The effects of clay a m e n d m e n t and composting on metal speciation in digested sludge liang qiao

~)

Pergamon
PlI: S0043-1354(96)00290-4

Wat. Res. Vol. 31, No. 5, pp. 951-964, 1997
© 1997ElsevierScienceLtd. All rights reserved
Printed in Great Britain
0043-1354/97$17.00+ 0.00

THE EFFECTS OF CLAY A M E N D M E N T AND
COMPOSTING ON METAL SPECIATION IN DIGESTED
SLUDGE
LIANG QIAO @ and GOEN HO *@
Institute for Environmental Science, Murdoch University, Murdoch 6150, Western Australia, Australia
(First received October 1995; accepted in revisedform September 1996)
Abstract--Sewage sludge usually contains significant heavy metals that may limit its land application.
Heavy metals in municipal solid waste have been shown to be less mobile by amendment with bauxite
refining residue (red mud) prior to the composting process. In the present research a sequential step
extraction was employed to investigate metal speciation (into exchangeable, bound to carbonate, to Mn
and Fe oxides, to organic matter and in residue phase) and the effect of red mud on metal speciation in
compost of sewage sludge for Cr, Cu, Ni, Pb and Zn. The effect of red mud addition and composting
process on metal distribution in sewage sludge compost is significant.Red mud addition generally reduces
metal leachability and therefore the potential hazard of releasing metals from sludge compost through
adsorption and complexation of the metals on to inorganic components to different extents for the
different metals. Red mud cannot desorb, however, metals bound to organic matter in the sludge. The
composting process breaks down organic matter in the sludge and may release the heavy metals. The
addition of red mud prior to the sludge composting binds the released metals on to the red mud for those
not strongly readsorbed by the remaining organics (Cu, Ni and Zn). © 199"7 Elsevier Science Ltd
Key words--bauxite refining residue (red mud), composting, heavy metals, metal mobility, metal
speciation, :~ewagesludge

INTRODUCTION

(MSW) compost when the red mud was added at the
beginning of the composting process (Hofstede,
1994).
The addition of a clay material, such as red mud,
to sewage sludge and composting the mixture is
expected to reduce the mobility of heavy metals along
similar lines to MSW compost (Hofstede and Ho,
1992; Hofstede, 1994). Differences exist between
sewage sludge and MSW because heavy metals in
sewage sludge have been in contact with the organic
matter in the sludge for a longer period of time. With
digested sludge, anaerobic digestion of the sludge
means bacterial processes may have transformed
readily mobile metals into more stable complexes. It
is desirable therefore to investigate the effect of red
mud addition on heavy metal mobility during sewage
sludge composting by determining the metal speciation during the composting process. The results of the
study are reported in this paper. The red mud
addition has been shown to improve the composting
process of digested sludge (Qiao and Ho, 1997).

In a previous experiment (Qiao et al., 1993) the effect
of bauxite refining residue (red mud) on the metal
speciation in sewage sludge, without composting the
red mud sludge mixture, was investigated. The effect
of red mud is delx,ndent on the metal distribution in
the sludge. In general, the exchangeable fraction of

metals can be effectively reduced and converted into
more stable forms, by precipitation and adsorption
into oxides of the red mud, either directly or
indirectly through complexation. The ratio of
exchangeable meta.l to total metal content in sludge
is therefore an important factor in assessing the role
of clay addition in controlling the mobility and plant
availability of metals.
The metals in sludge are generally in very stable
insoluble forms and most of the metals are bound to
the organic fraction that cannot be desorbed by red
mud or extracted by DTPA. The latter is usually used
as a measure of ple,nt available metals. The action of
microorganisms in composting may make the metals
more available due to metal release from the
decomposed organic matter in the sludge. Red mud
amendment significantly reduced the mobility and
plant availability of metals in municipal solid waste

M T RA S N M T O S
A E I LA D E H D

*Author to whom correspondence should be addressed
[Fax: (61) 9 310 z[997].

Samples of sludge compost
Sludge compost samples were taken from the sludge
composting experimental mixtures in which 0, 10 and 20%
red mud were added to the raw materials before the
composting process (Qiao and Ho, 1997). To obtain

successful composting and drying 0.5 kg of sugar was added

951


Liang Qiao and Goen Ho

952

as the carbon source, and also starting with drier mixtures.
Only results obtained with addition of 0.5 kg of sugar are
reported here, since the results obtained with drier mixtures
are largely similar (Qiao, 1997). A sample was taken from
each compost incubator every 10 days with the content of
the incubator thoroughly mixed before the sample was
taken. Because drying a sample changes metal speciation,
the metals in the moist sample were extracted immediately.

acid digestion (HNO~-HC104) to estimate leachable, plant
available, and total metal content, respectively (Hofstede,
1994). Red mud neutralised with gypsum was also analysed
to find out the metal speciation in the mud. Samples and
extractants were placed in closed centrifuge tubes shaken on
a Coulter mixer for 12 h, which was enough time to reach
solution equilibrium, and the residue was separated by
Sorvall RC-5B ultra centrifuge at 10,000 rpm for 20 min.
The supernatant was passed through a GF/C fibre glass
filter and stored in a cool room. The residue was subjected
to the next step extraction.
Six metals (Cd, Cr, Cu, Ni, Pb, Zn) were chosen for

analysis because they represent heavy metals of interest in
sewage sludge. The metals were analysed in duplicate on a
GBC atomic absorption spectrometer. All reported metal
figures in this paper are based on dry weight unless
otherwise specified.

