Abstract
EDTA tetrasodium (Ethylenediaminetetraacetic acid tetrasodium salt, EDTA·Na₄) is a high-efficiency aminopolycarboxylate chelating agent widely applied in industrial circulating cooling water, boiler feed water, membrane process water and industrial wastewater treatment. Its core value relies on unique chelating coordination structure to realize dual functions of preventing new scale precipitation and dissolving formed inorganic scale, meanwhile inhibiting metal catalytic corrosion induced by trace heavy metal ions in water. This paper systematically elaborates its anti-scaling & descaling mechanism, applicable working conditions, dosing specifications for typical industrial water systems and matched technical notes.
EDTA-metal chelate structure
EDTA tetrasodium molecular formula
1 Basic Physicochemical Properties of EDTA Tetrasodium
Appearance: White crystalline powder, easily soluble in cold/hot water; 1% aqueous solution pH=11.0~12.5, inherent alkaline without extra alkali addition for alkaline water system.
Molecular structure: 2 amine nitrogen atoms + 4 carboxylate oxygen atoms provide six coordination sites, forming stable five-membered ring chelate with divalent/trivalent metal cations (Ca²⁺, Mg²⁺, Fe²⁺/Fe³⁺, Cu²⁺, Mn²⁺, Pb²⁺).
Chelate stability: Metal-EDTA complexes keep soluble under wide pH (7–13) and temperature ≤180℃, resistant to thermal decomposition and secondary precipitation; superior stability than citrate, gluconate and other organic chelants.
2 Scale Inhibition & Scale Removal Working Principles
2.1 Scale Inhibition Principle (Prevent New Scale Generation)
Scale in industrial water mainly originates from precipitation of CaCO₃, Mg(OH)₂, CaSO₄, iron oxide induced by hardness ions after water concentration, heating and pH rise.
Sequestration chelation mechanism (core action)
EDTA tetrasodium preferentially captures free Ca²⁺, Mg²⁺ in bulk water to form water-soluble stable chelate compounds, lowering effective concentration of free scale-forming cations; block the reaction between hardness ions and carbonate/sulfate/hydroxide radicals to avoid insoluble mineral salt precipitation and scale adhesion on pipe wall, heat exchanger and boiler tube surface. The reaction stoichiometry is approximate 1:1 molar ratio between EDTA tetrasodium and metal ions.
Crystal distortion auxiliary effect
Trace residual EDTA molecules adsorb on growing crystal lattice of nascent scale, destroy regular crystal arrangement; scale turns into loose amorphous sludge instead of compact hard scale, easy to be washed away by circulating water flow without wall attachment.
Anti-corrosion synergism for indirect scale control
Chelate trace Fe³⁺, Cu²⁺ in water which catalyze electrochemical corrosion of carbon steel equipment; reduce iron hydroxide corrosion products (one major component of composite scale) generated from pipeline corrosion, indirectly suppress composite scale formation containing iron oxide.
2.2 Scale Removal Principle (Dissolve Existing Hard Scale Deposits)
For mature inorganic scale (calcium carbonate scale, gypsum scale, iron rust composite scale) accumulated on equipment inner wall, EDTA tetrasodium realizes descaling via two pathways:
Scale surface ion exchange dissolution
EDTA anions diffuse into solid scale micro-pores, replace carbonate/sulfate radicals combined with Ca/Mg in scale; original insoluble mineral scale converts into soluble EDTA-metal chelate and dissolves into circulating cleaning solution gradually, scale layer peels off spontaneously.
Alkali synergistic transformation for refractory sulfate scale
For hard-to-dissolve CaSO₄ gypsum scale: mix EDTA tetrasodium with sodium carbonate; carbonate first converts CaSO₄ into loose CaCO₃ intermediate, then EDTA rapidly chelates calcium from converted carbonate scale to complete full dissolution, widely used for boiler & cooling tower heavy gypsum descaling.
3 Suitable Application Scenarios & Dosing Methods by Industrial Water Classification
3.1 Circulating Cooling Water System (Central air-condition cooling tower, industrial production cooling circuit)
Application target: Prevent Ca/Mg carbonate scale + iron mud scale on heat exchanger
Working condition: Normal circulating water pH=7.5~9.5, water temperature 30~55℃, concentration multiple 2.0~4.5
Dosing mode: Continuous on-line drip feeding into cooling water inlet pool
Standard dosage:
① Low-hardness raw water (total hardness＜200mg/L as CaCO₃): 8~25 mg/L (based on circulating water volume)
② Medium-high hardness raw water (200~500mg/L): 30~60 mg/L; cooperate with small amount organic phosphonate scale inhibitor to cut EDTA cost
Operation note: Periodically test residual free EDTA content in circulating water (control residual 3~8mg/L) to adjust dosage; residual chelant can continuously inhibit secondary scaling.
