Environ. Sci. Technol. 1995, 29, 2647-2653
Influence of Dissolved Humic Acid and Ca-Montmorillinite Clay on Pesticide Extraction Efficiency from Water Using Salid=Phase Extraction Disks S C O T T A . S E N S E M A N , * T E R R Y L. LAVY, JOHN D. MATTICE, AND EDWARD E. GBUR The Altheimer Luboratory, Pesticide Residue Research, University of Arkansas, Fayetteville, Arkansas 72704
~
Intermittent rain can influence the sediment load in surface runoff from agricultural fields, thereby causing variability in amounts of sediment and dissolved organic matter (DOM) in the water that could adversely affect extraction efficiency and ultimately the method sensitivity of pesticide analyses in water monitoring studies. Therefore, a study was conducted to determine the effect of purified sediment components, Ca-montmorillinite clay and commercial humic acid, on extraction efficiency of 12 pesticides from water using solid-phase extraction (SPE) disks. Batches of water at pH 6.0 and 8.0 were prepared at an ionic strength of 3 x M. Individual water samples (250 mL) at each pH containing 2Opg L-I each pesticide were amended with all possible combinations of (a) commercial humic acid at either 0, 5, 10, or 25 mg L-I dissolved organic carbon (DOC) and (b) Camontmorillinite amounts of either 0, 0.01, 0.1, or 1 g. Samples were prefiltered to remove clay and then extracted using solid-phase extraction (SPE) disks. Pesticides eluted from disks were analyzed by gas chromatographylmass spectroscopy (GUMS). Pesticides within chemical families reacted similarly to treatments of pH, Ca-montmorillinite, and humic acid. The effects of Ca-montmorillinite and humic acid were generally pH-dependent and acted independently in affecting extraction efficiency. Lower recovery of most pesticides was observed at pH 8 when Ca-montmorillinite was 20.1 g and was attributed to greater dispersion of clay, increased surface area, and subsequent adsorption. Concentrations of DOC in humic acid had less effect on extraction efficiency when water was at pH 8 compared to water at pH 6, which was probably due to greater nonpolar interactions of pesticides to the chargeneutralized humic acid polymer.
0013-936W95/0929-2647$09.00/0
0 1995 American Chemical Society
Introduction Surface water monitoring and surface runoff studies continue to be important in providing assessments of potential environmental problems due to pesticide use. These studies typically involve the collection of large quantities of water samples. When these samples are collected, the amount of sediment and dissolved organic matter (DOM) cannot be controlled due to intermittent rainfall and can cause variability in volume and content of surface runoff containing these components. Nonpolar pesticides have been shown to interact with both sediment (1-4) and dissolved organic matter (5-8). These interactions could reduce extractable pesticides from aqueous solutions,adverselyaffectmethod sensitivity,and ultimately make it difficult to draw conclusions from the environmental assessment. Also, if trace pesticide concentrations are present in water potentially used for irrigation or mammalian consumption, it is important to have knowledge concerning the bioavailable pesticide fraction in the water. These studies would provide insight into differentiating between the pesticide amount in the solution versus the amount in the adsorbed phase. It is unclear whether the clay fraction of the sediment or the dissolved organic carbon (DOC)in the water is more important in adsorption of nonpolar pesticides. Lee et al. (3) suggested that additions of dissolved organic matter (DOM)to Na-montmorilliniteclay contributed to decreased napropamide sorption by competing for sorption sites in a clay slurry (3).If both sediment and DOM exist in water samples,uncertainty of results may be encountered due to variable adsorption from these two factors. A study involving the interaction of increased solution concentrations of DDT (l,l,l-trichloro-2,2-bis(4-chlorpheny1)ethane)with dissolved humic substancesindicated that, at pH 6, more DDT was bound to humic acid than at pH 9.2 (5).The difference in adsorption has been explained by a more hydrophobic form of the humic polymer partitioning the hydrophobic compound more effectively when negative charges are neutralized at pH 6 (5, 9, 10). Possible interferences due to varying levels of humic acid on extraction efficiency from water were evaluated using solid-phase extraction (SPE) cartridges containing octadecyl (C18) bonded silica (11). Significantly lower pesticide recovery from water containing humic acid was attributed to lower pesticide affinity to CIScolumns when associated with humic acid in suspension (11). SPE disks containing octadecyl (Cia) bonded silica have provided many analytical laboratories with reproducible extraction from water samples and is becoming a widely used analytical technique. This technique provides (a) reduced volume of potentially hazardous solvent used and disposed compared to liquidlliquid extraction (LLE)(121, (b) decreased sample preparation time and labor needed than LLE (121,and (c)increased stability of pesticides after filtration compared to pesticides stored in water (13, 14). SPE disks have been proven to be superior to SPE cartridges for extraction of analytes from water. SPE disks * Author to whom correspondence should be addressed at his present address: Department of Soil & Crop Sciences, Texas A&M University, College Station, TX 77843-2474; e-mail address:
[email protected].