Metal extraction

Around 1 g samples (based on dry matter) were emlSloyed
for the metal extraction. A sequential step extraction was
carried out employing 1 M MgC12 (exchangeable fraction);
1 M HOAc/NaOAc at pH 5 (carbonate fraction); 0.04 M
NH2OHHCI at 96°C (reducible or bound to oxides
fraction); 30% H202 at pH 2 and 85°C/3.2 M NH4OAc
(bound to organic fraction) extractions and acid digestion
by concentrated HNO3, HC104 and HC1 (residue fraction)
(Tessier et al., 1979). Metals bound to sulphides in this
extraction scheme would be included in the organic bound
fraction. Two batch extractions were also conducted
employing 0.01 M CaCl2 and 0.1 M DTPA followed by an

RESULTS AND DISCUSSION
In order to assess for each extraction the
significance o f the effects o f red m u d addition and

60

650

50

~

~--~ 55O

40
30
20

450
~ 350

10

"

!

•

10

I

30

"

250

!

50

I

0

Time (days)

"

I

"

10

I

30

"

!

50

Time (days)

16

70
60

12
~= 8

E

40

~" 4

~

30

0

!

0

!

,

10

30

•

20

!

o

50

T i m e (days)

•

1'o

3.0

!

so

T i m e (days)

300

Red M u d Addition
-

A
•

100

I

!

!

10

30

10%
20%

I

0

0%

50

Time (days)
Fig. 1. The total metal content in compost of digested sludge (each point represents an average of three
types of measurement; each half bar represents one standard deviation).

Metal speciation of digested sludge

953

Table I. The effect of factoring in the dilution by red mud addition on total metal content in sludge compost and the metal
content associated with the silicates
Metals
(rag kg t),[
At day 0 of composting
At 50th day of composting
Associated with silicates
RM %--,
1,9%
10%*
20%
20%*
10%
10%*
20%
20%*
0%
10%
20%
Total Cr
35
38
45
51

37
37
46
51
1.7
2.6
4.8
Total Cu
397
446
314
400
490
543
430
487
0.8
2.5
2.0
Total Ni
7.3
10.6
5.2
10
12
13
14
13
2.2
3.1

3.4
Total Pb
45
43
51
42
47
47
55
46
3
3.4
4.7
Total Zn
196
221
151
198
222
258
203
231
0.8
2
2.1
Note: RM % = percentage of red mud addition; * = calculated from metal in compost and in red mud; metals in RM can be
seen in Fig. 5.

sludge composting process, a multiple analysis of
variance was carried out using SPSS-X program on

a V A X computer. The results of statistical analysis
show red mud addition and sludge composting
process had statistically significant effect on all
measured metal concentrations at ct < 0.05.

the beginning and the end of the composting
experiment (50 days). The results o f the total metal
content averaged for the three measurements and their
standard deviations are shown in Fig. 1.
The total Cd concentration in the sludge compost
was below the detection limit (0.1 mg k g - ' ) . F r o m
Fig. 1, the total amount of metals had a slight but
significant increase as the composting process
progressed as a result of a reduction o f the dry matter
with composting amounting to 19, 18 and 16% for
the 0, 10 and 20% red mud addition, respectively
(Qiao and Ho, 1997).
Red mud addition diluted the metal concentration
except for Cr and Pb. The metal content of red mud
is shown in Fig. 5.
A comparison with calculated metal concentration
when the red mud dilution effect was factored in is
shown in Table 1. The calculated metal concentration
is derived from the metal concentrations of the
components. The average recovery rate of total metal
from sludge compost is 97% for Cr, 87% for Cu,
81% for Ni, 85% for Zn and 111% for Pb. The
deviation of the recovery rate from 100% may be due
to experimental error, but it appears that in general
heavy metals in sludge compost become less

extractable after amendment with red mud. In an
experiment to ascertain whether any could be bound
by the silicates, the residue after the last extraction
was dissolved in hydrofluoric acid (5% solution).
Some additional metals were dissolved and shown in
Table 1 confirming that indeed some metals were
associated with silicates. Some solids residue still
remained even after reaction with HF.

