Will Glycolyisis Continue or Not in the Absence of Electron Acceptor

Microbial Fuel Cells as a Platform Technology for Sustainable Wastewater Treatment

Veera Gnaneswar Gude , in Progress and Recent Trends in Microbial Fuel Cells, 2018

18.5.2 Electron Acceptors

The electron acceptor contributes to overcome the potential losses existing on the cathode, thus it is one of the major factors influencing power generation in MFCs. The conditions of being a good electron acceptor comprises possessing a high redox potential, presenting fast kinetics, being economically valuable, and preferably having sustainability and easy availability [42]. Oxygen is one of most promising electron acceptors in MFCs [43]. However, with the rapid progress of MFC technology as well as a better understanding of its principle, there is a broad awareness that the cathode process is far more than just an oxygen reduction reaction (ORR). Various alternative electron acceptors, such as nitrate (NO₃ ⁻), metal ions, perchlorate, nitrobenzene, and azo dyes, have been intensively explored to achieve bioremediation in MFCs [44].

In order to increase the oxygen reduction kinetics and reduce cathodic activation overpotential, different kinds of catalysts have been used in the cathode [45]. Platinum offers the highest catalytic performance with increased oxygen affinity and reduced activation loss, and is the most commonly used catalyst for ORR [42]. Logan et al. demonstrated that Pt-based MFCs could achieve a fivefold increase in power output compared to a MFC with a plain carbon cathode [46]. Other catalysts that are cheaper and more sustainable, such as lead dioxide [47], Fe/Fe₂O₃ [48], cobalt [49], manganese dioxide [42], or even activated charcoal [50] have also been explored for oxygen reduction reactions at the cathode of MFCs. Some studies also showed the possibility of utilizing metallic oxidants (e.g., U, Cd, Cr, Cu) that can be reduced to a less toxic oxidation state. The ORR remains one of the main bottlenecks of this technology, due to its high over potential and low kinetics that are encountered [41].

Biocathodes represent an innovative approach to produce sustainable cathodes using microbes as catalysts to facilitate electrochemical reduction on the cathode surface. The biocathodes eliminate the need for expensive chemical catalysts, lower construction, and operational costs, and offer flexibility in producing valuable commodities [51]. The biocathode requires optimal physiological conditions that promote microbial growth on the cathode surface. Unlike anode respiring bacteria in the anode, the microbes in the biocathode should have the ability to receive electrons from the cathode surface (i.e., electrotrophic). The growth of biofilms on the cathode may be achieved with special techniques such as an electrical inversion of the electrodes. Few scientific studies have confirmed that the biofilm-laden cathode can be conditioned under an oxic environment and then switched to a current generating anode. This indicates that both exoelectrogenic and electrotrophic microorganisms can be maintained in the electrode biofilms when the cathode is switched from oxic to anoxic conditions. Different approaches were taken to improve the kinetics at the cathode surface by using mediators such as ferricyanide or strong oxidants such as permanganate, catalytic electrodes with a platinum catalyst, bacteria catalyzing the oxidation of transition metals, and the bacteria catalyzing the reduction of the final oxidant (i.e., electron acceptor through either direct or indirect electron transport mechanisms including the metabolic products).

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Oxidation

Jay K. Kochi , in Comprehensive Organic Synthesis, 1991

7.4.2.2 Reduction Potentials of Oxidants (Electron Acceptors)

The electron-acceptor properties of oxidants are most readily evaluated by the reversible potentials E°_red for the one-electron reduction, i.e. equation (6b). Values of E°_red for many types of oxidants, particularly those based on transition metal cations, have been tabulated, and some of the more common ones in water are listed in Table 2. ⁴⁴ However there are a number of useful oxidants that undergo a multiple electron change, e.g. Tl^{3   +}   +   2e⁻ → Tl⁺, O₂Cr^{2   +}   +   3e⁻ → Cr^{3   +} , etc., and E°_red is known only for the overall change. With these oxidants, the one-electron potential of relevance to electron-transfer oxidation must be evaluated separately by such transient electrochemical techniques as linear-sweep microvoltammetry. ⁴⁵ Reduction potentials are also highly dependent on the solvent, particularly in those oxidants undergoing a pronounced change in charge. Since the values of E°_red are generally unattainable in organic solvents, an alternative measure of the electron-acceptor properties of A can be evaluated from the irreversible cathodic peak potential E _c. For a series of related compounds the values of E _c can parallel the gas-phase electron affinities (E a). ⁴⁶ (Note the same limitations apply to the use of E _c as those described above for the anodic counterpart.) Moreover, there are a number of stable organic and nonmetallic radicals that are useful in electron-transfer oxidations. Table 3 also includes several varieties of organic acceptors that afford persistent radical anions. Owing to their use as photochemical quenchers, the enhanced values of the reduction potentials E s and E t for the excited singlet and triplet acceptor species, respectively (see hv a in Figure 1), are also included in Table 3. ^47–59

