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

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

© 1997 Elsevier Science Ltd 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 A N D

C O M P O S T I N G ON METAL SPECIATION IN DIGESTED

S L U D G E

LIANG QIAO @ and GOEN HO *@

Institute for Environmental Science, Murdoch University, Murdoch 6150, Western Australia, Australia

(First received October 1995; accepted in revised form 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, :~ewage sludge

INTRODUCTION

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

*Author to whom correspondence should be addressed

[Fax: (61) 9 310 z[997]

(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 specia- tion 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)

MATERIALS AND METHODS

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

952 Liang Qiao and Goen Ho

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

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

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

RESULTS AND DISCUSSION

In o r d e r to assess for each extraction the significance o f the effects o f red m u d a d d i t i o n a n d

30

10 " ! • I " ! 250 I " I " I " !

60

12

0 1 0 30 50

T i m e ( d a y s )

o 1'o 3.0 so

T i m e (days)

300

Time (days)

Red M u d Addition

- 0 %

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 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

953

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%

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 o f

variance was carried out using SPSS-X program on

a V A X computer The results o f statistical analysis

show red m u d addition and sludge composting

process had statistically significant effect on all

measured metal concentrations at ct < 0.05

Total metal concentration

To ascertain the concentration o f total metal in

compost as the reference for the metal distribution in

the compost, three different kinds o f independent

measurement were carried out They are the direct

measurement for moist samples, for dried and ground

samples, and the sum o f the metal in sequential

extraction fractions for moist samples It was

anticipated that the sum o f the metal fractions

in sequential extraction would have the largest

analytical error a m o n g the three measurements due

to its multiple e~tractions, analyses and the more

heterogeneous (difficult to mix) nature o f the moist

samples The dried and ground samples were more

h o m o g e n e o u s and should give more reliable total

metal concentrations

The differences in the total metal values were,

however, small a m o n g the three measurements

compared to the total metal content The dried and

ground samples gave about 96% o f the average value

o f 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 o f the metal fractions were

available for all samples, whereas for dried and ground

samples and for moist samples they were determined at

the beginning and the end o f 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 c o m p o s t was below the detection limit (0.1 mg k g - ' ) F r o m Fig 1, the total a m o u n t o f metals had a slight but significant increase as the composting process progressed as a result o f 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 o f red m u d

is shown in Fig 5

A comparison with calculated metal concentration when the red m u d dilution effect was factored in is shown in Table 1 The calculated metal concentration

is derived from the metal concentrations o f the components The average recovery rate o f 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 b o u n d

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 H F

40

E 20

0 %

T ! • | w

10 20 30 40 50

T i m e ( d a y s ) T i m e ( d a y s )

Fig 2 The leachable copper and zinc in sludge compost

954

5O

~ 3o

• D

-~ ~0

0

Liang Qiao and Goen Ho

|

[] Zn R"2=0.83

• Cu R^2=0.55

m, ~ , ~

pH

Fig 3 The relationship of leachable metal and pH in sludge

compost

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

too

8

,!

• I " i " I • | • a

10 20 30 40 50 0 10 20 30 40 50

12"

10%

20

0 10 2 0 3 0 4 0 5 0 0 10 2 0 3 0 4 0 5 0

, Red mud addition

T i m e ( d a y s ) T i m e ( d a y s )

Fig 4 The plant available heavy metals in sludge compost

%

#168

100

80

60

40

20

Cr

33

Metal speciation of digested sludge

# = Total metals

(rag&g)

[ ] Exchangeable [ ] Carbonates [ ] Iron oxides bound [ ] Organic fraction [ ] Residue

Oa Ni Po Zn No detectable Cd in red mud

Metals

Fig 5 The speciation of metals in red mud

955

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

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 wash- ing) 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 decompo- sition 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 (Hofst- ede, 1994) The speciation of metals in the mixture

29 499 535

20

48 47

o 40

0 r

" - - O t -

10o

8o

e,O

Metals

* - Total metal (mg/kg)

[ ] Exchangeable [ ] Carbonates [ ] Oxides bound [ ] Organic fraction

I l l Residue

# -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

956 Liang Qiao and Goen Ho

ae

8

*53 23 29 27 29 29 33 33 31 34 38 37 37 38

[ ] Carbonates

[ ] Residue

m # ~ o o o o =E o o o o =E o o o o

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

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

L

G)

100

80

60 4O

* - Total Cr (mg/kg)

[ ] Exchangeable I~1 Carbonate [ ] Oxides bound Organic fraction [ ] Residue

R e d M u d A d d i t i o n Fig 8 Effect of red mud on the Cr speciation in sludge compost after factoring out the red mud dilution

effect

O

Metal speciation of digested sludge

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

Time (days) end red mud addition (%) Fig 9 The variation of Cu speciation in sludge composting with the red mud addition

[ ] Exchangeable [ ] Carbonates [ ] Oxides bound [ ] Organic fraction [ ] Residue

