JEQ Journal of Natural Resources and Life Sciences Education
HOME HELP FEEDBACK SUBSCRIPTIONS ARCHIVE SEARCH TABLE OF CONTENTS
QUICK SEARCH:   [advanced]


     

This Article
Right arrow Abstract Freely available
Right arrow Figures Only
Right arrow Full Text (PDF)
Right arrow Alert me when this article is cited
Right arrow Alert me if a correction is posted
Services
Right arrow Related articles in JEQ
Right arrow Similar articles in this journal
Right arrow Similar articles in ISI Web of Science
Right arrow Similar articles in PubMed
Right arrow Alert me to new issues of the journal
Right arrow Download to citation manager
Citing Articles
Right arrow Citing Articles via ISI Web of Science (14)
Right arrow Citing Articles via Google Scholar
Google Scholar
Right arrow Articles by Osuna, M. B.
Right arrow Articles by Lens, P. N. L.
Right arrow Search for Related Content
PubMed
Right arrow PubMed Citation
Right arrow Articles by Osuna, M. B.
Right arrow Articles by Lens, P. N. L.
Agricola
Right arrow Articles by Osuna, M. B.
Right arrow Articles by Lens, P. N. L.
Related Collections
Right arrow Heavy Metals
Right arrow Water Pollution
Published in J. Environ. Qual. 33:1256-1270 (2004).
© ASA, CSSA, SSSA
677 S. Segoe Rd., Madison, WI 53711 USA

TECHNICAL REPORTS

Heavy Metals in the Environment

Effect of Cobalt Sorption on Metal Fractionation in Anaerobic Granular Sludge

M. Begoña Osunaa,b, Eric D. van Hullebuscha, Marcel H. Zandvoorta, Jon Izab and Piet N. L. Lensa,*

a Sub-Department of Environmental Technology, University of Wageningen, NL-6700-EV Wageningen, the Netherlands
b Chemical and Environmental Engineering Department, School of Engineering, University of Basque Country, E-48013 Bilbao, Spain

* Corresponding author (piet.lens{at}wur.nl).

Received for publication February 24, 2003.

   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
A sequential extraction procedure was applied to two anaerobic methanogenic sludges (Eerbeek and Nedalco) to examine the speciation of micro- and macronutrients in the sludges after cobalt sorption by exposing the sludge to a 1 mM Co solution for 4 d at pH 7 and 30°C. The effect of different physicochemical conditions on cobalt sorption was studied as well: effect of pH (6–8), effect of competition by a second trace element (Ni or Fe), modification of the granular matrix by glutaraldehyde or heat treatment, and EDTA (ethylenediaminetetraacetic acid) addition. Sorbed Co was found to distribute between the carbonates, organic matter + sulfides, and residual fractions. Cobalt adsorption resulted in an antagonistic interaction with other metals present in the granular matrix, evidenced by the solubilization of other trace elements (e.g., Ni, Cu, and Zn) as well as macronutrients (especially Ca and Fe). Modification of the sludge matrix by glutaraldehyde or heat treatment, or exposure to EDTA, led to serious modifications of the Co sorption capacity and strong interactions with multivalent cations (i.e., Ca2+ and Fe2+).

Abbreviations: TS, total solids • UASB, upflow anaerobic sludge bed


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
TRACE ELEMENTS play an important role in the growth and metabolism of anaerobic microorganisms (Takashima and Speece, 1989; Goodwin et al., 1990). Trace metals present in wastewater can accumulate in anaerobic sludge aggregates through physical, chemical, or biological processes. The distribution of these metals into different chemical species (organic or inorganic, free-ion or chelated) determines their availability for the metabolic active microorganisms present within these granules (Alibhai et al., 1985).

As the free-ion activity controls the reactions of the metal with the surface sites of the anaerobic granules, it also affects the bioavailability of the metal (Callendar and Barford, 1983). In anaerobic reactors, trace elements are not only present as free ions, but as different chemical species because of the typical process conditions that prevail in these reactors, for example, pH, redox potential, alkalinity, and concentrations of phosphorous and sulfur (Theis and Hayes, 1978). Mineral precipitates such as CaCO3, Ca5OH(PO4)3, and FeS are present in the anaerobic granular sludge matrix as a result of metabolic activities and physicochemical reactions (Dolfing et al., 1985). These precipitates accumulate either inside the granules or on their surface. The minerals (such as calcium carbonate and iron sulfide) play an important role on the metal accumulation (Angelidis and Gibbs, 1989), thus also affecting their bioavailability. The behavior of metals in anaerobic sludge granules is further complicated by their biosorption to extracellular polymers (Artola et al., 1997). Thus, measurement of the total trace element concentration provides insufficient information about the bioavailability and toxicity of trace elements (Oleszkiewicz and Sharma, 1990). Consequently, there is a need for speciation or selective dissolution procedures to study the fate of trace elements in anaerobic bioreactors.