(6b)

Table 2. Reduction Potentials of Some Common Metal Oxidants ^a

Oxidant (A)	E°red	Oxidant (A)	E°red
AgII	2.00	RuIV	0.86
CoIII	1.81	AgI	0.80
O4Bi2 IV	1.59 b	TIIII	1.26 b
CeIV	1.61	O4ReVII	0.77
(5-NOphen)FeIII	1.53 c	FeIII	0.77
ORhIV	1.43	(phen)2FeIII	1.33 c
HgII	0.91 b	O4RuVII	0.59
RuIV	1.01	O4MnVII	0.57
AuIII	1.4 b	(NC)6FeIII	0.55
O2VV	1.00	CuI	0.52
O4RuVIII	1.00	(NC)8WV	0.46
PbIV	1.65 b	WVI	0.26
PuVI	0.92	O4OsVII	0.18 c
Cl6IrIV	0.87	CuII	0.17
XeF2	2.2 b	OTiIV	0.1
PdII	0.92 b	O2ClIV	0.06

a

One-electron potential (NHE) in water with oxo and aquo ligands, unless indicated otherwise. ⁴⁴

b

Two-electron potential.

c

MeCN.

Table 3. Reduction Potentials of Organic Electron Acceptors ^a

Acceptor (A)	E°red	Ref.
Thianthrenium ClO4 −	1.28	47
2,4,6-Triphenylpyrilium ClO4 −	−0.29 (2.8)	48
Tropylium BE4 −	−0.18	49
Nitrosonium BE4 −	1.28	50
Tris-p-bromophenylaminum BE4 − (BA+)	0.80	51
2-Phenylpyrrolinium ClO4 −	—(2.9)	52
Nitronium BE4 −	1.27	50
1,2,4,5-Tetracyanobenzene (TCB)	−0.65 (3.83)	53
9,10-Dicyancyanoanthracene (DCA)	−0.98 (2.88)	53
2,6,9,10-Tetracyanoanthracene (TCA)	−0.45 (2.82)	53
1,4-Dicyanonaphthalene (DCN)	−1.28 (3.45)	53
9-Cyanoanthracene	−1.39 (2.96)	53
1-Cyanonaphthalene (CN)	−1.98 (3.75)	54
1,4-Dicyanobenzene (DCB)	−1.60 (4.2)	53
Chloranil (CA)	0.02 [2.70]	55
Dioxygenyl (O2 +) SbF6 −	5.3	56
2,4,4,6-Tetrabromocyclohexa-2,5-dienone	0.29	57
Tetracyanoethylene (TCNE)	0.24	58
Tetracyanoquinodimethane (ICNQ)	0.19	53
Tetranitromethane (TNM)	∼0.0	59
Dioxygen (O2)	−0.78 (0.98)	55
1,2-Benzoquinone	0.12 [2.3]	53
Dichlorodicyano-1,4-benzoquinone (DDQ)	0.52	55
1,4-Dinitrobenzene	−0.69 [2.6]	55
Nitrobenzene	−1.15	55
N,N′-Dimethyl-4-bipyridinium (MV2   + ) ClO4 −	−0.45 [3.1]	53

a

In V versus SCE in MeCN solution; E _S (parentheses) and E _T [brackets] in eV.

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Heterogeneous Photocatalysis

Vittorio Loddo , ... Leonardo Palmisano , in Current Trends and Future Developments on (Bio-) Membranes, 2018

List of Acronyms and Symbols

A

Electron acceptor

A_g, B_g

Molecules in the gas phase

AOP

Advanced oxidation process

C_A

Concentrations of A in the solution

CB

Conduction band

c

Speed of light

C_Ox

Oxygen concentration in the solution

C_S

Substrate concentration

C_S0

Initial substrate concentration

D

Electrons donor

D_b

Bulk defects

D_s

Irregular surface states

e⁻ _(CB)

Electron in the conduction band

E

Energy

e⁰

Exciton

E_C

Energy of CB

E_F,redox

Fermi level of the redox couple

E_F

Fermi level energy

E_G

Energy bandgap

E_V

Energy of VB

F

Vacancies occupied by electrons

$\stackrel{\mathbf{ˆ}}{\mathbf{\phi }}$

Specific rate of photon absorption, i.e., the moles of absorbed photons per unit time per unit mass of photocatalyst

h

Planck constant

h⁺ _(VB)