957

more than countered what appeared to be the

conversion o f 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

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 o f 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

CJ

*535

100

80

6 0

40

20

o 0%

451 434 515 504 508

* - Total Cu (mg/kg)

I-I Exchangeable

[ ] Carbonates [ ] Oxides bound

Ill Organiofraction

[ ] Residue

Red M u d A d d i t i o n

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

effect

958 Liang Qiao and Goen Ho

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 C r > P b > C u > C d >

Ni > Zn Kaolinite and illite Pb > Cu > Zn > Cd Clay material Pb > Ni > Zn Amorphous AI hydroxide Cu > Pb > Zn > Ni > Cd Silanol groups of silica Pb > Cu > Zn > Ni = Cd

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 a l (1986 cited in Schmitt and Sticher, 1991) Hofstede (1994) Saeki et e l (1993) Mitchell (cited in Jorgensen and Jensen, 1984)

Kinniburgh et al (1976 cited in McBride, 1989) 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) T h e C u c o m p l e x e d with soluble

fuivic or h u m i c acids w o u l d be available to leaching

a n d to p r e s u m a b l y also plant

F a c t o r i n g o u t the red m u d d i l u t i o n effect the

c h a n g e o f C u speciation by red m u d a d d i t i o n was n o t

significant, b u t the effect o f c o m p o s t i n g process was

significant (Fig 10) W i t h c o m p o s t i n g the organic

b o u n d C u was c o n v e r t e d into c a r b o n a t e s , oxides

b o u n d a n d e x c h a n g e a b l e fractions The red m u d

a d d i t i o n i n h i b i t e d the increase o f e x c h a n g e a b l e C u in

red m u d sludge c o m p o s t t h r o u g h increasing p H to

precipitate C u a n d increasing the i n o r g a n i c oxides

surface to a d s o r b Cu, which also reduced the m e t a l

mobility in sewage sludge w h e n red m u d was a d d e d

to it (Qiao a n d Ho, 1996)

Ni T h e Ni c a t i o n has the stability o f complexes

with organic ligands j u s t less t h a n Cu 2÷ in the

transition metal cations according to the Irvine- Williams 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 m e t a l speciation to b e c o m e m o r e available w h e n the sludge was mixed with the sawdust a n d recycled sludge c o m p o s t , w h i c h m a y be caused by the decline

o f p H f r o m 8.3 to 5.2 after the sludge storage

T h e c o m p o s t i n g process significantly c h a n g e d the

Ni speciation in the sludge c o m p o s t Because the total

Ni increase was m o r e t h a n the r e d u c t i o n o f dry

m a t t e r d u r i n g the c o m p o s t i n g , the p e r c e n t a g e o f residue Ni a p p e a r e d 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 r e m a i n e d the same to the 50th day o f c o m p o s t i n g

F a c t o r i n g o u t the d i l u t i o n effect, the red m u d

a d d i t i o n significantly affected the Ni speciation in sludge c o m p o s t (Fig 12), particularly the c o n v e r s i o n

o f e x c h a n g e a b l e Ni into organic b o u n d with a d d i t i o n

#

i

100

80

60

40

20

0

*27 4 13 16 17 20 10 13 14 16 9 13 15 16

o o o o = o o g o

Time (days) end red mud addition (%)

* - Total Ni (mg/kg)

[ ] Exchangeable [ ] Carbonates [ ] Oxides bound

ml Organic fraction [ ] Residue

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

"13

100

80

60

40

20

0 0%

Metal speciation of digested sludge

IIIIII

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

* - Total Ni (mg/kg)

[ ] Exchangeable

~1 Carbonates [ ] Oxides bound ITS Organic fraction [ ] Residue

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

959

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

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 signifi- cantly 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

e l

.186 76 47 45 49 48 44 40 44 43 45 35 41 42

•

°JW

20

Time (days) and red mud addition (%) Fig 1:3 The variation of Pb speciation in sludge composting with the red mud addition

- Total Pb (mg/kg)

[ ] Exchangeable [ ] Carbonates [ ] Oxides bound

m Organic fraction [ ] Residue

960 Liang Qiao and Goen Ho

100

80

60

*47 47 50 45 45 47

4O

20

0 •

0 % 10% 20% ii 0% 10% 20%

* - Total Pb (mg/kg)

[ ] Exchangeable [ ] Carbonates [ ] Oxides bound [ ] Organic fraction [ ] Residue

Initial after 50 days

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

matter in the mixture was oxidised during the composting process, Zn was converted from organi- cally 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

Ze

I -

N

"901 229

100

80

60

40

20

0

242 254 221 277 212 200 204 211 201 234 217 259

* - Total Zn (mg/kg)

[ ] Exchangeable [ ] Carbonates

IN Oxides bound II1 Organic fraction

I ~ Residue

Times (days) end red mud addition (%)

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