Ferrous Chloride (FeCl2) in Wastewater Treatment: Guide

“Phèn sắt 2” is the Vietnamese trade name for ferrous chloride, FeCl2 (iron(II) chloride, CAS 7758-94-3), a salt of divalent iron. Loc Thien makes it as a 30% green solution. In wastewater treatment FeCl2 reduces Cr⁶⁺ to Cr³⁺, forms flocs for coagulation, precipitates phosphate and sulfide, and supplies Fe²⁺ for the Fenton reaction with H2O2. The same name is also used for FeSO4·7H2O (green vitriol), which is a different chemical.

Scope: this article covers FeCl2 for industrial wastewater treatment. If you are looking for a way to treat well water that contains iron, that is the opposite problem: in well water Fe²⁺ is the contaminant to remove, not something to add. The general definition, safety and properties are covered in Ferrous chloride (FeCl2): overview and industrial applications.

Contents

• What is “phèn sắt 2”: FeCl2 or FeSO4?
• What is FeCl2? Is it a salt or an acid?
• Physical properties: the FeCl2·4H2O crystal is not the 30% solution
• Chemical properties to remember in operation
• Using “phèn sắt 2” in wastewater treatment: where it belongs
• Common misunderstandings about “phèn sắt 2”
• Safety when handling FeCl2: protection and first aid
• Storage, tank materials and transport
• Buying 30% FeCl2 from Loc Thien
• Frequently asked questions
• Request a quote for 30% FeCl2

What is “phèn sắt 2”: FeCl2 or FeSO4?

“Phèn sắt 2” is not an official chemical name. It is an industry nickname, and two different chemicals carry it. The table below helps you tell them apart before ordering or calculating a dose.

In nomenclature FeCl2 is not an “alum”. Alums are hydrated double sulfate salts, for example potassium aluminium sulfate KAl(SO4)2·12H2O. The name “phèn sắt 2” stuck to FeCl2 only through habit in the water treatment trade. The surest way to know which chemical you are holding is to read the formula and CAS number on the label or SDS rather than guess from the name. This article is about FeCl2; FeSO4 appears only in the table above so that the two are not confused.

What is FeCl2? Is it a salt or an acid?

FeCl2 is a salt: an ionic compound of the Fe²⁺ cation and the Cl⁻ anion, formed from the strong acid HCl and the weak base Fe(OH)2. It dissolves completely in water and is a reducing agent, because Fe²⁺ readily gives up an electron to become Fe³⁺.

FeCl2 is not an acid by definition, but a solution of the salt reacts weakly acidic. The Fe²⁺ ion partly hydrolyses in water and releases H⁺ (in fact the hydronium ion H3O⁺):

[Fe(H2O)6]²⁺ ⇌ [Fe(H2O)5(OH)]⁺ + H⁺

However, Loc Thien’s commercial 30% FeCl2 solution is very strongly acidic according to its SDS (pH below 1) because it still contains free acid; the Fe²⁺ hydrolysis adds only weak acidity. At that acidity and concentration the solution corrodes skin, eyes and many metals, so it must be handled as a corrosive (class 8) chemical, not as an inert salt.

Physical properties: the FeCl2·4H2O crystal is not the 30% solution

Many sources give “phèn sắt 2” a density of 1.93 g/cm³, a melting point of 105 °C and a molar mass of 198.81 g/mol. These three figures belong to the tetrahydrate crystal FeCl2·4H2O, not to the 30% solution sold on the market. If you use 1.93 to convert litres to kilograms, the mass of the solution is overestimated by about 48%, and the dose is wrong by the same amount.

The table gives the figure used most often in dosing: 1 litre of 30% solution weighs about 1.3 kg and contains about 0.4 kg of pure FeCl2. The density is written as “about 1.3 g/cm³” because the measured value depends on the actual concentration of each batch and on temperature; when you need an exact figure, measure the batch in use with a hydrometer.

Chemical properties to remember in operation

Easily oxidised. Fe²⁺ in contact with oxygen from the air turns into Fe³⁺. The standard potential E°(Fe³⁺/Fe²⁺) = +0.771 V is much lower than that of oxygen in acidic medium (+1.229 V), so the reaction can occur spontaneously. Fe³⁺ hydrolyses into yellow-brown hydroxo compounds. A clean solution is light green; a yellow-brown tint means some Fe²⁺ has become Fe³⁺. The rate rises with open tanks, tanks that are mostly empty and full of air, high temperature or rising pH. The acceptable level of oxidation depends on storage conditions and the process step, so check the colour and measure Fe²⁺ on site rather than relying on a fixed threshold.

