Environmetal Soil Properties and Behaviour
DisciplinaControle e Remediação da Poluição dos Solos5 materiais • 18 seguidores
and zeta potential measurements for a kaolinite. pHiep from zeta potential measurements is 4.1, and pHpzc is about 4.08 from titration test results. (Data reported by Yong, R.N., 2001, Geoenvironmental Engineering: Contaminated Soils, Pollutant Fate, and Mitigation, CRC Press, Boca Raton, FL, 307 pp.) 50 Environmental Soil Properties and Behaviour potential \u3c2 refers to the electric potential developed at the solid\u2013liquid inter- face as a result of movement of colloidal particles in one direction and coun- terions in the opposite direction. The distinction between pHpzc and pHiep is relevant not only because slightly differing definitions exist in the literature for the pzc and iep, but also because the possible presence of potential determining ions (pdis) would influence the respective pH values obtained. What this means is that these differences in the pH values are related to methods of determination of these particular charge density relationships and the influence of counterions in the inner and outer Helmholtz planes (discussed in Chapter 3) on these mea- surement. The methods for determining the influence of the charge densities and (influence) of the ions in the inner and outer Helmholtz planes are not the same. The zeta potential \u3c2 is computed from experimental measurements obtained with a zetameter using the Helmholtz\u2013Smoluchowski relationship. One could reasonably argue that pHpzc and pHiep are operationally defined. The pzc and iep can also be distinguished according to whether or not spe- cifically adsorbed cations or anions are considered, as demonstrated in the graphical relationship shown in Figure 2.10. When H+ and OH\u2212 ions constitute the only potential-determining ions, the pH condition at which the adsorp- tion densities of H+ and OH\u2212 ions and their complexes are equally balanced is characterized as the pHiep. In terms of \u393, the surface excess concentration, pH 0 + \u2013 wa\u393H \u2013 \u393OH wc\u393H \u2013 \u393OH \u393H \u2013 \u393OH Point of zero net proton charge, pznpc Point of zero net proton charge with specifically adsorbed cations, pznpcwc Point of zero net proton charge with specifically adsorbed anions, pznpcwa Su m m at io n of S ur fa ce E xc es s C on ce nt ra tio n, \u3a3 \u393 FIguRE 2.10 Net charge (\u3c3h) curves as determined by proton balance, with and without specifically adsorbed cations and anions. The solid curve represents the condition with only H+ and OH\u2212 ions as pdis. The subscripts wc and wa refer to with specifically adsorbed cations and with specifically adsorbed anions, respectively. (Adapted from Yong, R.N., 2001, Geoenvironmental Engineering: Contaminated Soils, Pollutant Fate, and Mitigation, CRC Press, Boca Raton, FL, 307 pp.). 51Nature of Soils this means that the pHiep is obtained when |\u393H| = |\u393OH|. This distinguishes it from the situation where adsorbed ions from the porewater contribute to particle surface charges resulting in changes in the potential of the particle. Specifically adsorbed cations will decrease the pznpc, whereas specifically adsorbed anions will increase the pznpc, as shown by the net proton sur- face charge density relationship \u3c3h in Figure 2.10 The solid curve in the fig- ure represents the proton balance condition where H+ and OH\u2212 ions are the only pdis. Note that when the specifically adsorbed ions are cations (wc), a lower pznpc is obtained (pznpcwc), and when the specifically adsorbed ions are anions (wa), a higher pznpc is obtained (pznpcwa), as shown by the lower and upper dashed lines in the figure, respectively. Strictly speaking, since the point of zero charge is really the point of zero net charge, one should use pHpznc in place of pHpzc. However, we will follow common usage and continue with the use of pHpzc to mean point of zero (net) charge. 2.5.3 Exchangeable Cations and Cation Exchange Capacity (CEC) Exchangeable cations are the positively charged ions from salts in the pore- water that are attracted to the surface of clay particles because of the net negative surface charge exhibited by the particles. They are termed exchange- able because one cation can be replaced by another of equal valence, or by two of one-half the valence of the original cation. By and large, the majority of exchangeable cations in clays are calcium and magnesium, with much smaller proportions of potassium and sodium. Geological origin and environmental and regional controls (initial and subsequent leaching, etc.) are the major factors that determine which exchangeable ions will be present in a soil. Aluminium and hydrogen are the predominant exchangeable ions in acid soils. Clays derived from sedimentation-consolidation in seawater will show a predominance of magnesium and sodium. Whilst calcareous soils will contain mainly cal- cium, extensive leaching will remove the cations that form bases (calcium, sodium, etc.), leaving a clay with acidic cations, aluminium, and hydrogen. As an example of the exchange process, we consider the case where a clay containing sodium as the exchangeable cation is washed with a solution of calcium chloride. Each calcium will replace two sodium ions, which are then removed from the soil through the washing process. This process, which is called cation exchange, or base exchange, is written as Na2 Clay + CaCl2 \u2194 Ca Clay + 2NaCl In short, cation exchange occurs when positively charged ions in the porewa- ter are attracted to the surfaces of the clay fractions because of the net negative charge imbalance of the charged reactive surfaces of the clay particles. This stoichiometric process responds to the need to satisfy electroneutrality in the system, that is, replacing cations to satisfy the net negative charge imbalance of 52 Environmental Soil Properties and Behaviour the charged reactive surfaces of the clay particles. The number of charged sites considered as exchange sites are determined by isomorphous substitution in the layer lattice structure of the clay minerals. The quantity of exchangeable cations held by a clay is called the cation exchange capacity (CEC) of the clay. The CEC is usually expressed as milliequivalents per 100 g of clay (mEq/100 g soil), as shown for example for the various clay minerals listed in Table 2.1. For a clay containing solids whose net surface charges are pH dependent, the CEC of the clay is a function of the pH of the system. The clay solids that are included in this list are 1:1 and 2:1 layer-silicate minerals, natural organic matter, and the various oxides or amorphous materials. In kaolin- ites, for example, the values of CEC can vary by a factor of 3 between the CEC at pH 4 to pH 9 (Yong and Mulligan, 2004). A common technique used for measurement of CEC in clays is to use ammonium acetate (NH4OAc) as the saturation fluid. In theory, since cation sorption should occur on all available sites, one is required to determine that it occurs without the presence of artefacts. Reactions between the saturating cation solution and clay fractions can produce erroneous results. The dissolution of CaCO3 and gypsum in carbonate-rich clays when ammonium acetate (NH4OAc) is used as a saturation fluid is an example of such an artefact. Because variations in measured CEC can occur due to experimental conditions, the results obtained are sometimes referred to as operationally defined values. The relative energy with which different cations are held at the clay surface can be found by measuring their relative ease of replacement or exchange by a chosen cation at a chosen concentration. These measurements show that a small amount of calcium easily replaces exchangeable sodium, but the same amount of sodium does not replace much exchangeable calcium. The valence of the cation has a dominant influence on the ease of replacement.