Hole in the valence band

hν

Energy of the photon

I

Radiation flux

I₀

Incident radiation flux

k

Surface rate constant

k₁, k₂, α

Fitting constants

K_A

Adsorption equilibrium constant

k_LH

Surface rate constant obtained by the Langmuir–Hinshelwood model

K_Ox

O₂ equilibrium adsorption constant

L_b

Lattice

LH

Langmuir–Hinshelwood model

L_s

Regular surface states

M

Molecule

M⁻

Reduced adsorbed molecule

M⁺

Oxidized adsorbed molecule

M_s

Adsorbed molecule

NHE

Normal hydrogen electrode

η

Quantum yield

η_F

Effectiveness factor

n-SC

n-type semiconductor

Ox

Oxidized species

PC

Photocatalysis

PMR

Photocatalytic membrane reactor

r

Initial reaction rate

r_A

Initial reaction rate related to A species

${\stackrel{\mathbf{ˆ}}{\mathbf{r}}}_{\mathbf{A}}$

Reaction rate per unit of photocatalyst mass

Red

Reduced species

S∗

Excited state of surface site

S⁻

Negative surface active centers

S

Surface site

S⁺

Positive surface active centers

SC

Semiconductor

UV

Ultraviolet

V

Vacancies occupied by holes

VB

Valence band

δ

Ratio between the rates of the two processes carried out individually under the same operating conditions

θ_A

Fractional sites coverages by the species A on the surface of the semiconductor particles

θ_Ox

Fractional sites coverages by O₂ on the surface of the semiconductor particles

ν

Frequency

φ

Local volumetric rate of photon absorption

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Fundamentals: Ligands, Complexes, Synthesis, Purification, and Structure

A.J.L. Pombeiro , V.Yu. Kukushkin , in Comprehensive Coordination Chemistry II, 2003

1.29.2.1 Electron-acceptor/donor Properties of the Metal Center

The electron acceptor and donor properties of a metal center relative to a particular ligand depend, apart from the ligand itself, on a variety of factors associated with the metal (such as its position in the periodic table, its oxidation state and coordination number), to the co-ligands (e.g., their electronic donor/acceptor ability) and to the overall coordination entity (namely the electron count and the net charge). Naturally, all such factors also affect the reactivity of the ligand which is therefore determined by a complexity of combined effects whose relative weight is often not easy to predict.

The first feature of the binding metal center which commonly strikes a coordination chemist's attention is the metal itself and therefore its position in the periodic table, although this is not necessarily the main aspect. General correlations between the electron acceptor and donor properties of transition metal complexes and their position in the periodic table cannot be readily established, but some comments can be made, assuming similar effects of the other factors. The σ-acceptor character is favored by an increase in the atomic number due to the stabilization of the d-orbital energy levels along any transition metal period, whereas the π-back-bonding capacity (to a π*-orbital of an unsaturated ligand) is promoted by a decrease of atomic number along the period, thus occurring preferably to the left of the transition groups which, however, have a smaller number of filled d-orbitals. Hence, effective π-electron donation will result from a compromise between these two tendencies. In accord, the most effective activation of N₂ toward protonation, to give, e.g., hydrazido(2−), M(NNH₂), derivatives or even ammonia, by coordination to a single phosphinic metal center occurs principally for the group 6 (d ⁶) metal sites {ML₄} ⁿ (M   =   Mo, W; n  =   0; L   =   organophosphine or 1/2 diphosphine) or, to a lesser extent, to the isoelectronic V(−1) site (M   =   V, n  =   −1). ^30–35 For a poorly defined related Fe⁰, d ⁸, center, ammonia formation occurs in a much lower yield, ^30,31,35 whereas it is not observed with other transition metals.

Obviously, aside from its position in the periodic table, other factors are also important, such as the metal oxidation state and the charge of the complex, higher values of both of which will favor σ-acceptance while lower values will enhance π-donation. Also important is the nature of the co-ligands which can promote such properties or compete for them. The IR stretching frequency ν(XY) value of an unsaturated XY (NN, NCR, CNR, CO) ligand can provide a convenient probe for the study of those factors and, e.g., in the series trans-[Mo(N₂) L(dppe)₂] ⁿ (dppe   =   Ph₂PCH₂CH₂PPh₂) ν(NN) decreases in the order L   =   CO (n  =   0)   >   CNPh (n  =   0)   >   N₂ (n  =   1)   >   N₂ (n  =   0)   >   NCR (n  =   0), ^36–38 which reflects the increase of the π-donor ability of {MoL(dppe)₂} ⁿ toward the ligated N₂. Further, ligand protonation has not been reported for the cases of L   =   CO, CNR (n  =   0) or N₂ (n  =   1), in contrast to those for L   =   N₂ (n  =   0) or NCR (n  =   0) ³⁷ in which these ligands are readily protonated.

