Wastewater Treatment Basics: What Is Sodium Hypochlorite?

Wastewater Treatment Basics: What Is Sodium Hypochlorite?

Sodium hypochlorite is the salt formed by the acid-base neutralization of the strong base sodium hydroxide (NaOH) and the weak acid hypochlorous acid (HClO). Because it is a salt of a strong base and a weak acid, it undergoes hydrolysis in aqueous solution (the solution is weakly alkaline). Based on the dissociation constant of hypochlorous acid at 25 °C (Ka = 2.9×10^-8) [8], the hydrolysis equilibrium constant of NaClO at the same temperature can be calculated as about 3.45×10^-7:

NaClO + H₂O → HClO + NaOH
Kh = Kw / Ka = 1.0×10^-14 / 2.9×10^-8 = 3.45×10^-7

Sodium hypochlorite can therefore undergo neutralization reactions with acids. Taking dilute hydrochloric acid, concentrated hydrochloric acid and carbonic acid (CO₂ in water) as examples:

NaClO + HCl (dilute) → NaCl + HClO
NaClO + 2HCl (concentrated) → NaCl + Cl₂↑ + H₂O
CO₂ + H₂O + 2NaClO → Na₂CO₃ + 2HClO

This is why household bleach (sodium hypochlorite solution) and toilet cleaner (main ingredient HCl) must never be mixed: NaClO and HCl undergo a comproportionation reaction that releases toxic, yellow-green chlorine gas, which damages the respiratory tract and lungs if inhaled.

Similarly, fabric soaked in bleaching water bleaches better after being left in the air for a while because it reacts with atmospheric CO₂ to form hypochlorous acid, HClO (a strong acid displacing a weak acid). Note: the first dissociation constant of H₂CO₃ is 4.5×10^-7.

Sodium hypochlorite is relatively stable at low temperatures (15–25 °C) [9]. It decomposes into sodium chloride and oxygen when exposed to light (especially ultraviolet light) and heat; the higher the temperature, the faster the decomposition. At 70 °C it generates reactive oxygen species [10], which can trigger explosive decomposition:

2NaClO → 2NaCl + O₂↑
NaClO + O₂ → NaClO₃
NaClO → NaCl + [O]

Industrially, sodium hypochlorite should therefore be stored in a cool, dark environment; in laboratories it is usually kept in brown bottles.

Key physicochemical parameters (computational reference values [3]): XlogP: none; hydrogen-bond donors: 0; hydrogen-bond acceptors: 1; rotatable bonds: 0; tautomers: 0; topological polar surface area: 23.1 Ų; covalent units: 2; formal surface charge: 0; complexity: 4.8; heavy atoms: 3; defined/undefined stereocenters and bond stereocenters: 0.

Sodium hypochlorite is inexpensive, easy to produce and chemically reactive. It is widely used in bleaching, industrial wastewater treatment, papermaking, textiles, pharmaceuticals, fine chemicals, sanitary disinfection and many other fields, for example:

• Bleaching of paper, textiles, chemical fibers and starch; production of sulfur vat dyes

• Manufacture of hydrazine hydrate, monochloramine and dichloramine; chlorinating agent for cobalt and nickel

• Water-treatment agent (removal of trihalomethanes [11]), bactericide (e.g., against coliform bacteria [9]) and disinfectant

• Manufacture of chloropicrin (CCl₃NO₂); removal of sulfur and phosphorus compounds from crude acetylene

• Deodorizer for fruits and vegetables, feedlots and livestock barns

• Disinfection of food and food-processing machinery

Note: sodium hypochlorite must not be used in food production processes with sesame as the raw material.

Depending on its physical form and available-chlorine content, sodium hypochlorite is supplied in different specifications [10].

According to the raw materials used, the production of sodium hypochlorite can be divided into four routes: the precipitation (metathesis) method, the brine-electrolysis method, the soda-ash chlorination method and the caustic-soda chlorination method. The caustic-soda chlorination method is currently the dominant chemical production method.

Precipitation (metathesis) method: bleaching powder (active ingredient calcium hypochlorite) and soda ash (sodium carbonate) react to precipitate calcium carbonate and form sodium hypochlorite:

Ca(ClO)₂ + Na₂CO₃ → CaCO₃↓ + 2NaClO

Because calcium hypochlorite is a soluble salt, a large excess of soda ash is required to remove calcium ions and keep it out of the product, so this route is not suitable for industrial-scale production.

Electrolysis method: sodium chloride solution is used as the raw material; the electrolysis products sodium hydroxide (cathode) and chlorine (anode) are mixed internally to form a mixed solution of NaClO and NaCl:

2NaCl + 2H₂O → 2NaOH + Cl₂↑ + H₂↑
Cl₂ + 2NaOH → NaCl + NaClO + H₂O
2H⁺ + 2e^- → H₂↑
2Cl^- - 2e^- → Cl₂↑

This route consumes large amounts of electric power and yields a low available-chlorine content (only about 5%), so it has not become a mainstream production method.

Soda-ash chlorination method: chlorine gas is passed into a sodium carbonate solution to generate NaClO:

Cl₂ + H₂O → HClO + HCl
Na₂CO₃ + HClO + HCl → NaCl + NaClO + H₂O + CO₂↑
Cl₂ + Na₂CO₃ → NaCl + NaClO + CO₂

This route can produce NaClO solution with an available-chlorine content of up to 10%, but it releases large quantities of CO₂, making carbon-emission control more difficult.

Caustic-soda chlorination method: sodium hydroxide, commonly known as caustic soda, is the main raw material; it yields NaClO solution with an available-chlorine content of up to 13%:

Cl₂ + 2NaOH → NaCl + NaClO + H₂O

In recent years, researchers have also developed continuous sodium hypochlorite production units based on microchannel reactors [12], which offer better energy-saving and environmental benefits.

Transport and handling: packaging must be intact at dispatch and loads secured. During transport, containers must not leak, topple, fall or be damaged. Mixing and co-transport with acids, food chemicals and similar goods is strictly prohibited. Transport vehicles should be equipped with leak-emergency equipment, protected from exposure to sun, rain and high temperature; on highways, vehicles must follow the designated routes and must not stop in residential or densely populated areas.

First aid: Skin contact: remove contaminated clothing and rinse with plenty of running water. Eye contact: lift the eyelids and rinse with running water or physiological saline, then seek medical attention immediately. Inhalation: quickly move the victim to fresh air, keep the airway open; give oxygen if breathing is difficult and perform artificial respiration immediately if breathing stops, then seek medical attention. Ingestion: drink plenty of warm water; do not induce vomiting; seek medical attention promptly.

Hazard characteristics: decomposes under high heat to give toxic, corrosive fumes; corrosive. Harmful combustion products: chlorides. Firefighting: use water mist, carbon dioxide or sand.

Spill response: quickly evacuate personnel from the contaminated area to a safe zone and isolate it, strictly restricting access. Emergency responders should wear self-contained positive-pressure breathing apparatus and acid/alkali-resistant protective clothing, and should not touch the spilled material directly. Cut off the source of the leak as far as possible. Small spills: absorb with sand, vermiculite or other inert materials. Large spills: build dikes or dig pits to contain; cover with foam to reduce vapor hazards; pump to tank trucks or dedicated collectors for recovery or disposal at a waste-treatment facility.