Large application of fertilizers has been practiced on arable lands to improve crop production and recover inherent and induced soil nutrient deficiency (Kassir et al., 2012).
Contamination of agricultural soils with trace elements, such as cadmium (Cd), lead (Pb), zinc (Zn) and copper (Cu), and with fluorine can occur as these elements are transferred during manufacturing from phosphate rock to phosphate fertilizers (Camelo et al., 1997).
The accumulation of heavy metals in soil is of interest because of the adverse effect heavy metals may pose to food quality, soil health and the environment (Butkowska et al., 2015). While Cd and Pb are considered nonessential and toxic elements having no specific biological functions, Zn and Cu are micronutrients that could become toxic when exceeding certain limits (Kassir etal., 2012).
Factors Affecting the Mobility of Trace Elements in Soils
The mobility of trace elements in soils depends on their interactions between the solid and liquid phases, which determine their partitioning.
The underlying mechanisms regulating the partitioning of these trace elements include physicochemical and biological processes, which in turn are controlled by several factors including soil pH, chemical speciation, soil organic matter, fertilizer and soil amendments, redox potential, clay content and soil structure.
Soil pH
It is generally viewed that pH is the main variable controlling the solubility, mobility and transport of trace elements, as it controls metal hydroxide, carbonate and phosphate solubility (Carrillo-Gonzalez et al., 2006).
Soil pH controls the movement of trace elements from one soil compartment to another, since trace elements can be held in lattice of secondary minerals (1:1 and 2:1 clay minerals), adsorbed on Fe and Mn oxides, and carbonates, or precipitated as carbonates.
For instance, while Maskall and Thornton (1998) found increases in the proportion of readily mobile form of Pb and Zn as pH fell below 5, Cattlet etal. (2002) observed a decrease of the Zn2+ activity in the soil solution as pH increased.
They concluded that the organic matter adsorption and the formation of franklinite accounted for this trend (Carrillo-Gonzalez etal., 2006).
Many soil processes including trace element sorption are affected by soil pH. Cadmium sorption was observed to double for each 0.5 increase in pH from 3.8 to 4.9 (Boekhold etal., 1993). In sandy soils, a unit increase in pH produced a 2- to 10-fold increase in ion sorption.
Nickel removal from the soil solution by pyrophyllite increased strongly when pH went from 6 to 7.5, or even higher (Scheidegger et al., 1996). While the retention and release varied little for various cationic elements, they manifested large differences for those trace elements that form anionic chemical species such as As, Cr, or Se.
The concentration of arsenate in solution, that is, the predominant inorganic species of as decreased at low pH because of its adsorption (Manning and Golberg, 1996). An increase was observed in the concentrations of As, Se, Mo, Cr, Sb, and U in soil solutions with increasing pH (Tyler and Olsson, 2001).
While the solubility of naturally occurring Cd and Zn from mineral soils depends upon pH, in some situations dissolved concentrations of Cd, Cu, and other elements, such as Pb, may not follow a single relationship with pH for polluted soils (Carrillo-Gonzalez etal., 2006).
Chemical Speciation
Although the total trace element content determines the extent of elemental partitioning between the aqueous and solid phases in soils, the chemical speciation is likely one of the most important factors that influences trace element availability, solubility, and mobility (Carrillo- Gonzalez etal., 2006).
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Trace element ions can combine with organic and inorganic ligands or substances in soil solution or in the rhizosphere. The ligands can be hydroxyl, carbonates, sulfate, nitrate, chloride, dissolved organic matter, or chelating agents.
The distribution of metal ion species is apparently controlled by redox reactions, pH, and solubility of hydroxides, carbonates, oxides, and sulfides. Ion pairs, soluble metal-organic ligand complexes, and chelation are the three kinds of soluble complexes that can be formed between metal ions and ligands (Gao etal., 2003).
The first type is a weak electrostatic association while the second is a strong association that includes covalent bonding.
Trace element mobility is strongly restricted by carbonates in calcareous soils, presumably because of chemisorption or precipitation.
