Ecological Chemistry. St. Petersburg, Russia

SIMULATION OF HEAVY METALS MIGRATION IN PEAT DEPOSITS

I.L. Kharkhordin and F.G. Atroshchenko

St. Petersburg department of the Geology Institute of Russian Academy of Sciences,

196199 St. Petersburg

Interdepartmental Scientific Centre of Hydrogeoecology, St. Petersburg State University,

199178 St. Petersburg

(Accepted for publication September 30, 1998)

The problems of computer simulation of heavy metals migration in swamp deposits is considered taking as
an example a site at slime storage of the Arkhangelsk Thermal Power Station. Some principal results of the
simulation were found. Heavy metals in swamp and bog deposits are mainly in the sorbed state. The ratio of
sorbed to dissolved forms is 10

3

–10

4

. In low3lying swamps with near3neutral medium, peat exhibits consid3

erable buffer capacity with respect to both acid and alkaline solutions. In the initial migration state, heavy
metals concentration in water is virtually independent of their content in waste and is determined by the
composition of the exchange complex in peat. At low heavy metals concentrations in waste, interaction with
peat can lead either to their sorption or to their accumulation in solution. If the initial metal concentration
exceeds the first milligrams per litre, processes of self3purification predominate.

Key words: swamp deposits, heavy metals, slime storage, ion exchange, computer simulation.

Introduction

The prediction of changes in the chemical

composition of water in its interaction with
swamp deposits is necessary for solving many
practical problems. On the one hand, swamp
can accumulate heavy metals [1,2], radioactive
substances [3], and oil products. Therefore,
they can be used as slime storage spaces and
places of incompletely purified waste and drill3
ing water discharge. On the other hand, the
change in physico3chemical conditions in
swamp deposits under the effect of anthropo3
genic factors (drainage or flooding of swamps,
acid rains, etc.) can lead to desorption of toxic
elements previously accumulated in peat. The
aim of the present paper is to develop approach3
es to computer simulation of these processes
taking as an example a site of slime storage of
the Arkhangelsk Thermal Power Station
(ATPS).

Characteristic of the research location

Slime storage of ATPS is located in the delta of the

Northern Dvina River in the region of extensive swamp
deposits. It was constructed in the 1970s and is of a typ3
ical dike character. The discharged material differs in
chemical composition, volume, and degree of toxicity.

Therefore, the storage is divided in three sections: 1)
chemically purified water (CPW) about 130 thousand
m

3

in volume, 2) neutralized water (NW) — 50 thousand

m

3

in volume, 3) acid boiler washing water (AW) —

15 thousand m

3

in volume. At present only section CPW

and AW are used for discharging slime, and section NW
is temporarily closed. Taking into account the ratio of
solid to liquid phases in the discharged pulp, it was cal3
culated that in section CPW only 20% of capacity is filled
with slime, in section AW — about 10%, and in section
NW — not more than 2%. However, waste volume dis3
charged into these sections is such that if water leaks
from them is excluded, as is required the Norms and
Rules of designing these constructions [4], it would be
necessary to build a new slime storage every year. At
present the level in this storage is approximately con3
stant. This implies that leak volume corresponds to the
volume of discharged waste and atmospheric precipita3
tion. In order to evaluate the effect of leaks filtered
through the dike, extensive investigations were carried
out including testing of waste and pore solutions of
swamp deposits.

Chemical analyses were performed in the laboratory

of joint3stock society “Mekhanobr” by standard methods.
The averaged initial composition of swamp water and
discharge to sections CPW and AW taken for simulation
is given in Table 1. We can note the following changes in
chemical composition near sections CPW and AW found
from data on hydrogeochemical testing.

1. Section CWP. In underground water the concen3

trations of magnesium and sulphate ions increased twice.
Manganese concentration increased four times, and Ni,
Co, and Zn appeared in quantities exceeding LAC two and

66

I.L. Kharkhordin and F.G. Atroshchenko

three times, although their concentrations in the section
itself are not high.

2. Section AW. During filtering swamp deposits, wa3

ter from this section is transformed indo weakly acid and
neutral water and hydrobionts appear. The concentra3
tions of Mo, V, Zn, and Cu decrease to LAC and the con3
centrations of Ni and Co to background values charac3
teristic of water of swamp deposits. The content of Co,
Mn, and Al changes only slightly.

3. Section NW. The transformation of chemical com3

position of water of this section in the swamp deposits is
accompanied by increasing concentration of Mn and Zn,
whereas Ni, Co, Mo, and Cu content decreases to LAC or
to background values.

