Question

a) Suppose that 1 g of dry soil is mixed vigorously with 100 mL of 0.5 M ammonium acetate solution. The soil and water mixture is filtered to remove the soil. The concentrations of calcium, magnesium, potassium, and hydrogen ions in the filtrate are measured. The concentrations of the cations found in the extraction solution are as follows: Ca2+-25.8 μg/mL, Mg2 3.4 ug/mL, K -2.2 ug/mL. Calculate the CEC of the soil. This is the same problem from Discussion 2. You should find the CEC comes out to be 163 cmol/kg (remember those weird units!). Review slides 6 and 7 for the step-by-step calculations c) Estimate the residence time of a fly ash particle, with diameter 2.0 um and density 1.0x106 g/m3, which is released from a 200 meter tall smoke stack. Use the settling velocity equation given by Stokes law to calculate the setting velocity. Convert this number into units of meters/day (from meters/second). Divide that number into the height of the smoke stack to determine the residence time - that is, the total period of time a particle stays suspended above ground. You should find that the residence time is 20 days

Answer 1

Ca 25.8 ug/ml Mg",-341 g/mL K 2.2 ug/mL All of the cations in this solution were bound to the soil by ion exchange sites Some of the cations have a charge of +1 and others a charge of +2. To calculate the CEC, it is necessary to calculate the concentration of each cation and then multiply the concetation by the positive charge of each ion. When this value for each ion is summed, it equals the number of negative sites on 1 g of soil. Both the concentration and charge must be accounted for Calcium: Determine the micromoles, remembering that 1 mol of calcium has an atomic weight of 40.1 g/mol or 40.1 μg/μ mol. (25.8 μg/mL)/(40.1 μg/umol) = 0.643 μmol/mL Each calcium ion has a +2 charge and occupies two negative sites on the soil. (0.643 umol/mL) 1.29 umol/mL This concentration can be multiplied by 100 mL to give the micromoles of total positive charge that the exchangeable calcium ions exerted: (1.29 μmol/mL)(100 mL)-129 umol By convention, this value is converted to centimoles per kilogram soil: (129 umol) (1,000 g/kg soil) (1.0 x 10 cmol/umol) 12.9 cmol/kg Caz- 12.9 cmol/kg Magnesium: Using this method, the magnesium ion contribution is Mg 2.8 cmol/kg Potassium Determine the micromoles. (2.2 ug/mL)/(30.1 μg/μmol)-0.056 μmol/mL Each potassium ion has a +1 charge and occupies one negative site on the soil. ()(0.056 umol/mL) 0.056 umol/mL This concentration can be multiplied by 100 mI to give the micromoles of total positive charge exerted by the exchangeable calcium ion: (0.056 μmol/mL) (100 mL)-5.6 umol By convention, this value is converted to centimoles per kilogram soil: 10° cmol/μmol) (5.6 μmol)(1,000 g/kg soil)(1.0 The CEC is now computed by taking the sum of all three cations: 0.56 cmol/kg CEC (12.9 2.8 0.6) cmol/kg- 16.3 cmol/kg The CEC for this soil is 16.3 cmol/kg or 16.3 mEq/100 g.