Abstract--Parathion and methyl parathion adsorbed at high loads on clays and similar surfaces exhibited properties of a bulk phase, such as a well-defined ...
Clays and Clay Minerals, Vol. 33, No. 1, 62-70, 1985.
SURFACE CONDENSATION OF ORGANOPHOSPHATE ESTERS ON SMECTITES U . MINGELGRIN AND F. TSVETKOV Institute of Soils and Water, Agricultural Research Organization The Volcani Center, Bet Dagan 50250, Israel Abstract--Parathion and methyl parathion adsorbed at high loads on clays and similar surfaces exhibited properties of a bulk phase, such as a well-defined solubility and a characteristic X-ray powder diffraction (XRD) pattern, different from those of the free substances. The aqueous solubility of parathion condensed at these surfaces was as low as 1.2 #g/ml as compared to 14 tzg/ml for free parathion. The BET isotherm described the behavior of the investigated surface condensates even though these condensates were different from the multilayer adsorption for which the BET isotherm was originally developed. The X R D patterns of parathion and methyl parathion adsorbed on Na-bentonite exhibited reflections which were shifted towards higher angles as compared with the X R D patterns of the free compounds. This is in agreement with the lower solubility of the surface condensates. The properties of these condensates were easily controlled by the choice of adsorbent as well as by the choice of conditions under whch they were prepared. The existence of such surface-condensed phases and the manipulation of their properties can be utilized for the slow-release formulation of organophosphate esters which is important, for example, in pesticides application. Key Words--Adsorption, Organophosphate ester, Parathion, Pesticide, Smectite, Solubility, X-ray powder diffraction.
d e s p i t e t h e differences b e t w e e n t h i s c o n d e n s a t i o n a n d t h e m u l t i l a y e r a d s o r p t i o n for w h i c h t h e B E T i s o t h e r m was d e v e l o p e d ( B r u n a u e r et al., 1938).
INTRODUCTION T h e widely u s e d p e s t i c i d e s p a r a t h i o n ( O , O - d i e t h y l O-4-nitrophenyl phosphorothionate) and methyl parathion (O,O-dimethyl-O-4-nitrophenyl phosphorot h i o n a t e ) h a v e t h e following structures:
o~N~
MATERIALS A N D METHODS Materials Na-, Ca-, Mg-, and Al-bentonites were prepared from Wyoming bentonite (Fisher B-235, Fisher Scientific Co., Fairlawn, New York) by the procedure of Shainberg and Otoh (1968). Deuterated parathion and methyl parathion synthesized in the Geoisotopes Department of Weizmann Institute of Science were used for the X-ray powder diffraction (XRD) analysis, and 14C-labeled parathion (Amersham Radiochemicals Centre) was used in the desorption studies.
S
0-P~0--R ll~O--R
w h e r e R = C2H5 for p a r a t h i o n a n d C H 3 for m e t h y l p a r a t h i o n . A n u m b e r o f studies (e.g., B o w m a n , 1973; B o w m a n a n d Sans, 1977; Y a r o n a n d S a l t z m a n , 1972; G e r s t l a n d Y a r o n , 1978; Y a r o n , 1978) o n t h e i n t e r a c t i o n o f t h e s e a n d o t h e r o r g a n o p h o s p h a t e esters w i t h v a r i o u s clays i n a q u e o u s a n d n o n p o l a r m e d i a h a v e s h o w n t h a t t h e a d s o r p t i o n o f o r g a n o p h o s p h a t e esters o n a clay surfaee is highly d e p e n d e n t o n t h e e x c h a n g e able c a t i o n a n d its h y d r a t i o n state. P r e v i o u s studies i n v e s t i g a t e d t h e low p a r t o f t h e a d s o r p t i o n i s o t h e r m s ; yet, i f clays are to be u s e d as carriers for pesticides o r o t h e r b i o a c t i v e s u b s t a n c e s , it is i m p o r t a n t , for e x a m ple, to s t u d y t h e i n t e r a c t i o n s o f o r g a n o p h o s p h a t e esters o n clays at h i g h surface c o n c e n t r a t i o n s . In the present work the interactions of parathion and m e t h y l p a r a t h i o n at h i g h loads were s t u d i e d b y exa m i n i n g : (1) d e s o r p t i o n i s o t h e r m s o f p a r a t h i o n i n t o w a t e r f r o m v a r i o u s h o m o i o n i c b e n t o n i t e s ; (2) X - r a y p o w d e r diffraction s p e c t r a for p a r a t h i o n i n liquid, a d sorbed, a n d frozen p h a s e s a n d for m e t h y l p a r a t h i o n in solid a n d a d s o r b e d phases. T h e B E T i s o t h e r m h a s b e e n f o u n d to d e s c r i b e t h e type o f c o n d e n s a t i o n e n c o u n t e r e d Copyright 9 1985, The Clay Minerals Society
Procedures Desorption isotherms. Clay samples were oven-dried at 105~ for 24 hr or equilibrated in desiccators at 15% and 32% relative humidity for three weeks. Parathion solutions in n-hexane having initial concentrations in the range 50-5000 ppm were then shaken with the homoionic bentonite at a clay to solvent ratio of 1:100. Clay portions of 0.5 g were used in each run. After the clay had been shaken for 48 hr, it was allowed to settle, and the hexane was pipeted out. The hexane traces left were determined gravimetrically, and the clay was then dried in an air-stream at room temperature. No parathion was lost during the drying as determined by the twoliquid phase extraction procedure of Saltzman et al. (1976) which gives a complete recovery of the adsorbed parathion. The dried parathion-treated bentonite was suspended in water at a parathion to water ratio of 8 ppm. This fixed water to adsorbate ratio prevented large differences in the concentrations of the solute released into the solvent upon desorption. If a fixed water to adsorbent ratio had been used, both saturation of the solution at high surface concentrations and bulk concentrations below the reliably detectable limits at low surface concentration might have occurred during desorption. The aqueous bentonite suspensions were shaken for 24 hr 62
Vol. 33, No. 1, 1985
Condensation of organophosphate esters on smectites
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