Bergaya F. Handbook of Clay Science

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The organo-clay formulations exhibited a signiﬁcant reduction in leaching, which

implied maintaining high and relatively enhanced herbicidal activity in the topsoil,

and little activity at larger soil depths. Thes e tests included formulations of alachlor

(El-Nahhal et al., 1997, 1998, 2000), metolachlor (El-Nahhal et al., 1999a, 2000;

Nennemann et al., 2001), norﬂurazon (Undabeytia et al., 2000a), acetochlor

(El-Nahhal et al., 2001) and fenuron (Hermosin et al., 2001). The studies on met-

olachlor (Nennemann et al., 2001) and norﬂurazon (Undabeytia et al., 2000b) achieved

good slow release formulations by employing acid-activated pillared montmorillonite.

The organo-clay formulations of alachlor (El-Nahhal et al., 1998 and acetochlor

(El-Nahhal et al., 2001 ) were also shown to reduce leaching and enhance and pro-

long herbicidal activity under ﬁeld conditions. An illustration is given in Fig. 11.2.3.

11.2.2. REDUCTION OF VOLATILIZATION

Volatilization is a major facto r limiting the lifetime of certain pesticides in the

ﬁeld, e.g., carbamothiate herbicides (Wilkinson, 1988). Margulies et al. (1994) suc-

ceeded in signiﬁcantly reducing the volatility of the herbicide S-ethyl dipropylcarba-

mothiate (EPTC) by sorbing it on montmorillonite or sepiolite. At 30 1C, the half-life

1=2

) of the herbicide adsorbed on montmorillonite was extended to more than ﬁve

days, in comparison to 10 h for the herbicide in its free form. Oat bioassays showed

that the herbicidal activity of EPTC adsorbed on the clay mineral was extended by

more than 1 week.

A reduction in volatilization was also observed for organo-clay formulations of

alachlor and metolachlor (El-Nahhal et al., 1999b). We expect that, in general,

100

120

com. 5/5/0.05

Root growth inhibition (%)

soil not diluted

soil diluted 10 times

Fig. 11.2.3. Root growth inhibition of a test plant (sorghum) at a soil depth of 0–5 cm

sprayed with the SFM commercial formulation and formulation 5/5/0.05 (5 g clay/L;5 mM

ODTMA; 0.05 mM SFM).

Chapter 11.2: Clays and Pesticides682

slowing the release of pesticides in water by employing organo-clay formulations will

also reduce pesticide volatility, but more studies are needed to support this deduction.

11.2.3. CLAY-MICELLE FORMULATIONS

The starting point was a concentrated solution (several millimolar) of organic

cations which include a large hydrophobic part and consequently have a very

small critical micelle concentration (CMC), such as octadecyltrimethylammonium

(ODTMA), whose CMC equals 0.3 mM. The anionic organic molecules are bound

by the positively charged micelles whose interior also provides a hydrophobic en -

vironment for the hydrophobic moiety of the organic anion. The mixed micelles,

which carry a net positive charge adsorb on negatively charged clay mineral particles

such as montmorillonite.

The adsorption of hexadecyltrimethylammonium (HDTMA) and ODTMA

(Fig. 11.2.1b) on montmorillonite was studied above an d below their CMC (Mi-

shael et al., 2002a). At added concentrations above the CMC, the loading exceeded

the CEC of the clay mineral and indicated higher afﬁnity of the cation with the

longer alkyl chain. An adsorption model could adequately simulate ad sorption at

added concentrations below the CMC, and yielded fair predictions for the effect of

ionic strength. The model indicated that above the CMC, adsorbed micelles con-

tributed signiﬁcantly to the amount of ODTMA adsorbed. Evidence for adsorption

of ODTMA micelles on montmorillonite was provided by freeze-fracture electron

microscopy, XRD and dialysis measurements. Electron microscopy showed a high

concentration of micelles in the size range of about 7 to 17 nm. ODTMA added

inside a dialysis bag to a clay suspension outside the dialysis bag gave similar fea-

tures of the clay particles as observed for the clay alone. The freeze-fracture meas-

urements conﬁrmed the expected outcome that no micelles would be seen on the clay

added outside the dialysis bag, since only monomers can pass through the bag and

adsorb.

The basal spacing of the untreated montmorillonite was 1.48 nm. When adding

ODTMA at 6 mM and 1.6 g/L clay inside a dialysis bag (where micelle adsorption

was expected) a well-deﬁned series of peaks, due to the micelles and an additional

peak at 2.07 nm were observed. When the clay was added outside the dialysis bag

and only monomer adsorption was expected, only one strong peak was observed at

1.83 nm. Such spacing might be ascribed to a bilayer of ODTMA cations lying

parallel to the surface of the clay mineral.

It should be emphasized that the amount of ODTMA adsorbed was similar when

the clay was added inside or outside the dialysis bag. However, in the presence of

micelles (clay inside the dialysis bag) the basal spacing was 2.07 nm indicating a

difference in the montmorillonite ODTMA structure.

