Buffer Capacity of Soil

The formation of complexes modifies the metal species dissolved in pore fluid and those sorbed on soil particle surfaces interactively. The formation of complexes reduces the concentration of free metal ions in the dissolved phase. As a result, concentration-dependent properties of metals are altered. The effects include (a) modification of solubility, toxicity, and possibility biostimulatory properties of metals (b) modification of surface properties of solids and (c) sorption of metals from solution (Snoeyink and Jenkins, 1980). As a result, chelating agents can enhance the electrochemical remediation of subsurface contamination. 

The chelating agents used by researchers to enhance electrochemical remediation include EDTA, citric acid, oxalic acid, ammonia, iodide/iodine, potassium iodide solution, sodium chloride solution, l-hydroxyethane-l,l-diphosphonic acid (HEDPA), HPCD, and so on. EDTA is the most frequently used chelating agent in electrochemical remediation. The complexation chemistry of EDTA is briefly outlined as an illustration. 

Let Y denote the ethylenediaminetetraacetate ion EDTA. With a metal ion M, it can form a complex MY, a protonated complex MHY, a hydroxo complex MY(OH)n, and a mixed complex MYX, where X is a monodentate ligand. The complexation reactions involved are 

The results of Suer and Lifvergren (2003) indicate that mercury was removed from a field-contaminated soil by a combination of redox and complexation processes with iodide/iodine and electrokinetic mobilization. Iodide added to the cathode compartment was transported into the soil and oxidized to iodine near the 

The experimental results of Popov et al. (2001) reveal the influence of the chelating agent HEDPA on the electroosmotic flow rate in a natural sod-podzolic soil contaminated with 0.003 g phenol per gram of dry soil. The introduction of a small amount of HEDPA into the system increases the coefficient of electroosmotic conductivity of the soil from 4 x 10 to 11.2 x 10 mW-s. Up to 80%-95% of the phenol was removed after 30-50 h of treatment, and the average electroosmotic flow rate remained constant throughout the tests. 

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The presence of acidic functional groups, mostly carboxyl and phenolic OH groups, in the molecular structure of soil HS renders them major players in the acid-base buffering capacity of soils and in the fate, bioavailability, and physico-chemical behavior of macro- and micronutrients, toxic metal ions, and several xenobiotic organic compounds in soil (Ritchie and Perdue, 2003 Senesi and Loffredo, 2005). Consequently, the effects of amendment on the acid-base properties of soil HAs and FAs is a subject of considerable interest. 

Soil pH. This is only important when the pH buffering capacity of soil is very large (B approaches zero). Generally, when initial pH is high and soil B is very low, maximum NH3 is lost (Avnimelech and Laher, 1977). 

Soil acidity is known to exert adverse effect on crop growth by its effect on nutrient availability and microbial activity. Measurement of soil pH only is not a true representation of soil acidity. This is due to the fact that soil pH which is a measure of active acidity is subject to change due to a number of factors. Substantial amoimts of soil acidity reside in the soil solid phase, in the interlayer spaces, as solid phase minerals and in the fimctional group of soil-organic fraction. All these contribute to the pool of active acidity in response to any shift in the thermodynamic equilibrium and thus helps in maintaining the buffering capacity of soils. 

The lime potential is a very reliable estimate to predict the buffering capacity of soils. Smaller the value of lime potential, greater will be the buffering capacity of the soil. 

The same principles apply equally to solid matrices, both natural and artificial, and the pH both of the matrix and the solution of the toxicant should be carefully taken into consideration. In general, pH control of the matrices is difficult to maintain and reliance must be made on the natural buffering capacity of soils containing humic material. 

This buffering capacity of soils is of great practical importance. If it did not exist it would be difficult to conceive of agriculture as we know it today. We would have a... 

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