Persistence and Phase Distribution in Sediment - ACS Publications

Chapter 10

Persistence and Phase Distribution in Sediment 1

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J. Gan , S. Bondarenko , and F. Spurlock

Downloaded by UNIV LAVAL on June 7, 2017 | http://pubs.acs.org Publication Date: August 19, 2008 | doi: 10.1021/bk-2008-0991.ch010

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Department of Environmental Sciences, University of California, 2258 Geology Building, Riverside, C A 92521 California Department of Pesticide Regulation, 1001 I Street, Sacramento, CA 95812 2

The pyrethroids are all strongly hydrophobic and therefore are found associated primarily with bed sediment after entry into water bodies. Therefore, their persistence and phase distribution in sediment greatly influences their fate and effects. This chapter provides an up-to-date review of data on the persistence and partitioning of pyrethroids in sediment. Information from recent studies is summarized, and half-lives (T ), K and K values are tabulated. Pyrethroids display differing persistence in sediment, with bifenthrin being more persistent than the other compounds. However, the bioavailable concentrations of pyrethroids decrease quickly in sediment due to the aging effect, with bioavailable T ≤2 months, suggesting diminishing toxicity over time. K s from earlier literature may have been underestimated due to incomplete phase separation. Recent studies using selective methods such as solid phase microextraction show that K s and K s of pyrethroids are in the 10 range, with K s a few times smaller than K . We also identify information gaps that may serve as topics for future research. 1/2

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1/2

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6

D0C

D0C

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© 2008 American Chemical Society Gan et al.; Synthetic Pyrethroids ACS Symposium Series; American Chemical Society: Washington, DC, 2008.

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Introduction The persistence and phase distribution (i.e., adsorption/desorption behavior) of a pesticide are two of the fundamental processes which control its fate and its effects in the environment. Researchers in earlier studies have extensively considered the degradation and persistence of various pyrethroid compounds in soil, and to a lesser degree, phase partitioning such as soil adsorption (1-5, 7). In comparison, only a limited number of studies have been reported for sediments. Because offsite movement such as runoff will most likely transport pyrethroid residues into the bed sediment via erosion of soil particles bearing residues and subsequent deposition, the potential ecotoxicological effects of pyrethroids are expected to depend closely on their persistence and phase distribution in sediment. In this chapter, we provide an up-to-date review of the data available for describing the persistence and degradation in sediment, the partition between the sediment and water phases as defined by K and K , and the partition between the dissolved organic carbon (DOC) and water phases as defined by ^DOC- To provide information for comparison, selected physical-chemical properties such as aqueous solubility and /C w, and soil persistence and sorption data are briefly summarized at the beginning. Information gaps that merit further research are highlighted. 6

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Basic Properties and Behavior in Soil Basic Properties The values for some basic properties of pyrethroids varied drastically in literature. For instance, the water solubility values for pyrethroids cited in the Pesticide Properties Manual (6) are often higher than those listed in the review by Laskowski (7) (Table I). Given the more recent date that the Laskowski review was published and the number of sources from which the values were compiled, it is likely that the data in the Laskowski review are generally more reliable than those in many earlier references. According to Table I, pyrethroids are essentially insoluble in water, and are strongly hydrophobic, as is apparent from their very large K w values. The extremely low solubility and strong hydrophobicity is a source of challenge to researchers due to analytical artifacts caused by the tendency for pyrethroids to sorb to surfaces of glassware, and the fact that imperfect separation between the octanol and water phases could lead to abnormally high aqueous phase concentrations and thus artificially low K s. This likely has had an impact on 0

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Gan et al.; Synthetic Pyrethroids ACS Symposium Series; American Chemical Society: Washington, DC, 2008.

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Table I. Literature solubility and / f w values for pyrethroids 0

Solubility (jug/L) Compound


0

Pesticide Laskowski Manual

Pesticide Manual

Bifenthrin

100

0.014

1*10

Cyfluthrin

2

2.3

-

X-cyhalothrin

5

5

10x10

Cypermethrin

10-200

4

4x10

Deltamethrin

2

0.2

2.7xl0

Esfenvalerate

300

6

1.6xl0

Fenpropathrin

330

10.3

lxlO

Permethrin

200

5.5

1.26xl0

0

a

/C w Laskowski Laskowski (measured) (calculated) 6

6.4xl0

6

5.97xl0 s

6

7.0xl0

s

6

7.2 x l O

6

,6.4 x l O

6

6.1xl0

6

6.54xl0

6

6.1xl0

6

5

4.53xl0

6

6.5xl0

6

6

5.62xl0

6

6.8x10

6

60xl0 6

6

6

5.7xl0

6

6

6.9xl0

6

6.9x10

b

Ref. (6); Ref. (7).