VOL. 29, NO. 10, 1995 / ENVIRONMENTAL SCIENCE &TECHNOLOGY 12647
TABLE 1
Characteristics of Pesticides Analyzer water soil solubility half-life
pesticide alachlor atrazine cya nazi n e methyl parathion metolachlor metri buzin n orfl urazo n pendimethalin profenofos propanil simazine trifluralin
(maL-')
(4
240 33 170 60 530 1220 28 0.3 28 200 6.2 0.3
15 60 14 5 90 40 30 90
8 1
60 60
soil sorption, L
C
170 100 190 5100 200 60 700 5000
2000 149 130 8000
vapor pressure
(MPa) 1.9 x 3.9 x 2.1 x 2.0 x 4.2 ~1.3 x 2.7 x 1.2 x 1.2 x 5.3 x 2.9 x 1.5 x
10-11 10-13 10-9
10-9
lo-'* lo-*
a Data extracted from Wauchope et al. (20). These data represent values measured at 20-25 "C.
contain smaller silica particlesthat are more densely packed and more uniform (12). These particles enmeshed in a Teflon matrix eliminate bed channeling of silica particles, which is expected to cause lower recoveries when extracting with SPE cartridges (12). If extractionis more efficient with SPE disks, it is possible that extraction efficiency may not be affected as much by potential sediment components, such as Ca-montmorillinite and humic acid, as extraction efficiency of SPE cartridges. Therefore, a study was conducted to (a) observe the effect of Ca-montmorillinite clay and DOM from commercial humic acid on extraction efficiency of various pesticides and (b) determine any interactiveeffects from Ca-montmorillinite and humic acid within typical pH ranges and ionic strengths of surface water.
Experimental Section General Information. Twelve pesticides were included in the analysis representing the chemical families of dinitroanalines, triazines, acetanilides, organophosphates, chloroacetamides, and pyradazines (Table 1). Analytical standards ( > 98%purity)were used to prepare fortificationand standard solutions. These pesticides were chosen based on common occurrence in pesticide monitoring studies and their differences in chemical and physical behavior. The form of solid-phase extraction used was the 47-mm diameter Empore disk for environmental analysis (3M Industrialand Electronic Sector,New Products Department, St. Paul, MN; distributed by J. T. Baker Inc., Phillipsburg, NJ), Methanol, methylene chloride,and ethyl acetate used in the extraction were high-pressure liquid chromatography-grade solvents obtained from Fisher Scientific Company (Fairlawn,NJ). Preparation of Water Samples. Batches of 6 L of water fortified with KH2P04/NaOHbuffer solutionwere prepared at pH 6.0 and 8.0with an ionic strength of 3 x M. This ionic strength was derived for the water samples based on results of ionic concentrations from a surface water monitoring study previously done in the state of Arkansas (15). The concentrationsof KHaP04/NaOHbuffer solution to be added at the desired ionic strength and fixed pH of 6 and 8 were calculatedbyMINTEQA2/PRODEFA2software (16). ForpH6.0water, 150mLofO.l MKH2P04and24mL of 0.1 M NaOH were added to 6 L of deionized water; for 2648
ENVIRONMENTAL SCIENCE &TECHNOLOGY / VOL. 29, NO. 10,1995
pH 8.0,63mL of 0.1 M KH2P04 and 60 mL of 0.1 M NaOH were added to 6 L of deionizedwater. Four separate batches of water at both pH levels were prepared to provide four replications of water preparation at each pH. The pH was checked by pH meter after the addition of buffer and adjusted with no more than 5 drops of 0.1 M NaOH or HC1 from a Pasteur pipet. A humic acid solution with a dissolved organic carbon (DOC) concentration of 250 pg mL-l was prepared by dissolving 598 mg of the sodium salt of Aldrich humic acid (Aldrich Chemical Company, Inc., Milwaukee, WI) containing 42% carbon in 1 L of deionized water. Aliquots of 25,10, and 5 mL of this solution were removed and placed in the appropriate 250-mL volumetric flasks and brought to volume with either pH 6 or pH 8 water to create DOC concentrations of 25,10, and 5 pg mL-'. These concentrations of DOC were used according to relative DOC concentrations used and detected in reported work from the literature ( 5 1I,17). Also, water samples were prepared at each pH without addition of humic acid (0 mg L-' DOC). The water samples in the 250-mLvolumetricflasks were transferred to 500-mL Erlenmeyer flasks. Ca-montmorillinite (Clay Minerals Repository, Department of Geology, University of Missouri, Columbia,MO) in amounts of either 0, 0.01,O. 1,or 1g was then added to the water samples. The pH of each sample was measured after the addition of Camontmorillinite and humic acid to ensure that each sample was within 0.2 pH unit of the desired pH. The samples were covered and temporarily stored in a cold room at 4 "C overnight. The following day, the water samples amended with Ca-montmorillinite and humic acid were removed from the cold room