Cobalt was shown to play a key role in the methanogenic activity of granular sludges from upflow anaerobic sludge bed (UASB) reactors treating methanol (Florencio, 1994; Zandvoort et al., 2002) and volatile fatty acids (Osuna et al., 2003). These studies showed that cobalt accumulated in the sludge granules, but its fate and speciation within the granular matrix is not known. The objective of this study was to determine the distribution of trace metals present in anaerobic methanogenic granules from UASB reactors by using a chemical sequential extraction procedure. The distribution of metals over these fractions was studied in detail with two types of granular sludge. The effects of pH, competing metal ions during sorption, complexing agents, and changes in the matrix structure on the speciation of micronutrients (i.e., Co, Ni, Cu, and Zn) and macronutrients (i.e., Ca, Mg, P, and Fe) were determined.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 REFERENCES
 
Experimental Setup
Sorption experiments were performed in 117-mL serum bottles filled with 50 mL of NaHCO3 buffer (0.1 mM) containing 1 mM of CoCl2 and around 2 g volatile suspended solids L–1 granular sludge. As standard conditions for the sorption experiments, pH 7.0 (±0.2) and 4 d of contact time were chosen. The bottles were closed with butyl rubber stoppers (Rubber B.V., Hilversum, the Netherlands), flushed with N2 and CO2 (70:30%) for 5 min, and placed in a temperature-controlled water bath (30°C) equipped with a Haake (Karlsruhe, Germany) heater and shaken at 100 rpm. After the 4-d contact time, granules were harvested for total and sequential extraction to determine the metal distribution within the granules. The metal concentration in each fraction was related to the amount of biomass.

To obtain information about the kinetics of the cobalt sorption process, intact granules were exposed for 100 h to cobalt at concentrations of 1 µM, 10 µM, 100, µM, and 1 mM. The depletion of cobalt from the soluble fraction was monitored in time.

The effect of metal competition for adsorption sites during cobalt sorption was studied in the presence of 1 mM NiCl2 or 1 mM FeCl2. In this experiment, nickel was added simultaneously with cobalt (at t = t0). For Ni and iron, the effect of the retarded addition (i.e., 2 d after cobalt addition, at t = t0 + 2 d) was also investigated. The effect of the physicochemical conditions on cobalt sorption was investigated by contacting granular sludge with cobalt (1 mM) (i) at different pH values (pH 6, 7, and 8); (ii) after changing the structure of the granular matrix by exposing granules to glutaraldehyde (2.5%, 2 d) or a heat treatment (55 and 74°C, 4 d); and (iii) in the presence of different concentrations of EDTA (1, 10, and 50 mM EDTA).

Source of Biomass
Methanogenic granular sludges were obtained from a full-scale expanded granular sludge bed (EGSB) reactor treating alcohol distillery wastewater (Nedalco, Bergen op Zoom, the Netherlands; Gonzalez et al., 2001) and from a full scale UASB reactor treating wastewater of four paper mills (Industriewater Eerbeek B.V., Eerbeek, the Netherlands; Lens et al., 1999).

Total Extraction
The total metal concentration in the samples (expressed as mg metal kg–1 total solids [TS]) was determined after microwave destruction (Model 2100; CEM, Matthews, NC) of pre-dried samples (40°C), subjected to digestion with aqua regia (HCl to HNO3, 3:1). After digestion, samples were paper-filtered (Model 589; Schleicher & Schuell, Dassel, Germany), stored at 4°C, and analyzed on an Elan 6000 inductively coupled plasma–mass spectrometer (ICP–MS; PerkinElmer, Wellesley, MA).

Sequential Extraction Procedure
The sequential fractionation method proposed by Tessier et al. (1979) and, later on, modified by Modak et al. (1992) and Veeken (1998), was used to determine the speciation (Table 1). The supernatant of a specific fraction was analyzed after centrifuging for 10 min at 4000 rpm. The nonsolubilized fraction of the sludge was then submitted to the next, more stringent extraction step. The samples of the residual fraction were not centrifuged, but were filtered over a 0.45-µm filter to remove the particles present, if any, which can be detrimental to the ICP–MS.


View this table:
[in this window]
[in a new window]
  		Table 1. Extraction solutions for each fraction applied in the sequential extraction method.

 
Analysis
Total suspended solids and volatile suspended solids content were determined as described by standard methods (American Public Health Association, 1995). All chemicals were of analytical grade and purchased from Merck (Darmstadt, Germany). The metal content in the sludge is expressed as mg kg–1 on dry weight basis (TS).