The general implications of the electronic properties of the binding metal center (which are involved in redox potential parametrizations, see below) on the activation of unsaturated small molecules other than N₂ are also noteworthy. In particular, transition metals of later groups (typically Pt or Pd) behave as efficient σ-acceptors and activate organonitriles ³⁹ (see Chapter 1.34) or isocyanides ⁴⁰ toward nucleophilic attack, whereas those of earlier groups (namely Mo, W, or Re, in low oxidation states) exhibit a strong π-electron releasing character and can activate those substrates to electrophilic attack. ^39,41–43 The former type of addition is much more common since ligands normally undergo an increase in electrophilicity upon coordination. This is usually observed for organonitriles, in spite of the fact that as free ligands they are susceptible to electrophilic addition reactions, e.g., with hydrohalic acids. As an example, the hydrolysis of organonitriles is dramatically accelerated by metal ions, usually by a factor of 10⁶ to 10¹⁰ and occasionally by 10¹⁸. ^44–47 Similarly, the metal-promoted hydrolysis of urea (or derivatives) to cyanate (which can undergo further hydrolysis) has been suggested as an alternative mechanism for the urease-catalyzed hydrolysis of urea to ammonia and carbamate ions. ^48–52 Some other examples of metal-mediated hydrolysis include imines, ^53–56 oximes, amides (these studies are relevant to investigations of the peptidases), ^57–62 thiourea, ^63,64 phosphate esters, ^65–72 and thiophosphoric acid esters (well-known pesticides). ⁷³

Nucleophilic attack on a ligand is favored by a positive charge on the complex. The effect is particularly important when a reagent also bears a charge, as observed for the nucleophilic oxygenation of metal-bound CO ligands by trimethylaminoxide, Me₃N  ⁺ O⁻. Neutral metal carbonyls participate in these oxygenation reactions to give free CO₂ only if IR ν(CO)   >   2,000   cm ⁻¹. However, the reaction can also occur for lower wavenumbers (in principle corresponding to a weaker CO activation) if the complex is cationic as a result of the increase of its electrophilicity by the positive charge. ⁷⁴

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Stationary Phases in Gas Chromatography

In Journal of Chromatography Library, 1991

8.14.1 Silver Nitrate

Being an electron acceptor, Ag ⁺ is able to interact with suitable donor molecules (olefins, aromatics) and to retain them selectively in the column. In the early days of gas chromatography, silver nitrate was added to benzyl cyanide or oligomeric glycols and used for the separation of branched from unbranched and cis- from trans-olefins. The solutions of silver nitrate in these nitrite or glycol media become unstable above about 6S°C, and the desired labile adducts with the olefins can no longer be formed. The formation of these AgNO₃-olefin complexes was investigated by Schnecko [1004].

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Photoacoustic Probes for In Vivo Imaging

Weijie Chen , ... Guang-Fu Yang , in Methods in Enzymology, 2021

Abstract

Benzobisthiadiazole as a typical electron acceptor, has been widely used to design fluorescent dyes and photoacoustic (PA) agents. With the strategy of constructing donor-acceptor-donor (D-A-D) type of electron characteristics, benzobisthiadiazole derivatives tend to behave stable in near-infrared absorption and emission, which is beneficial to PA imaging. In this chapter, two molecular design strategies are combined to improve the photoacoustic imaging effects of new PA contrast agent IR-1302 NPs, by installing strengthened conjugated bridges and electron donors. The nanoparticles exhibit high-contrast noninvasive photoacoustic imaging in tumor models with longer wavelength absorption and emission and show potential as a clinic contrast agent.

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Mine-Impacted Water and Biochemical Reactors

Mark Fitch , in Food, Energy, and Water, 2015

Sulfate Reduction Rate

Using sulfate as an electron acceptor requires a substantial energy investment rather than yielding energy. It is thermodynamically favorable only at Eh below about −200  mV, which is why oxygen, nitrate, ferric iron, and some fermentations occur before sulfate reduction is observed in a system. Sulfate reduction competes with methanogenesis to come in last as a useful compound to which organisms transfer electrons. However, in the absence of these higher energy electron acceptors and with electron donor ("food," generally organic) present, SRB produce sulfide at some rate.

Most BCRs receive ample sulfate. Therefore, the SRB are limited for electron donor. That electron donor is generated by bVSS degradation, so the rate of bVSS degradation will determine the rate at which sulfide is produced. When considering woody substrates, the stoichiometry of that conversion is not completely clear because anaerobic cellulose degraders may produce succinate, propionate, ethanol, or other compounds.

Considering the range of potential sulfide yields and assuming the analysis of Hemsi et al. ⁴¹ broadly applies to cellulose degradation, that is, about half the carbon goes to cell mass, then the amount of evolved sulfide may be calculated to be equivalent to 6–14   mmol   S per   g   VSS consumed. Given a rate of VSS consumption of 100   g/m³-d, a midrange value in Tables 1–3, the rate of sulfide generation predicted from stoichiometry is 600–1400   mmol   S/m³-d. This value is surprisingly close to observed values considering the string of approximations required to arrive at the value. A summary of reported sulfate reduction rates is provided in Table 4.