and fortified with 1 mL of a 5-pg mL-' methanol fortification solution containing all pesticide analytes. This brought the concentration of all analytes in water to 20pg L-I. Blank quality-controlsamples received 1 mL of methanol and contained no pesticides. Fortified quality control samples included samples fortified at concentrations of 5 and 20 pg L-' for each pesticide. One fortified quality control sample of each concentration plus two blank samples were included with every batch of 16 samples. The samples were then placed on an orbitalshaker (Lab Line Instruments, Inc. Melrose Park, IL) for 24 h at 90 rpm to allow pesticides, Ca-montmorillinite, and humic acid to equilibrate. The water samples were removed from the orbitalshaker after 24 h and prefiltered through Whatman 47-mm glass microfiber filters GF/C (1.2pm particleretention) to remove clay particles. The GFlC filterswere held by stainless steel 47-mm pressure filter holders with 340-mL reservoirs (Millipore Products Division, Bedford, MA). The filtrate was caught in 250-mL Erlenmeyer flasks and set aside for pesticide extraction. Pesticide Extraction. A 47-mm extraction disk was placed on a sintered glass filter funnel apparatus and placed on one station of a six-stationvacuum manifold extraction manifold (3M Industrial and Consumer Sector, St. Paul, MN) attached to an active vacuum source to facilitateliquid filtration. A total of 10 mL of 1:l methylene chloride/ethyl acetate solventwas added to the filter funnel, and the solvent was drawn through the disk at approximately 2 mL s-l to remove any contaminants from the disk. Subsequently, air was drawn through for 1 min. A total of 10 mL of methanol was then added. As the solvent was drawn through, the vacuum was removed when a film of methanol
'
covered the disk. The filmprevented drying and subsequent slow filtration. Deionized water (10 mL) was added to the film of methanol and drawn through until a thin film of deionized water covered the disk the vacuum was again removed. The entire 250-mL water sample containing the pesticides was then added to the filter funnel and drawn through at approximately 25-30 mL min-l. After the water sample had been drawn through, the vacuum was left on for 5 min to allow the disk to dry. Borosilicate glass vials (20-mL capacity) were then placed in the base of the vacuum manifold to catch the eluate. The pesticides were eluted from the disks with four 5-mL portions of ethyl acetate. During each application of ethyl acetate, the vacuum was applied and removed quickly to allow some ethyl acetate to penetrate the entire thickness of the disk for an equilibration time of 2 min. The vacuum was then reapplied, and the remainder of the ethyl acetate was eluted into the glass vials. Anhydrous sodium sulfate (4 mL) was added to the vial to remove any excess water. The ethyl acetate was decanted into a calibrated test tube along with three rinses of ethyl acetate. The final volume was decreased to 2 mL of ethyl acetate by a stream of dry nitrogen while immersing the vials in a 30-35 "C water bath. Samples were vortexed twice at a slow speed to rinse the sides of the calibrated test tube. A 1.5-mLaliquot was transferred to a sample vial and sealed by Teflon septa and vial cap before analysis. Analytical Methodology. All pesticides in the samples were identified and quantified by a Varian 3400 gas chromatograph/mass spectrometer (GCIMS) equipped with a 0.25 mm i.d. x 30 m DB-5 column. The chromatograph temperature program employed was an initial oven temperature of 82 "C for 2.5 min increased to 300 "C at 14 "C min-l and held for 1min for a 19-minrun time. A 1-pL sample was injected at a rate of 0.2 ,uL s-l using a programmable injector. The initial temperature of the injector was 57 "C, held for 0.25 min, then increased to 260 "C at 180 "C min-I, and held for 2 min before returning to 57 "C. Mass spectrometer conditions included a manifold temperature of 220 "C, ionization time of 100 ms, and 0.75 s scan-'. Method of ionization was electron impact. Statistical Analysis. The experiment was analyzed as a split-plot with pH representing the main-plot effect while Ca-montmorilliniteamount and humic acid provided the subplot effects. Treatments were replicated four times. Means of percentage recovery of each pesticide from SPE disks were separated by Fisher's least significant difference (LSD)at a 5% level of significance ( I S ) . When interactions were significant between the main plot of pH and subplots of Ca-montmorillinite or humic acid, two LSD's were calculated. One LSD provided a critical difference for any two means between pH levels. The other provided a critical difference of any two means within pH level.
