Chromium species behaviour in the activated sludge process

Chemosphere 52 (2003) 1059–1067 www.elsevier.com/locate/chemosphere

Chromium species behaviour in the activated sludge process Athanasios S. Stasinakis a, Nikolaos S. Thomaidis a,*, Daniel Mamais b, Marianna Karivali a, Themistokles D. Lekkas a a

b

Laboratory of Air and Water Quality, Department of Environmental Studies, University of the Aegean, University Hill, Mytilene 81 100, Greece Department of Water Resources, Faculty of Civil Engineering, National Technical University of Athens, 5 Iroon Polytechniou Str., Zografou, Athens 15773, Greece Received 19 March 2002; received in revised form 18 March 2003; accepted 21 March 2003

Abstract The purpose of this research was to compare trivalent chromium (Cr(III)) and hexavalent chromium (Cr(VI)) removal by activated sludge and to investigate whether Cr(VI) reduction and/or Cr(III) oxidation occurs in a wastewater treatment system. Chromium removal by sludge harvested from sequencing batch reactors, determined by a series of batch experiments, generally followed a Freundlich isotherm model. Almost 90% of Cr(III) was adsorbed on the suspended solids while the rest was precipitated at pH 7.0. On the contrary, removal of Cr(VI) was minor and did not exceed 15% in all experiments under the same conditions. Increase of sludge age reduces Cr(III) removal, possibly because of Cr(III) sorption on slime polymers. Moreover, the decrease of suspended solids concentration and the acclimatization of biomass to Cr(VI) reduced the removal efficiency of Cr(III). Batch experiments showed that Cr(III) cannot be oxidized to Cr(VI) by activated sludge. On the contrary, Cr(VI) reduction is possible and is affected mainly by the initial concentration of organic substrate, which acts as electron donor for Cr(VI) reduction. Initial organic substrate concentration equal to or higher than 1000 mg l
1 chemical oxygen demand permitted the nearly complete reduction of 5 mg l
1 Cr(VI) in a 24-h batch experiment. Moreover, higher Cr(VI) reduction rates were obtained with higher Cr(VI) initial concentrations, expressed in mg Cr(VI) g
1 VSS, while decrease of suspended solids concentration enhanced the specific Cr(VI) reduction rate. Ó 2003 Elsevier Science Ltd. All rights reserved. Keywords: Chromium; Speciation; Activated sludge; Reduction; Adsorption; Precipitation

1. Introduction Chromium is usually encountered in the environment in the oxidation states of (III) and (VI) and is released by effluent discharge from steelworks, chromium electroplating, leather tanning and chemical manufacturing. Each of the above oxidation states has very different biological and chemical properties. Cr(VI) is very solu-

*

Corresponding author. Tel.: +30-22510-36226; fax: +3022510-36099. E-mail address: [email protected] (N.S. Thomaidis).

ble and toxic, while Cr(III) is more stable and presents lower toxicity (IPCS, 1988). The increasing trend towards combining industrial and municipal wastewater for treatment in sewage plants increases the possibility of contamination of the influent by metal ions. As a result, Cr is often detected in biological treatment plants (Nielsen and Hrudey, 1982). The presence of heavy metals in activated sludge plants is of great importance primarily because certain metal concentrations may adversely affect the operation of biological treatment processes (Harper et al., 1996; Stasinakis et al., 2001; Stasinakis et al., 2002; Stasinakis et al., 2003b). Moreover, it was shown that activated

0045-6535/03/$ - see front matter Ó 2003 Elsevier Science Ltd. All rights reserved. doi:10.1016/S0045-6535(03)00309-6