Process of interaction between metal

ions and swamp deposits. Evaluation of

model parameters

For quantitative description and prediction

of directions of further changes in chemical
composition of underground water, computer
simulation of migration of solutions contain3
ing heavy metals in swamp deposits was devel3
oped.

In spite a long history of studying the proc3

esses of solutions interaction with humus ma3
terial, an effective procedure for their mathe3
matical description is not yet available. This
mainly due to complex structure of humic com3
pounds, which contains various functional,

groups [5]. It is also important that the com3
position of these compounds is variable. There3
for, it is impossible in principle to isolate indi3
vidual humic compounds and one must work
with these complex mixtures. For instance,
according data in ref. [6], fulvic acids in natu3
ral waters migrate in the form of associates
with molecular weight from 300 to 60000 (de3
pending on their concentration and pH of so3
lution). These authors suggest that monomers
of fulvic acids contain two or three carboxylic
(K

1

= 2

⋅

10

–3

, K

2

= 5

⋅

10

–5

) and one or two phenol

groups. The study of fulvic acids fractionation
on ion3exchange materials has shown that the
fraction relatively enriched with nitrogen is
retained by cation exchangers at pH = 2 [7].
This fact indicates that nitrogen is predomi3
nantly contained in amino3groups. Cationic
groups in natural organic compounds are prob3
ably sorbed by clay minerals [8]. It is possible
that in this process hydrogen bonds and bonds
with the participation of metal ions are formed
It is established that mercapto3groups are
present in peat. Their concentration depend on
redox conditions [10].

The complexation model is usually applied

for the mathematical description of interaction
of dissolved fulvic and humic acids with metal
ions. However, in this case equilibrium con3
stants are tentative and depend on the solution
composition, in particular on pH [6]. This
makes it difficult to calculate the composition
of multicomponent systems. On the whole the
stability constants of hydroxyfulvic metal
complexes range from 10

3

to 10

11

increasing in

the following series:

Sr(II) < Ca(II) < Fe(II) < Ce(III) < Y(III) <

Cu(II) < Ru(IV) < Fe(III) < Sb(III) < Au(III) <
Hg(II) [11].

The model of cation exchange is generally

used to describe metal sorption on humic com3
pounds in the solid state of swamp deposits.
Either the ternary system Me

I

3Me

II

3H

+

is con3

sidered or the cation3exchange system is re3
garded as a function of pH and solution com3
position. The principal investigation methods
are potentiometric titration of humic materi3
al in salt solution and experiments on ion ex3
change. It is observed that the mechanism of
chemical bonding of metal with humic acids
and with peat are not very different [12]. More3

Table 1

Model compositions of solutions

Waste waters

Index

Measuremen

t units

CWP

AW

Swamp

waters

(background)

t,

°

C

6.0

6.0

6.0

pH

9.0

3.6

7.03

HCO

3

–

mg

⋅

l

–1

206

2

206

Cl

–

–“–

74

500

27

SO

4

2–

–“–

399

750

20

Ca

2+

–“–

112

180

43

Mg

2+

–“–

25

45

2.5

Na

2+

–“–

148

364

34

Mn

2+

–“–

0.05

1.87

0.26

Ni

2+

–“–

0.002 17.37

0.001

Zn

2+

–“–

0.001

7.17

0.5

Cu

2+

–“–

0.001

0.71

0.0003

Pd

2+

–“–

0.002 0.086

0.002

Cd

2+

–“–

0.015 0.028

0.005

67

Simulation of Heavy Metals Migration in Peat Deposits

over, most metal ions retain their hydrate
shells and the bond is of electrostatic charac3
ter, whereas copper ions form predominantly
more stable covalent bonds. The absolute rate
of metals adsorption on peat decreases in the
following series: Pb

2+

> Cu

2+

> Cd

2+

> Zn

2+

>

Ca

2+

and the time of half3reaction of sorption

(desorption) ranges from 5 to 15 s [13].

In this work for the description of metals

migration in swamp deposits, the model of cat3
ion exchange is also used. Note that the model
of surface complexation is probably better suit3
ed to the natural process but at present in the
published literature the quantity of experi3
mental data is not sufficient to evaluate the
required constants.

