Development and application of passive samplers based on polymer inclusion membranes for evaluating the fate of trace metals polluted by acid mine drainage

Abstract

The shortcomings of conventional sampling methods such as grab sampling have led to the development of alternative approaches especially those based on passive samplers. The challenge of passive samplers despite being cheap to use and being integrative is that there are limited commercial suppliers. Further, technical development is also still ongoing on the types of design and membrane materials. In this work, the technical development, optimization and preliminary application of a polymer inclusion membrane passive sampler is reported. Compared to commercial passive samplers, our passive sampler is reusable, rugged and able to tolerate harsh environmental conditions ensuring maximum efficiency for target compounds. The passive sampler consisted of a chamber made of polytetrafluoroethylene (PTFE) with a lumen that holds 5.5 mL of a receiver solution. A polymer inclusion membrane (PIM) composed of polyvinyl chloride as the base polymer and diethyl hexyl phosphoric acid (D2EHPA) as the carrier was then placed as a sealer for the receiver phase and a barrier to separate the receiver from the source solution. A polytetrafluoroethylene screw cap was used to hold the membrane and the chamber together. The final assembled polymer inclusion membrane passive sampler housing has a length of 46 mm. Target compounds diffuse from the sample solution through the membrane into the receiver solution. The membrane is incorporated by a mobile carrier that transports the target metal species from the sample into the receiver solution. The driving force in this case is the proton in the acceptor solution that is back transported across to the sample solution. The polymer inclusion membrane was characterized by atomic force microscopy and scanning electron microscopy, and results showed that the PIM’s roughness increased after the sampler had been deployed for 12 days. Fourier transform infrared revealed the presence of carrier molecules. The contact angle temperature (~78.80oC) showed that the polymer had hydrophilic surfaces due to the carrier functional groups. The effect of variables such as the carrier composition, pH, membrane thickness, receiver phase concentration and the stability of PIM were optimised to calculate the time-weighted average concentration of first and second row transition metal elements which are Ni (II), Co (II), Cu (II), and Cd (II). The PIM optimum conditions were composed of 40% D2EHPA as a carrier and 60% PVC as a base polymer, sample pH of 3-7, 1 mol L−1 HNO3 as acceptor solution and membrane thickness of 150 µm. The stability of PIM as passive was tested up to three cycles and showed remarkable stability. The stability was further followed using thermal gravimetric analysis and Phosphorus-31 NMR spectroscopy. The developed passive sampler showed a large time lag up to five days for all the metals and then uptake became linear up to 13 days. Thus slow release of the metals from the PIM into the bulk acceptor solution was the rate limiting factor. The passive sampler is best suited to be deployed for two weeks instead of a short period of time. The passive samplers were deployment in dam water and acid mine drainage (AMD) in the laboratory and in the field to study the transport and fate of these metals. The PIM-based passive sampler was able to extract the metals as it seemed to tolerate high metal content in AMD. The release of H+ on the source phase from the acceptor solution also prevented any fouling. Linear uptake of the metals was observed in PIM sampler but laboratory-based uptake in AMD failed because of precipitation that reduced the amount of the bioavailable fraction. Metals like Fe and Al were found to control these reactions along with redox potential. The PIM based sampler showed high versatility as it could tolerate AMD matrix.