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Nova Biotechnologica 10-1 (2010) 53
SORPTION OF Co2+, Zn2+, Cd2+ AND Cs+ IONS
BY ACTIVATED SLUDGE OF SEWAGE
TREATMENT PLANT
JANA MAREŠOVÁ, MIROSLAV HORNÍK,
MARTIN PIPÍŠKA, JOZEF AUGUSTÍN
Department of Biotechnology, Faculty of Natural Sciences, University of SS. Cyril and
Methodius, J. Herdu 2, Trnava, SK-917 01, Slovak Republic
(jana.maresova@ucm.sk)
Abstract: Sludges are byproducts of sewage treatment process. Land application of sewage sludge is one
of the final steps of waste water treatment, but solubilization of toxic metals restricts this method of sludge
disposal. In our paper cobalt, zinc, cadmium and cesium sorption by suspension of non-treated activated
sewage sludge (14 g/dm3, dry wt.) from waste water spiked with 60CoCl2, 65ZnCl2, 109CdCl2 or 137CsCl
were determined in laboratory experiments at 20°C. Activated sludge supplied by the municipal sewage
treatment plant in Zeleneč (Trnava region, Slovakia) showed high efficiency to sorb Co2+, Zn2+, Cd2+ and
Cs+ ions from waste water pH 6-7. The process can be characterized by the concentration equilibrium
(Csolid/Cliquid) typical for sorption processes. Efficiency of the sorption increased in the order
Cs < Co < Zn < Cd. Metal sorption process was not inhibited by pretreatment of the sludge with 0.2%
formaldehyde or thermal inactivation at 60°C, what confirms that the process was not dependent
on metabolic activity of the sludge. Cobalt, zinc, cadmium and cesium were easily removable from the
sludge by washing with diluted HCl, EDTA or water solutions of the corresponding metal ions, but with low
efficiency by deionized water.
Key words: activated sludge, sorption, cobalt, zinc, cadmium, cesium, radiotracer analysis
1. Introduction
Several physico-chemical methods were used for removal of heavy metals from
industrial liquid wastes, such as ion-exchange, chemical precipitation, chemical
reduction and adsorption. There are still some problems associated with these methods
since these are cost-expensive and they themselves can produce other wastes, which
will limit their industrial applications. Among the available treatment processes, the
application of biological sorbents is the most promising due to the following reasons:
requirement of chemicals for the treatment process is reduced, low operation costs,
eco-friendly and cost-effective alternative of conventional techniques and high
efficiency at low levels of contaminations.
Sludge of municipal waste water treatment plants are produced in huge amounts
and generally represent one of the main problems of European countries (FYTILI and
ZABANIOTOU, 2008). Many experimental studies are applied worldwide
to determine the extractable trace metals in sludge to assess the bio-available metal
fraction and the potential mobility of trace metals from polluted sludge (ALVAREZ
et al., 2002; FUENTEZ et al., 2004; IAQUINTA et al., 2006). However, the activated
sludge can be considered also as a biosorbent capable to bind toxic metals from liquid
wastes of industrial origin.
54 Marešová, J. et al.
In our previous papers we described sorption characteristics of Co, Zn and Cd
binded by dry biomass of lichen E. prunastri (PIPÍŠKA et al., 2008) and moss
R. squarrosus (PIPÍŠKA et al., 2010) biomass. The objective of this study was
to obtain quantitative data of cobalt, zinc, cadmium and cesium sorption by non-
treated activated sludge of the municipal wastewater treatment plant.
2. Materials and methods
2.1 Sorption and desorption experiments
Activated sludge supplied by a municipal sewage treatment plant in Zeleneč
(Trnava region, Slovakia) was kept in refrigerator at 4°C for 30 d and used for
experiments. Suspension of activated sludge pH 6.9 pre-concentrated by centrifugation
contained 14.0 g/dm3 biomass (dry wt.). In batch experiments, the test tubes with 1 ml
of activated sludge suspensions in waste water were spiked with 60CoCl2,
65ZnCl2,
109CdCl2 or
137CsCl (0.5-50 mmol/dm3) and shaken at 20°C. In time intervals, the
biomass was separated by centrifugation (20 min at 12 000 × g), supernatant was
removed and sediment was used for radiometric determination of sorbed metals.
In desorption experiments, the biomass after metal sorption was separated (20 min
at 12 000 × g) and resuspended by wortexing for 30 min in 1.0 ml deionized water,
0.1 M EDTA, 0.1 M HCl, 0.5 mM CoCl2, 0.5 mM ZnCl2, 0.5 mM CdCl2 or 0.5 mM
CsCl. Concentration of metals in biomass was estimated by similar way as in the
sorption experiments. Biomass dry weight was estimated by drying for 24 h at 60 °C.
