Paraquat in soils

Adverse effects of paraquat in sensitive species of terrestrial plants and soil microflora have been documented at application rates of 0.28 to 0.6 kg/ha (death, inhibited germination of seeds, reduced growth), at soil concentrations of 10 to 25 mg/kg (growth inhibition), and at soil-water concentrations as low as 1.6 mg/L (reduced growth, inhibited synthesis of protein and RNA). Among terrestrial invertebrates, certain species of mites were sensitive to paraquat at recommended rates of application, and the sensitive honey bee died when its diet contained 100 mg/kg. However, paraquat in soils was not accumulated by earthworms and other species of soil invertebrates after applications up to 112 kg/ha. These points, and others listed in this section, are discussed in greater detail later. 

The degradation rate of paraquat in certain soils can be slow, and the compound can persist for years — reportedly in a form that is biologically unavailable. But data are missing or incomplete on flux rates of paraquat from soil into food webs and on interaction dynamics of paraquat with other herbicides frequently applied at the same time. It seems prudent at this time to keep under close surveillance the residues of paraquat in soils in situations where repeated applications have been made over long periods of time (Summers 1980). 

Calderbank and Yuens [178] and Pope and Benner [179] have described a spectrophotometric method for the determination of paraquat in soil in amounts down to 0. mg kg 1. Paraquat, Trifluralin and Diphenamid have also been determined gas chromatographically in soil. 

To determine Paraquat in soil Payne et al. [180] separate the sediment from the sample (2L) by adding calcium chloride to aid flocculation, leaving the mixture overnight in a refrigerator for the sediment to settle, then decanting and filtering through a Whatman No. 42 paper under suction on a Buchner funnel. The wet sediment and soil core samples, are mixed for 4h... 

The limit of detection of the assay for soil extracts is 0.02pg ml-1, which corresponds to 0.2mg kg-1 of Paraquat in soil for a typical extraction procedure. This level of sensitivity is satisfactory for virtually all determinations of Paraquat in soil and lower limits of detection are rarely required. 

However, paraquat in soils was not accumulated by earthworms and other species of soil invertebrates after applications up to 112.0kg/ha. These points are discussed later. 

Some Physico-chemical Interactions of Paraquat with Soil Organic Materials and Model Compounds. II. Adsorption and Desorption Equilibria in Aqueous Solutions, I. G. Bums, M. H. B. Hayes, and M. Stacey, Weed Res., 13 (1973) 79-90. 

Paraquat is not degraded significantly in soil during incubation periods up to 16 months at 25°C by chemical or microbiological vectors (Smith and Mayfield 1978). For example, paraquat dichloride applied once annually at 4.48 kg/ha, or 4 times annually at 1.12 kg/ha, remained essentially undegraded in the soil for 6 years (Fryer et al. 1975 Moyer and Lindwall 1985). Massive applications... 

Fryer, J.D., R.J. Hance, and J.W. Ludwig. 1975. Long-term persistence of paraquat in a sandy loam soil. Weed Res. 15 189-194. 

Moyer, J.R. and C.W. Lindwall. 1985. Persistence and availability of paraquat in a Lethbridge clay loam soil. Can. Jour. Soil Sci. 65 523-529. 

Smith, E.A. and C.I. Mayfield. 1978. Paraquat determination, degradation, and mobility in soil. Water Air Soil Pollut. 9 439-452. 

This technique has been used to determine the following types of organic compounds in soil polychlorobiphenyls, chlorinated insecticides, triazine herbicides, paraquat and diquat. 

Electrophoretic and isotachoelectrophoretic techniques are gaining in popularity in soil analysis with applications to polyaromatic hydrocarbons, polychlorobiphenyls, tetrahydrothiophene and triazine herbicides, Paraquat and Diquat and growth regulators. Other lesser-used techniques include spectrophotometric methods (five determinants), spectrofluorimetric methods (two determinants), luminescence methods (one determinant), titration methods (one determinant), thin-layer chromatography (five applications), NHR spectroscopy (two applications) and enzymic immunoassays (one determinant). 

