Boron: Soil Contaminant
Boric acid is moderately soluble in water. Its solubility increases markedly with temperature due to the large negative heat of dissolution. Boron is considered as a typical metalloid having properties intermediate between the metals and the electronegative non-metals. Boron has a tendency to form anionic rather than cationic complexes. Boron chemistry is of covalent В compounds and not of B3+ ions because of its very high ionization potentials. Boron has five electrons, two in the inner spherical shell (Is2), two in the outer spherical shell (2s2), and one in the dumbbell shaped shell In the hybrid orbital state, the
three electrons in the 2s and 2p orbitals form a hybrid orbital state (2sx2plp^, where each electron is alone in an orbit whose shape has both spherical and dumbbell characteristics. Each of these three orbits can hold one electron from another element to form a covalent bond between the element and В (BX3). This leaves one 2p electron orbit that can hold two electrons, which if filled would completely fill the eight electron positions (octet) associated with the second electron shell around В. BX3 compounds behave as acceptor Lewis acids toward many Lewis bases such as amines and phosphines. The acceptance of two electrons from a Lewis base completes the octet of electrons around B. Boron also completes its octet by forming both anionic and cationic complexes.1'1 Therefore, tri-coordinate В compounds have strong electron-acceptor properties and may form tetra- coordinate В structures. The charge in tetra-coordinate derivatives may range from negative to neutral and positive, depending upon the nature of the ligands.
For the unshared oxygen atoms bound to B, they are, probably, always OH groups. Thus, in accordance with the electron configuration of B, boric acid acts as a weak Lewis acid:
The formation of borate ion is spontaneous. The first hydrolysis constant of B(OH)3, Kh,, is 5.8 x 10 10 at 20°C,121 and the other Kh2 and Kh3 values are 5.0 x 10 13 and 5.0 x 10~14, respectively.131 A dissociation beyond B(OH)4 is not necessary to explain the experimental data, at least below pH 13.14,51 Boron species other than B(OH)3 and B(OH)4, however, can be ignored in soils for most practical purposes. The first hydrolysis constant of B(OH)3 varies with temperature from 3.646 x 10 10 at 178 К to 7.865 x 10 10 at 318 KJ61
Both B(OH)3 and B(OH)4 ion species are essentially monomeric in aqueous media at low В concentration (< 0.025molL '). However, at high В concentration, polyborate ions exist in appreciable amount.1"1 The equilibria between boric acid, monoborate ions, and polyborate ions in aqueous solution are rapidly reversible. In aqueous solution, most of the polyanions are unstable relative to their monomeric forms B(OH)3
and B(OH)4.|s| Results of nuclear magnetic resonance191 and Raman spectrometry1101 lead to the conclusion that B(OH)3 has a trigonal-planar structure, whereas the B(OH)4 ion in aqueous solution has a tetrahedral structure. This difference in structure can lead to differences in the affinity of clay for these two В species.
The elemental form of boron (B) is unstable in nature and found combined with oxygen in a wide variety of hydrated alkali and alkaline earth-borate salts and borosilicates as tourmaline. The total В content in soils, however, has little bearing on the status of available В to plants.
Boron can be specifically adsorbed by different clay minerals, hydroxy oxides of Al, Fe, and Mg, and organic matter.1111 Boron is adsorbed mainly on the particle edges of the clay minerals rather than the planar surfaces. The most reactive surface functional group on the edge surface is the hydroxyl exposed on the outer periphery of the clay mineral. This functional group is associated with two types of sites that are available for adsorption: Al(III) and Si(IV), which are located on the octahedral and tetrahedral sheets, respectively. The hydroxyl group associated with this site can form an inner sphere surface complex with a proton at low pH values or with a hydroxyl at high pH values. The В adsorption process can be explained by the surface complexation approach, in which the surface is considered as a ligand.1121 Such specific adsorption, which occurs irrespective of the sign of the net surface charge, can occur theoretically for any species capable of coordination with the surface metal ions. However, because oxygen is the ligand commonly coordinated to the metal ions in clay minerals, the В species B(OH), and B(OH)4 are particularly involved in such reactions. Possible surface complex configurations for В—broken edges of clay minerals—were suggested by Keren, Grossl, and Sparks.1121
Keren and Bingham1111 reviewed the factors that affect the adsorption and desorption of В by soil constituents and the mechanisms of adsorption. Soil pH is one of the most important factors affecting В adsorption. Increasing pH enhances В adsorption on clay minerals, hydroxy-Al and soils, showing a maximum in the alkaline pH range (Figure 1).