Table 4. Sulfate Reduction Rates in Biochemical Reactors (BCRs)

Reference	Substrate a	SRR (mol/m3/d)	Influent pH	Flow Condition	Influent (mg/L)	Notes
49	100% Spent mushroom compost from 11-year-old VFP	−0.2 to 0.05	5.3–6.0	Semibatch, water volume and rate not reported	Ca: 181   ±   14 Cd: 0.09   ±   0.03 Fe: 67.8   ±   8.3 Mg: 65.3   ±   4.7 Mn: 7.3   ±   1.0 Na: 44.8   ±   5.5 Pb: 0.26   ±   0.08 Zn: 3.9   ±   0.6 SO4: 817   ±   80	6-Month semibatch experiment in 1-L cubitainers
	67   vol% horse manure/33% sawdust mix "with high-quality limestone" from 8-year-old VFP	0.2–0.6
	100% Spent mushroom compost from 3-year-old VFP	−0.6 b to 0.3
	45% Spent mushroom compost, 45% hardwood chips, 10% limestone sand from 1-year-old VFP	0.7–3.5 c
	Cattail marsh sediment from natural wetland	−0.8 b to 0.2
66	71.5% Aged chipped wood, 3.5% manure, 5.5% ag limestone, 19.5% alfalfa hay d	0.02–0.14, median ∼0.05   mol/m3/d	3.4	Downflow, 0.21   m3m−2d−1 EBCT: 8   d	Fe: 96 Cu: 0.22 Zn: 1.8 As: 0.09 Ni: ND Co: ND Al: 24.7 Mn: 2.8 SO4: 554	Dual cells in parallel; values during 4   years of operation
41	45% Sand, 5% limestone, 10% fresh dairy manure, 40% alfalfa hay	0.55   ±   0.04 (SD)	6.0	Upflow, 0.015   m3m−2d−1 EBCT: 12   d	SO4: 1000 Zn: 50 Mn: 50	0.38-L column; values after 100   days operation
	45% Sand, 5% limestone, 10% fresh dairy manure, 40% corncob	0.59   ±   0.05 (SD)
	45% Sand, 5% limestone, 10% fresh dairy manure, 40% oak	0.23   ±   0.04 (SD)
	45% sand, 5% limestone, 10% fresh dairy manure, 40% pine	0.15   ±   0.04 (SD)
50	10% Hay, 50% wood chips, 30% limestone, 10% cow manure	Not reported	3.8–7.1	Downflow 0.8   m3m−2d−1 EBCT: 1.3   d	Fe: 5.2 Cu: 0.26 Zn: 26 Cd: 0.14 Pb: 0.5 Mn: 11 SO4: 281	No flow ∼15   weeks during reported year
51	100% Spent mushroom compost	0.42	4.1	Downflow 0.4   m3m−2d−1 EBCT: 1.25   d (median values; EBCT for compost layer)	Fe: 202 Mn: 11.4 Cd: 0.01 Pb: 0.12 Zn: 0.13 Na: 1378 SO4: 5688	Full-scale, 1   year of operational data
	100% Spent mushroom compost	0.25	7.2		Fe: 16 Mn: 8.8 Cd: 0.01 Pb: 0.06 Zn: 0.02 Na: 1277 SO4: 4902
52	92   wt% walnut shell, 8   wt% corn stover	0.13–0.24	5.2–5.8	Upflow 0.015–0.03   m3m−2d−1 EBCT: 10–20   d	Fe: LT 0.1 Zn: 50 SO4: 700	0.59-L lab-scale columns
		0.28	6.3–6.5	0.015   m3m−2d−1 EBCT: 10   d	Fe: LT 0.1 Zn: 1.5 SO4: 300
53	75% Sugar beet waste, 25% +10 mesh taconite ore	0.65 0.93   w/ethanol	7.9–8.6	Downflow 0.2   m3m−2d−1 EBCT: 3.3   d HRT: 0.7–2   d	Iron ore tailings basin water. Fe: GT 100 SO4: 810–660	Lab-scale columns ∼12   L. Columns with ZVI not included here
	75% Peat, 25% +10 mesh taconite ore	0.27 2.2   w/ethanol
	10% Manure, 40% sawdust, 25% feed corn, 25% +10 mesh taconite ore	2.5–0.6
	25% Manure, 40% sawdust, 10% hay, 25% +10 mesh taconite ore	1.5–0.6
	25% Biosolids, 40% sawdust, 10% hay, 25% +10 mesh taconite ore	2.2–3.1
54	∼20% Spent mushroom substrate, 80% fine inert grit	0.09–0.15	7.8	Surface flow 0.07–0.13   m3m−2d−1EBCT: 30–55   d	Pb: 0.1 Zn: 5.0 SO4: 900	Pair of 16   m2, 2   m deep wetlands, substrate in 0.5 and 1   m beds
