## 3.4.2. Sorption

Authors: Joop Hermens

Reviewers: Kees van Gestel, Steven Droge, Philipp Mayer

Learning objectives:

You should be able to:

- understand why information on sorption is important for risk assessment
- give examples that illustrate the importance of sorption for risk assessment
- understand the concept of sorption isotherms
- be familiar with different sorption isotherms (linear, Freundlich, Langmuir).

Keywords: Sorption isotherm, absorption and adsorption, organic matter, Freundlich model, Langmuir model, organic carbon content

Introduction

Sorption processes have a major influence on the fate of chemicals in the environment (Box 1). In general, sorption is defined as the binding of a dissolved or gaseous chemical (the sorbate) to a solid phase (the sorbent) and this may involve different processes, including:

(i) binding of dissolved chemicals from water to sediments and soils

and (ii) binding of gaseous phase chemicals from air to soils, plants, and trees.

Information about sorption is relevant because of a number of reasons:

- sorption controls the actual fate and thereby the risk of (many) organic and inorganic contaminants in the environment,
- sorbed chemicals cannot evaporate, are not available for (photo)chemical or microbial breakdown, are not as easily transported as dissolved/vapor phase chemicals, and are not available for uptake by organisms,
- sorption also plays an important role in toxicity tests, affecting exposure concentrations.

Box 1. The Biesbosch is a wetland area in the Netherlands, an area in between the Rivers Rhine and Meuse and estuaries that are connected to the North Sea. The water flow is relatively low and as a consequence there is a strong sedimentation of particles from the water to the sediment. Chemicals present in the water strongly sorb to these particles, which in the past were polluted with hydrophobic organic contaminants such as dioxins and PCBs. The concentrations of these organic compounds in sediment are still relatively high because they are highly persistent. The reason for this persistence of these compounds is that these sorbed compounds are not easily available for degradation by bacteria. Also, the concentrations in organisms that live close to or in the sediment are high. These concentrations are so high that fishing on eel, for example, is not allowed in the area. This example shows the importance of sorption processes on fate, but also on effects in the environment.

Figure 1. Measurement of sorption coefficients.

Measurement of sorption is a simple procedure. A chemical X is spiked (added) to the aqueous phase in the presence of a certain amount of the solid phase (sediment or soil). The chemical sorbs to the solid phase and when the system is in equilibrium, the concentrations in the sediment (Cs) and in the aqueous phase (Ca) are measured. The solid phase is collected via centrifugation or filtration.

The sorption coefficient Kp (equation 1 and box 2) gives information about the degree of sorption of a chemical to sediment and is defined as:

(1)

Box 2: The concentration of a chemical X in sediment (Cs) is 30 mg/kg and the concentration in the aqueous phase (Ca) is 0.1 mg/L. The sorption coefficient Kp = Cs / Ca = 30 mg/kg / 0.1 mg/L = 300 L/kg Note the units of a sorption coefficient: L/kg In the environmental risk assessment of chemicals, it is very useful to understand the fraction of the total amount of chemical (Atotal) in a system that is sorbed (fsorbed) or dissolved (fdissolved) (e.g. due to an accidental spill in a river): fdissolved = Adissolved / Atotal , and thus fsorbed = 1 - fdissolved This is related to the sorption coefficients of X and the volume of the solvent and the volume of the sorbent material. The equation derived for calculating fdissolved is based on the mass balance of chemical A, which relates the concentration of X (C) to the amount of X (A) in each volume (V): C = A / V, and thus A = C ⋅ V which for a system of water and sediment (air not included for simplification) relates to: Atotal = Adissolved + Asorbed = Cwater ⋅ Vwater + Csediment ⋅ Vsediment = Cwater ⋅ Vwater + (Kp ⋅Cwater)⋅Vsediment fdissolved = Adissolved / Atotal = Cwater ⋅ Vwater / (Cwater ⋅Vwater + Kp ⋅Cwater⋅Vsediment) This way of separating out Csediment from the equation using Kp can result, after rearranging (by dividing both parts of the ratio by Cwater ⋅ Vwater) to the following simplified equation: fdissolved = 1 / (1 + Kp⋅(Vsediment / Vwater)) in this equation, ‘sediment’ can be replaced by any sorbent, as long as the appropriate sorption coefficient is used. Let’s try to calculate with chemical X from above, in a wet sediment, where 1L wet sediment contains ~80% water and 20% dry solids. The dissolved fraction of X with Kp = 300 kg/L, is only 0.013 in this example. Thus, with 1.3% of X actually dissolved, this indicates that 98.7% of X is sorbed to the sediment.

Sorption processes

There are two major sorption processes (see Figure 2):

- Absorption - partitioning (“dissolution”) of a chemical into a 3-D sorbent matrix. The concentration in the sorbing phase is homogeneous.
- Adsorption - binding of a chemical to a 2-D sorbent surface. Because the number of sorption sites on a surface is limited, sorption levels off at high concentrations in the aqueous phase.

A sorption isotherm gives the relation between the concentration in a sorbent (sediment) and the concentration in the aqueous phase and the isotherm is important in identifying a sorption process.

Figure 2. Two sorption processes: absorption and adsorption.

Absorption of a chemical is similar to its partitioning between two phases and comparable to its partitioning between two solvents. Distribution of a chemical between octanol and water is a well-known example of a partitioning process (see Section 3.4.1 on Relevant chemical properties for more detailed information on octanol-water partitioning). The isotherm for an absorption process is linear (Figure 3A) and the slope of the y-x plot is the sorption coefficient Kp.