the data quality of some earlier measurements of the physical-chemical properties for pyrethroids, as discussed by Laskowski (7). Measuring the water solubility of pyrethroids can be technically challenging because of the potential for the formation of pyrethroid suspensions during saturation of the water phase when a stirring technique is used to achieve saturation by equilibration of water with excess chemicals. Laskowski (7) noted that approximately half the experiments used a column saturation technique that does not produce the suspension artifact, providing water solubility values usually lower than those achieved with stirring. According to Table I, water solubility values for pyrethroids are generally of the order of 0.001-0.01 ppm, or 1 to 10 ppb, with that of bifenthrin at 0.01 ppb. Therefore, the detection of pyrethroids in surface water or porewater samples at levels higher than the specific solubility is likely a result of either an artifact of analysis or enhanced "apparent"solubility caused by the combined analysis of solubolized pyrethroids with those bound to dissolved organic carbon (DOC) and fine particles. Difficulties similarly exist in accurately measuring the /C w for pyrethroids. As noted by Laskowski (7), because of the high hydrophobicity of pyrethroids, chemical concentrations in the octanol phase are many orders of magnitude higher than those in the water phase, making it difficult to prevent the contamination of the water phase. This could result in apparently high waterphase concentrations that do not reflect the true partitioning behavior and thus create an artificially low /C w- In Table 1, both measured /C w values are listed, as well as values calculated from molecular structure.,The calculated A^owS are all > 5.7 x 10 , further demonstrating that all pyrethroids are extremely hydrophobic and tend to bind strongly to organic matter in sediment. 0

0

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6


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Persistence and Sorption in Soil A number of different degradation studies were reported on pyrethroids using soils as media in earlier literature. When reviewing the data for the persistence of pyrethroids in soil, Laskowski (7) employed a rating procedure to account for the potential effects of the use of large quantities of organic solvents in soil spiking or the use of unrealistically high initial pesticide concentrations. Under aerobic conditions, the half-lives of pyrethroids ranged from a low of 3.3 d for tralomethrin to a high of 96.3 d for bifenthrin. Pyrethroids generally display half-lives under anaerobic conditions similar to those under aerobic conditions. Therefore, in aerobic or anaerobic soils, pyrethroids have short to moderate persistence, and variations between the different pyrethroids suggest that bifenthrin is relatively more persistent than the other pyrethroids. Measuring the sorption of pyrethroids to soil is prone to several complications that likely have contributed to the relatively low K s reported in earlier studies (Table II). As previously mentioned, pyrethroids tend to sorb to surfaces of glass or plastic centrifuge tubes and other containers. In addition, because K is the ratio of chemical concentration in soil (C ) over that in water (C ), incomplete phase separation may lead to an exaggerated C and consequently an artificially low K or K . One cause for incomplete separation, as observed in Lee et al. (8), includes the inability of centrifugation to exclude all fine particles and D O C from the aqueous phase. Because pyrethroids are preferentially sorbed on fine particles and D O C over large particles, and it is generally these smaller particles that remain suspended, a very small quantity of fine particles and D O C remaining in the supernatant after centrifugation could increase C by many times. Laskowski (7) used a rating procedure to account for the variation in the quality of sorption data. The A^>cS listed in Table II are from the analysis of 392 adsorption ( R v a l u e s . Data for esfenvalerate are absent from the table because sorption experiments were not available for this chemical at the time of the publication. The K values in Table II indicate that all the pyrethroids are sorbed exceptionally strongly to soil, with the exception of fenpropathrin, which has a K lower than the rest. Laskowski (7) further noted that, contrary to most findings, the expression of sorption as K in Table II had little or no impact on reducing the variability of sorption from one soil to another. This suggests that qualitative differences in soil O C may have greatly impacted the K and a single K c may not apply across different soil types for the same pyrethroid. It must be noted that although K s in the Laskowski review were generally higher than the earlier values, these data were obtained using conventional batch equilibration methods. As discussed later, some degree of continuing underestimation in those "higher quality" batch-determined A^ c is likely. oc

d

s

w

w

d

oc

w

oc

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0Ci

0

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s

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207 Table II. Literature AQC values for pyrethroid adsorption in soil PAN Pesticides Database"


Chemical

USDA-NRCS (WIN-PST) h

Bifenthrin

6,314

2.4x10

Cyfluthrin

8,930

1.0x10

X-cyhalothrin

2,341

1.8xl0

Cypermethrin

82

4.2xl0

Deltamethrin

6,291

1.9xl0

Esfenvalerate

-

5.3xl0

3

Fenpropathrin

-

5.0xl0

3

2.3x105

1.0x10

Permethrin b

Laskowskf

5

2.4x10

5

s

1.2xl0

5

5

3.3xl0

5

3

3.1x10

s

5

7.0xl0

5

-

s

0.4x10

s

2.8x10

s

c

Ref.(9); Ref(70); Ref.(7).