Results and Discussion Analytical Methodology. Mean percent recovery of eight quality control extractions for the 12 pesticides along with retention times are reported in Table 2. Trifluralin and pendimethalin recovery was 69% and 83%, respectively, but all other compounds exhibited mean recoveries above 90%.
Statistical Significance of Treatments. Sources of variation and associated P values for percentage recovery of pesticides from SPE disks are listed in Table 3. The main effect of pH and the three-way interaction of pH-Ca-
TABLE 2
Retention Times and Percent Recovery of QualiControl Samples of Anawes Fortified at 20 pg 1-l and Analyzed by GC/MS YO recovery. pesticide
retention time (min)
mean
standard error
alachlor atrazine cyanazine methyl parathioin metolachlor metribuzin norfiurazon pendi m et ha Iin profenofos propanil simazine trifluralin
13.4 12.1 13.9 13.2 14.0 13.2 16.5 14.5 15.4 13.1 12.0 11.5
91.9 93.6 101.8 94.3 91.7 98.1 92.3 83.3 99.1 100.6 100.3 69.1
4.4 4.8 5.3 5.2 4.3 5.4 6.1 4.7 5.7 3.8 3.5 4.4
a
Values obtained from eight extractions.
montmorillinite-humic acid were not significant for any of the pesticides analyzed. The Ca-montmorillinite-humic acid interaction was marginally significant for propanil; however, erratic percent recovery for propanil resulted in statistical differences but no consistent trends and, therefore,was omitted from interpretationof the data. According to the P values, statistical significance occurred for each pesticide for the interactions of either pH-Ca-montmorillinite, pH-humic acid, or both. Several conclusionscan be derived from this information. First, these dataindicate that clay content as Ca-montmorillinite and dissolved organic carbon from humic acid influenced the amount of pesticide recovered but both factors were pH-dependent. Secondly, since the Ca-montmorillinite-humic acid and pH-Ca-montmorillinite-humic acid interactionswere not significant with the exception of propanil, the pH-dependent effects of Ca-montmorilliniteand dissolved humic acid affected the pesticide extraction efficiency independently. The dinitroanilines and organophosphates studied were exempt from this scenario because P values displayed statisticalsigdicance for only one interaction. These data showed that pH did not alter the effects of increased humic acid on the recovery of the dinitroanilines nor did pH alter the effect of increased clay content on the recovery of the organophosphates. The P values provided more insight of the behavior of various chemical families due to the specific treatments (Table 3). Statistical significance (P < 0.05) of the same sources of variation was demonstrated within the dinitroaniline, triazine, acetanilide, and organophosphate chemicalfamilies. This indicated that multiple pesticides within a given chemical family reacted similarly to amendments of pH, Ca-montmorillinite,and humic acid in water samples that were extracted using SPE disks. Extraction Efffciency. Dinitmanilines. Due to the differences in response to the treatments between some of the chemical families, the results are grouped according to familiesthat responded similarlyto pH, Ca-montmorillinite, and humic acid. Differencesin the mean percent recovery of pesticides from SPE disks due to interactive effects of Ca-montmorillinitecontent in water and pH are shown in Table 4. Within both pH levels,percent recovery decreased as Ca-montmorillinite content in the water sample increased. For these compounds, a significant reduction in VOL. 29, NO. 10,1995 I ENVIRONMENTAL SCIENCE &TECHNOLOGY
2649
TABLE 3
Sources of Variation rad Associated Statistical Si5nificanee levels for Percentage Recovery of Pesticide Analytes from SPE Disks P dinitroanalined source
PH CM' HAg pH-CM pH-HA CM-HA pH-CM-HA
df c
TRd
PE
triazines
SI
AT
CY
ME
1
NSe
NS
NS
NS
NS
NS
3 3 3 3 9 9
0.0001 0.0001 0.0240
0.0001 0.0001 0.0369
0.0032
0.0257
0.0004
0.0004
NS NS NS
NS NS NS
45.73 14.99
50.51 13.10
mean square error
cv
NS
NS
NS
NS
0.0223 0.0453
0.0355 0.0124
0.0211 0.0416
0.0245 0.0135
NS NS
NS NS
NS NS
NS NS
75.95 10.34
82.18 9.89
81 .OO 9.62
82.85 10.18
P acetanilides source
PH CM HA pH-CM pH-HA CM-HA pH-CM-HA mean square error
cv
df
A1
organophosphates
MT
MP
PF
PRh
NO'
1
NS
NS
NS
NS
NS
NS
3 3 3 3 9 9
0.0003
0.0002
0.0016 0.0443
0.0001 0.0071
0.0004 0.0178 0.0170 0.0250 0.0426
0.0011
NS
NS
0.0180 0.0407
0.0282 0.0269
NS
NS
0.0257
0.0084
NS NS
NS NS
NS NS
NS NS
63.88 9.73
59.98 9.38
77.81 10.39
84.63 10.91
NS 0.0133 0.0307
NS
NS NS
74.53 9.77
62.94 9.09
* Results for which the reported P value was less than 0.05 indicate statistical significance at the 5% level. Herbicide family. df, degrees of freedom. Key: TR, trifluralin; PE, pendimethalin; NO, norflurazon; ME, metribuzin; MP, methyl parathion; PF, profenofos; CY, cyanazine; SI, simazine; AT, atrazine; AL, alachlor; MT, metolachlor; PR, propanil. e NS, reported Pvalue was greater than 0.05, therefore, not significant at the 5% level of significance. CM, Ca-montmorillinite clay. 0 HA, humic acid. PR, propanil is in the chloracetamide chemical family. NO, norflurazon is in the pyradazine chemical family.
