Solid–liquid phase equilibrium of glyphosate in selected solvents

doi:10.1016/j.fluid.2012.05.003

Fluid Phase Equilibria

Volume 327, 15 August 2012, Pages 1-8

https://doi.org/10.1016/j.fluid.2012.05.003 Get rights and content

Abstract

Solid–liquid equilibrium (SLE) data for the glyphosate in methanol, ethanol, n-propanol, 2-propanol, acetone, benzene and ethyl acetate, and aqueous sodium chloride solutions were measured with a temperature range from (283 to 352) K. The hypothetical enthalpy of fusion and melting temperature of glyphosate were estimated. The Wilson model, the nonrandom two-liquid (NRTL) model, the universal quasi-chemical (UNIQUAC) model, and the Scatchard–Hildebrand (SH) model were applied to correlate the solid–liquid equilibrium. It is shown that the Wilson model can give better results than NRTL and UNIQUAC model. The solubility parameter of glyphosate was obtained based on the SH model. In aqueous sodium chloride solutions, the solubility of glyphosate increases with the molality of sodium chloride increasing. The system of glyphosate + sodium chloride + water is found to exhibit a synergistic effect.

Highlights

► The solubility of glyphosate in selected solvents was measured. ► Experimental data were well correlated with the NRTL models. ► The solubility parameter of glyphosate was obtained based on the SH model.

Introduction

Excessive release of toxic organic wastewater into the environment due to industrialization has created great global concern in recent years. Glyphosate (N-(phosphonomethyl) glycine, CAS registry no. 1071-83-6) is a highly effective post-emergence, broad-spectrum and non-selective herbicide that has been widely applied in agriculture [1], [2]. Glyphosate contains a phosphonate, a carboxylate and an amine group, all capable of taking part in stable five-membered chelate rings (Fig. 1). Glyphosate was prepared by the reaction of chloromethylphosphonic acid with glycine in the sodium hydroxide solution, then acidified using hydrochloric acid, and finally purified from crystallization. The wastewater from the crystallization process usually contains 1–2% glyphosate, 10–20% NaCl and a high concentration of organic by-products [2]. Currently, several strategies are being pursued to prevent the pesticide plant from offering 10% glyphosate aqueous solution. How to deal with this wastewater and effectively reclaim glyphosate has already been an important research topic. Because of the toxicological effects of glyphosate and by-products, its subsequent removal from aqueous solution has been mandatory.

A number of methods have been proposed and reported for the removal of glyphosate including advanced oxidation technologies [3], [4], [5], [6], biodegradation [7], [8], ion exchange and adsorption [9], [10], [11], [12], [13]. Recently, interest has increased in further development of adsorption technology. It is noteworthy that the adsorption capacity of the adsorbent is strongly dependent on its property and the solubility of the glyphosate. In order to effectively separate the glyphosate from a reactive mixture by physical or chemical method and assess environmental partitioning of different compounds, it is necessary to determine the solubility of glyphosate in different solvents. The solubility of glyphosate in water, ethanol + water, 1-propanol + water, and 2-propanol + water has been reported by Fu et al. [14]. However, the solubilities of glyphosate in organic solvents and in aqueous sodium chloride solutions have not been found in the literature. In this study, for the purpose of further investigations on the solubility of glyphosate as well as the investigation on the synergistic effect, the measurements were carried out in methanol, ethanol, n-propanol, 2-propanol, acetone, benzene, ethyl acetate, and aqueous sodium chloride solutions. The sodium chloride molality, h (moles of sodium chloride in kg of water, mol kg⁻¹) was range from 0.0 to 4.5 over a temperature range from (283 to 352) K. The Wilson model, the nonrandom two-liquid (NRTL) model, and the universal quasi-chemical (UNIQUAC) model were applied to describe the experimental solid–liquid equilibrium (SLE) data. Finally, the possible reason for the relatively smaller discrepancy appearing in the Wilson model was discussed.

Section snippets

Materials

A white crystalline powder of glyphosate (C₃H₈NO₅P, with a molar mass 169.07 g mol⁻¹) was purified by twice recrystallizing from water. Its mass fraction purity, determined by high-performance liquid chromatography (HPLC) [13]. Other reagents are analytical research grade reagents. All of the solvents used in the experiments have a minimum purity of 99.5% (Table 1).

Glyphosate characterization

The melting points and enthalpy of fusion of glyphosate were studied using a Q100 (TA Instruments) differential scanning calorimeter

Solubility and correlation

The measured solubility data of glyphosate in water and the seven organic solvents methanol, ethanol, n-propanol, 2-propanol, acetone, benzene and ethyl acetate at different temperatures are listed in Table 2 and graphically plotted in Fig. 3. As can be seen from Fig. 3, the solubility of glyphosate increases with temperature increasing. At constant temperature, each of the organic solvents except ethyl acetate shows lower solubility than water. The solubility of glyphosate in the water

Conclusions

The solid–liquid equilibrium (SLE) data for the glyphosate in methanol, ethanol, n-propanol, 2-propanol, acetone, benzene and ethyl acetate at a temperature range from (283 to 352) K have been investigated. The solubility of glyphosate in all solvents increases with temperature and show the strongest dependency on temperature. Because of the thermal instability of glyphosate, the conventional calorimetric test failed to measure its melting properties. The melting enthalpy of glyphosate

Acknowledgments

This research was funded by the Shandong Provincial Natural Science Foundation, China (Grant No. ZR2011BL013) and the Science Technology Development Program of Binzhou, China (Grant No. 2011ZC0801).

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View full text

Solid–liquid phase equilibrium of glyphosate in selected solvents

Abstract

Highlights

Introduction

Section snippets

Materials

Glyphosate characterization

Solubility and correlation

Conclusions

Acknowledgments

Environ. Pollut.

J. Hazard. Mater.

J. Hazard. Mater.

Water Res.

Water Res.

Chemosphere

Appl. Clay Sci.

J. Hazard. Mater.

Fluid Phase Equilib.

J. Pharm. Sci.

Int. J. Photoenergy

Bioresour. Technol.

Clays Clay Miner.

Environ. Sci. Technol.

Water Environ. Res.

J. Chem. Eng. Data

J. Chem. Eng. Data

J. Struct. Chem.

Phosphorus Sulfur