3 Behavior and Phytotoxicity of Inorganic Arsenicals in Soils L. M. WALSH and D. R. KEENEY Department of Soil Science, University of Wisconsin, Madison, Wisc. 53706 Introduction Inorganic arsenicals have been used in agriculture as a pesticide or plant defoliant for many years. In localized areas oxides also contaminate soils by fallout from smelting ore, especially sulfide ores, and from burning coal. Past use of arsenicals has not resulted in serious problems with human or animal poisoning but severely reduced plant growth due to As phytotoxicity has been observed, especially on former orchards and cotton fields. Once toxicity is observed, i t persists for several years even though no additional As is added. Most attempts to ameliorate As toxicity by the addition of salts which might reduce the level of available As in soils have not been very successful. Historically, inorganic arsenicals (lead arsenate, calcium arsenate) were used as insecticides beginning in the latter part of the 17th century (1). Paris Green (which contains cuprous arsenite) was successfully used to control the Colorado Potato Beetle in eastern United States by 1867 (2). Lead arsenate has been used for insect control on a variety of fruit trees and, for many years, was the only insecticide that would successfully control codling moth in apple orchards and horn worm on tobacco. Calcium arsenates have been applied to cotton and tobacco fields to protect these crops from boll weevils, beetles and other insects. Arsenic trioxide has been widely used as a soil sterilant (3) and sodium arsenite has been used for aquatic weed control (4) and as a defoliant to kill potato vines prior to tuber harvest (5). In recent years use of inorganic arsenicals has decreased markedly. Sodium arsenite has been banned and can no longer be used as a vine-killer or defoliant (6). Organic arsenicals have largely replaced inorganic As salts as herbicides and since they are applied at a lower rate of application, this has reduced the amount of As applied to the soil. Also, much lower quantities of lead arsenate are now used in orchards because fruit growers rely primarily on carbamates and organic phosphates to control 35
ARSENICAL PESTICIDES
36
i n s e c t s . However, where long-term r e s i d u a l i n s e c t i c i d a l a c t i v i t y i s needed t o p r o t e c t orchards from chewing i n s e c t s , l e a d arsenate i s s t i l l being used t o a l i m i t e d extent by some growers. A l s o , i n o r g a n i c a r s e n i c a l s continue t o be used on g o l f greens and f a i r ways i n some areas o f the United States t o c o n t r o l annual b l u e grass. The primary o b j e c t i v e s o f t h i s paper are: (a) t o review the r e l a t i o n s h i p between a r s e n i c s o r p t i o n and s o i l p r o p e r t i e s and the p o s s i b l e mechanisms o f As s o r p t i o n i n s o i l s , (b) t o review v a r i ous s o i l t e s t methods i n terms o f t h e i r a b i l i t y t o p r e d i c t As uptake and p h y t o t o x i c i t y , ( c ) t o review r e l a t i o n s h i p s between s o i l As and p l a n t uptake which r e s u l t i n p h y t o t o x i c i t y and p o t e n t i a l hazards t o humans or animals consuming p l a n t s grown on s o i l s cont a i n i n g high l e v e l s o f As, and (d) t o review methods o f amelior a t i n g As t o x i c i t y . Occurrence and D i s t r i b u t i o n o f As i n S o i l s A r s e n i c i s ubiquitous, i n nature and i s found i n d e t e c t a b l e amounts i n n e a r l y a l l s o i l s and i n many rocks and minerals. Surveys o f s o i l s i n the United States show t h a t As l e v e l s range from 0.2 t o 40 ppm f o r uncontaminated s o i l s , and up t o 550 ppm f o r Ast r e a t e d s o i l s (7., 8.)· Comparisons o f As-treated vs. untreated s o i l s were made by s e v e r a l r e s e a r c h e r s and are r e p o r t e d i n Table 1. These data i n d i c a t e t h a t untreated s o i l s seldom c o n t a i n more than 10 ppm As. In f a c t , F l e i s c h e r (9) r e p o r t s t h a t the average As content o f untreated s o i l s i s only 5 ppm. Even though s o i l s can vary g r e a t l y i n t h e i r n a t i v e As content, the l e v e l s o f As i n noncontaminated s o i l s g e n e r a l l y have not been high enough t o cause p h y t o t o x i c i t y and they do not represent a s i g n i f i c a n t h e a l t h hazard (10). The As content o f most rocks and minerals i s s i m i l a r to t h a t found i n s o i l s , except f o r s u l f i d e o r e s , sediment a r y Fe and Mn o r e s , and rock phosphate which o c c a s i o n a l l y cont a i n as much as 2000 ppm As ( 9 ) . Coal u s u a l l y contains about 25 ppm o f As (11). Even though s o i l s and s o i l parent m a t e r i a l s vary c o n s i d e r a b l y i n t h e i r n a t i v e content o f As, no c l e a r l y d e f i n e d r e l a t i o n s h i p has been noted between the As content o f s o i l s and the c l i m a t i c c o n d i t i o n s or g e o l o g i c a l formations i n which the s o i l s were developed. Under f i e l d c o n d i t i o n s high l e v e l s o f t o t a l s o i l As have been observed more o f t e n where crops have been t r e a t e d with an a r s e n i c a l i n s e c t i c i d e as compared to a d e f o l i a n t . T h i s i s due to the d i f f e r e n c e i n the t o t a l amount of As a p p l i e d . For i n s t a n c e , s o i l a n a l y s i s i n d i c a t e s t h a t some orchards have r e c e i v e d over 1000 kg As/ha (12) as compared t o about 10 kg As/ha where sodium a r s e n i t e was used as a potato v i n e d e f o l i a n t (13). Fate o f A r s e n i c i n S o i l s A r s e n i c Chemistry. While As chemistry i s i n many ways s i m i -
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l a r t o the other Group V elements, i t i s more m e t a l l i c ( i . e . more l a b i l e i n behavior) than P, which i s e s s e n t i a l l y c o v a l e n t . In Table 1. A comparison o f As l e v e l s i n A s - t r e a t e d and uncontaminated s o i l s i n North America Sampling site