Total metal concentration
To ascertain the concentration of total metal in
compost as the reference for the metal distribution in
the compost, three different kinds of independent
measurement were carried out. They are the direct
measurement for moist samples, for dried and ground
samples, and the sum of the metal in sequential
extraction fractions for moist samples. It was
anticipated that the sum of the metal fractions
in sequential extraction would have the largest
analytical error among the three measurements due
to its multiple e~tractions, analyses and the more
heterogeneous (difficult to mix) nature of the moist
samples. The dried and ground samples were more
homogeneous and should give more reliable total
metal concentrations.
The differences in the total metal values were,
however, small among the three measurements
compared to the total metal content. The dried and
ground samples gave about 96% of the average value
of the three measurements, the sum of the five fractions

in the sequential extraction 104% and moist samples
107%. It should bt; noted that the total metal content
data obtained from the sum of the metal fractions were
available for all samples, whereas for dried and ground
samples and for moist samples they were determined at

15

50

S

40
10

~,~
E

~,~

30
20

Red mud addition
-•
o

10"
T

0

10

20

Time

30

40

(days)

50

0

!

•

|

10

20

30

40

Time

w

50

(days)

Fig. 2. The leachable copper and zinc in sludge compost.

0%
10%
20%


Liang Qiao and Goen Ho

954
5O
~

|

4o
~
D

•

[]

Zn R"2=0.83

•

Cu R^2=0.55

3o

~

20

-~

~0
~,~

m,

0
4.5

6.5

5.5

7.5

pH

Fig. 3. The relationship of leachable metal and pH in sludge
compost.

Leachability of heavy metals (extraction with 0.01 M
CaCI2)
The leachable Cr, Pb, and Ni in sludge compost
were below detection limits even without red mud
amendment, even though the total amount of metals
in the sludge compost is significant (Fig. 1). The
decrease of leachable Cu and Zn by red mud addition
was significant, particularly the leachable Zn (Fig. 2).
This result agrees with the finding for red mud MSW
compost (Hofstede, 1994).
The leachable Cu increased during the composting
process following the thermophilic stage and was
likely due to the release of the organically bound Cu.
Red mud addition slowed down the increase of
leachable Cu with the composting process. About
80% of Cu in sludge compost was organically bound

(see below), so it is strongly affected by organic
matter degradation. This is consistent with the
finding of Wong (1995) and Hofstede (1994).
Although the leachable Cu increased with the
composting process, the magnitude of leachable Cu
in sludge compost without red mud amendment after
50 days of composting was less than 2.3% of its total

content indicating the low leachable metal content in
sludge compost. Garcia et al. (1990) extracted less
metals by CaCl2 during the composting of aerobic
digested sludge, but the same conclusion was drawn
by them that the quantities of metals extracted by
CaCl2, expressed as a percentage of total metal
content, were small.
pH of the sludge compost also affected the
solubility of the metal hydroxides and carbonates,
and the lower pH values increased the soluble heavy
metals in the sludge compost (Fig. 3). Since the initial
pH of the sludge mixture with sugar addition was
under 5.2 (Qiao and Ho, 1997), Cu and Zn
hydroxides could not form. Cu and Zn hydroxides
theoretically form in pure solution at pH above 5.6
and 7.1, respectively. The addition of red mud and
the composting process increased pH to over 7 in the
red mud sludge compost with sugar addition, so the
soluble Zn was precipitated as Zn hydroxide. The
increased pH also enhanced the precipitation of metal
carbonates, thus reducing the exchangeable metal
concentration, which can be seen from the Zn
speciation described below.

300

8

~

200

,!

too

•

I

"

i

10 20

"

I

•

|

30 40

•

a

50

0

10

Time (days)

20

30

40

50

Time (days)

12"

, Red mud addition
_.=

so

=

a
20
0

10

20

30

T i m e (days)

40

50

0

10

20

30

40

50

T i m e (days)

Fig. 4. The plant available heavy metals in sludge compost.

0%

10%
20%


Metal speciation of digested sludge
100

#168

33

20

40

955

20
# = Total metals

(rag&g)
80
%

[]
[]
[]
[]
[]

60
40
20

Oa

Cr

Ni

Po

Exchangeable
Carbonates
Iron oxides bound
Organic fraction
Residue

No detectable Cd in red mud.

Zn

Metals

Fig. 5. The speciation of metals in red mud.

Plant availability of heavy metals (extraction with
DTPA)
The plant available Cr in sludge compost was

below detection limit. The reduction of plant
available Cu, Ni, Pb, and Zn in the sludge compost
by red mud addition was significant, especially for Zn
(Fig. 4).
The composting process increased the plant
available Cu and Ni, but the plant available Pb was
dramatically decreased. The finding is similar to that
of Garcia et al. ,(1990) who extracted more plant
available metals by DTPA after composting of
aerobic digested sludge except for Pb even though the
total metal concentration increased due to the
organic decomposition and therefore reduction of
sludge mass. The variation of DTPA extracted metals
seems to be related to the changes in metal speciation
and will be discussed below.

