Feature articleAdsorption of heavy metal ions on soils and soils constituents
Introduction
Soil is one of the key elements for all terrestric ecosystems. It provides the nutrient-bearing environment for plant life and is of essential importance for degradation and transfer of biomass. Soil is a very complex heterogeneous medium, which consists of solid phases (the soil matrix) containing minerals and organic matter and fluid phases (the soil water and the soil air), which interact with each other and ions entering the soil system [1]. The ability of soils to adsorb metal ions from aqueous solution is of special interest and has consequences for both agricultural issues such as soil fertility and environmental questions such as remediation of polluted soils and waste deposition.
Heavy metal ions are the most toxic inorganic pollutants which occur in soils and can be of natural or of anthropogenic origin [2]. Some of them are toxic even if their concentration is very low and their toxicity increases with accumulation in water and soils. Adsorption is a major process responsible for accumulation of heavy metals. Therefore the study of adsorption processes is of utmost importance for the understanding of how heavy metals are transferred from a liquid mobile phase to the surface of a solid phase.
The most important interfaces involved in heavy metal adsorption in soils are predominantly inorganic colloids such as clays [3], metal oxides and hydroxides [4], metal carbonates and phosphates. Also organic colloidal matter of detrital origin and living organisms such as algae and bacteria provide interfaces for heavy metal adsorption [5], [6], [7], [8]. Adsorption of heavy metals onto these surfaces regulates their solution concentration, which is also influenced by inorganic and organic ligands. Those ligands can be of biological origin such as humic and fulvic acids [9], [10], [11] and of anthropogenic origin such as NTA, EDTA, polyphosphates, and others [12], [13], [14], [15], which can be found frequently in contaminated soils and wastewater.
The most important parameters controlling heavy metal adsorption and their distribution between soil and water are soil type, metal speciation, metal concentration, soil pH, solid: solution mass ratio, and contact time [16], [17], [18], [19], [20]. In general, greater metal retention and lower solubility occurs at high soil pH [21], [22], [23], [24], [25].
To predict fate and transport of heavy metals in soils both conceptual and quantitative model approaches have been developed. These models include the determination of the nature of the binding forces, the description of the chemical and physical mechanisms involved in heavy metal–surface reactions and the study of the influence on variations of parameters such as pH, Eh, ionic strength and others on adsorption. The scope of this article covers the theoretical background on adsorption mechanisms, empirical and mechanistic models, description of surface functional groups and of basic parameters influencing adsorption of heavy metals by soils and soil constituents such as clay minerals, metal (hydr)oxides, and humic acid. Also the quantitative description of adsorption processes through adsorption isotherms and the individual adsorption behavior of selected heavy metals (Pb, Zn, Cd, etc.) in soils will be taken into account.
Section snippets
Adsorption of heavy metal ions: background
First theoretical models for adsorption of metal ions on oxides surfaces appeared approximately 30 years ago connected with experimental studies of oxide surfaces such as titration [26], [27], [28]. Theoretical models have been increasingly applied to adsorption data and since the 1990s experimental confirmation of surface stoichiometries is possible by using surface spectroscopic techniques such as TRLFS (time-resolved laser-induced fluorescence spectroscopy), EXAFS (extended X-ray adsorption
Adsorption of heavy metal ions: model approaches
There are two different approaches to adsorption modelling of heavy metal adsorption. The empirical model approach aims at empiric description of experimental adsorption data while the semiempirical or mechanistic model approach tries to give comprehension and description of basic mechanisms [35], [36]. In the empirical model, the model form is chosen a posteriori form the observed adsorption data. To enable a satisfying fitting of experimental data the mathematical form is chosen to be as
Empirical models
Empirical models are usually based upon simple mathematical relationships between concentration of the heavy metal in the liquid phase and the solid phase at equilibrium and at constant temperature. This equilibrium can be defined by the equality of the chemical potentials of the two phases [37]. These relationships are called isotherms. Monolayer adsorption phenomena of gases on homogeneous planar surfaces were first explained mathematically and physically by Langmuir in 1916 [38]. Langmuir‘s
Mechanistic (semiempirical) models
General purpose adsorption isotherms do not take into account the electrostatic interactions between ions in solution and a charged solid surface as it is the case in most surfaces encountered when dealing with soils such as clay minerals, metal (hydr)oxides, and others. Adsorption as a function of pH and ionic strength is described as a competition for adsorption sites only. The effects of modifying the electric properties of the surface due to the adsorption of charged ions and its effect on
Sorption mechanisms in soils
As the retention mechanism of metal ions at soil surfaces is often unknown, the term “sorption” is preferred [79], which in general involves the loss of a metal ion from an aqueous to a contiguous solid phase and consists of three important processes: adsorption, surface precipitation, and fixation [4].
Adsorption is a two-dimensional accumulation of matter at the solid/water interface and is understood primarily in terms of intermolecular interactions between solute and solid phases [80]. These
Surface functional groups
The existence of surface functional groups is vital for adsorption. Surface complexation theory describes adsorption in terms of complex formation reactions between the dissolved solutes and surface functional groups. In general, a surface functional group is defined as a chemically reactive molecular unit bound into the structure of a solid phase at its periphery such that the reactive components of this unit are in contact with the solution phase [80]. The nature of the surface functional
Surface complexes
In aqueous solutions, metals can act as a Lewis acid (i.e., an electron acceptor). An electron-pair donating surface functional group (such as OH, SH, and COOH) and an electron-pair acceptor metal ion (such as Me2+) form Lewis salt-type compounds. For an oxide (e.g., ferric oxide) the functional surface hydroxo groups FeOH may act as Lewis basis in deprotonated form (FeO−) to bind a Lewis acid metal ion Me2+: Metal oxianions (e.g., HAsO42−) may release OH− ions
Parameters influencing adsorption
Adsorption of heavy metal ions on soils and soil constituents is influenced by a variety of parameters, the most important ones being pH, type and speciation of metal ion involved, heavy metal competition, soil composition and aging [5]. The influence of these factors is discussed separately.
Cadmium
The occurrence of cadmium in natural soils is largely influenced by the amount of cadmium in the parent rock. Average cadmium concentration in soils derived from igneous rocks is reported to be in the range from <0.10–0.30 ppm, while soils derived from sedimentary rocks contain 0.30–11 ppm Cd [125]. Adsorption is the main operating mechanism of the reaction of Cd at low concentrations with soils. Most studies conducted found that adsorption behavior of Cd in soils can be described by either the
Summary
Soil is one of the key elements for all terrestric ecosystems and is a very complex heterogeneous medium consisting of soil matrix, soil water, and soil air. Heavy metal ions are the most toxic inorganic pollutants which occur in soils and can be of natural or of anthropogenic origin. Adsorption is a major process responsible for their accumulation. The most important interfaces involved in heavy metal adsorption in soils are predominantly inorganic colloids such as clays, metal oxides and
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