T o t a l As Content Uncontaminated Treated soil soil* ppm As 1.3-2.3 Colorado 13-69 8 Florida 18-28 Idaho 0-10 138-204 Indiana 2-4 56-250 Maine 9 10-40 19-41 Maryland 21-238 New Jersey 10.0 92-270 New York 3-12 90-625 North C a r o l i n a 4 1-5 10-124 Nova S c o t i a 0-7.9 Ontario 1.1-8.6 10-121 Oregon 2.9-14.0 17-439 3-32 4-103 6-13 Washington 106-830 106-2553 8-80 4-13 48 Wisconsin 2.2 6-26
Crop
orchard potato orchard orchard blueberry orchard orchard orchard tobacco orchard orchard orchard orchard orchard orchard orchard potato
Source o f data
(61) (12) (12) (12) (12) ( 8) (12) (12) (30) (28) (62) (50) (12) (34) (12) (44) (54)
* These are r e s u l t s from s o i l s t h a t had been r e p e a t e d l y t r e a t e d with an As p e s t i c i d e o r d e f o l i a n t . S o i l s treated experimentally are not i n c l u d e d . s t r o n g l y reducing environments, elemental As and a r s i n e (-3) can e x i s t , but arsenate (+5) i s the s t a b l e o x i d a t i o n s t a t e i n oxygenated environments. Under moderately reduced c o n d i t i o n s such' as f l o o d e d s o i l s , a r s e n i t e (+3) may be the dominant form (14). E v i dence e x i s t s (15) t h a t d i m e t h y l a r s i n i c a c i d may be an important and u b i q u i t o u s ~ s compound which has not h e r e t o f o r e been i d e n t i f i e d due t o l a c k o f methods f o r d e t e c t i o n and q u a n t i f i c a t i o n . B i o l o g i c a l Transformations. The -3, +3, and +5 valence s t a t e s can form compounds c o n t a i n i n g the C-As bond and a r e r e a d i l y i n t e r c o n v e r t e d by microbes, although there i s no e v i dence the energy produced by o x i d a t i o n i s used f o r m i c r o b i a l growth (16_, 17_). F l e i s h e r (9) and Wood (16) have suggested that m i c r o b i a l formation o f v o l a t i l e a r s i n e o r other v o l a t i l e reduced As compounds might p l a y a r o l e i n the discharge o f As t o the a t mosphere. Reed and S t u r g i s (18) i n d i c a t e d t h a t a r s i n e may be l o s t from f l o o d e d s o i l s . Wood (16) pointed out the marked s i m i l a r i t i e s between the Hg and As c y c l e s . According t o Wood, i n
38
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reduced environments such as sediments, arsenate i s reduced t o a r s e n i t e and methylated t o methylarsenic a c i d or d i m e t h y l a r s i n i c a c i d . These compounds, which are extremely t o x i c , may be f u r t h e r methylated ( t r i m e t h y l a r s i n e ) o r reduced (dimethylarsine) and may v o l a t i l i z e t o the atmosphere where o x i d a t i o n r e a c t i o n s w i l l r e s u l t i n reformation o f d i m e t h y l a r s i n i c a c i d . Thus s t u d i e s which do not monitor the v a r i o u s p o s s i b l e forms o f As can r e s u l t i n misleading information. While p r e c i s e information o f the o x i d a t i o n - r e d u c t i o n potent i a l a t which a r s i n e , a r s e n i t e , and arsenate dominate i s not a v a i l a b l e , Turner (17_) c a l c u l a t e d the E' f o r the arsenate-arsen i t e r e a c t i o n to be +77 t o +167 mV (pH 7.0). He suggested t h a t b a c t e r i a l o x i d a t i o n o f a r s e n i t e i n v o l v e d an adaptive a r s e n i t e dehydrogenase system. I t i s p o s s i b l e t h a t t h i s enzyme complex evolves as a p r o t e c t i v e mechanism. The optimum pH f o r o x i d a t i o n was between 6.0 and 6.7, and 15 s t r a i n s o f h e t e r o t r o p h i c b a c t e r i a capable o f c a r r y i n g out t h i s r e a c t i o n were r e a d i l y i s o l a t e d . Deuel and Swoboda (14) obtained experimental evidence t h a t a r s e n i t e o x i d a t i o n occurs a t about +100 mV. This work shows t h a t a r s e n i t e w i l l be o x i d i z e d i n aerated s o i l s , where the o x i d a t i o n r e d u c t i o n p o t e n t i a l commonly i s i n the range o f +400 t o 600 mV. However, the r a t e o f a r s e n i t e o x i d a t i o n i n s o i l s i s a l s o o f importance i n e s t i m a t i n g the r e s i d u a l a c t i v i t y o f a r s e n i t e . While r a t e data have not been obtained, the p e r f u s i o n experiments o f Quastel and S c h o l e f i e l d (19_) showed complete o x i d a t i o n o f .0025M sodium a r s e n i t e i n 8 days i n an unenriched s o i l . When the s o i l was enriched by p e r f u s i o n with sodium a r s e n i t e u n t i l constant o x i d a t i o n r a t e s were obtained, complete o x i d a t i o n occurred i n about 4 days. Thus a r s e n i t e w i l l not p e r s i s t f o r long p e r i o d s i n w e l l aerated s o i l s which have s i g n i f i c a n t b i o l o g i c a l a c t i v i t y . Sorption o f A r s e n i c i n S o i l . In most s o i l systems, the chemi s t r y o f As becomes the chemistry o f arsenate. This i o n has p r o p e r t i e s s i m i l a r t o phosphate i n c l u d i n g formation o f i n s o l u b l e s a l t s with a number o f c a t i o n s , and s o r p t i o n by s o i l c o n s t i t u e n t s . For a comprehensive d i s c u s s i o n o f the behavior o f s o i l and f e r t i l i z e r phosphorus, see Black (20) and r e f e r e n c e s c i t e d therein.One major d i f f e r e n c e between P and As appears t o be t h a t while s o i l s c o n t a i n a p p r e c i a b l e organic-combined phosphate, they do not cont a i n measurable l e v e l s o f organic As (21). S o l u b i l i t y r e a c t i o n s may p l a y a r o l e i n As r e t e n t i o n by s o i l s . For example, i r o n arsenate i s extremely i n s o l u b l e , maint a i n i n g an arsenate c o n c e n t r a t i o n o f only about 10' i n solut i o n , compared t o about 10~5Af f o r the Ca or Mg arsenates (22). The modified Chang and Jackson (23) procedure f o r f r a c t i o n a t i o n of i n o r g a n i c s o i l P was f i r s t a p p l i e d to As-contaminated s a i l by Johnson and H i l t b o l d (21). They found t h a t about 90% of the As was a s s o c i a t e d with the c l a y f r a c t i o n o f a C h e s t e r f i e l d sandy loam p r e v i o u s l y t r e a t e d with organic a r s e n i c a l s , and that most of t h i s As was a s s o c i a t e d with s o i l A l . However, t h i s approach
3.
