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Journal of Soil Science, 1992,43,99%111
Carbendazim adsorption on montmorillonite, peat and soils
G. DIOS CANCELA, E. R O M E R O TABOADA& F. SANCHEZ-RASERO Estacibn Experimental del Zaidin, Profesor Albareda I , 18008 Granada, Spain
S U M M A R Y
The adsorption of carbendazim by peat and montmorillonite was studied as a function of the exchangeable cations and temperature. The adsorption on soils was also studied.
The kinetics of carbendazim adsorption on peat showed that adsorption equilibrium was reached within 1 h. The order of adsorption of carbendazim on peat was as follows: H+-peat > Cu2+-peat > Co2+-peat > Mg2+-peat > K+-peat, and the thermodynamic parameters appeared to suggest an adsorption mechanism involving hydrogen bonds, although in the H+, Cu2+ and Co2+ samples a protonation process and adsorption of the protonated species were also likely.
The kinetics of carbendazim adsorption on montmorillonite (mont.) showed that equilibrium was reached within 1 h. The order of adsorption was: H+-mont. > Cu2+- mont. >Co2+-mont. >Ca2+-mont., the adsorption on the H + and Cu2+ samples being much greater than that on the other samples. For the H + and Cu2+ samples, the thermo- dynamic parameters appeared to suggest a double mechanism: physical adsorption, and protonation and adsorption by ion exchange. The most probable mechanism for the adsorption of carbendazim on the Co2+ and Ca2+ samples was physical bonding.
The capacity for adsorption of this fungicide on soil was dependent on the organic matter, nitrogen and clay content, as well as on the cation exchange capacity. No significant correlation was found with pH, C/N ratio or free iron content.
I N T R O D U C T I O N
Carbendazim is the IS0 common name for methyl 1H-benzimidazol-2-ylcarbamate (C.A.). It is a systemic fungicide that controls a wide range of pathogens of cereals, vegetables, fruits, grapes and ornamental plants.
In some solvents, and in contact with water or under moist soil conditions (Chiba & Cherniak, 1978; Singh & Chiba, 1985), dissociation of benomyl can occur to form carbendazim. The Environmental Protection Agency of the USA has pointed to the possible mutagenicity, terato- genicity and potential for reduction in spermatogenic activity of benomyl and carbendazim under defined conditions. On the other hand, because of the slow rate of degradation and low solubility in water of carbendazim, this fungicide may remain in the soil for a long time in an immobilized state due to interactions with soil colloids. These facts emphasize the need for adsorption studies with carbendazim, since this process significantly affects its movement and persistence in the soil environment.
Previous studies (Aharonson & Kafkafi, 1975aJ) on carbendazim have shown that clay minerals and organic matter play a prominent role in the adsorption of this fungicide by soils. These authors suggest that carbendazim may be adsorbed on montmorillonite and soils by ion exchange. Release of this compound from adsorption surfaces will depend on the nature of the adsorption mechanism (physical or chemical adsorption). A direct estimate of the adsorption strength can be obtained from the effect of temperature on the equilibrium adsorption constant. Weber et al. (1965) and Talbert & Fletchal ( I 965) suggest that temperature affects the bonding of basic herbicides on soil colloids.
99
100 G. Dios Cancela et al.
Other workers (Bailey et al., 1968; Sullivan & Felbeck, 1968; Weber et al., 1969; Weber 1970a,b; Senesi & Testini, 1980, 1982, 1984; Muller-Wegener & Ziechmann, 1980; Senesi, 198 1; Kozak et al., 1983) studied the adsorption of weak basic organic compounds, such as s-triazines, on soil and its constituents, and concluded that organic matter and clay minerals are the components mainly responsible for the adsorption process in soils. The most probable mechanisms for retention were considered to be physical bonding or van der Waals forces, protonation and ion exchange.
In this paper, the adsorption of carbendazim on montmorillonite, peat and soils is described. In order to determine the thermodynamic parameters of the adsorption process and to establish the retention mechanism, the effect of temperature on the adsorption process was determined. The possible effect of the exchangeable cation on the adsorption process was measured and, finally, the adsorption of carbendazim on soils of different composition was studied and a correlation with the different soil components was established.