'
percent recovery was exhibited when 0.1 g of Ca-montmorillinitewas added to water samples compared to when 0 g of Ca-montmorillinite was added. Even lower recovery occurred as Ca-montmorillinite amount was increased to 1 g. These data suggest that increased adsorption may have occurred as the pH and Ca-montmorilliniteincreased, causing lower recoveries as the Ca-montmorillinite was removed during prefiltering. This increase in adsorption is possibly due to a greater negative charge of the Camontmorillinite at the higher pH, causing an increase in the dispersion of the clay in solution (19). With an increase in dispersion, more surface area of the clay could be exposed, resulting in greater adsorption of the pesticide on the clay surface. Since the water solubility of the dinitroanilines is low compared to the other pesticides studied, these compounds probably interact more with nonpolar surfaces of clay than with water. An inverse relationship with recovery was also evident with the dinitroanilines as the concentration of DOC from humic acid increased, but the relationship was not pHdependent as found with the other pesticides; therefore, the mean recovery for both pHs was pooled and represented graphically (Figure 1). This suggests that the innate hydrophobic character of trifluralin and pendimethalin was probably the overriding factor in binding to hydrophobic sites of humic acid rather than adsorption enhancements of the humic acid polymer by changes in pH. An approximate 20%difference in percent recovery of the dinitroanilines was evident between quality-control samples (Table 2) and samples that had received 0 g of Ca-montmorillinite(Table 4). These samples were identical 2650
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 29, NO. 10,1995
except that the prefltering step was eliminated for qualitycontrol samples. It is probable that this discrepancy in dinitroanhe recovery of the water samples was due to these compounds adsorbing to the stainless steel reservoirs during prefiltering. This discrepancy should be taken into consideration when analyzing for these compounds in turbid water samples, realizing that a substantial reduction in recovery and adverse effects on method sensitivity could be caused by removal of the compounds during prefiltering. Perhaps using glass or porcelain prefiltering reservoirs would reduce the loss through adsorption of these compounds. Triazines, Acetanilides, Propanil, and Norflurazon. These pesticides showed similar results to dinitroanilines regarding percent recovery from SPE disks. All of these compounds displayed statisticallysignificant interactions for pH-Ca-montmorillinite and pH-humic acid (Table3). No differenceswere detected when comparing recoveries of these pesticides between pH levels of increasing Camontmorillinite amounts (Table 4). Also, no differences were detected between recoveries from treatments of increasing Ca-montmorillinite amounts when water was at pH 6. Differences in mean percent recovery were observed from data of pH 8 water as the amount of Camontmorillinite added to the water sample increased to 0.1 g; however, percent recovery of these pesticides was not statistically lower at pH 8 when the amount of clay added to the water sample was increased to 1 g. These data suggest that more available adsorption sites at pH 8 resulted from a more disperse clay suspension with more surface area as clay content increased to 0.1 g as discussed
TABLE 4
TABLE 5
Mean Percentage Recoveries of Pesticides from SPE Disks Demonstrating a Relationship to Ca=Montmorillinite Clay Content as a Function of PH
Mean Percentage Recoveries of Pesticides from Water using SPE Disks Showing a Response to DOC Concentration as a Function of pH
dinitroanilines' clay amount (9)
pH
6
0
8
0.01 0.1 1 0 0.01 0.1 1
LSD(0.05)bpHC LSD(O.O~),,H~
PE
TRb
Organophosphates'
triazines AT
SI
CY
(YO) (YO) (YO) (YO) (YO) 50 49 44 42 52 50 41 34
60 58 54 49 63 60 50 42
86 87 84 86 90 87 78 76
91 94 90 94 96 96 87 86
17.4 4.3
17.3 22.6 22.1 3.9 5.6 6.2
93 96 91 92 101 98 87 86
pH
ME (Oh)
90 94 88 90 95 94 83 81
23.5 23.3 5.7 6.2
triazines
DOC
MPb
PF
(mg L-l)
(YO)
(YO) (YO) (Oh) (YO) (%)
0
6
5 10 25
0
8
5 10 25 LSD(0.05)bpHC LSD(0.05)wp~d
AT
SI
CY
ME
89 92 84 81 87 81 82 85
85 94 81 78 89 83 83 83
89 91 84 82 86 80 81 84
22.5 5.3
26.5 5.9
20.5 25.1 21.5 25.0 5.2 6.7 5.3 6.5
96 98 89 86 94 89 89 94
98 99 91 89 96 90 91 95
94 96 88 85 91 85 87 91
acetanilides acetanilides
PH
clay amount (9) AL (YO)M T (YO) P R O (YO) NOf(%)
PH
6
0
8
0.01 0.1 1 0 0.01 0.1 1
LSD(O.05)bpH LSD(o.o5),p~
83 86 82 82 88 86 77 74
84 86 82 82 88 87 77 75
90 92 88 89 98 93 84 82
88 92 87 90 92 91 81 81
21.2 5.1
19.5 4.5
23.1 5.5
17.3 6.1
Chemical family. * Key: TR, trifluralin; PE, pendimethalin; AT, atrazine; SI, simazine; CY, cyanazine; ME, metribuzin; AL, alachlor; MT, metolachlor; PR, propanil; NO, norflurazon. LSD (0.05)b,n, least significant difference at 5% probability level calculated to compare percent recovery means of any two treatment combinations between pH levels. LSD (0.05)wp~, least significant difference at 5% probability level calculated to compare percent recovery means within pH levels. PR, propanil is in thechloracetamidechemical family. NO, norflurazon is in the pyradazine chemical family. a