WALSH AND KEENEY
Inorganic Arsenicals in Soil
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assumes that As extracted by ammonium f l u o r i d e i s Al-bound and t h a t removed by sodium hydroxide i s Fe-bound. T h i s i s not necess a r i l y the case as i t has been shown that some o f the P r e l e a s e d by ammonium f l u o r i d e i s subsequently resorbed by other s o i l cons t i t u e n t s and then removed by the sodium hydroxide e x t r a c t i o n (2J+). This was confirmed f o r As by Jacobs et a l . (25) who a l s o a p p l i e d the Williams et a l . procedure (which c o r r e c t s f o r P r e s o r p t i o n ) to As-contaminated s o i l s . They obtained r e s u l t s sugg e s t i n g t h a t most of the As was sorbed by amorphous Fe and A l components i n s o i l s . Woolson e t _ a l . (12) conducted an extensive survey of As forms i n As-contaminated s o i l s from throughout the United S t a t e s . They found t h a t , as would be p r e d i c t e d , s o i l s with a high r e a c t i v e Fe (oxalate e x t r a c t a b l e ) content had predominately Fe-bound As, but that when the r e a c t i v e Fe was low, the dominant form o f As was c o n t r o l l e d by the r e l a t i v e l e v e l s o f exchangeable Ca and r e a c t i v e A l (see a l s o r e f e r e n c e 26). A number o f r e p o r t s have i n d i c a t e d that As i s more t o x i c on coarse-textured ( i . e . , sandy) than on f i n e - t e x t u r e d ( i . e . , high i n c l a y ) s o i l s (3_, 27^, 28_, 29_, 3£, 31_, :32, 33, 34^ 315). This i s exp l a i n e d i n p a r t by the marked increase i n surface area as the c l a y content i n c r e a s e s . A l s o , as would be p r e d i c t e d from the As f r a c t i o n a t i o n work discussed p r e v i o u s l y , s o r p t i o n of arsenate i n creases with the amount o f " a c t i v e " ( e x t r a c t a b l e ) Fe or A l cont e n t o f s o i l s (12_, 25, 2£, 30, 36, 37_, 38). Sorption o f As by s o i l s i s time dependent. I n d i c a t i o n s o f r e v e r s i o n to l e s s s o l u b l e (and hence l e s s t o x i c ) forms o f As have been obtained i n the f i e l d (39), greenhouse (35), and l a b o r a t o r y (26, 40). T h i s i s i l l u s t r a t e d by F i g u r e 1 (from reference 25) which shows the amount of As extracted from a sand ( P l a i n f i e l d ) and a s i l t y c l a y loam (Waupun) a f t e r e q u i l i b r a t i o n f o r up to 12 months. More As was extracted by the Bray P - l (0.025$ hydroc h l o r i c a c i d + 0.3N ammonium f l u o r i d e ) than by 1.0N ammonium acet a t e (pH 7.0) and more As was removed from the sand than the s i l t y c l a y loam. From 3 t o 6 months e q u i l i b r a t i o n was r e q u i r e d before As reached an apparent e q u i l i b r i u m with these s o i l s . Simil a r l y , Woolson ejt a l . (26) found that s o l u b l e (ammonium acetate o r ammonium c h l o r i d e e x t r a c t a b l e ) As decreased t o a constant value a f t e r about 4 months, and that the r a t e o f decrease d i f f e r e d among s o i l s . Iron-bound As continued to form even a f t e r the A l bound f r a c t i o n had reached a maximum l e v e l and begun to d e c l i n e . D e c l i n e i n As t o x i c i t y with time i n f i e l d and greenhouse s t u d i e s have been reported (3_, 27, 35). However, i n a f i e l d study, Steevens et ad. (13) found l i t t l e d e c l i n e i n As p h y t o t o x i c i t y to peas or potatoes with time. Arsenic Movement and P e r s i s t e n c e
in Soil
While downward movement ( l e a c h i n g ) o f phosphate i n s o i l s i s b e l i e v e d t o be very l i m i t e d (20) s e v e r a l workers have i n d i c a t e d
ARSENICAL
40
PKSTICiDES
that arsenate l e a c h i n g may be a s i g n i f i c a n t f a c t o r i n reducing As t o x i c i t y o f surface s o i l s (3_, 13_, 2£, 33_, 4 J J . The p r o f i l e d i s t r i b u t i o n o f As i n a P l a i n f i e l d sand which had r e c e i v e d up t o 720 kg/ha o f As i n 1967 was i n v e s t i g a t e d by Steevens e_t aJL. (13). These r e s u l t s , along with 1974 p r o f i l e analyses by the authors, are presented i n Table 2. Table 2. P r o f i l e d i s t r i b u t i o n with time o f t o t a l s o i l As i n a P l a i n f i e l d sand a f t e r s u r f a c e a p p l i c a t i o n of sodium a r s e n i t e i n 1967. [ J . Environ Qual. (13)] LSD As Applied Depth , cm .05 68-83 Year 23-38 kg/ha 0-23 38-53 53-68 A o
-ppm
0 45 90 180 720
3.0 11.0 23.0 73.0 250.0
— — — — --
— — — —
1968
0 45 90 180 720
3.0 19.0 26.0 63.0 150.0
— — — — —
— — — — —
1970
0 45 90 180 720
3.6 14.1 27.0 45.0 100.0
1967
— —
—
— — — — — — — — —
— — — — — — — — — —
1.3 1.9 1.2 1.7 3.3
1.2 1.6 1.4 1.1 2.0
0.8 1.1 3.2 4.5 4.2
1.3 .5 1.6 1.0 0 1.4 8.1 1.2 .7 45 14.4 1.8 1.6 1.3 90 180 2.8 1.5 1.1 21.6 50.6 19.6 82.8 2.6 720 * Walsh and Keeney, unpublished d a t a , 1974.