When adding 5 mM of ODTMA to a concentrated clay suspension (10 g/L), we

predicted that the monomers would adsorb and, due to the high clay concentration,

11.2.3. Clay-Micelle Formulations 683

micelles would decompose to monomers. Indeed, this sample exhibited only a basal

spacing of 1.48 nm without any additional peaks, indicating monomer adso rption.

In the presence of 5 mM ODTMA and 5 g/L clay, 95% of the anionic herbicide

sulfometuron (SFM) adsorbed (Mishael et al., 2002b), whereas the anionic herbicide

sulfosulfuron yielded 99% adsorption (Mishael et al., 2002c). A very small fraction

of these herbicides adsorbed when ODTMA was inside a dialysis bag and the clay

was outside, despite the similarity in the adsorbed amounts of ODTMA in both

cases. Thus, the complex form ed between ODTMA and montmorillonite in the

presence of an excess of micelles is very different from the complex formed in the

exclusive presence of ODTMA monomers, as shown by electron microscopy, XRD

and adsorption measurements. Unlike the monomer–clay complex, which was not

efﬁcient for the adsorption of anionic organic molecules, such as SFM or SFS, the

micelle–clay complex was highly efﬁcient.

At a 1% (w/w) water suspension, only 0.5% of the adsorbed SFM was released at

times varying from hours to nine days. An analytical test in Seville soil showed that

under excessive irrigation (400 mm), 100% of the commercial formulation leached,

whereas the micelle–clay formulations showed only 50–65% elution. A plant bio-

assay in Rehovot soil showed that the commercial dispersible granule formulation

(Oust, 75% ai sulfometuron methyl) yielded only 23% root elongation inhibition

at the top 5 cm of the soil, whereas complete inhibition was achieved with the

micelle–clay formulation. The detected concentration of SFM for the micelle–clay

formulation at a depth of 15–20 cm was half of that detected for the commercial one,

indicating a reduction in leaching when applying the micelle–clay formulation. A

10-fold reduction in the applied dose of SFM in the micelle–clay formulations re-

sulted in good herbicidal activity of 60–87% inhibition (Fig. 11.2.3).

The adsorbed amount of the anionic herbicide sulphentrazone on ODTMA-mont-

morillonite was 99% of added at 2.5 mM ODTMA and 0.2% clay; desorption of the

herbicide was 0.3% after 24 h. After 10 washings in funnels with thin layer of soil only

2.6% of the herbicide was released from ODTMA-montmorillonite formulations versus

100% release from the commercial formulation. The ODTMA-clay formulations of

sulfentrazone yield almost complete growth inhibition of green foxtail, at 700 ga.i./ha

versus 40% inhibition by commercial formulations applied at the same rate. (Polubesova

et al., 2003). Hence the slow release from micelle–clay formulations of sulfentrazone

promotes its biological activity and reduces environmental contamination.

These characteristics make the new formulations promising from the environ-

mental and economic points of view.

11.2.4. PHOTOSTABILIZATION OF PESTICIDES BY ORGANO-CLAYS

An organic cation adsorbed on the clay mineral can act as an energy acceptor of

the photo-excited molecule of the pesticide, which returns to its ground state before

its photodecomposition occurs, thus becoming photostabilized.

Chapter 11.2: Clays and Pesticides684

Margulies et al. (1987) observed that the insecticide bioresmethrin adsorbed on

montmorillonite at a load of 0.2 mmol/g of the dye methylgreen retained its activity

after three days of exposure to sunlight whereas the commercial insecticide lost its

activity after one day of irradiation. However, an increase in the loading of the dye

for the same concentration of the insecticide (0.2 mmol/g) did not achieve any pho-

tostabilization. Similarly, Margulies et al. (1988b) obtained a high degree of pho-

tostabilization of the insecticide NMH by using the dye thioﬂavin-T (TFT). The

highest photoprotection was obtained for samples containing 0.5 mmol TFT and

0.2 mmol NMH/g clay and again, an increase in the loading of the organic cation on

the clay mineral surface abolished the photostabilization of the pesticide molecules.

These patterns indicate the role of speciﬁc molecular interactions between the

organic cation and the pesticide molecules; evidence was obtained by infrared spec-

troscopy (Margulies et al., 1985 ). Because the efﬁciency of energy transfer processes

depends on the matching of energy levels of donor and acceptor chromophores, the

distance between them, and their relative orientations (Margulies et al., 1993), an

excessive loading of the clay mineral surface would impair the molecular interactions

between the pesticide and the organic cation, and less photostabilization would be

obtained.

