Posselt, H . S., Anderson, F. J., Weber, W. J., Jr., Enuiron. Sci. Technol., 2, 1087 (1968a). Posselt, H. S., Reidies, A. H., Weber, W. J., Jr., J . Amer. Water Works Ass., 60,48 (1968b). Posselt, H . S., Reidies, A. H., Weber W. J., Jr., ibid., 1968c, p 1366. Skogg, D. A,, West, D. M., “Fundamental of Analytical Chemistry,”Holt, Rinehart and Winston, N.Y., 437 (1963). Stumm, W., Huang, C. P., Jenkins, S. R., Croat. Chem. Acta, 42, 223 (1970)
Stumm, W., O’Melia, C . R., J . Amer. Water Works Ass., 60, 515 (1968). Van Lier, J. A,, deBruyn, P. L., Overbeck, J. T. G., J . Phys. Chem., 64,1675 (1960).
Received for reuieu, January 19, 1972. Accepted Nouember 1, 1972. This work u’as supported by the C‘S. Public Health Service Research Grant W P 00098. Presented a t the Division of Water, Air, and Waste Chemistry, I61st Meeting, A C S , Los Angeles, Calif March 1971.
‘
Persistence and Reactions of 4 ~ - ~ a c oicdAcid y ~ in Soils Edwin A, Woolson’ and Philip C. Kearney Agricultural Environment Quality Institute, Agricultural Research Center, Agricultural Research Service, U.S. Department of Agriculture, Beltsville. Md. 20705
Carbon-14-labeled cacodylic acid (hydroxydimethylarsine oxide) was prepared by reacting l4C-methyl iodide with methyl dichloroarsine. Concentrations of 1, 10, and 100 ppm of cacodylic acid were established in three soils of varying iron and aluminum content. At 2, 4, 8, 16, 24, and 32 weeks, soils were analyzed for 14C and total arsenic in the water-soluble (ws), calcium (Ca), iron (Fe), and aluminum (Al) fractions. Initially, cacodylic acid was distributed in the following fractions: ws >> A1 > Fe > Ca. After 32 weeks, the distribution was ws > A1 > Fe > Ca. In contrast, inorganic arsenate ( 5 + ) was largely present in the Fe and A1 fractions. Cacodylic acid persistence was a function of soil type and after 32 weeks the following amounts of 14C were recovered in each soil type by combustion: Christiana (23%), Hagerstown (%TO), Lakeland (62%). A decrease in both total 14C and total arsenic occurred in all soils with time. A pungent garlic odor was detected in soils receiving 100 ppm, suggesting the production of a volatile alkyl arsine. The loss of arsenic suggests that one route of cacodylic acid loss from aerobic and anaerobic soils is by alkyl arsine volatility. Degradation under aerobic conditions also occurred by cleavage of the C-As bond, presumably yielding COz and A s O ~ ~ - . This degradation is presumably due to microbiological action.
Cacodylic acid (CA), (hydroxydimethylarsine oxide) is a nonselective, postemergent, foliar contact herbicide. The other important organic arsenical herbicides are disodium methanearsonate (DSMA) and monosodium methanearsonate (MSMA). Several studies have been conducted with MSMA, including persistence and metabolism in soils (Dickens and Hiltbold, 1967; Von Endt et al., 1968), phytotoxicity to plants (Schweizer, 1967; Sachs and Michael, 1971), and metabolism in plants (Duble et al., 1968; Sckerl and Frans, 1969). Metabolism of CA in plants was examined by Sachs and Michael (1971). Inorganic arsenate (Jacobs et al., 1970b; Woolson et al., 1971a,b) has been characterized by different extractants and designated as water-soluble-, Fe-, Al-, and Ca-bound fractions. Arsenic in the fractions may not come entirely from the designated forms. Johnson and Hiltbold (1969) T o whom correspondence should be addressed.
extracted soils with nine successive extracting solutions to characterize arsenic residues from DSMA. Comparatively little is known about the persistence and metabolism of CA in soils. The object of this study was to examine the chemical distribution of cacodylic acid into water-soluble (ws), iron (Fe), aluminum (Al), and calcium (Ca) fractions in three soils, under aerobic and anaerobic conditions and to determine the persistence of CA in these soils. The mechanisms by which CA was transformed in soils were examined.