.6 .7 1.3 1.3 3.7
.9 4.5 5.1 9.9 24.9
1974*
1.2 2.3 3.4 4.8 65.0
1.8 1.4 1.5 1.9 8.6
Through 1970, these p l o t s r e c e i v e d about 75 cm o f p r e c i p i t a t i o n and 50 cm o f i r r i g a t i o n per year. A l f a l f a has been grown on the experimental area during the past 3 y e a r s , but i t was not harv e s t e d . The p l o t s were not i r r i g a t e d s i n c e 1970. Phytotoxicity p e r s i s t e d at the 180 and 720 kg/ha As p l o t s from 1967 to 1970, and the 720 kg/ha p l o t s are s t i l l e s s e n t i a l l y barren i n 1974, 7 years a f t e r As a p p l i c a t i o n on a coarse textured s o i l . However, there has been a s i g n i f i c a n t d e c l i n e i n t o t a l As with time i n the surface s o i l o f a l l As-treated p l o t s . Part of t h i s decrease can be explained by As l e a c h i n g ; by 1970 As had d e f i n i t e l y been leached t o the 38-53 cm depth i n the 720 kg/ha p l o t s , and l e a c h ing r a t e was r e l a t e d t o the amount a p p l i e d i n agreement with the work o f Tamines and de L i n t (33). By 1974, the surface s o i l of
3. WALSH AND KEENEY
Inorganic Arsenicals in Soil
41
the high As-treated p l o t s had decreased f u r t h e r , but the r a t e o f decrease was much l e s s than i n previous years. S i g n i f i c a n t amounts o f As had accumulated i n the s u b s o i l , and i n the 720 kg/ha p l o t s , t o t a l As was above background t o 83 cm. These r e s u l t s show that As w i l l l e a c h i n a sandy p r o f i l e , and t h a t i n time s u f f i c i e n t As w i l l be leached t o s i g n i f i c a n t l y reduce p h y t o t o x i c i t y . However, the r a t e o f As l o s s from the surf a c e s o i l decreased markedly with time, i n d i c a t i n g t h a t As i s p e r s i s t e n t even i n a s o i l with low s o r p t i o n c a p a c i t y . When t h e p r o f i l e r e s u l t s a r e summed, a l l o f the As a p p l i e d i n 1967 cannot be accounted f o r as t o t a l As i n the p r o f i l e . One p o s s i b i l i t y i s that s i g n i f i c a n t amounts o f As a r e l o s t from t h i s s o i l by wind e r o s i o n (40). Tammes and de L i n t (33) c a l c u l a t e d an average h a l f l i f e o f 6.5 ± 0.4 years f o r As p e r s i s t e n c e on two Netherland s o i l s . Since q u a n t i t a t i v e p r o f i l e As balances on the p l o t s reported i n Table 2 have not been achieved, h a l f - l i f e c a l c u l a t i o n s with these data a r e not p o s s i b l e . However, i n t h e 720 kg/ha p l o t s , t o t a l As i n the surface s o i l had d e c l i n e d to 40% o f the o r i g i n a l value, i n only 3 y e a r s , and t o 33% by 7 years. Arsenic P h y t o t o x i c i t y S o i l "unproductiveness" due to a r s e n i c a l poisoning r e s u l t i n g from heavy a p p l i c a t i o n o f a r s e n i c a l p e s t i c i d e s has been noted by many r e s e a r c h e r s (3_, 28_, 42_, 43_, 44_). In reviewing the l i t e r a t u r e , i t i s apparent t h a t the most s e r i o u s problems occur i n f i e l d s where orchards a r e removed and then r e p l a n t e d t o an agronomic o r h o r t i c u l t u r a l crop (28_, 44_). S u b s t a n t i a l amounts o f a r s e n i c a l h e r b i c i d e s have been used on g o l f greens but As t o x i c i t y has not been reported on bent grass o r c o a s t a l bermuda g r a s s . A r s e n i c i s not an e s s e n t i a l p l a n t n u t r i e n t but o c c a s i o n a l l y s m a l l y i e l d i n c r e a s e s have been observed a t low l e v e l s o f As, esp e c i a l l y f o r t o l e r a n t crops such as potatoes, corn» r y e and wheat (40, 45, 46). L i e b i g et_ al_. (47_) r e p o r t e d that r o o t growth o f lemon p l a n t s i n s o l u t i o n c u l t u r e was enhanced by 1 ppm As as a r senate or a r s e n i t e ; 5 ppm o f e i t h e r form o f As was t o x i c and adv e r s e l y a f f e c t e d both t o p and r o o t growth. Woolson (46) p o s t u l a ted t h a t As response was due t o s t i m u l a t i o n o f p l a n t systems by small amounts o f As s i n c e other p e s t i c i d e s (e.g. 2, 4-D) stimul a t e p l a n t growth a t s u b l e t h a l dose l e v e l s . Another reason f o r the y i e l d i n c r e a s e might be that arsenate would d i s p l a c e phosphate from the s o i l with a r e s u l t a n t increase i n phosphate a v a i l a b i l i t y (40). Plant S e n s i t i v i t y . P l a n t s vary c o n s i d e r a b l y i n t h e i r t o l e r ance t o high l e v e l s o f s o i l As. In h i s review on As, L i e b i g (43) included the f o l l o w i n g c l a s s i f i c a t i o n o f vegetables and small f r u i t s i n terms o f t h e i r t o l e r a n c e t o water-soluble As: Very t o l e r a n t : Asparagus, potato, tomato, c a r r o t , tobacco, dewberry, grape, and r e d r a s p b e r r y .