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sludge processes can remove substantial quantities of metals (Petrasek and Kugelman, 1983; Shafer et al., 1998). This removal is succeeded by precipitation, with the precipitate settling either independently by flocculation together with bacterial cells or by binding to activated sludge flocs and extracellular polymers (Brown and Lester, 1979). It is well documented that metal removal efficiency is influenced by several parameters including plant operating conditions (sludge age), physicochemical (pH, metal ion concentration, metal ion solubility, metal valency, concentration of complexing agents) and biological factors (concentration of extracellular polymers) (Brown and Lester, 1979; Nelson et al., 1981; Battistoni et al., 1993; Su et al., 1995). Previous studies, evaluating total Cr removal efficiency in wastewater treatment plants, reported that its removal efficiency was higher than 80% (Nielsen and Hrudey, 1982; Shafer et al., 1998). Imai and Gloyna (1990), investigating the effect of chromium speciation on its removal in the activated sludge process, reported that this was differentiated by Cr oxidation state. More specifically, at pH values 7 and 8, Cr(VI) average removal efficiency was less than 25%, whereas more than 90% of Cr(III) was removed. So far, the effect of factors such as the sludge age, the suspended solids concentration and the acclimatization of biomass on the removal efficiency of chromium species have not been investigated. Conventional waste management options for Cr(VI) removal include chemical reduction to Cr(III) followed by precipitation under alkaline conditions or removal by ion exchange and adsorption (Patterson, 1985). Recently, the research for new and innovative technologies has focused on biotransformation of Cr(VI) by microorganisms. The reduction of Cr(VI) to Cr(III) was shown to be possible, using pure and mixed cultures enriched from soil sources, under either aerobic or anaerobic conditions (Shen and Wang, 1993; Lovley and Phillips, 1994; Wang and Xiao, 1995; Schmieman et al., 1998). Two kinds of enzymatic reduction have been proposed. The aerobic activity of Cr(VI) reduction is generally associated with a soluble protein fraction utilizing nicotinamide adenine denucleodite (NADH) as an electron donor (Ishibashi et al., 1990), while under anaerobic conditions, Cr(VI) may act as a terminal electron acceptor through a membrane-bound reductase activity (Wang et al., 1990). Moreover, factors such as the initial cell concentration, Cr(VI) concentration, pH and temperature affected the rate of Cr(VI) reduction (Shen and Wang, 1994; Philip et al., 1998). In spite of the extended research for Cr(VI) reduction by pure and mixed cultures of microorganisms, data on Cr(VI) reduction by activated sludge are scarce. Only Imai and Gloyna (1990) reported that a part of Cr(VI) was reduced to Cr(III) in a fed-batch activated sludge system. However, no investigation of the factors that could af-

fect Cr(VI) reduction by activated sludge has been realised. All the above reported observations for Cr(VI) reduction by pure or mixed cultures and activated sludge were concluded by determining Cr speciation in the dissolved phase only, using the well-documented d1phenylcarbazide method (APHA, 1989). So far, Cr speciation in the particulate phase has not been realised. The purpose of the present study was to estimate Cr(III) and Cr(VI) removal during the activated sludge process and to investigate whether adsorption or precipitation is the main removal mechanism. Moreover, the effect of sludge age, concentration of suspended solids and acclimatization of biomass on the removal efficiency of chromium species was investigated. Data were fit to the Freundlich isotherm model and adsorption constants were determined in order to compare adsorption capacity of activated sludge with other potential chromium biosorbents reported in the literature. In addition, Cr(III) oxidation and Cr(VI) reduction by activated sludge was investigated in a series of batch experiments. Moreover, the effect of organic substrate concentration, suspended solids concentration and initial Cr(VI) concentration on Cr(VI) reduction was studied. For the investigation of Cr biotransformation, a novel analytical procedure was used, determining Cr speciation in the dissolved and the particulate phase.

2. Materials and methods 2.1. Cr(VI) and Cr(III) removal by activated sludge A sequencing batch reactor (SBR) was used to simulate activated sludge process and to provide biomass for the metal uptake experiments. The SBR was operated in a 24-h fill and draw cycle, each cycle consisted of four stages: FILL (10 min), REACT (22.5 h with aeration), SETTLE (1 h) and DECANT (20 min). Activated sludge from a municipal wastewater treatment plant, that received no industrial wastewater (Plomari, Lesvos), was used to seed the reactor. Synthetic wastewater with known chemical oxygen demand (COD) concentration was used as feed and a phosphate buffer was added to maintain a constant pH of 7.0  0.2 (Stasinakis et al., 2002). When biomass was acclimatized to the feed (time equal to 3 sludge age, hc ), activated sludge samples of known mixed liquor suspended solids (MLSS) concentration were introduced from the SBR into batch reactors (250 ml conical flasks). The reactors were put on an ORBIT shaker bath at 20.0  1 °C, while a phosphate buffer was added to maintain pH at 7.0 and a potassium dichromate (K2 Cr2 O7 , Merck) or chromium nitrate (Cr(NO3 )3 , Merck) solution was added to provide the desirable concentrations of Cr(VI) or Cr(III) respectively.

