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Effect of soil composition and dissolved organic matter on pesticide sorption
The Science of the Total Environment 298 (2002) 147–161
Effect of soil composition and dissolved organic matter on pesticide sorption K.M. Spark1,*, R.S. Swift Department of Soil Science, University of Reading, Whiteknights, Reading, Berkshire, RG6 2AA, UK Received 13 August 2000; accepted 1 May 2002
Abstract The effect of the solid and dissolved organic matter fractions, mineral composition and ionic strength of the soil solution on the sorption behaviour of pesticides were studied. A number of soils, chosen so as to have different clay mineral and organic carbon content, were used to study the sorption of the pesticides atrazine (6-chloro-N 2-ethyl-N 4 isopropyl-1,3,5-triazine-2,4-diamine), 2,4-D ((2,4-dichlorophenoxy)acetic acid), isoproturon (3-(4-isopropylphenyl)1,1-dimethylurea) and paraquat (1,19-dimethyl-4,49-bipyridinium) in the presence of low and high levels of dissolved organic carbon and different background electrolytes. The sorption behaviour of atrazine, isoproturon and paraquat was dominated by the solid state soil components and the presence of dissolved organic matter had little effect. The sorption of 2,4-D was slightly affected by the soluble organic matter in the soil. However, this effect may be due to competition for adsorption sites between the pesticide and the soluble organic matter rather than due to a positive interaction between the pesticide and the soluble fraction of soil organic matter. It is concluded that the major factor governing the sorption of these pesticides is the solid state organic fraction with the clay mineral content also making a significant contribution. The dissolved organic carbon fraction of the total organic carbon in the soil and the ionic strength of the soil solution appear to have little or no effect on the sorptionytransport characteristics of these pesticides over the range of concentrations studied. 䊚 2002 Elsevier Science B.V. All rights reserved. Keywords: Pesticides; Sorption behaviour; Organic matter state; Mineral content
1. Introduction The retention and mobility of a pesticide in soil is determined by the extent and strength of sorption reactions, which are governed by the chemical and *Corresponding author. Tel.: q61-7-5460-1336. E-mail address: [email protected] (K.M. Spark). 1 Present address: University of Queensland, Gatton Campus, Gatton Qld 4343, Australia
physical properties of the soils and pesticides involved. The sorption interactions of pesticides in the soil environment may involve either the mineral or organic components, or both. For soils that have higher organic matter levels ()5%), the mobility of the pesticides has been related to the total organic matter content, with the nature of the organic matter having little apparent influence on sorption processes (Bailey and White, 1964;
0048-9697/02/$ - see front matter 䊚 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 4 8 - 9 6 9 7 Ž 0 2 . 0 0 2 1 3 - 9
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Hayes, 1970; Riise et al., 1994; Arienzo and Buondonno, 1993; Jenks et al., 1998; Bekbolet et al., 1999). Total organic matter includes both the soluble and insoluble fraction of organic matter, although the proportion of soluble organic matter in a soil is usually very small. For soils, which have low organic matter contents, the mobility of the pesticide is often related to the active components of the inorganic fraction, which is predominantly the clay-sized fraction. An increase in the clay content results in decreasing mobility of the pesticide, with the composition of the clay and the identity of the major cations in the soil solution also being important (Walker and Crawford, 1968; Borggaard and Streibig, 1988; Murphy et al., 1992; Barriuso et al., 1992; Welhouse and Bleam, 1992; Baskaran et al., 1996a). There is considerable evidence that pesticides can interact with the soluble form of soil organic matter in the absence of the solid components of the soil. The extent and nature of this interaction depends on factors such as molecular weight and polarity of the pesticide (Burns et al., 1973; Gamble et al., 1986; Chiou et al., 1986; Madhun et al., 1986; Kan and Tomson, 1990; Maquedo et al., 1993; Spark and Swift, 1994; Devitt and Wiesner, 1998). Pesticides such as atrazine have been shown to be capable of solution-phase complexing with dissolved organic matter (Devitt and Wiesner, 1998; Gamble et al., 1986); paraquat associates with dissolved organic matter (Burns et al., 1973; Maquedo et al., 1993; Spark and Swift, 1994) and there is evidence for the association of hydrophobic pesticides with dissolved organic matter (Kan and Tomson, 1990; Kulovaara, 1993; Fitch and Du, 1996). There is some evidence that, in the presence of soils, an increase in the concentration of dissolved organic matter results in an increase in the mobility of atrazine (Gao et al., 1997) and 2,4-D (Baskaran et al., 1996b), and of hydrophobic organic chemicals such as DDT (Kan and Tomson, 1990; Fitch and Du, 1996). As many of these pesticides are found in groundwaters it is apparent that there is significant transport of these chemicals through the soil profile (Johnson et al., 1995; Masse et al., 1994; Miller et al., 1995; Ro et al., 1997). This transport may be the result of processes such as:
the formation of soluble complexes with soil solution components such as dissolved organic matter; or the incomplete interaction of these pesticides with the solid state organic or inorganic matter in the soil. Pesticides which interact with organic matter will react with both the soluble and solid phase fractions, therefore, competitive effects, the reversibility of these two types of interaction, and mass action effects will govern the distribution of the pesticides between the solid and soluble phases of the organic matter. For example, the sorption of atrazine on clays is generally reversible whereas the sorption from organic matter is less so (Harris and Warren, 1964; Moreaukervevan and Mouvet, 1998). As discussed above, it is has been shown that there is a strong relationship between total organic carbon in the soil and the mobility of many pesticides. However, the extent to which the soluble organic carbon is involved in the transport process is not known. If the interaction with the soluble organic matter relative to the insoluble organic matter is significant, the transport of the pesticides through the soil may be enhanced by an increase in soluble organic matter moving through the soil profile. The concentration of soluble organic matter would therefore influence the concentration of the pesticides in the drainage waters, surface waters and ground waters associated with a particular soil. The aim of this work was to determine the relative importance of soil components on the adsorption of a variety of pesticides. The research was carried out using a range of soils differing in organic matter content as well as clay mineral content, and varying the concentration of the soluble organic matter in the soil solution. 2. Materials Reagents were analytical grade unless otherwise stated. All solutions were made up in distilled deionised water and kept in airtight containers. 2.1. Soil characterisation The UK soils used in this work were from the Sonning and Thames series (Jarvis, 1973) from
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Table 1 Summary of characteristics of soils Sonning I
Sonning II
Sonning III
Broad
Denchworth
Small Moderate Moderate
Moderate Moderate Moderate
Moderate Small Small
Large Small Small
2.3 28.3 29.5 15.3 9.4
2.3 28.1 25.7 12.2 12.9
7.0 0.26 51 1.8 0.00090 0.158 3.1
7.0 0.27 60 2.8 0.0014 0.162 2.7
7.3 0.80 117 10.0 0.0050 0.211 1.8
3.30 0.18
6.3 0.19
a
Clay minerals Smectite Small Micas Moderate Kaolinite Moderate Particle size (% content of total soil) )600 mm 1.1 212–600 mm 30.7 63–212 mm 29.8 2–63 mm 12.4 -2 mm 10.8 Soil solution componentsb pH 7.6 Cond. mSycm 0.19 DOC(ppm) 23 Soluble Ca(II) (mmolyg) 2.6 (Molyl) 0.0013 Absorbance @ 400 nm 0.051 Abs400nmyDOC (=1000) 2.2 Organic carbon characteristics % OC (total soil) 1.25 DOCyOC (%) 0.18 a b
2.05 0.25
5.4 6.4 15.9 19.6 32.7
0.8 2.7 7.4 16.3 44.2 7.4 0.37 55 4.4 0.0022 0.083 1.5 3.25 0.17
X-Ray diffraction analysis of the clay mineral fraction. Note: All samples contain quartz. Soluble components in the supernatant of the soil suspension prepared as described for the sorption studies.