42
ARSENICAL PESTICIDES
F a i r l y t o l e r a n t : Strawberry and sweet corn, beet and squash. Low or no t o l e r a n c e : Snap bean, l i m a bean, onion, pea, cucumber, a l f a l f a and other legumes. L i e b i g (43) a l s o i n d i c a t e d t h a t r y e and Sudan grass were v e r y t o l e r a n t to s o l u b l e As. Recently, Deuel and Swoboda (29) r e ported t h a t soybeans were more s e n s i t i v e t o As t o x i c i t y than c o t ton. Jacobs e t _ a l . (40) found that crop t o l e r a n c e t o As was i n the f o l l o w i n g order: potatoes > peas > sweet corn > snap beans. Woolson et a l . (32) reported the As t o l e r a n c e o f s i x vegetable crops t o be as f o l l o w s : Cabbage > tomato > r a d i s h > spinach > lima beans > green beans. R e l a t i o n s h i p o f E x t r a c t a b l e As to P l a n t Growth. Many r e searchers have attempted t o r e l a t e the amount o f As i n the s o i l with p l a n t growth. As would be expected t o t a l s o i l As i s not a good p r e d i c t o r o f water s o l u b l e As or As p h y t o t o x i c i t y when these r e l a t i o n s h i p s are compared among s o i l s with widely d i f f e r i n g chara c t e r i s t i c s (12, 40, 48). However, t o t a l As a c c u r a t e l y p r e d i c t s p h y t o t o x i c i t y when s t u d i e s have been l i m i t e d t o a narrow range o f experimental t r e a t e d s o i l s (40, 49). In g e n e r a l , a h i g h l y s i g n i f i cant r e l a t i o n s h i p has been observed between some form o f e x t r a c t a b l e As and p l a n t growth. E x t r a c t a n t s commonly used t o measure a v a i l a b l e As i n c l u d e hot water, Bray P - l , mixed a c i d (0.05/17 hydroc h l o r i c a c i d + 0.025# s u l f u r i c a c i d ) , 0.5N h y d r o c h l o r i c a c i d , IN ammonium c h l o r i d e , IN ammonium acetate (pH 7.0) and 0.5N sodium bicarbonate. The r e s u l t s o f a few r e c e n t s t u d i e s , summarized i n Table 3, show t h a t s o i l type and p l a n t specie i n f l u e n c e the " c r i t i c a l " l e v e l of s o i l As a t which a y i e l d depression might be expected. The " c r i t i c a l " l e v e l ranged from 25 to 85 ppm As and from 3 t o 28 ppm As f o r t o t a l s o i l As and f o r water s o l u b l e As, r e s p e c t i v e l y . The c r i t i c a l l e v e l s f o r a v a i l a b l e As as measured by the Bray P - l , the mixed a c i d or sodium bicarbonate ranged from 10 to 22 ppm As. Greenhouse work by Woolson e t _ a l . (46) c o n c l u s i v e l y shows that p l a n t growth on As-contaminated s o i l s i s r e l a t e d to t o t a l As and a v a i l a b l e As. A l l c o r r e l a t i o n s presented i n F i g u r e 2 are h i g h l y s i g n i f i c a n t , however, use o f the a v a i l a b l e t e s t s more e f f e c t i v e l y p r e d i c t e d p l a n t response as compared to t o t a l As. The three a v a i l a b l e As t e s t s compared i n F i g u r e 2 were e q u a l l y e f f e c t i v e i n p r e d i c t i n g response to As. In a r e l a t e d study Woolson (32) found the growth o f s i x vegetable crops were reduced by 50% at l e v e l s o f 6.2 t o 48.3 ppm o f a v a i l a b l e As as measured with the mixed a c i d procedure. These data presented i n Table 4 a l s o show that the a v a i l a b l e As l e v e l s account f o r 64 t o 83% o f the v a r i a t i o n i n y i e l d f o r s i x greenhouse-grown vegetable crops. Greenhouse experiments were a l s o conducted by Deuel and Swoboda (29) i n which experimentally t r e a t e d s o i l s were used to r e l a t e l e v e l s o f As extracted from the s o i l s with y i e l d s o f c o t ton or soybeans. These r e s u l t s , presented i n Table 5, show t h a t
WALSH AND KEENEY
Inorganic Arsenicals in Soil
24QE
200
PLAINFIELD--BRAY PI
160-
120
80 •
WAUPUN—BRAY PI PLA INFIELO —NH4OAC
40 WAUPUN—NH4OAC
EQUILIBRATION TIME, MONTHS Soil Science Society of America, Proceedings
Figure 1. Extractability of added As (320 pg/g) from Plainfield sand and Waupun silty clay loam by Bray P-l and NH OAc as related to equilibration time (25) /f
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Figure 2. A comparison of four methods for the estimation of As availability by a correlation between growth reduction (4-week-old corn) and the log of As concentration in soils (46)
44
ARSENICAL PESTICIDES
hot water, IN ammonium c h l o r i d e and 0.5/1? h y d r o c h l o r i c a c i d e x t r a c ted As l e v e l s that were s i g n i f i c a n t l y c o r r e l a t e d with p l a n t growth. Table 3. Levels o f s o i l As a t which s i g n i f i c a n t y i e l d depressions occur. L e v e l o f As a t which Source Crop Soil s i g n i f i c a n t y i e l d deof name type p r e s s i o n s occurred data T o t a l Water A v a i l a b l e As soluble As As - ppm As Colton Blueberry — 44 loamy sand 6 C+9) Cotton Amarillo f i n e —— — _ sandy loam 8 (29) Cotton Houston Black (29) clay 28 Soybean Amarillo f i n e (29) sandy c l a y 3 Soybean Houston Black clay (29) 12 Potatoes, Plainfield — (to) 68 sweet corn loamy sand 22t Snap beans, P l a i n f i e l d — peas loamy sand lot 25 Corn (Avg. o f 13 — 85 soils) 10* 0*6) t E x t r a c t e d with Bray P - l (0.025/7 HCL + 0.3i^ NH F ) # E x t r a c t e d with e i t h e r O.SN NaHC0 o r 0.05/7 HC1 + 0.025/7 ^ S O ^ 3