Figure 3. Sorption isotherms for absorption: linear model (3A), and for adsorption: Langmuir model (3B) or Freundlich model (3C).

In an adsorption process, where the sorbing phase is a surface with a limited number of sorption sites, the sorption isotherm is non-linear and may reach a maximum concentration that is adsorbed when all sites are occupied. A mechanistic model for adsorption is the Langmuir model. This model describes adsorption of molecules to homogeneous surfaces with equal adsorption energies, represented by the adsorption site energy term (b) and a limited number of sorption sites (Cmax) that can become saturated (Figure 3B). The Langmuir adsorption coefficient (Kad) is equal to the product (b ⋅ Cmax) at relatively low aqueous concentrations, where the product (b ⋅ Caq) << 1 (note that the denominator term will then be ~1). Indeed, the isotherm curve on a double log scale plot shows a slope of 1 at such low concentrations, indicating linearity.

Another mathematical approach to describe non-linear sorption is the Freundlich isotherm (Figure 3C), where KF is the Freundlich sorption constant and n is the Freundlich exponent describing the sorption process non-linearity. Using logarithmic values for aqueous and sorbed concentrations, the Freundlich isotherm can be rewritten as:

Log Cs = n ⋅ log Caq + log KF (eq. 2)

This conveniently yields a linear relationship (just as y = a⋅x + b) between log Cs and log Caq, with a slope equal to n and the abscissa (crossing point with the Y-axis) equal to log KF. This allows for easy fitting of linear trend lines through experimental data sets. When n = 1, the isotherm is linear, and equals the one for absorption. In case of saturation of the sorption sites on the solid phase, 1/n will be smaller than 1. The Freundlich isotherm can, however, also yield a 1/n value > 1; this may occur for example if the chemical that is sorbed itself forms a layer that serves as a new sorbing phase and examples are described for surfactants.

Sorption phases

Soils and sediments may show large variations in composition and particle size distribution. The major components of soils and sediments are:

Sand | 63 – 2 mm
Silt | 2 – 63 µm
Clay | <2 µm
Organic matter | includes e.g. detritus, humic acids, especially associated with the clay and silt fractions
CaCO3 |

Figure 4 gives a schematic picture of a sediment or soil particle. In addition to the presence of clay minerals and (soil or sediment) organic matter (SOM), sediment and soil may contain soot particles (a combustion residue).

Figure 4. Structure of a soil or sediment particle showing the major components: organic matter and clay mineral and soot. Modified from Schwarzenbach et al. (2003) by Steven Droge.

Organic matter is formed upon decomposition of plant material and dead animal or microbial tissues. Upon decomposition of plant material, the first organic groups to be released are phenolic acids, some of which have a high affinity for complexation of metals. One example is salicylic acid (o-hydroxybenzoic acid), which occurs in high concentrations in leaves of willows, poplar and other deciduous trees. Further decomposition of plant material may result in the formation of humic acids, fulvic acids and humin. Humic and fulvic acids contain a series of functional groups, such as carboxyl- (COOH), carbonyl- (=C=O), phenolic hydroxyl- (-OH), methoxy- (-OCH3), amino- (-NH2), imino (=NH) and sulfhydryl (-SH) groups (see for more details the section on Soil).

Hydrophobic organic chemicals mainly sorb to organic matter. Because organic matter has the characteristics of a solvent, the sorption is clearly an absorption process and the sorption isotherm is linear. Because binding is mainly to organic matter, the sorption coefficient (Kp) depends on the fraction of organic matter (fom) or the fraction of organic carbon (foc) present in the soil or sediment. Please note that as a rule of thumb, organic matter contains 58% organic carbon (foc = 0.58⋅fom). Figure 5A shows the increase in sorption coefficient with increasing fraction organic carbon in soils and sediments. In order to arrive at a more intrinsic parameter, sorption coefficients are often normalized to the fraction organic matter (Kom) or organic carbon (Koc). These Koc or Kom values are less dependent of the sediment or soil type (Figure 5B).

(3)

(4)

Figure 5. The relationship between the sorption coefficient (Kp) (left) and the organic carbon normalized sorption coefficient (Koc) (right) and the fraction organic carbon (foc). Data from Means et al. (1980). Drawn by Wilma Ijzerman.

Hydrophobic chemicals can have a very high affinity to soot particles relative to the affinity to SOM. If a sediment contains soot, Kp values are often higher than predicted based on the fraction organic carbon in the organic matter (Jonker and Koelmans, 2002).

References

Schwarzenbach, R.P., Gschwend, P.M., Imboden, D.M. (2003). Environmental Organic Chemistry. Wiley, New York, NY, USA.

Means, J.C., Wood, S.G., Hassett, J.J., Banwart, W.L. (1980). Sorption of polynuclear aromatic-hydrocarbons by sediments and soils. Environmental Science and Technology 14, 1524-1528.

Jonker, M.T.O., Koelmans, A.A. (2002). Sorption of polycyclic aromatic hydrocarbons and polychlorinated biphenyls to soot and soot-like materials in the aqueous environment mechanistic considerations. Environmental Science and Technology 36, 3725-3734.

Suggested reading

van Leeuwen, C.J., Vermeire, T.G. (Eds.) (2007). Risk Assessment of Chemicals: An Introduction. Springer, Dordrecht, The Netherlands. Chapter 3 and 9.

Schwarzenbach, R.P., Gschwend, P.M., Imboden, D.M. (2003). Environmental Organic Chemistry. Wiley, New York, NY, USA. chapters 9, 11. Detailed information about sorption processes and sorption mechanisms.