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Biosorption of chromium(VI) ions by Mowital®B30H resin immobilized activated sludge in a packed bed: comparison with granular activated carbon
Process Biochemistry 38 (2002) 175 /186 www.elsevier.com/locate/procbio
Biosorption of chromium(VI) ions by Mowital† B30H resin immobilized activated sludge in a packed bed: comparison with granular activated carbon Zu¨mriye Aksu *, Ferda Go¨nen, Zafer Demircan Hacettepe University, Chemical Engineering Department, Beytepe, 06532 Ankara, Turkey Received 21 July 2001; received in revised form 24 January 2002; accepted 19 February 2002
Abstract The potential use of Mowital† B30H resin immobilized dried activated sludge as a substitute for granular activated carbon for removing chromium(VI) was examined in a continuous packed bed column. The effect of operating parameters such as flow rate and inlet metal ion concentration on the sorption characteristics of each sorbent was investigated. From the batch system studies the working sorption pH value was determined as 1.0 for both sorbents and packed bed sorption studies were performed at this pH value. The total adsorbed quantities, equilibrium uptakes and total removal percents of chromium(VI) related to the effluent volumes were determined by evaluating the breakthrough curves obtained at different flow rates and different inlet chromium(VI) concentrations for each sorbent. Data confirmed that the total amount of sorbed chromium(VI) and equilibrium chromium(VI) uptake decreased with increasing flow rate and increased with increasing inlet chromium(VI) concentration for both immobilized dried activated sludge and granular activated carbon systems. The results also indicated that the sorption process could only deal with lower flow rates and lower concentrations of chromium(VI) solutions if a high percentage removal was required for extended periods for both sorbents. The suitability of the Freundlich and Langmuir adsorption models to the column equilibrium data was also investigated for each chromium(VI)-sorbent system. The results showed that the equilibrium data for both the sorbents fitted the Langmuir model best within the concentration range studied. # 2002 Elsevier Science Ltd. All rights reserved. Keywords: Biosorption; Chromium(VI); Immobilized activated sludge; Mowital† B30H resin; Granular activated carbon; Packed bed column
1. Introduction The existence of copper, nickel, chromium, cadmium, etc., in wastewaters may cause toxic and harmful effects to living organisms in water and also to the consumers of it. All of the reported metals are widely used materials in daily life. For example, chromium compounds are added to cooling water to inhibit corrosion. They are employed in the manufacture of inks, industrial dyes and paint pigments, in chrome tanning, aluminum anodizing, leather and mining industries and other metal cleaning, plating and electroplating operations. The wastewaters from these industries contain undesirable amounts of chromium(VI) ions. Conventional methods
* Corresponding author. Tel.: /90-312-2977434; fax: /90-3122992124. E-mail address: [email protected] (Z. Aksu).
for removing dissolved heavy metal ions include chemical precipitation, chemical oxidation and reduction, ion exchange, filtration, electrochemical treatment and evaporative recovery. However, these high-technology processes have significant disadvantages, including incomplete metal removal, requirements for expensive equipment and monitoring systems, high reagent or energy requirements or generation of toxic sludge or other waste products that require disposal. New technologies are required that can reduce heavy metal concentrations to environmentally acceptable levels at affordable costs [1 /4]. The process of adsorption is a well established and powerful technique for treating domestic and industrial effluents. Activated carbon is the most widely and effectively used adsorbent. A typical activated carbon particle, whether in a powdered or granular form, has a porous structure consisting of a network of intercon-
0032-9592/02/$ - see front matter # 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 0 3 2 - 9 5 9 2 ( 0 2 ) 0 0 0 5 3 - 5
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nected macropores, mesopores, and micropores that provide a good capacity for the adsorption of heavy metals due to its high surface area. The surface chemistry of activated carbon and the chemical characteristics of adsorbate, such as polarity, ionic nature, functional groups and solubility determine the nature of bonding mechanisms as well as the extent and strength of adsorption. A variety of physicochemical mechanisms/forces, such as van der Waals, H-binding, dipole / dipole interactions, ion exchange, covalent bonding, cation bridging and water bridging, can be responsible for adsorption of heavy metal ions in activated carbon. In spite of these characteristics, activated carbon suffers from a number of disadvantages. It is quite expensive and the higher the quality, the greater the cost. Both chemical and thermal regeneration of spent carbon is expensive, impractical on a large scale and produces additional effluent and results in considerable loss of the adsorbent [5,6]. Biosorption of heavy metals by microbial cells has been recognized as a potential alternative to existing technologies for recovery of heavy metals from industrial waste streams and natural waters. Many aquatic microorganisms such as bacteria, yeast and algae can take up dissolved metals from their surroundings and can be used to remove heavy metal ions successfully. Since of this property, more economical, practical and efficient techniques are being developed for the treatment of industrial wastewaters. Metal ion binding to non-living cells occurs rapidly by cell surface (passive) adsorption and is called ‘biosorption’. The mechanism of binding metal ions by inactivated biomass may depend on the chemical nature of metal ion (species, size, ionic charge), the species of microorganism and environmental conditions (pH, temperature, ionic strength, existence of competing organic or inorganic metal chelators) [2 /4,7 /28]. Activated sludge is a well known biomass used for the purification of some industrial effluents and domestic wastes. Part of the microorganisms over grown in such wastewater systems can be separated and utilized for removal of heavy metal ions as an abundant and cheaper biosorbent. Activated sludge from wastewater systems contains both bacteria and protozoa. The cell wall of bacteria essentially consists of various organic compounds such as carboxyl, acidic polysaccharides, lipids, amino acids and other components. The protozoa are unicellular, motile, relatively large eucaryotic cells that lack cell walls. They can absorb components through their outer membranes that contain proteins and lipids [4,24]. The use of dead microbial cells in biosorption is more advantageous for water treatment in that dead organisms are not affected by toxic wastes, they do not require a continuous supply of nutrients and they can be regenerated and reused for many cycles. However, the use of dead biomass in powdered form has some
