Adsorption/Desorption

Overview

Adsorption relates to the process where a solute in a liquid phase becomes bonded to the surface of a solid (Linde, 2002). It can occur in three major pathways: physical adsorption, electrostatic adsorption and specific adsorption.

Desorption, the opposite process of adsorption, is likely to depend on salinity, as metals may be released from particles as they traverse the salinity gradient and encounter dissolved seawater ions, which compete for sorption sites or complex favourably with sorbed metals. Changes in pH and redox conditions, bacterial or chemical degradation of particulate organic matter (Martino et al, 2002) can also be accounted for desorption.

In some cases precipitation and dissolution can be compared as adsorption and desorption, respectively. They are different processes but with some similar practical results, removing a constituent from the dissolved phase. Precipitation is called to the process solid formation from the combination of two or more solutes. A subset of precipitation is chemical substitution, or co-precipitation, when a separate trace element becomes included in the crystal structure of the precipitating solid (Linde, 2002).

Partition coefficient

One way to parameterise the distribution of a constituent in the aquatic environment is by determination of the ratio between the adsorbed particulate concentration and the dissolved concentration. This ratio is a general approach in contaminant transport modelling (Johansson et al., 2001; James, 2002), and is known as the partition coefficient. Physically, the partition coefficient, as widely described by literature (Johansson et al., 2001), illustrates particle affinity and represents the chemical equilibrium of numerous processes such as sorption onto particulate matter, precipitation and dissolution.

This model can be applied to trace elements, as the limitation of adsorption sites in particulate matter, relatively to the low metal concentrations, can be considered not to be critical. Depending on the reversibility of these processes the partition coefficient should not be regarded as a constant but rather as a variable (Johansson et al., 2001). Again literature widely describes the factors influencing the equilibrium as being, for example, pH, salinity, concentration of suspended particulate matter, redox conditions, biogenic silica and concentration of dissolved organic matter. Examples of substances for which the partition coefficient has been either determined or modelled are: trace metals, organic micro pollutants, phosphorus and radionuclides (Johansson et al, 2001).

References

• James, I.D., 2002, Modelling pollution dispersion, the ecosystem and water quality in coastal waters: a review, Environmental Modelling & Software, 17, 363-385

• Johasson, H., Lindstrom, M., Hakanson, L., 2001, On the Modelling of particulate and dissolved fractions of substances in aquatic systems – sedimentological and ecological interactions, Ecological Modelling, 137, 225-240

• Linde, K., 2002, Assessment of Contaminant Availability in a Shallow Wetland, PhD thesis in Environmental Chemistry, University of Western Australia, Centre for Water Research

• Martino, M., A. Turner, M. Nimmo, G. E. Millward, 2002, Resuspension, reactivity and recycling of trace metals in the Mersey Estuary, UK, Marine Chemistry, 77, 171-186