55	30% Wood chip and sawdust, 30% leaf compost and poultry manure, 40% sand, creek sediment, urea, and calcium carbonate	0.2–1.6 (7.3   d HRT) 0.09–1.3 (10   d HRT)	5.7–2.9	Downflow HRT: 7.3 and 10   d	Ca: 372   ±   75 Cd: 9.8   ±   1.8 Fe: 504   ±   83 Mg: 85.8   ±   10.5 Mn: 10.1   ±   2.6 Na: 625   ±   231 Ni: 13.7   ±   1.0 Zn: 14.5   ±   2.1 SO4: 4022   ±   583	3.5-L columns
56	Aquifer material: 25% mica sand, 67% gravel	0.05–0.4 (planted) 0.009–0.18 (unplanted)	6.72   ±   0.25	Horizontal flow 0.23   m3m−2d−1 EBCT: 14   d	Ca: 394   ±   10 Fe: 1   ±   0.6 SO4: 957   ±   85	3   m3 wetlands, one with reeds, other unplanted, 4–5   years operation
60	Wetland: 20% clay, 70% sand	0.6 (Winter) 0.03 (Spring) 0.07 (Fall)	9–10	Horizontal, free water surface EBCT: 9   d	Fe: 0.8–34 Cr: 0.05–0.4 Ni: 0.01–0.036 Zn: LT 0.01–0.09 BOD: 11–94 SO4: 1650 (Winter), 270 (spring), 750 (fall)	2000   m3 sediment in ∼3000   m3 total FWS wetland w/Typha domingensis (southern cattail), treating tool facility wastewater
31	100% Fresh mushroom compost	0.2	3.0	Horizontal flow 0.2   m3m−2d−1 EBCT: 15   d	Al: 4 Ca: 375 Cd: 0.03 Cu: 0.57 Fe: 43.2 Mn: 31 Ni: 0.2 Pb: 0.02 Zn: 9.1 SO4: 1700	Four 9–19   m3 BCRs studied over 11   months
	0.67   m mushroom compost under 0.3   m hay	0.04		Upflow 1.3   m3m−2d−1 EBCT: 11   d
		0.01		Upflow 0.01   m3m−2d−1 EBCT: 36   d
		0.05		Downflow 0.04   m3m−2d−1 EBCT: 11   d
	33% Peat, 33% aged steer manure, 33% composted wood shavings and sawdust	0.1		Downflow 0.1   m3m−2d−1 EBCT: 10   d
57		−1.2 to 1.8 Median: 0.16 Average: 0.09	3.0–6.8	Vertical flow 0.08–0.65   m3m−2d−1 EBCT: 0.3–100   d	Al: 0–48 Ca: 86–320 Fe: 0.2–193 Mn: 2.4–57 SO4: 455–1372	23 Vertical flow SAPS, field-scale. Values calculated on compost layer volume
58	180-day-old MSW compost, limestone	−0.25 to 0.37 Median: ∼0.12	4.5–5.0	Upflow 0.6–2.0   m3m−2d−1 EBCT: 0.9–2.7   d	Co: 3–4 Zn: 3–4 Ni: 80–100 Cu: 40–50 SO4: 1500–2000	0.78-L columns operated 5   years after 2–4   years use of substrate in barrel scale
	180-day-old MSW compost	−0.27 to 0.3 Median: ∼0.1		1.3, then 0.3   m3m−2d−1 EBCT: 1.3, 6.5   d
	Yard waste compost	−0.2 to 0.25 Median: ∼0.04		1.3   m3m−2d−1 EBCT: 1.3   d
	45-day-old MSW compost	−0.25 to 0.7 Median: ∼0.04		0.6–3.1   m3m−2d−1 EBCT: 0.9–6.5   d
Liquid substrate
61	None (upflow anaerobic sludge blanket reactor)	20	6–7	Upflow EBCT: 1   d	SO4: 2000	Lab-scale 3-L UASB fed lactate
59	100% Ceramic rings	5 (Acetate), 7–8 (propionate)	7–8	Upflow with recycle ratio of 4 EBCT: 1.3   d	SO4: 1200	Lab-scale 1 1/3-L reactor fed propionate or acetate
53	100% +10 Mesh taconite ore	2.4–3.1 (Ethanol) 2.0 (Molasses) 1.3 (Methanol)	7.9–8.6	Downflow 0.2   m3m−2d−1 EBCT: 3.3   d HRT: 0.7–2   d	Iron ore tailings basin water. Fe: GT 100 SO4: 810–660	Lab-scale columns ∼12   L
	90% Oxidized ore, 10% coarse tailing	2.8 (Ethanol) 3.0 (Molasses) 1.6 (Methanol)

