Introduction

The issue of contaminants in the groundwater supply has peaked as an environmental issue, especially those contributed by hazardous wastes dumped on land. The environmental behavior of organic compounds depends on a number of mechanisms responsible for their migration, or attenuation at the surface and in the subsurface of the soil. Whether the organic compound moves readily or tends to remain fixed in association with the soil depends on the properties of the particular chemical and the soil characteristics. The movement of the contaminant into groundwater is controlled by the distribution coefficient of soils (KD), which is a measure of the amount of interaction between the organic compound and the make up of separate soil types (Dolan et al. 1998):

$$ K_{\text{D}} = \frac{S} {C} $$
(1)

where S is the amount of organic compound adsorbed in the soil and C is the amount of the organic compound soluble in water. The amount of organic compound adsorbed by the soil depends on the fraction of organic carbon within the soil (Dolan et al. 1998), assuming that the amount of organic carbon content is

$$ K_{{\text{OC}}} = K_{\text{D}} \times f_{{\text{OC}}} $$
(2)

where KOC is the organic carbon–water distribution coefficient and fOC is the fraction of organic carbon in a given type of soil. The plot of KD versus fOC is linear; KOC is the slope of the linear plot. Anything other than a linear plot would imply that the amount of chemical adsorbed into the groundwater is affected by the non-organic make up of soil. This study confirmed that the extent of groundwater contamination by hazardous organics depends on the local soil type (i.e., the organic content of soil). Such soils can be selected as sites for dumping the hazardous waste generated by industries and can be treated by cost-effective methods like bioremediation.

Experimental details

Collection of soil samples

Soil samples were collected from three different sites in Patancheru Industrial Area (Hyderabad, Andhra Pradesh, India). Samples were taken from the top 15–20 cm of the soil. A V-shaped hole was dug and sliced from the three sides and this sample was placed in a bucket. This core represented an individual sample and 15 such samples were collected. A composite sample was made by mixing the 15 samples, which was representative of the entire area selected.

After collection of the soil samples, the lumps were broken and stones removed. The soil was completely mixed and stored in polythene bags and taken to the laboratory for analysis. The soils were air dried under shade, sieved through a 2-mm sieve and used for the studies.

Materials

Analar grade phenol, p-nitrophenol, 4-chloro-2-nitrophenol, 2,4-dichlorophenol were used for the adsorption studies. Different concentrations of the compound were prepared in the range of 5–25 ppm in 0.01 M CaCl2. The characteristics of the organic compounds are reported in Table 1.

Table 1 Characteristics of hazardous organics selected
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Analysis of percent organic carbon

The soil organic carbon content was determined by the wet digestion method of Walkey and Black (1934) as described by Jackson (1967), which involves oxidation with dichromate and back titration of excess of dichromate with ferrous ammonium sulphate and expressed in percentages 0.5 g of the soil was taken in a 250 ml Erlenmeyer flask. To the flask, 10 ml of 1 N K2Cr2O7 and 20 ml of conc. H2SO4 was added and allowed to stand for half-hour. After the reaction time was completed 200 ml of distilled water was added along with 10 ml of H3PO4, 0.2 g of NaF and 1 ml of diphenyl amine indicator. The contents of the flask were titrated against 0.5 N ferrous ammonium sulphate until the solution in the flask turned a persistent bottle green color. A similar procedure was followed for a blank.

Adsorption studies on 2-mm sieved soils (whole soils)

For the adsorption isotherm studies, batch experiments were conducted by equilibrating the soils with the organics. Soils were shaken at 37°C for 24 h. Identical soil blanks minus the organic were also maintained in every case. The suspensions were centrifuged at 5,000 rpm for 15 min and filtered through Whatman no.1 filter paper. The amount adsorbed was determined by analyzing the equilibrium concentration of the organic compounds by measuring the absorbencies at their respective λ maximum and calculating the difference between them and initial concentration after correcting for soil blanks. The experimental variables considered were: (a) initial concentration of hazardous organics, 5–50 mg/l (b) pH 7 (c) weight of soil, 3 g (d) volume of hazardous organic, 50 ml and (e) soils selected: M, I and B. The amount adsorbed by the soil for the four selected organics is given in Table 2.

Table 2 Adsorption of hazardous organics on 2-mm sieved soils
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Estimation of distribution coefficient values of the identified organics in the soil water system

The distribution coefficients KD and KOC have been estimated by using Eqs. 1 and 2, respectively. The organic carbon has been determined by using the experimental method as given above and the corresponding fraction is calculated as fOC. The important assumption is that the organic matter fraction of the soil is solely responsible for sorption and retention of the hazardous organics. Table 3 lists the KOC ranges, corresponding KD (assuming 1% organic carbon) and Rf values for the selected organic compounds on the three soils. The KOC has been calculated using concentrations μg/g (in soil) and μg/ml (in water) and therefore has the units of ml/g. The Rf (Morril et al. 1982) function is borrowed from chromatography and is a measure of the fractional transport of the organic compound compared to the water solvent. When KOC=0, Rf=1 and there is no interaction with soil. Consequently, the compound moves freely into water. When KOC=0, Rf=1 and there is no interaction with soil. Therefore, the compound moves freely into water. When KOC is very large Rf approaches 0, signifying that the compound is completely immobilized.

Table 3 Distribution coefficients and mobility characteristics of the hazardous organics
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$$ {\text{R}}_f = \frac{{{\text{Rate of movement of solute}}}} {{{\text{Rate of movement of aqueous phase}}}} $$
(3)

According to the above discussion, it is found that the KOC is an integral component of determination of the mobility properties in accordance with KD and Rf (Vanloon and Duffy 2000). Hence, based on the KOC and KD values the mobility of the four selected compounds are determined by comparing with the standard values (Vanloon and Duffy 2000) given in Table 4.

Table 4 Distribution coefficients and mobility properties of various classes of organic compounds in soil
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Summary and conclusions

The soils used in this experiment contain organic carbon in the following order: soil B (2.4%) < soil I (1.5%) < soil M (0.93%). The data gathered in these experiments support the theory that soil with more organic carbon adsorbs organic compounds better than soil with little or no organic carbon (Table 2). The plot of the distribution coefficient versus the fraction of organic carbon appears to be linear (Figs. 1, 2, 3, 4) supporting the idea that the organic content of soil increases the amount of organic compound adsorbed into the soil (Borggard and Streibig 1988). Among the four compounds studied it was found that adsorption was maximum for 2,4-dichlorophenol due to the high hydrophobicity (Gao et al. 1998) and least for phenol (Table 2).

Fig. 1
figure 1

Distribution coefficient (KD) as a function of organic carbon (phenol) (fOC)

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Fig. 2
figure 2

Distribution coefficient (KD) as a function of the fraction of organic carbon (p-nitrophenol) (fOC)

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Fig. 3
figure 3

Distribution coefficient (KD) as a function of organic carbon (4-chloro-2-nitrophenol) (fOC)

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Fig. 4
figure 4

Distribution coefficient (KD) as a function of the fraction organic carbon (2,4-dichlorophenol) (fOC)

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The study shows that groundwater supply could be saved from hazardous organic contamination if the soil type adsorbs most of the organic compounds. The soils containing high organic carbon content adsorbs most and therefore are the most likely to prevent contamination of the groundwater supplies. Such soils can be used as sites for dumping the hazardous waste generated by industries allowing their further treatment by methods like bioremediation and phytoremediation.