3.2 Industrial Boiler Feed Water & Boiler On-line Descaling
A. Preventive scale inhibition for daily boiler operation
Applicable: Medium-low pressure industrial boiler (working pressure＜2.5MPa, feed water hardness ≤150mg/L)
Dosing: Dissolve EDTA tetrasodium into feed water tank first, continuous dosing with feed water pump; dosage calculated by feed water total hardness: molar ratio EDTA : total hardness =1.05~1.1:1 (slight excess for surplus chelation margin).
B. Off-line chemical cleaning for boiler old scale removal
Cleaning formula: 3%~8% EDTA tetrasodium aqueous solution, add NaOH to adjust cleaning liquid pH=10.0~11.5 (optimal chelation pH range for EDTA-4Na)
Construction: Circulation soaking cleaning at 50~70℃ for 8~24h; monitor calcium content in cleaning waste liquid, stop cleaning when calcium concentration keeps stable without rising (scale fully dissolved); drain waste liquor and flush boiler with clean water.
3.3 RO/NF Membrane Feed Water Treatment (Anti-membrane fouling)
Function: Chelate hardness & heavy metal ions to prevent inorganic scale blocking membrane pores; avoid membrane element fouling from calcium salt precipitation
Dosage: 5~15mg/L continuous dosing before security filter of RO unit; match with antiscalant to extend membrane service life.
3.4 Heavy Metal Contained Industrial Wastewater Treatment (Electroplating, surface pickling wastewater)
Application: Capture Cu²⁺, Ni²⁺, Zn²⁺, Pb²⁺ to form stable soluble chelate; subsequent coagulation precipitation or membrane separation removes heavy metal from wastewater to meet discharge standard
Dosage: Calculate by total heavy metal molar concentration in wastewater, EDTA excess 5%~10% molar ratio; adjust pH=9~11 for optimal chelation effect.
4 Key Influencing Factors of Service Effect & Technical Precautions
4.1 Influencing Factors
pH value: Optimal working pH for EDTA tetrasodium: 9.5~12; pH＜7 leads to EDTA protonation and reduced chelation capacity; alkaline environment guarantees full dissociation of carboxylate groups for maximum coordination performance.
Temperature: Chelation rate rises with temperature; 40~80℃ is best for descaling construction; over 200℃ causes partial EDTA molecular decomposition.
Dosage control: Low dosage only achieves threshold anti-scaling effect; insufficient EDTA cannot fully sequester hardness; excessive long-term over-dosage will slightly corrode passive iron oxide film on carbon steel equipment surface, accelerate mild metal corrosion.
4.5 Practical Operation Notes
Formula compounding optimization: For high-hardness high-sulfate raw water, compound EDTA tetrasodium with polyacrylate, organic phosphonic acid to reduce single EDTA dosage and lower running cost.
Waste liquid disposal: EDTA-metal chelate wastewater is difficult for conventional biochemical degradation; high-concentration descaling waste liquid adopts acidification precipitation to recover solid EDTA for cyclic reuse after filtration, reduce chemical consumption and environmental pollution.
Storage requirement: Sealed dry storage to avoid moisture deliquescence; avoid mixing with strong acid chemicals to prevent EDTA precipitation failure.
5 Advantages & Limitations in Industrial Water Treatment
Advantages
Broad-spectrum chelation: Effective for carbonate, sulfate, iron oxide composite scale simultaneously, multi-scenario universal agent;
Wide temperature & pH adaptability, stable chelate performance under variable water quality fluctuation;
Descaling construction nearly non-corrosive to carbon steel, copper, stainless steel base material compared with strong acid pickling, reduce equipment corrosion damage risk.
Limitations
High raw material cost, not economical for ultra-large open circulating water with super high hardness alone; mostly used as auxiliary agent or concentrated system descaling agent;
Poor biodegradability in natural water body, excess discharged EDTA causes heavy metal re-mobilization in sediment; strictly control discharge concentration of treated wastewater.
Conclusion
EDTA tetrasodium depends on multi-site coordination chelation to realize integrated prevention & removal of inorganic scale in industrial water; its alkaline characteristic matches most industrial alkaline water systems without extra pH adjustment. In practical engineering, select independent application or compound formula according to raw water hardness, system temperature and equipment type; reasonable dosing control can maximize anti-scaling & descaling efficiency while cut comprehensive water treatment cost.