SAPS, successive alkalinity producing systems; VFP, vertical flow ponds; SRR, sulfate reduction rate; EBCT, empty bed contact time; HRT, hydraulic residence time; ZVI, zero valent iron; FWS, free water surface; MSW, municipal solid waste.

a

Percentages are volume unless otherwise noted.

b

Substrate dried before experiments, may have oxidized sulfide and thus released sulfate.

c

Steady decline from 3.5 to 0.7 ±0.2 over 6-month batch experiment.

d

Estimated assuming 130   kg/m³ hay.

As previously noted, a stoichiometric approach that considers the source of electrons might allow the designer to estimate the sulfide generation rate and to determine how that rate will decline as the organic is depleted. More data would be required for the specific plant matter used and to estimate the change in degradation with time, as no rigorous long-term study has shown values for such degradation rates.

One attraction of BCRs in which electron donor is added in a liquid phase is that the rate of sulfate reduction may be controlled based on the addition of that BOD, and the achievable rate is much greater than relying on slowly degrading cellulosic material. A recent review of such rates was provided by Sanchez-Andrea et al. ² For simple organics such as methanol, formate, and lactate, reported rates span a range of 0.6–151   mol/m³/d.

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Electron Spin Polarization in Photosynthetic Reaction Centers

Seth W. Snyder , Marion C. Thurnauer , in Photosynthetic Reaction Center, 1993

A Electron spin polarization in green sulfur bacteria

A general analogy between the electron acceptor pathways in green sulfur bacteria and PSI is believed to exist. Although ESP behavior is observed readily in a variety of PSI samples, ESP is much more difficult to observe in green sulfur bacteria. Heathcote and Warden (1982) reported the observation of ESP in whole cells and membrane fractions of Chlorobium limicolaf. thiosulpatophilum in a buffer containing 10 mM ascorbate. An e/a* pattern was observed for these ESP EPR spectra.

In membrane fractions of Chlorobium vibrioforme under conditions in which the iron–sulfur centers (probably F_a/b but not F_x) are reduced (Miller et al., 1992), we also observed an e/a* ESP signal (Fig. 22). Samples that were not reduced did not exhibit ESP. Sample reduction probably is required to increase recombination from electron acceptors that occur earlier in the chain than the iron–sulfur centers. The observed e/a* ESP behavior more closely resembles the CIDEP observed with quinone-replaced iron-containing purple bacterial RCs than the CRPP observed with PSI. The meaning of this comparison and the nature and role of the electron acceptors in green sulfur bacteria are still under investigation.

FIGURE 22. A comparison between the X-band LM-EPR spectra from membrane fractions of (A) green sulfur bacterium Chlorobium vibrioforme and (B) reaction centers of R26 iron-containing, quinone-replaced (2-t-butyl-AQ).

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Membrane-Associated Energy Transduction in Bacteria and Archaea

G. Schäfer , in Encyclopedia of Biological Chemistry (Second Edition), 2013

Aerobic Organotrophs

Oxygen is a superior electron acceptor for aerobic microbes that use reduced substrates or even molecular hydrogen as donors of reducing equivalents. Organotrophs deliver electrons into a respiratory chain ( Figure 3 ) via ubi- or menaquinone-reducing membrane-anchored or integral membrane protein complexes. A major complex is the FMN-dependent type-I NADH dehydrogenase (complex I), composed of 14 different polypeptides (e.g., Paracoccus and Escherichia coli). It couples the translocation of (probably four) protons to the reduction of quinone and consists of a peripheral substrate oxidizing subcomplex facing the cytosol, a membrane integral transducer subcomplex with several FeS centers, and a receiver subcomplex transferring electrons to the quinone. Alternatively, in many bacteria and in archaea, type-II NADH dehydrogenases are found; these have less complex composition and cannot couple ion translocation to substrate oxidation. Several alkaline-adapted bacteria couple the translocation of Na⁺ instead of protons to NADH oxidation. As another quinone-reductase succinate dehydrogenase (complex-II) is found in the plasma membranes of all aerobic microbes. The small change in free energy of this reaction is insufficient to drive energy-conserving ion translocation. The reoxidation of reduced quinones is either catalyzed directly by heme/Cu-type quinol oxidases as, for example, in E. coli, Paracoccus denitrificans, and Rhodobacter sphaeroides, or by a second energy-conserving module of respiratory chains, the bc ₁ complex (complex-III). The essential redox catalysts of this module are an [2Fe₂S] iron–sulfur protein (Rieske protein), a diheme b-type cytochrome, and a c-type cytochrome c ₁. Complex-III couples the stepwise reoxidation of QH₂ to the transport of 2H⁺ across the membrane for each electron transduced to cytochrome c ₁. The complex reaction mechanism is known as Q-cycle. The mobile redox carrier cytochrome c associated with the outer surface of the plasma membrane accepts electrons from cytochrome c ₁ and serves as the connector to the terminal oxidase module, the heme/Cu-type cytochrome c oxidase (⇒). All oxygen-reducing heme/Cu oxidases are proton pumps. The path of electrons from NADH to oxygen can thus conserve the energy equivalent to 12H⁺ pumped across the membrane per [H₂]. Depending on the genetic disposition, either one of the energy-transducing complexes can be missing, except a terminal oxidase. The modular structure of microbial respiratory chains allows a variety of special adaptations. Besides an aerobic respiratory chain, all modules for denitrification and even several terminal oxidases can be present in parallel (e.g., Figure 3 ); their individual contribution to energy transduction is usually regulated via the expression of the respective genes in response to nutritional and environmental conditions.