   RESULTS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 REFERENCES
 
Metal Composition of the Granules
Figure 1 shows the metal speciation of the inoculum of the two anaerobic granular sludges investigated. The major element content of the paper mill sludge was much higher compared with that of the distillery sludge (Table 2) and revealed a different metal distribution (Fig. 1). For instance, the paper mill sludge contained an initial cobalt concentration of 30 mg Co kg–1 TS, of which 17% was present in the exchangeable, 20% in the carbonates, 33% in the organic matter + sulfides, and 30% in the residual fraction (Fig. 1A). The total cobalt concentration in the distillery sludge was 16 mg Co kg–1 TS, of which 56% was present in the exchangeable, 9% in the carbonates, 29% in the organic matter + sulfides, and 6% in the residual fraction (Fig. 1B).



View larger version (31K):
[in this window]
[in a new window]
  		Fig. 1. Distribution of the metal concentration (mg kg–1 total solids [TS]) in granules from the paper mill sludge (A and C) and the distillery sludge (B and D). From left to right, bars represent the following fractions: exchangeable, carbonates, organic matter, and residual. (A) and (B), micronutrients; (C) and (D), macronutrients.

 

View this table:
[in this window]
[in a new window]
  		Table 2. Effect of pH and the presence of competitive ions (Ni or Fe) on cobalt sorption and the concentration of other trace elements and macronutrients in intact granules from paper mill (Eerbeek) and distillery (Nedalco) sludge.

 
The distillery sludge showed relatively high concentrations of Mn, Cu, and Zn (about 350 mg kg–1 TS), which were mainly associated with the organic matter + sulfides fraction (Fig. 1B). The distillery sludge contained only low amounts of metals (i.e., the micronutrients Co, Mo, Ni, Se, and Mn were present at concentrations below 15 mg kg–1 TS; Fig. 1B). Moreover, their distribution over the different fractions was also quite different between both sludges (Fig. 1C and 1D). Paper mill sludge presented a nickel distribution mainly related to the residual fraction, whereas distillery sludge showed, also, the importance of the carbonates fraction on nickel accumulation in aggregates. The total calcium content in the paper mill sludge was 10 times higher than in the distillery sludge (Table 2). Total sulfur and total phosphorus content in the Eerbeek sludge was twice the content in the Nedalco sludge (Table 2). Iron present in Eerbeek sludge was three times higher than in Nedalco (Table 2); these high levels of iron were extracted in the carbonates fraction (Fig. 1C and 1D).

Intact Granules
Cobalt Sorption Capacity
Exposing the sludges to medium containing Co concentrations of 10 µM, 100 µM, and 1 mM showed that after 100 h of contact time, the Co concentration of the soluble fraction was already below 10% (Fig. 2). It was decided for further experiments to use a Co concentration of 1 mM and a contact time of 4 d (96 h).



View larger version (20K):
[in this window]
[in a new window]
  		Fig. 2. Evolution of soluble cobalt concentration of the fraction when exposing the sludges from the paper mill sludge (A) and the distillery sludge (B) to medium containing cobalt concentrations of 10 µM (diamonds), 100 µM (squares), and 1 mM (triangles), represented on the secondary axis.

 
Exposure to a 1 mM Co concentration for 96 h showed similar adsorption capacities and distribution patterns for both sludges (Fig. 3, intact granules). The addition of cobalt led to its adsorption into the sludge to a total concentration (after 4 d) of 1276 and 1558 mg Co kg–1 TS for, respectively, the paper mill and the distillery sludge (pH of approximately 7). The largest amount of sorbed Co was located in the exchangeable fraction for both inocula. Only 10% (paper mill) and 4% (distillery) of Co was associated with the residual fraction (Fig. 3). A significant fraction of cobalt was found in the carbonates fraction of the paper mill sludge (Fig. 3), and also in the organic matter + sulfides fraction in the case of the distillery sludge. Moreover, paper mill sludge presented a high amount of Co bound to the organic matter + sulfides fraction.



View larger version (24K):
[in this window]
[in a new window]
  		Fig. 3. Effect of physicochemical conditions on cobalt speciation (mg kg–1 total solids [TS]) in paper mill (Eerbeek) (A) and distillery (Nedalco) (B) granules. Intact granules: (a), (b), and (c), effect of pH value 6, 7, and 8, respectively; (d) and (e), effect of 1 mM Ni addition at time t0 and t0 + 2 d, respectively; (f), effect of 1 mM Fe addition at time t0 + 2 d. Modified granules: (a), (b), and (c), effect of EDTA addition 1, 10, and 50 mM, respectively; (d), effect of glutaraldehyde treatment; (e) and (f), effect of temperature exposure to 74 and 55°C, respectively. From left to right, bars represent the following fractions: exchangeable, carbonates, organic matter + sulfides, and residual.