problems, such as difficulty in the separation of biomass after biosorption, mass loss after regeneration and low strength and small particle size, which make it difficult to use in column applications [2,3,13,19,27]. To solve these problems, dead biomass can be immobilized in a supporting material. Much of the bioremoval literature deals with artificially immobilized biomass. Researchers have recognized that immobilizing nonliving biomass in a biopolymeric or polymeric matrix may improve biomass performance, biosorption capacity, increase mechanical strength and facilitate separation of biomass from metal-bearing solution. Immobilization also allows higher biomass concentration, resistance to chemical environments and column operations and immobilized systems may be well suited for non-destructive recovery. Indeed, the use of immobilized biomass has a number of major disadvantages. In addition to increasing the cost of biomass pre-treatment, immobilization adversely affects the mass transfer kinetics of metal uptake. When biomass is immobilized the number of binding sites easily accessible to metal ions in solution is greatly reduced since the majority of sites will lie within the bead. So a good support material used for immobilization should be rigid, chemically inert and cheap, should bind cells firmly, should have high loading capacity and should have a loose structure for overcoming diffusion limitations. In most biomass immobilization processes described in the literature, the cells are embedded in a gelatinous biomatrix such as sodium or calcium alginate, agarose or polymers such as polypropylene, polyacrylamide, polysulfone or cross-linked copolymer of ethyl acrylide-ethylene glycol dimethacrylate, poly(hydroxyethyl methacrylate) or silica for selective, efficient and low-cost metal removal [2,3,7 / 13,15,16,18 /20,23,26 /28]. Mowital† B30H resin (a polyvinyl butyral based polymer) is a well known polymer used extensively in painting and coating industries. While it is abundant, extremely cheap than other immobilizing agents, non toxic and chemically inert, more suitable for preparation of porous beads, the solid and rigid support, showing little change in volume under any conditions, with dimensional stability under pressure, produced in the desired size and thus highly competitive with ion exchange resins and activated carbon [30] so it can be used for immobilization of microorganisms for biosorption. Activated sludge has been immobilized in Mowital† B30H resin in order to produce a biosorbent material with proper characteristics for use in typical chemical engineering operations such as fixed beds. It has been used to demonstrate the potential for removal of chromium(VI) ions from waste streams and to compare the chromium(VI) sorption characteristics of granular activated carbon in a continuous packed bed column as a function of flow rate and inlet metal ion concentration and to describe the column chromium(VI)
Z. Aksu et al. / Process Biochemistry 38 (2002) 175 /186
sorption equilibrium by the use of Langmuir and Freundlich models. To our knowledge, this is the first attempt to immobilize biomass in Mowital† B30H resin in a fixed column and to test its use as an biosorbent for the removal of metal ions.
2. Mathematical description
Co Qttotal
(3)
1000
Total removal percent of metal ion (column performance) with respect to flow volume can be also found from the ratio of total adsorbed quantity of metal ion (qtotal) to the total amount of metal ion sent to column (mtotal; Eq. (4)). Total Removal %
For continuous operation with granular activated carbon or immobilized biomass, the most convenient configuration is that of a packed column, much like that used for ion exchange. Continuous packed bed sorption has a number of process engineering advantages including high yield operations and relatively easy scaling up from a laboratory scale procedure. The stages in the separation protocol can also be automated and high degrees of purification can often be achieved in a single step process. A large volume of wastewater can be continuously treated using a defined quantity of sorbent in the column. Reuse of microorganism is also possible. After metal loading the metal may be concentrated in a small volume of solid material or desorbed into a small volume of eluant for recovery, disposal or containment [2,11,13,16,17,19,23,26]. Much of the information needed to evaluate column performance is contained in plots of adsorbed metal ion concentration (Cad /inlet metal ion concentration (Co) /outlet metal ion concentration (C )) or normalized concentration defined as the ratio of effluent metal ion concentration to inlet metal ion concentration (C /Co) as a function of flow time (t) or effluent volume (Veff) calculated from Eq. (1). Veff Qt total
mtotal
(1)
where ttotal and Q are the total flow time (min) and volumetric flow rate (ml min 1). The general position of the breakthrough curve along the volume axis depends on the capacity of the column with respect to the feed concentration and flow rate. The breakthrough curve would be a step function for favorable separations, i.e. there would be an instantaneous jump in the effluent concentration from zero to the feed concentration at the moment the column capacity is reached [2,23,27,29]. The area under the breakthrough curve (A ) can be obtained by integrating the adsorbed concentration (Cad) versus t plot. Total adsorbed metal ion quantity (qtotal) in the column for a given feed concentration and flow rate is calculated from Eq. (2):
177
qtotal mtotal
100
(4)
Adsorption is also a well-known equilibrium separation process for wastewater treatment. Equilibrium studies on adsorption give information about the capacity of the sorbent or the amount required to remove a unit mass of pollutant under the system conditions. Equilibrium metal uptake (qeq) in the column is defined by Eq. (5) as the total amount of metal ion sorbed (qtotal) per g of sorbent (X ) at the end of total flow time. qeq
qtotal
(5)
X
Unadsorbed metal ion concentration at equilibrium in the column can be defined by Eq. (6): Ceq
mtotal qtotal Veff
1000
(6)
The most widely used isotherm equation for modeling the equilibrium is the Langmuir equation which is valid for monolayer sorption on to a surface a finite number of identical sites [2,11,22,25,29]. The empirical Freundlich model based on sorption on a heterogeneous surface is also more widely used but provides no information on the monolayer adsorption capacity, in contrast to the Langmuir model [2,11,22,25,29].
3. Experimental 3.1. Microorganism The waste activated sludge collected from the wastewater treatment system of Meteksan Company/Ankara Paper and Board Mill, Turkey was used in this study. The harvested cells were washed thoroughly with sterile distilled water, centrifuged at 5000 rpm for 5 min, dried at 60 8C for 24 h and then powdered before immobilization.
t ttotal
qtotal
QA 1000
Q 1000
g
Cad dt
(2)
t 0
Total amount of metal ion sent to column (mtotal) is calculated from Eq. (3).