However, the presence of humic acids increases Cd, Co, Cu, and Zn adsorption even at low pH, while at high pH they reduced the precipitation of trace elements, apparently due to the formation of metal humate species (Sparks etal., 1997a,b).
The stability of the metal-organic matter complexes is influenced by pH. Copper, Pb, and Cr form stable complexes, while Cu complexes dissociate at low pH.
The association of trace elements to ligands in the soil is controlled by pH, with the ligand species ionic concentration increasing with higher pH (Carrillo-Gonzalez etal., 2006).
Soil Organic Matter
Soil organic matter (OM) can play a dual role in trace elements solubility (Carrillo-Gonzalez et al., 2006). Particulate OM, by virtue of its high cation exchange capacity (CEC), can effectively adsorb trace elements.
High-molecular-weight organic compounds can also bind and strip trace elements from the solution, because they can be insoluble and therefore semi-immobile (Schmitt et al, 2002).
Taylor and Theng (1995) reported that humic acids can increase Cd retention on kaolinite four times and the formation of stable organo-metallic complexes can lead to relatively lower mobility of Cu, Pb, Ni, Zn, and Cd.
It has also been observed that insoluble organic molecules reduced the availability of some elements, such as Cu or Pb, by the formation of insoluble complexes (Bataillard et al., 2003).
Conversely, Temminghoff et al. (1998) found that humic acids enhanced Cu mobility, but the process was strongly affected by Ca concentration and pH of the soil solution.
In general however, low-molecular-weight compounds, such as fulvic acids, could remain in the soil solution and thus increase the mobility of bound metals.
It has been found that the naturally occurring DOM can increase the mobility of some elements such as Cd (Lassat, 2002). OM reduced Zn, Pb, and Fe adsorption onto kaolinite and montmorillonite at pH 5 and 7, presumably due to metal-complexes formation (Schmitt etal., 2002).
Fertilizers and Soil Amendments
Although fertilizers have been identified as a source of trace elements, the amounts of trace elements derived from fertilizers typically do not significantly increase trace element uptake by plants (Carrillo-Gonzalez etal., 2006). The main exception are possibly phosphate fertilizers.
According to He et al. (2005), phosphate rocks contain on average 11, 25, 188, 32, 10, and 239 mg kg-1 of As, Cd, Cr, Cu, Pb, and Zn, respectively.
Cadmium is probably the main element of concern in this case since it can vary from near zero to more than 150 mg Cd kg-1 in some phosphate fertilizers. Cadmium is the most susceptible to be of concern in terms of crop accumulation from fertilizers and soil amendments.
Moreover, application of fertilizers can further affect soil properties related to metal availability. Ammoniacal nitrogen fertilization has been shown to decrease soil pH in the rhizosphere, which could modify trace elements (Zn, Cu, and Mn) availability.
Soluble phosphate, a rock phosphate, fertilizers such as monoammonium phosphate and diammonium phosphate decrease Cd, Pb, and Zn mobility, probably due to formation of metal minerals (McGowen et al., 2001).
However, DOM present in the solution can coat the phosphate surfaces and thus inhibit the sorption on phosphate compounds, reducing the amount and rate at which phosphate becomes available for precipitation (Carrillo-Gonzalez etal., 2006).
Application of limestone and alkaline waste by-products such as beringite, a modified aluminosilicate produced from the fluidized bed burning of coal refuse, to the soil has increased pH and precipitated metals. Beringite depresses trace elements mobility, apparently by precipitation, ion exchange and crystal growth (Adriano etal., 2004).
Redox Potential
Redox processes are controlled by the aqueous free electron activity, but certain microorganisms can modify and mediate most redox reactions in aquatic and terrestrial environments. Several elements, such as As, Cr, Mn, Fe, V, Mo, and Se manifest different oxidation states in the environment.
Arsenic is found in -3, 0, +3, and +5 oxidation states (Carrillo-Gonzalez etal., 2006). At the soil surface, oxidizing conditions are favoured, so it allows the formation of either As(V) or As(III).
