Competitive sorption and desorption of trace elements by Tunisian Aridisols Calcorthids.

The sorption and retention processes play an important role in determining the bioavaibility and fate of trace elements in soils. Sorption and desorption of Pb(2+), Zn(2+), Ni(2+), Cu(2+), and Co(2+) in three Tunisian Aridisols Calcorthids (AR1, AR2, and AR3) were studied using batch experiments. Sorption and retention capacities were determined by means of K r parameter and they were related to soil properties. The results showed that in all studied soils, K r values for Pb(2+) and Cu(2+) were higher than those of Zn(2+), Ni(2+), and Co(2+) indicating that soils have higher affinity for the first ones. The high sorption and retention capacity of the three studied soils is ascribed to their alkaline pH and their high carbonates contents favoring the precipitation of these elements. Moreover, bivariate correlation analysis showed that sorption and retention of the studied cations was also strongly correlated with clay fraction and Fe oxides contents. All soils show high sorption irreversibility of Pb(2+), Zn(2+), Ni(2+), Cu(2+), and Co(2+). The soils with highest sorption capacity show also the highest irreversibility.

Competitive and synergistic effects in pH dependent phosphate adsorption in soils: LCD modeling.

The pH dependency of soluble phosphate in soil was measured for six agricultural soils over a pH range of 3-10. A mechanistic model, the LCD (ligand charge distribution) model, was used to simulate this change, which considers phosphate adsorption to metal (hydr)oxides in soils under the influence of natural organic matter (NOM) and polyvalent cations (Ca(2+), Al(3+), and Fe(3+)). For all soils except one, the description in the normal pH range 5-8 is good. For some soils at more extreme pH values (for low P-loading soils at low pH and for high P-loading soils at high pH), the model over predicts soluble P. The calculation shows that adsorption is the major mechanism controlling phosphate solubility in soils, except at high pH in high P-loading soils where precipitation of calcium phosphate may take place. NOM and polyvalent cations have a very strong effect on the concentration level of P. The pattern of pH dependency of soluble P in soils differs greatly from the pH effects on phosphate adsorption to synthetic metal (hydr)oxides in a monocomponent system. According to the LCD model, the pH dependency in soil is mainly caused by the synergistic effects of Ca(2+) adsorption to oxides. Adsorption of Al(3+) to NOM adsorbed plays an important role only at a pH < 4.5. Presence of NOM coating strongly competes with phosphate for the adsorption and is an important factor to consider in modeling phosphate adsorption in natural samples.

Speciation of Se and DOC in soil solution and their relation to Se bioavailability.

A 0.01 M CaCl(2) extraction is often used to asses the bioavailability of plant nutrients in soils. However, almost no correlation was found between selenium (Se) in the soil extraction and Se content in grass. The recently developed anion Donnan membrane technique was used to analyze chemical speciation of Se in the 0.01 M CaCl(2) extractions of grassland soils and fractionation of DOC (dissolved organic carbon). The results show that most of Se (67-86%) in the extractions (15 samples) are colloidal-sized Se. Only 13-34% of extractable Se are selenate, selenite and small organic Se (<1 nm). Colloidal Se is, most likely, Se bound to or incorporated in colloidal-sized organic matter. The dominant form of small Se compounds (selenate, selenite/small organic compounds) depends on soil. A total of 47-85% of DOC is colloidal-sized and 15-53% are small organic molecules (<1 nm). In combination with soluble S (sulfur) and/or P (phosphor), concentration of small DOC can explain most of the variability of Se content in grass. The results indicate that mineralization of organic Se is the most important factor that controls Se availability in soils. Competition with sulfate and phosphate needs to be taken into account. Further research is needed to verify if concentration of small DOC is a good indicator of mineralization of soil organic matter.

Donnan membrane technique (DMT) for anion measurement.

Donnan membrane technique (DMT) is developed and tested for determination of free anion concentrations. Time needed to reach the Donnan membrane equilibrium depends on type of ions and the background. The Donnan membrane equilibrium is reached in 1 day for Cl(-), 1-2 days for NO(3)(-), 1-4 days for SO(4)(2-) and SeO(4)(2-), and 1-14 days for H(2)PO(4)(-) in a background of 2-200 mM KCl or K(2)SO(4). The strongest effect of ionic strength on equilibrium time is found for H(2)PO(4)(-), followed by SO(4)(2-) and SeO(4)(2-), and then by Cl(-) and NO(3)(-). The negatively charged organic particles of fulvic and humic acids do not pass the membrane. Two approaches for the measurement of different anion species of the same element, such as SeO(4)(2-) and HSeO(3)(-), using DMT are proposed and tested. These two approaches are based on transport kinetics or response to ionic strength difference. A transport model that was developed previously for cation DMT is applied in this work to analyze the rate-limiting step in the anion DMT. In the absence of mobile/labile complexes, transport tends to be controlled by diffusion in solution at a low ionic strength, whereas at a higher ionic strength, diffusion in the membrane starts to control the transport.

Heavy metal sorption and desorption capacity of soils containing endogenous contaminants.

Soils on serpentinites in some regions of northwestern Spain have been the subject of agricultural management practices involving the use of fertilizers and various types of organic waste containing heavy metals. Although such practices have facilitated crop growth, they have also raised the natural contents in heavy metals of the soils. In this work, three ferralic Cambisols and another three mollic Leptosols with high Cr and Ni contents were used to study competitive sorption and desorption of six heavy metals via K(d100), which was employed as a measure of the ability of the soils to adsorb and retain each metal. Lead was found to be the metal sorbed and retained to the greatest extent, and Cd, Ni and Zn those sorbed and retained in the smallest amounts. Although the ferralic Cambisols were found to contain greater amounts of natural heavy metals, they exhibited an increased ability to adsorb and retain the body of metals relative to the mollic Leptosols by effect of their increased contents in clay and Fe, Mn and Al oxides, in addition to their higher ion-exchange capacity. Based on the results, Pb and Cu are strongly bound, and Zn, Cd and Ni weakly bound, to the soils. The ferralic Cambisols exhibited an increased capacity to adsorb and retain Cd, Ni, Zn and--especially--Cr than the mollic Leptosols; the latter, however, proved more effective in adsorbing and retaining Cu and Pb by virtue of their increased organic matter contents. Copper sorption and retention, and Pb retention, were found to be correlated with the content in organic matter and that in vermiculite--which was only present in the mollic Leptosols--in the clay fraction.