 
Table 2 presents the total metal content for the experiments at different pH values. At pH 6 the Co concentration in the granules of both sludges was lower than at pH 7 and 8. The differences in the Co distribution between pH 6 and 8 were small (Fig. 3). For the paper mill sludge, the exchangeable fraction decreased somewhat with increasing pH (from 480 to 290 mg Co kg–1 TS) and the residual fraction increased slightly. The distillery sludge showed only an increase in the Co associated with the organic fraction (from 345 to 620 mg Co kg–1 TS) with increased pH.

Upon exposure to 10 mM Co, the paper mill sludge accumulated 17-fold more Co than in the 1 mM Co sorption experiments (data not shown). The distillery sludge showed only a 4-fold increase of its Co concentration. The distillery sludge showed no change in Co concentration in the residual fraction, whereas in the paper mill sludge this fraction increased from 189 up to 1862 mg Co kg–1 TS on exposure to 10 mM Co for 4 d.

Sorption of Cobalt in the Presence of Nickel or Iron
The trace element concentrations in the sludge granules were affected by the simultaneous exposure to Co (1 mM) with Ni (1 mM) or Fe (1 mM), either supplied together from the start or after 2 d of exposure to Co alone (Table 2). The Co concentration in the paper mill sludge increased from 1270 mg Co kg–1 TS in the absence of Ni to 2400 and 3250 mg Co kg–1 TS in the presence of 1 mM Ni for 4 and 2 d of contact time, respectively. This is related to the high Co associated with the residual fraction of the paper mill sludge, up to 840 and 1950 mg Co kg–1 TS in the presence of 1 mM Ni for 4 and 2 d of contact time, respectively (Fig. 3, intact granules). Similarly, the total Co concentration increased when 1 mM Fe was added during the sorption incubation (up to 3120 mg Co kg–1 TS) (Table 2). This increase mainly occurred in the residual fraction (Fig. 3). For distillery sludge, the Co and Fe addition decreased the total Co concentration (down to 620 mg Co kg–1 TS) (Table 2). In this case, all fractions presented low values, except the exchangeable fraction, which kept about the same concentration (Fig. 3). Moreover, the Co concentration decreased drastically in the distillery sludge, from 1558 mg Co kg–1 TS in the absence of Ni to 472 and 223 mg Co kg–1 TS in the presence of Ni with a contact time of 4 and 2 d, respectively (Table 2).

Nickel after its addition distributed mainly into the more labile fractions (i.e., exchangeable and carbonates fractions; Fig. 4), independently of the contact time, to a final concentration of about 2400 and 600 mg Ni kg–1 TS for the paper mill and distillery sludge, respectively (Table 2).



View larger version (38K):
[in this window]
[in a new window]
  		Fig. 4. Effect of Co sorption under different physicochemical conditions on nickel speciation (mg kg–1 total solids [TS]) in paper mill (Eerbeek) (A) and distillery (Nedalco) (B) granules. For denomination of sorption conditions, see Fig. 3. I, inoculum. Intact granules: (d) and (e) represented as black bars on the secondary axis for 1 mM Ni addition. From left to right, bars represent the following fractions: exchangeable, carbonates, organic matter + sulfides, and residual.

 
Modified Granules
Effect of Glutaraldehyde
Cobalt sorption onto glutaraldehyde-treated granules led to an increase of the total amount of Co absorbed onto both sludges (Table 3). Both sludges showed very similar Co sorption capacities, with 2310 and 2170 mg Co kg–1 TS for the paper mill sludge and distillery sludge, respectively. The Co in the exchangeable and carbonates fractions considerably increased compared with untreated granules (both sludges). In contrast, the residual fraction remained intact (Fig. 3).


View this table:
[in this window]
[in a new window]
  		Table 3. Effect of EDTA, glutaraldehyde, and temperature treatment on cobalt sorption and the concentration of other trace elements and macronutrients in the methanogenic aggregates investigated.

 
Effect of Heat Treatment
Heat-treated paper mill sludge lost its sorptive capacity (Table 3) and contained only 100 mg Co kg–1 TS, with proportional reductions in all fractions (Fig. 3). In contrast, the distillery sludge retained its sorption capacity after the heat treatment. At both temperatures, a concentration exceeding 2500 mg Co kg–1 TS was obtained, which is about twofold that of intact sludge. This increase of Co was associated mainly with the increase in the carbonates fraction (Fig. 3).

Effect of EDTA
The addition of a chelating agent (EDTA) during the sorption reduced the Co adsorption in the sludge. In the presence of 1 mM of EDTA, the total amount of Co adsorbed decreased from 1276 to 712 mg Co kg–1 TS, and from 1558 to 1440 mg Co kg–1 TS for the paper mill and the distillery sludge, respectively (Table 3). The paper mill sludge showed a Co washout with a homogeneous final distribution between all fractions. In contrast, the distillery sludge presented a change in the Co speciation from almost no Co located in the residual fraction toward nearly all of the Co located in this residual fraction (Fig. 3, modified granules).

