~ Pergamon

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Wal. Sci. Ttch. Vol. 35, No.7, pp. 139-145, 1997. C 1997 IA WQ. Published by ElsevIer Science Lid

Pnn.ed in Oreal Bri.wn. 0273-1223197 $17'00 + 0-00

TRANSPORT OF ORGANOCHLORINE PESTICIDES IN SOIL COLUMNS ENHANCED BY DISSOLVED ORGANIC CARBON

Jiann-Yuan Ding and Shian-Chee Wu

GradULJte Institute of Environmental Engineering, National Taiwan University. 71 Chou-Shan Road. Taipei 10770. Taiwan

ABSTRACT

The objective of this study is to quantify the effects of humic acid solution infiltration on the transport of organochlorine pesticides (OCPs) in soil columns using a three-phase transport model. From experimental results, it is found that the dissolved organic carbon enhances the transport of OCPs in the soil columns. In the OCPs-only column, the concentration profiles of OCPs can be simulated well using a two-phase transport model with numerical method or analytical solution. In the OCPs-DOC column. the migrations of aldrin. DDT and its daughter compounds are faster than those in the OCPs-only column. The simulation with the three-phase model is more accurate than that with the two-phase model. In addition. significant decrease of the fluid pore velocities of the OCPs-DOC column was found. When DOC leachate is applied for remediation of soil or groundwater pollution. the decrease of mean pore velocities will be a crucial afrecting factor. @ 1997 IA WQ. Published by ElseVIer Science Ltd

KEYWORDS

Dissolved organic carbon (DOC); humic acid; hydrophobic organic contaminants; organochlorine pesticides (OCPs); soil column; partition coefficient; three-phase transport model.

INTRODUCfION

Hydrophobic organic contaminants were considered moving slowly in the environment owing to their significantly low water solubility and high trend to associate with soils and sediments. However. recent investigations (Magee et al,. 1991; Dunnivant et al .• 1992; Ding. 1995; Ding and Wu. 1996) have indicated that when there is a significant amount of free-moving dissolved organic carbon (DOC) in the system. DOC may behave as a carrier. and the transport of hydrophobic organic contaminants may be enhanced. The objective of this study is to quantify the effects of humic acid solution infiltration on the transport of organochlorine pesticides (OCPs) in soil columns using a three-phase transport model.

MATERIALS AND METHODS

Mathematical models. The transport of hydrophobic organic solute in soil column can be described with a typical two-phase transport model:

139

140 J.-Y. DING and S.-C. WU

where e is the solute concentration in aqueous phase (ML-3), S is the solute concentration in the solid phase (MM-I), p is the bulk density of the solid phase (ML-3), e is the volumetric water content (L3L-3), Dc is the hydrodynamic dispersion coefficient of solute (L2'f- I), vp is the mean pore velocity (LT-I), t is the time coordinate (T), Z is the distance coordinate (L). When there is free-moving DOC in the system, the three•phase transport model for hydrophobic organic solute can be expressed as:

(2)

where M is the concentration of DOC in the aqueous phase (ML-3), Sm is the amount of solute associated with DOe (MM-I), Dm is the hydrodynamic dispersion coefficient of DOC (LlT-I). The term MSm, in the equation represents the amount of hydrophobic organic solute associated with the organic matter in the pore water, that is the mobile adsorptive phase or the third phase. The partition relationships of hydrophobic organic solute between aqueous phase and soil organic carbon or DOC can be expressed as:

(3)

Sm=Kdo<·C (4)

where Koc is the partition coefficient between water and soil organic carbon(L3M-I), foe is the fraction of organic carbon on soil (MM-I), K.toe is the partition coefficient between water and DOC (L3M·I).

The transport of DOC in soil column has to be quantified with an additional equation. It is similar to equation (1) and can be expressed as:

OM p 8M, i1M 8M -+-- = Dm---vp-tl- 0 tl- ~2 ~

(5)

where Ms is the amount of DOe adsorbed on solid phase (MM-I).