3.2. Granular activated carbon A commercial, 18 /20 mesh (within the particle range of 0.84 /1.00 mm) granular activated carbon (Sigma) with a specific surface area of 600 /650 m2 g 1 was used
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for packed bed column studies. The particles were oven dried at 110 8C for 24 h and stored in a desiccator until used. 3.3. Preparation of Mowital† B30H resin immobilized activated sludge The method used for the immobilization is based on solvent evaporation. For each batch immobilization, 1 g Mowital† B30H (Hoechst) was dissolved in 8 ml of chloroform (Merck). When Mowital† B30H resin was completely dissolved in this solution, 1 g of dried and powdered activated sludge biomass was added to this medium mixed continuously by means of an agitator. 4 g of polyvinyl alcohol (Sigma) used as the stabilizator was solved in 500 ml of distilled water. After that, 0.3 g of sodium dodecyl sulphate (Sigma) used as the emulsifier was added to this mixture and mixed thoroughly. At the last step, the first prepared mixture was added to this mixture. The mixing operation was continued for 6 h at 750 rpm to obtain the porous, uniform and spherical immobilized particles and to evaporate chloroform. Finally, the particles were removed from the immobilization medium and washed with distilled water. These particles had 1.0 mm of mean diameter and contained 50% dried activated sludge by weight [30]. 3.4. Preparation of chromium(VI) solutions Chromium(VI) solutions were prepared by diluting 1 g l 1 of stock chromium(VI) solution which was obtained by dissolving exact quantity of K2Cr2O7 (Merck) in double distilled and de-ionized water. The range in concentrations of chromium(VI) prepared from stock solution varied between 50 and 500 mg l 1. Before the adsorption/biosorption in the continuous column system, the pH of each test solution was adjusted to the required value with diluted and concentrated H2SO4 and NaOH solutions.
uptake value (qeq) was determined as mg of sorbed chromium(VI) per g of sorbent. Continuous fixed bed column studies were performed in a fixed bed mini column reactor with an inside diameter of 0.96 cm, a bed depth of 6 cm and 3 g of immobilized cells containing 1.5 g of dried activated sludge biomass. For the adsorption of chromium(VI) to granular activated carbon, a fixed bed mini column reactor with an inside diameter of 0.96 cm, a bed depth of 10.4 cm and 4.2 g of granular activated carbon was used. The column was preconditioned to pH 1.0 for chromium(VI) (by eluting the column with 1 M H2SO4). The metal ion solution at a known concentration and flow rate was passed continuously through the stationary bed of sorbent. The flow rate was regulated with a variable speed pump by a Masterflex L/S digital drive and easy-load pump head. Samples were taken from the effluent at timed intervals and analyzed for chromium(VI) ions as described below. The experiment was continued until a constant concentration of metal ion was obtained. The experiments showed that no chromium(VI) was taken up by Mowital† B30H beads in the absence of entrapped biomass. For both the sorption systems, the studies were performed at a constant temperature of 25 8C to be representative of environmentally relevant conditions. All the experiments were carried out in duplicates and the average values were used for further calculations. 3.6. Analysis of chromium(VI) ions The concentration of free chromium(VI) ions in the effluent was determined spectrophotometrically by using diphenyl carbazide as the complexing agent. One milliliter of a 0.2% (w/v) of diphenyl carbazide solution prepared in 95% ethyl alcohol and 1 ml of 1/5 H2SO4 solution was added to the sample (1 ml) containing less than 100 mg l 1 of chromium(VI) ions and diluted to 100 ml with double-distilled water. The absorbance of the purple colored solution was read at 540 nm after 10 min [31].
3.5. Sorption studies in the batch and column systems The effect of initial pH on chromium(VI) adsorption/ biosorption by granular activated carbon and Mowital† B30H resin immobilized activated sludge were examined in a batch system. For the batch system studies, 0.05 g of granular activated carbon or immobilized particles containing 0.05 g of dried activated sludge were contacted with the known concentration of 100 ml metal-bearing solution in Erlenmayer flasks at the desired pH. The flasks were agitated on a shaker for 120 h which is more than ample time for sorption equilibrium. Three milliliter samples of solution were taken in definite intervals and analyzed for the residual chromium(VI) ions. The equilibrium chromium(VI)
4. Results and discussion 4.1. Effect of initial pH The most important single parameter influencing the sorption capacity is the pH of adsorption medium. The initial pH of adsorption medium is related to the adsorption mechanisms onto the adsorbent surface from water and reflects the nature of the physicochemical interaction of the species in solution and the adsorptive sites of adsorbent. The variation of equilibrium chromium(VI) uptake with initial pH obtained from batch system studies was given in Fig. 1. It was
Z. Aksu et al. / Process Biochemistry 38 (2002) 175 /186
179
Fig. 1. The effect of initial pH on the equilibrium chromium(VI) uptake in the batch system (Temperature, 25 8C; initial pH 1.0; Co, 100 mg l 1; X , 0.5 g l 1; agitating rate, 150 rpm).
observed that the removal of chromium(VI) ions from aqueous solution was more efficient with decreasing pH and was the greatest at pH 1.0. The equilibrium chromium(VI) uptakes were 72.2 and 61.0 mg g 1 for granular activated carbon and immobilized activated sludge, respectively, at this pH value. Packed bed column studies were also performed at the same pH. pH affects the surface properties of the sorbent, i.e., surface charge of the cells used as biosorbent. The isoelectric point of activated sludge, is usually between pH 1 and 3. From Fig. 1, as the pH is lowered, the overall surface charge on the cells will become positive and the interaction of metal ions such as chromium(VI), in anionic form, with the biosorbent will be primarily electrostatic in nature. At very low pH values, the surface of sorbent would also be surrounded by the hydronium ions which enhance the chromium(VI) interaction with binding sites of the biosorbent by greater attractive forces. As the pH increased, however, the overall surface charge on the cells became negative and biosorption decreased [2,4]. Chromium(VI) anions were bound to positively charged sites on activated carbon surface which is a collection of organic functional groups such as carboxyl, phenolic, alcoholic and quinone groups at pH 1. In addition, hydrogen bonding by the metal anion with the oxygen present on the carbon surface is also quite probable. The adsorption decreased with further increasing of pH due to repulsive forces between the negative surface charge of the activated carbon and anionic chromium(VI) ions [6,32].
