Basics of Electrochemistry for Beginners & its Applications in W&WWT Systems
Generation of hypochlorite from brine or seawater – Overall Reaction
The process of electrolysis of brine or seawater is a well-established technology based on Faraday’s law of electron-equivalence. The production of sodium hypochlorite (source of total residual chlorine (TRC)) is based on passing direct current (electricity) through an aqueous solution of sodium chloride, NaCl (dissociated to Na+ and Cl-). In the Electrolytic cells, seawater is exposed to a moderate voltage, high amperage direct electrical current across the electrodes to induce a chain of chemical reactions with a net result of:
Brine based Electrolytic Process:
Salt (NaCl) + H2O + DC current → NaOCl (liquid) + H2 (gas)
Seawater based Electrolytic Process:
Seawater (NaCl) + DC current → NaOCl (liquid) + H2 (gas)
Generation of hypochlorite from brine or seawater – Individual Reactions
The electrical potential in the cells causes the dissolved sodium chloride (salt) present in the seawater to form chlorine (Cl2) gas that is rapidly hydrolyzed to form hypochlorous acid (HOCl) and elemental sodium (Na0 – no valence). The elemental sodium reacts nearly instantaneously with water in the bulk stream to form sodium hydroxide (NaOH). The sodium hydroxide and hypochlorous acid react to form sodium hypochlorite (NaOCl). Hydrogen gas is produced by the reaction of sodium with the water. Since the hydrogen gas does not recombine with any other chemicals, it becomes a gas-phase waste by-product.
The following chemical and electrochemical reactions occur:
a. Free chlorine is generated at the anode:
2Clˉ → Cl2 + 2eˉ
b. Hydrogen is evolved at the cathode with the corresponding formation of hydroxyl ion:
2H2O + 2eˉ → 2 OHˉ + H2 (gas)
c. The overall electrochemical reaction is:
2Clˉ + 2 H2O → Cl2 + H2 + 2OHˉ
d. Chlorine and hydroxyl ions react chemically producing hypochlorite and chloride:
2OHˉ + Cl2 → ClOˉ + Clˉ + H2O
e. The overall chemical reaction can be expressed as follows:
2NaOH + Cl2 → NaClO + NaCl + H2O
Or
NaOH (Sodium hydroxide) + HOCl (Hypochlorous acid) → NaClO (Sodium hypochlorite) + H2O (Water)
Electrochlorination Efficiency
Side reactions, both chemical and electrochemical, take place simultaneously with the basic reactions, such as the thermal decomposition of hypochlorite to chloride (2ClO- →2Cl- + O2), the anodic oxidation of hypochlorite to chlorate (trace amounts) (2ClO- → ClO3- + 2Cl-), the cathodic reduction of hypochlorite to chloride (ClO- + H2O + 2e- → Cl- + 2OH-), and anodic evolution of oxygen (4OH- → 2H2O + O2 + 4e-). All these side reactions affect the current efficiency so that actual DC power required to produce hypochlorite is slightly higher than theoretical.
Scale Build Up in Electrolytic Cells
Moreover, some cations present in seawater (e.g., calcium, magnesium, and other dissolved metal ions) form hydroxides, sulphates, and carbonates resulting in suspended solids, are mostly removed with the chlorinated seawater stream out of the electrolyzers. However, these cations tend to accumulate on cathode surface and eventually close the electrode gap, causing reduced electrolysis and electrode degradation. Consequently, frequent acid cleaning or reversal of electrode polarity is required to protect the electrodes. Otherwise, a probable reduction in cell electrode life could result.
Equivalence of Sodium hypochlorite to Total Residual Oxidants (TRO)
The Electrolytic Generator produces a seawater mixture containing hypochlorous acid (HOCl), hypochlorite ion (OCl-), hypobromous acid (HOBr), hypobromite ion (OBr-), and hydrogen gas (by-product). Chlorine and bromine species can undergo reactions with ammonia naturally present in seawater and may result in the formation of low concentrations of chloramines and bromamines. Chloramine and bromamine are also disinfectants but less effective (i.e., weak oxidants) compared with free chlorine and free bromine. The amount of chloramine and/or bromamine potentially formed is related to the amount of ammonia present, amount of oxidant present, reaction time, and pH of the water treated. In treated discharge, chloramines and bromamines are not determined analytically as methods capable of measuring combined halogen species are not readily available. In addition, combine chlorine and bromine species are included in the total residual oxidants (TRO) measurement.
By industry standards and convention, all chlorine present in a water sample, regardless of form, is referred as Free Available Chlorine (FAC). FAC includes Cl2, HOCl, and OCl-. When including chloramines, the term Combined (or Total) Chlorine is used. For the ballast water application, TRO is used to describe the total of all oxidants present in treated water. To clarify:
Free Available Chlorine (FAC) = Cl2 + HOCl + OCl-
Total Chlorine = FAC + Chloramine
Free Available Bromine (FAB) = Br2 + HOBr + OBr-
Total Bromine = FAB + Bromamine
Total Residual Oxidants (TRO) = Total Chlorine (FAC + Chloramine) + Total Bromine (FAB + Bromamine)
The chemical methods used for analysing TRO often state the measurement is calculated as mgCl2/L. However, these methods include all of the oxidants present in a water samples as mentioned above, not just Cl2.
Applications of in-situ Electrochemically generated Sodium hypochlorite in Water & Wastewater Treatment Systems:
Application 1: Municipal Water & Wastewater Treatment systems: On-site hypochlorite generation systems (OSHGs) which produces a 0.6-0.8 wt% Sodium hypochlorite concentration is typically used.
Application 2: Ballast Water Treatment Systems: OSHGs which produces a 1,000 – 1,500 ppm Sodium hypochlorite concentration is typically used.
Application 3: Offshore and Marine Sewage Treatment Systems: OSHGs which produces a 100-500 ppm Sodium hypochlorite concentration is typically used with other treatment processes.
Application 4: Treatment Systems for Power, Coastal and Industrial Biofouling Controls, Desalination Water Disinfection, and LNG Terminals: OSHGs which produces a 500-2,000 ppm sodium hypochlorite concentration is typically used.
Other Applications: On-shore oil & gas frac water treatment and recycling, to create enhanced advanced oxidation process (EAOP) for treating wastewater containing emerging contaminants (such as PFASs) and persistent organic pollutants, treatment of industrial wastewaters from textile mills, pulp & paper industry, wood preserving, leather industry, etc.