Pest Management Science Pest Manag Sci 56:565±570 (2000) Removal of paraquat and atrazine from water by montmorillonite-(Ce or Zr) phosphate cross-linked compounds E Gonza´lez-Pradas,1* M Villafranca-Sa´nchez,1 F Del Rey-Bueno,2 MD Uren˜a-Amate1 and M Ferna´ndez-Pe´rez1 1Department of Inorganic Chemistry, University of Almerı´a, La Can˜ada San Urbano s/n, 04120 Almerı´a, Spain 2Department of Inorganic Chemistry, University of Granada, Campus Fuentenueva, Avda Severo Ochoa, s/n, 18071 Granada, Spain Abstract: The adsorption of paraquat (1,1'-dimethyl-4,4'-bipyridilium dichloride) and atrazine (6- chloro-N 2-ethyl-N 4-isopropyl-1,3,5-triazine-2,4-diamine) from aqueous solution onto two mont- morillonite-(Ce or Zr) phosphate cross-linked compounds at different temperatures (288K and 308 K) has been studied using batch experiments. The adsorption isotherms obtained for paraquat on both adsorbents may be classi®ed as H-type of the Giles classi®cation, which suggests that paraquat molecules are strongly adsorbed on the samples. For the adsorption of atrazine, L-type isotherms were obtained for both montmorillonite-(Ce or Zr) phosphate compounds, which suggests that these compounds have a medium af®nity for this herbicide. The increase in temperature from 288K to 308K did not have any clear effect on the adsorption process of paraquat on either adsorbent whereas atrazine adsorption decreased slightly as temperature increased, possibly due to a mainly physical process. Fourier transform infrared (FTIR) spectroscopic studies revealed that at the pH generated by the adsorbents, the cationic herbicide interacted to a greater extent with the negatively charged surface of the adsorbents than did atrazine. For both herbicides, the Ce-montmorillonite adsorbent showed a higher adsorption capacity than the Zr-montmorillonite adsorbent. # 2000 Society of Chemical Industry Keywords: atrazine: paraquat; montmorillonite; phosphate; adsorption 1 INTRODUCTION solids of controllable porosity for possible use as The vast agricultural use of pesticides in Southern adsorbents.2 Phosphates of tetravalent metals are Spain has important implications on the contamina- another type of inorganic layered compound that has tion of ground water systems which are used both for been widely studied because of their catalytic and ion- human consumption and for crop irrigation. Areas exchange properties. These compounds have been such as AlmerõÂa (southeastern AndalucõÂa) have low compared with some smectites due to their structural rainfall and an intensive horticultural production similarities.3 So the use of cross-linked compounds based on plastic greenhouses, so this contamination obtained using montmorillonite (a layered silicate of aquifer systems is an increasingly serious problem. belonging to the group of smectites) and phosphates Water analyses have shown pesticide residues at of tetravalent metals has gained widespread accep- concentrations of 0.01±0.5mg litre1 in the AlmerõÂa tance as a technique to eliminate contaminants in wells.1 One of the main reasons that removal and natural waters, as a result of the improvement of their disposal of these potentially hazardous waste chemi- surface properties (speci®c surface areas and porosity), cals is such a complex problem arises from the wide with regard to those exhibited by each compound range of chemical compounds which are used as independently. This allows the new compounds to be pesticides. This makes it extremely dif®cult to produce used as better adsorbents in aqueous media. The a single method for pesticide disposal that applies association between montmorillonite and phosphates universally. Therefore, several speci®c methods for the of tetravalent metals yields a material which does not removal and disposal of these chemicals may be disperse in water and undergoes only a very low degree required to solve the problem. of hydrolysis of its phosphate groups.4 The use of these There is a growing interest in the application and compounds in preference to others such as anionic study of clays as precursors of cross-linked com- exchangers and some types of clay (natural or pounds, especially since with these one can now obtain modi®ed), can be justi®ed by the superior results * Correspondence to: E Gonza´lez-Pradas, Department of Inorganic Chemistry, University of Almerı´a, La Can˜ada San Urbano s/n, 04120 Almerı´a, Spain E-mail: [email protected] (Received 26 January 1999; revised version received 15 October 1999; accepted 14 February 2000) # 2000 Society of Chemical Industry. Pest Manag Sci 1526±498X/2000/$17.50 565 E GonzaÂlez-Pradas et al obtained in the removal of contaminants from Analytical grade atrazine (99%) and paraquat water.5,6 (99%) were purchased from Riedel-De HaeÈn, (the As adsorption on solid substrates is one of the latter obtained as the dichloride salt) and used without methods which has been used for removing pesticides further treatment or puri®cation. from water,7 we considered it useful to study the Atrazine and paraquat adsorption experiments were sorption processes