Pesticide sorption onto aquifer sediments

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Abstract

This study aims to investigate the nature and extent of sorption of herbicides by sandy aquifer sediments. Three phenylurea herbicides (linuron, diuron and isoproturon), two triazine herbicides (atrazine and simazine), and one phenylurea metabolite and one triazine metabolite (dichlorophenylurea and desethylatrazine, respectively) were investigated. The approaches taken were: (1) batch experiments to determine Kd values followed by simple linear regression against sediment properties; (2) theoretical consideration of sorption, using partial molar free energies and binding-site calculations. The results demonstrated that the sands had a powerful sorption capacity for all of the compounds studied. Sorption of the phenylurea compounds indicated a probable dual-mode sorption process in which partitioning and site-directed interactions occur.

Introduction

The transport and ultimate fate of herbicides in soil and sediment systems is controlled to a large extent by the process of sorption. Sorption ultimately determines mobility and availability for bio- and chemical degradation (e.g. Karickhoff, 1984; Weber et al., 1993; Murphy and Zachara, 1995).

The interactions of herbicides with the solid phases of soils and sediments are many, poorly understood, and often difficult to differentiate (Karickhoff, 1984; Nicholls, 1991). The chemical and physical properties of a particular herbicide, and the chemical and physical properties of the sorbent, determine the exact nature and extent of the sorptive process (Rao and Davidson, 1980).

Within a soil matrix, organic carbon (OC) is regarded as being the key control on sorption (Nicholls, 1991), whereas for sediments, which often have low OC, numerous properties have been related to herbicide sorption. These normally include: particle-bound OC content; water pH; sediment surface area (SA); temperature; mineralogy; sediment cation exchange capacity (CEC); and particle size distribution (Karickhoff, 1984). Of these properties OC content, pH, SA and CEC have been identified as the most important (Schwarzenbach and Westall, 1981; Karickhoff, 1984). The OC content is normally the dominant factor governing the hydrophobic sorption of herbicides in the majority of soils and sediments (Karickhoff, 1984). In such cases, the apparent sorption coefficients (Kd′) can be normalised to the OC content, yielding the organic-carbon-normalised sorption coefficient (Koc′), and so reducing the inherent variability when comparing the extent of sorption in different soil and sediment types.

This relationship does however not apply under all conditions. Adsorbents with low organic carbon contents (<0.1% OC), e.g. many aquifer sediments, have apparent Koc′ values which can be elevated because of nonhydrophobic sorption (mineral phase) processes (Karickhoff, 1984). Some authors have suggested in these cases that nonhydrophobic (mineral phase) bonding may be even more prevalent than hydrophobic (organic) sorption (Karickhoff, 1984; Piwoni and Banerjee, 1985; Barber et al., 1992).

To date there has been little published information on nonhydrophobic sorption processes. Previous work has focused on relating OC and expandable clay mineral content of sediments to the extent of herbicide sorption (Karickhoff, 1984).

This paper reports the findings of an investigation into herbicide sorption in sandy aquifer sediments with a low organic carbon content; these sediments have previously been shown to exhibit large sorption capacities for commonly used herbicides (Long, 1996). The study was designed to evaluate the extent of herbicide sorption and to try to identify sediment components which could be used to describe the sorptive process in sediments with low organic carbon contents.

Section snippets

Sediments

Sediment cores were obtained from the Shortlands Site (SHL) in the Eocene Thanet Sands Formation which forms part of the London Basin aquifer system (Long, 1996). Dry-percussion drilling yielded core sections of approximately 1 m length. Core sections were halved longitudinally and the centre channel of one half-section was excavated and homogenised. Fifteen sediment samples were taken over the depth range 3.9–19.25 m below surface. The material was placed in pore-fluid extraction centrifuge

Partial molar free energy

The free-energy changes that may occur when a chemical is adsorbed can be used as a measure of the extent or driving force of the reaction (Bailey et al., 1968). The change in the partial molar free energy can be calculated from the thermodynamic relationship:−F=RTln(Ce/Co)where R is molar gas constant, T is absolute temperature in degrees Kelvin, Ce is the equilibrium concentration and Co is the initial concentration of the solution prior to sorption.

It follows therefore that the greater the

Sediment characteristics

The results of the analysis of the core are given in Table 1; a dramatic change in core properties for some parameters (pH, conductivity, SO42−, NO3−, Na+, K+, Mg2+, Ca2+, Fe) occurs at a depth of approximately 11 m. This marks the boundary between the overburden, consisting of fluvial sediment, and the Thanet Sands Formation which is the aquifer material proper. The low pH values (down to 2.7) within the sands and the correspondingly high concentrations of iron and sulphate (up to 689.0 and

Discussion

Linear correlation analysis of the sorption data from the experiments where the ionic strength was 10 mM CaCl2 suggested that the sorption of the three phenylurea herbicides (isoproturon, diuron and linuron) was most significantly related to five properties: fc (fraction of carbon); CEC; SA; silt fraction and pHeqm (Fig. 1, Fig. 2, Fig. 3). The relationships between the sediment properties to one another were analysed using simple linear correlation. From this it is apparent that the silt size

Conclusions

(1) Triazines and metabolite compounds were completely adsorbed by the sediments. This may be indicative of protonation of the compounds, making them cationic in nature owing to a decrease in the solution pH as the batch equilibration progresses.

(2) Sorption isotherms for three phenylurea compounds exhibited a degree of nonlinearity as reflected by the Freundlich number (N). The results are consistent with the aquifer sediments being a dual-mode sorbent in which partitioning and site-directed

Acknowledgements

The authors would like to thank the NERC for the ROPA award which allowed this research to be performed.

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Present address: Environmental Science Department, I.E.N.S., Lancaster University, Lancaster, LA1 4YQ, UK.

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