Adsorption

Adsorption is the result of chemical bonding between a gas, nonaqueous liquid, or dissolved compound and the solid sediment mineral matrix or organic matter. The strength and reversibility of this interaction can vary substantially depending upon the nature of the bonding. The most important substrates for adsorption in sediments are generally clay minerals, organic matter, and iron and manganese oxyhydroxides.

Organic compounds with hydrophobic regions often undergo adsorption onto sediment organic matter or mineral faces by Van der Waals or hydrophobic interactions. Adsorption of metals and other ions is generally via ionic or electrostatic interactions, with bonding sometimes being more covalent in nature. Metals that are complexed by DOM will also be retained by sediments if the organic matter with which they are associated is adsorbed onto sediment particles.

The adsorptive behavior of sediment is controlled by its organic content, surface area, and thus particle size, pH, and the type and density of adsorption sites. Sediments with more organic coatings generally absorb relatively large amounts of organic and hydrophobic compounds. Fine-grained sediments with relatively small particle sizes and large total surface areas have a relatively high number of adsorption sites, and thus relatively greater adsorption capacity than that of larger-grained sediments. Since clay particles exhibit both a high surface area as well as a net negative charge density, they are generally more effective at adsorbing dissolved metals and other cations than coarser-grained sediments. The mineralogy of sediments also influences the density and type of adsorption and exchange sites present, as does their moisture content, which can play an important role in adsorption, with increasing water content being associated with decreasing retention of nonpolar compounds.

A stylized sequencing of some of those processes is illustrated in Figure 1. Chemical adsorption of cations can also be treated as a surface complexation reaction in which lone electron pairs of primarily oxygen, nitrogen, and sulfur atoms at the solid surface are donated to metals and other cations to form surface complexes. In this model, surface complexing sites compete with dissolved complexing agents for cations, both being capable of forming inner or outer sphere complexes. As previously noted, surface hydroxyl groups of iron and manganese oxyhydroxide solids exhibit strong affinities for many trace metals that are scavenged by sediments. But, the presence of dissolved ligands capable of outcompeting sediment surface complexing sites can lead to metal desorption and mineral dissolution.

The pH of sediment porewaters is one of the primary controls on the adsorption of compounds by sediments. The number of negatively charged surface sites decreases with pH as they are filled and neutralized by hydrogen ions. Thus, metal adsorption is generally low at low pH when the ratio of free adsorption sites to metal concentration is low. At higher pH, metals are much more effectively scavenged as more acidic functional groups on organic matter become deprotonated and available for complexation and less mineral surface adsorption sites are filled by hydrogen ions. The pH of sediment porewaters also exerts a control on adsorption of many compounds by its influence on the solubility of minerals, especially iron and manganese oxy-hydroxides, which are important adsorption substrates.

The partitioning of a compound between the dissolved and solid phase can be described by the ratio of its equilibrium concentrations in the sorbed phase, Cs, and in solution, Cw. This ratio is referred to as a partition coefficient, Kd, where Cs is in molkg—1, and Cw is in moll—1:

However, for nutrients and contaminants sorbed on suspended particulates in aquatic systems that partitioning is often defined differently:

mgl in dissolved phase

Because surface sorption of trace metals is often by surface complexation-like interactions with oxygen donor atoms at the solid surface in the form of Si-O, Fe-O, Fe-OH, Al-O, Al-OH, or Mn-O, or oxygen or nitrogen atoms in organic matter, the affinity of trace metals for binding sites on solids, and thus their Kd, has been found to often follow the thermodynamic stability of metal complexes described by the irving-Williams series. The series predicts that, for an oxygen-containing ligand, the general affinity for trace metals, under equivalent conditions, will be Pb > Cu > Ni > Co > Zn > Cd > Fe > Mn > Mg.

The relationship between the concentration of a compound in the dissolved phase and the adsorbed phase varies over any range of the compound's total concentration, and is known as a sorption isotherm. The shape of a sorption isotherm is compound and sorbent dependent. However, experimental data often exhibit behavior similar to mathematically derived isotherms such as the Freundlich or Langmuir isotherms, allowing the sorption behavior of compounds to be simplified and modeled or predicted under certain conditions.

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