Figure 3. Illustration of the diversity of energy-transducing membrane systems in bacteria and archaea. All boxed or encircled components are membrane-integral or membrane-associated. All red boxes are proton pumps acting according to principles (b) and (c) of Figure 1 . The scheme depicts the situation in P. denitrificans. The shaded block from left to right symbolizes a regular and complete aerobic respiratory chain as present, for example, in purple bacteria and also in eukaryotic mitochondria. The linear arrows indicate the direction of electron flux between donor and acceptor complexes or compounds. The symbols aa₃, ba₃, and bo₃ in the box quinoloxidases indicate the species-dependent variability of observed heme-compositions; calQ (caldariella quinone) applies to some archaea, as does the blue-Cu protein instead of c-type cytochromes. SDH, succinate dehydrogenase, complex II.

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Microbial Fuel Cells—Air-Cathode Electrodes

Q. Zhao , ... Z. Liang , in Encyclopedia of Interfacial Chemistry, 2018

Cathode material

Oxygen is the final electron acceptor in air-cathode MFCs because of its great potential for oxidation, wide availability, low in cost, and lack of waste. The previously mentioned anode materials can also be used as cathodes, including carbon paper, graphite, carbon cloth fabric, graphite particles, and brushes. The cathode differs from the anode in that it requires a catalyst. The application of a catalyst can increase the reaction rate of oxygen reduction, improving electricity generation overall.

In order to increase cathode performance, Pt was used as a catalyst. ⁷ The power density of the cathode with a supported Pt catalyst could be increased by 1 order of magnitude, as compared to those without Pt. The best platinum loading capacity of carbon paper was 0.1   mg   cm^{−   2} generally. Although Pt has good catalytic performance, its high cost limits its application. In addition to Pt, metal, metal oxides, and CoTMPP can also be used as catalysts for the modification of cathodes to improve cathode performance. ⁸

CeO₂ doped Pt/C electrodes had higher catalytic performance than pure Pt/C electrodes. Their maximum voltage was 580   mV, and their maximum power density was 840   ±   24   mW   m^{−   2}. With mixtures of different proportions of carbon black (C) and zirconium oxide (ZrO₂) as air-cathode electrode materials, the oxygen reduction reaction activity and stability of the MFC were improved. When ZrO₂ was loaded at 25   wt.%, the maximum power density was 600   mW   m^{−   2}. With Ag/Fe/N/C composites as MFC electrode materials to enhance oxygen reduction via cathodic biofilm inhibition, the maximum power density of 1791   mW   m^{−   2} was obtained at a temperature of 630°C, much larger than that of the Pt/C electrode (1192   mW   m^{−   2}). Air-cathodes modified by MnO₂ could increase the surface area of the holes in the cathode and improve catalytic activity. The maximum power density reached was 1554   mW   m^{−   2}. ⁹ Compared to the electrocatalytic activity of Fe₂O₃ and Mn₂O₃ nano powders, carbon black powder and Pt-cathode, the Mn₂O₃ cathode resistance was the lowest, and the power output of Pt was the highest, followed by Mn₂O₃, Fe₂O₃ and C with the corresponding volume power output of 90, 32, 15 and 8   W   m^{−   3}, respectively. Mn₂O₃ showed the best redox potential, while Pt showed the best volumetric power output. Carbon cathode MFCs were the most stable. The Mn₂O₃ electrode appears to be the most promising cathode for nonnoble metal catalysts. ¹⁰

The cheaper and nontoxic carbon materials can also be used as cathodic oxygen reduction catalysts. For example, the carbon nanotubes might replace the carbon dust as a carrier of Pt catalyst because of its good chemical stability, conductivity and large surface area. ¹¹ Compared with conventional carbon cloth cathodes, all nanotubes-modified cathodes exhibited higher electrochemical response performance and power generation in MFCs. MFCs with this material as an air-cathode electrode showed a maximum power density of 329   mW   m^{−   2}. The addition of a Pt catalyst significantly increased the current density of all cathodes, and the maximum power density of 1118   mW   m^{−   2} was obtained using Pt/CNTs cathode. ¹²

Multifunctional homogeneous carbon film was prepared with micro filtration, electron conduction, and oxygen reduction catalysis as a cathode material. The power density of MFCs with this air-cathode electrode was 581.5   mW   m^{−   2}, and the current density was 1671.4   mA   m^{−   2}. The removal rates of TOC, NH₄ ⁺-N, and total nitrogen were up to 93.6%, 97.2%, and 91.6%, respectively. These removal efficiencies were better than that of conventional wastewater treatment continuously operated for 20 days. Moreover, compared to traditional carbon membranes, microfiltration membranes are much cheaper, which indicates that multifunctional materials are promising for wastewater treatment.

It was also found that doping with nitrogen could greatly improve the catalytic activity of carbon nanotubes in oxidation reduction. Compared with untreated carbon powder, materials which underwent nitrogen treatment displayed significantly increased maximum power density. Two-step pretreatment using heat treatment and hydrochloric acid soaking could improve the maximum power density. Nitrogen-doped carbon powder exhibited high stability and long-term operation. ¹³

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