the University of Reading farm, Sonning, Berkshire and the Denchworth series (Avery, 1990) from Brimstone, Oxfordshire. A summary of the characteristics of the soil samples taken from these sites is shown in Table 1. Three samples with different organic carbon (OC) contents were taken of the Sonning soil, a free-draining, brown, coarse, sandy-loam soil. Sonning I soil (OCs1.25%) had been under continuous cultivation and cropping, Sonning II (OCs2.05%) was soil under natural grassland vegetation, and Sonning III soil (OCs 3.30%) was taken under woodland vegetation. The Denchworth soil (OCs3.25%) had a similar organic matter content to the Sonning III soil, but was a very fine clay from the Ap horizon. The Broad soil, from the Thames series, had the highest OC content in the soils studied here (OCs6.3%) and was a dark brownish-grey, alluvial, loamy-clay soil from a flat, flood-prone plain under grassland vegetation. The soils were dried, sieved to -1 mm and stored in sealed containers at room temperature. The soils were characterised for particle size using
sedimentation characteristics of the soil (Avery and Bascomb, 1982) and total carbon using a Leco SC-444 Analyser. The results of the particle size analysis and mineralogical analysis of the soils using X-Ray Diffraction are given in Table 1. The composition of the supernatant of the soils when suspended using the same conditions as for the pesticide adsorption experiments (described below) but in the absence of pesticides, was analysed to determine the pH, the background concentration of Ca2q (Perkin Elmer 3030 Atomic Absorption Spectrophotometer), the absorbance at 400 nm (Perkin Elmer Lambda 2 UV-Visible Spectrophotometer), the conductivity and the concentration of dissolved organic carbon (Shimadzu TOC 5000 Total Organic Carbon Analyser) (Table 1). 2.2. Leaching solution containing dissolved fulvic acid The soluble humic substance used in this work was a fulvic acid (FA) isolated from a water
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draining from a sapric histosol (Methwold Fens, Norfolk). The pH of the drain water was adjusted to two before passing it through XAD-8 resin to separate the FA from the other components of humic substances. Additional treatment associated with purification of the FA in the eluent is described in Watt et al. (1996). The FA solutions were made up daily at pH 7 and stored in airtight volumetric flasks. The adsorption of FA by the soils in the absence of pesticides was determined using the same method as for the pesticide adsorption studies (described below), with three types of background electrolytes; 0.005 M CaCl2, 0.01 M CaCl2 and 0.05 M NaCl, none of which caused precipitation of the FA from solution. The concentration of organic matter in solution was determined as described in the previous paragraph. 2.3. Pesticides The pesticides used in the studies were: atrazine (6-chloro-N 2-ethyl-N 4 -isopropyl-1,3,5-triazine-2, 4-diamine, technical grade (99.0%)) kindly supplied by Ciba-Geigy, UK; 2,4-D w(2,4-dichlorophenoxy)acetic acidx supplied by Sigma Chemical Company, St. Louis, USA; isoproturon w3-(4-isopropylphenyl)-1,1-dimethylurea, technical grade (99.0%)x kindly supplied by Rhone-Poulenc Agriculture Ltd., Essex, England; paraquat w1,19dimethyl-4,49-bipyridinium, technical grade (98%)x supplied by Aldrich Chemical Co. Ltd., Dorset, England. The radiolabeled chemicals of the above pesticides were atrazine (atrazine-ring-UL-14 C), kindly supplied by Ciba-Geigy, 2,4-D (2,4-dichlorophenoxyacetic acid-carboxyl-14C, )98% purity) supplied by Sigma Chemical Company, isoproturon (isoproturon-ring-UL-14 C), kindly supplied by Ciba-Geigy and paraquat (paraquat-methyl-14C dichloride,)98% purity), supplied by Sigma Chemical Company. 3. Methods 3.1. Adsorption studies Stock solutions of 2,4-D and paraquat contained 0.010 gyl of the pesticide dissolved in water passed
through a Milli-Q Reagent Grade Water System (Millipore Corporation, Bedford, Massachusetts). Stock solutions of atrazine and isoproturon were prepared by dissolving 0.010 g of the pesticide in 10 ml of AR methanol and diluting the solution to 1 l using Milli-Q water. The concentration of pesticides used in this study ranged from 250 ppb to 10 ppm, the background solution was an electrolyte (0.005 M Ca2q-, 0.01 M Ca2q, or 0.05 M Naq) and soluble organic matter (0 or 500 ppm FA). Adsorption experiments were carried out in triplicate using soil:solution ratio of 1 g of soil to 2 ml of solution unless otherwise stated. The adsorption solution was shaken on an end-overend shaker for 16 h. Trials carried out using Sonning I and Broad soil indicated that, for both soils, adsorption after 16 h was ;96% of that amount adsorbed after 24 h of shaking for atrazine and isoproturon. The corresponding values for 2,4D on Sonning I and Broad soils were ;88% and ;94%. Essentially 100% of the paraquat was adsorbed even at the shorter shaking time of 16 h. At the end of the 16 h allowed for adsorption, the samples were removed from the shaker and the soil suspension was centrifuged at 20 000=g for 20 min. Duplicate aliquots of the supernatant were removed for analysis. The amount of pesticide in the supernatant solution was determined using a liquid scintillation counter which had been calibrated for solutions containing 0–1000 ppm FA in solution to account for quenching effects of the FA. 3.2. Desorption studies For desorption experiments, the initial adsorption was as described above. After the removal of the two samples for analysis, further supernatant was removed until only 25% of the original volume of solution in the vessel remained. The solution in the vessel was then made up to the original volume with the same background electrolyte and either 0 or 500 ppm of added FA. The suspensions were equilibrated on a shaker for 16 h, centrifuged and the supernatant was again analysed. This process was repeated three more times in the desorption studies.
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4. Results 4.1. Characterisation of the soils The results of the characterisation of the soils are shown in Table 1. The three Sonning soils have a similar particle size distribution with 55– 60% of the particles being in the size range associated with coarse sand (63–600 mm) and approximately 10% of the particles being in the size range associated with fine clays (-2 mm). The clay composition (Table 1) in these samples is similar. However, the Sonning soil samples differ in the amount of organic matter present with 1.25, 2.05 and 3.30% total organic carbon for Sonning I, II and III, respectively. Broad and Denchworth soils are much heavier textured soils with a particle size distribution consisting of a coarse sand content of approximately 35 and 25%, respectively, and a fine clay content of 35 and 45%, respectively. The nature of the clay in these samples is similar (Table 1). The main difference between these two smectitic clay soils is that the Broad soil has approximately twice the total organic carbon to that of Denchworth (6.3 and 3.25%, respectively). The pH of the soil solutions were similar (Table 1), ranging from 7.0 to 7.6. The conductivity of these solutions ranged from 0.19–0.8 mSycm, the wDOCx ranged from 23–55 ppm, the soluble Ca2q extracted using water ranged from 0.18– 10.0 mmolyg soil and the absorbance at 400 nm ranged from 0.046–0.219 (Table 1). The values of DOCyOC (%) shown in Table 1 are relatively similar for all soils except Sonning II indicating that the organic matter in this soil is relatively more soluble, possibly due to lower wCa2qx (lowest of all soils). The values of abs400 nm yDOC w(absyppm)=1000x show a greater range. In general it has been observed that for the same DOC concentration the relative absorbance at 400 nm of humic acids is greater than fulvic acids (Senesi et al., 1989). The higher values of abs400 nm yDOC of the Sonning II and III soils suggests that the organic matter in these two soils has a higher ratio of humic:fulvic acid character than that of the other soils. And correspond-
151
ingly, it suggests that the Denchworth soil has the lowest ratio of humic:fulvic acid character. The adsorption of the FA by the soils was found to be directly proportional to the amount of FA added to the soil solution (Fig. 1). The slope of the straight line representing this linear proportionality Kfa for each of the soils is shown in Table 2, together with the initial absorbance of the soil solutions. The total amount of FA adsorbed was least in 0.005 M Ca2q (lowest Kfa value) and greatest in 0.01 M Ca2q (highest Kfa value) for all soils. (It is important to note that the fulvic acid fraction used in this work remained in solution in the presence of 0.01 M Ca2q background electrolyte solution in the absence of soils.) The amount of FA adsorbed for a particular type of background electrolyte, was also dependent on the nature of the soil, increasing in the order Sonning II-Sonning I s Sonning III-Broad-Denchworth. This order correlates with the order of relative abundance of clay fraction (-2 mm) in the soils (Table 1) with a statistical R 2 value for fit of 0.98, 0.94 and 0.98 for 0.005 M Ca2q, 0.05 M Naq and 0.01 M Ca2q, respectively. As Broad soil had the highest wCa2qx in soil solution the adsorbance of FA would appear to be more dependent on the clay content than the soluble calcium content. 4.2. Adsorption of pesticides The characteristics of the adsorption at low ionic strength (0.005 M Ca2q) of the pesticides, atrazine, 2,4-D and isoproturon for the soils are shown in Fig. 2a,b,c, respectively. Over the range of concentrations used the adsorption is close to a linear relationship between the amount of pesticide (x) adsorbed per g of soil (m) as a function of the concentration of the pesticide in solution at equilibrium (C). xymsKdp C Where Kdp is the distribution constant of the pesticide (over the equilibrium solution concentration of 0–2500 ppb) between the soil and the solution. The Kdp values of the plots for all five
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Fig. 1. The amount of FA sorbed from solution as a function of the initial concentration in the presence of the soils; Sonning I (m), Sonning II (h), Sonning III (j), Broad (s) and Denchworth (⽧).
soils are shown in Table 3. The order of adsorption on all soils is 2,4-D-atrazine-isoproturon-paraquat. The order of adsorption of the pesticides atrazine and 2,4-D on each of the soils is Sonning I-Sonning II-Sonning III f DenchworthBroad, which is the order of abundance of OC in the soils. This order is slightly different for isoproturon on the soils as the position of Broad and Denchworth are reversed i.e. Sonning I-Sonning II-Sonning III-Broad-Denchworth. Paraquat was completely adsorbed using these experimental conditions and concentrations (as shown in Fig.
2a) and so other graphical results and the Kdp values for this pesticide are not included here. The corresponding ratio of the Kdp value to the OC of the soil, Kdp yOC, is also given in Table 3. The range in these values for all soils is 0.56"0.16, 0.12"0.03 and 0.74"0.20 for atrazine, 2,4-D and isoproturon, respectively, (representing 25–30% variation in values). The three Sonning soils have similar mineralogy but different levels of organic matter (Table 1). Taking only the Kdp yOC values for the Sonning soils, and hence largely eliminating mineralogy as a factor in the
Table 2 Effect of the background electrolyte type and concentration on the absorbance (400 nm) of the soil solution (1:5 suspension; no added FA) and on the Kfa value (degree of sorbance of soluble FA from solution) for each of the soils Electrolyte
Ionic strength
Sonning I
Sonning II
Sonning III
Broad
Denchworth
Absorbance of soil solution
0.005 M Ca2q 0.01 M Ca2q 0.05 M Naq
0.015 0.03 0.05
0.012 0.016 0.043
0.135 nd nd
0.119 0.074 0.155
0.188 0.166 0.226
0.075 0.050 0.064
Kfa value
0.005 M Ca2q 0.01 M Ca2q 0.05 M Naq
0.015 0.03 0.05
0.26 0.57 0.39
0.19 nd nd
0.27 0.59 0.48
0.52 0.69 0.59
0.60 0.80 0.75
Note: nd means not determined.
