Journal of Membrane Science 203 (2002) 201–208

Facilitated transport and preconcentration of theherbicide glyphosate and its metabolite AMPA

through a solid supported liquid-membrane

Carolina Rios, Victòria Salvadó, Manuela Hidalgo∗Department de Quımica, Universitat de Girona, 17071 Girona, Spain

Received 31 May 2001; received in revised form 18 December 2001; accepted 19 December 2001

Abstract

A solid supported liquid-membrane (SLM) system for the transport and preconcentration of the herbicide glyphosate and itsmain metabolite aminomethylphosphonic acid (AMPA) has been designed and characterised. The influence of the chemicalcomposition of the system as well as membrane configuration on the transport properties were investigated. The best extractionconditions were achieved by adjusting the pH of the aqueous feed phase to 12 (0.1 M NaOH). Among the different optionstested, a 0.1 M HCl solution was found to be the most effective to strip glyphosate and AMPA from the loaded organic phase.The effect of carrier concentration and the nature of the organic solvent were also tested and the best results were obtainedwhen working with 0.2 M Aliquat 336 in dodecane modified with 4% dodecanol.

Under these chemical conditions some preconcentration experiments were run using two different geometries of themembrane: a hollow fiber liquid-membrane (HFSLM) and a laminar membrane system (LSLM) where the stripping solutioncan be recirculated. The best results were obtained for the liquid-membrane system in HF configuration, with concentrationfactors of 19 and 2.5 for glyphosate and AMPA, respectively. © 2002 Elsevier Science B.V. All rights reserved.

Keywords:Glyphosate; Liquid-membrane; Preconcentration; Aminomethylphosphonic acid (AMPA); Hollow fiber

1. Introduction

Glyphosate, N-(phosphonomethyl)glycine, is anon-selective, post-emergence organophosphorus her-bicide widely used in various applications for weedand vegetation control. Because of its relatively lowmammal toxicity, glyphosate has become one of themost widely used herbicides in the world. Therefore,there is a great interest in the monitoring of thiscompound and its major metabolite aminomethyl-phosphonic acid (AMPA).

∗ Corresponding author. Tel.:+34-972-418190;fax: +34-972-418150.E-mail address:manuela.hidalgo@udg.es (M. Hidalgo).

A great variety of methods for the determina-tion of glyphosate and AMPA in environmentalmatrices have been applied [1]. Although, manyof the techniques described in the literature arehighly sensitive, the analysis of these compoundsat residue levels requires the use of effective ex-traction/preconcentration and cleanup procedures inmany matrices [2,3]. However, their high water sol-ubility and polar nature makes their extraction andenrichment from aqueous samples a very difficulttask.

Several methods have been published concerningextraction and preconcentration of herbicides fromnatural waters using techniques such as liquid–liquidextraction (LLE), solid-phase extraction (SPE) and

0376-7388/02/$ – see front matter © 2002 Elsevier Science B.V. All rights reserved.PII: S0376-7388(02)00007-8

202 C. Rios et al. / Journal of Membrane Science 203 (2002) 201–208

anion or cation-exchange. An attractive alternativeto these sample pretreatment methods are supportedliquid-membranes (SLMs).

SLMs have the advantage of achieving the extrac-tion and stripping operation with very high enrich-ment factors in a single stage using low concentrationof extractants. In addition, they can be easily coupledon-line with the analytical system [4,5].

SLMs have been applied for separation and enrich-ment of metals [5,6], for extraction and enrichmentof pesticides such as sulfonylurea herbicides [7], tri-azines [8,9] and acidic herbicides [10] and for aminoacids [11–14].

For the acidic or basic compounds, a non-facilitatedtransport mechanism is possible, being the drivingforce for transport the differences of pH between thedonor and receiving aqueous phases.