The prediction of Co, Zn, Cd and Cs speciation in the solution as a function of pH
was performed using the Visual MINTEQ (version 2.53) program.
2.2 Radiometric analysis
The gamma spectrometric assembly using the well type scintillation detector
54BP54/2-X, NaI(Tl) (Scionix, the Netherlands) and the data processing software
Scintivision 32 (ORTEC, USA) were used for 60Co, 65Zn, 109Cd and 137Cs
determination in sediment of sludge and supernatant fluids at the energy of γ- photons
[keV]: 60Co- 1173.24, 65Zn – 1115.52, 109Cd – 88.04, 137Cs – 661.62. Counting time
600 s allowed obtaining data with measurement error <2 %, which do not reflect other
sources of errors. Standardized solutions of 65ZnCl2 (50 mg/dm
3 ZnCl2 in 3 g/dm
3
HCl), 60CoCl2 (20 mg/dm
3 CoCl2 in 3 g/dm
3 HCl), 109CdCl2 (50 mg/dm
3 CdCl2 in 3
g/dm3 HCl), 137CsCl (20 mg/dm3 CsCl in 3 g/dm3 HCl) were obtained from The Czech
Institute of Metrology (Prague, Czech Republic).
3. Results and discussion
3.1 Metal sorption
Sorption experiments showed that non-treated activated sludge is able to bind high
amounts of cobalt, zinc, cadmium and cesium (Fig. 1). At C0 = 50 mmol/dm
3,
Nova Biotechnologica 10-1 (2010) 55
sorptions Q (μmol/g; dry wt.) 611, 995, 1076 and 374 for cobalt, zinc, cadmium and
cesium respectively, were observed. However, at the initial concentration C0 = 50
mmol/dm3, the system was not saturated, and higher Qmax values can be expected.
10 100 1000 10000 100000
0,01
0,1
1
10
100
1000
Co
Zn
Cd
Cs
Q
[μ
m
ol
/g
]
C0 [μmol/dm
3]
Fig. 1. Cobalt (- - - -), zinc (-o-o-o-), cadmium (- - - -) and cesium (- - - -) uptake Q (μmol/g;
dry wt.) by non-treated activated sludge in dependence on the initial CoCl2, ZnCl2, CdCl2 and CsCl
concentration. Specific radioactivity in solutions [Bq/mL]: 60Co – 195; 65Zn – 156; 109Cd – 192 and
137Cs – 172. Data after 1h reaction at 20°C. Biomass concentration 14.0 g/dm3 (dry wt.). Data are the
arithmetic mean of three replicates.
3.2 The role of metabolic activity
Broad consortium of vital aerobic microbial population in activated sludge can
participate in metal uptake driven by metabolic processes. On the other hand, dead
microbial biomass and polymers such as proteins and polysaccharides can also bind
metals on the basis of sorption processes or complexing. Experiments showed that
treatment of activated sludge by formaldehyde or thermal inactivation at 60°C had
minimal effect on cobalt, zinc, cadmium and cesium uptake (Fig. 2). It means, that
metal uptake is not dependent on metabolic activity and physico-chemical processes
play decisive role in cobalt, zinc and cadmium binding by non-treated activated
sludge.
The sorption of metals is attributed to the bacterial cell wall (FEIN 2006; VULLO
et al., 2008) and to the exopolymeric substances (PAL and PAUL, 2008), which
contain a large number of negatively charged functional groups such as carboxyl,
phosphate and sulphate (WINGENDER et al., 1999). Exopolymers can be expected
56 Marešová, J. et al.
to buffer metal ion concentrations over wide ranges of concentrations and pH
(BHASKAR and BHOSLE, 2006). Comprehensive data dealing with metal binding
by activated sludge can be found in papers (MILNE et al., 2003; GUIBAUD et al.,
2009).
0
10
20
30
Co Zn Cd Cs
Q
[μ
m
ol
/g
]
Fig. 2. Uptake of cobalt, zinc, cadmium and cesium Q (μmol/g; dry wt.) by non-treated activated sludge
-- --, formaldehyde-treated sludge (0.5%, 30 min at 20°C, under wortexing) -- --, and thermally treated
sludge (30 min at 60°C) -- --. Specific radioactivity in solutions [Bq/mL]: 60Co – 198; 65Zn – 150;
109Cd – 167 and 137Cs – 162. Sorption 1 h under agitation at 20°C. Reaction conditions see Fig. 1.