Reducing power. Fe²⁺ reduces Cr⁶⁺ (Cr2O7²⁻) and activates H2O2 in the Fenton reaction. Fe³⁺ has no reducing power and so cannot reduce Cr⁶⁺, and it activates H2O2 far less effectively than Fe²⁺, so a heavily oxidised tank means the plant has lost part of the activity it pays for.

Acidity and the Cl⁻ ion. An acidic solution containing Cl⁻ attacks zinc, tin, aluminium, copper and their alloys (copper corrodes quickly). Do not mix with hypochlorite (bleach) or strong oxidisers: an acidic chloride-containing medium meeting a strong oxidiser can release toxic chlorine gas.

Fully soluble in water. This helps metering: the solution can be pumped directly, with no need to dissolve it as with solid chemicals.

Using “phèn sắt 2” in wastewater treatment: where it belongs

The table summarises the well-founded roles of FeCl2 in industrial wastewater treatment:

Cr⁶⁺ reduction in plating wastewater

This application has the clearest reaction. At acidic pH:

Cr2O7²⁻ + 6Fe²⁺ + 14H⁺ → 2Cr³⁺ + 6Fe³⁺ + 7H2O

Each mole of Cr⁶⁺ needs 3 moles of Fe²⁺. In mass terms: 3.22 g Fe²⁺ per gram of Cr⁶⁺, equal to 7.31 g of pure FeCl2. Worked example for 1 m³ of wastewater with 50 mg/L Cr⁶⁺ (50 g Cr⁶⁺): at least about 366 g FeCl2 is required, that is about 1.22 kg of 30% solution (about 0.94 litre at 1.3 g/cm³).

That is the theoretical lower limit. Real wastewater contains other species that also consume Fe²⁺ (dissolved oxygen, other reducing and oxidising agents), so the real dose is higher and is set by jar test and ORP monitoring. After reduction, the pH is raised with alkali to precipitate Cr(OH)3 together with iron hydroxide, followed by settling. The step-by-step procedure is in the article on Cr6+ reduction in plating wastewater with FeCl2.

Coagulation, floc formation and phosphate precipitation

To form good flocs, Fe²⁺ must be oxidised to Fe³⁺ or the pH raised enough to precipitate iron hydroxide. Performance therefore depends on pH, aeration time or an upstream oxidation step, not only on the amount of FeCl2 dosed. If your basin has no oxidation step, compare with FeCl3 before deciding: the article FeCl2 vs FeCl3 as iron coagulants sets out how the two salts differ. Fix every dose by jar test on a real sample.

Sulfide precipitation and H2S odour reduction

Fe²⁺ combines with sulfide to form sparingly soluble FeS, so FeCl2 is used on streams with sulfide or H2S odour. Performance is set by the amount of sulfide to be captured, so measure the sulfide in the stream before setting a dose.

Fenton for dark-coloured wastewater

Fe²⁺ with H2O2 generates hydroxyl radicals (·OH), which can break down many hard-to-degrade organic compounds and colour bodies. The reaction usually runs at acidic pH; around 3 is a common starting point, then fixed by experiment. For textile dyeing wastewater, Fenton is used for colour removal before or after biological treatment depending on the stream. The textile context is in the article on FeCl2 in the textile dyeing industry.

Common misunderstandings about “phèn sắt 2”

Safety when handling FeCl2: protection and first aid

The information below follows the SDS of Loc Thien’s FeCl2 solution. The SDS classifies the solution as harmful if swallowed, causing skin burns and eye damage, respiratory irritation, organ damage and long-term harm to aquatic life; the signal word is “Danger”. Oral LD50 is 984 mg/kg (rat, rabbit). Full text: FeCl2 SDS.

Protection when working: chemical-resistant goggles, gloves, clothing and boots; with vapour or mist, an activated-carbon respirator. Dose and dilute in a ventilated area.

These steps are initial first aid only. When someone is exposed, call emergency services and bring the SDS for the medical staff. Untreated wastewater containing FeCl2 must not be discharged directly, because the SDS places this chemical in the group with long-term harm to aquatic life.