4.2. Effect of flow rate In the first stage of removal studies in the continuousflow fixed column with granular activated carbon and Mowital† B30H resin immobilized activated sludge, the flow rate was changed from 0.8 to 3.2 ml min 1 while the inlet chromium(VI) concentration in the feed was held constant at 100 mg l 1 at pH 1.0. The plots of comparative normalized chromium(VI) concentration versus effluent volume at different flow rates are given in Fig. 2 for both sorbents. As the flow rate increased the effluent volume (or indirectly contact time) required for complete column saturation decreased for both sorbents due to less interaction between the metal ion and sorbent. The sorption data were evaluated for each sorbent and total amounts of chromium(VI) sent to column (mtotal), total sorbed quantities of chromium(VI) (qtotal), equilibrium chromium(VI) uptakes (qeq) and total removal percentages of chromium(VI) with respect to the volume of flow passed at different flow rates are presented in Table 1. In general mtotal values increased; qtotal, qeq and total chromium(VI) removal percentage values decreased with increasing flow rate and maximum values were obtained at 0.8 ml min 1 for each sorbent. When compared with the values of equilibrium uptake, total sorbed quantity and removal percentage of immobilized activated sludge beads, granular activated carbon particles showed higher uptake and yield values. The equilibrium uptakes and total chromium(VI) removal percentages were 96.5 mg g 1 and 42.2% and 16.4 mg g 1 and 22.4% by the activated carbon and
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180
Fig. 2. The breakthrough curves obtained at different flow rates for chromium(VI) biosorption/adsorption to Mowital† B30H resin immobilized activated sludge (open symbols) and granular activated carbon (filled symbols). (Temperature, 25 8C; initial pH, 1.0; Co, 100 mg l 1).
immobilized activated sludge beads, respectively, at 0.8 ml min 1 flow rate and 100 mg l 1 inlet chromium(VI) concentration. As may be seen from Fig. 2, the lowest flow rate gave a more gentle breakthrough curve for granular activated carbon. The breakthrough curves became steeper and the breakpoint time decreased with increasing flow rate. Early saturation and lower chromium(VI) removal were observed at the highest flow rate. The two main reasons for this behavior are the unsufficient adsorption equilibrium time in the column and the diffusion limitations of the solute into the pores of granular activated carbon.
For all flow rates studied, after the breakpoint, chromium(VI) removal was maintained over a long period showing approximately constant uptake. Apparently, the intraparticular uptake was still in progress long after the breakpoint. Fig. 2. also illustrated that breakthrough for granular activated carbon occurred much later because of its higher chromium(VI) uptake capacity and, therefore, the saturation time of activated carbon was also longer than that of immobilized biomass. As indicated in Fig. 2, much sharper breakthrough curves were obtained by immobilized activated sludge at
Table 1 The effect of flow rate on total amount of chromium(VI) sent to column (mtotal), total adsorbed quantity of chromium(VI) (qtotal), equilibrium chromium(VI) uptake (qeq) and total removal percentage of chromium(VI) for chromium(VI) adsorption to Mowital† B30H resin immobilized activated sludge and granular activated carbon Q (ml min 1)
t
total
(min)
Veff (ml)
mtotal (mg)
Mowital† B30H resin immobilized activated sludge 0.8 1800 1440 145.8 1.6 660 1056 107.5 3.2 210 672 69.3 Granular activated carbon 0.8 11700 1.6 4950 3.2 1065
9360 7920 3408
961.1 801.7 351.3
Total chromium(VI) removal (%)
qeq (mg g 1)
32.7 20.3 9.6
22.4 18.9 22.4
16.4 10.1 4.8
405.4 221.8 79.2
42.2 27.7 22.5
96.5 52.8 18.9
qtotal (mg)
Z. Aksu et al. / Process Biochemistry 38 (2002) 175 /186
all studied flow rates. The breakpoint time and total adsorbed chromium(VI) quantity also decreased with increasing flow rate. Unlike for chromium(VI) adsorption on granular activated carbon, saturation was reached within 210/1800 min at different flow rates from 0.8 to 3.2 ml min 1 and prolonged exposure time did not increase removal of chromium(VI) by Mowital† B30H resin immobilized activated sludge. At the lowest flow rate of 0.8 ml min 1, relatively higher uptake values were observed for chromium(VI) biosorption to the immobilized activated sludge at the beginning of column operation could be due to the magnitude of concentration driving force. But, as solution continued to flow, the concentration of chromium(VI) in the effluent rapidly increased, the bed became saturated with chromium(VI) and the concentration of solute in the effluent suddenly rose to inlet chromium(VI) concentration. This behavior can be explained that chromium(VI) biosorption by activated sludge biomass is also affected by insufficient residence time of the solute in the column, the diffusion of the solute into the pores of biosorbent and limited number of active sites and ionic groups of biomass for biosorption in the matrix. 4.3. Effect of inlet chromium(VI) concentration In the sorption of chromium(VI) to both the sorbents, a change in inlet chromium(VI) concentration affected the operating characteristics of the packed column. The sorption breakthrough curves obtained by changing inlet chromium(VI) concentration from 50 to 500 mg l 1 at 0.8 ml min 1 flow rate for granular activated carbon and Mowital† B30H resin immobilized activated sludge are given in Fig. 3. The breakpoint time decreased with increasing inlet chromium(VI) concentration as the binding sites became more quickly saturated in both the systems. The equilibrium chromium(VI) uptakes, total sorbed chromium(VI) quantities and chromium(VI) removal percentages of both systems related to the feed chromium(VI) concentration are also compared in Table 2. Although the equilibrium chromium(VI) uptake and amount of total sorbed chromium(VI) increased with increasing inlet chromium(VI) concentration from 50 to 500 mg l 1, the total chromium(VI) removal percentages for both the sorbents showed opposite trend. The removal efficiency was higher at low influent chromium(VI) concentration. Table 2. shows that for the inlet chromium(VI) concentrations of 50 and 500 mg l 1, 55.7 and 30.3 and 25.8 and 17.0% of total chromium(VI) applied to the column was adsorbed by the granular activated carbon and Mowital† B30H resin immobilized activated sludge beads, respectively. The equilibrium chromium(VI) uptake increased from 71.8 to 135.8 mg g 1 for activated carbon and from 15.3 to 18.5 mg g 1 for the immobilized biomass with an increase of inlet chro-
181
mium(VI) concentration from 50 to 500 mg l 1. The equilibrium chromium(VI) uptakes, total sorbed chromium(VI) quantities and adsorption yields were highest for the granular activated carbon, which was expected, because of the greater specific surface area and the porous structure of granular activated carbon. The biosorption of chromium(VI) to the immobilized activated sludge appeared to be significantly lower compared with the adsorption to granular activated carbon. The relatively low chromium(VI) retention for immobilized activated sludge can be attributed to the difference in the surface morphology. Immobilized activated sludge particles do not have many micro or macropores, so its low surface area also results in lower sorption capacity. As seen from Fig. 3 at lower inlet chromium(VI) concentrations breakthrough curves were dispersed, breakthrough occurred very late and the surface of the adsorbent was saturated with chromium(VI) after a longer time for granular activated carbon. At 50 mg l 1 of inlet chromium(VI) concentration and at the lowest flow rate of 0.8 ml min 1, the sorption, although continuous with time, was very efficient in the initial steps of the process. This fact is probably associated to the availability of reactional sites around or inside the granular activated carbon particles, able to capture chromium(VI) and sufficient retention time. In a second stage, with the gradual occupancy of these sites, the uptake become less effective. Even after the breakthrough occurred, the column was still capable of accumulating chromium(VI), although at a progressively lower efficiency. In continuous-flow column, equilibrium chromium(VI) uptake level was higher than the batch uptake value at 100 mg l 1 inlet chromium(VI) concentration and at 0.8 ml min 1 flow rate. The sorption behavior of immobilized cells for chromium(VI) was different from that of chromium(VI) adsorption by granular activated carbon. The beads adsorbed chromium(VI) very rapidly at the beginning of the biosorption but as the experiment continued, the concentration of chromium(VI) in the effluent increased rapidly, the bed became saturated with chromium(VI) and the concentration of solute in the effluent rose to the inlet chromium(VI) concentration Fig. 3 shows that much sharper breakthrough curves so very low chromium(VI) uptakes and removal yields were obtained especially at higher inlet chromium(VI) concentrations for the chromium(VI) biosorption to immobilized activated sludge. At 100 mg l 1 inlet chromium(VI) concentration when compared with the batch system biosorption, it was observed that equilibrium metal uptake level decreased from 61.0 to 16.4 mg g 1 in continuous packed bed studies at 0.8 ml min 1.