Metal speciation
To determine the effect of red mud on the
speciation of metals in sludge, the metal speciation

10o

"18

29

499 535

17 13

in red mud needs to be known first (Fig. 5). More
than 60% of the metals are in residue form except Zn
that was distributed more evenly into the five
fractions. This implies that the metals contained in
red mud were mainly in very stable forms even
though the Cr content in the red mud is as high as
168 mg kg-'. This fact is not surprising since red mud
has undergone processing (size reduction, Bayer
process caustic digestion, and countercurrent washing). It has also very little organic matter associated
with it.
After mixing with sawdust and recycled compost,
the speciation of heavy metals in the sludge was
shifted to more available forms (Fig. 6). This may be
caused by changes due to storage of the sludge and
to moisture change. The pH of sludge after storage
dropped suggesting that some anaerobic decomposition took place. A change in redox condition and
solid/solution ratio therefore occurred.
Red mud has a high pH, cation exchange capacity,
AI and Fe oxides and clay minerals and can
effectively adsorb free cations from solution (Hofstede, 1994). The speciation of metals in the mixture

48

47

210 242
* - Total metal (mg/kg)

8o

[]

Exchangeable

40

[]
[]

Carbonates
Oxides bound

20

[]
Ill

Organic fraction
Residue

e,O

o

0

r
"-O

.O

t-

Z

EL

N

Metals

#---value calculated
from the sludge and
the recycled compost

Fig. 6. Comparison of the metal speciation calculated from the metal in the mixture's components and
the speciation measured in the initial compost mixture.


Liang Qiao and Goen Ho

956
*53 23

29 27 29 29

33 33 31 34

38 37 37 38

100 " - ~

* - Total Cr (mg/kg)

ae 60

8

[]
[]
[]
[]
[]

40
20
0

.

.

m

.

#

~

.

o

.

.

o

o

.

o

=E

.

o

.

o

.

o

.

o

o

=E

.

o

Exchangeable
Carbonates
Oxidesbound
Organic fraction
Residue

.

o

o

o

o

# recycled compost

Time (days) and red mud addition (%)

P

Fig. 7. The variation of Cr speciation in sludge composting with the red mud addition (*total metal figure
is sum of metal fractions in the speciation study).

would therefore be affected by the addition of red
mud. Since the heavy metals have different properties
and different concentrations in the red mud and
compost mixtures, the speciation of metals and the
effect of red mud on the speciation are quite different
for each metal.
Cr. Cr 3÷ has an electron configuration closest to a
noble gas with a high spherical symmetry and its
polarisability is the lowest among the six tested
metals. It has a valency of three and therefore it has
a stronger electrostatic affinity for the sorption sites
than divalent cations. Consequently it forms the most
stable complexes among the six metals and dominates
in the residue and organic bound fractions (Fig. 7).
The exchangeable Cr in the initial mixture was
about 10% of total Cr and the carbonate fraction was
also about 10% total Cr, which means about 20% of
total Cr in the mixture may become leachable or
available with a changed environmental condition
such as a reduction in pH. Because of the high
competitive nature of Cr for adsorption sites
(Table 2) any released Cr will, however, displace

100

*29

21

13

29

other adsorbed metals. Therefore there was no
leachable and plant available Cr detected in the
sludge compost even though it contained 29 mg kg -~
total Cr.
The composting process affected the speciation of
Cr in the sludge compost though the changes were
relatively small. The carbonates and oxides bound Cr
were converted into the organic bound fraction
during the composting process perhaps as a result of
the competition of Cr with other metal cations for
limited humic organic ligands which were produced
during composting (Fig. 7). The effect of the
composting process on the Cr speciation was similar
to MSW composting (Qiao, 1997). This conversion
would make Cr more stably fixed in mature sludge
compost.
Factoring out the red mud dilution effect the total
Cr was reduced from 29 to 13 mg kg -~ for the 20%
red mud addition (Fig. 8). This seems to indicate that
the more red mud was added, the less Cr was

recovered due most probably to the irreversible
adsorption of Cr on to red mud. This irreversibility

21

12
* - Total Cr (mg/kg)

80
L.

G)

[ ] Exchangeable
I~1 Carbonate
[ ] Oxidesbound
Organic fraction
[ ] Residue

60
4O
2~ .

.
o%

.

.

lO%
Initial

20*/0

0%

I0% 20%
after 50 days

Red Mud Addition
Fig. 8. Effect of red mud on the Cr speciation in sludge compost after factoring out the red mud dilution

effect.


Metal speciation of digested sludge

957

"1.954 635 535 528 515 515 409438 413 476 352 363 346 406
100 -

* - Total Cu (mg/kg)

[]
[]
[]
[]
[]

O

ii
.