of two herbicides, atrazine and performed by using the batch-equilibration method. paraquat, as a function of temperature on two Duplicate samples of each adsorbent (0.1g) were montmorillonite-(Ce or Zr) phosphate cross-linked equilibrated in 100ml conical ¯asks with 50ml of compounds. aqueous solution of the herbicides with varying initial 2 4 4 2 Atrazine (6-chloro-N -ethyl-N -isopropyl-1,3,5- concentrations, ranging from 1Â10 to 7.5Â10 cM 4 2 triazine-2,4-diamine) is a systemic herbicide which for the 5C sample and from 1Â10 to 6.7Â 10 cM inhibits photosynthesis and is applied for general weed for the 5Z sample. The experiments were carried out control.8 Paraquat (1,1'-dimethyl-4,4'-bipyridinium in a thermostatic shaker bath at 288K and 308K. ion), normally applied in the form of the dichloride Preliminary experiments were conducted for various salt, is an extremely effective, non-selective herbicide time intervals to determine when sorption equilibrium which also interferes with the redox reactions related was reached. The time required for atrazine was 48h to photosynthesis.9 Its persistence and polar character and 168h for paraquat. Following the equilibration mean that this compound may be present as residues period, the adsorption systems were centrifuged at in surface water.10,11 Both herbicides are extensively 19000 rev min1 for 10min and the concentration of used in the AlmerõÂa region. the herbicide in the supernatant solutions (Ce) Taking into account the above, this study was determined by high performance liquid chromatogra- initiated to determine the effectiveness of the new phy (HPLC) using a Beckman liquid chromatographic cross-linked compounds in removing atrazine and the system equipped with diode-array detector and data cationic pesticide paraquat from aqueous solutions. station. This evaluation was carried out by studying the The HPLC operating conditions were as follows: sorption of the two pesticides in batch experiments, separation by isocratic elution was performed on a in order to obtain the corresponding sorption iso- 150Â3.9mm Nova-Pack LC-18 bonded-phase col- therms and sorption capacities, as well as to study the umn (Waters, Millipore Corporation); sample vo- effect of temperature on the adsorption process. lume, 20ml; ¯ow rate, 1.0ml min1; and the mobile phase, HPLC grade acetonitrile (Riedel-De HaÈen) demineralized water (milli-Q quality, Millipore Corp) 2 EXPERIMENTAL (6040 by volume) for atrazine measurements. For The materials used as adsorbents in this study were paraquat measurements, the mobile phase was pre- two samples of montmorillonite-Ce(IV) phosphate pared as follows: a solution containing the speci®c and montmorillonite-Zr(IV) phosphate cross-linked equivalent of 7.5mM sodium heptanesulphonate compounds (labelled as 5C and 5Z, respectively), (Sigma Chemical Co) and 0.10 M orthophosphoric which have been characterized by the present authors acid (85%, Panreac) was made up in 0.45mm ®ltered in a previous paper.4 Tetravalent metal phosphate-clay doubly distilled water. The pH was adjusted to 3.00 compounds were obtained by using a method adapted with triethylamine (99.6%, Merck), and the organic from the one proposed by Sterte,12 to obtain titanium modi®er, acetonitrile, was added to yield a 10% (v/v) oxide±montmorillonite compounds. The starting ma- proportion. Atrazine was analysed at 222nm and terial was a bentonite from `Los Trancos' deposit, paraquat at 257nm, their wavelengths of maximum Minas de Gador SA, Cabo de Gata, Almeria, Spain. absorption. The amount of pesticide adsorbed on the Samples of montmorillonite, the <2-mm fraction, 5C and 5Z samples, (X), was calculated from the (5g), were dispersed in 50ml of the corresponding difference between the initial (C0) and equilibrium tetravalent cation (Ce(IV) or Zr(IV)) solution at an herbicide solution concentrations (Ce). The pH of the appropriate concentration that exceeded by ®ve times adsorbent solutions was also measured before and the exchange capacity of the clay (90meq 100g1). after the adsorption experiments to determine if the The suspensions obtained were continuously stirred at pH was stable during the experiment. Blanks contain- room temperature for 9h and then allowed to stand for ing no pesticide were used for each series of experi- 12h. Next, they were re¯uxed under constant stirring ments. No degradation products of any herbicide were with a volume of 50ml of phosphoric acid at a found in the supernatant. concentration double that of the tetravalent cation The FTIR spectra were recorded on an ATI used to saturate the clay. The mixture was re¯uxed for Mattson spectrometer. An aqueous pesticide solution a further 7h and allowed to stand at room temperature (0.05 litre) with a concentration corresponding to the for 12h. The samples obtained were then washed with maximum used in the adsorption experiments was distilled water until sulphate (Ce sample) or chloride added under continuous stirring to 0.25g of each (Zr sample) was completely removed in the washing adsorbent in 100ml conical ¯asks.