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Fig. 2. (a) The amount of atrazine sorbed by the soil (gyg) as a function of the concentration of the pesticide in solution (background electrolyte 0.005 M Ca2q) for the soils; Sonning I (m), Sonning II (h), Sonning III (j), Broad (s) and Denchworth (⽧). The amount of paraquat sorbed by the soil (mgyg) as a function of the concentration of the pesticide in solution at low ionic strength (0.005 M Ca2q) for Sonning I soil (d); (b) The amount of 2,4-D sorbed by the soil (mgyg) as a function of the concentration of the pesticide in solution (background electrolyte 0.005 M Ca2q) for the soils; Sonning I (m), Sonning II (h), Sonning III (j), Broad (s) and Denchworth (⽧); (c) The amount of isoproturon sorbed by the soil (mgyg) as a function of the concentration of the pesticide in solution (background electrolyte 0.005 M Ca2q) for the soils; Sonning I (m), Sonning II (h), Sonning III (j), Broad (s) and Denchworth (⽧).
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Fig. 2 (Continued).
adsorption characteristics, results in a smaller coefficient of variation for isoproturon (now 0.75"0.06) but not for atrazine and 2,4-D.
solution also had no effect on the adsorption characteristics of these two pesticides in any of the three electrolyte solutions used in this work. However, these solution conditions did affect the adsorption of 2,4-D as shown in Fig. 3a, where it can be seen that increasing the ionic strength caused the adsorption of 2,4-D to increase on all four soils. The effect is greater for higher ionic strengths solutions containing Ca2q (I.S. 0.03) than Naq (I.S. 0.05) and is reduced in the presence of increased concentration of soluble FA. Paraquat was completely adsorbed under these experimental conditions and concentrations and therefore the effects of ionic strength and added fulvic acid on the adsorption were not detectable.
4.3. Effect of ionic strength and concentration of FA The ionic strength (I.S.) of the 0.005 M Ca2q solutions referred to in the above discussion is 0.015. Using background electrolyte solutions of higher ionic strength, 0.01 M Ca2q (I.S. 0.03) or 0.05 M Naq (I.S. 0.05) had no significant effect on the adsorption of atrazine and isoproturon on the four soils studied (Sonning I, Sonning III, Broad and Denchworth). Repeating the adsorption procedure using 500 ppm FA in the adsorption
Table 3 Kdp and KdpyOC values for the soilypesticide systems, where Kdp is the distribution constant of the pesticide between the soil and the solution and OC is the organic carbon content of the whole soil Pesticide Atrazine 2,4 D Isoproturon
Kdp KdpyOC Kdp KdpyOC Kdp KdpyOC
Sonning I
Sonning II
Sonning III
Broad
Denchworth
0.46 0.37 0.13 0.10 1.01 0.81
1.32 0.64 0.25 0.12 1.53 0.75
2.23 0.68 0.51 0.15 2.32 0.70
2.49 0.40 0.66 0.10 2.72 0.43
2.23 0.69 0.50 0.15 3.21 0.99
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Fig. 3. The effect of the absence or presence of FA (500 ppm) and the nature and concentration of the electrolyte (0.005 M Ca2q, 0.01 M Ca2q, 0.05 M Naq) on the sorption of 2,4-D by soils (Sonning I, Sonning III, Broad and Denchworth) at high (1:2) and low (1:16) soil to solution ratio (Fig. 3a,b, respectively).
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Fig. 4. Desorption of 2,4-D (n,m), atrazine (h,j) and isoproturon (s,d) from Sonning I soil in the absence (open symbolsydotted line) and presence (closed symbolsysolid line) of 500 ppm FA using the same electrolyte solution (0.005 M Ca2q) and soil to solution ratio (1:2) as that used in the initial adsorption solution containing 1000 ppb of the pesticide.
4.4. Effect of soilysolution ratio Decreasing the soil to solution ratio from 1:2 to 1:16 had no significant effect on the adsorption characteristics of the pesticides atrazine, isoproturon and paraquat by these soils in the presence of the electrolytes and FA solution as outlined in the Methods section. However, the adsorption of 2,4D was significantly greater from suspensions prepared with a lower soil:solution ratio for all soils for the two higher ionic strength solutions w0.01 M Ca2q(I.S. 0.03) and 0.05 M Naq (I.S. 0.05)x (Fig. 3b). The increased adsorption at the higher ionic strength was less in the presence of FA, especially in the presence of the Naq cations. 4.5. Desorption of pesticides from soils The desorption of the three pesticides from Sonning I is shown in Fig. 4. The desorption of paraquat was negligible and 2,4-D was only slightly desorbed under the conditions used here. Atrazine and isoproturon were significantly desorbed with a small amount of hysteresis. For all four pesticides the addition of 500 ppm FA to the desorption solution had no discernible effect on the desorption characteristics.
5. Discussion 5.1. Pesticide adsorption The extent of adsorption of the pesticide under any particular soil type and soil solution conditions will depend on the nature and properties of the soil and the pesticide. Atrazine and isoproturon both have functional groups capable of engaging in H-bonding, van der Waals bonding and ligand exchange. However, the presence of the C_ O bond in isoproturon considerably enhances the potential for H-bonding for this pesticide relative to that for atrazine. In addition the presence of the C_ O bond will result in the overall polarity of the isoproturon structure being greater than that for atrazine. A more polar molecule is more likely to move closer to charged surfaces increasing the likelihood of van der Vaals interactions. Paraquat and 2,4-D both exist as charged species at pH values approximately 7 and so both could engage in ionic bonding as well as the other bonding mechanisms referred to above. As soil organic matter tends to exhibit predominantly negatively charged adsorption surfaces (Beckett and
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Le, 1989) the delocalized positive charge associated with the paraquat ion would be expected to be considerably more interactive with soil components than the localised negatively charged carbonyl group of the 2,4-D ion. The charge interaction of the 2,4-D ionic species with the negatively charged soil mineral and organic matter surfaces is more likely to involve interaction with cationic species such as Ca2q in salt bridge arrangements, although it may also participate in hydrogen bonding interactions through the carbonyl group. The four pesticides studied vary in the extent of adsorption of the pesticide by the soil, from virtually complete adsorption for paraquat to limited adsorption for 2,4-D (Fig. 2a,b,c). The order of extent of adsorption of the pesticides, paraquat) isoproturon)atrazine)2,4-D exemplifies the above discussion regarding reactivity of isoproturon relative to atrazine and paraquat relative to 2,4-D. For the soils and conditions used in this study the ionic bonding associated with the positive delocalized charge of paraquat appears to be the most favourable mechanism of bonding to the soil components.
cators of the extent of adsorption of pesticides by soils, even for soils of similar clay content. An explanation for these discrepancies from conclusions generally made in the literature and outlined in the introduction, may be that the total carbon content of a soil does not account for the nature or location of the organic matter. It is quite possible that some of the organic matter may be inaccessible to the pesticide if it is associated within solid state humic particles or clay aggregates, reducing the effective level of active OC in the soil (Skjemstad et al., 1996; Barriuso and Koskinen, 1996). Alternatively, it is possible that some soils may have a lower proportion of reactive to unreactive organic matter present, again reducing the effective OC of the soil. The variation in the reactivity of the organic carbon may not be related just to the nature and origin of the organic matter but it may also be related to the nature of the mineral to which it is sorbed and the solution conditions prevailing when the sorption occurred (Murphy et al., 1992).
5.2. Total organic carbon content and adsorption
In previous studies (Spark and Swift, 1994) it was found that there is a strong interaction between paraquat and FA in solution, but no significant interaction between atrazine, 2,4-D and isoproturon and the solution FA. The results from this present work support these previous results with respect to atrazine and isoproturon. From the values of abs400nm yDOC (Table 1) the humic:fulvic acid character of the pre-existing soluble organic matter in the Sonning soils decreases in the order Sonning II)Sonning III)Sonning I. However, there is no correlation between this order and the order of either the Kdp or Kdp yOC values (Table 3). In addition, the presence of high concentrations of FA had no effect on the adsorption properties of these pesticides by soils for the conditions used in these studies. Because of the strong adsorption characteristics of paraquat for all the soils at the concentrations used in this work, it is not possible to distinguish the effect of increased dissolved organic carbon on the adsorption of this pesticide by soils.