The addition of higher concentrations of EDTA (10 and 50 mM) had a similar effect on the Co sorption (i.e., a reduction of the amount of cobalt sorbed). The paper mill sludge contained a total amount of Co of around 415 and 551 mg Co kg–1 TS for 10 and 50 mM of EDTA, respectively. The distillery sludge also showed a reduction of the total amount of Co sorbed (660 and 870 mg Co kg–1 TS for 10 and 50 mM EDTA, respectively) in comparison with 1 mM EDTA, although no changes in the metal distribution were observed (Table 3 and Fig. 3).

Effect of Cobalt Sorption on Other Metal Speciation
When the paper mill sludge was exposed to 1 mM of cobalt, there were some small changes in the distribution of other trace metals, resulting in a slightly decreased concentration of these metals within the sludge granules (Table 2), indicating the solubilization of trace metals. The residual fraction was the most affected, showing reductions of 80% for Ni and Se and 60% for Mo. Copper (Fig. 5) and zinc (Fig. 6) concentrations in the organic matter + sulfides fraction decreased for the distillery sludge. For the paper mill sludge it seems that a significant amount of Cu is removed from the residual fraction to the liquid phase (Fig. 5). Zinc was redistributed from the residual fraction to the organic matter + sulfides and carbonates fractions (Fig. 6).



View larger version (75K):
[in this window]
[in a new window]
  		Fig. 5. Effect of Co sorption under different physicochemical conditions on copper speciation (%) in paper mill (Eerbeek) (A) and distillery (Nedalco) (B) granules. For denomination of sorption conditions, see Fig. 3. I, inoculum. From top to bottom, bars represent the following fractions: soluble, exchangeable, carbonates, organic matter + sulfides, and residual.

 


View larger version (70K):
[in this window]
[in a new window]
  		Fig. 6. Effect of Co sorption under different physicochemical conditions on zinc speciation (%) in paper mill (Eerbeek) (A) and distillery (Nedalco) (B) granules. For denomination of sorption conditions, see Fig. 3. I, inoculum. From top to bottom, bars represent the following fractions: soluble, exchangeable, carbonates, organic matter + sulfides, and residual.

 
The total amount of iron in both sludges decreased slightly after the addition of 1 mM Co (Table 2). However, there was a significant change of the Fe distribution in the different fractions (Fig. 7, intact granules). The Fe concentration in the organic matter + sulfides fraction of the paper mill sludge increased, at the expense of the exchangeable fraction, which decreased from 2.74 g Fe kg–1 TS (inoculum) to 0.60 g Fe kg–1 TS.



View larger version (35K):
[in this window]
[in a new window]
  		Fig. 7. Effect of Co sorption under different physicochemical conditions on iron speciation (mg kg–1 total solids [TS]) in paper mill (Eerbeek) (A) and distillery (Nedalco) (B) granules. For denomination of sorption conditions, see Fig. 3. I, inoculum. Intact granules: (f) represented as black bar for 1 mM Fe addition. From left to right, bars represent the following fractions: exchangeable, carbonates, organic matter + sulfides, and residual.

 
Calcium and magnesium distribute mainly exclusively between the carbonates and exchangeable fractions (Fig. 8 and 9). The total phosphorous concentration within the paper mill granules decreased by about 50% (Table 2), which was related to a reduction of the P concentration in the residual fraction (Fig. 10).



View larger version (65K):
[in this window]
[in a new window]
  		Fig. 8. Effect of Co sorption under different physicochemical conditions on calcium speciation (%) in paper mill (Eerbeek) (A) and distillery (Nedalco) (B) granules. For denomination of sorption conditions, see Fig. 3. I, inoculum. From top to bottom, bars represent the following fractions: soluble, exchangeable, carbonates, organic matter + sulfides, and residual.

 


View larger version (67K):
[in this window]
[in a new window]
  		Fig. 9. Effect of Co sorption under different physicochemical conditions on magnesium speciation (%) in paper mill (Eerbeek) (A) and distillery (Nedalco) (B) granules. For denomination of sorption conditions, see Fig. 3. I, inoculum. From top to bottom, bars represent the following fractions: soluble, exchangeable, carbonates, organic matter + sulfides, and residual.

 


View larger version (71K):
[in this window]
[in a new window]
  		Fig. 10. Effect of Co sorption under different physicochemical conditions on phosphorus speciation (%) in paper mill (Eerbeek) (A) and distillery (Nedalco) (B) granules. For denomination of sorption conditions, see Fig. 3. I, inoculum. From top to bottom, bars represent the following fractions: soluble, exchangeable, carbonates, organic matter + sulfides, and residual.