0
5
10
25
LSD (0.05)
Concentration of Dissolved Organic Carbon (mg L1)
FIGURE 1. Mean percent recovery of the dinitroaniline herbicides from water as a function of dissolved organic carbon (DOC) from Aldrich humic acid using solid-phase extraction disks. Means were averaged across clay treatments and pH levels.
with the dinitroanilines. Increased adsorption did not occur when 10timesmore Ca-montmorillinite was added. These data suggest that the affinity of these compounds to the Ca-montmorillinite was not as great as with the dinitroanilines and can be supported by observing the differences
6
8
LSD(0.05)bp~ LSD(0.05)wp~
DOC (mg L-l) 0 5 10 25 0 5 10 25
A 1 (YO) M T (YO) PRe (YO) NOf(%)
85 88 81 79 84 79 81 82
86 88 81 79 85 79 80 83
90 93 85 81 94 87 87 89
91 95 86 85 89 84 84 87
19.1 4.7
19.6 4.5
22.0 5.3
19.7 6.4
a Chemicalfamily. Key: MP, methyl parathion; PF, profenofos;AT, atrazine; SI, simazine; CY, cyanazine; ME, metribuzin; AL, alachlor; MT, metolachlor; PR, propanil; NO, norflurazon. c LSD(0.05)bpH,least significant difference at 5% probability level calculated to compare percent recoverymeansof anytwo treatmentcombinations between pH levels. LSD(O.OB),.,,H, least significant difference at 5% probability level calculated to compare percent recovery means within pH levels. e PR, propanil is in the chloracetamide chemical family. NO, norflurazon is in the pyradazine chemical family.
in relative KO,values for soil adsorption (Table 1). Also, it was observed that total dispersion of 1 g of Ca-montmorillinite was not achieved by orbital shaking; therefore, an insignificantchange in exposed surface area available and sorptive sites probably resulted from the 10-fold increase in Ca-montmorillinite from 0.1 to 1 g. Lower affinity coupled with an insignificant change in sorptive sites resulted in little change in percent recovery as Camontmorillinite amounts increased. These dataalso suggest that ifthese pesticides were being monitored in water at pH 8, prefilteringa sample to remove sediments would not result in an appreciable loss of pesticide analytes under the following conditions: (a) the 250-mL water sample contains less than 1 g of sediment, (b)the ionic strength of the water is approximately3 x M, and (c) the clay in the sediment is predominantly Camontmorillinite . Differences in mean percent recovery due to the pHdependent effect of humic acid on the extraction efficiency from SPE disksis shownin Table 5. No statisticaldifferences between mean percent recoverieswere observed from any two treatment means of a particular pesticide between pH levels. Within pH 6, triazine recovery was unaffected by humic acid until the concentration of DOC reached 10 mg L-l. Similar results were found for the acetanilides, organophosphates, propanil, and norflurazon for water extracted at pH 6. VOL. 29. NO. 10, 1995 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
2651
At pH 8, no differences in mean percent recovery were detected for simazine, metribuzin, and noflurazon while statisticaldifferences inextraction efficiencyweremarginal foratrazine, cyanazine,alachlor, metolachlor. and propanil lJable51. ThesedatasuggestthatatalowerpHthecharge on the dissolved humic acid polymers may have been reduced, creating a more nonpolar environmentconducive to increased partitioning of nonpolar compounds. Other researchers have statedthat anincreaseinH+concentration or ionic strength makes dissolved humic polymers more hydrophobic by neutralizing negative charges (5). Therefore, it is reasonable that the more hydrophobic form of the polymer would bind hydrophobic compounds more effectively through association with uncharged portions of the humic polymer (5). These data also suggest that if a water sample at pH 8 contains humic acid with a DOC concentration up to 25 mg L-I then the extraction efficiency probably would not be significantly reduced. Results at pH 6 in this study correlate with pesticide recovery from earlier work by Johnson et al. (11)where some ofthe same pesticides were extracted using CISbonded silica cartridges. In their study, percent recovery of triiluralin, simazine, atrazine, methyl parathion, and alachlor was generally lower in samples containing humic acid than in pure water: however, the pHandionicstrengthoftheextractedwaterwerenotstated, making further inferences irrelevant. Organophosphates. The Pvalues for methyl parathion and profenofos indicated similar responses to pH, Camontmorillinite, and humic acid (Table 31. A significant response to Ca-montmorillinite and a significant pHdependent response to humic acid was responsible for differencesin percent recovery. Similar trends in percent recovery to the triazines and acetanilideswere shown from results of pH 6 amended water and humic acid (Table 5). No statistical differences were observed between any two means of percent recovery between pH levels. Significant reductions in percent recovery occurred as DOC increased in water within pH 6. Within pH 8, profenofos showed marginal significance with increasing DOC concentration from humic acid, indicatingthat extractionefficiencywould not he greatly affected hy humic acid. A significant response to increasing amounts of Camontmorillinite was evident and is presented graphically in Figure 2. It appeared that the Ca-montmorillinite amounts in the water sample had to he at least 0.1 g to demonstrate a significant reduction in percent recovery of the organophosphates and did not significantly decrease as the clay amount increased 10-fold to 1 g. This is the same response described earlier with Ca-montmorillinite amounts for the other pesticides.