Z. Aksu et al. / Process Biochemistry 38 (2002) 175 /186
182
Fig. 3. The breakthrough curves obtained at different inlet chromium(VI) ion concentrations for chromium(VI) biosorption/adsorption to Mowital† B30H resin immobilized activated sludge (open symbols) and granular activated carbon (filled symbols) (Temperature, 25 8C; initial pH 1.0; Q , 0.8 ml min 1).
4.4. The application of the Langmuir and Freundlich models Analysis of the isotherm data is important in order to develop an equation which accurately represents the results of the column and which could be used for
column design purposes. Two kinds of several isotherm equations have been applied for this study, Langmuir and Freundlich isotherms. The linearized Freundlich and Langmuir adsorption isotherms of chromium(VI) obtained at 25 8C were shown in Figs. 4 and 5 for both sorbents. The Freundlich and Langmuir adsorption
Table 2 Total amounts of chromium(VI) sent to column (mtotal), total adsorbed quantities of chromium(VI) (qtotal), equilibrium chromium(VI) uptake (qeq) and total removal percentages of chromium(VI) for chromium(VI) adsorption to Mowital† B30H resin immobilized activated sludge and granular activated carbon at different inlet chromium(VI) concentrations Q (ml min 1)
t
total
(min)
Veff (ml)
Mowital† B30H resin immobilized activated sludge 51.4 2880 2304 101.2 1800 1440 251.5 840 672 503.1 540 432 Granular activated carbon 51.2 13200 102.7 11700 251.5 7800 502.3 4680
10560 9360 6240 3744
mtotal (mg)
qtotal (mg)
Total chromium(VI) removal (%)
qeq (mg g 1)
118.4 145.8 169.0 217.3
30.6 32.7 34.4 36.9
25.8 22.4 20.4 17.0
15.3 16.4 17.2 18.5
541.0 961.1 1569.6 1880.6
301.6 405.3 527.9 570.2
55.7 42.2 33.6 30.3
71.8 96.5 125.7 135.8
Z. Aksu et al. / Process Biochemistry 38 (2002) 175 /186
183
Fig. 4. The linearized Langmuir adsorption isotherms for chromium(VI) biosorption/adsorption to Mowital† B30H resin immobilized activated sludge and granular activated carbon.
constants evaluated from the isotherms with the correlation coefficients were also presented in Table 3. In view of the values of linear regression coefficients in the table, the Langmuir model exhibited a little better fit to the sorption data of both the chromium(VI)-sorbent systems than the Freundlich model in the studied concentration ranges. However, the Freundlich model also seemed to agree well with the experimental data of
chromium(VI) considering that obtained linear regression coefficients are greater than 0.972. An adsorption isotherm is characterized by certain constants the values of which express the surface properties and affinity of the sorbent and can also be used to compare the sorptive capacity of sorbents for the chromium(VI) in the fixed column. KF and n , the Freundlich constants, are indicators of adsorption
Fig. 5. The linearized Freundlich adsorption isotherms for chromium(VI) biosorption/adsorption to Mowital† B30H resin immobilized activated sludge and granular activated carbon.
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Z. Aksu et al. / Process Biochemistry 38 (2002) 175 /186
Table 3 The comparison of the Langmuir and Freundlich adsorption constants evaluated from the Langmuir and Freundlich isotherms for chromium(VI) adsorption to Mowital† B30H resin immobilized activated sludge and granular activated carbon Freundlich model KF [(mg g Immobilized activated sludge Granular activated carbon
1
)(mg l
Langmuir model 1 n
) ]
11.6 35.4
capacity and adsorption intensity, respectively. The closer the n value of Freundlich is to zero the more heterogeneous is the system. From Table 3, the high values of KF showed easy uptake of chromium(VI) ions from wastewater with high adsorptive capacities of these sorbents. Table 3 also indicates that n is greater than unity, indicating that chromium(VI) ions are favorably adsorbed by the sorbents. The obtained data in Table 3 also pointed out that granular activated carbon had a higher biosorption capacity than that of Mowital† B30H resin immobilized activated sludge and represented a greater heterogeneity than immobilized activated sludge. The Langmuir constants, Qo and b have been determined from Ceq/qeq versus Ceq plots obtained for each sorbent and the results are also tabulated in Table 3. The maximum capacity Qo defines the total capacity of each sorbent for chromium(VI) ions. The maximum sorption capacities of granular activated carbon and immobilized biomass were determined as 147.1 and 18.9 mg g 1, respectively, in the packed bed column. The value of Qo appears to be significantly higher for the chromium(VI)-granular activated carbon system in comparison with the uptake of chromium(VI) on immobilized activated sludge. But a large value of b , related to binding energy, implied strong bonding of chromium(VI) on to Mowital† B30H resin immobilized activated sludge. The Langmuir model makes several assumptions, such as monolayer coverage and constant adsorption energy while the Freundlich equation deals with heterogeneous surface adsorption. The applicability of both Langmuir and Freundlich isotherms to the chromium(VI)-granular activated carbon and immobilized activated sludge systems implies that both monolayer adsorption and heterogeneous surface conditions exist under the experimental conditions used. It was shown that Mowital† B30H resin immobilized dried activated sludge can be used as an adsorbent in the same way as granular activated carbon in a continuous packed bed column to separate chromium(VI) ions from aqueous solutions. The immobilized biomass was preferable over native biomass in packed column operations as the dried, powdered biomass was not rigid and strong enough to use in downtlow packed bed column operations and presents an unacceptable pressure drop
2
n
R
13.2 4.2
0.988 0.972
Qo (mg g 1)
b (l mg 1)
R2
18.9 147.1
0.0799 0.0370
0.999 0.999
to flow. Diffusion limitations causing a decrease in biosorption capacity is one of the major disadvantages of immobilized systems. Considerable research has been focused on reducing diffusion limitations, but no generally satisfactory solution has been found. The major requirement is to develop immobilization systems which are more open to hydraulic flow but still provide large contact areas and which have good mechanical properties for large-scale systems. The beads must be mechanically strong, chemically resistant, inexpensive and easy to regenerate and they must not swell and contract during use as do many ion exchange resins. The production of the beads must be examined in the hope of increasing the yield in the desired size range, improving porosity, and increasing capacity. The performance of polymer, which have been used to immobilize activated sludge must be significantly improved by modifying bead fabrication procedure to improve surface and/or interior properties. Mowital† B30H resin immobilized activated sludge can be used in continuous packed bed columns for chromium(VI) removal from waste waters because they satisfy most of the specifications defined above. They can be hardened, formed into beads and used highly efficiently in packed-bed bioreactors that resemble the column processes used with ion exchange resins and activated carbon systems. This suggests that if the porosity of the beads could be increased by modification of the biosorbent preparation method, then the contact time necessary for maximal biosorption could be significantly reduced.