A.S. Stasinakis et al. / Chemosphere 52 (2003) 1059–1067

To quantify Cr removal, homogenized samples of 5 ml of mixed liquor were collected periodically and were filtered through 0.45 lm membrane microfilter (Millipore). The filtrates were analyzed for residual dissolved Cr species concentration, using the analytical method described below, under Analytical Methods. In these experiments, total Cr species removal, due to adsorption and precipitation, was calculated. To isolate the role of precipitation in Cr species removal, the above removal procedure was repeated for concentrations of 1.0 and 10 mg l
1 of Cr(VI) or Cr(III), using four control batch reactors containing the filtrate from the activated sludge. This filtrate was obtained by filtering the harvested activated sludge through a 0.45 lm membrane filter (Millipore) (Su et al., 1995). The above described metal removal experiments were conducted using non-acclimatized activated sludge to Cr(VI) for sludge ages of 2, 5 and 10 days. Experiments were also repeated using acclimatized biomass to Cr(VI) (hc ¼ 2 days). The acclimatization of biomass to Cr(VI) was achieved by operating the SBR in the presence of 1 mg l
1 of Cr(VI) for a period equal to 6 days (3  hc ) (Stasinakis et al., 2002). 2.2. Adsorption isotherms Data from the batch reactors were fit to the Freundlich isotherm model. This is expressed as the equation: qe ¼ KF Ce1=n

ð1Þ

where qe ¼ amount of chromium adsorbed per unit weight of adsorbant at equilibrium (mg/g), Ce ¼ equilibrium residual concentration of adsorbate in solution after adsorption (mg l
1 ), KF ¼ constant related to adsorption capacity of adsorbent (activated sludge), 1=n ¼ constant related to adsorbent affinity to adsorbate. The constants KF and n were evaluated by plotting log qe against log Ce . 2.3. Biotransformation of Cr species by activated sludge Cr(VI) reduction and Cr(III) oxidation experiments were carried out in batch reactors (250 ml Erlenmeyer flasks) containing non-acclimatized activated sludge samples from the SBR. Added also were a phosphate buffer to maintain pH at 7.0, a K2 Cr2 O7 or a Cr(NO3 )3 solution to provide 1 mg l
1 of Cr(VI) or Cr(III) respectively, and a synthetic substrate (containing CH3 COOH and NH4 Cl) to provide 1000 mg l
1 as COD. The temperature within the batch reactors was kept at 20  1 °C using an ORBIT shaker bath, while the dissolved oxygen (DO) was maintained above 4.0 mg l
1 using porous ceramic diffusers. Samples were withdrawn

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from batch reactors periodically and Cr species were determined in the dissolved and particulate phase. To investigate the effect of organic substrate concentration, suspended solids concentration and initial Cr(VI) concentration on Cr(VI) reduction, the above procedure was repeated for different values of these parameters.

2.4. Analytical methods The operation of the SBR was controlled by monitoring influent and effluent COD and mixed liquor suspended solids. The measurement of suspended solids in Cr species removal and biotransformation experiments and determination of COD in Cr(VI) reduction experiments were done according to Standard Methods for the Examination of Water and Wastewater (APHA, 1989). Suspended solids samples were obtained after filtration through glass fiber filters (Whatman GF/C, UK). A simple method for the speciation of Cr in wastewater and sewage sludge was used, utilizing liquid anion exchange by Amberlite LA-2 and final determination by electrothermal atomization atomic absorption spectrometry (ETAAS) (Stasinakis et al., 2003a). Samples of the mixed liquor were filtered with a 0.45 lm membrane filter (Millipore) and Cr species were determined in the filtrate and in the suspended solids on the filter. Total Cr in the filtrate was determined directly by ETAAS, after appropriate dilution with ultrapure water, using a Perkin–Elmer model 5100PC spectrometer equipped with a transversely heated graphite atomizer (THGA) graphite furnace (Perkin–Elmer 5100ZL). For the determination of Cr(VI), the sample was diluted with a HCO
3 –H2 CO3 buffer (pH 6.4) and extracted with 1 ml of Amberlite LA-2 solution in methyl isobutyl ketone (liquid anion exchange solution, LAES). Cr(VI) was determined in the organic extract, while Cr(III) was determined by the difference between total Cr and Cr(VI). For the determination of Cr(VI) in the suspended solids, the filter was placed in a beaker, covered with 10 ml of an alkaline buffer solution (2% NaOH–3% Na2 CO3 , pH 12.7) and was placed in an ultrasonic bath at 70 °C for 30 min. An aliquot of the supernatant was diluted with a HCO
3 –H2 CO3 buffer (pH 6.4), extracted with LAES and Cr(VI) was determined in the organic phase. For the determination of total leachable Cr, the filter was subjected to ultrasonic agitation with 10 ml of dilute HNO3 (pH 1) at 70 °C for 30 min and the supernatant was subjected to ETAAS, after appropriate dilution with ultrapure water. Then, Cr(III) was determined by the difference. A more detailed description of the method could be found elsewhere (Stasinakis et al., 2003a).