.

.

.

.

.

.

.

-

.

.

.

.

.

.

.

.

.

.

Exchangeable
Carbonates
Oxides bound
Organic fraction
Residue

.

_

Time (days) end red mud addition (%)

Fig. 9. The variation of Cu speciation in sludge composting with the red mud addition.

more than countered what appeared to be the
conversion of the reversible part into more available
forms with the addition of red mud (Fig. 8).
Cu. Cu dominated in the organic bound fraction in

both sludge and slludge compost ( > 80%) due to Cu
forming very stable complexes with organic ligands
(Fig. 9). Furthermore Cu ion is directly bound to two
or more organic functional groups mainly carboxylic,
carbonyl and phenolic so that the ion is immobilised
in a rigid inner-sphere complex (McBride, 1989).
After mixing with the sawdust and recycled sludge
compost, part of Lhe Cu was converted from organic
bound into carbonates (Fig. 9), though only to a
minor extent.
The result of the Cu speciation is in agreement with
the literature (Carapanella et al., 1987; Duquet and
V~dy, 1991; also Table 2). Campanella et al. (1987)
gave the following order of stability of humic
complexes: Cu >> Zn > Mn in the research of the
metal speciation in urban sludge. Duquet and V~dy
(1991) concluded that in the case of Cu, the organic
matter is the first ,;olicited phase; then comes the Fe
oxides fraction and finally Mn oxides in the sludge

100

*535

451

434

515

compost; the clay plasma and the sandstone
oxyhydroxides sorb little Cu.
The composting process reduced the organic
bound Cu and transferred it into carbonate, oxides
and exchangeable fractions though the amount of Cu
transformed was small compared to the total Cu in
the mixture. The change may be caused by the
decomposition of organic matter in the sludge
compost releasing bound Cu. Dudley et al. (1987)
concluded that Cu was initially associated to the
greatest degree with the small molecular-size fraction
of high amide content in sludge amended soils. As the
pH increased and soluble amide content moieties
decreased, Cu remained in solution. Since the pH rose
slightly during the composting process (Qiao and Ho,
1997), this released Cu was precipitated as Cu
carbonate, oxides bound and complexed with soluble
fulvic acids increasing leachable and plant available
Cu with the composting process. Baham and Sposito
(1994) also found the amount of Cu adsorbed
decreased with an increase in the amount of dissolved
organic carbon in solution due to Cu forming stable
complexes in solution which had a tendency to

504

508
* - Total Cu (mg/kg)

80

60
40

[]
[]

20

CJ

I-I Exchangeable

Ill
[]

Carbonates
Oxides bound
Organiofraction
Residue

o
0%

10%
Initial

20%

0%

10% 20%
after 50 days

Red M u d Addition

Fig. 10. Effec, of red mud on the Cu speciation in sludge compost after factoring out the red mud dilution
t
effect.


Liang Qiao and Goen Ho

958

Table 2. The sequence of complex stability for the transition metal ions
Sequence of
Binding materials
complex stability
Reference
Organic matter

Cu > Ni = Zn > Cd

Organic substance

Pb > Cu > Ni > Zn

Soil organic matter

Cu > Pb > Ni > Zn

Soil organic matter

Pb > Cu ,> Cd > Zn

Red mud

Cr>Pb>Cu>Cd>
Ni > Zn
Pb > Cu > Zn > Cd
Pb > Ni > Zn

Kaolinite and illite
Clay material

Irving-Williams order in
McBride (1989)
Scheffer and
Schachtschabel (cited in
Joregensen and Jensen,
1984)
Schnitzer and Skinner at
pH 5 (1966, 1967)
Elliott et al. (1986 cited in
Schmitt and Sticher, 1991)
Hofstede (1994)

Saeki et el. (1993)
Mitchell (cited in
Jorgensen and Jensen,

1984)
Amorphous AI hydroxide Cu > Pb > Zn > Ni > Cd Kinniburgh et al. (1976
cited in McBride, 1989)
Silanol groups of silica
Pb > Cu > Zn > Ni = Cd Schindler et al. (1976 cited
in McBride, 1989)

remain in solution. The dissolved organic carbon was
significantly increased with the composting of sewage
sludge (Qiao, 1997). The C u complexed with soluble
fuivic or h u m i c acids would be available to leaching
a n d to presumably also plant.
F a c t o r i n g out the red m u d dilution effect the
change o f C u speciation by red m u d addition was not
significant, but the effect of composting process was
significant (Fig. 10). W i t h composting the organic
b o u n d C u was converted into carbonates, oxides
b o u n d a n d exchangeable fractions. The red m u d
addition inhibited the increase of exchangeable Cu in
red m u d sludge compost t h r o u g h increasing p H to
precipitate C u a n d increasing the inorganic oxides
surface to a d s o r b Cu, which also reduced the metal
mobility in sewage sludge when red m u d was added
to it (Qiao a n d Ho, 1996).
Ni. The Ni cation has the stability of complexes
with organic ligands just less t h a n Cu 2÷ in the