3.3 The role of pH
Acidic pH region is not typical for municipal waste waters as well as activated
sludges. It is generally known, that metal cation biosorption is diminished below pH 4
(PIPÍŠKA et al., 2010). In acidic pH region species of cobalt and zinc exist as Me2+
cations, and cesium as Me+ cation (Fig. 3).
In the case of cadmium the main ionic form within pH 4 -8 beside Cd2+ is CdCl+
with the ratio approx 60 to 40 (Fig. 4). Above pH 7 more complex ionic equilibrium
takes place, in dependence of the concentration of anion such as chloride, carbonate
and phosphate.
The concentration of Cd(OH)3
– anion is increasing above pH 9. During the sorption
of metals by sludges from alkaline liquid wastes besides sorption of Me2+ cations, also
the existence of other than ionic metal forms with different solubility
in water have to be taken into consideration. This fact is in agreement with our
observations that sorption efficiency of activated sludge increases above pH 8
(not shown). Strong sigmoidal pH dependence of Cd2+ sorption by polysaccharides
of activated sludge with inflection point at pH 8.6 was described by GUIBAUD et al.
(2009).
Nova Biotechnologica 10-1 (2010) 57
3 4 5 6 7 8 9 10 11 12
0
20
40
60
80
100
(M
en
+ /
M
e t
ot
al
)
x
10
0
pH
Co2+
Zn2+
Cd2+
Cs+
Fig. 3. Molar fraction of Co2+, Zn2+, Cd2+ and Cs+ in dependence on pH of synthetic waste water
(IAQUINTA et al., 2006) at 25°C and C0 1.0 mmol/dm3. Calculated by speciation program Visual MINTEQ
ver. 2.53.
3 4 5 6 7 8 9 10 11 12
0
10
20
30
60
70
80
CdCl+
CdCl
2
CdSO
4
Cd(SO4)
2
2-
CdOH+
Cd(OH)
2
Cd
2
OH3+
Cd(OH)
3
-
(M
eX
/
M
e t
ot
al
)
x
10
0
pH
Fig. 4. Speciation forms of cadmium in dependence on pH of synthetic waste water (IAQUINTA et al.,
2006) at 25°C and C0 1.0 mmol//dm3. Calculated by speciation program Visual MINTEQ ver. 2.53.
3.4 Desorption
High percentage of Co, Zn, Cd and Cs can be desorbed from the sludge by one step
washing with CoCl2, ZnCl2, CdCl2 and CsCl solutions respectively, as well as with
EDTA and HCl solutions (Tab. 1; Fig. 5B).
At the same time only 6, 2 and 7 per cent of sorbed cobalt, zinc and cadmium,
respectively, can be solubilized by washing the activated sludge with deionized water.
It means that for releasing of the above mentioned metals certain, ionic force
is necessary for replacing metal ions from binding sites. Cesium was easily removable
from the sludge even with deionized water. Generally, Cs+ ions show the highest
mobility in biological systems resembling behavior of K+ ions.
Cadmium showed the highest affinity to the sludge and the lowest extractability
with mineral acids and salt solutions (Fig. 5A, B). The explanation of this
58 Marešová, J. et al.
phenomenon will require a more detailed study oriented toward speciation
of cadmium in individual components of the sludge flocks.
Table 1. Removable portion of metals from activated sludge by single step washing of sludge with 0.1 M
HCl, 0.1 M EDTA or 0.5 mM CoCl2, ZnCl2, CdCl2 or CsCl. Specific radioactivity in solutions [Bq/mL]:
60Co – 205; 65Zn – 158; 109Cd – 160 and 137Cs – 146. Calculations based on volume radioactivity
of supernatant washing liquid.
Parameter Co Zn Cd Cs
Sorbed Q (μmol/g; dry wt.) * 26±0.7 30±0.9 31±1.1 11±0.2
Removed with water [%] 6 2 7 36
Removed with 0.5 mM salt [%] 46 50 15 72
Removed with 0.1 M HCl [%] 81 91 30 83
Removed with 0.1 M EDTA [%] 90 92 73 81
* Biosorption from C0 = 0.5 mmol/dm3 solutions at biomass concentration 14.0 g/ dm3 (dry wt.).