MATERIALS A N D M E T H O D S
Materials The < 2-pm fraction of a montmorillonite from Cab0 de Gata, Almeria, Spain, of mineralogical formula [Si,.,,, A10,82] [Al,,,, Fe,,,,, Mg,,,,,] 0,, (OH), X,,,,, was separated and saturated with H+ , Ca2+, Co2+ or Cu2+. The < 2-pm fraction of peat from Padul, Granada, Spain, was separated and saturated with H+ , K+, Mg2+, Co2+ or Cu2+. In both cases saturation was achieved by shaking a suitable weight of clay or peat (10 g) with a 1 N cation-acetate solution (100 cm3) for 1 h, followed by centrifugation of the suspension at 1900g for 15 min. The saturation process was repeated five times, and the excess acetate was then washed out using a water-methanol mixture of 40:60 (v/v). The samples were oven-dried at 60°C for 48 h and stored in vials above concentrated sulphuric acid.
The cation exchange capacities (CEC) for montmorillonite and peat are 800 mmol, kg-l and 1650 mmol, kgg', respectively, as determined according to method 5A6 of the Soil Conservation Service (1972). The specific surface area of the montmorillonite was determined, by the method of Dyal & Hendricks (1950), to be 734 m2 g-'. The specific surface area of the peat was 56 m2 g-I, as determined by nitrogen adsorption at 78°K according to the BET method.
Nine soils of the province of Granada (Spain) were studied. Their properties are shown in Table 1. The samples were from the A horizon, previously sieved to < 2 mm and saturated with calcium. Organic matter was determined by the Walkley & Black (1934) method. CEC in the calcium form was measured following method 5A1 of the Soil Conservation Service (1972). pH was determined on slurries with a soi1:water ratio of 1:2. For determination of the specific surface areas of soils, the BET method (nitrogen adsorption at 78°K) was used. Free iron was measured by method 6C2 of the Soil Conservation Service (1972), and the clay minerals present were determined by X-ray diffraction.
Organic compound Carbendazim, as an analytical standard of known purity, was a gift from BASF AG (Germany), HOECHST AG (Germany) and E. I. du Pont de Nemours and Co., Inc. (USA). Its solubility in water is about 8 pg ~ r n - ~ at pH 7.
Kinetic study Aliquots of 20 cm3 of a 6.0 pg cm-3 aqueous solution of carbendazim were added to 1 g of either peat or montmorillonite and incubated at 30°C for different time periods. At appropriate time intervals samples were removed, centrifuged at 17 200 g for 20 min and the carbendazim concentration in the supernatant analysed as described below.
Adsorption measurements The adsorption of carbendazim at 20°C and 30°C was measured. Carbendazim solutions of 0.6,1.2, 2.4, 3.6, 4.8 and 6.0 pg cm-3 were used. Aqueous suspensions of the samples were prepared by adding 20 cm3 of the different carbendazim solutions to the appropriate quantity of adsorbent (0.1-1 g) in 50-cm3 Erlenmeyer flasks. The Erlenmeyer flasks were mechanically shaken for 24 h in a
Adsorption of carbendazim
Table 1. Properties of soils
101
Organic Specific Free matter Nitrogen surface CEC iron
Soil (gkg-') (gkg-9 C/N pH (m'g-I) (mmol, kg-I) % clay (g kg-I)
P-8
P-9
P-10
P-11
P-12
M-106
M-I28
M- 130
M-272
15.0
6.0
24.0
13.2
18.0
104.1
4.0
12.0
68.9
1.7
0.7
1.4
1 .o
1.1
3.8
0.6
1 .o
3.3
11.0 8.10 10.0
7.0 7.10 20.0
9.0 8.40 28.0
6.0 8.20 20.0
8.0 8.40 39.0
15.9 7.45 30.1
3.7 7.45 15.0
12.4 7.87 13.0
12.1 1.74 36.8
193
81
338
128
186
662
155
265
554
9.8 KMCI"
13.5 KCI 20.7
KMCI 17.4 KCI 20.5
KMCI 24.5
KMCI 11.6 KCI 12.0
KMCI 32.6
KMCI
8.0
18.0
24.0
8.0
14.0
9.8
1.7
3.5
16.4
"K = kaolinite; M = montmorillonite; C = chlorite; I =illite.
thermostatic bath (kinetic experiments indicated that adsorption of carbendazim by the samples reached an apparent equilibrium after 1 h). The suspensions were then centrifuged at 17 200g for 20 min. The supernatant phases were removed and carbendazim analysed by reverse-phase HPLC (Sanchez-Rasero et al., 1990). The amount of carbendazim adsorbed was considered to be the difference between the amount of carbendazim initially present and that in the supernatant phase. Standard solutions were run simultaneously, under the same experimental conditions. The experiments were repeated twice.