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3. Results and discussion

1200

3.1. Kinetics of chromium species removal by activated sludge

1000

Removal kinetics of chromium species by activated sludge were studied in batch reactors, for hc of 2, 5 and 10 days. Neither adsorption of chromium species on the conical flask walls, nor reduction of Cr(VI) or oxidation of Cr(III) was observed in these batch experiments. The removal efficiency was calculated by dividing removed Cr to the initial Cr concentration. As it can be seen in Fig. 1, for all concentrations used (0.1–10 mg l
1 ), removal of Cr(III) was very high during the first 30 min of the experiment. It was reduced gradually and reached a steady-state level, 5 h after the beginning of the experiment, where almost 95% of the initial dosage of Cr(III) had been removed. The observed behaviour during Cr(III) removal from the activated sludge has also been reported for other metals (Su et al., 1995). In an early study, Cheng et al. (1975) proposed the existence of a two-step reaction model for metal uptake. The first step is a rapid uptake phase in which a large quantity of metal ions are adsorbed by the cell flocs, while the second one is a slow phase which may extend over many hours and is dependent on the viability of the sludge (Lamp and Tollefson, 1973; Cheng et al., 1975). The effect of activated sludge solids concentration on Cr(III) removal is illustrated in Fig. 2. According to the experimental results, residual Cr(III) concentration is reduced with increasing suspended solids concentration. Similar observations for the effect of suspended solids concentration to metal removal have been reported in previous studies (Cheng et al., 1975; Fristoe and Nelson, 1983). The higher metal removal in the presence of higher suspended solids concentration is due to the existence of more sorption sites available for metal uptake,

MLSS concentration (mg/l) 790 340

800

145

600

400

200

0 0

1

2

3

4

5

6

7

8

Fig. 2. Residual Cr(III) concentration for suspended solids concentrations of 145, 340 and 790 mg l
1 . The experiments were conducted for hc of 2 days and Cr(III) initial concentration of 1.0 mg l
1 .

either on the extracellular polymers or in the cytoplasm and cell wall. Fig. 3 shows Cr(III) residual concentration obtained at various sludge ages. According to this, Cr(III) removal appeared to decrease at higher sludge ages. The effect of sludge age on the removal of heavy metals in the activated sludge process has been investigated in previous studies but the results are often controversial. Nelson et al. (1981) suggested that the bacterial solids from an aged sludge culture exhibit greater affinity for Cd, Cu and Zn than cultures grown at a low sludge age. This behaviour was consistent with the hypothesis that greater production of extracellular polymers is observed in slower growing or endogenous cultures (Pavoni et al.,

12000 10000

100

Sludge Age (d)

8000

80 Initial Cr(III) concentration (mg/l)

2

0.1

60

5

6000

0.5

10

1.0

40

4000

3.0 5.0

20

2000

10.0

0 0

1

2

3

4

5

6

0 0

Fig. 1. Cr(III) removal efficiency (%) for initial Cr(III) concentrations of 0.1, 0.5, 1.0, 3.0, 5.0 and 10.0 mg l
1 . The experiments were conducted for hc of 2 days and MLSS of 800 mg l
1 .

2

4

6

8

10

Fig. 3. Residual Cr(III) concentration for sludge age of 2, 5 and 10 days. The experiments were conducted for Cr(III) initial concentration of 10.0 mg l
1 and MLSS of 800 mg l
1 .