In these studies, the role of the clay mineral was to keep the organic cation and the

pesticide molecules at an optimal distance and perhaps orientation as well for energy

transfer processes. However, in certain cases (e.g., the herbicide triﬂuralin (Margulies

et al., 1992)), the clay mineral itself could provide photostabilization. NMH ad-

sorbed on montmorillonite and nontronite decomposed more slowly than that ad-

sorbed on hectorite in line with their increasing iron content, which was attributed to

charge transfer from the excited pesticide molecules to Fe

ions in the clay miner al

structure (Rozen and Margulies, 1991). Banerjee and Dureja (1995) also found a

slightly lower rate of photodegradation for quinalphos adsorbed on palygorskite

than on kaolinite, in accord with the relatively low content of structural iron in

kaolinite.

Undabeytia et al. (2000b) obtained a high degree of photostabilization of the

herbicide norﬂurazon by preadsorbing TFT on montmorillonite. This was due to

energy transfer involving p–p* transitions between triplet states of the herbicide and

the organic cation, but an additional mechanism was also reported. Unlike the

insecticide NMH, the irradiation of norﬂurazon in the presence of montmorillonite

enhanced its photolysis, which was explain ed by the react ion of the herbicide mol-

ecules with hydroxyl and hydroperoxyl radicals formed on the clay mineral by ir-

radiation. Consequently, loading the clay mineral surface with the organic cation

may lower the levels of production of these radicals, and yield photostabili zation.

The photostabilization of the insecticide quinalphos was also due to modiﬁcation of

the catalytic properties of the clay mineral surface when covered with the organic

cation crystal violet (Banerjee and Dureja, 1995).

In other organo-clay complexes, the organic cation is not really involved in the

photo-protection of the herbicide. For example, El-Nahhal et al. (1999b) observed

11.2.4. Photostabilization of Pesticides by Organo-Clays 685

that the largest adsorption of the herbicides alachlor and metolachlor by montmo-

rillonite, preadsorbed with the organic cation BTMA, correlated with an optimal

reduction of photodecomposition. It appeared that the role of the organic cation was

to enhance the adsorption of the non-polar herbicides to the organo-clay complex,

whereas the photoprotection was provided by the clay mineral itself. Margulies et al.

(1992) suggested that the mechanism operating for the photostabilization of

triﬂuralin was steric hindrance by the clay mineral surface to the cyclization step

of the photochemical reaction, which leads to one of its photoproducts. This mech-

anism could also occur for the photostabilization obtained for triﬂuralin by using

sepiolite-TFT instead of montmorillonite-BTMA (Casal et al., 2001).

11.2.5. EXPLOITING CLAY–ORGANIC INTERACTIONS TO ENHANCE

THE EFFICACY OF BIPYRIDYL HERBICIDES

Paraquat (PQ) and diquat (DQ) are divalent organic bipyridyl herbicides, widely

used for rapid post-emergence weed control. Both herbicides are highly toxic, and

relatively expensive. Hence, it is advantageous to reduce their use rate and minimize

their costs and health hazard. A decrease in efﬁcacy of DQ and PQ is caused by dust

particles on leaves of weeds (Wicks and Fenster, 1996), which adsorb the herbicide,

reducing its amount available for biological activity, hence causing inactivation

(Calderbank, 1968). The adsorbed herbicide molecules interact with the negatively

charged sites on the surface of clay mineral particles, as was observed by several

analytical techniques (Rytwo et al., 1996a, 2002). The herbicides also bind to soils by

reaction with humic acids and organic matter (Wauchope et al., 1992).

Previous studies showed that certain monovalent organic cations can compete

with the bipyridyl divalent herbicides for adsorption to montmorillonite (Rytwo

et al., 1996a) or sepiolite (Rytwo et al., 2002). On this basis, Rytwo an d Tropp (2001)

proposed to add a monovalent organic cation to the herbicide formulation. The

main idea was to utilize the strong interactions between certain monovalent organic

cations and clay minerals in order to make more herbicide molecules available for

acting on the plant cells, rather than being irreversibly adsorbed by dust.

Based on this hypothesis, Rytwo (2000) and Rytwo and Tropp (2001) observed

that clay sprayed on plants reduced the herbicidal activity of bipyridyl herbicides.

This reduction which was ascribed to the adsorption of the herbicide on clay mineral

sites was larger in a high CEC clay mineral, such as montm orillonite, than in a low

CEC clay mineral, such as sepiolite. Addition of monovalent cations, such as

acriﬂavin or mepiquat (MQ) could overcome the inﬂuence of a clay dust. Such

addition gave good herbicidal activity (Fig. 11.2.4) with less than 1/3 of the normally

used amount of herbicide. MQ should be preferred, since it is a chemical allowed for

agricultural use.

Rytwo an d Tavasi (2003) showed a similar enhancing effect by using the mono-

valent herbicide difenzoquat (DZ). They observed a decrease in the efﬁcacy of the

Chapter 11.2: Clays and Pesticides686

herbicide when the plants were dusted with common soil either from mineral source

(57% clay, 4.5% organic matter) or organic peat (6% clay, 20% organic matter).

The proposed formulations exhibit both economic and environm ental advantage s.

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