Table 1. The properties of Taichung soil

soil pH 6.8 organic carbon content (%) 1.8 solid density (g/cm3) 2.62 sand (%) 2S silt (%) 40 clay (%) 3S texture clay loam

Soil and DOC. The soil for experiments was sampled from an agricultural research station located at Taichung. Soil was air-dried for 14 days, then ground and sieved. All material passing through the sieve with 2 mm cutoff of particle diameter was retained for experimental use. Soil analyses were performed in duplicate and included pH (at 1:1 soiVwater ratio), organic carbon content (Walkley-Black method, in Nelson and Sommers, 1982), and particle-size distribution (pipette method, in Gee and Bauder, 1986). The

Dissolved organic carbon 141

water content of air dried soil was less than 1 %. The properties of the soil are shown in Table 1. Aldrich humic acid sodium salt (Aldrich Chemical Co., WI, USA) was dissolved in M!lli-Q water (Millipore Corp., Bedford, MA., USA) and filtered through the 0.451lm filter paper (Millipore CO., USA). Aldrich humic acid solution was regarded as the dissolved organic carbon in this study.

Estimate the partition coefficients. The partition coefficients of OCPs between soil organic carbon and water (Koc) were evaluated in batch experiments. The partition coefficients of OCPs between DOC (Le. Aldrich humic acid) and water (Kdoc) were estimated with water solubility enhancement method. The methods and results of estimations are described in Ding and Wu (1995). The Kow' Koc and Kdoc of aldrin, p,p' are listed in Table 2.

Table 2. The partition coefficients of organochlorine pesticides

aldrin 5.52 A 5.38 5.05 p,p'-DDr 6.36 B 5.68 5.53 p,p'-DDD 6.02 A 5.12 p,p '-DOE 5.69 c 4.42 4.88

A Garten and Trabalka (1983), B Chiou and Schmedding (1982), c Mackay (1982), other data from Ding and Wu (1995)

Adsorption of Aldrich humic acid on Taichung soil. The adsorption of Aldrich humic acid on Taichung soil was simulated with the second-order kinetics model (Hiester and Vermeulen, 1952).

(6)

where kKin is the rate constant, K is Langmuir constant, Q is the maximum amount of adsorption. The parameters of kKin, K and Q were evaluated with the recycle adsorption experiments. This process was described by Ding (1995) and is not shown here.

Soil columns and leaching experiments. Two 10 cm soil columns were packed with Taichung soil, and the soil of the top I cm layer of the soil column was spiked with aldrin and p,p'-DDT (Riedel-de Haen, Germany). The diameter of the column was 4.4 cm. Each I cm layer of the column was packed with 23.0 g air-dried soil. The bulk density of the soil column was 1.51 g/cm3. The solid density of soil was 2.62 glcm3. Under saturated conditions, the volumetric water content of the soil column may be calculated from the solid density of the soil and bulk density of the soil column. The volumetric water content of the soil column was equal to 0.423 cm3/cm3• The length of the soil column was 10 cm. The pore volume of soil column (Vo) was 64.32 cm3. There were two leaching solutions, one was DOC-free solution and the other was Aldrich humic acid solution. Both of them contain 0.002% NaN3 to inhibit the microbial activities and contain 10 mg/l Br•as tracer. Before the leaching experiments, the soil columns were saturated with Milli-Q water about two days. Then the soil columns were leached with the two leaching solutions, respectively. Schematic of the experimental apparatus is shown Figure I. The total organic carbon (TOC) concentrations of the effluent were measured with TOC analyzer (0.1. Company, model 700, College Station, TX, USA). Bromide concentrations of the effluent were also measured with an ion chromatograph (Dionex Corp .• USA) using a Dionex AS4A column. The leaching experiments were performed under 25 ± 3°C and stopped after about 40 days. Soil column was sampled in I cm intervals carefully, and was extracted with solvent. The amounts of OCPs on soil were measured with GC-ECD (Hewlett Packard, HPS890Il, Avondale. PA, USA) equipped with a DB-60S capillary column (30 m x 0.32 mm x 0.5 Ilm) (J&W Scientific, Folsom, CA, USA).

