Sorption

Sorption is the phenomenon that makes soluble substances accumulate at the solid–

liquid interface on the surface of pulp fibers. Since with pulp the phenomenon

of adsorption (i.e., the retention of solutes from a solution by a solid surface) and

absorption (i.e., the uptake and retention of solutes within the mass of the solid)

are difficult to distinguish one from another, the term “sorption” is often used in

the pulp and paper industry to cover both events. Sorption occurs when molecules

accumulate on the pulp fiber because this represents their most stable situation

with the lowest free energy.

Traditionally, sorption has been described by the Langmuir isotherm, which

assumes a state of saturation at high solute concentrations due to space constraints

on the sorbent surface [1,17]:

A _ A max

K c

1 _ K c _10_

where A = sorbed quantity of substance per unit mass of pulp (kg odt–1); A max=maximum

quantity of sorbed substance per unit mass of pulp (kg odt–1); K = equilibrium

constant (m3 kg–1);and c = equilibrium concentration of substance remaining in

solution (kg m–3).

The saturation approach determines that A max is related to the sorbent’s surface

area. Even if this relationship is not linear, the maximum sorption rate is substantially

lower for undamaged fibers than it is for fines. In contrast, it increases massively

with the accessibility of internal pore surfaces.

The equilibrium constant K depends on the charge difference between the sorbent

and sorbate. It is also most fundamentally affected by changes in pH. Since

the net charge of pulp surfaces is generally negative, the sorption of cations is of

primary interest in pulp washing.

In kraft cooking, the predominant cation is obviously sodium. Early laboratory

tests on hardwood and softwood pulps have confirmed that the pH has a significant

influence on the amount of sorbed sodium [18].

The sorption behavior shown in Fig. 5.6 is characteristic of that obtained with a

bifunctional ion exchanger, suggesting that sodium sorption on pulp results from

the presence of two sets of functional groups [18]. While carboxylic and other

acidic functional groups are made responsible for sorption already at acidic pH,

phenolic hydroxyls account for the additional sorption under alkaline conditions.

The amount of sodium sorbed depends on the wood species, and also on the cooking

process. For a given species, sorbed sodium increases with the lignin content

because higher-kappa pulps contain more carboxylic and phenolic groups.

In the multi-component system usually found in an industrial liquor, other cations

compete with sodium for available sorption sites. Not all cations have the

same affinity for cellulose, but there is a ranking in affinity as follows [19]:

H+ > Zn2+> Ca2+> Mg2+ > Cs+ > K+ > Na+

5 Pulp Washing

3 4 5 6 7 8 9 10 11 12

Sorbed sodium [g/kg o.d. pulp]

pH

Fig. 5.6 Effect of pH on sodium sorbed by slash pine at kappa number 80 [18].

There is also a temperature-dependence of the sorption equilibrium in a way

that, at higher temperatures, the ions are bound more weakly to the fiber [20].

A more recent approach to predicting the ion-exchange behavior of pulps is

based on the Donnan equilibrium model, which describes the unequal distribution

of ions between two parts of an aqueous system [21].

Fiber wall External solution

Fig. 5.7 Model of the water-swollen fiber wall in contact with an external solution [21].

Conceptually, there are two separate solutions in a pulp suspension, that is, a

small volume of solution contained inside the fiber walls and a large volume of

solution external to the fibers (Fig. 5.7). Within the fiber walls, the readily dissociable

groups are associated with a lignin-hemicellulose gel which is located between

the microfibrils. Once dissociated, the negatively charged groups cause an

unequal distribution of mobile ions in a way that cations show higher concentrations

within the fiber wall than in the external solution.

5.2 Pulp Washing Theory

The Donnan theory relates the concentrations of the mobile ions in the two solutions

in the form of:

k _

_ H __ F

_ H __ S _

_ M __ F

_ M __ S _ __ M __2_____ F _

_ M 2__ S

_ _ ___ _

_ Mz __ F

_ Mz __ S _ _

1 z

_11_

where k= constant; [ H +] = hydrogen ion concentration (kmolm–3);[ M +] = monovalent

metal ion concentration (kmol m–3);[ M 2+] = divalent metal ion concentration

(kmol m–3);a nd [ M z+] = concentration ofmultivalent metal ion (valency z) (kmolm–3).