Methods and Materials 14C-Cacodylic Acid Synthesis a n d Purification. Methyldichloroarsine (CH3AsC12) was prepared by reacting an HCl solution of methylarsonic acid with KI and SOz. The purified methyldichloroarsine (bp = 132°C/760 mm) was added to 100 mg NaOH dissolved in several drops of water and contained in a 16 cm X 5 cm i.d. thick-walled tube sealed a t one end and restricted at the other. 14C-methyl iodide (0.25 mCi) was dissolved in 0.5 ml cold Et20 and transferred to the reaction vessel after 0.15 ml of CH3AsC12 was added. The tube was sealed and heated at 70-75°C for 24 hr. The CA was purified by tlc on cellulose plates developed in MeOH:NH40H (8:2). The CA had a specific activity of 2.7 mCi/mM. Soil Studies. Three soils (50 grams), Lakeland loamy sand, Hagerstown silty clay loam, and Christiana clay loam, were treated with 0.2 FCi 14C-CA and sufficient unlabeled CA to achieve final concentrations of 1, 10, and 100 ppm. The CA was added in MeOH and the soils thoroughly stirred after solvent evaporation. The soils were contained in 50-ml covered beakers. The soils in three replications were brought to 75% of field capacity and incubated at 25°C for 32 weeks. Grab samples (0.5 gram dry wt) were taken periodically and analyzed for the chemical distribution of CA into ws, Fe, Al, and Ca fractions (Petersen and Corey, 1966; Woolson et al., 1971a) by I4C-liquid scintillation analysis. The extracts from the 100-ppm CA treatment were analyzed for arsenic (As). The extracting solutions are described in Table 11. One-gram samples were taken periodically for total As analysis (Woolson et al., 1971b) and 0.1-gram samples were combusted for total 14C content. W02 was trapped in 2-methoxyethanolmonoethanol amine and diluted with PPO and POPOP dissolved in toluene. Volume 7, Number 1, January 1973
47
Table I. Physical and Chemical Properties of Experimental Soils
Hagerstown silty clay loam Lakeland loamy sand Christiana clay loam
Water, field capacity,
Clay C0.002mm,
Organic matter,
PH
%
%
%
5.5
29.1
30.0
6.2
8.7
4.4 19.7 aOxalate extractable (Schwertmann, 1964) b l N NaOH extractable (Yuan, 1959; Yuan and Fiskell 1959). C0.5NNaC2H302extractable (pH = 8.2) (Woolson et al., 1970)
AI
2.50
933
10.5
0.90
24.4
0.99
-..I‘\
AI.CI
--
I
ok---0 2
4
6
6 ’
S0L:CA
I _ -
I
12
16
24
32
WEEKS
Figure 1. Chemical distribution and disappearance of 14C-CA from Lakeland sandy loam (average of 1 , 10, and 100 ppm CA)
TOTAL E 0
N 0 0 \
In t
z
3
0 U
0
2
4
6
6
12
16 WEEKS
24
32
Figure 2. Chemical distribution and disappearance of 14C-CA from Hagerstown silty clay loam (average of 1, 10, and 100 ppm CA)
-:“E;;
hl
?
140,
120
-----.---
\,. --\.
TOTAL C-14 SUM OF FRACTIONS WATER S0L:CA 1I.U FE.CA
WEEKS
Figure 3. Chemical distribution and disappearance of 14C-CA from Christiana clay loam (average of 1, 10, and 100 p p m CA) 48
Environmental Science & Technology
Total As,
Fea
C.14 - TOTAL SUM OF FRACTIONS WATER
Available cations, me/100 g CaC
PPm
6.3
6.0
4.5
135
1.9
0.5
1.2
1402
2.4
10.5
3.5
A second set of soils was treated identically but was flooded to maintain 1in. of water above the soil surface. COZ Evolution Studies. A 10-gram sample of Christiana clay loam from the previous experiment and a fresh 10-gram sample were treated with 0.05 pCi 1%-CA and 0.1 gram of unlabeled CA. The adapted soil had 23% more I4C (residual 14C) present than the fresh soil. The soils, in two replications, were placed in biometer flasks and incubated at 25°C. W 0 2was trapped in 10 ml of 0.1N KOH and sampled at three- to seven-day intervals. A 1-ml sample was taken for 14C analysis and was added to a watermiscible scintillation solution containing PPO, POPOP, naphthalene, 2-ethoxyethanol, and 1,4-dioxane. Percent of l4CO2 evolved was based on activity in each flask. Occasionally, the remaining 9 ml was digested with “03, HC104, and and analyzed for As (Woolson et al., 1971b). Properties of the three soils used in this experiment are given in Table I.