100

*27 4

13 16 17 20

transition metal cations according to the IrvineWilliams order (Table 2). Therefore Ni dominated in
the organic bound and residue fraction in the sludge
and the compost mixture (Fig. 11). There was a shift
in metal speciation to become more available when
the sludge was mixed with the sawdust a n d recycled
sludge compost, which m a y be caused by the decline
o f p H from 8.3 to 5.2 after the sludge storage.
The composting process significantly changed the
Ni speciation in the sludge compost. Because the total
Ni increase was more t h a n the reduction o f dry
m a t t e r during the composting, the percentage o f
residue Ni appeared to decrease even t h o u g h the
c o n c e n t r a t i o n o f residue Ni remained the same to the
50th day o f composting.
Factoring out the dilution effect, the red m u d
addition significantly affected the Ni speciation in
sludge compost (Fig. 12), particularly the conversion
o f exchangeable Ni into organic b o u n d with addition

10 13 14 16

9

13 15 16
* - Total Ni (mg/kg)

80

#

60

i

40

[]
[]
[]
ml
[]

20
0

x = o o o o =

Exchangeable
Carbonates
Oxidesbound
Organicfraction
Residue

oooo=oogo
~-

Time (days) end red mud addition (%)

Fig. 11. The variation of Ni speciation in sludge composting with the red mud addition.


Metal speciation of digested sludge
100

10

"13

9.8

20

17

959

18

* - Total Ni (mg/kg)

80

IIIIII

[ ] Exchangeable
~1 Carbonates
[ ] Oxides bound

60
40
20

ITS Organic fraction
[ ] Residue

:::::::::::::::::::::

0
0%

10%
Initial

20%

0%

10%
20%
after 50 days

T i m e (days) and Red M u d addition (%)

Fig. 12. Effect of red mud on the Ni speciation in sludge compost after factoring out the red mud dilution
effect.
of red mud in the initial sludge mixture and into
oxides bound Ni with composting. The composting
process humified the organic matter in the sludge

releasing Ni from the bound sites on the organic
matter. The released Ni was likely to form complexes
with the oxides surfaces in the red mud preventing the
Ni to complex with the humic substance in the
compost.
Pb. Lead is also a metal cation, but it has a stronger
affinity to the adsorption sites on the clay materials
such as silanol groups of silica and amorphous Al
hydroxide (Table 2). Therefore Pb dominated in the
residue and organic fractions in the sludge, but it
was more evenly distributed in the carbonates,
organic and oxide:~ bound fractions in the compost
mixture.
The composting process significantly stabilised the
Pb in the mixture. The 50 days composting process
converted all the Pb in the exchangeable and
carbonates forms into organic bound fraction.
Baham and Spo:sito (1994) suggested that Pb
facilitated the removal of dissolved organic carbon in
sewage sludge through the adsorption of positively

47 45 49 48

.186 76

charged Pb-organic complexes, cation bridge, or
hydrophobic interactions with the clay surface as a
result of a lowering of the negative charge on the
dissolved organic carbon in sewage sludge through
complex formation. Simeoni et al. (1984) also found

that the composting of sludge decreased the Pb
mobility and plant availability.
The changes of Pb speciation in red mud sludge
compost was still significant after factoring out the
red mud dilution effect (Fig. 14). Red mud converted
the exchangeable, carbonates and oxides bound Pb
into the residue and organic bound fractions in the
sludge compost. The composting process had the
same effect on the speciation of Pb. Therefore the
mobility and plant availability of Pb were significantly reduced, because the leachability and plant
availability of metals can be expressed as the
exchangeable, carbonates and oxides bound metal
species (see below).
Zn. Zn in the sludge and sludge compost was
evenly distributed in the carbonates, oxides, organic
and residue fractions as a result of the lowest value
of the standard electrode potential of Zn 2+ among the

45 35 41 42

44 40 44 43

•

el
a.

[]

.

.

.

.

.

,

•

•

•

°

•

•

•

•

Carbonates
Oxides bound

m
[]

20

Exchangeable

[]
[]

°JW
0

- Total Pb (mg/kg)

Organic fraction
Residue

•

~
oJ
Time (days) and red mud addition (%)

Fig. 1:3.The variation of Pb speciation in sludge composting with the red mud addition.