5. Conclusion The breakthrough curves for column sorption of chromium(VI) from dilute solutions using granular activated carbon and Mowital† B30H resin immobilized activated sludge have been measured at 25 8C. The obtained results showed that the sorption of chromium(VI) is dependent on the flow rate, the inlet chromium(VI) concentration and time and these parameters affect the saturation capacity of both sorbents directly. The breakpoint time and total sorbed chromium(VI) quantity and chromium(VI) removal yield decreased with increasing flow rate and chromium(VI) concentration in both systems. Although granular
Z. Aksu et al. / Process Biochemistry 38 (2002) 175 /186
activated carbon had a higher adsorption capacity for chromium(VI) (147.1 mg g 1), the experimental results indicated that dried activated sludge has also a considerable potential for the removal of chromium(VI) over a wide range of chromium(VI) concentration (18.9 mg g 1). Consequently, it can be said that for a given type of immobilized biosorbent, the saturation capacity is greater under conditions of lower concentrations of chromium(VI) and lower flow rates. By adjusting the operating characteristics of the packed column, for example the flow rate, inlet chromium(VI) concentration, particle size, biomass quantity very rapid and efficient chromium(VI) uptake can be achieved for the system. The Freundlich and Langmuir adsorption models were used for the mathematical description of the adsorption of chromium(VI) ions to granular activated carbon and Mowital† B30H resin immobilized activated sludge in a continuous packed bed column and the isotherm constants were determined to compare the adsorption capacity of granular activated carbon and Mowital† B30H resin immobilized activated sludge for chromium(VI) ions. It was seen that the adsorption equilibrium data fitted very well to Langmuir adsorption model. It seems that although the immobilization process decreased the metal biosorption properties of the biomass, the use of Mowital† B30H immobilized activated sludge technology for wastewater treatment will offer an attractive alternative for various industries. If processing time is important, recycle or linked-column series may be also employed in order to obtain higher chromium(VI) removal yields at higher flow rates and higher inlet chromium(VI) concentrations. If the quantity of biomass per unit volume of the column is increased (decrease of the void fraction of the column) chromium(VI) uptake also will be higher for concentrated chromium(VI) solutions because of the increasing of active adsorption sides.
Appendix A: Nomenclature Co
inlet (feed) chromium(VI) concentration in the column (mg l 1) C effluent chromium(VI) concentration in the column (mg l 1) Cad( /Co / adsorbed chromium(VI) concentration in C) the column (mg l 1) Ceq nonadsorbed chromium(VI) concentration at equilibrium in the column (mg l 1) mtotal total amount of chromium(VI) ions sent to column (mg) qeq equilibrium chromium(VI) uptake in the column or in the batch system (mg g 1)
qtotal Q t ttotal Veff X
185
total adsorbed quantity of chromium(VI) in the column (mg) flow rate ( ml min 1) flow time (min) total flow time (min) effluent volume (ml) amount of sorbent in the column (g) or sorbent concentration in the batch system (g l 1)
References [1] Patterson JW. Wastewater treatment technology. USA: Ann Arbor Science Pub. Inc, 1977. [2] Aksu Z. Biosorption of heavy metals by microalgae in batch and continuous systems. In: Tam N, Wong Y-S, editors. Algae for waste water treatment. Germany: Springer-Verlag and Landes Bioscience, 1998:37 /53. [3] Wilde EW, Benemann JR. Bioremoval of heavy metals by the use of microalgae. Biotech Adv 1993;11:781 /812. [4] Aksu Z, Kutsal T, Gu¨n S, Haciosmanoglu N, Gholaminejad M. Investigation of biosorption of Cu(II), Ni(II) and Cr(VI) ions to activated sludge bacteria. Environ Tech 1991;12:915 /21. [5] Perrich JR. Activated carbon adsorption for wastewater treatment. USA: CRS Press, 1981. [6] Aksu Z, Akpinar D. Simultaneous adsorption of phenol and chromium(VI) from binary mixtures onto powdered activated carbon. J Environ Sci Health 2000;A35:379 /405. [7] Darnall DW, Greene B, Hosea JM, McPherson RA, Henzyl M, Alexander MD. Recovery of heavy metals by immobilized algae. In: Thomson R, editor. Trace metal removal from aqueous solutions. USA: Whitstable Litho Ltd, 1986:1 /24. [8] Huang C-P, Huang C-P, Morehart AL. The removal of Cu(II) from dilute aqueous solutions by Saccharomyces cerevisiae . Wat Res 1990;24:433 /9. [9] Macaskie LE. An immobilized cell bioprocess for the removal of heavy metals from aqueous flows. J Chem Tech Biotech 1990;49:357 /79. [10] da Costa ACA, Leite SGF. Metals biosorption by sodium alginate immobilized Chlorella homosphaera cells. Biotechnol Lett 1991;13:559 /62. [11] Zhou JL, Kiff J. The uptake of copper from aqueous solution by immobilized fungal biomass. J Chem Tech Biotech 1991;52:317 / 30. [12] Trujillo EM, Jeffers TH, Ferguson C, Stevenson HQ. Mathematically modeling the removal of heavy metals from wastewater using immobilized biomass. Environ Sci Tech 1991;25:1559 /65. [13] Tzesos M, Deutschmann AA. The use of a mathematical model for the study of the important parameters in immobilized biomass biosorption. J Chem Tech Biotech 1992;53:1 /12. [14] Holan ZR, Volesky B, Prasetyo I. Biosorption of cadmium by biomass of marine algae. Biotech Bioeng 1993;41:819 /25. [15] Nikolaus FE, Nestle I, Kimmich R. NMR imaging of heavy metal adsorption in alginate, immobilized cells, and kombu algal biosorbents. Biotechnol Bioeng 1996;51:538 /43. [16] Spinti M, Zhuang H, Trujillo ER. Evaluation of immobilized biomass beads for removing heavy metals from wastewaters. Water Environ Res 1996;67:943 /52. [17] Aksu Z, Kutsal T. Determination of kinetic parameters in the biosorption of copper(II) on Cladophora sp . in a packed bed column reactor. Process Biochem 1998;33:7 /13.
