sequestering agents

SEQUESTERING AGENTS

Name : MAZADUL HASAN SHESHIRID: 2010000400008Batch: 13th Batch (Session 2009-2013)Department: Wet Processing Technology Email: [email protected]: www. Textilelab.blogspot.com Southeast UniversityDepartment of Textile Engineering

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Sequestration or ChelationThe principles behind sequestration is the formation of a water soluble complex between a sequestering agent and a polyvalent metal ion. The technique can be used for softening water; however, it is more often used as a component in many textile wet processing steps to remove metallic ions that interfere with the process.

Sequestration or Chelation

A sequestering or chelating agent is a complex forming auxiliary chemical with metals such as Iron, Copper, Nickel, Zinc and Magnesium that are present in water and affects the textile processing in many way.

Certain organic compounds are capable of forming coordinate bonds with metals through two or more atoms of the organic compound; such organic compounds are called chelating agents. The compound formed by a chelating agent and a metal is called a chelate. A chelating agent that has two coordinating atoms is called bidentate; one that has three, tridentate; and so on. EDTA, or ethylenediaminetetraacetate, (−O2CH2)2NCH2CH2N(CH2CO2−)2, is a common hexadentate chelating agent. Chlorophyll is a chelate that consists of a magnesium ion joined with a complex chelating agent; heme, part of the hemoglobin in blood, is an iron chelate. Chelating agents are important in textile dyeing, water softening, and enzyme deactivation and as bacteriocides.

What is a Sequestering Agent?

Sequestering agent is a dyeing auxiliaries which is used during dyeing for removing hardness of water.Sequestering agents combine with calcium and magnesium ions and other heavy metal ions in hard water. They form molecules in which the ions are held so securely (sequestered) that they can no longer react. The most undesirable impurities in Fibre, Common salt, Glauber salt, Caustic Soda and Soda ash are the di- and tri-valent cations, e.g., Ca++, Mg ++ Cu ++, Fe+++ etc. These ions increase hardness of the process bath and generate iron oxides in the bath. Calcium and Magnesium reacts with alkali and precipitates as a sticky substance on the textile material, which creates patchy dyeing and discoloration of the fibre. The ferric oxide with cellulose and creates small pinhole on the fibres also damages the machinery by scale formation in the nozzles and base.

To overcome these deleterious effects in the scouring and bleaching bath adequate amount of sequestrant must be used. Sequestrants prevent di-and tri-valent metal ions, e.g., Cu++, Fe +++ , Mn ++, Ca++, Mg++ etc from interfering with the chemical processing of the textile material. It prevents catalytic damage of cellulosic fibres in bleaching hath during hydrogen peroxide bleaching.

Use of Sequestering agent

Thus, unwanted metal salts cause a lot of problems in processing. Now, with the focus on minimising costs and maximising efficiency, consistency and fastness are two important parameters that every dyer would like to achieve first time. This reduces reprocessing costs, making him competitive.

Sequestering agentThe dyer has to use a suitable sequestering agent in the process, wherever it is required. Selection of the right sequestering agent is very important. First and foremost, the sequestering agent should chelate offending metal ions under the given condition and should form a stable complex, which does not decompose over a prolonged processing period.

There are some main type of commercial sequestering agents are:

1. Aminocarboxylic acid base products

2. Phosphates and Phosphonates

3. Hydroxy carbroxylates

4. Polyacrylates

5. Sugar acrylates

Type of commercial sequestering agents

CLASSES OF SEQUESTERING AGENTS

2. Important Polyphosphates

A. Polyphosphates1. Formation of PolyphosphatesPolyphosphates are derivatives of phosphoric acid and are made by reacting phosphorous pentoxide with phosphoric acid. The formation of the polyphosphates can be seen by the following dehydration of orthophosphoric acid.

3. Advantages of Inorganic PhosphatesThey sequester metal ions and contribute to detergency by suspending and dispersing soils. They require less than the stoichiometric amount predicted to keep ions in solution (threshold effect).

They will break down to sodium phosphate in water over time losing their ability to chelate, especially in hot water. They are foods for algae causing rapid growth in streams and ponds. Algae growth depletes stream's oxygen supply causing fish kill.

B. Organophosphonic Acids

1. Ethylenediaminetetra (methylenephosphonic Acid) EDTMP

Advantages and Disadvantages:They will sequester metal ions and aid detergency by dispersing and suspending soil. They are more stable than inorganic polyphosphates in hot water and exhibit threshold effect. They are more expensive than inorganic polyphosphates.

C. Aminocarboxylic Acids

1. Disodium-Ethylenediaminetetraacetic acid (EDTA)

Advantages and Disadvantages:They form very stable complexes with most metal ions. They reacts stoichiometrically and can be used to quantitatively determine calcium and magnesium by titration. They do not contribute to detergency nor do they exhibit a threshold effect.

EDTA: Good sequestering agent for calcium and magnesium at alkaline pH but no sequestering agents on Fe3+ at alkaline pH. Not stable with oxidising agents. Low solubility in acidic medium.Some ligands can bond to a metal atom using more than two pairs of electrons. An example is ethylenediamminetetraacetate ion (EDTA4-), the Lewis structure of which is shown below. EDTA4- forms very stable complexes with most of the transition metals.