960

Liang Qiao and Goen Ho

*47

47

50

45

45

47

100

* Total Pb (mg/kg)
-

80
60
4O

20
0 •
0%

10%

20%

ii

0%

[]
[]
[]
[]
[]
10%

Exchangeable
Carbonates
Oxides bound
Organic fraction
Residue

20%

after 50 days

Initial

Red Mud Addition
Fig. 14. Effect of red mud on the Pb speciation in sludge compost after factoring out the red mud dilution
effect.
six tested metals. Based on the redox potential for the
redox reaction with other metal ions, the Zn 2+ can be
expected to stay in ionic form in solution. Since the
total concentration of Zn was high in sludge and most
soluble Zn was in free ionic form (Behel et al., 1983),
Zn was evenly distributed in carbonate, organic and

oxides bound fractions in the sludge as result of
adsorption equilibrium relationships.
This result agrees with the finding of Duquet and
V6dy (1991) who studied a sludge compost and soil
system and with other findings reported in the
literature (Table 3). Duquet and V6dy (1991) found
for the sludge compost that the affinity of Zn for the
Fe oxide fraction was high (76%) and increased
(83%) in the mature compost. It should be noted that
this Fe oxide fraction includes the exchangeable and
carbonate fractions in our research. Like Cu, Zn was
not greatly fixed on sandstone oxyhydroxydes and
clay (Duquet and V6dy, 1991), and similarly Zn in the
residual fraction in this research was less than 5%
(Fig. 15).
The composting process converted the organic and
oxides bound Zn into exchangeable, and red mud
addition inhibited this conversion. Since the organic
"901 229

100

matter in the mixture was oxidised during the
composting process, Zn was converted from organically bound into exchangeable and tended to be more
mobile as a result of the increase of the redox
potential and decrease of the sulphides (Saeki, 1993).
The change in redox potential more strongly affected
the speciation of Zn than that of Cu and Pb. The
addition of red mud buffered the changes of Zn
speciation during the composting, and the buffer was

more effective for the more mature compost than the
raw mixture,
Factoring out the dilution effect, red mud addition
converted the exchangeable Zn into carbonate
fraction due to the increase of pH, and suppressed the
changes of Zn speciation during the composting
process. The exchangeable Zn during the composting
was precipitated as Zn hydroxide and carbonate by
red mud addition through raising pH from 5 to 7.2.
Zn hydroxide dissolves under pH7.2, so the
exchangeable Zn was effectively controlled by pH in
the sludge compost.
Table 3 gives the predominant metal species in
sludges by chemical sequential extraction, and shows
that there are differences between the reported

242 254 221 277 212 200 204 211 201 234 217 259
* - Total Zn (mg/kg)

80
Ze

[]
[]
IN
II1
I~

60

I-

N

40
20

Exchangeable
Carbonates
Oxides bound
Organic fraction
Residue

0

Times (days) end red mud addition (%)

Fig. 15. The variation of Zn speciation in sludge composting with the red mud addition.


Metal speciation of digested sludge
1 O0 "

233

*242
~

~

260
~

277
~

233

961

319

~

* - Total Zn (mg/kg)

80"
[]

40

Exchangeable

[]

60-

Âg

Carbonates

Oxides bound

DI

IT[] Organic fraction

2O

[]

0

ã .....

0%

ã

10%
Initial

Residue

~-11.

20%

0%
*

10% *
20% *
after 50 days

Time (days) and Red Mud Addition (%)
Fig. 16. Effect of red mud on the Zn speciation in sludge compost after factoring out the red mud dilution
effect.
results. The differences of the metal speciation are
mainly caused by the different chemical reagents and
extraction scheme~,; used. Although these differences
make it difficult to compare results, it can be clearly
seen from Table 3 that Cu dominated in the
organically bound and sulfide (oxidisable) fraction;
Pb was mainly present in the organically bound and
residue fractions; and Zn was distributed in the
carbonates, oxides and organic bound fractions in the
sludge, and the results obtained in the present
research are in general agreement with these.
Since sludge contains a significant amount of
organic matter and Cu has a high affinity to the
organic matter, the extracted Cu in the exchangeable
carbonates or oxid,~s fraction is likely readsorbed on
the organic fraction during the procedure of

extraction (Saeki et al., 1993). Similarly Pb has a high
affinity to the residue fraction. Therefore it may be
assumed that the results for Cu, Pb and Zn speciation
in the sludge mixture was governed by readsorption
during extractions.
During the sludge composting process the organic

matter in the sludge is humified causing variation in
metal speciation in the sludge. Duquet and V6dy
(1991) reported that nearly 58% of Cu and Zn were
located in the coarse fraction (=50-2000/~m) in
sewage sludge, and the metals shifted into the fine
fraction ( < 50 #m) as a result of humification of the
coarse fraction and became more leachable with the
composting. Canarutto et al. (1991) also reported
that a correctly carried out composting process
increased the humic acids with respect to the fulvic

15

"

lO

==

0% RM (R^2=0.82)
10% RM (R^2=0.18)

ã

20% RM (R^2=0.01)

O

o
0

10

20

Exchangeable

Â

5O

B

40
U

~:=

=

2o

0

20

40

60

80

100

10%RM(FIA2=0.29)

o

m

0%RM (PP2=0.a4)

ã

30

20%RM (RA2=0.84)

120

Exchangeable
Fig. 17. Comparison of leachable and exchangeable fractions.


