Biomedical Engineering Reference
In-Depth Information
albumin-containing aqueous solutions has been previously investigated (Annesini et al.,
2005, 2008, submitted) and expressions for the adsorption isotherms, accounting for the
effect of albumin concentration, are available. It is worth remarking some of the main re-
sults of those papers. Firstly, on all of the adsorbent media tested, bilirubin molar uptake
was largely greater than albumin molar uptake (which was, at least for anionic resins, neg-
ligible), indicating that bilirubin is mainly, if not only, adsorbed in its unbound form. Fur-
thermore, a significant reduction of bilirubin adsorbed amount was always observed when
albumin concentration was increased in the solution. Finally, comparing all the results, it
can be stated that the adsorption capacity for bilirubin of the three different media increases
in the following order
polymeric non ionic resin
<
activated carbon
<
anionic resin
The general expression of the bilirubin adsorption isotherm obtained by Annesini et al.
(2005, 2008, submitted) was derived accounting for albumin-toxin association in the liquid
phase and free toxin adsorption on the adsorbent solid. The same expression will be derived
here in a simpler way, by the chemical thermodynamic approach followed throughout this
chapter for the description of bilirubin phase partition.
For tightly albumin-bound toxins like bilirubin, adsorption on a solid medium can be
viewed as an exchange reaction involving the adsorption sites on the solid:
AB
+
S
⇋
A
+
BS
(28)
at thermodynamic equilibrium:
C
AB
(1−Θ)
C
A
Θ
K
ADS
=
(29)
where
Θ
is the fraction of saturated binding sites or the ratio between the bilirubin adsorbed
amount and the maximun adsorption capacity of the adsorbent for the free toxin (
Θ =
N
BIL
/N
M,B
). With the approximations (8), the adsorption isotherm may be written in an
apparent Langmuir form as
C
BIL
K + C
BIL
N
BIL
= N
BIL
(30)
with the following apparent constants:
N
M,B
1−K
ADS
N
BIL
=
(31)
K
ADS
1−K
ADS
C
ALB
= M
BIL
C
ALB
K =
(32)
According to this model, the apparent Langmuir constant
K
should be an increasing
function of albumin concentration, whereas the maximum adsorption capacity should be
independent of it. Furthermore, if
C
BIL
/C
ALB
≪M
BIL
a linear adsorption isotherm is obtained
N
BIL
M
BIL
C
ALB
C
BIL
N
BIL
=
(33)
