Trace Enrichment of Triazine Herbicides using the Supported Liquid Membrane Extraction Technique: Applications to the Study of Waters in Ethiopian Lakes
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Date
2000-04
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Addis Ababa University
Abstract
Methods for sample preparation of the s-triazine herbicides and their degradation products
in environmental water samples using supported liquid membrane (SLM) has been
developed. The herbicide compounds were selectively extracted from the flowing donor
solution into a porous polytetrafluoroethylene (PTFE) membrane impregnated with nundecane,
di-n-hexylether alone or with 25% 6-undecanone and 50% of n-undecane. After
diffusion through the hydrophobic membrane the s-triazine herbicide compounds were
irreversibly trapped into acidic acceptor phase of lower pH. The Effects of different
experimental variables governing the efficiency of the extraction process have been
thoroughly studied and optimized. These include the pH of both the donor and acceptor
solutions, donor flow rate, ionic strengths and the analyte trapping conditions.
Furthermore, the conditions for attaining maximum enrichment factor from the membrane
extraction have been investigated.
Selectivity of the membrane technique developed was compared with one of the
most frequently used technique for sample preparation of organic pollutants, solid phase
extraction (SPE) in off-line mode. It was observed that the chromatograms obtained using
the SLM methodology were cleaner, indicating the good selectivity of the extraction
method. The liquid membrane serves as a barrier for interfering compounds, and neutral
molecules may not be enriched. The limit of detection for the extracts using SLM
technique was also comparable to that of the SPE. The possibility of determining the
extraction efficiency, and thus the enrichment factor, from the donor waste was also
investigated. The results obtained from both the acceptor concentrate and the donor waste
agreed well, which in turn indicated efficient penneation through the liquid membrane
thereby giving minimum memory effects.
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The applicability of the method for extraction of th e compounds of s-tri azine
herbicides from water samples containing complex matrices was studi ed by spi king analyte
sample solutions both in reagent water and ri ver water. The resulting chromatograms were
not differing much, which also refl ects the selectivity of the membrane extraction
developed.
Water samples collected from Southern Ethiopian lakes were pre-treated and
extracted in the same manner. There are a number of agricultural fi elds in this region
where herbicides are periodically applied. Thus, the lake waters certainly contain
considerable quantities of these residues and degradation products. The water samples, in
which the phosphate buffer was dissolved to obtain neutral pH, were extracted at high
donor flow rate, 5.0 mLimin, for 10 h. Low quantities of atrazine and terbutryn were
identified in Awassa Lake, while only atrazine could be seen in the extracts from Chamo
Lake. In both cases the concentration range of these compounds were estimated to vary
from 0.02 to 0.05 Ilg/L. None of these herbicides or other s-triazine compounds could be
detected in the extracts of the water samples collected from Abbaya lake.
A mixture of the parent compounds and the degradation products of the s-triazine
herbicides were also successfully extracted in the liquid membrane system containing
carriers, e.g., trioctyl phosphine oxide (TOPO). Low detection limits were obtained for the
aqueous samples spiked in river water, water containing humic acid and also in urine.
Effects of the flowing donor channel, while the acceptor concentrate collected, on analyte
permeation was studied in detail and better analyte results were obtained than the stagnant
donor system particularly for the more polar compounds.
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Applications to the Study of Waters in Ethiopian Lakes