142

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f .. pu:-+£~_

--- -

constant-head device

soil column

,,-humic acid ~

r~ , .. O"".~~O" .~, r. . U collector

FIgure I. Schemallc dIagram of the expenmental apparatus.

RESULTS AND DISCUSSION

The mean pore velocities of leachate In the soil columns were measured from the volume of the effluent. The mean pore velocities of the soil columns are shown as Figure 2. Owing to the vanation of the mean pore velocities the transport model was solved with a numerical method. In addition, an average velocity can be calculated from the mean pore velocities with time average. Then, the two-phase transport model may get an analytical solution.

1 ... • OCPo - only column -;: ! 14 1-. • OCPo - DOC column

.[12 .. . . .. -. ". ~10

. . . • X

;i 8 .. .. ! • .. e. .. .... c 4 .... I •••• .. .. ::E 2 .. ..

0 0 5 10 15 20 21 30 35 40

nme (day.)

Figure 2 The mean pore velocities of the soil columns

The hydrodynamic dispersion coefficIents of solute and DOC in the soil column were regarded as same and estimated with the breakthrough curve of tracer (not shown here) by the curve-fitting method. The dispersivities of the OCP-only and OCP-DOC soil columns are 1.92 and 2.20 cm, respectively. The difference of the hydrodynamic dispersion coefficients of soil columns may result from different pore structure of soils owing to different leaching conditions. The variation of the mean pore velocities with time in the soil columns also reflects the modification of soil properties by artificial infiltration. In Figure 2, the significant decrease of the mean pore velocities of the OCPs-DOC column was shown. The behavior of the

Dissolved organic carbon 143

transport of DOC deserves attention. When the mean pore velocities are so small that the convective effect of th~ transpo~ can be neglected, they wi~1 redu~e the enhanced effect on the transport of hydrophobic organIc contanunants. When DOC leachate IS applIed for the purpose of remediation of soil or groundwater pollution, the reduction of the flow rate will be an important consideration.

The transport of DOC in the soil column was simulated with equation 5 combined with equation 6. The concentration of organic carbon in influent is 948.5 mg-CII. Concentration of DOC as high as this is not found frequently in nature, but may be so in contaminated soil or groundwater. The parameters of kKin, K and Q are 0.567 d' J, 0.008 cm3/llg, and 2600.3 ~g-C1g-soil, respectively. The breakthrough curve of DOC is shown as Figure 3. The discrepancy between model simulation and observed data after the 7th number of pore volume (VNo) is obvious. It seems to be an underestimation of adsorption of Aldrich humic acid on Taichung soil. However, the model simulation of the transport of Aldrich humic acid in soil column is reasonable for the rest of the experimental period.

1.0r---------:::===:"'l

0.' 0.8

0.7

0°.& U 0°·5

0.4

0.3

0.2

. .

DCf'I.OOC_

• 0111, dill 01 DOC -lIodeIlillulllld

0.1

o.oH,....;~~_T__r""T"..,........---r-...-.,......_r_~ ° 1 2 3 4 I 1 7 • • 10 11 12 13 14 11 11

VIVo

Figure 3. The breakthrough curve of Aldrich humic acid in the Taichung soil column.

p,p'-DDT may convert to p,p'-DDD by reductive dechlorination or to p,p'-DDE by release of a HCI molecule (Tinsley, 1979). In fact, p,p'-DDD and p,p'-DDE were found in this study. Therefore, the summation ofp,p'•DDT, p,p,-DDD, and p,p'-DDE was expressed IDDT.

