First-Order Degradation Exercises

 

  1. A pesticide is applied to one field at a rate such that its concentration is 60 mg L-1. If the half-life of the pesticide is 40 days, how long will it take for the concentration to be reduced to 45, 30, 20, 15, 10, 5, and 1 mg L-1?
  2. If the same pesticide is applied to an identical field but at a higher rate so that the initial concentration is 120 mg L-1, how long will it take to reach each of the concentrations listed in problem 1?
  3. A pesticide was applied so that it had an initial concentration of 100 mg L-1. Ninety days later, the concentration was reduced to 25 mg L-1. What is the half-life of the pesticide?
  4. Two pesticides were applied to a field. Pesticide 1 had an initial concentration of 200 mg L-1 and a half-life of 28 days. Pesticide 2 had an initial concentration of 50 mg L-1 and a half-life of 90 days. At what time after application will the concentrations of the two pesticides be the same? What will be the concentration at that time? What will be the concentration of each pesticide one year after application?
  5. Hornsby, Wauchope, and Herner published summary data of pesticide properties measured by different scientists. Half-life values for atrazine reported in the scientific literature included 18, 64, 120, 48, and 60 days. What would be the expected concentrations of the pesticide at the end of 365 days using each of the half-life values if the initial concentration was 200 mg L-1?
  6. A degradation experiment was conducted using 5 samples of the same soil. The initial concentration in the soil was 500 mg L-1. At the end of 200 days, the measured concentrations in the 5 samples were 120, 80, 70, 135, and 150 mg L-1.
    1. Calculate the half-life for each sample.
    2. Calculate the average half-life value.
    3. Calculate the average concentration in the soil at the end of the 200 days.
    4. Calculate the half-life based on this average concentration.
    5. If you were asked to predict the concentration of this product 1 year after application, how would you do it? Why? What answer would you give?