Soil modifies water chemistry or quality through the processes of 1. Surface-exchange hydrolysis
2. Dispersion by monovalent metal ions
3. Soil's catalytic role in many chemical and/or electrochemical reactions 4. Precipitation reactions of heavy metals through hydroxylation
5. Oxidation reactions of organics and inorganics 6. Hydrolysis reactions of organics and inorganics 7. Condensation reactions of organics
8. Physical adsorption of metals and metalloids 9. Chemical reactions with metalloids
10. Soil-dissolution reactions
Overall, soil systems behave as complex biomolecular sieves. It is the purpose of this book to elucidate these soil processes in the following chapters.
PROBLEMS AND QUESTIONS
1. Explain (a) coordination, (b) coordination number, (c) tetrahedral, and (d) octa- hedral.
2. What is the driving force in cation coordination?
3. Name the various clay mineral groups present in soil.
4. What are 2: 1, 1: 1, and 2:2 clay minerals? How do they differ?
5. Explain how and why the surface charge properties of various 2: 1 clay minerals might differ?
6. What is a clay interlayer? How does it form? What is its role in ion adsorption?
PROBLEMS AND QUESTIONS 165 7. What are primary minerals and how do they differ from secondary clay minerals?
8. Explain isomorphous substitution and its practical significance.
9. Name the various cation exchange sites on clay minerals.
10. What is soil organic matter? How does it form?
11. Explain the two major groups of organic compounds found in soil.
12. What are (a) outer-sphere complexes and (b) inner-sphere complexes? What is their practical significance in soil?
13. Explain the potential reactivity of soil organic matter with (a) cations and (b) clay minerals. Explain the practical significance of this reactivity.
14. How does humic acid differ froIJ1 fulvic acid? What is the practical significance of this difference in soil?
15. Consider a surface characterized by the following reactions:
SOH; ~ SOH + H+ pKa = 4.0 SOH ~ SO-+ H+ pKa = 8.0
where S denotes charged mineral surface.
a. Plot net surface charge as a function of pH.
h. What is the pH of zero point of charge?
c. Describe briefly (2-3 sentences maximum) how would you determine the pKa values of this surface?
16. Explain the relationship between variably charged soils and surface electric potential and the relationship between constant charge soils and surface electric potential. Define the role of potentially determining ions in variably and constant charged soils. Discuss the practical meaning of the above.
17. Ten grams of soil were displaced with 250 mL of 1 M ammonium acetate and made to a final volume of 1 L. Analysis of the finall-L solution showed 20 mg L-1 Ca, 2 mg L-1 Mg, 1 mg L-1 K, and 0.5 mg L-1 Na. Estimate exchangeable cations in meq/IOO g soil.
18. After the I-M ammonium acetate extraction in problem 17, the sample was rinsed twice with distilled water and the NH! concentration of the second rinse was determined to be 180 mg L -I. The rinsed moist sample (60% moisture by weight) was then displaced with 250 mL of 1 M KCI solution and made to a final volume of 1 L. Analysis of the finall-L solution showed 30 mg L-1 NH:. Estimate the CEC (in meqll 00 g) of the soil.
19. Based on your calculations in problems 17 and 18, estimate the percent base saturation of the sample and discuss its practical significance.
166 SOIL MINERALS AND THEIR SURFACE PROPERTIES 20. Explain how one may determine the PZSE. How is it related to the PZC? What is
the practical significance of the PZC?
21. Explain when and why PZC equals the IEP of a mineral.
22. Explain the difference between the PZNC and the PZSE. When would one expect PZNC to be the same as PZSE? How would you determine that PZNC and PZSE differ?
23. Explain what type of soil mineral would not show an apparent PZSE.
24. Calculate the change in ionic strength when the thickness of the double layer is suppressed by 50% (see Eq. 3.12).
25. Give two reasons for the observed decrease in CEC of soil organic matter when in the presence of some soluble AI.
26. Give an explanation for the constant CEC of soil organic matter when in the presence of soluble sodium.
27. Name the various functional groups of (a) clay mineral surfaces and (b) soil organic matter. Explain which of these functional groups exhibits constant charge or variable charge behavior and discuss the practical significance of this behavior.
28. Based on the double layer theory, explain the potential effect of temperature on soil CEC.
29. Based on the classical double layer theory, name two assumptions that are now known not to be valid when one tries to predict ion adsorption on the basis of surface electrical potential.
30. How did the Stern model get around the limitations of the double-layer or Gouy-Chapman model?
31. On what basis does the Stern model distinguish the potential location of ions near a surface under the influence of an electrical potential?
32. Based on the classical double layer theory, name all parameters affecting CEC and explain how they affect it.