A.S. Stasinakis et al. / Chemosphere 52 (2003) 1059–1067

1972). On the contrary, Battistoni et al. (1993) suggested that while Ni uptake increases with sludge age, Cd affinity is lower in older sludge. The latter further suggested that the ratio of extracellular polymers to total solids remains constant and thus is independent of the sludge age. Similar findings are also reported by other authors (Brown and Lester, 1982). However, a different distribution between capsular and slime extracellular polymers is expected for different sludge ages. According to Saunders and Dick (1981) concentration of capsular polymers increase and slime polymers decrease with increasing sludge age. Therefore, the decrease of Cr uptake at higher sludge age seems to indicate a higher affinity of Cr(III) for slime extracellular polymers. Contrary to what was observed for Cr(III), removal of Cr(VI) was slow during the first hours of the experiment and reached a steady level 10 h after the start of the experiment, as opposed to 5 h for Cr(III). Cr(VI) removal was significantly lower than that observed for Cr(III) and did not exceed 15% of the initial concentration. No effect of suspended solids concentration on Cr(VI) removal was observed. Similarly, the effect of sludge age on Cr(VI) removal was negligible. The observed chromium species removal efficiencies were consistent with that reported in the literature. Imai and Gloyna (1990) reported that mean Cr(VI) removal efficiency was less than 25%, while more than 90% of Cr(III) was removed, at pH values between 7 and 8. To investigate Cr species removal efficiency by activated sludge that was cultured in the presence of Cr(VI), batch experiments were repeated using biomass acclimatized to Cr(VI). Activated sludge was considered as acclimatized, after operation of the sequencing batch reactor in the presence of 1 mg l
1 of Cr(VI) for a period equal to 3hc . When Cr(VI) was added to the batch reactors, Cr(VI) removal was similar to that reported for non-acclimatized biomass, without exceeding 15% of the initial concentration. On the contrary, when Cr(III) was added, the results were differentiated. Comparing residual Cr(III) concentration, 8 h after the start of each batch experiment, in the presence of acclimatized and non-acclimatized activated sludge (Fig. 4), one can observe that acclimatized biomass presented significantly lower removal efficiency for Cr(III). If acclimatization of biomass had been achieved using Cr(III), then this observation would have been justified and could have been attributed to the fact that in acclimatized biomass a part of sorption sites has already been occupied and is no longer available for the metal uptake. In this experiment, acclimatization of biomass was achieved using Cr(VI). To explain the lower removal efficiency for acclimatized biomass, a reduction of Cr(VI) to Cr(III) in the SBR reactor could have been performed. To investigate this, few mixed liquor samples were taken from the SBR reactor. The results revealed that a part of Cr(VI) reduced to Cr(III) accumulated mainly on the suspended solids.

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Fig. 4. Residual Cr(III) concentration at the end of each batch experiment (t ¼ 8 h) for non-acclimatized and acclimatized biomass. The experiments were conducted for hc of 2 days and MLSS of 500 mg l
1 .

It was mentioned above that Cr(VI) reduction was not observed during the batch experiments. However, Cr(VI) reduction was observed in the SBR, an observation consistent with that reported in a previous study by Imai and Gloyna (1990). The factors that affect Cr(VI) reduction by activated sludge are discussed in a following paragraph of this study. Use of the Lagergren equation (2) showed that the kinetics of Cr(III) removal follows the first-order expression (Namasivayan and Yamuna, 1995): logðqe
qÞ ¼ log qe
ðkad  tÞ=2:303

ð2Þ

where q is the amount of Cr(III) adsorbed (mg/g) at time t, qe is the amount of Cr(III) adsorbed (mg/g) at equilibrium time, kad is the rate constant of adsorption (min
1 ). Linear plots of logðqe
qÞ versus time show the applicability of the above equation for different Cr(III) concentration (Fig. 5). Thus, Cr(III) adsorption is a first-order process. Moreover, kad values (the slopes of Initial Cr(III) concentration

2

(mg/l) 1

0.5 1.0

0

3.0 5.0

-1

10

-2 -3 0

50

100

150

200

250

300

350

-4 -5

Fig. 5. Lagergren plots for the adsorption of Cr(III) by activated sludge at Cr(III) concentrations of 0.5, 1, 3, 5 and 10 mg l
1 . The experiments were conducted for hc of 2 days and MLSS of 500 mg l
1 .