In the case of amino acids, the presence of a car-rier in the membrane is necessary to achieve thetransport. Due to the zwitterionic character of thesecompounds, they are present in an anionic form in theaqueous donor solution at pH greater than 9. Whenusing a chloride quaternary ammonium salt as a ex-tractant an ion-exchange reaction takes place at theliquid-membrane interface in which the amino acidin the negative form is exchanged for the counter-ionof the extractant. The complex amino acid-carrierdiffuses through the membrane and in the side ofthe aqueous stripping phase, it exchanges again theamino acid for the anion that is present in this aque-ous solution [13]. Because of the resemblance ofglyphosate to the amino acid glycine, the same mech-anism could be applied but taking into account themore hydrophilic character of glyphosate and AMPA.To our knowledge, there is only one work reportedin the literature in which this technique is appliedfor the preconcentration of glyphosate [15]. No datahave been found on the transport of AMPA throughliquid-membranes.

The aim of this work is to develop an SLMsystem for the extraction and preconcentration ofglyphosate and AMPA in order to facilitate their de-tection at residue levels. The influence of the differentchemical parameters affecting the transport of thesecompounds through the SLM is examined. The per-formance of the liquid-membrane system in differentgeometries, i.e. hollow fiber and flat-sheet, is alsocompared.

2. Experimental

2.1. Chemicals

The extractant, tricaprylylmethylammonium chlo-ride (Aliquat 336), was supplied by Fluka Chemie.Cumene (Merck), decaline (Sigma), kerosene (Fluka)and dodecane (Merck) modified with 1-dodecanol(Merck) were used as organic solvents.

Glyphosate was obtained from Dr. Ehrenstor-fer GmbH (Germany) and AMPA from LancasterSynthesis (UK). All the aqueous solutions were pre-pared by using water purified with a Milli-Qplussystem (Millipore, Barcelona, Spain).

2.2. Analysis

A model LC-9A high-performance liquid chro-matograph (Shimadzu, Kyoto, Japan) equipped witha UV–VIS detector set a 240 nm was used to separateand determine glyphosate and AMPA as describedin [16]. Samples were injected via a Rheodyne valve7725 equipped with a 20�l loop. The HPLC sepa-ration was performed on a Hypersil ODS-5 column(200 mm× 4.6 mm i.d.) with a mobile phase con-sisting of 0.06 M phosphate buffer (pH 2.30 adjustedwith sodium hydroxide (Panreac)), acetonitrile (CarloErba) (85:15,v/v) at a flow rate of 1 ml/min at roomtemperature.

The sample was derivatized by neutralising a1 ml aliquot with NaOH or HCl to which 0.5 mlof 0.4 M phosphate buffer (pH 11.0) and 0.2 mlof p-toluenesulphonyl chloride (Aldrich) solution(10 mg/ml in acetonitrile) were added. This mixturewas heated in water bath at 50◦C for 5 min. A 20�laliquot of the reaction mixture was injected into theHPLC column.

2.3. Membrane equipment and procedure

Three different membrane units were used (Fig. 1).For the design of the liquid-membrane system, the ex-periments were carried out by using a membrane cellprovided with two separated compartments (for theaqueous feed and stripping solutions) connected bya circular window where the impregnated flat-sheetmembrane was placed (Fig. 1a). The microporous sup-port for the liquid-membrane used in the experiments

C. Rios et al. / Journal of Membrane Science 203 (2002) 201–208 203

Fig. 1. Schematic diagram of the cells and module used in the experiments. (a) Cell used for the design of the liquid-membrane system.(b) Three compartment unit containing two laminar membranes where the stripping solution recirculated (1: stirring motors; 2: membranes;3: stripping compartment; 4: feed compartment). (c) Hollow fiber module.

was a polydifluoroethylene film (Millipore GVHP)with a thickness of 125�m, a porosity of 75% andan average pore size of 0.2�m. The support was im-pregnated with a solution of Aliquat 336 in an organicsolvent.

The volumes of both aqueous feed and strippingsolutions were 200 ml in all experiments and they bothwere stirred at 1000 rpm. The instant when the stirringmotors were started was taken as the zero time ofthe experiment. The concentration of glyphosate andAMPA was followed by withdrawing samples from thefeed and stripping solutions at given time intervals andanalysing them by HPLC as described previously. Allexperiments were performed in a thermostatic roomat 22± 1◦C.

