Production of single superphosphate labeled with 34S

Produção de superfosfato simples marcado com 34S

NOTE

Production of single superphosphate labeled with ³⁴S

Produção de superfosfato simples marcado com ³⁴S

Alexssandra Luiza Rodrigues Molina Rossete^I; Josiane Meire Toloti Carneiro^I; José Albertino BendassolliII, ^*; Claudineia Raquel Oliveira Tavares^II; Carlos Roberto Sant'Ana Filho^II

^IUSP/CENA - Programa de Pós-Graduação em Ciências

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

ABSTRACT

Single superphosphate is currently one of the mostly used fertilizers as an alternative source for phosphorus and sulphur. Sulphur presents four stable isotopes (³²S, ³³S, ³⁴S, and ³⁶S) with natural abundances of 95.00; 0.76; 4.22; and 0.014% in atoms, respectively. Single superphosphate labeled with the ³⁴S isotope was obtained from a chemical reaction in stoichiometric amounts between Ca(H₂PO₄)₂ and Ca³⁴SO₄.2H₂O. Calcium sulphate (Ca³⁴SO₄.2H₂O) was enriched with 5.85 ± 0.01 atoms % of ³⁴S. The Ca(H₂PO₄)₂ reagent was obtained from a reaction between CaCl₂.2H₂O and H₃PO₄. The reaction between the Ca(H₂PO₄)₂ thus produced and the labeled Ca³⁴SO₄.2H₂O compound was then performed to obtain the ³⁴S-labeled single surperphosphate. The thermal decomposition of the labeled superphosphate for the production of gaseous ³⁴SO₂ was carried out under a vacuum line at 900ºC in the presence of NaPO₃. The isotopic determination of S (atoms % of ³⁴S) was carried out on an ATLAS-MAT model CH-4 mass spectrometer. The production yield of Ca(H₂PO₄)₂ and labeled single superphosphate were approximately 97 and 99% respectively, and the purity level of the labeled single superphosphate was estimated as 96%. No isotopic fractionation was observed in the production process of ³⁴S-labeled single superphosphate.

Key words: isotopic determinations, sulphur, stable isotopes, mass spectrometry, labeled compounds

RESUMO

O superfosfato simples é um dos fertilizantes mais utilizados atualmente como fonte de fósforo e uma alternativa para enxofre. O enxofre apresenta quatro isótopos estáveis, ³²S, ³³S, ³⁴S e ³⁶S, com abundância natural de 95,00; 0,76; 4,22 e 0,014% em átomos, respectivamente. O superfosfato simples marcado com ³⁴S foi obtido a partir da reação química em proporção estequiométrica entre o Ca(H₂PO₄)₂ e o Ca³⁴SO₄.2H₂O. O Ca³⁴SO₄.2H₂O foi enriquecido com 5,85 ± 0,01% em átomos de ³⁴S. O Ca(H₂PO₄)₂ foi obtido a partir da reação entre CaCl₂.2H₂O com o H₃PO₄. A decomposição térmica do superfosfato marcado para produção do ³⁴SO₂ gasoso foi realizada em linha de vácuo a 900ºC na presença de NaPO₃. A determinação isotópica do S (% em átomos de ³⁴S) foi realizada no espectrômetro de massas. O rendimento da produção do Ca(H₂PO₄)₂ e do superfosfato simples marcado foi em média 97 e 99%, respectivamente, e a pureza do superfosfato marcado foi estimada como 96%. Não foi observado fracionamento isotópico no processo de produção do superfosfato simples marcado com ³⁴S.

Palavras-chave: determinação isotópica, enxofre, isótopos estáveis, espectrometria de massas, compostos marcados

INTRODUCTION

Phosphorus is essential for plant development, helping in cell division and stimulating root development. In the industry, the decomposition of rock phosphate with sulphuric acid yields many different kinds of phosphate compounds, including the Single Superphosphate (SSP), which is one of the most important fertilizers applied as a phosphorus source for plants. These compounds also contain sulphur (S) (Havlin et al., 2005), an element presenting deficiency in different soils and plants (Bissani & Tedesco, 1988; Tisdale et al., 1986). Isotope dilution techniques and the use of compounds labeled with stable isotopes have been widely employed in order to obtain information about the cycle of many elements especially nitrogen and sulphur (Awonaike et al., 1993).