Results and Discussion Chemical Distribution. Changes in chemical distribution of CA in each soil are presented in Figures 1-3. Analysis is based on 14C solubilized from the soil by each extracting solution. The separations are somewhat empirical since some redistribution of arsenic among various chemical forms occurs during the successive extractions (Jacobs et al., 1970a). Metabolized 14C could also be included in any extract. The figures are averaged over all three concentrations since there was little difference between treatments at the end of the experiment. However, until the 16-week sampling time, all soils had the highest percentage of ws-CA at the 100-ppm treatment. After 16 weeks, only the Lakeland and Hagerstown soils had higher ws-CA levels at the 100-ppm level than at 1 or 10 ppm. The ws-CA in the Lakeland soil (Figure 1) decreased for the first eight weeks and then remained unchanged. The A1-CA increased with time reaching a maximum in 8-12 weeks and then decreased. The Fe-CA gradually increased over the course of the experiment although it represented less than 5% of the total 14C-CA (62%) present at the end of the experiment. Little Ca-CA was detected. The extractable 14C (sum of fractions) was about 15% less than the combusted W O Z . The Hagerstown soil (Figure 2), with higher available Fe and A1 levels, bound more CA than the Lakeland soil. As a result, there was less ws-CA in the Hagerstown soil than in the Lakeland soil, particularly after the 12-week incubation period. The ws-CA continued to decrease in the Hagerstown soil after the ws-CA in the Lakeland soil reached an equilibrium level. The A1-CA reached a maximum around eight weeks and then decreased. The Fe-CA increased over the course of the experiment and represented about 5% of the total 14C-CA (53%). Again, little Ca-CA was detected. The extractable 14C-CA was only
42% of the total 14C02 obtained by combustion. This would indicate that CA is held more tightly by the Hagerstown soil than by the Lakeland soil. The greater clay and available Fe and A1 content of the former could account for this difference. The ws-CA in the Christiana clay loam (Figure 3) decreased most rapidly of the soils studied and reached a nondetectable level a t 24 weeks. The A1-CA reached a maximum around six weeks and then decreased to a nondetectable level at 24 weeks, while the Fe-CA increased slowly during the course of the experiment. The difference between extractable and total 14C-CA is not as great as it was in the Hagerstown soil, but the total 14C-CA remaining was less (23%). The rate of application had no appreciable effect on disappearance of CA. However, the rate of degradation or disappearance of 14C-CA was soil dependent. About 23% of the total 14C was left in the Christiana, 53% in the Hagerstown soil, and 62% in the Lakeland soil after the 32-week incubation period. Since the total 14C-CA content generally paralleled the disappearance of ws-CA, the rate of degradation appeared to be a function of the ws-CA concentration and consequently of the amount of CA available for microbial action. CA in the insoluble A1-CA form appeared to return to solution since as the ws-CA level decreased the AI-CA level also decreased. This is indicative of an equilibrium between adsorbed and soluble CA. The Fe-CA did not appear to be in equilibrium since it continued to increase during the incubation period. However, the amount of CA present in the Fe form may have been too small to observe the equilibrium shifts which might have occurred. The difference in distribution of CA and inorganic arsenate is striking. Inorganic arsenate reacts with Fe, Al, Ca, or Mg salts to form insoluble arsenates Mx(H2As04),. Arsenate may also react with hydrous Fe and A1 oxides which coat the soil particles. In a previous study, As present in the Fe-As fraction was greater than that in the AI-As fraction of the Hagerstown soil (Woolson, 1972), and accounted for about 60% of the As applied a t 100 ppm in the soil. The A1-As fraction was predominant in the Lakeland soil and also accounted for about 60% of the applied As a t the 100-ppm As level. The ws-As was the smallest of the three fractions except a t very high application rates (1667 ppm As). In contrast, the abundance of CA fractions is ws-CA > AI-CA > Fe-CA. This means
