Removal of picloram herbicide from an aqueous environment using polymer inclusion membranes
Introduction
Herbicides have harmful effects on the environment and non-target organisms such as plants and animals. They have a range of effects on animals including carcinogenicity and endocrine disruption [1,2]. The contamination of environmental waters with herbicides is a direct consequence of chemical weed control mainly in agriculture production. Poor water quality caused by these chemicals has been listed as one of the major environment-related health threats [3]. The persistence of herbicides in water is determined by their chemical characteristics. Pyridine herbicides are resistant to natural degradation processes or metabolic reactions in plants or animals. As a result, they remain active in environmental waters for a long time and are continuously recycled through the decomposition of organisms [4,5].
Picloram (4-amino-3,5,6-trichloro-2-pyridinecarboxylic acid) is the most persistent pyridine herbicide which is sometimes referred to as a chlorinated derivative of picolinic acid. It functions by mimicking the auxin hormone, a growth regulator, causing unorganized and uncontrolled growth of cells that leads to plant death. The herbicide is used at low and high application rates to control the growth of deeply rooted herbaceous and annual broadleaf weeds, respectively. Picloram (pKa 2.3) exists as an anion at the pH of most soils and waters. Consequently, it has high water mobility and poor adsorption to most soils. Picloram is classified as an herbicide with high leaching potential and presents a significant risk for water contamination. It has been detected in fresh waters at concentrations of 0.3–437 μg/L [6]; just below the maximum permissible level of 500 μg/L in drinking water as established by some regulatory agencies [7]. The main process by which picloram is naturally removed from the environment is by microbial degradation, although, in most cases, this is a slow and incomplete process. The degradation of picloram by other chemicals present in the environment is estimated to have a half-life of 9–166 years [8]. Additionally, picloram is not readily removed from the environment by volatilization because it has a low vapor pressure of 6.16 × 10−7 mm Hg at 35 °C [9]. Thus, alternate and viable methods/techniques to remove picloram from environmental waters are required. Previous methods and techniques have included heterogeneous photo-catalysis, chemical oxidation, electrochemical oxidation, and microbial degradation that produced reasonable removal efficacies [[10], [11], [12], [13], [14]]. Others have prepared picloram in a form of ionic liquids with the purpose of reducing its effects by increasing soil adsorption and decreasing water solubility and leaching ability [15]. However, most of these methods and techniques contribute to secondary pollution due to the formation of other chemicals or byproducts. Thus, developing a method or technique with minimized environmental pollution, high removal/recovery efficiency and less energy consumption is deemed necessary.
The use of a liquid membrane is a highly selective and effective method/technique for wastewater treatment and recovery of various chemicals [[16], [17], [18]]. Similar to other types of liquid membrane, polymer inclusion membranes (PIMs) are regarded as an environmentally friendly alternative because of their reusability, reduced use of and exposure to organic chemicals, and a lesser risk of chemicals leaking into the environment. Moreover, their versatility, ease of preparation and selectivity make them even more attractive. The use of PIMs for the extraction and transport of various metal species has been extensively studied because of their potential commercial applications [16]. Similar studies of organic compounds using PIMs have not been as prolific but have recently received attention [17,19,20]. PIMs containing CTA and Aliquat 336 have demonstrated ability to extract various organic compounds (Table 1). However, until this time, no study has reported the use of PIMs for the removal of picloram from the environment. In this paper, we contribute to this later growing field of research by reporting the removal of picloram, a typical pyridine-based herbicide, using PIMs containing CTA, Aliquat 336 and NPOE. The viability of a membrane for the transport and recovery of picloram from environmental waters was also evaluated.
Section snippets
Chemicals and reagents
Cellulose triacetate (CTA), Aliquat 336 (Aliquat), 2-nitrophenyl octyl ether (NPOE) (≥99.0%), tris-(2-ethylhexyl) phosphate (TEHP) (97%), dioctyl phthalate (DOP) (≥99.5%), picloram (4-amino-3,5,6-trichloro-2-pyridinecarboxylic acid) (BioReagent), 2,4-D (2,4-dichlorophonoxyacetic acid) (97%), triclopyr (3,5,6-trichloro-2-pyridinyloxyaceticacid) (PESTANAL®, analytical reagent, 99.9%), sodium monohydrogen phosphate dihydrate (98–101.0%), sodium dihydrogen phosphate dihydrate (98–100.5%), methanol
Transport mechanism
A facilitated counter-coupled mechanism for the transport of picloram (pKa = 2.3) using a PIM containing Aliquat carrier at pH 7 is shown in Fig. 2. Picloram in an anionic form undergoes an ion-exchange with chloride of Aliquat (Q+Cl−) at the feed/membrane interface, forming an Aliquat–picloram ion-pair (Q+P−) that permeates and diffuses through the membrane phase. The reverse ion-exchange process (stripping) is driven by a ‘high concentration’ of chloride at the receiving/membrane interface.
Conclusion
This research has detailed the development of a PIM with an optimum composition of 25 wt% CTA, 30 wt% Aliquat and 45 wt% NPOE for the effective competitive and non-competitive extraction and transport of picloram from aqueous test solutions and spiked environmental water samples. A maximum flux and transport efficiency of 294 (±14) × 10ˉ8 mol mˉ2 sˉ1 and 95 ± 1%, respectively, were achieved for picloram using an optimized receiving solution of 0.25 M NaCl. The membrane was stable and reuseable
Funding
This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.
CRediT authorship contribution statement
Alinanuswe J. Mwakalesi: Conceptualization, Methodology, Investigation, Writing - original draft, Validation, Formal analysis. Ian D. Potter: Conceptualization, Methodology, Supervision, Project administration, Writing - review & editing, Resources.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
Alinanuswe J. Mwakalesi thanks La Trobe University for the awards of a La Trobe University Postgraduate Research Scholarship (LTUPRS) and La Trobe University Full Fee Research Scholarship (LTUFFRS) to undertake this project.
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