No degradation of carbendazim was observed in the absence of adsorbent material, since a constant concentration of the fungicide was observed throughout the experiment, and no new peak appeared on the chromatograms.
Thermodynamic calculations The thermodynamic equilibrium constant, KO, and the AGO, AHo and ASo values associated with the adsorption of carbendazim by montmorillonite and peat were calculated according to the method of Biggar & Cheung (1973), which has also been used more recently by other authors (Moreale & van Bladel, 1979; McCloskey & Bayer, 1987).
RESULTS A N D DISCUSSION
Adsorption of carbendazim on peat The kinetics of carbendazim adsorption on Mg2+ and Cu2+-peat are shown in Fig. 1. A rapid adsorption was observed during the first few minutes, followed by a slower process which, in all cases, reached an equilibrium within 1 h. The experimental data followed a hyperbolic equation:
102 G . Dios Cuncelu et ul.
8ool# ~ ,;m+ ~ -,
-- 600 12 000
- i 400
I (51
w
* 200 4000
0 1 2 3 4 0 1 2 3 4
f ( h ) f ( h )
0 1 2 3 4
f ( h )
Fig. 1. Rateofadsorption ofcarbendazimonpeatandrnontrnorilloniteat 30°C: (a)Mg2+-peat, (b)Cu2+-peat,(c) Cu2+-montmorillonite. The symbols represent experimental data and the lines are calculated from Equation (1).
as proposed by Biggar et al. (1978) for the adsorption of picloram and parathion by soils. Xt is the adsorbed quantity at time t , X,,, is the maximum adsorbed quantity, t is the solid-solution contact time, and B is a constant. This purely empirical model enables us to calculate values of X,,, by fitting the experimental data to a linear form of Equation (1):
1 /Xt = (B/X,,,)t + 1 /Lax. (2)
The experimental data were fitted to Equation (l), with a correlation coefficient r =0.99, and the values of X,,, for Mg2+ and Cu2+-peat were 610 pg g-' and 1 1.4 mg g-', respectively.
The differential, dXJdt, was determined at different times between 3 and 60 min. The rate of adsorption has been considered to be proportional to the difference X,,, - X,, according to the power equation given by Moreale & van Blade1 (1979):
dXJdt = k(X,,, - X,)". (3)
The parameters k and n are the rate constant and the order of the reaction, respectively. The data were found to fit this power model in its logarithmic form ( r = 0.99). It is a second-order reaction and the k values were 1.3 x lo-' and 3.9 x (pgg-')l-"min-l for the Mg2+ and Cu2+ samples, respectively. These values suggest that there are different bonding mechanisms for each sample.
Figure 2 shows the adsorption isotherms of carbendazim on peat saturated with H+, K+, Mg2+, Co2+ and Cu2+, at 20°C and 30°C. The empirical Freundlich relationship was used to describe the results of the carbendazim adsorption on peat (r=0.99). The linear form of this equation is as follows:
logx = l ogK+ (l/n)logC, (4) where x is the amount (pg g-') of pesticide adsorbed, C, is the pesticide concentration (pg cm-') in the solution at equilibrium, and Kand l/n are constants. Kis the amount ofpesticide adsorbed when the equilibrium concentration is equal to unity, and ljn reflects the degree of linearity of the adsorption trend. The values of Kand l /n are given in Table 2.
The adsorption data for the H+-peat sample fit the equation x=a+KC"", similar to that described by Bergmann & O'Konski (1963), for the adsorption of the methylene blue cation on montmorillonite. This equation describes two different adsorption processes: (i) ion exchange, a; and (ii) physical adsorption, KC"". The isotherm for the H+ sample belongs to the H type of Giles et al. (1960).
The values of l / n (Table 2) for carbendazim adsorption on K+ , Mg2+, Co2+ and Cu2+-peat were less than one, indicating a convex, or L-type of isotherm (Giles et al., 1960). The slope of the isotherms steadily decreases with increasing solute concentration, because vacant sites become less accessible as the surface becomes progressively covered. The curvilinear isotherms suggest that the number of available sites for adsorption becomes a limiting factor.