In the preconcentration experiments, other cellsor modules were used: a three compartment mem-brane unit containing two laminar membranes wherethe stripping solution recirculated and a hollow fibermodule.

In the first case, the membrane cell is shownschematically in Fig. 1b. Each of the two end com-partments were filled up with 200 ml of the feedsolution and they were separated from the centralcompartment, where 40 ml of the stripping solutionrecirculated using a peristaltic pump, by two laminarmembranes impregnated with the organic solutioncontaining the carrier.

In the second case, hydrophobic polypropylene hol-low fibers from Azko (Enka, E.G., Germany) were

used as the supports for the liquid-membrane. Thefiber had an i.d. of 0.3 mm, an external diameter of0.5 mm, a pore size of 0.2�m, a porosity of 75% anda length of 57 cm. This hollow fiber module is shownin Fig. 1c.

The HFSLM was prepared by a slow impregnationof the tubular microporous fiber, flowing a solutionof Aliquat 336 in dodecane containing 4% dodecanolthrough the lumen of the hollow fiber module. AGilson peristaltic pump (Pacisa, Barcelona, Spain) wasused to recirculate the aqueous phases, where 250 mlfeed solution was flowed through the fiber, while theshell-side contained the stripping solution (10 ml).

3. Results and discussion

3.1. Design of the liquid-membrane system

In order to select the best chemical condi-tions for the transport of glyphosate through theliquid-membrane, a set of experiments were carriedout by using the flat-sheet membrane cell shown inFig. 1a.

The chemical properties of glyphosate and AMPA,i.e. their hydrophilic nature and ionic character in abroad pH range, makes necessary the presence of acarrier in the membrane which facilitates the transport.In this work, the commercial extractant Aliquat 336is used. As it is a quaternary ammonium chloride, it

204 C. Rios et al. / Journal of Membrane Science 203 (2002) 201–208

is assumed that the extraction mechanism occurs viaan ion-exchange of glyphosate for the chloride ionpresent in the organic phase.

The composition of the feed solution was studiedto establish the optimum conditions for the transportof glyphosate. These experiments were performed byimpregnating the membrane support with Aliquat 336in dodecane modified with 4% dodecanol. The valuesfound in the literature corresponding to the acid disso-ciation constants of glyphosate are pK1 0.8, pK2 2.2,pK3 5.4 and pK4 10.2 [3] and therefore in order tohave glyphosate in a completely deprotonated form apH > 10 is needed.

The best results were obtained when working witha solution containing 5 mg l−1 of glyphosate in 0.1 MNaOH with a recovery of 37% after 3 h. Higher con-centrations of NaOH or the presence of other anionssuch as Cl− or NO3

− at 0.5 M concentration in the feedsolution greatly reduced the recovery of glyphosate to1%. Other solutions such as 0.4 M phosphate buffer atpH 7 and 11 were used with the same negative results.In these preliminary experiments, a 0.1 M citrate so-lution was used as stripping phase. These results canbe explained taking into account the non-selectivecharacter of Aliquat 336 which caused other an-ions presents in the feed solution to compete withglyphosate.

Fig. 2. Influence of the organic solvent on the transport of glyphosate. Feed solution: 5 mg l−1 glyphosate in 0.1 M NaOH; strippingsolution: 0.1 M citrate; carrier: 0.2 M Aliquat 336.

The effect of the organic solvent was tested using0.2 M Aliquat 336 as carrier. Aliphatic and aromaticdiluents and also mixtures of them were used. Asseen in Fig. 2, the recovery of glyphosate was higherwhen using cumene as a solvent than with dodecane,kerosene or kerosene–cumene mixture. This effectmay be related to solubility properties, but as it is re-ported, aromatic solvents deform polymeric supportsused in the impregnation [17], thus, dodecane modi-fied with 4% of 1-dodecanol to increase the solubilityof Aliquat 336 was used in subsequent experiments.