Conclusions Pesticides within chemical families reacted similarly to treatments of pH, Ca-montmorillinite,and dissolved humic acid in water extraction using SPE disks. Effects from Camontmorillinite and humic acid on pesticide recovery from disks were pH-dependent for the triazines, acetanilides, propanil. and noflurazon. Statistics suggested that the pH-dependent effects of Ca-montmorillinite and humic acid acted independently for these pesticides. When pH interactions with Ca-montmorillinite were significant, increases in the Ca-montmorilliniteamount added to water at pH 6 did not significantlydecrease percent recovery. At pH 8, a significant decrease was observed as Ca-mont2652 m ENVIRONMENTAL SCIENCE &TECHNOLOGY I VOL. 29. NO. 10.1995
"
0
0.01
0.1
1
1 ~~~
~~E LSD (0.05)
Amount of Ca-montmorillinile added (9)
FIGUREL Mean percent recovery of organophosphate insecticides from water as a function of the amount of Ca-montmorillinite added to the fortified water sample prior to filtering through solid-phase emaction disks. Meanswere averaged across humic acidtreatments and pH levels.
morillinite increased. Increasing humic acid did not decrease percent recovery as much at pH 8 as at pH 6. Prefilteringwater samplesto remove Ca-montmorillinite caused lower recoveries of dinitroanilines at the high clay content and was further enhanced when water was at pH 8. It was postulated that greater dispersion of the clay occurred at the higher pH, resulting in more surface area and available sites for adsorption. Similar effects were noticedwiththeotherpesticides,but reductions in pesticide recovery from the SPE disks were not as great. Although it has been determined that typically humic acid and Ca-montmorillinite were pH-dependent and affected extraction effiency independently, a general comparison of the magnitude of their effects should he further explored. Depending on the pesticide, with the exception of the dinitroanilines, at least 74% recovery resulted regardless of pH or clay content. The lowest recoveries due to effects of dissolved humic acid were >78% regardless of DOC concentrationfrom humic acid or pH. These data suggest that additions of Ca-montmorillinite reduced pesticide recovery slightly more than humic acid amendments. This difference might he attributed to the removal ofpesticide associatedwiththe clay throughthe prefiltering step. Only a small percentageof the humic acid was removed by prefilteringas deduced by no observable change in the brown color of the filtrate. Also, a dark brown residue was observed on the SPE disk during and after extraction from humic acid, signifying that removal of humic acid from water occurred by partitioning onto the CISmaterial of the SPE disk. By partitioning a percentage of humic acid onto the disk, the portion of the pesticide that was hound to humic acid would have been subjected to the ethyl acetate disk rinses and possibly eluted, thereby minimizing the loss of the pesticide even though it had been in association with the humic acid polymer. Sincepesticide analysesofwater are generally concerned with concentrations in solution rather than the amounts adsorbed onto sediments, prefiltering samples to remove these sediments prior to extraction may be needed. Although the dinitroanilines were affected to a larger extent than the other pesticides in the study, the observation was made that approxhately20%reduction in percent recovery
from adsorption onto the stainless steel prefilteringreservoir and 20% loss due to Ca-montmorillinite or humic acid amendments could be expected for dinitroaniline compounds if prefiltered prior to extraction with SPE disks. This reduction is substantial enough to adversely affect method sensitivity of these compounds and demonstrates the importance of including field fortified samples when surface water samples are collected to account for losses due to sediment, dissolved organic carbon from humic acid, and pH differences in water that may affect pesticide analyses. It is not known what effect other types of clay or other sources of dissolved organic matter would have on extraction efficiency using SPE disks. Further research using kaolinite and fulvic acids may lead to more informationon potential interferences of these components on extraction efficiency and method sensitivity. An expanded database containing information such as adjustmentsin the variables of pH or ionic strength of water samples may lead to prediction and avoidance of problems with method sensitivity caused by sediment and dissolved organic carbon in water samples by using postsampling amendments to change these variables.