that CA is not bound as readily in the soil as inorganic As and is more likely to be leached. Metabolism of CA. The chemical distribution of total extractable As and extractable 14C a t 100 ppm CA treatment is presented in Table 11. This comparison was made to determine whether As and 14C ratios were the same in each fraction as in the original sample or if As from metabolized CA was present. Amounts of As and CA present as ws-As and ws-14C-CA appeared to he about the same and decreased with time. However, the A1-As and Fe-As were significantly greater than A1-CA and Fe-CA. The difference between the two became larger with increased incubation time indicating an increase in CA metabolism. Since the ratio of As to 14C increased, the products of metabolism must be volatile compounds containing no As and inorganic As, probably 14CO2 and A s O ~ ~ Ap-. proximately 31% of the applied 14C and 41% of the applied As were recovered from all treated soils in the extractable forms after a 32-week incubation period. Part of the remaining 59% of the As not extracted is found in a nonextractable form (Figures 1-3). The average difference between total 14C and the sum of extracted fractions is 20%. The fate of CA under anaerobic conditions was also investigated. A comparison of both 14C-CA and As in chemical fractions of soils held under aerobic and anaerobic conditions is presented in Table 111. The data are presented as percent of total residue (14C or As) remaining. A higher percentage of ws-CA was present in all flooded soils. Water apparently prevented fixation of some l4CCA in the soil. The amounts of Fe- and Ca-CA or Fe- and Ca-As appeared to be independent of moisture content while the formation of AI-CA or A1-As was inhibited under flooded conditions. Since A1-As is more soluble than Fe-As, this finding was not too surprising. The low ws- and high Ca-CA values for the Christiana soil were a result of very little 14C-CA remaining in this soil (Table IV). Only 9% of the 14C remained after 24 weeks and most of this residue was present in the A1 and Ca fractions. The other soils contained larger amounts of 14C and averaged 24% in all aerobic soils. Under anaerobic conditions, 39% of the 14C was left after 24 weeks. The same percent of As remained in the flooded soils indicating similar losses of As and 1%. It is possible that loss of a volatile derivative of As might account for this discrepancy. A garlic-like odor was generally prevalent over the
Table I I . Chemical Distribution of As and 14C-CA in Soil Treated with 100 ppm CA at Various Sampling Periodsa
Table I l l . 14C-CA and As Distribution in Three Soils Treated with 100 ppm CA After a 24-Week Incubation Period Held at Two Moisture Regimesa
Weeks incubation Chemical fractionb
6
8
12
16
Lakeland 24
Christiana
Average
a
a
an
49 3a 7 6
30 34 32 4
50 29 16 5
a2 87 64 72 3 74 4 3 Fe-CA a io 0 5 AI-CA 11 5 20 1 0 55 18 Ca-CA 3 4 9 8 42 4 “Av of three replications. ’a = 75% field capacity moisture; an = flooded soil.
49 4 29 18
7a 6 11 5
32
ab
an
46 34 16 30 2 16 0 4
45 35 22 27 2 14 1 1
36 32 16 19 3 22 1 0
27 22 22 27 5 14 1 1
a
an
an
Yo of residue remaining
YOof applied ws-14C-CA WS-AS Al-14C-CA AI-AS Fe-14C-CA Fe-As Ca-14C-CA Ca-As
Hagerstown
16 17 1 28 5 14 2 2
19 11 5 34 5 27 2 4
sum 14C-CA 64 70 56 55 24 31 sum As a4 77 73 64 61 76 aAveraged over three soils and three replications. ’WS, AI. Fe. and Ca forms are extracted in N NH4CI, 0 . 5 N NH4F. 0 . l N NaOH, and 0.5N H2S04, respectively (Petersen and Corey, 1966; Woolson et al., 1971a).
As WS-AS Fe-As AI-AS Ca-As
51 2a 19 1
58 26 8 7
14 36 43 6
42 24 32 2
24 38 33 4 14C-CA
WS-CA
Volume 7, Number 1, January 1973
49
14, Table I V . Total Residual As and 14C-CA After 24 Weeks Incubation in Aerobic and Anaerobic Soilsb 12' Average Lakeland Hagerstown Christiana ~a an a an 10 a0 an a an
/
% of applied remaining
As
60
14C-CA
37
30 26
71 27
47 43
65
% of
applied b s t as
9
41 48
65 24
39 39
Volatile organoarsenical
40 70 29 53 35 59 35 61 A s O ~ ~ - 23 4 44 4 56 0 41 0 OAv of three replications, includes both extractable and nonextractable As and 14C-CA treated at 100 p p m CA. ba = 75% field capacity moisture; an = flooded soil.
