A new approach for arsenic speciation in natural waters through complexation with a dithiophosphinic acid derivative incorporated in a polymer inclusion membrane

Arsenic is a toxic element found in water mainly from natural sources. Ground water contains inorganic arsenite (As(III)) and/or arsenate (As(V)), being the first the most toxic form. Thus, it is necessary to develop reliable methodologies for the selective detection of As in natural waters. In literature, studies on As(V) extraction are reported. However, to the best of our knowledge, only few works concerning As(III) extraction are available, but not in natural matrices but in synthetic samples. Accordingly, in this study we present the possibility of using the extractant Bis(2,4,4-trimethylpentyl)dithiophosphinic acid, Cyanex 301, to complex arsenic species and to investigate whether it is possible to discriminate between the two chemical forms, As(III) and As(V). Moreover, a simple and novel analytical methodology is also proposed incorporating this complexing agent in a polymeric matrix to determine arsenite content in natural water samples. Preliminary solvent extraction experiments (using a 0.1 M Cyanex 301 solution in toluene) were performed in 0.1 M HCl solutions containing 10 mg L-1 of As(III) or As(V). It was found that As(III) was completely extracted after 30 minutes whereas 24 hours were required for the complete extraction of As(V). This different kinetic behaviour can be exploited for speciation purposes. These results encouraged us to attempt the extraction by using polymer inclusion membranes (PIMs), which need a very small amount of organic components, and thus, are an eco-friendly option of the solvent extraction process. PIMs consist of a polymer, which provides mechanical strength, the carrier, which is the responsible of the extraction process, and sometimes also a plasticizer can be used to provide elasticity. These membranes are easy to prepare and thanks to their versatility can be used in many analytical applications, especially in the removal of pollutants from waters. PIMs were prepared by incorporating the extractant in a cellulose triacetate (CTA) polymeric matrix with a final composition of 50% CTA-50% Cyanex 301 (% in mass). Results demonstrated that when Cyanex 301 is entrapped in a polymer the quantitative extraction of As(III) requires 3 hours while As(V) was not extracted, even after 24 hours. Therefore, we extended the use of PIMs to natural waters, in particular well water and tap water samples acidified with HCl. Despite the complexity of the matrix, As(III) extraction was complete, showing the good performance of these membranes containing Cyanex 301. Taking advantage of the characteristics of the PIMs, we investigated the possibility of determining arsenite preconcentrated in PIMs by Energy Dispersive X-ray fluorescence spectrometry (EDXRF). Loaded PIMs were directly analyzed by EDXRF. Results demonstrated that As signal increases when increasing the metal concentration initially present in the aqueous solution. This strong relationship can be used as a calibration curve for future determination of As(III) when contained in water samples. Thus, this methodology combining As extraction by PIM and EDXRF determination can be viewed as a promising approach to perform not only As(III) quantification in natural waters but also speciation studies.