 
At higher pH values, higher concentrations of micronutrients and macronutrients were bound to the residual fraction. The exchangeable fraction for Ni, Fe, Cu, Zn, Ca, Mg, and P remained almost constant as a function of pH (Fig. 4–10). The same trend was observed for Mn, Se, Mo, and W (data not shown).

The simultaneous Co and Ni or Fe addition had no significant effect on the distribution of Cu (Fig. 5) or Zn (Fig. 6), compared with when Co was added alone. However, the Mn (about 80%) and Ca (by a factor of 10) concentration decreased (Table 2).

Addition of the complexing agent EDTA induced a washout of the other trace elements and macronutrients (Table 3), especially macronutrients such as Fe (Fig. 7) and Ca (Fig. 8) from the exchangeable, carbonates, and organic matter fractions. Moreover, when using glutaraldehyde to modify the physicochemical properties of the sludge, the content of micronutrients decreased in both sludges (Table 3) (e.g., Ni, Cu, and Zn). Mobilization of macronutrients in the paper mill sludge was observed, especially for Ca and Mg, with concentrations five times lower compared with granules without glutaraldehyde treatment (Table 3). The heat treatment led to a decrease of the total concentration of other trace elements and macronutrients as well (Table 3), accompanied by a redistribution toward the residual fraction (Fig. 4–10).


   DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
REFERENCES
 
Trace Metal Speciation in Anaerobic Sludges
The trace element (Co, Ni, Cu, Zn, Mn, Se, and Mo) and macronutrient (Ca, Mg, Fe, P, and S) content of the paper mill sludge was much higher, compared with that of the distillery sludge (Table 2), with an initial total metal stock of 83.30 and 19.75 g kg–1 TS for the paper mill and distillery sludge, respectively. Moreover, both sludges also display a quite different metal distribution (Fig. 1), which resulted in a different metal behavior during the Co sorption experiments (Fig. 3).

Figure 3 shows that a significant fraction of cobalt was found in the carbonates fraction of the Eerbeek sludge. This is in accordance with the observations of Espinosa et al. (1995), in which the carbonates fraction was shown to play an important role in the cobalt retention in anaerobic granular sludge. Moreover, cobalt was found predominantly bound to the organic matter + sulfides and residual fractions. The organic matter + sulfides fraction in the case of the distillery sludge, and also the residual fraction in the paper mill sludge, are, respectively, the most important fractions involved in the cobalt accumulation. Sulfur compounds, mainly found in the organic matter + sulfides and residual fractions, are known to play a key role in scavenging metals like cobalt. Insoluble sulfide precipitates, linked to the biological sulfide production by sulfate-reducing bacteria, are a main removal mechanism for the trace elements Co, Ni, Fe, Cu, and Zn. Their degree of removal is reflected in the trend of the solubility products of the respective metal sulfides, with log Ks of NiS, CuS, and ZnS equivalent to –27.98, –40.94, and –28.39, which are lower than FeS at –22.39 (Di Toro et al., 1990).

Nickel has a high trend to associate with the soluble fractions (Legret et al., 1987). This study shows that the carbonates fraction played an important role on nickel retention in the distillery sludge (Fig. 4). However, in the paper mill sludge, nickel is mainly found in the residual fraction (Fig. 4), presumably in the form of metal sulfides, as the sludge contained a significant amount of sulfur (29.3 g kg–1 TS).

Both sludges showed that a great part of iron is bound to the carbonates fraction (Fig. 7), associated with the high carbonate concentrations in anaerobic bioreactors (van Langerak et al., 2000; Yamaguchi et al., 2001). Moreover, iron was also found in the organic matter + sulfides and residual fractions (Fig. 7) associated with ferrous sulfide precipitates. Ferrous iron is usually precipitated as relatively insoluble ferrous sulfide during anaerobic digestion, which is important in regulating the free metal concentration in the effluent and the distribution of metals between the different sludge fractions during anaerobic digestion. Liu and Fang (1998) showed by X-ray spectrometry analyses that the metals precipitated into granules were mainly composed of sulfur, plus iron and nickel presumably in the form of metal sulfides. Zandvoort et al. (unpublished data, 2003) showed that the supply of a sulfur source to the UASB influent shifted the metals (Ni, Co, Cu, and Zn) accumulated in the exchangeable and carbonates fractions to the organic matter + sulfides fraction. This is due to the higher affinity of the organic matter + sulfides fraction for metals such as cobalt and nickel compared with the exchangeable and carbonates fractions (van Hullebusch et al., 2004a).