The effect of the receiving phase composition wasevaluated by testing different solutions. As can beseen in Table 1, HCl was found to be the most ef-fective compound for the stripping of glyphosate.This could be attributed to the fact that this reagenthad the effect of exchanging the chloride anion forglyphosate and besides it could protonate glyphosatein order to convert the analyte to a non-anionic form.When using citrate as a stripping solution, the onlyeffect which influenced in the recovery of glyphosatewas the exchange of the anion. The role of chlorideanions exchange as a driving force for the transport isconfirmed by the results obtained with H3PO4 and amixture of H3PO4and NaCl as a stripping solutions.The percentage of recovery increases from 1 to 48%by the addition of NaCl.

C. Rios et al. / Journal of Membrane Science 203 (2002) 201–208 205

Table 1Effect of stripping solution composition on the recovery ofglyphosate

Stripping solution Recovery of glyphosate (%)

0.1 M HCl 520.1 M H3PO4 10.1 M H3PO4 + 0.075 M NaCl 480.1 M citrate 360.015 M citrate 22

Feed solution: 5 mg l−1 of glyphosate in 0.1 M NaOH; organicphase 0.2 M Aliquat 336 in dodecane+ 4% dodecanol; length ofthe experiment: 3 h.

Based on these results, the stripping solution wasfixed as a 0.1 M HCl.

The influence of carrier concentration on the trans-port of glyphosate was also studied. The experimentaldata were expressed in terms of glyphosate permeabil-ity coefficient,P, defined by

P = −(

dC

dt

) (V

Q

) (1

C

)(1)

whereC is the concentration of glyphosate at timet, Vthe volume of the feed solution (200 ml) andQ is theeffective membrane area (geometric area corrected forporosity) which had a value of 8.5 cm2 in these experi-ments (laminar membrane unit shown in Fig. 1a). Thepermeability coefficient is obtained from the graphln(C/C0) versus time by determining the slope of thecorresponding linear relationship for the experimental

Fig. 3. Glyphosate permeability vs. carrier concentration. Feed solution: 5 mg l−1 glyphosate in 0.1 M NaOH; stripping solution: 0.1 MHCl; organic phase: Aliquat 336 in dodecane+ 4% dodecanol.

data according to the integrated form of Eq. (1):

ln

(C

C0

)= −

(Q

V

)Pt (2)

whereC0 is theC value at time zero.Fig. 3 shows the permeability coefficients for the

transport of glyphosate through an SLM impregnatedwith Aliquat 336 in the range 0.1–0.3 M in dodecanemodified with 4% 1-dodecanol. The permeability co-efficient increased with carrier due to an increase inextractability into the liquid-membrane.

With the best conditions obtained for glyphosate,some experiments in which its metabolite AMPAwas added were performed and the maximum recov-ery achieved for this compound was about 8%. Thismay be explained because of the more hydrophiliccharacter of AMPA compared to glyphosate whichmakes the extraction into the organic phase verydifficult.

3.2. Enrichment experiments

Experiments were performed with different mem-brane units that allow preconcentration of glyphosateand AMPA.

3.2.1. Hollow fiber moduleMost of experiments were carried out with the mod-

ule shown in Fig. 1c which contained one coiled hol-low fiber which is impregnated with a solution of

206 C. Rios et al. / Journal of Membrane Science 203 (2002) 201–208

Fig. 4. Influence of the flow rate on the transport of glyphosate and AMPA. Feed solution: 5 mg l−1 glyphosate+ 5 mg l−1 AMPA in 0.1 MNaOH; stripping solution: 0.1 M HCl; organic phase: 0.2 M Aliquat 336 in dodecane+ 4% dodecanol.

0.2 M Aliquat 336 in dodecane modified with 4% ofdodecanol.

Previously to the preconcentration experiments, thehydrodynamic conditions of the system were opti-mised. In these experiments, the same volume of feedsolution and stripping solution were used (200 ml ofeach one in these experiments). From the results ob-tained, which are collected in Fig. 4, it can be seenthat increasing the flow rate the recovery of glyphosateand AMPA is slightly higher. However, at higher flowrates, the time of life of the fiber decreased, therefore,it was fixed at 0.85 ml/min.