M6 4
t
b 0
20
40
flooded soils. This may be ascribed to the evolution of dimethyl arsine or other volatile organo-arsenical compounds. Methylated and oxidized derivatives of dimethyl arsine are known (Dehn and Wilcox, 1906). Among these are partially and completely demethylated compounds, [(CH~AS)~*AS As4, ~ O As2031, ~, as well as a dimer [(CH3)2As12, and oxygenated products, [CH3)2As]20 and (CH3)2AsOOH (cacodylic acid). The aerobic soils contained 65% of the As initially applied (100 ppm) while the anaerobic soils contained only 39%. The loss of As (35%) from the aerobic soils can only be through volatilization as AsH3, CH3AsH2, (CH3)2AsH, (CH3)3As, or some other volatile organo-arsenic compound. In addition to the loss of As, CA is metabolized since more 14C is lost than As under aerobic conditions. Thus, the loss of 76% of 14C-CA can be divided into two portions, the 35% lost through volatilization and 41% through metabolism, probably W02 and A s O ~ ~ Since -. 39% of both 14C and As remained in the flooded soil after 24 weeks, the formation of 14C02 and AsO43- was not significant under anaerobic conditions, and loss was by formation of a volatile organo-arsenic compound. Microbial Degradation. Figure 4 reveals that 14C02 was given off by the W-CA-treated soils. Further, since there was a lag phase in W02 generation, the degradation was probably microbiological. The soil, which had received CA previously and presumably had an adapted microbiological population, metabolized the second CA addition much more readily than fresh soil and released about 13% of the 14C after 98 days. The fact that the fresh soil released only 2% of the 14C in 98 days may indicate that the microbial population was indeed low or that it may become adapted to CA with time. In any event, the degradation of CA appears to be slow. The KOH trapping solution was assayed for As in case the volatile organo-arsenical compound was trapped in the KOH solution instead of W02.The high rate was used to allow for detection of As if the rate of volatilization was very slow. Since 0.002% of the CA applied could be detected in any sampling period in the KOH, the lack of As indicates that 14C02 was evolved and trapped in the KOH and not a 14CAs compound.
Conclusions In conclusion, the degradation of CA in soils proceeds by two mechanisms. Under anaerobic conditions, 61% of the applied CA was converted to a volatile organo-arseni-
50
Environmental Science & Technology
60
80
100
D AYS ~.
l4CO2+
Figure 4. E v o l u t i o nof
14c02
from an adapted (a) and a non-
adapted (b) Christiana (10 gram) soil treated with 0.05 KCi CA and 0.1 gram of unlabeled CA. cal within a 24-week period and was lost from the soil system. Under aerobic conditions, 35% was converted to a volatile organo-arsenical compound and 41% to 14C02 and As043- within the same 24-week period. Under aerobic conditions, CA may be reduced to a volatile organo-arsenical derivative, possibly dimethyl arsine or metabolized to C02 and A s O ~ ~ If - . the derivative is dimethyl arsine, it is extremely unstable and may be oxidized to the oxide or back to CA by air and return to the earth by fixation to plants or soils or by rainfall. The h o d 3 - and CA are bound to the Fe and A1 oxides which coat the soil colloids. The ultimate environmental fate of the arsenic from cacodylic acid appears to be metabolized to inorganic arsenate which is bound as insoluble compounds in the soil.
Acknowledgment The authors thank J. R. Plimmer and J. B. Yount for preparing the 14C-CA, Virginia P. Williams for purification of the 14C-CA, P. D. J. Ensor for the chemical fractionations, and Juanita B. Yates for the As analyses. Literature Cited Dehn, W. M., Wilcox, B. B., Amer. Chem. J., 35,9 (1906). Dickens, R., Hiltbold, A. E., Weeds, 15,299 (1967). Duble, R. L., Holt, E . C., McBee, G. G., Weed Sci., 16, 421 (1968). Jacobs, L. W., Keeney, D. R., Walsh, L. M., Agron. J., 62, 588 (1970a). Jacobs, L. W., Syers, J . K., Keeney, D. R., Soil Sci. SOCAmer. h o c . , 34,750 (1970b). Johnson, L. R., Hiltbold, A. E., ibid., 33,279 (1969). Petersen, G. W., Corey, R. B., ibid., 30,563 (1966). Sachs, R. M., Michael, 3.L., Weed Sci., 19,558 (1971). Schweizer, E. E., Weeds, l5,72 (1967). Schwertmann, U., 2. Pflanzenernaehr. Dueng. Bodenk., 105, 194 (1964). Sckerl. M. M.. Frans. R. E.. Weed Sci.. 17,421 (1969) Von Endt, D. W., Kearney, P. C., Kaufman, D. D., J. Agr. Food Chem., 16,17 (1968). Woolson, E . A., Axley, J. H., Kearney, P. C., Soil Sci., 109, 279 (19701. > \ - -
Woolson, E. A., Axley, J. H., Kearney, P . C., Soil Sci. SOC. Amer. Proc., 35, 101 (1971a). Woolson, E . A,, Axley, J. H., Kearney, P. C., Soil Sci., 111, 158 (1971b). Woolson. E. A.. unwblished data. 1972. Yuan, T.'L., Soil Sei., 88,164 (1959). Yuan, T. L., Fiskell, J . G. A,, Soil Sci. SOC.Amer. Proc., 23, 202 (1959).
Receiced for reuieu April 28, 1972 Accepted October30, 1972