The response of В adsorption on clays to variations in pH can be explained as follows. Below pH 7, B(OH)3 predominates and since the affinity of the clay for this species is relatively low, the amount of
FIGURE 1 Boron adsorption isotherms for a soil as a function of solution В concentration and pH. Bold lines— calculated values.
Source: Mezuman and Keren.1281
FIGURE 2 Boron concentration in soil solution as a function of solution-to-soil ratio for a given total amount of B. (a) No interaction between В and soil, (b) Boron adsorption account for.
Source: Mezuman and Keren.!281
adsorption is small. Both B(OH)4 and OH- concentrations are low at this pH; thus, their contribution to total В adsorption is small despite their relatively strong affinity for the clay. As the pH is increased to about 9, the B(OH)4 concentration increases rapidly. Since the OH concentration is still low relative to the В concentration, the amount of adsorbed В increases rapidly. Further increases in pH result in an enhanced OH- concentration relative to B(OH)4, and В adsorption decreases rapidly due to the competition by OH- at the adsorption sites. Adsorption models for soils, clays, aluminum oxide, and iron oxide minerals have been derived by various workers.113-171
In assessing В concentration in irrigation water, however, the physicochemical characteristics of the soil must be taken into consideration because of the interaction between В and soil. Boron sorption and desorption from soil adsorption sites regulate the В concentration in soil solution depending on the changes in solution В concentration and the affinity of soil for B. Thus, adsorbed В may buffer fluctuations in solution В concentration, and В concentration in soil solution may change insignificantly by changing the soil-water content (Figure 2). When irrigation with water high in В is planned, special attention should be paid to this interaction because of the narrow difference between levels causing deficiency and toxicity symptoms in plants.
Boron is an essential micronutrient element required for growth and development of plants.
Many of the experimental data suggest that В uptake in plants is probably a passive process. There are clear evidences, however, that В uptake differs among species.1181 Several mechanisms have been postulated to explain this apparent paradox.118-201 Boron deficiency in plants initially affects meristematic tissues, reducing or terminating growth of root and shoot apices, sugar transport, cell-wall synthesis and structure, carbohydrate metabolism and many biochemical reactions.121221 Tissue В concentrations associated with the appearance of vegetative deficiency symptoms have been identified in many crop species. It is essential to remember that for B, as for phosphorus and several other plant nutrient elements, deficiency may be present long before visual deficiency symptoms occur.
Excess and toxicity of boron in soils of semi-arid and arid areas are more of a problem than deficiency. Boron toxicity occurs in these areas either due to high levels of В in soils or due to additions of В in irrigation water. A summary of В tolerance data based upon plant response to soluble В is given by Maas.1231 Bingham et al.1241 showed that yield decrease of some crops (wheat, barley, and sorghum) due to В toxicity could be estimated by using a model for salinity response, suggested by Maas and Hoffman.1251
There is a relatively small difference between the В concentration in soil solution causing deficiency and that resulting in toxicity symptoms in plants.1111 A consequence of this narrow difference is the difficulty posed in management of appropriate В levels in soil solution.
The suitability of irrigation water has been evaluated on the basis of criteria that determine the potential of the water to cause plant injury and yield reduction. In assessing the В in irrigation water, however, the physicochemical characteristics of the soil must be taken into consideration because the uptake by plants is dependent only on В activity in soil solution.126,271 Boron uptake by plants grown in a soil of low-clay content is significantly greater than that of plants grown in a soil of high-clay content at the same given level of added В (Figure 3). This knowledge may improve the efficacy of using water of different qualities, whereby water with relatively high В levels could be used to irrigate В-sensitive crops in soils that show a high affinity to B. Such water can be used for irrigation as long as the equilibrium В concentration in soil solution is below the toxic concentration threshold (the maximum permissible concentration for a given
FIGURE 3 Relationship between В content in wheat shoot and the amount of В added to soil, for three ratios of soil-sand mixtures.
Source: Keren et al.1261
crop species that does not reduce yield or lead to injury symptoms) for the irrigated crop. The existing criteria for irrigation water, however, make no reference to differences in soil type.
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