By using a volume of the stripping solution muchsmaller than the feed solution volume, the analytescan be concentrated. The results presented in Table 2where obtained when working with 250 ml volumefeed and 10 ml volume stripping solution. The con-centration factor F(t) is defined as F(t) =Cst (t)/Cs0,where Cst (t) is the concentration of the analytein the stripping solution at timet, and Cs0 is the

Table 2Concentration factor values using the hollow fiber module (HFSLM)

[Solute]feed,0

(mg l−1)a[AMPA] strip,t

(mg l−1)Concentrationfactor AMPA

[Glyphosate]strip,t

(mg l−1)Concentration factorglyphosate

5 12.3 2.6 97 18.72 6.7 2.4 23.5 9.90.5 1.1 2.2 4.0 8.1

Feed solution: glyphosate and AMPA in 0.1 M NaOH (250 ml); stripping solution: 0.1 M HCl (10 ml); organic phase: 0.2 M Aliquat 336in dodecane+ 4% dodecanol; flow rate: 0.85 ml/min; time of experiment: 24 h.

a AMPA and glyphosate single initial concentrations in the source solution.

concentration in the feed solution at the beginning ofthe experiment. The highest glyphosate and AMPAenrichment was obtained with a feed solution whichcontained 5 mg l−1 of each analyte; the concentrationfactors were 18.7 and 2.6, respectively.

3.2.2. Flat-sheet liquid-membrane flow cellThese experiments were performed by using the

cell shown in Fig. 1b. A 200 ml of feed solution wereplaced in each of the two end compartments whichmeant that the total feed volume was 400 ml, and40 ml of stripping solution recirculated in the cen-tral compartment. Table 3 shows the concentrationfactors obtained when working with different initialconcentrations of glyphosate and AMPA in the feedsolution. It can be seen that the concentration fac-tor for glyphosate increases slightly when the initialconcentration of the analyte decreases. This result isinteresting for the application of this system to naturalwaters where the concentration of glyphosate is low.

C. Rios et al. / Journal of Membrane Science 203 (2002) 201–208 207

Table 3Concentration factor values using the flat-sheet liquid-membrane flow cell

[Solute]feed, 0

(mg l−1)a[AMPA] strip,t

(mg l−1)Concentrationfactor AMPA

[Glyphosate]strip,t

(mg l−1)Concentrationfactor glyphosate

5 2.0 0.4 37.0 7.42 0.7 0.3 15.4 7.70.5 0.5 0.9 4.0 8.0

Feed solution: glyphosate and AMPA in 0.1 M NaOH (400 ml); stripping solution: 0.1 M HCl (40 ml); organic phase: 0.2 M Aliquat 336in dodecane+ 4% dodecanol; flow rate: 0.85 ml/min; time of experiment: 24 h.

a AMPA and glyphosate single initial concentrations in the source solution.

For AMPA, it can also be observed that the con-centration factor increased when the initial concentra-tion was lower although no enrichment was obtainedin the range studied, i.e. concentration factor valueswere lower than 1.

If we compare the two systems used in the precon-centration experiments, we can conclude that the bestresults can be achieved using the hollow fiber mod-ule because higher enrichment factors can be obtainedfor both analytes, moreover, the ratio of the volumesof the feed solution and stripping can be increased inorder to have better results.

4. Conclusions

This study shows that the herbicide glyphosate canbe transported from an aqueous donor phase at pH 12consisting of a solution of 0.1 M sodium hydroxide toan hydrochloric acid acceptor phase through a solidSLM consisting of a solution of Aliquat 336 in dode-cane containing 4% dodecanol.

In these conditions, high enrichment factors can beachieved for glyphosate when using a flat-sheet mem-brane unit in which the stripping solution recirculatesor a hollow fiber module. AMPA can also be trans-ported, but the concentration factors obtained werelower due to its polar nature and high water solubilitythat makes the extraction into the organic phase verydifficult.

Acknowledgements

The present work has been carried out under CYCIT(Spanish Commission for Research and Development)financial support, projects no. AMB97-0859-C02-02

and QUI1999-0749-C03-03. C. Rios also thanks theUniversity of Girona funding through a research grant(reference: BR99/792).

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