Analysis of the Distribution Coefficient of Simazine in Organic Soils vs. Non-organic Soils

Lindsay Braun

Abstract

Soil samples of high organic and low organic content were collected in this study to analyze the distribution behavior of the herbicide, Simazine in different soil types.  Simazine is the active ingredient in the commercial herbicide known as Princep Calibur 90.  High Performance Liquid Chromatography was used to analyze the herbicide absorption capacity of the soil samples.  Results show that the amount of herbicide absorbed by the soil varies linearly with the fraction of organic carbon.  Soils that are highly organic compared to those with low organic matter content retain the herbicide to a greater extent.  The determination of the extent of absorption of the herbicide by different kinds of soil is important since the smaller amount of absorption by the soil increases the risk of contamination of the groundwater supply.

 Introduction

The issue of contaminants in the groundwater supply has peaked as an environmental issue, especially of those contributed by organic crop herbicides.  The movement of the contaminant into groundwater is controlled by the distribution coefficient (K_d) of soils, which is a measure of amount of interaction between the herbicide and the makeup of separate soil types, (Dolan, 1998), 

K_d = S                                      (equation 1)

                                       C

where S is the amount of herbicide retained in the soil and C is the amount of herbicide absorbed by the water supply.  The amount of herbicide absorbed by the soil depends on the fraction of organic carbon within the soil (Dolan, 1998).  Assuming that the amount of herbicide  retained by the soil is directly proportional to its content of organic carbon, K_d can be expressed in terms of the soil�s  organic carbon content:  

K_d = K_oc x f_oc                                  (equation 2)

where K_oc is the organic carbon-water distribution coefficient and f_oc is the fraction of organic carbon in a given type of soil.  The plot of K_d versus f_oc is linear; K_oc is the slope of the linear plot.  Anything other than a linear plot would imply that the amount of chemical absorbed into the groundwater is affected by the non-organic makeup of soil (Dolan, 1998).

            The herbicide used in this experiment is Simazine (2-chloro-4,6-bis-ethyl-amino-s-triazine), which is the active ingredient of Princep Calibur 90.  The chemical makeup of Princep is closely related to that of Atrazine (2-choro-4-ethylamino-6-isopropylamino-6-isopropylamino-s-trazine).  The concentrations of Atrazine found in the groundwater supply has been under observation in numerous experiments.  Even though Princep is widely used in agricultural settings just as is Atrazine, it had not been given the attention that Atrazine had as an environmental hazard.  This study confirms that Princep seems to react similar to Atrazine with the organic matrix of soils. 

            In this study, High Performance Liquid Chromatography (HPLC) is used to evaluate the amount of Simazine absorbed by the organic components of soil. Three soil types are analyzed.  The two organic soil samples used are the commercial topsoil and the soil gathered from outside of Vandalia, IL.  Sand is chosen to represent the non-organic soil used in this study.   

            The procedure followed in this experiment is similar to that of Dolan, Zhang, and Klarup who used the herbicide, Atrazine (Dolan, 1998). The purpose of this experiment, in reference to the experiment involving Atrazine, is to see how closely these two herbicides are related.  This study confirmed that the extent of groundwater contamination by Princep depends on the local soil type (i.e. the organic content of soil), similar to the experimental results obtained with the respect to Atrazine.

Materials

Equipment used:
Buck Scientific BLC-20-UV-VIS Integrated HPLC System
Drying Oven
Degassing Apparatus
Whatman Syringe Filters (with 0.2 micrometer size membrane)

Chemicals used:
Mobile Phase:  (1:1), Acetonitrile : Water
Princep (56 ppm in Acetonitrile/Water)

Method

            Three soil samples are gathered including two rich in organic (commercial topsoil  and farm ground from Vandalia, IL) and one rich in non-organic components (sand).  Ten grams of each soil sample is heated overnight at one hundred five degrees Celsius, then for ten hours at two hundred fifty degrees Celcius, to determine the percent moisture and the fraction of organic matter, respectively. 

            To analyze the amount of Simazine retained in the soil, each of the three soil samples are treated with 50 mL of 56 ppm solution of Princep in acetonitrile/water mixture.  During this procedure, the solutions are swirled continuously for ten minutes followed by occasional swirling for twenty-four hours.  Afterwards, the solutions are centrifuged for ten minutes followed by filtration, first, with #42 Whatman filter paper and, then, using a syringe filtration technique.  Finally, all of the samples are analyzed by HPLC.  Reference solutions for each soil sample without Princep are also analyzed by HPLC for comparison. 

Data:  Chromatograms for different soil types in the presence and absence of Princep are shown below: 

 Blank Solution (without soil) Containing Princep*

Chromatogram I

* The only peak measurable (with a good signal/noise ratio) appeared at the retention time of 4.316 minutes.  The peak was assigned to Princep.

Filtered Sand Solution Containing Princep*

Chromatogram II

*This chromatogram analyzes the amount of Princep not retained by sand.  The peak area eluted at 4.333 minutes is directly proportional to the amount of Princep present in the solution.

Filtered Farm Ground Solution Containing Princep*

Chromatogram III

*Princep peak area at 4.416 minutes is smaller than that for �sand� which indicates that more amount of herbicide is retained by �farm ground� than by �sand�.