Acknowledgments The authors acknowledge the financial support of the 3M Corp. and the Cooperative State Research Service in this research endeavor.
(4) McDowell, L. L.; Willis, G. H.; Murphree, C. E.; Southwick, L. M.; Smith, S. 1.Environ. Qual, 1981, 10, 120-125. (5) Carter, C. W.; Suffet, I. H. Environ. Sci. Technol. 1982, 16, 735740. (6) Landrum, P. F.; Nihart, S. R.; Eadie, B. J.; Gardner, W. S. Environ. Sci. Technol. 1994, 18, 187-192. (7) McCarthy, J. F.; Jimenez, B. D. Environ. Sci. Technol. 1985, 19, 1072- 1076. (8) Pennington, K. L.; Harper, S. S.; Koskinen, W. C. WeedSci. 1991, 39, 667-672. (9) Chen, S.; Inskeep, W. P.; Williams, S. A.; Callis, P. R. Environ. Sci. Technol. 1994, 28, 1582-1588. (10) Chiou, C. T.; Malcolm, R. L.; Brinton, T. I.; Kile, D. E. Environ. Sci. Technol. 1986, 20, 502-508. (11) Johnson, W. E.; Fendinger, N. J.; Plimmer, J, R. Anal. Chem. 1991, 63, 1510-1513. (12) Hagen, D. F.; Markell, C. G.; Schmitt, G. A.; Blevins, D. D. Anal. Chim. Acta 1990,236, 157-164. (13) Senseman, S A . ; Lavy, T. L.; Mattice, J, D.; Myers, B. M.; Skulman, B. W. Environ. Sci. Technol. 1993, 27, 516-519. (14) Johnson, W. G.; Lavy,T. L.; Senseman, S. A. J. Environ. Qual. 1994, 23, 1027-1031. (15) Senseman, S . A.; Lay, T. L.; Mattice, J. D.; Skulman, B. W. Unpublished data, 1994. (16) Mison, J. D.; Brown, D. S.; Novo-Gradac, K. J, Mintequ2/
Prodefd: A geochemical assessment model for environmental systems, Version 3.0 User'smanual; Unitedstates Environmental Protection Agency, Office of Research Athens, GA, 1990. (17) Collins, M. R.; Amy, G. L.; Steelink, C. Environ. Sci. Technol. 1986, 20, 1028-1032. (18) SAS Institute Inc. SAS User's Guide: Statistics, Version 6,3rd ed.; SAS Institute Inc.: Cary, NC, 1988. (19) Brady, N. C. The Nature and Properties of Soils; Macmillan Publishing Company: New York, 1984; Chapter 5, pp 141-188. (20) Wauchope, R. D.; Buttler, T. M.; Hornsby, A. G.; Augustijn-
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literature Cited (1) Isensee, A. R. In Treatment and Disposal of Pesticide Wastes; Krueger, R. F., Seiber J. N., Eds.; ACS Symposium Series 259; American Chemical Society: Washington, DC, 1984; pp 261277. (2) Laird, D.A.; Yen, P. Y.; Koskinen,W. C.; Steinheimer, T. R; Dowdy, R. H. Environ. Sci. Technol. 1994, 28, 1054-1061. (3) Lee, D.; Farmer, W. J.; Aochi, Y. J. Environ. Qual. 1990,19,567573.
Received for review March 3, 1995. Revised manuscript receivedJune 11, 1995. AcceptedJune 11, 1995.@ ES950148W
@Abstractpublished in Advance ACS Abstracts, August 1, 1995.
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