Targeting the removal of Pyridine Herbicides from an Aqueous environment using polymer inclusion membrane

Abstract

The beneficial use of herbicides on pastures to increase crop yields is offset by associated risks from chemical runoff contaminating environmental waters. Most commonly used methods to remove residues from environmental waters are ineffective, while some produce degradation by-products which can also affect the environment. The successful use of polymer inclusion membranes (PIMs) as a simple remediation method to remove pyridine herbicides, such as picloram (4-amino-3,5,6-trichloro-2-pyridinecarboxyIic acid), and related herbicides from an aqueous environment was reported. An optimum PIM composition of 25 wt%CTA, 30 wt% Aliquat 336 and 45 wt% NPOE was used to transport picloram with an initial flux of 294 (± 14) * 10 8 mol.m 2.s 1 and transport efficiency of 95 ± 1%. The PIM was reused in five consecutive transport cycles with a negligible change in flux during later cycles. The results indicate that PIMs provide a potential alternate method for the removal of troublesome herbicides fr om environmental waters. Degradation products from herbicides are considered emerging contaminants and studies on effective removal methods from environmental waters are limited. The successful tiansport of 4-amino-2-chloropyridine (ACP) as a model degradation product of pyridine- based herbicides using PIMs was demonstrated. An optimal membrane composition of 20wt%CTA, 40 wt% Aliquat and 40wt%NPOE producing an initial flux of 413 (± 7) 10 8 mol.m“2.s 1 and 98 ± 1% transport efficiency was identified. Preferential transport of ACP, involving the formation of aggregates with the carrier, over picloram was noted. The results indicate the opportunity for future investigations of the extraction and transport of degradation products of pyridine-based herbicides using PIMs. The use of natural compounds from plants as active chemicals for various applications is regarded as a “green chemistry approach” because of die reduced risk of contaminating the environment. However, there are limited investigations on the use of iplant-based compounds as components in PIMs. Anacardic acid (AA) derived from cashew nut shell liquid was investigated as a “green cationic carrier’’ for some representative organic compounds. An 20 wt% dodecanol optimal and membrane composition 40 wt% NPOE of producing 30 wt% CTA. an initial 10 wt% A A. flux of 364 (± 16) x to x mol.m 2.s 1 and transport efficiency of 98 ± 1% for the transport of ACP as a model compound was demonstrated. The results were comparable with using a commercial carrier, bis-(2-ethylhexyl) phosphoric acid. The membrane also displayed effective competitive transport of ACP, paraquat and diquat with an average transport efficiency of 97±l%. Therefore, the potential use of plant derived natural compounds as “green chemicals” in membranes is an exciting novel development and worthy of further investigations. PIMs have emerged as a powerful tool for the pre-treatment of samples because of their versatility and easy fabrication. The optimal membranes were tested for the pre­ concentration of picloram, ACP. paraquat and diquat as representative compounds from a low initial concentration of 500 pg/L in samples of natural water. The effective pre­ concentration of the compounds was indicated by the significant increase in HPLC signal intensities of the post-concentrated samples. The results indicate the suitability of PIMs as an alternate method of sample pre-treatment and the potential for fabrication and application in passive sampling devices. The results from this thesis indicate the potential of PIMs as a viable method to remove and measure problematic herbicides and degradation products from natural waters using commercial and “green” carriers. However, more investigations simulating real-world conditions, such as long-term use of similar membrane chemistry in hollow fibre extractors and passive sampling devices, are needed to fully demonstrate this exciting potential. Statement of Authorship Except where reference is made in the text of the thesis, this thesis contains no material published elsewhere or extracted in whole or in part from a thesis accepted for the award of any other degree or diploma. No other person’s work has been used without acknowledgment in the main text of the thesis. This thesis has not been submitted for the award of any degree or diploma in any other tertiary institution.