Storage, tank materials and transport

• Temperature and packaging: keep at room temperature, below 35 °C, in a closed tank or container; avoid heat sources and alkali.
• Suitable materials: composite or PVC tanks; rigid PVC or PE drums for transport.
• Materials to avoid: zinc, tin, aluminium, copper and alloys (including valves, fittings and metal pipework).
• Transport: UN 1760, class 8, packing group III according to Loc Thien’s SDS; delivered in 5–30 tonne tankers.
• Warehousing: keep away from alkali and bleach stores; keep closed to limit Fe²⁺ turning into Fe³⁺ (see the chemical properties section).

Loc Thien Technical Department: “Before switching to FeCl2 dosing, a plant should check two points. First, convert litres to kilograms with about 1.3 g/cm³ for the 30% solution, not the 1.93 of the crystal. Second, keep the tank closed so that Fe²⁺ does not turn into Fe³⁺ before it reaches the reaction basin. The 30% solution holds about 13.2% Fe²⁺. For Cr⁶⁺, theory alone already needs 3.22 g Fe²⁺ per gram of Cr⁶⁺, and the real dose is always higher, so set it by jar test.”

Buying 30% FeCl2 from Loc Thien

Loc Thien is the direct manufacturer of 30% FeCl2 solution and delivers it in 5–30 tonne tankers. The product comes with an SDS and a ZDHC MRSL V3.1 test report from TÜV Rheinland (code 248162921-02a, result: pass). Product details are on the page 30% FeCl2 (iron(II) chloride).

Delivery times: inner Ho Chi Minh City 4 hours; southern and Mekong Delta industrial parks 24–48 hours; northern and central Vietnam 2–3 days. The unit price depends on volume, delivery area, packaging and contract form, so Loc Thien quotes per order rather than listing one general price.

For a quote within 15 minutes, send five items: the chemical and concentration needed (FeCl2 30%), expected quantity per delivery, industrial park or province of delivery, purpose of use (Cr⁶⁺ reduction, coagulation, Fenton…) and invoicing details.

Frequently asked questions

What is “phèn sắt 2”?

It is the Vietnamese trade name for ferrous chloride, FeCl2 (CAS 7758-94-3), a salt of divalent iron, usually sold as a green solution. Some people use the same name for FeSO4·7H2O (green vitriol), so check the formula and CAS number on the label or SDS before buying.

Is FeCl2 a salt or an acid?

FeCl2 is a salt, formed from the strong acid HCl and the weak base Fe(OH)2. A solution of the salt is only weakly acidic, because Fe²⁺ hydrolyses and releases H⁺. Loc Thien’s commercial 30% solution has a pH below 1 according to the SDS because it still contains free acid, so it is corrosive and requires protective equipment.

What is the density of 30% FeCl2 solution?

About 1.3 g/cm³. The 1.93 g/cm³ and 105 °C often seen online belong to the tetrahydrate crystal FeCl2·4H2O. Converting litres to kilograms with the crystal figure is off by nearly half for the solution.

Can “phèn sắt 2” be used for well water that contains iron?

No. Iron-rich well water already contains Fe²⁺, and the aim is to remove it, so no more iron salt is added. Commercial FeCl2 is for industrial wastewater such as plating and textile dyeing, at a dose set by jar test.

How much FeCl2 is needed to reduce Cr6+?

In theory 7.31 g of pure FeCl2 per gram of Cr⁶⁺ (3 mol Fe²⁺ : 1 mol Cr⁶⁺), about 24 g of 30% solution. That figure is only a lower limit; the real dose is higher and needs a jar test on the actual wastewater.

Does it matter if FeCl2 solution turns yellow-brown?

The yellow-brown colour shows that some Fe²⁺ has been oxidised to Fe³⁺. That part no longer reduces Cr⁶⁺ and activates Fenton far less effectively. Keep the tank closed, below 35 °C, and re-check the dose once the colour has changed.

How do I order 30% FeCl2?

Go to the contact page or call 0979 891 929 with the quantity, the industrial park or province of delivery and the purpose of use. Loc Thien quotes within 15 minutes and sends the SDS with it.

Request a quote for 30% FeCl2

Need 30% FeCl2 for Cr⁶⁺ reduction, coagulation or Fenton? Send your request at /en/lien-he or by email to [email protected]; quotes within 15 minutes. For more on coagulation dosing and applications, read FeCl2 in wastewater treatment: dosing guide.