Adsorption of carbendazim 103
- - '0 6000
J
0 0.1 0.2 0.3 0 10 20 30 40
Ce(pg ~ r n - ~ ) Ce(/.Lg ~ r n - ~ )
- j 5 O o E 4 Y
1" cn
4 3 6000
2000
0 I 2 3 4 5
Ce (pg ~ r n - ~ )
Fig. 2. Adsorption isotherms of carbendazim on peat at 20°C (0) and 30°C ( x ) : (a) H+-peat, (b) K+-peat, (c) Mg2+-peat, (d) Co2+-peat, (e) Cu2+-peat.
From the K-values shown in Table 2, it is deduced that the carbendazim adsorption capacity follows the order H+-peat > Cuz+-peat > Coz+-peat > Mg2+-peat = K+-peat, at 20°C and 30°C. However, no clear relationship between adsorption capacity and the crystal-chemical character- istics of the saturating cation, such as polarizing power, ionic potential or electronic configuration, was found. The significantly higher values found for H + and Cu2+-peat suggest that a different bonding mechanism is operative for these two samples.
104 G . Dios Cancela et al. Table 2. Freundlich constants (Kand I/n) and correlation coefficient ( r ) for the adsorption
of carbendazim on peat
H +
K’
Mg2+
20 33 156 30 31 290
20 113 30 83
20 30
110 69
20 299 30 247
20 4498 30 3890
co2+
cu*+
1 .oo 1 .oo 0.581 0.642
0.547 0.620
0.728 0.750
0.635 0.712
4013 0.997 3404 0.997
- 0.999 - 0.998
- 0.998 - 0.995
- 0.999 - 0.998
- 0.999 - 0.998
The pH value of the H+-peat and Cu2+-peat suspensions was 6.0, but for the other samples it was 7.5. At those pH levels the carbendazim molecules must be predominantly in an (uncharged) molecular state (pK, = 4), and so may be mainly adsorbed by physical forces. However, it is possible that exchangeable protons on the adsorbent may protonate the carbendazim molecules, which could then be adsorbed by ion exchange. This could explain the relatively high adsorption capacity of the H + - peat. Protonation ofcarbendazim and other basic pesticides by organic matter, montmorillonite and soil has already been described (Bailey et al., 1968; Weber et al., 1969; Mortland, 1970; Aharonson & Kafkafi, 1975a.b; Senesi & Testini, 1980, 1982, 1984; Kozak et a[., 1983).
A similar mechanism may occur to some degree in the Cuz+-peat and the Co2+ sample although the source of protons, in this case, may be those liberated by the hydrolic reaction: [M(0H2)J2+ +[M(OH,),- ,OH] + + H + (Mortland, 1970).
An increase in temperature from 20°C to 30°C (Table 2 and Fig. 2) results in a decrease in pesticide adsorption. One would also expect the standard enthalpy associated with the adsorption process to be exothermic. The variation in adsorption with temperature differs between the five systems. The effect of temperature was greater for the K+ and Mg*+ samples than for the other samples.
The values of AG“, AW and AS”, associated with the adsorption process of carbendazim by peat, are shown in Table 3. The values of AG“ for carbendazim adsorption on peat were negative in all cases (Table 3), as expected from spontaneous reactions. In general, the numerically higher values of AGO corresponded to the samples with the greater adsorption capacity. The corresponding values of AS” were positive in all cases, except for the Mg2+ sample. The net entropy changes of such reactions are the result of several interactions. Thus the removal of carbendazim molecules from solution (i.e. adsorption) would produce a positive entropy change due to carbendazim-water interactions or solubility effects. The desorption of water molecules from peat surfaces in response to carbendazim adsorption would also result in a positive entropy change (due to the greater number of degrees of freedom in bulk water relative to adsorbed water). The overall entropy change would then be positive (McCloskey & Bayer, 1987).