The concentration profiles of aldrin and IDDT in the OCPs-only and OCP-DOC soil columns after the leaching experiments (Figures 4 and 5) were simulated with the two-phase and three-phase transport model, respectively. If the concentration of DOC (M) in the system or the partition coefficient ~oc is so small that it can be neglected, then the three-phase model can be simplified to become the two-phase model. In other words, equation 2 can be simplified to equation 1. In the OCPs-only column, OCPs were retained in the top few centimetres of the column after a period of the leaching time. The concentration profiles of OCPs can be simulated well with a two-phase transport model with numerical method or analytical solution. The difference between the numerical and analytical solutions is attributed to the difference between the real mean pore velocities and the average velocity. In the OCPs-DOC column, the migrations of aldrin and IDDT are faster than that in the OCPs-only column. In Figure 4, the migration of aldrin in the OCPs-DOC column is simulated well with the three-phase transport model. In Figure 5, a few observed data of IDDT are faster than the model simulation. The result was attributed to the smaller partition coefficients of p,p'•ODD and p,p'-DDE than those of p,p'-DDT (see Table 2). The partition coefficients of p,p'-DDT (Koc and ~oc) were applied in the model calculation only. Therefore, there is some difference between the observed data and model simulation. However, the migration of IDDT can be simulated well with the three-phase transport model well. In OCPs-DOC column, it is found that the simulation with the two-phase transport model will underestimate the migration of OCPs. The simulation with the three-phase model is more accurate than that with the two-phase model.

144

a6

f.\1 is .. I I I

~4 Ii -l!3 :g It 8.2 "0 ~I 1\ u

0 0

J.-Y. DING and S.-C. WU

7

aldrin l6 ;-; ':-1 ~s '1 E :1

OCPs-only column 14

:1 :1

- initial condition g , • observed data -83 i

:g I -rwo·phase model (Num.) 12

, - - two-phase model (Ana.) "0 t

51 ° 2 3 7 9 10

Depth of soil column ( em )

aldrin

OCPs-OOC column

- . initial condition

.. observed data

- three-phaHe model

... twn-phlll'C model

3 4 S 6 8 9 10

Depth of lOil column ( em )

Figure 4. The concentration profiles of aldrin in the OCPs-only column and OCPs-DOC column.

0.4O-,---------_____ -. . ~40,---------------_, ~0.3S ~:

i°.30 :::'0.2S 'i gO.2O -8 1°·15 'a 0.10

50·0S 1 •

OCPs-only column - initial condition

• ohKCrYcd dalll

-two-phase model (Num.)

.... lWO-pha"C model (Ana.)

1\ O.OO+-~--,......,..-,......,.._.-..,.......,.-,...-..f

o 8 10 Depth of lOiI column ( em )

iO.35 jO.30 ;;0.2S 51 gO.2O

:1 '1

~ I I

-!I 10.1S '0°.10 ...1..00 ...... -.

Ii 8 0.05

LOOT

OCPs-OOC column

- initial condition

• ob~erved data

-three-phase model

- - two-phase model

O.OO+--.,.-r---r---;::!1110.,........,..a...,......,..-,.--t o 23467 B 9 W

Deptb of lOa column ( em )

Figure S. The concentration profiles of I:OOT in the OCPs-only column and OCPs-DOC column.

CONCLUSION

In our experiments. it is found that the dissolved organic carbon can enhance the transport of organochlorine pesticides in the soil columns. The prediction with the three-phase model is more accurate than that with the two-phase model. From this study. two conclusions can be drawn. One is that dissolved organic carbon cim enhance the transport of hydrophobic organic contaminants in the environment. The other is that when there is a significant amount of free-moving dissolved organic carbon in the system, the two-phase transport model is impractical and will underestimate the results. The three-phase transport model will be a better choice. In addition, when DOC leachate is applied for remediation of soil or groundwater pollution. the behavior of the decrease of mean pore velocities must be taken into consideration.

ACKNOWLEDGEMENT

The authors express their gratitude to the National Science Council. Taiwan, for its financial support. (Contract No: NSC82-0421-E-002-033)

Dissolved organic carbon 145

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