Sulphur presents four stable isotopes (³²S, ³³S, ³⁴S, and ³⁶S) with natural abundances of 95.02; 0.75; 4.21; and 0.02% in atoms, respectively (Krouse et al., 1996). Most studies based on the application of labeled S have used the ³⁵S radioisotope (Sharma & Kamath, 1991), however, the application of compounds labeled with a stable isotope like ³⁴S presents advantages especially due to the fact of not being radioactive. Today it is important to stress a world tendency to use stable isotopes as tracers to replace radioactive techniques, especially in studies involving field experiments (Zhao et al., 2001). The first studies based on the application of ³⁴S-labeled stable isotopes were developed by Hamilton et al. (1991), Awonaike et al. (1993), and Trivelin et al. (2002). Studies aiming at the separation of the ³⁴S isotope developed by Bendassolli et al. (1997) were able to produce many different labeled compounds such as (¹⁵NH₄)₂³⁴ SO₄, K₂³⁴SO₄, H₂³⁴SO₄ and Ca³⁴SO₄.2H₂O (Maximo et al., 2005; Rossete et al., 2006).

The aim of the present work was to produce a single superphosphate labeled with the ³⁴S isotope in the form of 3Ca(H₂PO₄)₂ + 7Ca³⁴SO₄.2H₂O and the subsequent S isotopic determination (atoms % of ³⁴S) by mass spectrometry.

MATERIAL AND METHODS

The ATLAS-MAT model CH4 mass spectrometer was equipped with an admission system by molecular flow and a single ion collector with a Faraday cup. The vacuum line was built of Pyrex® and quartz, and the system comprised: an Edwards model 2M8 mechanical vacuum pump; an Edwards model E050 high vacuum diffusion pump; an Edwards active vacuum gauge display (AGD), and an Edwards model APG-M Pirani vacuum filament display. The following reagents, all of analytical grade, were used: CaSO₄.2H₂O; CaCl₂.2H₂O; H₃PO₄ and NaH₂PO₄.H₂O.

Production of ³⁴S-labeled single superphosphate (SSP)

The SSP labeled with the ³⁴S isotope was obtained from a chemical reaction in stoichiometric amounts between Ca(H₂PO₄)₂ and Ca³⁴SO₄.2H₂O. Calcium sulfate (Ca³⁴SO₄.2H₂O) enriched with 5.85 ± 0.01 atoms % of ³⁴S was initially produced as shown by Rossete et al. (2006). The Ca(H₂PO₄)₂ reagent was obtained from the chemical reaction between CaCl₂.2H₂O and H₃PO₄ in stoichiometric amounts, according to the reaction shown in Equation 1.

After running for approximately 72 h, the produced Ca(H₂PO₄)₂ was dried at 60ºC to eliminate water and the excess acid possibly formed during the reaction, and its yield production was evaluated by gravimetry. Next, 10 mg Ca(H₂PO₄)₂ were dissolved into 100 mL water and phosphate/calcium concentrations were determined by ICP-AES (Gine et al., 2004). To verify the yield production of contaminant in the production process, the chloride concentration was determined by spectrometry (Zagatto et al., 1981). Next, the reaction between the produced Ca(H₂PO₄)₂ and the ³⁴S-labeled Ca³⁴SO₄.2H₂O (5.81 ± 0.01 atoms % of ³⁴S) was developed. After running for 7 days, the labeled SSP was dried at 60ºC and its yield production was evaluated by gravimetry.

Sample preparation and S isotopic determination

The high vacuum line used in the production and purification of SO₂ with subsequent ³⁴S isotopic determination (atoms % of ³⁴S) by mass spectrometry is shown in Figure 1. According to the proposed procedure, 10.00 mg of labeled SSP (approximately 1.35 mg S) and 60 mg sodium metaphosphate were placed inside a quartz tube of 30 cm length. The sodium metaphosphate was obtained by submitting approximately 10 g NaH₂PO₄.H₂O at 200ºC for 2 h to remove structural water and possible organic compounds formed. The mass ratio between superphosphate and NaPO₃ was 1:6 (w/w) (Halas & Wolacewicz, 1981). The quarts tube (QT) was then connected into the high vacuum line (Figure 1) and the vacuum system was activated by mechanic (MP) and diffusion (DP) pumps. A bottle containing dry ice plus ethanol solution (-70ºC) and another containing liquid nitrogen (-196ºC) were introduced around the Tr₁ and Tr₂ traps, respectively. The objective of the traps was to retain water vapor in Tr₁ and SO₂ (formed during the combustion process) in Tr₂. The high vacuum line configuration was based on the method developed by Bailey & Smith (1972).