However, microbial activity could promote methylation, demethylation, or change in the oxidation state, while the presence of clay minerals, Fe, Al, Mn oxides, and OM can also modify the oxidation state.
The most stable As chemical species are H3AsO4 up to pH 2.2, H2AsO4- in the pH range approximately between 2 and 7, and HAsO42- above pH 7.
Chromium, Hg, Se, and Mn occur in more than one oxidation state, with their solubility in the soil depending on pH and mineral content. Cr (III) is an essential nutrient, it has a low solubility, it is mainly trivalent, it is specifically sorbed by Fe, Mn, and clay minerals, and its concentration in solution decreases with increasing pH and soil OM content.
Cr (VI) on the other hand is anionic, relatively soluble and rep[resents a very mobile ion. Combined with its toxicity and carcinogenicity, this element certainly warrants careful speciation to differentiate trivalent from hexavalent chromium.
The mobile and reactive chemical species of mercury are Hg0, (CH3)2Hg. Hg2- and HgXn2-n-, where X could be OH-, Cl-, Br-, or organic ligands, hence more than one oxidation state could be present in the same environmental matrix. Selenate Se (VI) (HSe04-) is the most mobile form of Se that can be leached to groundwaters.
Manganese occurs in two oxidation states: Mn(IV), which is the most stable in neutral to slightly alkaline conditions, and Mn(II), which is stable in reducing conditions.
The solubility of Mn is highly sensitive to redox conditions; under oxidizing conditions Mn is precipitated as nodules or concretions of Mn oxides, but reduction of Mn oxides increases Mn solubility.
Trace elements such as Cu, Co, Cr, Ni, Pb, and Zn associate to Mn oxides through coprecipitation and substitution, so when Mn is reduced the solubility of Pb, Zn, Cu, and Ni increases (Carrillo-Gonzalez etal., 2006).
Clay Content and Soil Structure
Soils rich in clay generally have higher retention capacity than soils with little or no clay. Cation sorption on clay minerals varies depending on clay nature and cation properties.
The adsorption of Pb and Cu was higher than the adsorption of Zn, Ni, and Cd on illite, beidellite, and montmorillonite (Carrillo-Gonzalez et al., 2006).
Desorption followed the trend Pb>Cd>Cu>Ni>Zn for beidellite and Pb>Cd=Cu>Ni>Zn for illite and montmorillonite (Rybicka etal., 1995).
Selectivity of trace element cation adsorption varies with clay minerals. Vermiculite is very effective for adsorbing Cu2+, Pb2+, Cd2+, Zn2+, and Ni, and the selectivity is greater than in montmorillonite, apparently due to more specific adsorption sites (Malla, 2002).
However, selectivity changes with cations as Brigatti etal. (2004) reported that montmorillonite adsorbed greater amount of Hg than vermiculite.
Tiller etal.(1984) identified three reaction types, each having different affinities for cations: (1) those concerned with iron oxides, which appeared to be controlled by metal ion hydrolysis; (2) those associated with organic colloids; and (3) those associated with 2:1 clay minerals with lower sensitivity to pH.
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Main factors affecting mobility or bioavailability of trace elements in soils are given in Table I. The most important factors affecting trace elements release from soil are pH, OM including DOM, and chemical speciation, while clay content and redox potential are less important.
In summary, from the above, it can be concluded that the most important factors affecting trace elements release from soil are pH, OM including DOM, and chemical speciation, while clay content and redox potential are less important.
pH is the main variable controlling the solubility, mobility and transport of trace elements, as it controls metal hydroxide, carbonate and phosphate solubility
Trace element mobility in soil is largely controlled by several factors including soil organic matter, redox potential, soil pH, clay content and soil structure;
With the exception of phosphate fertilizers, the amounts of trace elements derived from fertilizers typically do not significantly increase trace element uptake by plants. Cation sorption on clay minerals varies depending on clay nature and cation properties.