Metal Sequestration (English version)
The term “sequestration” was first used in relation to Chemistry by R.E. Hall [1] in a patent concerning the
properties of certain phosphates. With the introduction of the amino carboxylic acids and particularly
ethylene diamino tetracarboxylic acid (EDTA) the term has become increasingly associated with the
products having similar action. Sequestration is the suppression of a property or reaction of a metal
without the removal of that metal from the system or phase by any process f precipitation, solvent
extraction, or technological process employing for example ion exchange or chelating resins. It can be
affirmed that sequestration consists in the formation of some co-ordination state of the metal ion in
solution which is stronger than the previous co-ordination condition. Therefore it can be assumed that the
metal sequestration is a chemical process occurring in solution by which the metal ion remains into solution
with a free concentration noticeably reduced, depending on the strength of the sequestering agent, i.e. on
the stability of complex species formed by the metal ion with the ligand added or natural occurring in
solution (such as, for example, the humic substance in natural waters). The reduction of free metal ion
concentration influences strongly the mobility, bioavailability and toxicity of dangerous metal ions in
natural systems, including the human bodies. On the basis of the above consideration is of great
importance to know the acid-base behavior of metal ions in aqueous solution, especially in the presence of
ligands of different strength: that means to know (or to determine by experimental measurements) the
formation constants of hydrolytic species and of all possible complex species formed by the metal ions with
the different classes of ligands, in order to have a whole picture of the thermodynamic stability of the
aqueous systems. After the more stable metal-ligand species in solution has been found, a question arises:
which is the necessary ligand concentration to sequester the metal ion ? To answer this question, we used
the following sigmoid type Boltzman equation
x
1
1  10
( pL  pL0.5 )
where pL stands for the –log (total ligand concentration) and x is the fraction of the metal ion complexed by
the ligand that is given by
x
[M]
tot
 [M] free   [M] other
[M]
tot
with [M]tot = analytical concentration of the metal,
[M]free = free metal concentration and [M]other = sum of species formed by the metal with other ligands
present in the solution (such as the anion of the ionic medium or the hydroxide ion).
[1]
Smith R.L., Marriott R.H., Aiken J.K. Peck D.R:, Chalk A.J. and Smith J.F. (1956). Letters to the Editor on the
meaning of sequestration. Chem. & Ind., 320, 462-463
Some recent articles published on this topic
1) A. Gianguzza, D. Piazzese et al. Quantitative parameters for the sequestering capacity of polyacrylates
towards alcaline earth metal ions. Talanta, 61, (2003), pp. 181-194.
2) A. Gianguzza, D. Piazzese et al. Sequestration of biogenic amines by alginic and fulvic acids. Biophysical
Chem., 122, (2006), 221-231
3) A. Gianguzza. A. Pettignano et al. Sequestration of organometallic compounds by natural organic
matter. Binding of trimethyltin(iv) by fulvic and alginic acids. Appl. Organomet. Chem., 20, (2006), 706717.
4) A. Gianguzza, A. Pettignano et al. Sequestering ability of polyaminopolycarboxylic ligands towards
dioxouranium(VI) cation. Journal of Alloys and Compounds, 424, (2006), 93-104.
5) A. Gianguzza, A. Pettignano et al. Sequestration of organometallic compounds by synthetic and
naturally occurring polycarboxylate ligands. Binding of monomethylmercury(II) by polyacrylic and
alginic acids. Chem. Spec. Bioavailab., 19(3), (2007), 131-142.
6) A. Gianguzza, A. Pettignano et al. Interaction of methyltin(IV) compounds with carboxylate ligands. Part
2: formation, thermodynamic parameters, predictive relationships and sequestering ability. Appl.
Organomet. Chem., 22, (2008) 30-38.
7) A. Gianguzza, A. Pettignano et al. Palladium(II) sequestration by phytate in aqueous solution –
speciation analysis and ionic medium effects. Environ. Chem. 2010, 7, 259–267.
8) A. Gianguzza, A. Pettignano, D. Piazzese et al. Uranium(VI) sequestration by polyacrylic and fulvic acids
in aqueous solution. J. Radioanal. Nucl. Chem., (2011) 289:689–697.
9) S. Cataldo, A. Gianguzza, A. Pettignano et al. Sequestration of (CH3)Hg+ by amino‐polycarboxylic
chelating agents. J. Molec. Liquids, 172 (2012) 46–52.
10) A. Gianguzza, A. Pettignano, D. Piazzese et al. Sequestering ability of some chelating agents towards
methylmercury(II). Anal. Bioanal. Chem., (2013) 405:881–893.