Distillery sludge (sulfur content of 10.2 g kg–1 TS) presented high amounts of copper and zinc associated with the organic matter + sulfides fraction (Fig. 5 and 6). This is in accordance with the results of Stover et al. (1976), who described a high affinity of Zn for the organic matter fraction, and Angelidis and Gibbs (1989), who reported significant amounts of Zn (85%) bound to the oxidizable (organic matter + sulfides) phase. In contrast, paper mill sludge presented a significant percentage of both metals associated with the carbonates fraction.

Cobalt Sorption onto Granular Sludge
This study showed that the sorption of cobalt onto UASB sludge granules depends on both the chemical (Co, Ni, and Fe concentration, pH) and physical (matrix composition: organic and inorganic fractions) conditions (Tables 2 and 3; Fig. 3). This has also been reported for other biosorption materials, such as algae or activated sludge (Nestle and Kimmich, 1996). Figure 3 shows that the exchangeable, carbonate, organic matter + sulfides, and residual fractions are mainly involved in the cobalt sorption in the paper mill sludge. In the distillery sludge, the residual fraction played a minor role. The extraction scheme showed that a significant fraction of the sorbed cobalt is found in the carbonate fraction of both sludges. This observation is in accordance with the observations of Espinosa et al. (1995). Thus, the carbonate species plays an important role in the cobalt retention in anaerobic granular sludge.

According to the importance of extracellular polymers and biomass in anaerobic granular sludges, a high sorption capacity can be expected (van Hullebusch et al., 2004a). In the present study, it is not possible to determine which fraction of the sequential extraction scheme contributed most to the release of metals from biomaterials (bacterial cells and extracellular polymeric substances associated with minerals). However, cobalt bound to the biomaterials is more likely mainly released in the exchangeable and organic matter + sulfides fractions. The organic matter + sulfides and residual fractions are also shown to play a big role in cobalt sorption in anaerobic granular sludges. Morse and Luther (1999) showed that cobalt tends to adsorb on or coprecipitate with the iron sulfide phase and can be incorporated into pyrite. Domènech et al. (2002) studied the oxidative dissolution of pyritic sludge and showed that Co dissolves congruently with sulfur. The latter authors confirmed that cobalt is mainly incorporated into pyrite.

Cobalt sorption onto both sludges resulted in an antagonistic interaction, as the concentration of the micro- and macronutrients originally present in the sludge decreased (Fig. 4–10). Some of the metals remained in the exchangeable fraction, displacing other metals from the fixation sites and/or producing a cation exchange effect, thus inducing a metal solubilization from the sludge. Mandal et al. (2000) found that, when using humic materials, the metal uptake by ion exchange of aqueous metallic cations occurs with metals such as Ca and Mg associated with anionic sites and by proton displacement by complexation.

When Ni or Fe were added together with Co, the excess of ions in the soluble fraction, which is directly influenced by the ionic strength (Tessier and Turner, 1995), resulted in a lower amount of Co sorbed by the distillery sludge compared with the addition of solely Co (Fig. 3). However, there was Co accumulation into the residual fraction for the paper mill sludge. Belzile et al. (1989) reported that if there were kinetic effects, in combination with sites not readily available for the reaction, increasing time would lead to increasing adsorption. Indeed, the Ni addition after 2 d of Co exposure led to a slight increase in total Co concentration (Fig. 3 and 4), whereas the total Ni concentration in the sludge decreased as a consequence of the competition for sorption sites between Ni and Co. The latter is in agreement with Schneider et al. (1995), who found that Ni2+ is a competitive metal for the binding of cobalt ions to cells of Propionibacterium arabinosum. Van Hullebusch et al. (2004a) showed that competition for sorption sites occurred between Ni and Co in anaerobic granular sludge. They showed that in binary conditions (both added at to), the sorption capacity of cobalt and/or nickel is strongly reduced in the exchangeable, carbonates, and organic matter + sulfides fractions compared with the monometal conditions. For instance, the cobalt maximal sorption capacity of the distillery sludge decreased from 3.09 g kg–1 TS in cobalt monometal condition to 1.76 g kg–1 TS in binary condition in the exchangeable fraction. The contrasting results of Co and Ni sorption on the same anaerobic sludges under binary conditions (synergistic in this study versus antagonistic in Van Hullebusch et al., 2004a) is most probably due to differences in the contact medium composition. This warrants further research into the Co and Ni sorption mechanisms and the way these are influenced by counterions (e.g., HCO–3, Ca2+, HS–, and SO2–4) and metabolic activity.

Effects of pH on Cobalt Sorption
The pH may influence the metal distribution into the sludges (Fig. 3–10). The pH conditions affect directly the metal speciation, and indirectly the biological surface chemistry (Wang et al., 1999). Due to the indirect effects (e.g., the amount of protons or bicarbonate ions present), competition for the binding sites on the membranes or matrix proteins could occur during the sorption. However, no significant differences in the Co sorption were found between pH 6 and 8, typical for the operation of anaerobic reactors. This is in agreement with van Hullebusch et al. (2004b), who described the cobalt and nickel sorption capacity of the anaerobic granular sludge used in the present study at different pH conditions between 6 and 8. In contrast, the concentration of the nonsupplemented trace elements and macronutrients of both sludges significantly decreased as a function of pH (Table 1), although the pH hardly affected the Ca content and distribution (Fig. 8).