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[18] Di Simine D, Finoli C, Vecchio A, Andreoni V. Metal ion accumulation by immobilized cells of Brevibacterium sp . J Ind Microbiol Biotech 1998;20:116 /20. [19] Brady JM, Tobin JM, Roux J-C. Continuous fixed bed biosorption of Cu2 ions: application of a simple two parameter mathematical model. J Chem Tech Biotech 1999;74:71 /7. [20] Veglio F, Beolchini F, Boaro M, Lora S, Corain B, Toro L. Poly(hydroxyethyl methacrylate) resins as supports for copper(II) biosorption with Arthrobacter sp. matrix nanomorphology and sorption performances. Process Biochem 1999;34:367 /73. [21] McKay G, Ho YS, Ng JCY. Biosorption of copper from waste waters: a review. Sep Pur Methods 1999;28:87 /125. ¨ ztu¨rk A, Kutsal T. A comparative study [22] Do¨nmez G, Aksu Z, .O on heavy metal biosorption characteristics of some algae. Process Biochem 1999;34:885 /92. [23] Aksu Z, Egretli G, Kutsal T. A comparative study for the biosorption characteristics of chromium(VI) on Ca-alginate, agarose and immobilized C. vulgaris in a continuous packed bed column. J Environ Sci Health 1999;A34:295 /316. [24] Aksu Z, Akpinar D. Modelling of simultaneous biosorption of phenol and nickel(II) onto dried aerobic activated sludge. Sep Pur Tech 2000;21:87 /99.
[25] Sag Y, Atac¸oglu I, Kutsal T. Equilibrium parameters for the single and multicomponent biosorption of Cr(VI) and Fe(III) ions on R. arrhizus in a packed column. Hydrometallurgy 2000;55:165 /79. [26] Volesky B. Detoxification of metal-bearing effluents:biosorption for the next century. Hydrometallurgy 2001;59:203 /16. [27] Yan G, Viraraghavan T. Heavy metal removal in a biosorption column by immobilized M. rouxii biomass. Biores Tech 2001;78:243 /9. [28] Jang LK, Lopez SL, Eastmen SL, Pryfogle P. Recovery of copper and cobalt by biopolymer gels. Biotech Bioeng 1991;37:266 /73. [29] Ruthven DM. Adsorption kinetics. In: Rodrigues AE, et al, editor. Adsorption: science and technology (Nato ASI Series, Series E: Applied Sciences). The Netherlands: Kluwer Academic Publishers, 1989:87 /114. [30] Tanyola/c+/ D, Ozdural A. A new low cost magnetic material: magnetic polyvinyl butyral microbeads. React Funct Polym 2000;43:279 /86. [31] Snell FD, Snell CT. Colorimetric methods of analysis, vol. 2, 3rd ed.. Toronto: D. Van Nostrand Company, 1959. [32] Mohan D, Chander S. Single component and multi-component adsorption of metal ions by activated carbons. Colloids Surf A Physicochem Eng Aspects 2001;177:183 /96.
Biosorption of chromium(VI) ions by Mowital† B30H resin immobilized activated sludge in a packed bed: comparison with granular activated carbon Zu¨mriye Aksu *, Ferda Go¨nen, Zafer Demircan Hacettepe University, Chemical Engineering Department, Beytepe, 06532 Ankara, Turkey Received 21 July 2001; received in revised form 24 January 2002; accepted 19 February 2002
Abstract The potential use of Mowital† B30H resin immobilized dried activated sludge as a substitute for granular activated carbon for removing chromium(VI) was examined in a continuous packed bed column. The effect of operating parameters such as flow rate and inlet metal ion concentration on the sorption characteristics of each sorbent was investigated. From the batch system studies the working sorption pH value was determined as 1.0 for both sorbents and packed bed sorption studies were performed at this pH value. The total adsorbed quantities, equilibrium uptakes and total removal percents of chromium(VI) related to the effluent volumes were determined by evaluating the breakthrough curves obtained at different flow rates and different inlet chromium(VI) concentrations for each sorbent. Data confirmed that the total amount of sorbed chromium(VI) and equilibrium chromium(VI) uptake decreased with increasing flow rate and increased with increasing inlet chromium(VI) concentration for both immobilized dried activated sludge and granular activated carbon systems. The results also indicated that the sorption process could only deal with lower flow rates and lower concentrations of chromium(VI) solutions if a high percentage removal was required for extended periods for both sorbents. The suitability of the Freundlich and Langmuir adsorption models to the column equilibrium data was also investigated for each chromium(VI)-sorbent system. The results showed that the equilibrium data for both the sorbents fitted the Langmuir model best within the concentration range studied. # 2002 Elsevier Science Ltd. All rights reserved. Keywords: Biosorption; Chromium(VI); Immobilized activated sludge; Mowital† B30H resin; Granular activated carbon; Packed bed column
1. Introduction The existence of copper, nickel, chromium, cadmium, etc., in wastewaters may cause toxic and harmful effects to living organisms in water and also to the consumers of it. All of the reported metals are widely used materials in daily life. For example, chromium compounds are added to cooling water to inhibit corrosion. They are employed in the manufacture of inks, industrial dyes and paint pigments, in chrome tanning, aluminum anodizing, leather and mining industries and other metal cleaning, plating and electroplating operations. The wastewaters from these industries contain undesirable amounts of chromium(VI) ions. Conventional methods
* Corresponding author. Tel.: /90-312-2977434; fax: /90-3122992124. E-mail address: [email protected] (Z. Aksu).