Apart from the clay mineralogy, the main differences between the soils are the nature and total amount of organic matter present. While the humic substances in any given soil will vary widely in composition, most samples will contain similar structural units and functional groups (Stevenson, 1972), although the relative proportions of these may vary. As the three Sonning soils have similar mineralogy, differences in interaction of the pesticides with the Sonning soils would be expected to be related generally to the nature and quantity of carbon present. From the varying Kdp yOC values for the Sonning soils for atrazine and 2,4-D it is apparent that the adsorption of the two pesticides is not dependent only on the total carbon in the soil. For both of these pesticides the Kdp yOC value is less for Sonning I than for the other two Sonning soils. From the results presented in this work it would appear that Kdp yOC values are not reliable indi-
5.3. Dissolved organic carbon content and adsorption
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2,4-D was the only pesticide to show a change in sorption behaviour (sorption decreased) with the soils as a result of an increase in soluble organic matter, although this pesticide exhibited no discernible interaction with FA in the absence of soil (Spark and Swift, 1994). Compared with the other pesticides studied here, the adsorption of 2,4-D from solution by all five soils is relatively small. 2,4-D will be negatively charged above pH 4 (PKas2.73, Tomlin 1997) and would experience repulsion by the predominantly negatively charged surface or organic matter in the soil matrix. Despite this, the adsorption of 2,4-D is usually associated most strongly with the organic matter content of the soil (Reddy and Gambrell, 1987; Hermosin and Cornejo, 1993; Bekbolet et al., 1999). Adsorption of 2,4-D is not likely to involve Hbonding for these soil conditions as H-bonding will be limited to acid conditions where carboxyl groups are not ionised (Barriuso et al., 1992). The adsorption of 2,4-D on humic acid is thought to involve physical adsorption possibly involving van der Waals forces and hydrophobic bonding, with the rate limiting step for the initial process being the diffusion of the pesticide molecules to the surface of the humic acid and for the second process being the diffusion of the pesticide molecules into the humic particles (Khan, 1973). The increased adsorption of 2,4-D at high Ca2q suggests that the bonding mechanism for part of the uptake could involve a salt bridge with the Ca2q cation acting as the bridge between the pesticide and the negatively charged organic matter or negatively charged clay particles. The salt bridge would be favoured at pH)7, and would overcome the problem of repulsion between the negatively charged 2,4-D and the negative humic substances or clay minerals. As the adsorption of 2,4-D is reduced with increase in FA concentration it is consistent to propose that the negatively charged FA added to the soil suspension competes with the negatively charged pesticide for adsorption sites involving the salt bridging. The effect of increased ionic strength on the adsorption of 2,4-D at 1:2 soil:solution ratio is more strongly evident when using a soil:solution ratio of 1:16 for the Broad and Denchworth soils (Fig. 3b). These two soils contain significantly
more clay-sized particles per gram of soil than the Sonning soils, and hence would be expected to have significantly more charged mineral surface area in solution. At the Naq higher ionic strength the thickness of the double layer around the charged soil particles would be reduced, enhancing the likelihood of the 2,4-D overcoming the electrostatic repulsion enough to be adsorbed via the physical adsorption processes mentioned above. 5.4. Soil mineralogy and adsorption characteristics For soil suspensions of equivalent mass, the quantity and nature of the smaller particle size fractions will contribute most to the surface area and hence largely control the overall adsorption characteristics of the soil. This fraction will consist mainly of clays and aluminium and iron oxides. With these considerations, the mineral components of Denchworth and Broad soils would be expected to adsorb the pesticides to the greatest extent based on surface area availability, with the Sonning soils adsorbing the least amount of pesticide. The results indicate that, similar to the discussion on soil organic matter content, the sorption of the pesticides is not simply a function of the clay content as the adsorption characteristics of the Sonning soils vary by as much as a factor of 5 (e.g. atrazine) when the difference in clay content is at the most a factor of 1.5. There are frequent reports in the literature that the adsorption of pesticides is not solely related to either the organic matter content or the clay content although it appears to be positively correlated with both of these soil components. Celis et al. (1999) concluded that for atrazine and simazine adsorption the extent of adsorption on individual soil components (montmorillonite, ferrihydrite and humic acid) did not relate directly to the adsorption of the pesticides on inter-associated combinations of two or more of these individual components. It was concluded that the adsorption sites for the pesticide may be reduced in number or change in character as a result of the association between soil components. Spark et al. (1997) found similar results for the sorption of heavy metals on individual minerals or humic acid compared with that on minerals coated with humic acid.
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5.5. Desorption characteristics The desorption of atrazine and isoproturon from Sonning I showed similarities (Fig. 4) despite the differences in adsorption characteristics (Table 3). In accordance with the findings of other workers, the desorption of these pesticides although significant, exhibited a hysteresis, which indicates that the interaction is not truly reversible (Clay et al., 1988; Piccolo et al., 1992). Harris and Warren (1964) reported that the desorption of atrazine from organic matter was less than that from clays. The use of increased concentrations of FA in the desorption solution had very little effect on the extent of desorption of these two pesticides in agreement with the findings of Clay et al. (1988) for atrazine. The desorption of 2,4-D was relatively small, compared to that of atrazine and isoproturon. As for atrazine and isoproturon, the higher concentrations of FA in the desorption solution had no effect on the characteristics of desorption for 2,4-D. As ionic bonding involving a salt bridge such as Ca2q would be expected to be reversible, it would appear that the 2,4-D was involved in a more specific interaction with the humic surface following the initial salt-bridge bonding process. Isaacson and Frink (1984) found similar results for 2,4-D with up to 90% of the pesticide being irreversibly adsorbed. Brusseau et al. (1989) suggested that the sorption non-equilibrium of 2,4-D may be a result of intra-organic matter diffusion as they found that rate constants showed an inverse relationship to organic matter content. There is considerable evidence that paraquat interacts strongly with soluble organic matter, as discussed above. The results in this work show that there is negligible desorption of paraquat from the soils in the absence or presence of increased content of soluble fulvic. This lack of desorption would result in the paraquat having limited interaction with the soluble fraction of organic matter in the soil. 6. Conclusion The adsorption of pesticides by soils is dominated by the interaction with the solid state of the
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soil. From the results presented here and those of others, the solid state interactions involves both the organic and inorganic fractions, the relative importance of which will depend on the relative abundance of these two fractions. The soluble organic matter in the soil solution had no effect on the sorption characteristics of atrazine, isoproturon and paraquat. This fraction did affect the sorption characteristics of 2,4-D but this is thought to be due to competitive effects of the organic matter with the sorption of the pesticide rather than due to the interaction of 2,4-D with the soluble fraction of the organic matter. Acknowledgments This work was funded by a research grant from the BBSRC, UK. We are grateful to John Cooke for technical assistance. References Avery, BW. Soils of the British Isles. CAB International, Wallingford, UK, 1990. Avery, BW., Bascomb, CL. Soil Survey Laboratory Methods. Harpenden, UK, 1982. Arienzo M, Buondonno A. Adsorption of paraquat by Terra Rossa soil and model soil aggregates. Toxicol Environ Chem 1993;39:193 –199. Bailey GW, White JL. Review of adsorption and desorption of organic pesticides by soil colloids with implications concerning pesticide bio-activity. J Agric Food Chem 1964;2:324 –332. Barriuso E, Baer U, Calvet R. Dissolved organic matter and adsorption-desorption of dimefuron, atrazine, and carbetamide by soils. J Environ Qual 1992;21:359 –367. Barriuso E, Koskinen WC. Incorporating nonextractable atrazine residues into soil size fractions as a function of time. Soil Sci Soc Am J 1996;60:150 –157. Baskaran S, Bolan NS, Rahman A, Tillman RW. Pesticide sorption by allophanic and non-allophanic soils of New Zealand. New Zealand J Agric Res 1996a;39:297 –310. Baskaran S, Bolan NS, Rahman A, Tillman RW. Effect of exogenous carbon on the sorption and movement of atrazine and 2,4-D by soils. Aust J Soil Res 1996b;34:609 –622. Beckett R, Le NP. The role of organic matter and ionic composition in determining the surface charge of suspended particles in natural waters. Colloid Surf 1989;44:35 –49. Bekbolet M, Yenigun O, Yucel I. Sorption studies of 2,4-D on selected soils. Water Air Soil Pollut 1999;111:75 –88. Borggaard OK, Streibig JC. Atrazine adsorption by some soil samples in relation to their constituents. Acta Agric Scan 1988;38:293 –301.
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Brusseau ML, Rao PSC, Jessup RE, Davidson JM. Flow interruption: A method for investigating sorption non-equilibrium. J Contam Hydrol 1989;4:223 –240. Burns IG, Hayes MHB, Stacey M. Studies of the adsorption of paraquat on soluble humic fractions by gel filtration and ultrafiltration techniques. Pest Sci 1973;4:629 –641. Celis R, Hermosin MC, Cox L, Cornejo J. Sorption of 2,4dichlorophenoxyacetic acid by model soil particles simulating naturally occurring soil colloids. Environ Sci Technol 1999;33:1200 –1206. Chiou CT, Malcolm RL, Brinton TI, File DE. Water solubility enhancement of some organic pollutants and pesticides by dissolved humic and fulvic acids. Environ Sci Technol 1986;20:502 –508. Clay SA, Allmaras RR, Koskinen WC, Wyse DL. Desorption of atrazine and cyanazine from soil. J Environ Qual 1988;17:719 –723. Devitt EC, Wiesner MR. Dialysis investigations of atrazineorganic matter interactions and the role of a divalent metal. Environ Sci Technol 1998;32:232 –237. Fitch A, Du J. Solute transport in clay media: Effect of humic acid. Environ Sci Technol 1996;30:12 –15. Gamble DS, Haniff MI, Zienius RH. Solution phase complexing of atrazine by fulvic acid: A batch ultrafiltration technique. Anal Chem 1986;58:727 –731. Gao JP, Maguhn J, Spitzauer P, Kettrup A. Distribution of pesticides in the sediment of the small Teufelsweiher pond. Water Res 1997;31:2811 –2819. Harris CI, Warren GF. Adsorption and desorption of herbicides by soil. Weeds 1964;12:120 –126. Hayes MHB. Adsorption of triazine herbicides on soil organic matter including a short review on soil organic matter chemistry. Residue Rev 1970;32:131 –174. Hermosin MC, Cornejo J. Binding mechanism of 2,4-dichlorophenoxyacetic acid by organo-clays. J Environ Qual 1993;22:325 –331. Isaacson PJ, Frink CR. Non-reversible sorption of phenolic compounds by sediment fractions: the role of sediment organic matter. Environ Sci Technol 1984;18:43 –48. Jarvis, MG. Soils of the Wantage and Abingdon District (Memoirs of the Soil Survey of Great Britain-England and Wales) Agricultural Research Council, Harpenden, 1973. Jenks BM, Roeth FW, Martin AR, McCallister DL. Influence of surface and subsurface soil properties on atrazine sorption and degradation. Weed Sci 1998;46:132 –138. Johnson AC, Haria AH, Cruxton VL, Batchelor CH, Williams RJ, Heppell K, Burt T. Isoproturon and anion transport by preferential flow through a drained clay soil. Pesticide movement to water. In: Walker A, Allen R, Bailey SW, Blair AM, Brown CD, Gunther P, Leake CR, Nicholls PH, editors. Proceedings of a symposium held at the University of Warwick, Coventry, UK 1995. p. 105 –110. Kan AT, Tomson MB. Ground water transport of hydrophobic organic compounds in the presence of dissolved organic matter. Environ Toxicol Chem 1990;9:253 –263.