The subscripts F and S denominate the concentrations in the solution inside the

fibers and in the external solution, respectively.

The numerical determination of the factor k is somewhat lengthy, and requires

a certain knowledge about the functional groups involved [21]. Nevertheless, the

charm of the theory lies in the exchangeability of metal ions in Eq. (11). When k

has been calculated over a range of pH values from a set of experimental distribution

data available for a particular metal ion, then the pH-dependent distribution

of any other metal ion can be predicted, if only the total amount of this other metal

is known, by:

_ Mz __ S _

V _ F k z _ _ 1_

Mz _ _12_

_ Mz __ F _ k z _ Mz __ S _13_

where V = total volume of solution inside and outside fibers per unit amount of

pulp (m3 odt–1); F = total volume of solution inside fibers per unit amount of pulp

(m3 odt–1);and M z+ = total amount of cation with valency z per unit amount of

pulp (kmol odt–1).

Equation (12) is easily obtained by combining the overall mass balance for the

metal ion [Eq. (14)] with Eq. (11):

Mz _ _ F _ Mz __ F __ V _ F __ Mz __ S _14_

The total volume of solution inside the fibers F (also called the fiber saturation

point) has been determined to be about 1.4 m3 per oven-dried ton (odt) for northern

softwood pulps [21].

It becomes apparent from Eq. (13) that the valency of the ion has a decisive

influence on the distribution of a metal between the fiber wall and the external

solution. For the softwood pulp investigated by Towers and Scallan [21], k was

about 15 in the higher pH range. A monovalent ion such as sodium can then be

found at concentrations in the fiber wall which are about one order of magnitude

higher than the concentrations in the surrounding liquor (Fig. 5.8). For a divalent

ion such as manganese, the ratio increases to two orders of magnitude (Fig. 5.9).

5 Pulp Washing

0.01

0.1

0 2 4 6 8 10 12

Na concentration [mmol/L]

External solution pH

[Na+]S

[Na+]F

Fig. 5.8 Experimental data for the distribution of sodium

between the fiber wall (F) and the external solution (S) [21].

0.001

0.01

0.1

0 2 4 6 8 10 12

Mn concentration [mmol/L]

External solution pH

[Mn2+]S

[Mn2+]F

Fig. 5.9 Experimental data for the distribution of manganese

between the fiber wall (F) and the external solution (S) [21].

Models based on the Donnan theory enjoy increasing popularity. The prediction

accuracy for the metal distribution can be increased, for instance, by extending

Eq. (11) with activity coefficients, and by considering the pH-dependency of the

fiber saturation point [22], as well as by including models for complexation and

kinetics [23].

Certain metal ions, predominantly manganese and iron, play a role in chlorinefree

bleaching. They need to be removed in order to avoid catalytic decomposition

5.3 Principles of Washing

of peroxide. On the other hand, the presence of magnesium ions is desirable

because they serve as an inhibitor to cellulose degradation [20]. Figure 5.10 shows

the manganese concentration on pulp as a function of the pH, both with and without

addition of the chelant DTPA to the dilute pulp slurry prior to dewatering.

0 2 4 6 8 10 12

Sorbed manganese [g/kg o.d. pulp]

pH

0.4% DTPA

no chelant

Fig. 5.10 Manganese sorbed on pulp, with and without chelant [24].

In practice, sorption can place a limit on the washing efficiency that can be

reached. Since sorbed substances are not accessible to normal washing, they will

remain in the washed pulp and adversely affect the washing result. Depending on

the process environment, the amount of a sorbed substance may become almost

insensitive to its concentration in the surrounding liquor and cannot reasonably

be affected by changes in the wash liquor quantity.

However, the amount of a substance which is sorbed on pulp can be influenced

by changing the pH – for example, by acidification with sulfuric acid or carbon

dioxide, by changing the pulp surface charge with the aid of surfactants, or by

means of additives which bind the substance otherwise in the surrounding liquor

(e.g., chelating agents).

It must be well noted that sorption is a reversible process. Substances that readily

desorb from pulp under changing conditions will as readily redeposit on the

pulp when the environment is returned to the original conditions.

5.3

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