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the plots) decreased with increased initial Cr(III) concentration, which means that adsorption is slower at high Cr(III) concentration. Namasivayan and Yamuna (1995), studying the biosorption of chromium by biogas residual slurry, observed a similar behaviour. 3.2. Mass balance of Cr species removal in batch reactors In the batch reactors, metal removal can occur through both sorption and precipitation. To investigate the contribution of metal sorption and precipitation to metal removal occurring in samples, batch tests were carried out for metal additions of 1 and 10 mg l
1 . After 12 h in a shaking water bath, samples were filtered and the metal content of these filtrates was analyzed. Generally, the following mass balance is performed (Su et al., 1995): Mt ¼ Ms1 þ Mb þ Mp

ð3Þ

where Mt ¼ total mass of metals, Ms1 ¼ mass of metals in solution, Mb ¼ mass of adsorbed metals, Mp ¼ mass of precipitated metals. In control batch tests where only precipitation occurs, the mass balance is: Mt ¼ Ms2 þ Mp

ð4Þ

where Ms2 ¼ mass of metals in solution in control reactor. Control batch experiments were conducted only with filtrate. Combining Eqs. (3) and (4) results in: Mb ¼ Ms2
Ms1

Fig. 6. Metal distribution for: (a) 1 mg l
1 of Cr(III) initial concentration; (b) 10 mg l
1 of Cr(III) initial concentration.

Adsorbed 14%

Precipitated 0%

ð5Þ

The distribution of Cr(III) and Cr(VI) is presented in Figs. 6 and 7, respectively. Referring to Cr(III), at a concentration of 1 mg l
1 , the major part of it was adsorbed (90%), 9% was precipitated, while only 1% was in the dissolved phase (Fig. 6a). When 10 mg l
1 of Cr(III) was added to the reactor, the precipitation increased to 14% (Fig. 6b). These two Cr(III) removal mechanisms, adsorption on activated sludge and precipitation as Cr(OH)3 , have already been reported in a previous study (Imai and Gloyna, 1990). In contrast to Cr(III), a major part of Cr(VI) was in the dissolved phase (>85%) while the rest was adsorbed (Fig. 7). The precipitation of Cr(VI) was negligibly independent of its initial concentration (Fig. 7a and b).

(a)

Dissolved 86%

Precipitated 0% Adsorbed 11%

(b)

Dissolved 89%

Fig. 7. Metal distribution for: (a) 1 mg l
1 of Cr(VI) initial concentration; (b) 10 mg l
1 of Cr(IV) initial concentration.

3.3. Freundlich isotherms for biosorption Adsorption isotherms were used in this study to represent equilibrium distributions of chromium species between solution and bacterial solid phases at 20  1 °C. They are adequately described by Freundlich model (Eq. (1)), as shown by high values of the correlation coefficient (r2 ) given in Table 1. This model was tested for a

range of loadings of 0.1–11 mg Cr g
1 MLSS, in the absence of organic substrate to exclude biomass propagation in the system and possible biological transformation of Cr(VI) to Cr(III). Since sludge age and acclimatization of biomass on Cr(VI) had a significant effect on adsorption equilibrium, Freundlich constants

A.S. Stasinakis et al. / Chemosphere 52 (2003) 1059–1067 Table 1 Freundlich isotherms constants for Cr(III) and Cr(VI) biosorption Sludge age (days)

Cr(III) KF

N

r2

KF

n

r2

10 5 2 2a

0.80 6.5 49.3 13.1

1.3 1.3 1.4 1.8

0.999 0.97 0.96 0.98

0.06 0.05 0.16 –

1.4 0.97 1.0 –

0.999 0.88 0.99 –

a

Cr(VI)

Acclimatized biomass in 1 mg l
1 of Cr(VI).

