2. Substrate combines with enzyme to form activated complex, which can decay to give catalytic product Michaelis-Menten equation gives rate of forward reaction k + is rate constant, m E enzyme concentration, m A substrate concentration, and K A = (k 2 +k 3 )/k 1 is the half-saturation constant (molal). Enzymatic Catalysis

3. Michealis-Menten Kinetics mAmA r+r+ substrate limited enzyme limited

4. Reaction proceeds at rate r = r + – r – of the forward less the reverse reaction. Overall rate is given m P is product concentration, K P = (k 2 +k 3 )/k 4 is half-saturation constant for reverse reaction, Q and K are reaction’s activity product and equilibrium constant. Thermodynamic Consistency

5. Benzene actively biodegraded under oxic conditions Monod equation gives degradation rate [X] is biomass concentration, in mg kg –1. Reduces to Michaelis-Menten if [X] holds constant. Aerobic Degradation of Benzene

6. Alvarez et al. (1991) observed the rate of benzene mineralization in natural sediment from an oxic aquifer. k + = 8.3 (g benzene) (g cells) –1 day –1 or 1.2 × 10 –9 mol (g cells) –1 s –1. K A = 12.2 mg kg –1 or.16 × 10 –3 molal. For [C 6 H 6 ] ≤ 100 mg kg –1. Reverse reaction is negligible. Kinetics of Benzene Biodegradation Alvarez, P.J.J., P.J. Anid and T.M. Vogel, 1991, Biodegradation 2, 43–51.

7. Copy from thermo.com.V8.R6+.tdat Edit → Copy “−” sign indicates left side Log Ks at principal temperatures: New Redox species Benzene is formed from the Basis species

8. Paste into thermo.tdat… Edit → Paste File → Save As… → thermo+benzene.tdat … and save as thermo+benzene.tdat

9. Task 1 — Benzene Biodegradation Aquifer contaminated with 1000 μg kg –1 C 6 H 6 for 2 years, then flushed. Biomass concentration is 10 –3 (g cells) (kg water) –1, or 1 mg kg –1. Compare to non-reacting case.

10. 02004006008001000 0 1 0 1 0 1 0 1 0 1 X position (m) C 6 H 6 in fluid (mg kg –1 ) t = 2 yr 4 yr 6 yr 8 yr 10 yr flow non-reacting biodegrading

11. In aquifers containing significant amounts of natural organic matter, benzene sorbs to the organic surfaces. Retardation varies with organic content. Benzene Sorption

12. Task 2 — Sorption and Biodegradation Retardation factor R F in aquifer is 2. So K d ’ is.16 × 10 –3 mol (g sediment) –1. Compare to biodegradation alone.

13. 02004006008001000 0 1 0 1 0 1 0 1 0 1 X position (m) C 6 H 6 in fluid (mg kg –1 ) t = 2 yr 4 yr 6 yr 8 yr 10 yr flow non-reacting biodegrading sorbing and biodegrading

14. play movie X2t Models Non-reactive. Biodegrading. Sorbing and biodegrading.

15. Craig M. Bethke and Brian Farrell © Copyright 2016 Aqueous Solutions LLC. This document may be reproduced and modified freely to support any licensed use of The Geochemist’s Workbench® software, provided that any derived materials acknowledge original authorship.