The interaction of basic organic chemicals with organic and inorganic soil colloids usually involves more than one mechanism, including chemical adsorption, physical adsorption, hydrogen bonding, protonation and ion exchange (Weber, 1966,1970~; Bailey et al., 1968; Russell et al., 1968; Hance, 1969; Weber et a[., 1969; Gwo-Chen Li & Felbeck, 1972; Aharonson & Kafkafi, 1975a,b; Senesi & Testini, 1980; Kozak et al., 1983). However, a single mechanism may dominate, depend- ing on factors such as the physical and chemical properties of the adsorbent, the experimental conditions, and the herbicide concentration.
Adsorption of curbenduzim 105
Table 3. Values of AGO, AH" and ASo associated with the adsorption of carbendazim on peat
- AG" In KO (kJ mol-I)
- AH" A T Sample 20°C 30°C 20°C 30°C (kJmol-') (Jmol-I k-l)
H' 14.78f0.02 14.68k0.02 36.20+0.04 37.16k0.04 7.5k2.9 +97.8f 10.0 K + 9.48f0.01 9.19+0.01 23.20k0.04 23.28+0.04 21.3k 1.2 +6.7&4.2
co2+ 10.20k0.01 10.02kO.01 25.00k0.04 25.37k0.04 13.4+ 1.2 +39.3 k4 .2 cu2+ 13.12k0.02 12.88f0.02 32.10+0.04 32.60k0.04 17.9k2.9 +48.1k 10.0
Mg2+ 9.66k0.01 9.20k0.01 23.66k0.04 23.28k0.04 34.3k 1.2 -36.4k4.2
The error in In(&) was calculated as the uncertainty in the extrapolation of In (.x/C,) against C,. The errors in A G and AH" were calculated from the uncertainty in In KO. The error in ASo was calculated from the errors in AGO and AH".
Chemical bonds are of relatively high energy ( > 60 kJ mol-I), compared to hydrogen bonds (1 7-21 kJ mol-I) and van der Waals bonds (&8 kJ mol-I). Adsorption by ion exchange.can involve a similar or greater energy than adsorption by physical forces.
According to the thermodynamic parameters shown in Table 3, the AW values for the adsorp- tion of carbendazim on the K + and MgZ+-peats were 21.3 kJ mol-I and 34.3 kJ mol-I, respectively, suggesting a physical bonding mechanism involving hydrogen bonds between the carbendazim molecule and the peat surface. These values are similar to those found by Gwo-Chen Li & Felbeck (1972) for the adsorption of atrazine on humic acids, which they also attributed to hydrogen bonds. Similar results were found by Sullivan & Felbeck (1968), for the adsorption of s-triazines by humic acids.
The finding that the pH for K + - and Mg2+-peat suspensions was 7.5, so that carbendazim molecules were (mainly) in the molecular state, is consistent with adsorption by physical forces.
The AH" value for the H+-peat was 7.5 kJ mol-', which is consistent with bonding by ion exchange after protonation (Weber et al., 1969; Senesi & Testini, 1980,1982,1984). The pH for H+- peat suspensions was 6.0, although the peat surface must be more acidic, which would justify a mechanism involving carbendazim protonation and adsorption by ion exchange.
The AW values for CuZ+ and Co2+ samples were 17.9 and 13.4 kJ mol-I, respectively, which accords with a double adsorption mechanism: physical adsorption and adsorption by ion exchange after protonation. This type of bonding mechanism was suggested for the adsorption of other basic pesticides on smectites (Weber et al., 1965; Bailey et al., 1968; Weber, 1970~).
Adsorption of carbendazim on montmorillonite The kinetics of carbendazim adsorption by Cu'+-montmorillonite are shown in Fig. 1. The exper- imental data, fitted to Equation (l), gave a value of 1.020 pg gg' for X,,,. The values of n and k , calculated by fitting the experimental data to Equation (3) (r=0.99), were 2.0 and 4.7 x (pg gg')-' min-', respectively.
Figure 3 shows the adsorption isotherms ofcarbendazim on montmorillonite saturated with H', CaZ+, Co2+ and Cuz+ at 20°C and 30°C. As for peat, the empirical Freundlich equation (r =0.99) describes the adsorption well; Table 4 shows the values of K and l jn. It can be seen that all the l /n values are < 1, indicating convex or L-type isotherms (Giles et al., 1960). Isotherms (Fig. 3) and values of K (Table 3 ) show that the adsorption capacity decreases in the order H+-mont. > Cu2+- mont. > Caz+-mont. > Co2+-mont., with very high values for H+ and Cu*+-montmorillonite.