KF and n were evaluated for sludge ages of 2, 5 and 10 days for the non-acclimatized biomass and 2 days for the acclimatized biomass. Referring to Cr(III), values of KF ; the constant related to adsorption capacity, were higher at sludge ages of 2 and 5 days. The values of n obtained for Cr(III) were not significantly different between sludges with different age. The n constant is related to sorption affinity and values higher than unity denote favourable adsorption. The n values found in this study were slightly higher than unity, indicating that Cr(III) adsorption is favourable and that Cr(III) sorption mechanism is the same for all experiments. The KF and n values determined in this study are consistent with those reported by Chua et al. (1999) for Cr(III) adsorption by activated sludge within similar age range. So far, few recent studies have reported the potential of living and dead biomass to adsorb Cr(VI) from solutions (Gupta et al., 2001; Bai and Abraham, 2002). However, values of KF determined in this study were significantly lower than those reported in the literature for several adsorbents (Bai and Abraham, 2002), indicating that activated sludge cannot serve as an efficient bioadsorbent for Cr(VI), unless Cr(VI) reduction to Cr(III) is achieved. The parameters that affect Cr(VI) reduction by activated sludge are investigated in the next paragraph. 3.4. Investigation of biotransformation of Cr species by activated sludge To investigate whether Cr(III) oxidation or Cr(VI) reduction is possible, aerobic batch experiments were carried out in Erlenmeyer flasks containing non-acclimatized activated sludge from the SBR. For better simulation of the activated sludge process, except for the appropriate amount of buffer and K2 Cr2 O7 or Cr(NO3 )3 solution, a synthetic substrate was added at 1000 mg l
1 as COD. During the entire duration of the experiment (48 h), no oxidation of Cr(III) into Cr(VI) was observed both in the dissolved and particulate phases. This observation is consistent with the results reported by Imai and Gloyna (1990) for Cr(III) behaviour in a fed-batch

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activated sludge plant. In an earlier study, Schroeder and Lee (1975) suggested that the oxidation of Cr(III) by dissolved oxygen is so slow that Cr(III) may be adsorbed on solids before being oxidized. However, when Cr(VI) was added, most of it was reduced to Cr(III) during the experiment. As it was expected, determination of Cr speciation on the dissolved and particulate phase revealed that the part of Cr(VI) that was not reduced, remained mainly in the dissolved phase, while the produced Cr(III) was found on the particulate phase. Use of control batch experiments, containing only the substrate, buffer and Cr(VI) solution and no activated sludge, showed that no abiotic Cr(VI) reduction occurred (Chirwa and Wang, 1997). The reduction of Cr(VI) to Cr(III) under aerobic or anaerobic conditions has been mentioned in several previous studies using pure and mixed cultures of microorganisms (Wang and Shen, 1997; Schmieman et al., 1998). Moreover, Imai and Gloyna (1990) observed Cr(VI) reduction in a fed-batch activated sludge treatment plant. According to Imai and Gloyna, Cr(VI) may be adsorbed on the bacterial surface through specific adsorption and afterwards it is reduced inside the activated sludge flocs, and specifically on the surface or inside the bacterial cell. The fact that Cr(VI) reduction was not observed during the previous batch tests, conducted for the investigation of Cr(VI) removal by activated sludge, is probably due to the absence of a carbon source in these experiments. Previous studies with pure cultures of microorganisms have shown that Cr(VI) reduction is limited in case of insufficient substrate, which acts as electron donor for Cr(VI) reduction (Shen and Wang, 1995; Wang and Xiao, 1995). To fully understand the effect of organic substrate concentration on Cr(VI) reduction by activated sludge, experiments were repeated for initial COD concentrations of 100, 500, 1000 and 2000 mg l
1 . During the duration of the experiments, all of the feed COD was oxidized in all reactors. Percentage Cr(VI) reduction was calculated by dividing Cr(VI) reduced (initial Cr(VI) minus residual Cr(VI) in solution) to the initial Cr(VI) concentration. As it can be seen in Fig. 8, reduction of 5 mg l
1 was nearly completed after 24 h of incubation with 1000 and 2000 mg l
1 COD. The presence of lower initial COD concentrations resulted in lower Cr(VI) reduction, suggesting that Cr(VI) reduction may be limited by the electron donor (acetic acid in this study). This was further proved by spiking acetic acid (COD concentration equal to 1000 mg l
1 ) in reactors C and D, 24 h after the start of each experiment. This addition permitted complete reduction of the residual Cr(VI) during the next 12 h of the experiment. The effect of initial Cr(VI) concentration on the rate of Cr(VI) reduction was examined under an initial COD concentration of 1000 mg l
1 and initial Cr(VI) concentration ranging from 0.5 to 10 mg l
1 . The time