Khan SU. Equilibrium and kinetic studies of the adsorption of 2,4-D, and picloram on humic acid. Canad J Soil Sci 1973;53:429 –434. Kulovaara M. Distribution of DDT and Benzowax(pyrene between water and dissolved matter in natural humic water. Chemosphere 1993;27:2333 –2340. Madhun YA, Young JL, Freed VH. Binding of herbicides by water-soluble organic materials from soil. J Environ Qual 1986;15:64 –68. Maquedo C, Morillo E, Martin F, Undabeytia T. Interaction of pesticides with the soluble fraction of natural and artificial humic substances. J Environ Sci Health B 1993;28:655 – 670. Masse L, Prasher SO, Khan SU, Arjoon DS, Barrington S. Leaching of metolachlor, atrazine, and atrazine metabolites into groundwater. Trans ASAE. 1994;37:801 –806. Miller JJ, Foroud N, Hill BD, Lindwall CW. Herbicides in surface runoff and groundwater under surface irrigation in southern Alberta. Canad J Soil Sci 1995;75:145 –148. Moreaukervevan C, Mouvet C. Adsorption and desorption of atrazine, deethylatrazine, and hydroxyatrazine by soil components. J Environ Qual 1998;27:46 –53. Murphy EM, Zachara JM, Smith SC, Phillips JL. The sorption of humic acids to mineral surfaces and their role in contaminant binding. Sci Total Environ 1992;117y118:413 –423. Piccolo A, Scheunert I, Schroll R. Supercritical vs. normal methanol mass extraction of 14C labelled atrazine bound to humic substances. Fresen Environ Bull 1992;1:627 –631. Reddy KS, Gambrell RP. Factors affecting the adsorption of 2,4-D and methyl parathion in soils and sediments. Agric Ecosystems Environ 1987;18:231 –241. Riise G, Eklo OM, Lode O, Pettersen MN. Mobility of atrazine and tribenuron methyl in the soil water system lysimeter experiments. Contamination of pesticides from agriculture and industrial areas to soil and water. Norw J Agric Sci Supp 1994;13:31 –41. Ro KS, Chung KH, Chung YC, Tsai FJ. Pesticides and herbicides. Water Environ Res 1997;69:664 –667. Senesi N, Miano TM, Provenzano MR, Brunetti G. Spectroscopic and compositional comparative characterisation of IHSS reference and standard fulvic acid and humic acids of various origin. Sci Total Environ 1989;81y2:143 –156. Skjemstad JO, Clarke P, Taylor JA, Oades JM, McClure SG. The chemistry and nature of protected carbon in soil. Aust J Soil Res 1996;34:251 –271. Spark KM, Swift RS. Investigation of the interaction between pesticides and humic substances using fluorescence spectroscopy. Sci Total Environ 1994;152:9 –17. Spark KM, Wells JD, Johnson BB. Sorption of heavy metals by mineralyhumic acid systems. Austral J Soil Res 1997;35:113 –122. Stevenson FJ. Organic matter reactions involving herbicides in soils. J Environ Qual 1972;1:333 –343. Tomlin CDS. The Pesticide Manual, 11th Edition. British Crop Protection Council, Surrey, UK, 1997.
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substances in waters from different watersheds in Britain and Ireland. Water Res 1996;30:1502 –1516. Welhouse GJ, Bleam WF. NMR spectroscopic investigation of hydrogen bonding in atrazine. Environ Sci Technol 1992;26:959 –964.
Effect of soil composition and dissolved organic matter on pesticide sorption K.M. Spark1,*, R.S. Swift Department of Soil Science, University of Reading, Whiteknights, Reading, Berkshire, RG6 2AA, UK Received 13 August 2000; accepted 1 May 2002
Abstract The effect of the solid and dissolved organic matter fractions, mineral composition and ionic strength of the soil solution on the sorption behaviour of pesticides were studied. A number of soils, chosen so as to have different clay mineral and organic carbon content, were used to study the sorption of the pesticides atrazine (6-chloro-N 2-ethyl-N 4 isopropyl-1,3,5-triazine-2,4-diamine), 2,4-D ((2,4-dichlorophenoxy)acetic acid), isoproturon (3-(4-isopropylphenyl)1,1-dimethylurea) and paraquat (1,19-dimethyl-4,49-bipyridinium) in the presence of low and high levels of dissolved organic carbon and different background electrolytes. The sorption behaviour of atrazine, isoproturon and paraquat was dominated by the solid state soil components and the presence of dissolved organic matter had little effect. The sorption of 2,4-D was slightly affected by the soluble organic matter in the soil. However, this effect may be due to competition for adsorption sites between the pesticide and the soluble organic matter rather than due to a positive interaction between the pesticide and the soluble fraction of soil organic matter. It is concluded that the major factor governing the sorption of these pesticides is the solid state organic fraction with the clay mineral content also making a significant contribution. The dissolved organic carbon fraction of the total organic carbon in the soil and the ionic strength of the soil solution appear to have little or no effect on the sorptionytransport characteristics of these pesticides over the range of concentrations studied. 䊚 2002 Elsevier Science B.V. All rights reserved. Keywords: Pesticides; Sorption behaviour; Organic matter state; Mineral content
1. Introduction The retention and mobility of a pesticide in soil is determined by the extent and strength of sorption reactions, which are governed by the chemical and *Corresponding author. Tel.: q61-7-5460-1336. E-mail address: [email protected] (K.M. Spark). 1 Present address: University of Queensland, Gatton Campus, Gatton Qld 4343, Australia
physical properties of the soils and pesticides involved. The sorption interactions of pesticides in the soil environment may involve either the mineral or organic components, or both. For soils that have higher organic matter levels ()5%), the mobility of the pesticides has been related to the total organic matter content, with the nature of the organic matter having little apparent influence on sorption processes (Bailey and White, 1964;
0048-9697/02/$ - see front matter 䊚 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 4 8 - 9 6 9 7 Ž 0 2 . 0 0 2 1 3 - 9
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Hayes, 1970; Riise et al., 1994; Arienzo and Buondonno, 1993; Jenks et al., 1998; Bekbolet et al., 1999). Total organic matter includes both the soluble and insoluble fraction of organic matter, although the proportion of soluble organic matter in a soil is usually very small. For soils, which have low organic matter contents, the mobility of the pesticide is often related to the active components of the inorganic fraction, which is predominantly the clay-sized fraction. An increase in the clay content results in decreasing mobility of the pesticide, with the composition of the clay and the identity of the major cations in the soil solution also being important (Walker and Crawford, 1968; Borggaard and Streibig, 1988; Murphy et al., 1992; Barriuso et al., 1992; Welhouse and Bleam, 1992; Baskaran et al., 1996a). There is considerable evidence that pesticides can interact with the soluble form of soil organic matter in the absence of the solid components of the soil. The extent and nature of this interaction depends on factors such as molecular weight and polarity of the pesticide (Burns et al., 1973; Gamble et al., 1986; Chiou et al., 1986; Madhun et al., 1986; Kan and Tomson, 1990; Maquedo et al., 1993; Spark and Swift, 1994; Devitt and Wiesner, 1998). Pesticides such as atrazine have been shown to be capable of solution-phase complexing with dissolved organic matter (Devitt and Wiesner, 1998; Gamble et al., 1986); paraquat associates with dissolved organic matter (Burns et al., 1973; Maquedo et al., 1993; Spark and Swift, 1994) and there is evidence for the association of hydrophobic pesticides with dissolved organic matter (Kan and Tomson, 1990; Kulovaara, 1993; Fitch and Du, 1996). There is some evidence that, in the presence of soils, an increase in the concentration of dissolved organic matter results in an increase in the mobility of atrazine (Gao et al., 1997) and 2,4-D (Baskaran et al., 1996b), and of hydrophobic organic chemicals such as DDT (Kan and Tomson, 1990; Fitch and Du, 1996). As many of these pesticides are found in groundwaters it is apparent that there is significant transport of these chemicals through the soil profile (Johnson et al., 1995; Masse et al., 1994; Miller et al., 1995; Ro et al., 1997). This transport may be the result of processes such as:
the formation of soluble complexes with soil solution components such as dissolved organic matter; or the incomplete interaction of these pesticides with the solid state organic or inorganic matter in the soil. Pesticides which interact with organic matter will react with both the soluble and solid phase fractions, therefore, competitive effects, the reversibility of these two types of interaction, and mass action effects will govern the distribution of the pesticides between the solid and soluble phases of the organic matter. For example, the sorption of atrazine on clays is generally reversible whereas the sorption from organic matter is less so (Harris and Warren, 1964; Moreaukervevan and Mouvet, 1998). As discussed above, it is has been shown that there is a strong relationship between total organic carbon in the soil and the mobility of many pesticides. However, the extent to which the soluble organic carbon is involved in the transport process is not known. If the interaction with the soluble organic matter relative to the insoluble organic matter is significant, the transport of the pesticides through the soil may be enhanced by an increase in soluble organic matter moving through the soil profile. The concentration of soluble organic matter would therefore influence the concentration of the pesticides in the drainage waters, surface waters and ground waters associated with a particular soil. The aim of this work was to determine the relative importance of soil components on the adsorption of a variety of pesticides. The research was carried out using a range of soils differing in organic matter content as well as clay mineral content, and varying the concentration of the soluble organic matter in the soil solution. 2. Materials Reagents were analytical grade unless otherwise stated. All solutions were made up in distilled deionised water and kept in airtight containers. 2.1. Soil characterisation The UK soils used in this work were from the Sonning and Thames series (Jarvis, 1973) from
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Table 1 Summary of characteristics of soils Sonning I
Sonning II
Sonning III
Broad
Denchworth
Small Moderate Moderate
Moderate Moderate Moderate
Moderate Small Small
Large Small Small
2.3 28.3 29.5 15.3 9.4
2.3 28.1 25.7 12.2 12.9
7.0 0.26 51 1.8 0.00090 0.158 3.1
7.0 0.27 60 2.8 0.0014 0.162 2.7
7.3 0.80 117 10.0 0.0050 0.211 1.8
3.30 0.18
6.3 0.19
a
Clay minerals Smectite Small Micas Moderate Kaolinite Moderate Particle size (% content of total soil) )600 mm 1.1 212–600 mm 30.7 63–212 mm 29.8 2–63 mm 12.4 -2 mm 10.8 Soil solution componentsb pH 7.6 Cond. mSycm 0.19 DOC(ppm) 23 Soluble Ca(II) (mmolyg) 2.6 (Molyl) 0.0013 Absorbance @ 400 nm 0.051 Abs400nmyDOC (=1000) 2.2 Organic carbon characteristics % OC (total soil) 1.25 DOCyOC (%) 0.18 a b
2.05 0.25
5.4 6.4 15.9 19.6 32.7
0.8 2.7 7.4 16.3 44.2 7.4 0.37 55 4.4 0.0022 0.083 1.5 3.25 0.17
X-Ray diffraction analysis of the clay mineral fraction. Note: All samples contain quartz. Soluble components in the supernatant of the soil suspension prepared as described for the sorption studies.