0,01
0,1
1
10
100
1000
10000
Q
[μ
m
ol
/g
]
C
0
[mmol/L]
5,0 50,00,5 0,1
1
10
100
1000
10000
Q
[μ
m
ol
/g
]
C
0
[mmol/L]
5,0 50,00,5
Fig. 5A. Cobalt (- -), zinc (- -), cadmium (- -) and cesium (- -) uptake Q (μmol/g; dry wt.) by non-
treated activated sludge in dependence on the initial CoCl2, ZnCl2, CdCl2 and CsCl concentration. Specific
radioactivity in solutions [Bq/mL]: 60Co – 195; 65Zn – 156; 109Cd – 192 and 137Cs – 172. Data after 1h
reaction at 20 °C. Biomass concentration 14.0 g/dm3 (dry wt.).
Fig. 5B. Remaining metal uptake after 30 min washing of the sludge with CoCl2, Zn Cl2, Cd Cl2 or CsCl
solution. Data are the arithmetic mean of three replicates.
According to HSIAU and LO (1998), sequential extraction revealed that
the percentages of the heavy metals of organically bound form and exchangeable form
in chemically fixed sludge samples were in the order of Cu > Pb > Cr > Zn.
However, mobility of cesium can be diminished by supplementing of activated
sludge with clay materials or bentonites. In that case, cesium released from activated
sludge will be bound irreversibly on clay minerals and will not be bio-available
or leachable under natural conditions and even under more drastic conditions such as
with alkali or acid solutions. Clays (DYER et al., 2000) and bentonites (see e.g.
A B
Nova Biotechnologica 10-1 (2010) 59
GALAMBOŠ et al., 2010; ZÁVODSKÁ and LESNÝ, 2006) are well known sorbents
for irreversible cesium binding, as well as for binding many heavy metals and
metalloids.
Activated sludge disposes much higher capacity for metal sorption calculated for
unit biomass, than the content of metals contained in municipal waste waters. It means
that the sludge biomass is not saturated and can be used as sorbent for reversibly
sorption of the metals from other liquid wastes.
Table 2. Sorption capacities (mg/g, dry wt.) of naturally occurring sorbents. According to BAILEY et al.
(1999), modified.
Sorbent Cs Co Zn Cd Pb Hg
Bark 32 182 400
Modified cotton 1000
Chitin 100
Chitosan 558 796 1123
Clay 16.5 58
Microbial biomass 28 116
Lignin 95 1865 150
Modified wool 87 135 632
Moss 46.7
Peat 5.1 230 16.2
Seaweed 215 344
33.3 18 1.2 Xanthate
Zeolite 84.3 155.4 150.4
Lichen biomass (1) 5.7 7.3
Moss biomass (2) 7.2 12.2 19.4
Green algae (3) 14.5
Activated sludge(4) 24.5
Activated sludge (5) 50 36 65 121
(1) PIPÍŠKA et al. 2008; (2) PIPÍŠKA et al., 2010; (3) HORNÍK et al., 2008; (4) CHOI and YUN, 2006; (5)
This paper.
In Tab. 2, the sorption capacity (Q) values of activated sludge obtained in this
paper are compared with Q values of other sorbents published. As we can see,
activated sludge showed the sorption capacity comparable with other low-cost
sorbents and therefore the sludge could be used for lowering concentration of toxic
metals originating from industrial effluents, subsequently dewatered or incinerated.
4. Conclusions
The non-treated activated sludge from aerobic phase of municipal waste water
treatment plant contains the whole spectrum of metals. However, the sorption capacity
of the sludge is much higher than actual concentration of heavy metals in treated waste
waters. Sludge can be used as an efficient sorbent for the removal of heavy metals
from industrial liquid wastes. Sorption capacity of activated sludge studied for Co, Cd,
60 Marešová, J. et al.
Zn and Cs from 0.5 mmol/dm3 solution was 36, 65, 121 and 50 mg/g (dry wt.),
respectively. The ability of non-treated activated sludge to sorb Co2+, Zn2+, Cd2+ and
Cs+ metal ions is based mainly on physical processes not dependent on the metabolic
activity of the sludge microflora. Recovery of the metals by washing with diluted HCl,
EDTA and salt solutions decreases in the order: Cs > Co = Zn > Cd. The obtained data
stress the potential of surplus production of activated sludge as a sorbent for binding
of toxic and radiotoxic metals and metaloides from liquid industrial wastes. The next
treatment of the sludge will depend on the concentration of sorbed metals and their
toxicity. Incineration will be one of the ways for final treatment.
Acknowledgement: We would like to thank Juraj Miština, M.A. for proofreading. Department
of English Language, Faculty of Natural Sciences, University of SS. Cyril and Methodius in
Trnava, Slovak Republic.
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