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Electrochemical related parameters and calculation methods

Time：2019-11-19

**Electrochemical related parameters and calculation methods**

** **

**1.O****xidation reduction potential:**

It is used to reflect the macroscopic oxidation-reduction properties exhibited by all substances in the water. The higher the oxidation-reduction potential, the stronger the oxidizability, and the lower the potential, the weaker the oxidizing property. A positive potential indicates that the solution exhibits a certain degree of oxidative properties, and a negative indicates that the solution exhibits reducibility.

**Voltage/Tank pressure:** The overall voltage after the cell is turned on,the unite is V.

**Current**:Generally refers to the current supplied to the cell, in units of A.

**Density of current:** It is the current per unit area and the unit is mA/cm2, A/m2, and 1 mA/cm2 = 10 A/m2.

The effective electrode area:It is neither the electrode area into the water nor the area of the electrode, it’s the area of attending the electrochemical reaction. The value is anode or the cathode and unite is

cm^{2}、m^{2}^{.}

**Electricity**: The amount of charge is called electricity. It is represented by the symbol Q. The unit is Coulomb C or Ah, 1Ah=3600C. For the convenience of calculation, more Ah is used, and the amount of electricity carried by 1 mol of electrons is 26.8 Ah.

2.**electrochemical calculation method**

When the waste-water treatment,The related parameters of el ectrochemical is based on 1ton. So it need the unit changing and calculation of related parameter is also based on it.

**2.1 Electricity**

Calculation formula: electricity Q=It/V.

When the electrolytic cell is operated with a current of 1 A and 1 t of waste-water is treated for 1 hour, the total amount of electricity supplied to the 1 t waste-water is 1 Ah/t.

**2.2 Current efficiency and COD removal**

COD(chemical oxygen Demand) is the oxygen equivalent of a substance that can be oxidized by a strong oxidizing agent, and the unit is mg/L.For example, if the oxygen equivalent of a substance which can be oxidized by a strong oxidizing agent in a waste-water is 300 mg/L, then the waste-water COD=300 mg/L.

When the waste-water is electrolyzing the calculation of the theoretical COD removal number is based on the electron reaction of O2. According to the definition of COD, the value of COD is equivalent to the combustion reaction of organic matter A and O2:

A+O_{2}=B+2O^{2-}

At this time, all O2 changes from 0 to -2. Therefore, 1 mol of O2 gives 4 mol of electrons, and it is necessary to supply a charge of 4*26.8=107.2 Ah. The mass of 1 mol of O2 is 32 g, then 1 mg of O2, the corresponding amount of electricity = 107.2 / 32000 = 0.00335 Ah. Therefore, the amount of 1 Ah corresponds to 1/0.00335 = 298.5 mg of O2. That is, 1 Lh of electricity is supplied to 1 L of waste-water, and theoretically COD removal is 298.5 mg/L.

Current efficiency can be measured by the ratio of actual degradation of COD to theoretically degraded COD. The electrochemical reaction on the surface of the electrode, in addition to the degradation of COD, also has side reactions such as oxygen evolution and chlorine evolution, so the actual degradation of COD is less than the theoretical value. For example, to provide 1 Ah of electricity to 1 L of water, the COD removal number is 100 mg/L, then the current efficiency = 100/298.5*100% = 33.55%. When the actual removal of COD is higher than the theoretical value, that is, the current efficiency is higher than 100%, there must be other effects, such as air flotation and flocculation.

2.3**Energy consumption and COD removal**

**Energy consumption **Calculation formula,unit energy consumption W=UIt/V. Unit of W is Wh•L-1, unit of U is V； unit I is A；unit t is h , unit V is L.

Unit conversion: 1W=1VA, 1 Wh•L-1=1VAh•L-1, 1kWh•m-3=1000Wh•m-3=1000Wh•1000L-1=1Wh•L-1; for wastewater 1m3≈1t, Therefore, 1 kWh per ton = 1 Wh•L-1.

**COD removal and energy consumption**

Time/h | 0.25 | 0.5 | 0.75 | 1 |

current/A | 2 | 2 | 2 | 2 |

voltage/V | 4 | 4 | 4 | 4 |

Water volume/L | 0.5 | 0.5 | 0.5 | 0.5 |

Electricity/Ah •L | 1 | 2 | 3 | 4 |

Energy consumption /Wh•L | 4 | 8 | 12 | 16 |

Energy consumption/KW.H .T | 4 | 8 | 12 | 16 |

Theoretical COD removal/mg•L | 298.5 | 597 | 895.5 | 1194 |

2.4**Calculation of the amount of iron dissolved in Fenton**

The biggest advantage of the electric Fenton is that the amount of dissolved iron is precisely controllable. The reason is that when the electrode and the electrolytic cell are designed reasonably, and the electrolysis conditions are properly controlled, the anode has almost no side reaction, and the current efficiency of the molten iron reaction can reach 100%. The reaction of anodic dissolved iron is:

Fe-2e-=Fe2+

1 mol Fe becomes 1 mol Fe2+, and 2 mol of electrons is required, that is, 2*26.8=53.6 Ah. The mass of 1 mol of Fe was 56 g, so the Fe2+ eluted at 1 Ah was 18.7 mmol = 1.04 g.

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