Table 4. Regression o f a v a i l a b l e a r s e n i c * on t o t a l d r y - p l a n t weight f o r s i x vegetable crops. [Weed Science (32)3 A v a i l a b l e As Regression Correlation at G R * Crop equationt coefficient 50
(r) Green beans y = 77-34 l o g x§ 0.89 0.83 Lima geans y = 107-55 l o g x Spinach y = 88-37 l o g x 0.91 y = 114-38 l o g x 0.80 Cabbage 0.87 Tomato y = 109-42 l o g x 0.81 Radish y = 96-36 l o g x * A v a i l a b l e A s - s o i l extracted with 0.05/7
(ppm) 6.2 10.9 10.6 48.3 25.4 19.0 H SO^ and 0.025/17 HC1
t C a l c u l a t e d by l e a s t squares method 4 GR = a v a i l a b l e s o i l As content necessary t o reduce p l a n t growth t o 50% o f t h a t grown on non-arsenate-treated s o i l § x = a v a i l a b l e s o i l As s
50
3. WALSH AND KEENEY
Inorganic Arsenicals in Soil
45
Table 5. C o r r e l a t i o n c o e f f i c i e n t s f o r y i e l d vs. a r s e n i c . [ J o u r n a l o f Environmental Q u a l i t y (29)] Soil Amarillo Houston Amarillo Houston
Crop Soybeans Soybeans Cotton Cotton
H0 2
-.943 -.968 -.951 -.954
extractable
Extractant* HCl NH^Cl -.914 -.938 -.960 -.918
-.915 -.931 -.830 -.895
* A l l o f the c o r r e l a t i o n c o e f f i c i e n t s a r e s i g n i f i c a n t a t t h e 0.05 level. Jacobs et_ ajU (40) s t u d i e d the r e l a t i o n s h i p between s o i l As and the y i e l d s o f s e v e r a l vegetable crops grown on an i r r i g a t e d P l a i n f i e l d Sand. A l l extractants were n e g a t i v e l y c o r r e l a t e d with crop y i e l d s a t the 0.01 p r o b a b i l i t y l e v e l (Table 6). Total As and ammonium acetate o r Bray P - l e x t r a c t a b l e As were e q u a l l y e f f e c t i v e i n p r e d i c t i n g reduced y i e l d s as l e v e l s o f s o i l As i n creased. Table 6. R e l a t i o n s h i p s between t o t a l o r e x t r a c t a b l e y i e l d o f vegetable crops. [Agronomy J o u r n a l (40)] As fraction
Potatoes
Peas
Crop* Snap beans
s o i l As and
Sweet corn
NH^OAc
-0.91
-0.85
-0.73
-0.91
Bray P - l
-0.91
-0.88
-0.77
-0.93
Total
-0.92
-0.87
-0.76
-0.03
" A l l c o r r e l a t i o n c o e f f i c i e n t s s i g n i f i c a n t a t the 0.01 p r o b a b i l ity level. P r i o r work c l e a r l y i n d i c a t e s that s e v e r a l s o i l t e s t s can be used t o p r e d i c t As p h y t o t o x i c i t y . Even though water s o l u b l e and t o t a l As s a t i s f a c t o r i l y p r e d i c t As p h y t o t o x i c i t y , s o i l t e s t i n g l a b o r a t o r i e s w i l l f i n d i t more convenient t o use Bray P - l , sodium bicarbonate or the mixed a c i d e x t r a c t a n t because these e x t r a c t a n t s a r e now r o u t i n e l y used f o r a v a i l a b l e P. Furthermore, they e x t r a c t more As than water, and e l i m i n a t e the d i g e s t i o n step needed f o r t o t a l As. P l a n t Uptake o f A r s e n i c . A p p l i c a t i o n o f many chemical e l e ments r e s u l t s i n s u b s t a n t i a l increases i n crop a s s i m i l a t i o n o f those elements. In the case o f As such bioaccumulation would be hazardous t o human beings o r animals because o f t o x i c i t y o f As and i t s p o s s i b l e r e l a t i o n s h i p t o cancer, a r t e r e o s c l e r o s i s and c h r o n i c l i v e r diseases (10). F o r t u n a t e l y , the e d i b l e p o r t i o n o f p l a n t s seldom accumulates a hazardous l e v e l o f As, p r i m a r i l y
46
ARSENICAL PESTICIDES
because most p l a n t s a r e s e n s i t i v e t o As t o x i c i t y and growth i s u s u a l l y s e v e r e l y reduced b e f o r e a l e v e l o f As hazardous t o man or animals accumulates i n t h e p l a n t . When As poisoning does occur, i t i s u s u a l l y due t o d i r e c t i n g e s t i o n o f a s u r f a c e r e s i d u e o f As or t h e i n g e s t i o n o f s p r i n g water o r muds c o n t a i n i n g abnormally high l e v e l s o f As (50, 51). The e d i b l e p o r t i o n o f most f r u i t s and vegetables grown i n Ast r e a t e d s o i l s c o n t a i n l e s s than the t o l e r a n c e o f 2.6 ppm as a l lowed by the U.S. P u b l i c Health s e r v i c e f o r A s - t r e a t e d f r u i t s and v e g e t a b l e s . The h i g h e s t l e v e l s o f As a r e found i n p l a n t r o o t s , the v e g e t a t i v e top growth i s intermediate, and e d i b l e seeds and f r u i t s c o n t a i n the