Degradation and Persistence in Sediment Knowledge of pyrethroid degradation and persistence in sediments is limited compared to soils. Earlier studies using mesocosms often stopped after measuring the dissipation of pyrethroids from the water column without further following their degradation and persistence in sediment. However, several recent studies have examined the degradation and persistence of pyrethroids in sediment in more depth. In a published study, we incubated field-contaminated sediments at room temperature and followed pesticide dissipation using exhaustive solvent extraction to measure the total sediment concentration. The sediments were collected from three different locations along a runoff drainage ditch at a commercial nursery in southern California (77). Due to continuous onsite use, the sediments contained elevated levels of bifenthrin and permethrin. The sediment samples were incubated under either flooded aerobic or flooded anaerobic conditions. Pesticide dissipation over time was fitted to a first-order decay model to estimate the first-order rate constant k (d' ) and half-life (T ) (Table III). Under aerobic conditions, noticeable differences in persistence were observed between the different pesticides and all dissipation rates were slower than in soil. Bifenthrin exhibited similar persistence in the different sediments, with T ranging 428-483 d, or 12-16 months. Degradation of permethrin isomers under the same conditions was markedly faster than for bifenthrin, with a T\/2 of 98-142 d (3-4.7 months) for c/s-permethrin and 60-312 d (2-10 months) for frafts-permethrin. Therefore, under aerobic conditions, while permethrin 1

U2

m


208 showed intermediate persistence in the sediments at 20 °C, bifenthrin was much more persistent. The degradation of bifenthrin was slightly enhanced under anaerobic conditions when compared to the aerobic treatments, with the T ranging 251498 d (8-16 months) in the same sediments. Degradation of c/s-permethrin was inhibited under anaerobic conditions when compared to the aerobic treatments, with the T extended from 98-142 d (or 3-4.7 months) to 209-380 d (or 7-13 months) under anaerobic conditions. Therefore, the oxidation state of sediment may affect the persistence of pyrethroids in sediment. Overall, the selected pyrethroids exhibited intermediate to long persistence in sediment, and bifenthrin was apparently more persistent than permethrin (11). m


m

1

Table III. First-order rate constant k (d ) and half-life T (d) for degradation of bifenthrin and permethrin isomers in sediments under aerobic conditions m

Sediment

3

Bifenthrin k

104M 166 M 210 M

0.0016 0.0016 0.0014

428 436 483

104 M 166 M 210 M

0.0014 0.0028 0.0025

498 251 280

cis-Permethrin k Tm Aerobic 0.0049 142 137 0.0051 0.0071 98 Anaerobic 209 0.0033 0.0028 245 0.0018 380

trans-permethrin k T, l7

0.0022 0.0031 0.0116

312 223 60

0.0025 0.0043 0.0040

276 160 175

Sampled at different distances along a drainage channel.

In a more recent study (unpublished data), we spiked four pyrethroids (bifenthrin, fenpropathrin, cyfluthrin and lambda-cyhalothrin) into two sediments and incubated the spiked sediments at room temperature under flooded aerobic conditions. One sediment was collected from San Diego Creek (SDC) in southern California and contained O C at 1.4%. The other sediment was sampled from a pond in Black Mountains (BM) in Paso Robles (central California) and contained O C at 5.0%. Pesticide dissipation in the sediment over time was fitted to a first-order decay model to estimate k and T . The selected pyrethroids showed differential degradation rates under the same conditions, with bifenthrin being the most persistent in both sediments, and cyfluthrin being relatively the least persistent (Table IV). The T values of bifenthrin ranged from 11 to 21 months, while with the exception of cyfluthrin in SDC sediment (T = 1 month), those of the other pyrethroids were mostly 3 to 5 months (Table IV). The m

m

m


209 persistence of bifenthrin observed in this study was in close agreement with that found in the previous degradation study using field-aged sediments. Therefore, although more pyrethroid compounds need to be included in future studies, bifenthrin appears to be one of the most persistent members of the pyrethroid family, which may partly contribute to its more frequent detections than the other pyrethroids in stream sediments. However, other pyrethroids have moderate and sometimes long persistence, which, along with other factors such as use patterns, may explain their presence in sediment.

