁺ form (wet, balanced in water) 2.2 L Na₂³⁴SO₄ 30 g L^-1 solution was used for total elution of the H⁺ ions, and a final solution containing 44.2 g H₂³⁴SO₄ in 2.2 L (0.2 mol L^-1) was obtained.

The chemical reaction of H₂³⁴SO₄ with Ca(OH)₂ was performed at a stoichiometric ratio. The tests were performed in three replications using the same acid, but on different days. We used 2.2 L H₂³⁴SO₄ in each test at a concentration of 0.2 mol L ¹, and 33.6 g Ca(OH)₂ were added slowly, theoretically forming a precipitate containing approximately 78.0 g Ca³⁴SO₄.2H₂O.

The Ca³⁴SO₄.2H₂O mass produced in each test was determined in the precipitate by a gravimetric method. Using the theoretical value for the Ca³⁴SO₄.2H₂O mass (78.0 g) and the mass obtained experimentally in the chemical reaction, the reaction yield and total mass of Ca³⁴SO₄.2H₂O lost in the process could be calculated. Table 1 shows the Ca³⁴SO₄.2H₂O mass produced, losses, and chemical reaction yields in each test. It can be observed that complete Ca³⁴SO₄.2H₂O precipitation did not occur in any of the three tests. On average, 73.7 ± 0.6 g Ca³⁴SO₄.2H₂O were produced in these tests, representing a mean yield of 94.6 ± 0.8%. There was no pronounced variability in Ca³⁴SO₄.2H₂O mass loss; on average, this fraction represented 5.4 ± 0.8%. This is a relatively high value, since it is a labeled material with high added value.

Because complete gypsum precipitation did not occur, the supernatant of each test was analyzed in order to determine the mass lost in the reaction. The pH readings in the supernatant were in the range of 6.0 to 7.0, possibly indicating the complete consumption of H₂³⁴SO₄ in the chemical reaction with Ca(OH)₂. Therefore, the H⁺ concentration in the solution (supernatant) was very reduced (10^-6 to 10^-7 mol L^-1).

The S-³⁴SO₄^2- concentration in the supernatant was measured in the three tests, to estimate the Ca³⁴SO₄.2H₂O mass solubilized in the volume. The data shown in Table 1 demonstrates that part of the 2.7 g Ca³⁴SO₄.2H₂O not recovered in the chemical process were present in the supernatant volume. This solubilized Ca³⁴SO₄.2H₂O fraction can be recovered in the chemical reaction process using ³⁴S-enriched H₂³⁴SO₄.

From the results presented in Table 1, it can be verified that the Ca³⁴SO₄.2H₂O mass lost in the chemical process was 1.5 g on average. Also, using the results from Table 1, the global mass balance for the chemical reaction process between H₂³⁴SO₄ and Ca(OH)₂ to produce Ca³⁴SO₄.2H₂O can be obtained. The mean for the global mass balance process can be observed in Table 2.

It can be observed from Table 2 that, on average, 64.3% of the Ca³⁴SO₄.2H₂O losses of the chemical reaction process occurred in the supernatant, while 35.7% were not identified. From the same Table, it can be verified that only 2% of the total mass (Ca³⁴SO₄.2H₂O) was really lost, which makes the process viable when ³⁴S is used.

From the result presented in Table 2 it can also be observed that 3.4% of the Ca³⁴SO₄.2H₂O mass lost in the process is solubilized in the supernatant, with a value equivalent to 50.2 mg ³⁴S.

The S-SO₄^2- concentration in the Ca³⁴SO₄.2H₂O obtained through the chemical process (three assays) was determined by turbidimetry, indicating a purity of produced Ca³⁴SO₄.2H₂O of 98%, on average.

The test using the ³⁴S-labeled Na₂³⁴SO₄ solution with an abundance of 5.85 ± 0.01 atoms % performed in system 2, containing 33 cm³ cation resin in the R-H⁺ form (balanced in water) allowed to obtain about 3.0 g of ³⁴S-labeled H₂³⁴SO₄.

From the chemical reaction between labeled H₂³⁴SO₄ and Ca(OH)₂, under stoichiometric conditions, 5.2 g Ca³⁴SO₄.2H₂O could be obtained in theory. The mass obtained in the reaction was 4.8 g Ca³⁴SO₄.2H₂O, with a 91% yield.

The ³⁴S determination in atoms % of ³⁴S was performed on three replicates for each test. For the evaluation of isotopic results a natural variation from -30 to +30‰ was taken into consideration (Krouse & Tabatabai, 1986). The natural variation values (-30 to +30‰) correspond to a variation from 4.15 to 4.38 atoms % of ³⁴S. It must also be considered that the analytical precision of the mass spectrometer is in the order of 1%.

The results for isotopic determination of S in the SO₂ gas samples obtained from the three Ca³⁴SO₄.2H₂O production tests (natural abundance), one CaSO₄.2H₂O p.a. sample (natural abundance), and ³⁴S-labeled Ca³⁴SO₄.2H₂O test can be observed in Table 3.

The values presented in Table 3 concerning samples (CaSO₄.2H₂O) with natural variation, labeled material, or the p.a. product, are in accordance with the expected values (4.15 to 4.38 atoms % of ³⁴S), demonstrating that the process for obtaining SO₂ in the presence of NaPO₃ and the isotopic determination of S in the mass spectrometer were suitable.

The result for isotopic determination of S (atoms % of ³⁴S) in the test that used labeled material (Na₂³⁴SO₄ 5.81 ± 0.01 atoms % of ³⁴S) demonstrated that there was no isotopic fractionation in the process for the production of ³⁴S-labeled Ca³⁴SO₄.2H₂O.

ACKNOWLEDGEMENT

The authors thank FAPESP for the financial support.

Received September 20, 2005

Accepted July 04, 2006

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Publication Dates

• Publication in this collection
  20 Aug 2007
• Date of issue
  Aug 2006

History

• Received
  20 Sept 2005
• Accepted
  04 July 2006