the University of Reading farm, Sonning, Berkshire and the Denchworth series (Avery, 1990) from Brimstone, Oxfordshire. A summary of the characteristics of the soil samples taken from these sites is shown in Table 1. Three samples with different organic carbon (OC) contents were taken of the Sonning soil, a free-draining, brown, coarse, sandy-loam soil. Sonning I soil (OCs1.25%) had been under continuous cultivation and cropping, Sonning II (OCs2.05%) was soil under natural grassland vegetation, and Sonning III soil (OCs 3.30%) was taken under woodland vegetation. The Denchworth soil (OCs3.25%) had a similar organic matter content to the Sonning III soil, but was a very fine clay from the Ap horizon. The Broad soil, from the Thames series, had the highest OC content in the soils studied here (OCs6.3%) and was a dark brownish-grey, alluvial, loamy-clay soil from a flat, flood-prone plain under grassland vegetation. The soils were dried, sieved to -1 mm and stored in sealed containers at room temperature. The soils were characterised for particle size using
sedimentation characteristics of the soil (Avery and Bascomb, 1982) and total carbon using a Leco SC-444 Analyser. The results of the particle size analysis and mineralogical analysis of the soils using X-Ray Diffraction are given in Table 1. The composition of the supernatant of the soils when suspended using the same conditions as for the pesticide adsorption experiments (described below) but in the absence of pesticides, was analysed to determine the pH, the background concentration of Ca2q (Perkin Elmer 3030 Atomic Absorption Spectrophotometer), the absorbance at 400 nm (Perkin Elmer Lambda 2 UV-Visible Spectrophotometer), the conductivity and the concentration of dissolved organic carbon (Shimadzu TOC 5000 Total Organic Carbon Analyser) (Table 1). 2.2. Leaching solution containing dissolved fulvic acid The soluble humic substance used in this work was a fulvic acid (FA) isolated from a water
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draining from a sapric histosol (Methwold Fens, Norfolk). The pH of the drain water was adjusted to two before passing it through XAD-8 resin to separate the FA from the other components of humic substances. Additional treatment associated with purification of the FA in the eluent is described in Watt et al. (1996). The FA solutions were made up daily at pH 7 and stored in airtight volumetric flasks. The adsorption of FA by the soils in the absence of pesticides was determined using the same method as for the pesticide adsorption studies (described below), with three types of background electrolytes; 0.005 M CaCl2, 0.01 M CaCl2 and 0.05 M NaCl, none of which caused precipitation of the FA from solution. The concentration of organic matter in solution was determined as described in the previous paragraph. 2.3. Pesticides The pesticides used in the studies were: atrazine (6-chloro-N 2-ethyl-N 4 -isopropyl-1,3,5-triazine-2, 4-diamine, technical grade (99.0%)) kindly supplied by Ciba-Geigy, UK; 2,4-D w(2,4-dichlorophenoxy)acetic acidx supplied by Sigma Chemical Company, St. Louis, USA; isoproturon w3-(4-isopropylphenyl)-1,1-dimethylurea, technical grade (99.0%)x kindly supplied by Rhone-Poulenc Agriculture Ltd., Essex, England; paraquat w1,19dimethyl-4,49-bipyridinium, technical grade (98%)x supplied by Aldrich Chemical Co. Ltd., Dorset, England. The radiolabeled chemicals of the above pesticides were atrazine (atrazine-ring-UL-14 C), kindly supplied by Ciba-Geigy, 2,4-D (2,4-dichlorophenoxyacetic acid-carboxyl-14C, )98% purity) supplied by Sigma Chemical Company, isoproturon (isoproturon-ring-UL-14 C), kindly supplied by Ciba-Geigy and paraquat (paraquat-methyl-14C dichloride,)98% purity), supplied by Sigma Chemical Company. 3. Methods 3.1. Adsorption studies Stock solutions of 2,4-D and paraquat contained 0.010 gyl of the pesticide dissolved in water passed
through a Milli-Q Reagent Grade Water System (Millipore Corporation, Bedford, Massachusetts). Stock solutions of atrazine and isoproturon were prepared by dissolving 0.010 g of the pesticide in 10 ml of AR methanol and diluting the solution to 1 l using Milli-Q water. The concentration of pesticides used in this study ranged from 250 ppb to 10 ppm, the background solution was an electrolyte (0.005 M Ca2q-, 0.01 M Ca2q, or 0.05 M Naq) and soluble organic matter (0 or 500 ppm FA). Adsorption experiments were carried out in triplicate using soil:solution ratio of 1 g of soil to 2 ml of solution unless otherwise stated. The adsorption solution was shaken on an end-overend shaker for 16 h. Trials carried out using Sonning I and Broad soil indicated that, for both soils, adsorption after 16 h was ;96% of that amount adsorbed after 24 h of shaking for atrazine and isoproturon. The corresponding values for 2,4D on Sonning I and Broad soils were ;88% and ;94%. Essentially 100% of the paraquat was adsorbed even at the shorter shaking time of 16 h. At the end of the 16 h allowed for adsorption, the samples were removed from the shaker and the soil suspension was centrifuged at 20 000=g for 20 min. Duplicate aliquots of the supernatant were removed for analysis. The amount of pesticide in the supernatant solution was determined using a liquid scintillation counter which had been calibrated for solutions containing 0–1000 ppm FA in solution to account for quenching effects of the FA. 3.2. Desorption studies For desorption experiments, the initial adsorption was as described above. After the removal of the two samples for analysis, further supernatant was removed until only 25% of the original volume of solution in the vessel remained. The solution in the vessel was then made up to the original volume with the same background electrolyte and either 0 or 500 ppm of added FA. The suspensions were equilibrated on a shaker for 16 h, centrifuged and the supernatant was again analysed. This process was repeated three more times in the desorption studies.
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4. Results 4.1. Characterisation of the soils The results of the characterisation of the soils are shown in Table 1. The three Sonning soils have a similar particle size distribution with 55– 60% of the particles being in the size range associated with coarse sand (63–600 mm) and approximately 10% of the particles being in the size range associated with fine clays (-2 mm). The clay composition (Table 1) in these samples is similar. However, the Sonning soil samples differ in the amount of organic matter present with 1.25, 2.05 and 3.30% total organic carbon for Sonning I, II and III, respectively. Broad and Denchworth soils are much heavier textured soils with a particle size distribution consisting of a coarse sand content of approximately 35 and 25%, respectively, and a fine clay content of 35 and 45%, respectively. The nature of the clay in these samples is similar (Table 1). The main difference between these two smectitic clay soils is that the Broad soil has approximately twice the total organic carbon to that of Denchworth (6.3 and 3.25%, respectively). The pH of the soil solutions were similar (Table 1), ranging from 7.0 to 7.6. The conductivity of these solutions ranged from 0.19–0.8 mSycm, the wDOCx ranged from 23–55 ppm, the soluble Ca2q extracted using water ranged from 0.18– 10.0 mmolyg soil and the absorbance at 400 nm ranged from 0.046–0.219 (Table 1). The values of DOCyOC (%) shown in Table 1 are relatively similar for all soils except Sonning II indicating that the organic matter in this soil is relatively more soluble, possibly due to lower wCa2qx (lowest of all soils). The values of abs400 nm yDOC w(absyppm)=1000x show a greater range. In general it has been observed that for the same DOC concentration the relative absorbance at 400 nm of humic acids is greater than fulvic acids (Senesi et al., 1989). The higher values of abs400 nm yDOC of the Sonning II and III soils suggests that the organic matter in these two soils has a higher ratio of humic:fulvic acid character than that of the other soils. And correspond-
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ingly, it suggests that the Denchworth soil has the lowest ratio of humic:fulvic acid character. The adsorption of the FA by the soils was found to be directly proportional to the amount of FA added to the soil solution (Fig. 1). The slope of the straight line representing this linear proportionality Kfa for each of the soils is shown in Table 2, together with the initial absorbance of the soil solutions. The total amount of FA adsorbed was least in 0.005 M Ca2q (lowest Kfa value) and greatest in 0.01 M Ca2q (highest Kfa value) for all soils. (It is important to note that the fulvic acid fraction used in this work remained in solution in the presence of 0.01 M Ca2q background electrolyte solution in the absence of soils.) The amount of FA adsorbed for a particular type of background electrolyte, was also dependent on the nature of the soil, increasing in the order Sonning II-Sonning I s Sonning III-Broad-Denchworth. This order correlates with the order of relative abundance of clay fraction (-2 mm) in the soils (Table 1) with a statistical R 2 value for fit of 0.98, 0.94 and 0.98 for 0.005 M Ca2q, 0.05 M Naq and 0.01 M Ca2q, respectively. As Broad soil had the highest wCa2qx in soil solution the adsorbance of FA would appear to be more dependent on the clay content than the soluble calcium content. 4.2. Adsorption of pesticides The characteristics of the adsorption at low ionic strength (0.005 M Ca2q) of the pesticides, atrazine, 2,4-D and isoproturon for the soils are shown in Fig. 2a,b,c, respectively. Over the range of concentrations used the adsorption is close to a linear relationship between the amount of pesticide (x) adsorbed per g of soil (m) as a function of the concentration of the pesticide in solution at equilibrium (C). xymsKdp C Where Kdp is the distribution constant of the pesticide (over the equilibrium solution concentration of 0–2500 ppb) between the soil and the solution. The Kdp values of the plots for all five
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Fig. 1. The amount of FA sorbed from solution as a function of the initial concentration in the presence of the soils; Sonning I (m), Sonning II (h), Sonning III (j), Broad (s) and Denchworth (⽧).