The pH of the H+ and Cuz+-mont. suspensions was about 5.1, although the montmorillonite surface must be more acidic (Mortland & Raman, 1968). Under these conditions some of the carbendazim molecules must be protonated, and so could be adsorbed directly by ion exchange (Aharonson & Kafkafi, 1975a,b).
106 G. Dios Cancela et al.
Table 4. Freundlich constants (Kand I /n ) and correlation coefficient (r) for the adsorption of carbendazim on montmorillonite
Temperature K Sample (“C) (Pg g-7 1 in r
H+
Ca2+
co2+
cu2+
20 30
20 30
20 30
20 30
527 423
2.41 1.71
16.8 9.7
340 287
0.532 0.605
0.685 0.776
0.573 0.619
0.608 0.663
0.997 0.993
0.997 0.992
0.990 0.997
0.999 0.990
300
0 10 20 30 40 50 60 70 80
C, ( p g cme3 )
0 10 20 30 40
C, (pg cm-3
Fig. 3. Adsorption isotherms of carbendazim on montmorillonite at 20°C (0) and 30°C ( x): (a) H+-mont., (b) Ca2+-mont., (c) Co2+-mont., (d) Cu2+-mont.
Adsorption of curbenduzim 107
The pH of the Ca2+ and Co2+-montmorillonite suspensions was 7.0, so the number of protonated molecules would be much smaller and ion exchange mechanisms would be less important.
An increase in temperature from 20°C to 30°C (Table 4 and Fig. 3) results in a decrease in pesticide adsorption. It must be pointed out that the smaller effect of temperature on the adsorption capacity of the H + and Cu2+ samples compared to the Ca2' and Co2+ samples would be expected from the main adsorption mechanism assumed (ion exchange).
The AGO values for carbendazim adsorption on montmorillonite were, in all four cases, negative (Table 5) , as expected from spontaneous reactions. The AS' values were positive for the H+ and Cu2+ samples, and negative for the CaZt and Co2' samples, which again suggests that the adsorption mechanisms for the two groups of cations are different.
Table 5. Values of AGO, AW and A,T associated with the adsorption of carbendazim on montmorillonite
-AG" In KO (kJ mol-I)
- AW A S Sample 20°C 30°C 20°C 30°C (kJ mol-') (Jmol-' k-I)
H' 11.25k0.02 11.01 k0.02 27.54k0.04 27.88k0.04 17.9k2.9 +32.6* 10.0 Ca2+ 5.39k0.01 5.02k0.01 13.21 k0.04 12.71k0.04 27.6k 1.2 -48.9k4.2 co2+ 7.70k0.01 7.30k0.01 18.85k0.04 18.47f0.04 29.7k1.2 -36.8k4.2 cu2+ 10.52k0.02 10.28k0.02 25.75k0.04 26.04+0.04 18.4k2.9 +26.3& 10.0
The errors were calculated as in Table 3.
AH' values for the Ca2+ and Co2+ samples were 27.6 kJ mol-' and 29.7 kJ mol-', respectively, which indicate a physical bonding mechanism, possibly involving hydrogen bonds between the NH or CH, groups of carbendazim and oxygen on the surface of montmorillonite. A similar process was described by Weber et al. (1965) for the adsorption of other basic pesticides, such as prometon, on montmorillonite.
The AH" values for the Hf and Cu2+ samples were 17.9 kJmol-' and 18.4 kJ mol-', respect- ively, which is in agreement with a double adsorption mechanism, i.e. physical adsorption and adsorption by ion exchange after protonation (Weber, 1970a,b; Aharonson & Kafkafi, 1975a,b).
No significant adsorption of carbendazim on other clay minerals, such as illite and kaolinite, was observed under our experimental conditions.
Adsorption of carbendazim on soils Figure 4 shows the adsorption isotherm of carbendazim on the soils studied at 30°C. The empirical Freundlich equation (r=0.99) described the adsorption on the soils well. Table 6 shows the values of K and Ijn. It can be seen that all the ljn values were < 1, as for montmorillonite and peat.
Isotherms (Fig. 4) and K-values (Table 6) show that the adsorption capacity follows the sequence M-272 > M-106 > P-10> P-1 1 > P-12> M-130 > P-9> P-8. The M-272 and M-106 soils gave particularly high values.