Filtered Topsoil Solution Containing Princep*

Chromatogram IV

*The Princep peak area at 4.316 minutes is slightly smaller than that for �farm ground� which indicates that more amount of herbicide is retained by �topsoil� than by �farm ground�.

 Filtered Sand Solution without Princep*

Reference Chromatogram I

* Princep peak at 4.316 minutes is not observed.
Filtered Farm Ground Solution without Princep*

Reference Chromatogram II

*Peaks at 1.550 and 1.816 minutes represent the organic compounds present in �farm ground� soil.

Filtered Topsoil Solution without Princep*

Reference Chromatogram III

* It is clear that the amounts of organic acids present (at 1.583 and 1.816 minutes) are much larger than those observed in �farm ground�.

 Results

            The amount of mass lost upon heating for different soil samples are given in Table I.  Determinations of the percent moisture content and the fraction of organic matter are based on the mass losses at one hundred five degrees Celcius and two hundred fifty degrees Celcius, respectively.  The fraction of organic carbon (f_oc) was calculated by assuming that about 0.58 of the fraction of organic matter (f_om) consists of organic carbon (Dolan, 1998). 

Table I

            As indicated in Table I, the % moisture present in the soil increases in the order:  sand < topsoil < farm ground.  Also, the fraction of organic matter (f_om) and the fraction of organic carbon (f_oc) are found to be highest and lowest in �topsoil� and �sand,� respectively.

Table II gives the results of HPLC analysis (peak areas and retention times in minutes) for different soil samples after treatment with Princep.

Table II

Retention Times (min)

            As indicated by the peak areas of Princep in Table II, the amount of Princep not retained by soil increases in the order:  topsoil < farm ground <sand.

              Results of HPLC analysis of reference soil solutions without Princep are shown in Table III for comparison of retention times. 

Table III

Retention Times (min)

 Calculations of the Distribution Coefficient, K_d for different soil types:  

1.  Determining the amount of Princep retained in the soil (S):

            First, the amount of Princep in the solvent (C) was calculated using the chromatograms according to the following equation:

C = peak area of soil sample x amount of Princep in blank

peak area of the blank

 Then, S, the amount of Princep retained in each soil sample is calculated using the equation:

 S = (C_i � C) x 50mL

M_soil

where C_i is the total amount of Princep added to the soil samples in 50 mL of solvent, C is the amount of Princep not retained by the soil and M_soil is the dry soil mass.

2. K_d is found by taking the ratio of the amount of Princep retained by soil (S) to the amount absorbed by water (C).

The results are shown in Table 4.

Table IV

 The linear relationship between K_d and the fraction of organic carbon is shown on the graph below:

 Discussion

            The data gathered in this experiment supports the theory that soil with more organic carbon absorbs organic herbicides better than a soil with little or no organic carbon.    The topsoil used in this experiment contains the highest concentration of organic carbon.  The farm ground contains just slightly less organic carbon than the topsoil.  The sand contains hardly any organic carbon.  The plot of the distribution coefficient versus the fraction of organic carbon appears to be linear, supporting the idea that the organic carbon content of soil increases the amount of herbicide absorbed into the soil. 

            Peak 2 on the chromatograms represents Princep in the solution.  The peak area is proportional to the amount of Princep not retained by the soil sample.  As shown in Table II, Princep peak area decreases as the organic content of soil increases.  Peak 1a  represents humic acids, and  peak 1b represents fulvic acids.  These two organic components are portions of Humus (decomposed plant and animal matter), which are soluble.  Humic acids are present in a greater amount than fulvic acids in grassland soils (http://www.ar.wroc.pl/~weber/kwasy2.htm).  This fact is supported by the larger peak area of humic acids relative to that of fulvic acids on the chromatograms of topsoil and farm ground samples.  Also, comparison of the chromatograms (farm ground and topsoil) indicates that topsoil contains a much larger ratio of humic acids than farm ground does.

             This study shows that the groundwater supply could be saved from agricultural runoff if the soil type absorbs most of the herbicide.  The soils containing high organic carbon content absorb most and, therefore, are the most likely to prevent contamination of the groundwater supplies.

Works Cited

Chefez, B., Chen, Y., Clapp, C., Hatcher, P.  �Characterization of Organic Matter Soils.�  Soil Science of America Journal.  2000. (online)

Dolan, E., Zhang, Y., Klarup, D.  �The Distribution Coefficient of Atrazine with Illinois Soils.�  Journal of Chemical Education.  Vol. 75 No. 12.  1998.

Meloan, Clifton.  �Chemical Separations:  Principles, Techniques, and      Experiments.�  Jon Wiley and Sons Inc., 1999.

Pederson, T. L.  �Pesticide Residues in Drinking Water.�  June 1997.            (online)

Pike, David R.  �Reducing Herbicide Movement to Surface and Groundwater.�  December 11, 1998.  (online)

Roeth, F. W., Comfort, S. D.  �Questions and Answers About Atrazine.�     April 1996.  (online)

Vitosh, Maurice L. and Jacobs, Lee W.  �Nutrient Management to Protect Water Quality.�  January 1996.  (online)

�What are Humic Acids?�  http:  www.horizonag.com/art_2.htm. 1993.

�Properties of Humic Substances.�  http:  ar.wroc.pl/~weber/kwasy2.htm.