EDTA4-

This hexadentate ligand forms very stable complexes (usually octahedral structures) with most of the transition metals. The donor atoms in EDTA4- are the two N atoms, and the four, negatively charged O atoms.

EDTA

EDTA - Diammonium Salt of EDTA (Ethylene Diamine Tetra Acetic Acid)

EDTA - Disodium Salt of EDTA (Ethylene Diamine Tetra Acetic Acid)

EDTA - Chelated Ferric Sodium - (EDTA Ferric Sodium -13%)

EDTA - Ferric Ammonium Salt of EDTA (Ethylene Diamine Tetra Acetic Acid)

EDTA

2. Nitrilotriacetic Acid (NTA)

D. Hydroxycarboxylic Acids

Advantages and Disadvantages:Hydroxy acids are effective for sequestering iron. They are not effective for calcium or magnesium.

FORMATION OF COMPLEXESMost polyvalent ions can form complexes with certain ions or molecules. This type of complex formation is called coordination chemistry. The types of molecules or ions that form coordination complexes are called Ligands, abbreviated "L". Metal ions are electron acceptors (Lewis Acids) and Ligands are electron pair donors (Lewis Base). The bond that is formed is a Coordinate Covalent Bond. Formation Constant is a measure of the strength and stability of a complex. It is a measure of the extent the complex will form or dissociate when the system has reached equilibrium. Complex formation is an equilibrium process.

FORMATION OF COMPLEXES

A. Formation Constant

where K = equilibrium constant and log K = formation constant (stability constant). The higher the formation constant, the more strongly held is the metal ion in the complex. Therefore Ligand that give high log K values with a particular metal are very effective sequestering agents. Table 9 lists some formation constants for several chelating agents. The data shows the specificity of some agents, i.e. gluconic acid which is particularly effective for iron. Also the data shows that EDTA is effective across the board.

Formation Constants

4.Polyacrylates Polyacrylates are effective dispersants, with mild chelation values and protective colloid properties. The chelation values of polyacrylates have no demetallising effect on metal containing dyestuffs. They are completely non foaming. They are very suitable as dyebath conditioners, soaping agents and washing aids. Being non surface active agents they are easily rinsable and thus reduce the quantity of water required for removing their traces from the substrates, unlike all surfactants. The typical chelation values offered by polyacrylates do not come close to the chelation values offered by amino polycarboxylates or the phosphonates. This problem has been overcome by development of sugar acrylates.

Polyacrylates

5.Sugar Acrylates Sugar acrylates have sequestering values as high as amino polycarboxylates or the phosphonates. They are biodegradable, effective components in cellulosic fabric pretreatment during desizing, scouring, bleaching and mercerising. These products are characteristed by good chelation values from the acidic to the alkaline range and from temperatures of 45 to 115øC. They also exhibit no demetalising effect on metal-containing dyestuffs and are non-foaming. They are ideally recommended in pretreatment for desizing, scouring and bleaching and as dyebath conditioners during the cellulosic dyeing.

Sugar Acrylates

Factors to be taken into consideration while selecting a sequestering agent for the process :

1. Stability Constant: As chelation is a reversible reaction, the equilibrium is dependent on the process pH and the concentration of the metal ions and chelating agent, which react together to form a chelate. The stability of the metal complex is expressed in terms of its stability constant. If we represent chelation of metal ion, Mm+ with sequestering agent, An- as: Mm+ + An- MA(m-n) then the stability constant is Ks = MA (m-n) (Mm+) (An-) A high value of Ks indicates high sequestering effect. For example, in the case of aminopolycarboxylates, the stability constant for same metal iron increases in the order NTA, EDTA, DTPA.

In the case of metal ions, the stability constant increases in the order.

From the above information it can be deduced that the NTA-Mg2+ complex has the least stability, whereas DTPA - Fe3+ has the highest stability. Thus, in a process, if more than one metal ion is present, the metal ion having the highest stability will be chelated preferentially. If chelating agent is present in sufficient quantity, the metal with the highest stability constant will be chelated completely, followed by the next metal ion in te order given above. Even after chelation is complete in this order, if additional metal impurity, with metal having a higher stability constant, is introduced, then this metal ion will displace low stability constant metal ions from the complex. For example, Fe3+ displaces Ca2+ from a Ca2+ chelating agent complex. Of course, the chelating agent should be capable of chelating Fe3+ under given conditions.

Factors to selecting a sequestering agent

2. The pH of the Process: The pH of the system will influence the formation of the chelation complex. For example, NTA, EDTA cannot chelate Fe3+ under alkaline conditions, whereas DTPA can. HEDP can chelate Fe3+ up to pH 12, and so also gluconic acid.

3. Demetalisation: This property is particularly important for dyeing and printing with premetallised dyes - for example, some direct, reactive and premetallised metal complex dyes. If Ni2+, Cu2+, Cr3+, Co2+ or Fe3+ is present in premetallised dyes, these could be preferentially chelated ahead of Ca2+ and Mg2+, due to the higher stability constant of these metal ions. Therefore pretrials in the lab are required to establish the suitability of the chelating agent, and also to arrive at the optimum concentration for the given process, when premetallised dyes are to be used.


4. Other Features: Stability of chelate to prolonged process periods, dispersing properties, crystal-growth inhibition, effect on equipment, etc. are also to be considered when selecting a commercial sequestering agent.
