Sequestering agents form complexes with polyvalent metal ions that can interfere with textile processing. There are several types of commercial sequestering agents, including aminocarboxylic acids, phosphates, hydroxycarboxylic acids, polyacrylates, and sugar acrylates. The most important factors to consider when selecting a sequestering agent are the stability constant of the metal-chelate complex, the pH of the process, and whether the agent could remove metals from premetallized dyes. An effective sequestering agent forms stable complexes, works under the process pH conditions, and does not demetalize important process metals.

1. SEQUESTERING AGENTS

2. Name : MAZADUL HASAN SHESHIR
ID: 2010000400008
Batch: 13th Batch (Session 2009-2013)
Department: Wet Processing Technology
Email: mazadulhasan@yahoo.com
Blog: www. Textilelab.blogspot.com
Southeast University
Department of Textile Engineering
PREPARED BY ©right

3. Sequestration or Chelation
The 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

4. 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?

5. 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

6. 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 agent
The 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.

7. 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

8. CLASSES OF SEQUESTERING AGENTS
2. Important Polyphosphates
A. Polyphosphates
1. Formation of Polyphosphates
Polyphosphates 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.

9. 3. Advantages of Inorganic Phosphates
They 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.

10. 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.

11. 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.

12. 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

13. 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

14. 2. Nitrilotriacetic Acid (NTA)

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

16. FORMATION OF COMPLEXES
Most 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

17. 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.

18. Formation Constants

19. 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

20. 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

21. 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.

22. 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

23. 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.
Factors to selecting a sequestering agent

24. 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.
Factors to selecting a sequestering agent