1/24/2019 Ch 7 - Environmental Mineralogy Mineral = a naturally occurring inorganic substance with a characteristic internal structure and a chemical composition that is either fixed or varies within certain limits. Clay Minerals Clay = any material less than 2 um in diameter Clay Mineral = a fine-grained hydrous silicate composed of layers of tetrahedrally and octahedrally coordinated cations 1 1/24/2019 Octahedral Sheets Tetrahedral Sheets Kaolinite Clay Structures (1:1 Clays) T O 2 1/24/2019 Structure of Kaolinite Clay Minerals T O T O T O Montmorillonite Clay Structures (2:1 Clays) T O T 3 1/24/2019 Structure of Montmorillonite Clay Minerals T O T T O T T O T Ionic Substitutions, p. 211 1. Ions of one element can substitute for those of another in a crystal structure if their radii differ by less than about 15%. 2. Ions that differ by one charge unit substitute readily for each other as long as charge neutrality is maintained. Note that this requires a coupled substitution, such as occurs in the plagioclase solid solution series. If the charges differ by more than one unit, substitution is limited. + + 2+ 3+ 4 1/24/2019 Ionic Substitutions, p. 211 3. When two ions can occupy the same site in a electronegativity crystal structure, the ion with the higher ionic potential (I.P.) preferentially enters the site. I.P. = charge radius in Angstroms 4. Even if the size and charge of the minor and major ion are similar, substitution may be limited for the minor ion if it has a very different electronegativity and forms a bond of very different character from that of the major ion. EXAMPLE 7-3, p. 212 In galena (PbS, radius Pb = 1.26 Å) lead is in six-fold coordination with sulfur. Will any of the following trace elements – Mn2+, Ag+, or Np3+ - likely be found in galena? The ionic radius of S2- = 1.72 Å and the E.N. = 2.6. EN % Ionic Ionic (Table Character Subst Ions radius I.P. % Diff. Radii 7-2) Metal-S bond . ? (App III) (Table 7-3) +2/1.26= 2.6 - 1.8 = 0.8 Pb2+ 1.26 Å 1.8 --- --- 1.6 15% +2/0.75= 2.6 – 1.6 = 1.0 (1.26-0.75)/1.26 Mn2+ 0.75 Å 1.6 2.7 22% = 40% +1/1.23 = 2.6 – 1.9 = 0.7 (1.26-1.23)/1.26 Ag+ 1.23 Å 1.9 0.81 12% = 2.4% +3/1.10 = 2.6 – 1.3 = 1.3 (1.26-1.10)/1.26 Np3+ 1.10 Å 1.3 2.7 34% = 13% 5 1/24/2019 Ionic Substitutions in Clay Minerals Kaolinite Structure – Montmorillonite Structure – tetrahedral sheets octahedral sheets Positive Charge Deficiency - Table 7-5. Summary of the principal characteristics of the layered clay mineral groups Smectites Kaolinites lllites Vermiculites (Mont.’s ) Structure: Octaheral/ 1:1 2:1 2:1 2:1 Tetrahedral Cation exchange capacity Nil Low High High (CEC) in 3-5 10-40 80-150 100-150 meq/100 g clay + 3+ 2+ 2+ 3+ 2+ Formula Al2Si2O5(OH)2 K0.50.75Al2 M 0.7(Y ,Y )4-6 M 0.66(Y ,Y )6 little variation (Si,Al)2O10(OH)2 (Si,Al)8O20(OH)4 (Si,Al)8O20(OH)4 nH20 8H20 6 1/24/2019 Langmuir Isotherms – Ion Exchange Properties different metals are adsorbed more efficiently Q KC C = o ads 1 + KC mass adsorbed (mg/g) Cads = mass sorbent Qo = maximum adsorbed (mg/g) K = partition coefficient C = solution concentration (mg/L) Case Study 7-2, p. 224 Adsorption Characteristics of Clays for Various Heavy Metals Cads = 5.5 Cads = 8.0 mg/g mg/g Cads = 7.0 Cads = 2.5 mg/g mg/g Figure 7-C2-1. Adsorption isotherms for sepiolite. Adsorbent dose = 10 g L-1, agitation time = 3 h, pH = 4, and T = 22oC. 7.