tered Valentine soil and in the organic-free soils. ... thetic organic compounds (SOC) in soil include lysimeters ... leaching columns in the laboratory (Helling ... ographic column method to assess the mobility ..... ence, 3rd Ed. N. D. Camper (ed.).
0038-075X/92/1542-0145$03.00/0 SOIL
August 1992 Vol. 154, No. 2 Printed in U.S.A.
SCIENCE
Copyright © 1992 by Williams & Wilkins S O I L C H R O M A T O G R A P H I C S Y N T H E T I C
C O L U M N S T OA S S E S S T H E O R G A N I C
C O M P O U N D S
M O B I L I T Y O F
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N. B . S T O L P E , P . J . S H E A , D . T . L E W I S , A N D D . L . M C C A L L I S T E R
Soil thin-layer chromatography ( T L C ) is useful for rapid determination of the relative mobility ( R ) of herbicides and other synthetic organic compounds (SOC) in soil but fails for volatile compounds. An altérnate method using small soil chromatographic columns was developed to minimize SOC volatilization while maintaining the essential characteristics of soil T L C . The column method utilizes soil contained in narrow diameter (4 mm I.D.) glass tubes. Soil T L C and soil chromatographic columns were used to determine the relative mobility of aniline and benzoic acid in unaltered and organic-free Cecil, Holdrege, Sharpsburg, and Valentine soils. Both methods indicated that benzoic acid was more mobile than aniline in most of the soils. Variations in R valúes between methods generally were small and suggested that either method was acceptable for laboratory estimation of the relative mobility of SOC with low vapor pressures. Differences in R valúes may be attributed to differences in soil preparation, as T L C plates were prepared by drying thinly spread soil slurries while soil chromatographic columns contained powdered soil. Soil chromatographic columns allowed determination of R valúes for toluene, which is too volatile for soil T L C . Toluene was less mobile than benzoic acid in all soils, and less mobile than aniline in the unaltered Valentine soil and in the organic-free soils. F
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tance traveled by the capillary front of a n aqueous solution. T r a v e l distances are used to calcúlate a relative mobility ( R ) valué for the S O C i n the soil (Helling 1971). Soil T L C is valuable for rapid assessment of S O C mobility, but may not be suitable for S O C with higher vapor pressures because of volatilization from the plates. Soil columns have been used to study downward mobility of herbicides a n d other S O C i n response to leaching w i t h water (Weber et a l . 1986). A combination of capillarity and evapor a r o n also has been used to study S O C movement i n soils. H a r r i s (1967) used soil columns subirrigated i n 2 - L plástic containers to assess the relative mobility of several herbicides i n soils. Columns were subdivided after 3 days of subirrigation and evaporation, and herbicide displacement relative to that of monuron [3-(pchlorophenyl)-l,l-dimethylurea] was determined by oat (Avena sativa L . ) bioassay. F
Hubbs and L a v y (1990) used 2.5-cm-diameter soil columns to study the upward mobility ("wick effect") of C-norflurazon [4-chloro-5(methylamino)-2-(3-(trifluoromethyl)phenyl)3(2H)-pyridazinone] and C - a t r a z i n e [6-chloroAT'-(l-methylethyl)-l,3,5-triazine-2,4-diamine] in soil. T h e top of the column was open to the atmosphere for several days to promote evaporation a n d upward movement of water. T h e location of the herbicide was determined by subdividing the column into four segments and direct radioassay ( L a v y et a l . 1972) of the soil samples. R valúes were not calculated as herbicide movement continued after the capillary front had reached the top of the column, and water was evaporating from the open system. 14
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Methodologies used to study mobility of s y n thetic organic compounds ( S O C ) i n soil include lysimeters i n the field and thin-layer chromatography ( T L C ) , miscible displacement, and soil leaching columns i n the laboratory (Helling 1970; Weber and M i l l e r 1989). Soil T L C is a relatively simple, reliable procedure that measures distance traveled by a S O C relative to disContribution from the Dept. of Agronomy, Univ. of Nebraska, Lincoln, N E 68583. Published as Paper No. 9620, Jour. Series, Nebraska Agrie. Res. Div. 1
Received July 1, 1991; accepted Jan. 6, 1992. 145
D a t a generated from soil columns may not be comparable to soil T L C data because the methodologies (downward leaching or upward capillary movement, respectively) can produce differing results. Objectives of the present research were to (a) develop a n alternative soil chromatographic column method to assess the mobility of volatile S O C ' s that are not amenable to analysis by soil T L C and (b) compare the soil chromatographic column method w i t h soil T L C u s -
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S T O L P E , SHEA, L E W I S ,
ing the nonvolatile S O C aniline and benzoic acid. M A T E R I A L S
Soils and Synthetic
A N D
Organic
M E T H O D S
Compounds
Soils differing i n particle size distribution, clay mineral suites, and organic matter contení were selected. T h e soils included the A horizons of Cecil (clayey, kaolinitic, thermic T y p i c H a pludult), Holdrege (fine-silty, mixed, mesic T y p i c Argiustoll), Sharpsburg (fine, montmorillonitic, mesic T y p i c Argiudoll), and Valentine (mixed, mesic T y p i c Ustipsamment). Basic, acidic and nonionic S O C were selected, including C - a n i l i n e (Sigma, no. 3 1 , 178-2; specific activity 5.37 X 10 B q m o l ) , C-benzoic acid (Sigma no. 29, 717-8; 4.81 X 1 0 " B q m o r ) , and C-toluene (Sigma no. 31,435-8; 1.81 X 10 B q m o l ) . Diluents included deionized water for C - a n i l i n e , reagent grade methanol for C - b e n zoic acid, and reagent grade toluene for C toluene. Soils were air dried and dry sieved to
