1070-4272/04/7705-0756C2004 MAIK [Nauka/Interperiodica]
Russian Journal of Applied Chemistry, Vol. 77, No. 5, 2004, pp. 756!758. Translated from Zhurnal Prikladnoi Khimii, Vol. 77, No. 5,
2004, pp. 760!762.
Original Russian Text Copyright + 2004 by Bel’chinskaya, Tkacheva.
AND ION-EXCHANGE PROCESSES
Determining the Mechanism of Formaldehyde Adsorption
from an Aqueous Medium on the Basis
of the Diffusion Criterion
L. I. Bel’chinskaya and O. A. Tkacheva
Voronezh State Forestry Academy, Voronezh, Russia
Received April 16, 2003; in final form, October 2003
Abstract-Adsorption of formaldehyde from an aqueous medium on acid-treated clayey minerals was studied
under conditions of natural convection and under stirring of the solution.
One of important unsolved environmental problems
of the woodworking industry is removal of formalde-
hyde from liquid wastes of furniture factories. The
aim of this study was to establish the mechanism of
formaldehyde adsorption from an aqueous medium on
the basis of the diffusion criterion and to use this
parameter to determine ways to intensify the process.
Three systems constituted by formaldehyde and
clayey microporous minerals (montmorillonite, paly-
gorskite, and clinoptilolite) and having different types
of structure [1, 2] were studied.
To determine the specific features of the behavior
of the systems under study under conditions of natural
convection and under stirring, equilibrium and dynam-
ic processes were studied.
The adsorption of formaldehyde was carried out
on 1-g mineral samples (grain size 0.025 cm) from
0.025 l of 0.03 M formaldehyde solution. The time of
equilibration was 20 min. The samples were pre-
treated with a 15% solution of H
their adsorption characteristics .
The adsorption value, a (mg g
), was calculated
by the equation 
! c)/m, (1)
and c are the concentrations of the formal-
dehyde solution before and after adsorption, respec-
tively (mg l
); m is the adsorbent mass (g l
The values of a were obtained with a high correla-
tion factor (r = 0.98).
Typical dependences which supposedly reflect
processes of the rate-determining stage of diffusion
were analyzed: q3t and q3t
, where q = a/a
the fraction of adsorption in a certain interval of time
t; a and a
are the running and maximum adsorp-
tion values, respectively (mg g
The adsorption rate is determined by inner diffu-
sion if the dependence q3t is initially linear and then
gives way to the linear dependence q = f(t
the straight lines emerging from the origin of coordi-
nates . The obtained linear dependence q = f(t
assumes that the inner diffusion is the rate-determin-
ing factor in adsorption of formaldehyde on palygor-
skite, montmorillonite, and clinoptilolite.
When the rate of the process is determined by outer
diffusion, the dependence ln(1 3q)3t from the
ln(1 !G)=!>J, (2)
where b is the rate constant (s
), should be linear ,
which was found to be the case in an analysis of for-
maldehyde adsorption on the minerals under study.
The value of b was determined graphically from
the slope of the straight line in the coordinates
The bounds of the outer- and inner-diffusion re-
gions were discussed on the basis of an analysis of the
Bi criterion with the use of the rate constant b of the
adsorption process  and the inner-diffusion constant
D. The Bi criterion (ratio of the inner resistance to
mass transfer within an adsorbent grain to the outer
resistance to mass transfer from the bulk of the liquid
to the grain surface) was found from the equation
Bi = >r