Effects of Matrix Treatment on Cobalt Sorption
The treatment of granules with glutaraldehyde, as well as heat treatment, lead to serious increases in the metal sorption capacity (Fig. 3–10). Both methods significantly alter the diffusional matrix properties of granular sludge (Lens et al., 2003). Moreover, Azaredo et al. (1999) showed by microscopic observations that subjecting biofilm to a glutaraldehyde treatment induces the disappearance of the polymeric matrix that was formerly covering the cells. The matrix is known to play a key role in determining, via the extracellular polymers, the number of adsorption and ion-exchange sites, and thus the metal sorption capacity of the granules (Gould and Genetelli, 1978).

Addition of EDTA led to a reduction of the Co adsorption into the sludge (Fig. 3), as reported by Schneider et al. (1995). It also mobilized macronutrients from the sludge, such as Fe and Ca (Fig. 7 and 8), by forming strong water-soluble complexes (Elliott and Brown, 1989) and thus stimulated the dissolution of the metal fractions bound to the matrix (Fig. 3–10), as also shown by Alibhai et al. (1985) and Artola et al. (2000). Holding et al. (2003) showed that the EDTA-extractable metal content from biofilm was positively correlated with the amount of metals extracted in the exchangeable fraction. The treatment of biofilm with chelating agents such as EDTA was shown to interact strongly with multivalent cations (i.e., Ca2+ and Fe2+) involved in the cross-linking of the polymeric structure of biofilms, which influence biofilm cohesion (Caccavo et al., 1996).

Sequential Extraction and Practical Implications
Most of the speciation schemes classify fractions by lability, which is very suitable to determine the availability of metals for microorganisms (van Leeuwen, 1999). The bioavailability of the trace elements for the metabolic needs of the bacteria is usually unrelated to the total metal amount present (Oleszkiewicz and Sharma, 1990). Most of the trace metals remain inaccessible for microorganisms in the form of more stable species such as precipitates or chelates (Mosey et al., 1971; Speece, 1996). It is, thus, important to know the total concentration, and, more importantly, the speciation of metals within a granule, to assess the scarcity or excess of a metal for a metabolic process (Campbell, 1995). The sequential extraction applied in this work gives an approximation of the metal distribution into different chemical fractions and thus in different categories of bioavailability (Clevenger, 1990). Determination of the metal speciation within the granule matrix, and relating this to their microbial–ecological structure, requires more research using advance analytical techniques, for example, energy dispersive X-ray analysis (ED-X; Gonzalez, 2000) or electron probe X-ray microanalysis (EPMA; Yamaguchi et al., 2001). This has an economic significance for the operation of industrial anaerobic reactors, since the addition of trace metals may significantly increase the operating costs of the process. Thus, a more thorough understanding of the speciation of metals in the sludge will allow a more economic and effective metal dosage to anaerobic bioreactors.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 


Related articles in JEQ:

This Issue in Journal of Environmental Quality

JEQ 2004 33: 1177-1182. [Full Text]  




This Article
Right arrow Abstract Freely available
Right arrow Figures Only
Right arrow Full Text (PDF)
Right arrow Alert me when this article is cited
Right arrow Alert me if a correction is posted
Services
Right arrow Related articles in JEQ
Right arrow Similar articles in this journal
Right arrow Similar articles in ISI Web of Science
Right arrow Similar articles in PubMed
Right arrow Alert me to new issues of the journal
Right arrow Download to citation manager
Citing Articles
Right arrow Citing Articles via ISI Web of Science (14)
Right arrow Citing Articles via Google Scholar
Google Scholar
Right arrow Articles by Osuna, M. B.
Right arrow Articles by Lens, P. N. L.
Right arrow Search for Related Content
PubMed
Right arrow PubMed Citation
Right arrow Articles by Osuna, M. B.
Right arrow Articles by Lens, P. N. L.
Agricola
Right arrow Articles by Osuna, M. B.
Right arrow Articles by Lens, P. N. L.
Related Collections
Right arrow Heavy Metals
Right arrow Water Pollution

HOME HELP FEEDBACK SUBSCRIPTIONS ARCHIVE SEARCH TABLE OF CONTENTS
The SCI Journals Agronomy Journal Crop Science
Vadose Zone Journal Journal of Plant Registrations
Journal of Natural Resources
and Life Sciences Education		 Soil Science Society of America Journal