962

Liang Qiao and Goen Ho
3oo

200

r~

O%RM (R~-0.92)
10%RM (RA=-0.83)
20%RM (R~-0.70)

1°°
o
0

20

40

60

80

100

Sum of Exchangeable and Carbonate

0% RM (R^2=0.85)
10% RM (RA2=0.74)

.=

20% RM (RA2=0.61)

0

1

2

3

Sum of Exchangeable and Carbonates
12=.
[]

0% RM (R^2=0.98)
10% RM (R^2=0,99)

O

20% RM (R^2=0,98)

.=
_e~

4'

=h
0

•

0

•

•

,

•

5

•

•

•

•

10

•

•

•

15

Sum of Exchangeable and Carbonate

120

°

!
,o

0%RM (R^2=0.91)
10%RM (FIA2=0.66)
20%RM (RA2=0.61)

o
0

100

200

300

Sum of Exchangeable and Carbonates
Fig. 18. Comparison of metal extracted by DTPA and the sum of exchangeable + carbonate fractions.

acids in municipal solid waste. These variations in the
quantity and quality of humic substances also
influenced the speciation of heavy metals.

Relationship of metal speciation and leachable metal
Since a 1 M MgC12 solution is a stronger extractant
than a 0.01 M CaC12 solution for metals from cation

sites, more metals can be extracted by the former, so
the leachable metals is included in the exchangeable
metal fraction (Fig. 17). Differences exist between

metals, however, with Cr, Ni and Pb hardly extracted
by 0.01 M CaCI2, and Cu was more easily extracted
than Zn from compost unamended by red mud
(Fig. 17).

Relationship of metal speciation and plant available
metal
A comparison between the speciation of metals in
the sludge compost and the metals extracted by
DTPA is shown in Fig. ! 8. It can be seen that the


Metal speciation of digested sludge

963

Table3. Predominantmetalspeciesidentifiedin sludgesby meansof sequentialchemical
extraction
Predominantmetal species
Present
Stover Emmerich Legret
Oake
Lake
Metal
data
et al.*

et al.*
et al.*
et al.*
et al.*
Cd
-CO~
CO3
Oxid
COs
Org
Cu
Org
S
Org
Org
S
COs
Ni
Org/Res
CO3
CO3
Org
COs
Sol/Res
Pb
Org/Res
COs
-Res
Org
Org

Zn
Org/Oxid/CO3 Org
CO3
Oxid
Org
Org
Note: COt--carbonate;Org--organicallybound;Oxid--oxidesphases; Res--residual;
S--sulphide; Sol--soluble/exchangeable;
*---citedin Lester (1987).

DTPA could generally extract the heavy metals in the
exchangeable and carbonate bound fractions in the
sludge, except for Cr which was below the detectable
limit in the DTPA extraction. In the case of Ni and
Cu the agreement would be marginally better when
the oxides bound fraction is included. Petruzzelli
(1989) stated that the metals present in these fractions
are considered to be the most available forms to
plant.

CONCLUSIONS
(i) Red mud affects the speciation of heavy metals
through increasing the pH, solid-to-solution ratio,
and available adsorption sites. In general sludge
composting with red mud addition reduces the
leachability, plant availability and total metal
content. The effect of red mud is different for each
metal with a greater effect on Cr, Pb and Zn
speciation than on Cu and Ni speciation. More than
80% of Cu is tightly bound to the organic fraction,

and red mud addition hardly affects the Cu
speciation.
(ii) Although the red mud increases total Cr in
sludge compost, the leachable and plant available Cr
are undetected in the red mud compost. The Cr
remains in tightly bound fractions, and thus is
unlikely to be released into the environment.
(iii) Leachable metal as measured by 0.01 M CaC12
extraction and plant available metal as measured by
0.1 M DTPA extraction are related to the metal in
exchangeable and in exchangeable/carbonates fractions, respectively, except for Cr which cannot be
extracted by DTPA and CaCI2.
(iv) Composting process affects the metal speciation, with Pb in particular gradually becoming
insoluble in sludge compost even without red mud
addition. Cu, Ni anti Zn become more available with
composting, but the extent of availability is reduced
by red mud addition.
The results obtained in this paper suggest that the
addition of red mud to digested sludge prior to
composting should reduce metal mobility. In the
longer term the red mud should be able to retain
heavy metals released by the decomposition of
organic materials in the compost. This situation is
similar to compost applied to clayey soils; hence clays

with similar properties to red mud could be used as
amendment for composting digested sludge.
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