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Table IV. First-order rate constants ( d ) and half lives (d) for dissipation of total chemical concentration (A, t ) and rapidly desorbing concentration (k\ Txii) of pyrethroid compounds in sediments under aerobic conditions


m

Compound Bifenthrin Cyfluthrin Fenpropathrin Cyhalothrin

1.10 1.76 7.76 7.29

Bifenthrin Cyfluthrin Fenpropathrin Cyhalothrin

1.21 2.88 1.90 1.85

SDC k * 10" x 10x 10" x 10k' x 10 x IO" x IO" x IO' 3

2

3

3

-2

2

2

2

BM T,n 629 30 89 95 Tin 56 24 36 37

k 2.07 5.76 4.57 4.42

3

x 10" x 10'

3

x 10'

3

x 10°

k' 1.10 1.38 1.26 1.25

x IO

-2

x 10' x IO

2

-2

x 10'

2

Tm 335 120 152 157 Tm 63 50 55 55

To understand the role of microbial degradation in pyrethroid degradation, we isolated a large number of bacteria strains capable of degrading bifenthrin and permethrin from field-contaminated sediments (12). In solution media, the selected bacteria strains were able to effectively degrade both bifenthrin and permethrin, with the T ranging from 1.3 to 5.5 d for bifenthrin, and from 1.5 to 3.3 d for permethrin isomers. However, we further observed that in the presence of sediment, the ability of the same bacteria for degrading bifenthrin or permethrin greatly decreased, and the inhibition was attributed to the strong adsorption of these compounds to the sediment phase. Therefore, even though microbial degraders may be ubiquitous in sediment, the persistence of pyrethroids in sediment can be prolonged due to their strong affinity for the solid phase and consequentially reduced bioavailability. V2

Information Gaps From the current state of knowledge on the degradation and persistence of pyrethroids in sediment, the following gaps exist. First, knowledge on pyrethroid



210 persistence in sediment is incomplete and readily comparable half-life values are not available for all pyrethroids. In addition, different studies used different sediments or incubation conditions, which makes the comparison of parameters such as 7^1/2 difficult. Therefore, it is necessary to evaluate the persistence of most or all pyrethroids under the same conditions while using the same sediments. This may be achieved by spiking low levels of mixtures of all pyrethroids into the same sediment. Likewise the relationship between % C in sediments and pyrethroid biodegradation should be investigated, since bioavailability to microbial degradation may be similar to bioavailability to other sediment organisms such as benthic invertebrates. Information from such a study will provide valuable information on the relative persistence of the different pyrethroid compounds, which will help to predict which pyrethroids will likely appear and accumulate in sediment. The second noticeable gap is the lack of knowledge on the degradation and persistence of pyrethroids in urban compartments where the application of pyrethroid products first occurs. For instance, although the general perception is that application of pesticides around houses or on lawns contribute to pesticide runoff in urban areas, as pesticides from such uses may deposit onto impervious concrete surfaces and be subject to runoff, there is little data to validate this assumption. A study is needed to understand the dissipation and persistence of pyrethroids on concrete surfaces as a function of seasonality, application methods and formulations. Such knowledge will not only improve our understanding of how pyrethroids and other pesticides move from residential areas to urban streams, but may also reveal options that can be useful for reducing runoff-facilitated transport. Another important topic is the need to distinguish the persistence of pyrethroids as the total chemical concentration from its persistence as the bioavailable concentration. As will be discussed in more detail in the following section, the strong hydrophobicity of pyrethroids accentuates the importance of bioavailability. That is, the persistence of pyrethroids in sediment is more appropriately expressed in terms of bioavailable concentration, and which will change as the sediment-bound materials age. A few methods are available for measuring bioavailable concentrations of hydrophobic compounds in sediments, including sequential extractions with Tenax (75), and use of solid phase microextraction (SPME) (14). In a recent study (unpublished data), we used a sequential Tenax extraction procedure to measure the rapid desorption concentration (C id) of four pyrethroids (bifenthrin, fenpropathrin, cyfluthrin and lambda-cyhalothrin) in two sediments, with C id representing the bioavailable concentration for hydrophobic. compounds, as described by Hulscher et al. (75). We observed that C decreased more rapidly than the total chemical concentration as seen in the above degradation studies. Table IV shows that if the half-lives (Tm) were calculated for the decline in C they were shortened to

Persistence and Phase Distribution in Sediment - ACS Publications

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