lowest l e v e l o f As. Jones and Hatch (50) anal y z e d vegetable p l a n t s growing on A s - t r e a t e d s o i l s on s e v e r a l experimental farms i n Oregon and found t h e r o o t s , tops (stems and l e a v e s ) and e d i b l e p o r t i o n t o c o n t a i n an average o f 7.1, 5.0, and 1.2 ppm As, r e s p e c t i v e l y . Fleming e t a l . (52) and McLean e t a l . (53) observed t h a t vegetables grown on s o i l s t r e a t e d with h i g h l e v e l s o f l e a d arsenate seldom contained more than 1 ppm As i n the e d i b l e p o r t i o n s . A wide range o f forage and vegetable crops were grown by Jones and Hatch (50) on A s - t r e a t e d and adjacent untreated s o i l s i n Oregon. They found the As content i n the e d i b l e p l a n t p a r t s t o be only 1.2 and .41 ppm As i n the t r e a t e d and untreated areas, r e s p e c t i v e l y . On t h e other hand, Small and McCants (3£) found t h a t the c o n c e n t r a t i o n o f As i n f l u e - c u r e d t o bacco v a r i e d from 2 ppm As where no As was a p p l i e d t o 14.3 ppm where 54 kg As/ha or l e a d arsenate had been a p p l i e d t o the s o i l . A r s e n i c l e v e l s o f l e s s than 1.0 ppm were found by Jacobs e t a l . (40) i n potato tubers even where severe As t o x i c i t y occurred. The p e e l i n g , however, contained up t o 48 ppm As where the s o i l had been experimentally t r e a t e d with 720 kg As/ha. The authors suggested the high l e v e l s o f As i n t h e p e e l i n g s was due t o minute q u a n t i t i e s o f As-contaminated s o i l adhering t o the s u r f a c e o f the potato t u b e r , r a t h e r than a s s i m i l a t i o n o f As i n t o the p e e l i n g itself. A survey o f potato grower f i e l d s i n C e n t r a l Wisconsin where 8 t o 65 kg As/ha as sodium a r s e n i t e had been a p p l i e d over a p e r i o d o f s e v e r a l years r e v e a l e d t h a t the potato p e e l i n g s d i d not c o n t a i n more than 2-3 ppm As (54). I t was concluded, thus, t h a t past usage o f sodium a r s e n i t e i n Wisconsin had not caused harmful l e v e l s o f As t o accumulate i n potato tubers. High As l e v e l s have a l s o been observed i n washed, unpeeled r a d i s h e s (32). As with potatoes, p a r t o f the As may have been adsorbed on the s u r f a c e o f the r a d i s h r o o t . Woolson (32) found a s i g n i f i c a n t p o s i t i v e r e l a t i o n s h i p between a v a i l a b l e s o i l As and the c o n c e n t r a t i o n o f As i n the whole p l a n t f o r s i x greenhouse grown vegetable crops (Table 7 ) . Correl a t i o n s between a v a i l a b l e s o i l As and the c o n c e n t r a t i o n o f As i n the e d i b l e p l a n t p a r t s were g e n e r a l l y poorer, e s p e c i a l l y f o r p l a n t s i n which the seed or f r u i t was consumed (32). As p r e v i ously noted, p l a n t s tend t o exclude As from the seeds and f r u i t s . Hence, s o i l t e s t s f o r a v a i l a b l e As would not be a r e l i a b l e
3. WALSH AND KEENEY
Inorganic Arsenicals in Soil
47
i n d i c a t o r o f the amount o f As i n the e d i b l e p l a n t t i s s u e . Woolson (32) has pointed out t h a t where the e d i b l e p a r t o f a crop i s not the r o o t or whole p l a n t , i t i s u n l i k e l y t h a t the t o l e r a n c e l e v e l f o r As (2.6 ppm) would be exceeded even where As t o x i c i t y reduced growth by 50%. Table 7. Regression o f a v a i l a b l e s o i l As on the As content i n whole dry p l a n t o r e d i b l e d r y - p l a n t p a r t [Weed S c i . (32] Crop Green bean Lima bean Spinach Cabbage Tomato Radish
Regression equation
y y log y y y log y
= = = = =
Correlation coefficient
0.4 +4.2 l o g x* 0.5 + 1.2 l o g x -0.129 + 1.1 l o g x 0.4 + 1.8 l o g x -0.1 + 3.3 l o g x -0.147 + 1.4 l o g x
(r) 0.93 0.49 0.90 0.77 0.80 0.88
* x = a v a i l a b l e s o i l As t A v a i l a b l e As a t GR_ was used t o c a l c u l a t e As content table 4). n
b
Arsenic a t GR (ppm) 3.7t 1.7 10.0 3.4 4.5 43.8 (see
0
Wherever s o i l s have been t r e a t e d with s u b s t a n t i a l q u a n t i t i e s of As, the surface o f the a e r i a l p o r t i o n o f the p l a n t may be contaminated with dust. The work o f Jacobs e t a l . (40) showed t h a t the concentration o f As i n potato l e a v e s , stems and p e t i o l e s was r e l a t e d t o the surface area o f the p l a n t p a r t s and the p r e v a i l i n g wind i n r e l a t i o n t o p l o t s t r e a t e d with high r a t e s o f As. Plant As c o n c e n t r a t i o n bore l i t t l e r e l a t i o n s h i p t o As treatment; thus, these workers concluded t h a t s u b s t a n t i a l e x t e r n a l contamination of the p l a n t t i s s u e i n the f i e l d had occurred. In l i g h t o f t h i s work, i t appears t h a t some o f the data i n the l i t e r a t u