The PHREEQC program [14] freely distrib3

uted by the USA Geological Service is used for
the simulation. This program makes it possi3
ble to calculate physico3chemical equilibrium
in multiphase and multicomponent systems
taking into account complexation in solution,
precipitation (dissolution) of minerals and ion
exchange. The user can introduce several types
of ion exchangers with different values of ex3
change constants. Particular attention should
be paid to the form of expression of constants
for ion exchange equilibrium in this program:
interactions are written in the form of half3
reactions:

nX

–

+ Me

n+

= X

n

Me

and the law of mass action is corresponding3

ly expressed as

+

−

⋅

=

]

Me

[

]

X

[

]

Me

X

[

K

n

n

n

where [X] is the fictious activity of positions

in the exchange complex (value analogous to
electron activity in recording redox reactions
in the form of half3reactions), [Me

n+

] is the cat3

ion activity in solution, and [X

n

Me] is the ac3

tivity of the cation bound in the exchange com3
plex. This form of evaluating ion3exchange
processes is more convenient for mathemati3
cal simulation and makes it possible to avoid
restrictions appearing when usual constants
for exchange of one cation by another are ap3
plied. For example, it is necessary to enter a
certain non3zero concentration for the “basic”

cation through which the expressions of ex3
change constants are written for other ions.

Exchange constants were evaluated by us3

ing the experimental results published in lit3
erature. Andre and Pijarovski [15] have car3
ried out the most complete investigation of
exchange equilibrium in peat at different pH
values with the participation of K

+

, NH

4

+

, Ca

2+

,

and Mg

2+

[15]. Exchange constants calculated

on the basis of these data are given in Table 2.
It was possible to obtain satisfactory coinci3
dence of calculated and experimental data by
assuming the existence of three types of ex3
change parts corresponding to groups with dif3
ferent degree of acidity (real distribution of
exchange positions with respect to density of
bonding to hydrogen atoms is more complicat3
ed). Capacity ratio of strongly acidic, acidic
and weakly acidic groups is 7 : 12 : 11.

These ratios were used in estimating the ex3

change constants for heavy metals on the basis
of published experimental data [12,16–18, etc.].
Ion exchange constants on peat accepted in fur3
ther calculations are listed in Table 3.

The model is a series of 20 cells. At the zero

step, solution composition in cells corresponds
to that of water of swamp deposits, and the
composition of cations in the exchange com3
plex is calculated on the basis of the condition
of equilibrium with it. It should be pointed out
that the ratio of sorbed to dissolved forms for
different metals is 10

3

–10

4

. In other words,

under natural conditions heavy metals in
swamp deposits are mainly in the sorbed state.
These results are in good agreement with data
of geochemical peat tests. Further, it is as3

Table 2

Ion exchange constants for swamp deposits

calculated from experimental data [15]

Groups with different degrees of

acidity

Cation

Strongly

acidic

Acidic

Weakly

acidic

H

+

1.75

4.48

7.41

K

+

0.00

0.00

0.00

Ca

2+

1.0

2.27

1.7

Mg

2+

0.65

1.93

1.3

68

I.L. Kharkhordin and F.G. Atroshchenko

7.0

6.5

6.0

5.5

5

4.5

0

4

8

12

16

20

pH

Cell number

Fig. 1. Change in pH during waste filtration from the AW section in swamp deposits at the 10, 50, 300, and 10003th
calculated steps

10

50

300

1000

sumed that the solution corresponding to wa3
ter composition in the slime storage begins to
be discharged into the first cell. During one
calculated step, the solution passes to the next
cell. After each passage solution composition
was brought into equilibrium with that of the
exchange complex. In the simulating brine
migration from section AW, the possibility of
carbon dioxide dissolution with the formation
of hydrocarbonate ions absent in the initial so3
lution was also taken into consideration. Sat3
isfactory agreement between calculated and

measured concentrations of hydrocarbonate
ions in solution was obtained at lg [CO

2

] = – 1.5.

Simulation results and their discussion

Let us consider in greater detail the pecu3

liar features of transformation of the chemi3
cal composition of waste in swamp deposits.

Section AW.

In the initial stage of filtration

of waste water from slime storage, as a result
of interaction with swamp deposits their chem3
ical composition is transformed drastically:
the pH of solutions increases (Fig. 1) and the
concentrations of micro3 and macrocompo3
nents change. High buffer capacity of peat
with respect to acid and alkaline solutions over
a wide pH range is due to the presence of dif3
ferent groups in the exchange complex. Even
after repeated washing with an acid solution,
the pH of swamp deposits are 5.5–6. The
changes in concentrations of macrocompo3
nents are of a smoother character than for
rocks with an exchange complex of a single
type. In the initial stage calcium content
(Fig. 2) increases with respect to that in the pri3
mary solution, whereas the content of sodium

