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1 <br />' 7.2 Anaerobic <br /> Blodegraclation <br /> Microbial aerobic respiration occurs until dissolved oxygen concentrations are depleted below 0 5 <br /> mg/L, a condition that favors anaerobic microbial activity Depending on the types of electron <br /> acceptors present (e g , nitrate, iron (III), or sulfate), pH conditions, and oxidation-reduction <br /> (redox) potential, anaerobic biodegradation can occur by denitrification, iron (III) reduction, <br /> sulfate reduction Site conditions will ultimately determine which TEA-Ps will dominate, but in a <br /> typical aquifer that is devoid of dissolved oxygen, denitrification typically occurs first, followed by <br /> iron (III) reduction, and finally by sulfate reduction based on the amount of Gibb's free energy <br /> produced by the reaction The EAC for each anaerobic TEAR is determined from the degradation <br /> of benzene <br /> 7.2.1) Denitrification <br /> Nitrate can be used as an electron acceptor by facultative anaerobic microbes to degrade BTEX <br /> compounds under oxygen limited conditions via denitrification Denitrification of benzene occurs <br /> as follows <br /> 6NO3- + 6H+ + CH6 - 6CO? + 6 HBO + 3N,) <br /> From the stoichiometry of anaerobic denitrification of benzene, 4 8 mg of nitrate are required to <br /> completely metabolize 1 mg of benzene and 1 mg of nitrate is capable of facilitating the <br /> destruction of 0 21 mg of benzene Similar calculations can be made using the stoichiometry for <br /> toluene, ethylbenzene, and xylene denitnficat;or. Using the average mass ratio of nitrate to total <br /> BTEX from the individual compound biodegradation stoichiorietry, 1 mg of nitrate is required to <br /> metabolize 0 21 mg oFtota] BTEX The FAC orgrni„nddater ror denitrification is, determined as <br /> follows <br /> EACH = 0 21 (NB-NM) <br /> Where EAC;, =Expressed Assimilative Capacity, denitrification. <br /> 0 21 = mg,2.BTEX degraded per mgr nitrate consumed ratio <br /> Ns = hackg*o�u-nd nitrate concentration (mall) <br /> N,, = nitrate concentration in plume (mg/L) <br /> A reduction in nitrate and BTEX concentrations vrithin an existing BTEX plume is a strong <br /> .ndication that indigenous anaerobic microbes ate established a-id act-wely biudegi ttding reu o!eurn <br /> hydrocarbons .nd denitpficetton <br /> 7.2.2) iron (Ill) Reduction <br /> Iron_ (III) can be used as an electron acceptor by facultative anaerobic microbes to degrade BTFX <br /> 16 <br />