In the next step, the MF furnace adjusted to 900ºC is moved to the QT tube and is heated for approximately 10 min to produce SO₂ after the combustion reaction between SSP and NaPO₃. Depending on the O₂ partial pressure, an undesirable production of SO₃ may occur, with consequent isotopic fractionation. In order to retain oxide compounds formed during the combustion process and to convert SO₃ to SO₂ thus avoiding isotopic fractionation (Yanagisawa & Sakal, 1983; Rafter, 1957) a metallic copper ring supported by a piece of quartz wool was placed inside the QT tube approximately 2 cm above the volume occupied by the sample and the NaPO₃ reagent. The SO₂ gas retained inside the Tr₂ trap was then transferred into the stock tube (ST) by replacing the bottle containing liquid N₂ from the trap Tr₂ to the stock sample tube (ST). Next, the ST tube containing SO₂ gas was removed from the high vacuum line and connected to the mass spectrometer admission system and the isotopic determination of S (atoms % of ³⁴S) was accomplished (Bendassolli et al., 1997).

The mass spectrometer equipment worked with an admission system heated to 60ºC due to the polarity of the SO₂ molecules, thus avoiding interference on the analysis. A cryogenic trap (Tr₁) containing dry ice and ethanol (-70ºC) was adapted on the spectrometer admission system to retain water from coming the samples.

RESULTS AND DISCUSSION

With regard to the stoichiometric reaction between H₃PO₄ and CaCl₂.2H₂O, using 14.7 g of CaCl₂.2H₂O for each test and slowly adding 13.5 mL of concentrated H₃PO₄, it should be theoretically possible to obtain 27.0 g Ca(H₂PO₄)₂, considering the HCl mass formed in the process. Based on the theoretical value, the yield conversion (%) and total Ca(H₂PO₄)₂ mass lost in the process were calculated. Results are in Table 1, where the Ca(H₂PO₄)₂ mass produced and the yield conversion values for each test can be seen.

The concentrations of Ca²⁺ and PO₄^2- samples produced in Ca(H₂PO₄)₂ as determined by ICP-AES (Gine et al., 2004) were estimated to be 17 and 80% (w/w), respectively. Results were considered satisfactory and in agreement with the gravimetric method.

In relation to the chlorine determination exploring the flow injection analysis system (FIA), the contamination by chlorine in the Ca(H₂PO₄)₂ production process was approximately 5%. This contamination is not significant, thus allowing the application of the ³⁴S-labeled compound in agronomic studies without interferences. After the analysis related to the yield production of ³⁴S-labeled SSP by the gravimetric method, results showed yield conversion values of approximately 99%, with a purity level in the labeled compound estimated approximately at 96%, thus confirming the efficiency of the proposed method. In relation to the results obtained in the isotopic determination of S (atoms % of ³⁴S) in the labeled compound (Ca³⁴SO₄.2H₂O) by mass spectrometry, no isotopic fractionation was observed. The results obtained by S isotopic determination (atoms % of ³⁴S) in the mass spectrometry using CaSO₄.2H₂O p.a. with natural abundance and two different S compounds for the production of ³⁴S-labeled SSP can be observed in Table 2.

Results obtained with the CaSO₄.2H₂O analysis were satisfactory (Table 2), since the natural abundance of the compound is approximately 4.15 to 4.38 atoms % of ³⁴S. Results for ³⁴S determination (atoms % of ³⁴S) using different compounds prove the efficiency of the proposed method, especially in relation to the production of SO₂ in the presence of NaPO₃ and the isotopic determination of ³⁴S by mass spectrometry.

CONCLUSIONS

The proposed method is feasible and suitable for the production of ³⁴S-labeled single superphosphate by the reaction between Ca(H₂PO₄)₂ and Ca³⁴SO₄.2H₂O, and for the production of SO₂ in a high vacuum line with S isotopic determination (atoms % of ³⁴S) by mass spectrometry. The application of the ³⁴S stable isotope can help identify sulphur sources. Therefore, the production of ³⁴S-labeled single superphosphate represents an important alternative to the application of ³⁴S aiming at studies related to sulphur dynamics in the soil-plant system.

ACKNOWLEDGEMENTS

To FAPESP for partial support and infra-structure.

Received July 28, 2006

Accepted September 18, 2007

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Publication Dates

• Publication in this collection
  17 Jan 2008
• Date of issue
  Feb 2008

History

• Accepted
  18 Sept 2007
• Received
  28 July 2006