r e on p l a n t uptake o f As would be questionable unless the researcher very c a r e f u l l y washed the p l a n t t i s s u e t o remove adsorbed s o i l p a r t i c l e s . For i n s t a n c e , Jones and Hatch (50) reported t h a t the top growth o f vegetable p l a n t s growing on untreated s o i l adjacent t o As-treated s o i l contained 3.1 ppm As while the r o o t s from these p l a n t s contained only 1.1 ppm As. Since r o o t s normally accumul a t e more As than the top growth, i t appears t h a t the top growth must have been contaminated with s o i l p a r t i c l e s from the adjacent orchard s o i l which had been t r e a t e d with As f o r s e v e r a l years. A l l e v i a t i o n o f As T o x i c i t y Since r e g i s t r a t i o n o f inorganic a r s e n i c a l s f o r use on n e a r l y a l l vegetable and agronomic crops was c a n c e l l e d i n 1968 ( 6 ) , a high p r i o r i t y problem a t the present time i s t o f i n d ways t o r e s t o r e As-contaminated s o i l s t o t h e i r optimal l e v e l o f production (55). Several approaches have been attempted.
American Chemical Society Library 1155
l»f_ _L
v.
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ARSENICAL PESTICIDES
Molar P/As Ratio. One approach i s t o add s u f f i c i e n t phosphate t o the system t o depress the uptake o f arsenate by the plant. Hurd-Karrer (37) showed i n s o l u t i o n c u l t u r e work t h a t t h i s hypothesis was v a l i d and that a molar P/As r a t i o o f a t l e a s t 5 was needed t o p r o t e c t against As t o x i c i t y t o wheat. Rumberg et a l . (56) a l s o reported that phosphate w i l l improve growth i n n u t r i e n t s o l u t i o n s containing s u f f i c i e n t arsenate t o be t o x i c . The r e s u l t s with s o i l s systems, however, have been l e s s c l e a r , due i n part t o the d i f f i c u l t y o f evaluating " a v a i l a b l e " P and As. For example, Jacobs and Keeney (35) found that P a d d i t i o n s d i d not i n f l u e n c e As t o x i c i t y on a s i l t loam s o i l . T h i s s o i l had a high P f i x a t i o n c a p a c i t y and a v a i l a b l e P probably d i d not i n crease g r e a t l y . However, with a sandy s o i l , P a c t u a l l y enhanced As t o x i c i t y . They hypothesized t h a t , with the sand, phosphate may have d i s p l a c e d sorbed arsenate r e s u l t i n g i n the enhanced t o x i c i t y . S i m i l a r r e s u l t s were obtained by Woolson e t a l . (26) with a sandy loam s o i l . When s u f f i c i e n t phosphate was added to maintain an a v a i l a b l e P/As r a t i o o f about 7, improved y i e l d s r e s u l t e d . T h i s e f f e c t was not c o n s i s t e n t , however, and a t very high l e v e l s o f added As (1,000 ppm), phosphate d i d not overcome As t o x i c i t y even a t a P/As r a t i o o f 10. A d d i t i o n o f Fe o r A l Compounds. Since As i s sorbed by Fe and A l components o f s o i l s , another obvious approach i s t o ammend the s o i l with Fe or A l s a l t s . Large amounts ( 5 t o 10 metric tons/ha) o f f e r r o u s s u l f a t e or f e r r i c s u l f a t e have occas i o n a l l y reduced As t o x i c i t y (57_, 58, 59). Steevens et_ a l . (13) attempted to a l l e v i a t e As t o x i c i t y on a sandy s o i l by a p p l i c a t i o n of 4 metric tons/ha o f f e r r i c s u l f a t e or aluminum s u l f a t e . The Fe treatment had a s l i g h t b e n e f i c i a l e f f e c t while the A l t r e a t ment a c t u a l l y depressed y i e l d s f u r t h e r . Both treatments decreased As uptake by potatoes. C u l t u r a l P r a c t i c e s . Deep plowing t o d i l u t e the As concentrat i o n o f the surface s o i l and expose As t o more s i t e s f o r f i x a t i o n would seem t o be one o f the most economical methods o f decreasing t o x i c i t y . T h i s approach was suggested by McLean e t a l . (53) and A l b e r t (31). Vincent (27) a l s o suggested growing t o l e r a n t cover crops such as r y e or Sudan grass. When plowed under, these crops reduced subsequent As t o x i c i t y . Apparently As t o x i c i t y t o f r u i t t r e e s i s involved with induced Zn d e f i c i e n c y , as f o l i a r a p p l i c a t i o n o f zinc s u l f a t e or Zn chelates w i l l overcome As t o x i c i t y to peach t r e e s (42, 60). Leaching. Woolson et_ a l . (26) pointed out t h a t i n s o i l s i n which added phosphate desorbs arsenate, d e l i b e r a t e leaching o f the s o i l a f t e r a d d i t i o n o f phosphate may be a v i a b l e approach t o removing As from the r o o t zone. This would seem f e a s i b l e s i n c e the l i m i t e d data (13, 26, 35) i n d i c a t e s that phosphate most l i k e l y w i l l desorb arsenate on sandy s o i l s , which a l s o are e a s i e s t
3.
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