Khan [181] has described a method for determining Paraquat and Diquat in soils involving catalytic dehydrogenation of the herbicide followed by gas chromatography and also a pyrolytic method [182]. 

Niewola et al. [183, 185] have described a rapid, convenient and accurate method, based upon an enzyme-based immunosorbent assay (ELISA) for the determination of Paraquat residues in soil. Polystyrene plates, coated with paraquat-keyhole limpet haemocyanin (KLH) conjugate, are incubated with the test samples and a known amount of monoclonal antibody. Residual antibody that has not reacted with free Paraquat in the sample combines with paraquat-KLH on the plate. The determination of the fixed antibody is achieved by the addition of peroxidase labelled rabbit antimouse immunoglobulin G followed by reaction with a chromogenic substrate. The enzyme activity of the solid phase is determined from the absorbance measurements, which are inversely proportional to the concentration of Paraquat. The method shows high specificity and correlates well with the traditional ion exchange-spectrophotometric method for the determination of Paraquat [178]. 

The suitability of the ELISA for soil analysis was initially tested by assaying a number of control soil samples, fortified after extraction and neutralisation with Paraquat in the range 10-300mg kgy1. The results in Table 9.19 were close to the expected values and thus confirmed that natural soil components did not interfere with the determination. These results justified the further refinement of the method for soil analysis. 

Stransky [107] used isotachophoresis to determine Paraquat and Diquat in soils in amounts down to 10pg kgy1. 

Burclul SM, Hayes MHB, Greenland DJ (1981) Adsorption. In Greenland DJ, Hayes MHB (eds) Chemistry of soil processes. WUey, New York, pp 224 00 Bums IG, Hayes MHB, Stacey M (1973) Some physico-chemical interactions of paraquat with soil organic materials and model compounds. 11. Adsorption and desorption equilibria in aqueous suspensions. Weed Res 13 79-90... 

Kennedy VC, Brown TC (1965) Experiments with a sodium ion electrode as a mean to studying cation exchange rate. Clays Clay Minerals 13 351-352 Khachikian C, Harmon TC (2000) Nonaqueous phase liquid dissolution in porous media Current state of knowledge and research needs. Trans Porous Media 38 3-28 Kookana RS, Aylmore LAG (1993) Retention and release of diquat and paraquat herbicides in soils. Austral J Soil Res 31 97-109... 

Paraquat was determined in soil and water samples by the extraction and gas chromatographic procedure of King ( 9). 

Reports of other pesticide residues are less frequent. Propanil was present in 42% of the soil samples analyzed as well as ethyl parathion in 29%, methyl parathion in 17% and chlorpyrifos in 45% of the samples [14]. The concentrations of all three individual compounds were low (0.5-14 pg-kg ). Residues of 2,4-D were found only in a few samples but MCPB was present in the 60% of the sanq les, the levels of these two compounds varied between < 0.01 to 0.12 mg kg". In El Salvador, where paraquat is extensively used, different regions of the country were monitored. Residues in Sonsonate soil samples averaged 7.14 mg-kg, while in Ahuachapan, one sample contained as much as 16.47 mg-kg" paraquat [15]. Tatui soils from Sao Paulo State, Brazil, contained triflu-raline residues of 1.2 mg-kg" six weeks after application also methyl parathion residues up to 2 mg-kg" [16]. Reports on residues of newer pesticides in soil are rare. 

Sorption to mineral surfaces (as opposed to NOM) is generally viewed as more of a displacement than a dissolution phenomenon. Because mineral surfaces tend to be more polar than NOM, sorption to the former is more substantial for polar and ionic compounds than for those that are more hydrophobic (Curtis et al., 1986 Chiou, 1998). Furthermore, since most NOM and mineral surfaces exhibit either a neutral or negative charge, sorption to soils and sediments is considerably stronger for pesticide compounds that are positively charged in solution—such as paraquat or diquat—than for neutral species, and weaker still for anions. As a consequence, measured values in soils exhibit little dependence upon pH for pesticide compounds that are not Brpnsted acids or bases (Macalady and Wolfe, 1985 Haderlein and Schwarzenbach, 1993). 

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