СЕКЦИЯ 3. «ЗОЛЬ-ГЕЛЬ ПЛЁНКИ, ПОКРЫТИЯ И МЕМБРАНЫ»
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One of the important indicators in the absorption of metal ions into the chitosan membranes is
the nature of the buffer solution. Because, it allows absorption to partially change the selectivity of the
membrane sorbents and the composition of the formed membrane. The main reason for this is that the
composition of the solution affects the state of the functional groups of metal ions polymer membrane
sorbents.
In ammonium-acetate
buffer solution, sorption of metal ions in various pH environments to
the dissolved chitosan film-membrane was studied (Picture 1). the pH values of silver (I) and copper
(II) ions in the 6,5-7,5 range reached the maximum values of absorption. In this solution, it was found
that the sorption amount of other metal ions, namely zinc (II) and lead (II), was less absorbable than
silver (I) and copper (II) ions. In this case, copper (II) and silver (I) ions are obtained at the same pH
values as the maximum in the ammonium-acetate buffer solution. Since the number of sorption centres
is limited, this leads to a decrease in the degree of separation of metal ions from the ammonia-acetate
buffer solution in comparison with the ammonia buffer.
The dependence of sorption of metal ions on pH obtained in buffer
solutions with ammonia
and ammonium acetate is somewhat different (Picture 2). In particular, the absorption of copper (II)
and silver (I) ions more than ammonia-acetate solution from ammonia buffer solution can be seen
from the experimental results. In a buffer solution with ammonia, it
corresponds to the maximum
amount of ions under consideration, but the pH values do not match. At the same time, the maximum
of the same pH values of copper (II) and silver (I) ions in the ammonia-acetate buffer solution is
obtained in the range of 6.0–7.5. Since the number of sorption centres was limited,
the degree of
separation of metal ions into ammonia and ammonia-acetate buffer solution was compared. Sorption
of metal ions in ammonia-acetate buffer solution is passive.
Conclusion.
All buffer systems under consideration are characterized by a significant absorption of metal
ions in a strongly acidic and strongly alkaline environment. This condition, on the one hand, is
explained by the protonation
of amino groups in chitosan, which are altered in a strongly acidic
environment. On the other hand, the high pH values of the solution
correspond to the high
concentration of ammonia. Therefore, at pH> 8, metal cations combine,
forming strong ammonia
complexes with the maximum number of coordinates.
So, the high selectivity of chitosan-based sorbents corresponds to the principles of achieving
high selectivity described in this literature regarding the low stability of complexes formed by
functional groups of sorbents with mixed metal ions.
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