soils are shown in Table 3. The order of adsorption on all soils is 2,4-D-atrazine-isoproturon-paraquat. The order of adsorption of the pesticides atrazine and 2,4-D on each of the soils is Sonning I-Sonning II-Sonning III f DenchworthBroad, which is the order of abundance of OC in the soils. This order is slightly different for isoproturon on the soils as the position of Broad and Denchworth are reversed i.e. Sonning I-Sonning II-Sonning III-Broad-Denchworth. Paraquat was completely adsorbed using these experimental conditions and concentrations (as shown in Fig.
2a) and so other graphical results and the Kdp values for this pesticide are not included here. The corresponding ratio of the Kdp value to the OC of the soil, Kdp yOC, is also given in Table 3. The range in these values for all soils is 0.56"0.16, 0.12"0.03 and 0.74"0.20 for atrazine, 2,4-D and isoproturon, respectively, (representing 25–30% variation in values). The three Sonning soils have similar mineralogy but different levels of organic matter (Table 1). Taking only the Kdp yOC values for the Sonning soils, and hence largely eliminating mineralogy as a factor in the
Table 2 Effect of the background electrolyte type and concentration on the absorbance (400 nm) of the soil solution (1:5 suspension; no added FA) and on the Kfa value (degree of sorbance of soluble FA from solution) for each of the soils Electrolyte
Ionic strength
Sonning I
Sonning II
Sonning III
Broad
Denchworth
Absorbance of soil solution
0.005 M Ca2q 0.01 M Ca2q 0.05 M Naq
0.015 0.03 0.05
0.012 0.016 0.043
0.135 nd nd
0.119 0.074 0.155
0.188 0.166 0.226
0.075 0.050 0.064
Kfa value
0.005 M Ca2q 0.01 M Ca2q 0.05 M Naq
0.015 0.03 0.05
0.26 0.57 0.39
0.19 nd nd
0.27 0.59 0.48
0.52 0.69 0.59
0.60 0.80 0.75
Note: nd means not determined.
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Fig. 2. (a) The amount of atrazine sorbed by the soil (gyg) as a function of the concentration of the pesticide in solution (background electrolyte 0.005 M Ca2q) for the soils; Sonning I (m), Sonning II (h), Sonning III (j), Broad (s) and Denchworth (⽧). The amount of paraquat sorbed by the soil (mgyg) as a function of the concentration of the pesticide in solution at low ionic strength (0.005 M Ca2q) for Sonning I soil (d); (b) The amount of 2,4-D sorbed by the soil (mgyg) as a function of the concentration of the pesticide in solution (background electrolyte 0.005 M Ca2q) for the soils; Sonning I (m), Sonning II (h), Sonning III (j), Broad (s) and Denchworth (⽧); (c) The amount of isoproturon sorbed by the soil (mgyg) as a function of the concentration of the pesticide in solution (background electrolyte 0.005 M Ca2q) for the soils; Sonning I (m), Sonning II (h), Sonning III (j), Broad (s) and Denchworth (⽧).
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Fig. 2 (Continued).
adsorption characteristics, results in a smaller coefficient of variation for isoproturon (now 0.75"0.06) but not for atrazine and 2,4-D.
solution also had no effect on the adsorption characteristics of these two pesticides in any of the three electrolyte solutions used in this work. However, these solution conditions did affect the adsorption of 2,4-D as shown in Fig. 3a, where it can be seen that increasing the ionic strength caused the adsorption of 2,4-D to increase on all four soils. The effect is greater for higher ionic strengths solutions containing Ca2q (I.S. 0.03) than Naq (I.S. 0.05) and is reduced in the presence of increased concentration of soluble FA. Paraquat was completely adsorbed under these experimental conditions and concentrations and therefore the effects of ionic strength and added fulvic acid on the adsorption were not detectable.
4.3. Effect of ionic strength and concentration of FA The ionic strength (I.S.) of the 0.005 M Ca2q solutions referred to in the above discussion is 0.015. Using background electrolyte solutions of higher ionic strength, 0.01 M Ca2q (I.S. 0.03) or 0.05 M Naq (I.S. 0.05) had no significant effect on the adsorption of atrazine and isoproturon on the four soils studied (Sonning I, Sonning III, Broad and Denchworth). Repeating the adsorption procedure using 500 ppm FA in the adsorption
Table 3 Kdp and KdpyOC values for the soilypesticide systems, where Kdp is the distribution constant of the pesticide between the soil and the solution and OC is the organic carbon content of the whole soil Pesticide Atrazine 2,4 D Isoproturon
Kdp KdpyOC Kdp KdpyOC Kdp KdpyOC
Sonning I
Sonning II
Sonning III
Broad
Denchworth
0.46 0.37 0.13 0.10 1.01 0.81
1.32 0.64 0.25 0.12 1.53 0.75
2.23 0.68 0.51 0.15 2.32 0.70
2.49 0.40 0.66 0.10 2.72 0.43
2.23 0.69 0.50 0.15 3.21 0.99
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Fig. 3. The effect of the absence or presence of FA (500 ppm) and the nature and concentration of the electrolyte (0.005 M Ca2q, 0.01 M Ca2q, 0.05 M Naq) on the sorption of 2,4-D by soils (Sonning I, Sonning III, Broad and Denchworth) at high (1:2) and low (1:16) soil to solution ratio (Fig. 3a,b, respectively).
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Fig. 4. Desorption of 2,4-D (n,m), atrazine (h,j) and isoproturon (s,d) from Sonning I soil in the absence (open symbolsydotted line) and presence (closed symbolsysolid line) of 500 ppm FA using the same electrolyte solution (0.005 M Ca2q) and soil to solution ratio (1:2) as that used in the initial adsorption solution containing 1000 ppb of the pesticide.
4.4. Effect of soilysolution ratio Decreasing the soil to solution ratio from 1:2 to 1:16 had no significant effect on the adsorption characteristics of the pesticides atrazine, isoproturon and paraquat by these soils in the presence of the electrolytes and FA solution as outlined in the Methods section. However, the adsorption of 2,4D was significantly greater from suspensions prepared with a lower soil:solution ratio for all soils for the two higher ionic strength solutions w0.01 M Ca2q(I.S. 0.03) and 0.05 M Naq (I.S. 0.05)x (Fig. 3b). The increased adsorption at the higher ionic strength was less in the presence of FA, especially in the presence of the Naq cations. 4.5. Desorption of pesticides from soils The desorption of the three pesticides from Sonning I is shown in Fig. 4. The desorption of paraquat was negligible and 2,4-D was only slightly desorbed under the conditions used here. Atrazine and isoproturon were significantly desorbed with a small amount of hysteresis. For all four pesticides the addition of 500 ppm FA to the desorption solution had no discernible effect on the desorption characteristics.
5. Discussion 5.1. Pesticide adsorption The extent of adsorption of the pesticide under any particular soil type and soil solution conditions will depend on the nature and properties of the soil and the pesticide. Atrazine and isoproturon both have functional groups capable of engaging in H-bonding, van der Waals bonding and ligand exchange. However, the presence of the C_ O bond in isoproturon considerably enhances the potential for H-bonding for this pesticide relative to that for atrazine. In addition the presence of the C_ O bond will result in the overall polarity of the isoproturon structure being greater than that for atrazine. A more polar molecule is more likely to move closer to charged surfaces increasing the likelihood of van der Vaals interactions. Paraquat and 2,4-D both exist as charged species at pH values approximately 7 and so both could engage in ionic bonding as well as the other bonding mechanisms referred to above. As soil organic matter tends to exhibit predominantly negatively charged adsorption surfaces (Beckett and
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Le, 1989) the delocalized positive charge associated with the paraquat ion would be expected to be considerably more interactive with soil components than the localised negatively charged carbonyl group of the 2,4-D ion. The charge interaction of the 2,4-D ionic species with the negatively charged soil mineral and organic matter surfaces is more likely to involve interaction with cationic species such as Ca2q in salt bridge arrangements, although it may also participate in hydrogen bonding interactions through the carbonyl group. The four pesticides studied vary in the extent of adsorption of the pesticide by the soil, from virtually complete adsorption for paraquat to limited adsorption for 2,4-D (Fig. 2a,b,c). The order of extent of adsorption of the pesticides, paraquat) isoproturon)atrazine)2,4-D exemplifies the above discussion regarding reactivity of isoproturon relative to atrazine and paraquat relative to 2,4-D. For the soils and conditions used in this study the ionic bonding associated with the positive delocalized charge of paraquat appears to be the most favourable mechanism of bonding to the soil components.
cators of the extent of adsorption of pesticides by soils, even for soils of similar clay content. An explanation for these discrepancies from conclusions generally made in the literature and outlined in the introduction, may be that the total carbon content of a soil does not account for the nature or location of the organic matter. It is quite possible that some of the organic matter may be inaccessible to the pesticide if it is associated within solid state humic particles or clay aggregates, reducing the effective level of active OC in the soil (Skjemstad et al., 1996; Barriuso and Koskinen, 1996). Alternatively, it is possible that some soils may have a lower proportion of reactive to unreactive organic matter present, again reducing the effective OC of the soil. The variation in the reactivity of the organic carbon may not be related just to the nature and origin of the organic matter but it may also be related to the nature of the mineral to which it is sorbed and the solution conditions prevailing when the sorption occurred (Murphy et al., 1992).