Since the pH of all the carbendazim-soil suspensions was about 7.5, and Ca2+ was the dominant exchangeable cation, carbendazim molecules must have been adsorbed on soil colloids, mainly by physical forces, as for montmorillonite and peat. The high affinity of carbendazim for organic matter may account for the greater adsorption capacity of soils M-272 and M-106, which have particularly high organic matter contents. To check this hypothesis, simple linear correlations were made between the K-values (Table 6) and the various physico-chemical properties of the soils shown in Table 1. The correlation coefficients and their significance levels are shown in Table 7. Thus it may
108 G. Dios Cancela et al.
(D
In
* m
N 1 -
0 D u ) b N 0
0 0 0 0 0 t m N
Li 0 M c
Adsorption of curbenduzim
Table 6. Freundlich constants ( K and l / n ) and corre- lation coefficient (r) for the adsorption of carbendazim
on soils
109
P-8 P-9 P-10 P-11 P-12 M-106 M-128 M- 130 M-272
1.979 2.065 6.872 3.007 3.525
17.607
2.078 21.627
-
0.56 0.85 0.81 0.74 0.94 0.67
0.76 0.63
-
0.999 0.995 0.999 0.986 0.998 0.994
0.987 0.999
-
be concluded that carbendazim adsorption by soils is closely correlated with organic matter content ( r =0.92), significant at the 0.001 probability level. Thus organic matter content may explain 85 per cent (?) of the total variance shown in carbendazim adsorption.
Table 7. Correlation coefficients (r) between the Freundlich constant ( K ) and soil properties
Soil properties
Organic Specific Free matter Nitrogen pH surface CEC C/N Clay iron
*** ** * *** * *** Correlation coefficient, r 0.920 0.926 0.278 0.616 0.920 0.633 0.847 0.218
***P<O.OOl; **P<O.Ol; *P<0.05.
ThecorrelationofKwith theclay content ofthesoils(r= 0.85)isweakerthanthat with theorganic matter content. This finding may be related to the fact that the soils are low in montmorillonite and high in illite, which has a low affinity for carbendazim.
There is a strong correlation (I = 0.92) between K-values and cation exchange capacity (CEC), as would be expected due to the close correlation between CEC and the organic matter and clay content of soils.
The poor correlation found between K and soil pH may simply reflect the narrow range of this property in the soils studied. The correlation with the specific surface area of the soil is also poor, indicating that some regions of the soil surface are not useful for adsorption. Free iron does not appear to affect the adsorption process.
A multiple correlation of the soil organic matter and clay content with K-values (Table 6) gave a correlation coefficient, significant at the 0.001 probability level, of 0.97. Thus the content of organic matter plus clay in soils accounts for 94 per cent of the total variance, which is in agreement with the data of Aharonson & Kafkafi (19753). The regression equation between Kand percentage organic matter (0) and percentage clay (C) was:
K = - 6.6 + 1.220 + 0.53 C.
If the K-value in the Freundlich equation is replaced by the one found from Equation ( 5 ) , a simple relationship may be established between carbendazim adsorption and soil organic matter plus clay. This may be useful for calculation of the quantity of fungicide to be applied in order to achieve greater efficiency.
( 5 )
110 G. Dios Cuncelu et ul.
C O N C L U S I O N S
These adsorption studies suggest the following conclusions:
(i) carbendazim adsorption follows the Freundlich adsorption equation well, under our experimental conditions;
(ii) both on montmorillonite and on peat, the adsorption capacity is greater for acidic than for neutral o r basic surfaces;
(iii) the adsorption mechanism for carbendazim on acidic montmorillonite or peat is by protonation and ion exchange, whereas for neutral surfaces physical forces predominate;
(iv) the adsorption of carbendazim by soils is in direct proportion to their organic matter and clay content;
(v) most soils in southern Spain are low in montmorillonite and organic matter; consequently the adsorption of carbendazim is tow and the possibility of contamination of underground and surface waters is high.
A C K N O W L E D G E M E N T S
Financial support for preparation of this paper was provided by CAYCIT, under contract no. PR84-0160-C04-02. The valuable technical assistance of Ma D. Maroto is gratefully acknowledged.
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(Rece ived5 June 1990; accepted 12 Jury 1991)
