Rules of Thumb for pH Dependent Surface Charge

1. At high pH, a negatively charged surface prevails.

2. At low pH, a positively charged surface prevails.

3. At some intermediate pH, the pH dependent surface charge is zero. This pH is called the point of zero charge (pzc).

Cation and Anion Exchange Capacity (CEC and AEC)

Cations are attracted to negative sites on a solid surface. Cation exchange capacity (CEC) is defined as the total number of negatively charged sites in a material at which reversible cation adsorption and desorption can occur. Operationally, it is measured by determining the total concentration (usually in meq/100 g of dry soil) of all exchangeable cations adsorbed. Thus, CEC is a measure of the reversible adsorptive capacity of a material for cations. At equilibrium, the total adsorbed cation charge equals the total negative charge on the solid. The portion of CEC not affected by pH changes is caused by adsorption to permanently charged sites. The portion of CEC that increases with pH is caused by pH dependent charged sites. Below about pH 5, H+ ions are strongly bound to oxygen atoms at crystal edges, making these sites unavailable for cation adsorption. As pH increases above 5, H+ ions are increasingly released into solution, making new sites available for cation adsorption.

Anion exchange capacity (AEC) arises mainly from protonation of hydroxyl groups on the surface of minerals and organic particles. It is mostly pH dependent.

1. pH dependent CEC does not change much as pH increases up to about pH 5.

2. Above pH 5, CEC increases rapidly with pH.

3. AEC increases as pH decreases. Gibbsite, kaolinite, goethite, and allophane clays exhibit small AECs.

Kaolinite Surface Charge

figure 6.6 Ion exchange capacity dependence on pH. Cation exchange capacity (negative charge on the solid) is relatively constant up to about pH 5, due to permanent surface charge. It increases rapidly with pH above pH 5 due to pH dependent charge. Anion exchange capacity (positive charge on the solid) decreases with increasing pH due to pH dependent surface charge.

figure 6.6 Ion exchange capacity dependence on pH. Cation exchange capacity (negative charge on the solid) is relatively constant up to about pH 5, due to permanent surface charge. It increases rapidly with pH above pH 5 due to pH dependent charge. Anion exchange capacity (positive charge on the solid) decreases with increasing pH due to pH dependent surface charge.

Exchangeable Bases: Percent Base Saturation

The primary exchangeable bases (exchangeable metal cations) are Na+, Ca2+, Mg2+, and K+. They usually occupy the majority of CEC sites in natural environments. The remaining CEC sites are occupied mainly by H+. The surface concentration of H+ is pH dependent.

Percent base saturation is defined as the percent of primary exchangeable base cations relative to the total adsorbed cation concentration, at, or near, pH 7 (CECpH=7):

% base saturation = (primary exchangeable bases/CECpH=7) x 100.

If there is no pH dependent surface charge (i.e., only permanent surface charge), CECpH=7 will equal total exchangeable cations (including H+) at any pH.

Example 6.4

Suppose a clay is measured to have the following cations (in meq/100 g):

Ca2+ = 16.2, Mg2+ = 4.4, K+ = 0.1, Na+ = 1.6, and H+ = 10.2.

What is its percent base saturation?

Answer: At the pH of the CEC measurement, the total CEC = 16.2 + 4.4 + 0.1 + 1.6 + 10.2 = 32.5 meq/100 g. The 22.3 meq/100 g of base exchange capacity due to Ca2+, Mg2+, K+, and Na+ represent 68.6% of the total CEC. The 10.2 meq/100 g of exchangeable H+ is 31.4% of the total CEC. Assume that there is no pH dependent surface charge. Then,

CECpH=7 = 32.5 meq/100 g, and percent base saturation = 68.6.

Percent base saturation is related to the soil pH as follows:

• The higher the % base saturation, the higher the pH (more sites have been vacated by H+ and occupied by metal cations).

• The lower the % base saturation, the lower the pH (more sites are occupied by H+ and unavailable to metal cations).

Leaching of soils reduces base saturation but does not change CEC. Therefore, soil leaching tends to increase soil acidity.

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