5.2. Total organic carbon content and adsorption
In previous studies (Spark and Swift, 1994) it was found that there is a strong interaction between paraquat and FA in solution, but no significant interaction between atrazine, 2,4-D and isoproturon and the solution FA. The results from this present work support these previous results with respect to atrazine and isoproturon. From the values of abs400nm yDOC (Table 1) the humic:fulvic acid character of the pre-existing soluble organic matter in the Sonning soils decreases in the order Sonning II)Sonning III)Sonning I. However, there is no correlation between this order and the order of either the Kdp or Kdp yOC values (Table 3). In addition, the presence of high concentrations of FA had no effect on the adsorption properties of these pesticides by soils for the conditions used in these studies. Because of the strong adsorption characteristics of paraquat for all the soils at the concentrations used in this work, it is not possible to distinguish the effect of increased dissolved organic carbon on the adsorption of this pesticide by soils.
Apart from the clay mineralogy, the main differences between the soils are the nature and total amount of organic matter present. While the humic substances in any given soil will vary widely in composition, most samples will contain similar structural units and functional groups (Stevenson, 1972), although the relative proportions of these may vary. As the three Sonning soils have similar mineralogy, differences in interaction of the pesticides with the Sonning soils would be expected to be related generally to the nature and quantity of carbon present. From the varying Kdp yOC values for the Sonning soils for atrazine and 2,4-D it is apparent that the adsorption of the two pesticides is not dependent only on the total carbon in the soil. For both of these pesticides the Kdp yOC value is less for Sonning I than for the other two Sonning soils. From the results presented in this work it would appear that Kdp yOC values are not reliable indi-
5.3. Dissolved organic carbon content and adsorption
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2,4-D was the only pesticide to show a change in sorption behaviour (sorption decreased) with the soils as a result of an increase in soluble organic matter, although this pesticide exhibited no discernible interaction with FA in the absence of soil (Spark and Swift, 1994). Compared with the other pesticides studied here, the adsorption of 2,4-D from solution by all five soils is relatively small. 2,4-D will be negatively charged above pH 4 (PKas2.73, Tomlin 1997) and would experience repulsion by the predominantly negatively charged surface or organic matter in the soil matrix. Despite this, the adsorption of 2,4-D is usually associated most strongly with the organic matter content of the soil (Reddy and Gambrell, 1987; Hermosin and Cornejo, 1993; Bekbolet et al., 1999). Adsorption of 2,4-D is not likely to involve Hbonding for these soil conditions as H-bonding will be limited to acid conditions where carboxyl groups are not ionised (Barriuso et al., 1992). The adsorption of 2,4-D on humic acid is thought to involve physical adsorption possibly involving van der Waals forces and hydrophobic bonding, with the rate limiting step for the initial process being the diffusion of the pesticide molecules to the surface of the humic acid and for the second process being the diffusion of the pesticide molecules into the humic particles (Khan, 1973). The increased adsorption of 2,4-D at high Ca2q suggests that the bonding mechanism for part of the uptake could involve a salt bridge with the Ca2q cation acting as the bridge between the pesticide and the negatively charged organic matter or negatively charged clay particles. The salt bridge would be favoured at pH)7, and would overcome the problem of repulsion between the negatively charged 2,4-D and the negative humic substances or clay minerals. As the adsorption of 2,4-D is reduced with increase in FA concentration it is consistent to propose that the negatively charged FA added to the soil suspension competes with the negatively charged pesticide for adsorption sites involving the salt bridging. The effect of increased ionic strength on the adsorption of 2,4-D at 1:2 soil:solution ratio is more strongly evident when using a soil:solution ratio of 1:16 for the Broad and Denchworth soils (Fig. 3b). These two soils contain significantly
more clay-sized particles per gram of soil than the Sonning soils, and hence would be expected to have significantly more charged mineral surface area in solution. At the Naq higher ionic strength the thickness of the double layer around the charged soil particles would be reduced, enhancing the likelihood of the 2,4-D overcoming the electrostatic repulsion enough to be adsorbed via the physical adsorption processes mentioned above. 5.4. Soil mineralogy and adsorption characteristics For soil suspensions of equivalent mass, the quantity and nature of the smaller particle size fractions will contribute most to the surface area and hence largely control the overall adsorption characteristics of the soil. This fraction will consist mainly of clays and aluminium and iron oxides. With these considerations, the mineral components of Denchworth and Broad soils would be expected to adsorb the pesticides to the greatest extent based on surface area availability, with the Sonning soils adsorbing the least amount of pesticide. The results indicate that, similar to the discussion on soil organic matter content, the sorption of the pesticides is not simply a function of the clay content as the adsorption characteristics of the Sonning soils vary by as much as a factor of 5 (e.g. atrazine) when the difference in clay content is at the most a factor of 1.5. There are frequent reports in the literature that the adsorption of pesticides is not solely related to either the organic matter content or the clay content although it appears to be positively correlated with both of these soil components. Celis et al. (1999) concluded that for atrazine and simazine adsorption the extent of adsorption on individual soil components (montmorillonite, ferrihydrite and humic acid) did not relate directly to the adsorption of the pesticides on inter-associated combinations of two or more of these individual components. It was concluded that the adsorption sites for the pesticide may be reduced in number or change in character as a result of the association between soil components. Spark et al. (1997) found similar results for the sorption of heavy metals on individual minerals or humic acid compared with that on minerals coated with humic acid.
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5.5. Desorption characteristics The desorption of atrazine and isoproturon from Sonning I showed similarities (Fig. 4) despite the differences in adsorption characteristics (Table 3). In accordance with the findings of other workers, the desorption of these pesticides although significant, exhibited a hysteresis, which indicates that the interaction is not truly reversible (Clay et al., 1988; Piccolo et al., 1992). Harris and Warren (1964) reported that the desorption of atrazine from organic matter was less than that from clays. The use of increased concentrations of FA in the desorption solution had very little effect on the extent of desorption of these two pesticides in agreement with the findings of Clay et al. (1988) for atrazine. The desorption of 2,4-D was relatively small, compared to that of atrazine and isoproturon. As for atrazine and isoproturon, the higher concentrations of FA in the desorption solution had no effect on the characteristics of desorption for 2,4-D. As ionic bonding involving a salt bridge such as Ca2q would be expected to be reversible, it would appear that the 2,4-D was involved in a more specific interaction with the humic surface following the initial salt-bridge bonding process. Isaacson and Frink (1984) found similar results for 2,4-D with up to 90% of the pesticide being irreversibly adsorbed. Brusseau et al. (1989) suggested that the sorption non-equilibrium of 2,4-D may be a result of intra-organic matter diffusion as they found that rate constants showed an inverse relationship to organic matter content. There is considerable evidence that paraquat interacts strongly with soluble organic matter, as discussed above. The results in this work show that there is negligible desorption of paraquat from the soils in the absence or presence of increased content of soluble fulvic. This lack of desorption would result in the paraquat having limited interaction with the soluble fraction of organic matter in the soil. 6. Conclusion The adsorption of pesticides by soils is dominated by the interaction with the solid state of the
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soil. From the results presented here and those of others, the solid state interactions involves both the organic and inorganic fractions, the relative importance of which will depend on the relative abundance of these two fractions. The soluble organic matter in the soil solution had no effect on the sorption characteristics of atrazine, isoproturon and paraquat. This fraction did affect the sorption characteristics of 2,4-D but this is thought to be due to competitive effects of the organic matter with the sorption of the pesticide rather than due to the interaction of 2,4-D with the soluble fraction of the organic matter. Acknowledgments This work was funded by a research grant from the BBSRC, UK. We are grateful to John Cooke for technical assistance. References Avery, BW. Soils of the British Isles. CAB International, Wallingford, UK, 1990. Avery, BW., Bascomb, CL. Soil Survey Laboratory Methods. Harpenden, UK, 1982. Arienzo M, Buondonno A. Adsorption of paraquat by Terra Rossa soil and model soil aggregates. Toxicol Environ Chem 1993;39:193 –199. Bailey GW, White JL. Review of adsorption and desorption of organic pesticides by soil colloids with implications concerning pesticide bio-activity. J Agric Food Chem 1964;2:324 –332. Barriuso E, Baer U, Calvet R. Dissolved organic matter and adsorption-desorption of dimefuron, atrazine, and carbetamide by soils. J Environ Qual 1992;21:359 –367. Barriuso E, Koskinen WC. Incorporating nonextractable atrazine residues into soil size fractions as a function of time. Soil Sci Soc Am J 1996;60:150 –157. Baskaran S, Bolan NS, Rahman A, Tillman RW. Pesticide sorption by allophanic and non-allophanic soils of New Zealand. New Zealand J Agric Res 1996a;39:297 –310. Baskaran S, Bolan NS, Rahman A, Tillman RW. Effect of exogenous carbon on the sorption and movement of atrazine and 2,4-D by soils. Aust J Soil Res 1996b;34:609 –622. Beckett R, Le NP. The role of organic matter and ionic composition in determining the surface charge of suspended particles in natural waters. Colloid Surf 1989;44:35 –49. Bekbolet M, Yenigun O, Yucel I. Sorption studies of 2,4-D on selected soils. Water Air Soil Pollut 1999;111:75 –88. Borggaard OK, Streibig JC. Atrazine adsorption by some soil samples in relation to their constituents. Acta Agric Scan 1988;38:293 –301.
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