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A.S. Stasinakis et al. / Chemosphere 52 (2003) 1059–1067 Table 3 Effect of activated sludge concentration on specific Cr(VI) reduction rate (initial COD concentration 1000 mg l
1 ; initial Cr(VI) concentration 5 mg l
1 ; duration of the experiment 48 h)

120 100 80 Initial COD concentration (mg/l)

60

100 500 1000 2000

40 20 0 0

5

10

15

20

25

30

Fig. 8. Cr(VI) reduction (%) for initial concentrations of COD 100, 500, 1000 and 2000 mg l
1 . The experiments were conducted for Cr(VI) initial concentration of 5 mg l
1 and MLSS of 4000 mg l
1 .

Activated sludge concentration (g l
1 MLSS)

Specific Cr(VI) reduction rate (mg Cr(VI) g MLSS
1 h
1 )

10 5.6 3.4 1.7 1.2

0.010 0.019 0.030 0.061 0.087

microorganisms (Shen and Wang, 1994; Philip et al., 1998). 4. Conclusions

required for complete reduction of Cr(VI) increased with the initial Cr(VI) concentration. Nine hours after the start of the experiments, more that 90% of Cr(VI) was reduced in reactors with initial Cr(VI) concentrations of 0.5 and 1 mg l
1 . In batch reactors with initial concentrations of 3, 5 and 10 mg l
1 of Cr(VI), similar percentage reductions were observed after 24, 36 and 48 h, respectively. However, the average hourly rate of Cr(VI) reduction, observed during the first nine hours of the experiment, increased with increasing initial Cr(VI) concentration (Table 2). Similar results were previously reported in experiments with pure cultures by Shen and Wang (1994) and Chirwa and Wang (2000). The effect of activated sludge concentration on Cr(VI) reduction is presented in Table 3. For all activated sludge concentrations tested, near complete reduction occurred during the duration of the experiment (48 h). However, the specific Cr(VI) reduction rate, which is a measure of Cr(VI) reduction per unit weight of activated sludge per hour, was higher at relatively lower activated sludge concentrations. Similar trends have also been reported in studies with pure cultures of

Table 2 Effect of initial Cr(VI) concentration on average rate of Cr(VI) reduction observed 9 h after the start of the experiment (initial COD concentration 1000 mg l
1 ; initial MLSS concentration 4000 mg l
1 ) Initial Cr(VI) (mg l
1 )

Average rate of Cr(VI) reduction (mg l
1 h
1 )

0.5 1 3 5 10

0.052 0.103 0.24 0.33 0.71

The objective of this study was to estimate Cr(VI) and Cr(III) removal by activated sludge and to investigate whether Cr(VI) reduction or Cr(III) oxidation is possible. Batch experiments using non-acclimatized biomass showed that almost 95% of Cr(III) was removed, mainly by adsorption, while Cr(VI) removal was significantly lower and did not exceed 15% of its initial concentration. Sludge age and suspended solids concentration can be used to control Cr(III) uptake by bacterial solids. Metal affinity for the bacterial solids was found to increase as sludge age reduced from 10 to 2 days or as suspended concentration increased. Adsorption isotherms adequately represented the distribution of chromium species between bacterial solids and solution phases, and conditional adsorption constants derived from the isotherms were used to quantify the distribution. Batch experiments showed that Cr(III) cannot be oxidized to Cr(VI) by activated sludge, though Cr(VI) reduction to Cr(III) is possible. Results indicated that Cr(VI) reduction is affected by the initial organic substrate concentration. Higher rates of Cr(VI) reduction were observed at higher initial Cr(VI) concentrations. Moreover, the specific Cr(VI) reduction rate was higher at relatively lower activated sludge concentrations. The ability of activated sludge to reduce Cr(VI) to Cr(III) in combination with the high removal efficiency of Cr(III) by the suspended solids allows the future use of activated sludge as a detoxification process for wastewater containing Cr(VI) in the range of concentrations studied (0.5–10 mg l
1 ).

Acknowledgement A.S. Stasinakis would like to thank the Greek Scholarship Foundation for financial support of this work.

A.S. Stasinakis et al. / Chemosphere 52 (2003) 1059–1067

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