Table 3

Ion exchange constants for heavy metals

in swamp deposits

Groups with different degrees of acidity

Cation

Strongly

acidic

Acidic

Weakly

acidic

Mn

2+

0.8

2.0

1.6

Cd

2+

0.8

2.0

1.6

Zn

2+

1.0

2.4

1.8

Ni

2+

1.2

2.9

2.2

Pd

2+

1.3

3.2

2.4

Cu

2+

1.4

3.5

2.8

10

50

300

1000

Cell number

Ca,

m

g

⋅

l

–1

600

500

400

300

200

100

0

4

8

12

16

20

and magnesium decreases, later gradual sta3
bilization takes place. The transformation of
the macrocomponent composition is virtually
completed at the thousandth calculated step
(fifty3fold change of solution over the whole
simulation range). Effective purification of

waste water from heavy metals takes place.
Fig. 3 shows changes in nickel concentration
the initial content of which in the AW section
is about 17 mg

⋅

l

–1

. After interaction with

swamp deposits it decreases by more than four
order of magnitude. Similar changes in concen3

Cell number

Fig. 3. Change in nickel concentration during waste filtration from the AW section in swamp deposits at the 10, 50,
300, and 10003th calculated steps.

4

8

12

16

20

0

20

16

12

8

4

Ni,

m

g

⋅

l

–1

10

50

300

1000

Fig. 2. Changes in calcium concentration during waste filtration from the AW section in swamp deposits at the 10, 50,
300, and 10003th calculated steps.

70

I.L. Kharkhordin and F.G. Atroshchenko

Fig. 4. Change in zinc concentration during waste filtration from the CWP section in swamp deposits at the 10, 50,
300, and 10003th calculated steps.

Zn,

m

g

⋅

l

–1

4

8

12

16

20

Cell number

0

0.02

0.016

0.012

0.008

0.004

0.009

Cell number

Fig. 5. Change in plumbum concentration during waste filtration from the CWP section in swamp deposits at the 10,
50, 300, and 10003th calculated steps.

tration are characteristic of zinc, copper, and
lead: their contents decrease from 7.5, 0.7, and
0.11 mg

⋅

l

–1

, respectively, to a few hundredths

and thousandths of milligram per litre. Man3
ganese content slightly increases at first, then
decreases and is stabilized at the level of the
initial solution.

Section CWP

. Waste water in this section

is also drastically transformed interacting

with swamp deposits: pH decreases to neutral
value and heavy metals concentrations change.
It should be pointed out that the solution is
purified from some metals and is contaminat3
ed with others. For instance, in the initial stage
zone concentration increases almost 203fold
with respect to that in the initial solution
(Fig. 4). It decreases only after the filtered so3
lution has been changed many times as zinc is

10

50

300

1000

Zn,

m

g

⋅

l

–1

0.008

0.007

0.006

0.005

0.004

0.003

0.002

0.001

4

8

12

16

20

10

50

300

1000

71

Simulation of Heavy Metals Migration in Peat Deposits

discharged from the peat exchange complex.
Manganese behaves in a similar way. Cadmi3
um content slightly increases in the initial
stage, then it decreases and stabilizes at its
concentration in the slime storage. Lead behav3
iour deserves particular attention (Fig. 5). In
the initial stage near the slime storage, high
lead concentration zone is formed in solution.
Subsequently it moves slowly downstream and
its maximum concentration increases. This
increase is caused by changes in migration
forms: near the storage the pH values of water
are higher and lead is bound into carbonate
complex compounds to a considerable extent
but with increasing distance from the storage
pH decreases and lead fraction in the ionic form
increases. Hence, ion exchange processes take
place more actively and cause lead bonding in
the exchange complex. Consequently, a mobile
geochemical barrier is formed in which the
process of concentration of lead in solution in3
creases.

On the whole the simulation results are in

good agreement with data on testing peat and
water of swamp deposits in the ATPS region.

Conclusions

The results of computer simulation proved

that heavy metals in swamp deposits are main3
ly in the solved state: the ratio of sorbed to dis3
solved forms is 10

3

–10

4

.

It was established that in low3lying swamps

at pH @ 7 peat exhibits considerable buffer
capacity for both acidic and alkaline solutions.

In the initial migration stage, the concen3

tration of heavy metals in the bulk of peat lay3
er in solution virtually does not depend on their
initial content in waste water and is determined
by the composition of the peat exchange com3
plex. At low concentrations of compounds of
heavy metals in waste water, interaction with
peat can lead both to ion sorption and to their
accumulation in solution. If the initial metal
concentration exceed the first milligrams per
litre, the processes of self3purification occur
actively.

This work is supported by Russian Founda3

tion for Basic Researches, project codes 973053
64419 and 96305364338.

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