for removing dissolved heavy metal ions include chemical precipitation, chemical oxidation and reduction, ion exchange, filtration, electrochemical treatment and evaporative recovery. However, these high-technology processes have significant disadvantages, including incomplete metal removal, requirements for expensive equipment and monitoring systems, high reagent or energy requirements or generation of toxic sludge or other waste products that require disposal. New technologies are required that can reduce heavy metal concentrations to environmentally acceptable levels at affordable costs [1 /4]. The process of adsorption is a well established and powerful technique for treating domestic and industrial effluents. Activated carbon is the most widely and effectively used adsorbent. A typical activated carbon particle, whether in a powdered or granular form, has a porous structure consisting of a network of intercon-
0032-9592/02/$ - see front matter # 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 0 3 2 - 9 5 9 2 ( 0 2 ) 0 0 0 5 3 - 5
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nected macropores, mesopores, and micropores that provide a good capacity for the adsorption of heavy metals due to its high surface area. The surface chemistry of activated carbon and the chemical characteristics of adsorbate, such as polarity, ionic nature, functional groups and solubility determine the nature of bonding mechanisms as well as the extent and strength of adsorption. A variety of physicochemical mechanisms/forces, such as van der Waals, H-binding, dipole / dipole interactions, ion exchange, covalent bonding, cation bridging and water bridging, can be responsible for adsorption of heavy metal ions in activated carbon. In spite of these characteristics, activated carbon suffers from a number of disadvantages. It is quite expensive and the higher the quality, the greater the cost. Both chemical and thermal regeneration of spent carbon is expensive, impractical on a large scale and produces additional effluent and results in considerable loss of the adsorbent [5,6]. Biosorption of heavy metals by microbial cells has been recognized as a potential alternative to existing technologies for recovery of heavy metals from industrial waste streams and natural waters. Many aquatic microorganisms such as bacteria, yeast and algae can take up dissolved metals from their surroundings and can be used to remove heavy metal ions successfully. Since of this property, more economical, practical and efficient techniques are being developed for the treatment of industrial wastewaters. Metal ion binding to non-living cells occurs rapidly by cell surface (passive) adsorption and is called ‘biosorption’. The mechanism of binding metal ions by inactivated biomass may depend on the chemical nature of metal ion (species, size, ionic charge), the species of microorganism and environmental conditions (pH, temperature, ionic strength, existence of competing organic or inorganic metal chelators) [2 /4,7 /28]. Activated sludge is a well known biomass used for the purification of some industrial effluents and domestic wastes. Part of the microorganisms over grown in such wastewater systems can be separated and utilized for removal of heavy metal ions as an abundant and cheaper biosorbent. Activated sludge from wastewater systems contains both bacteria and protozoa. The cell wall of bacteria essentially consists of various organic compounds such as carboxyl, acidic polysaccharides, lipids, amino acids and other components. The protozoa are unicellular, motile, relatively large eucaryotic cells that lack cell walls. They can absorb components through their outer membranes that contain proteins and lipids [4,24]. The use of dead microbial cells in biosorption is more advantageous for water treatment in that dead organisms are not affected by toxic wastes, they do not require a continuous supply of nutrients and they can be regenerated and reused for many cycles. However, the use of dead biomass in powdered form has some
problems, such as difficulty in the separation of biomass after biosorption, mass loss after regeneration and low strength and small particle size, which make it difficult to use in column applications [2,3,13,19,27]. To solve these problems, dead biomass can be immobilized in a supporting material. Much of the bioremoval literature deals with artificially immobilized biomass. Researchers have recognized that immobilizing nonliving biomass in a biopolymeric or polymeric matrix may improve biomass performance, biosorption capacity, increase mechanical strength and facilitate separation of biomass from metal-bearing solution. Immobilization also allows higher biomass concentration, resistance to chemical environments and column operations and immobilized systems may be well suited for non-destructive recovery. Indeed, the use of immobilized biomass has a number of major disadvantages. In addition to increasing the cost of biomass pre-treatment, immobilization adversely affects the mass transfer kinetics of metal uptake. When biomass is immobilized the number of binding sites easily accessible to metal ions in solution is greatly reduced since the majority of sites will lie within the bead. So a good support material used for immobilization should be rigid, chemically inert and cheap, should bind cells firmly, should have high loading capacity and should have a loose structure for overcoming diffusion limitations. In most biomass immobilization processes described in the literature, the cells are embedded in a gelatinous biomatrix such as sodium or calcium alginate, agarose or polymers such as polypropylene, polyacrylamide, polysulfone or cross-linked copolymer of ethyl acrylide-ethylene glycol dimethacrylate, poly(hydroxyethyl methacrylate) or silica for selective, efficient and low-cost metal removal [2,3,7 / 13,15,16,18 /20,23,26 /28]. Mowital† B30H resin (a polyvinyl butyral based polymer) is a well known polymer used extensively in painting and coating industries. While it is abundant, extremely cheap than other immobilizing agents, non toxic and chemically inert, more suitable for preparation of porous beads, the solid and rigid support, showing little change in volume under any conditions, with dimensional stability under pressure, produced in the desired size and thus highly competitive with ion exchange resins and activated carbon [30] so it can be used for immobilization of microorganisms for biosorption. Activated sludge has been immobilized in Mowital† B30H resin in order to produce a biosorbent material with proper characteristics for use in typical chemical engineering operations such as fixed beds. It has been used to demonstrate the potential for removal of chromium(VI) ions from waste streams and to compare the chromium(VI) sorption characteristics of granular activated carbon in a continuous packed bed column as a function of flow rate and inlet metal ion concentration and to describe the column chromium(VI)
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sorption equilibrium by the use of Langmuir and Freundlich models. To our knowledge, this is the first attempt to immobilize biomass in Mowital† B30H resin in a fixed column and to test its use as an biosorbent for the removal of metal ions.
2. Mathematical description
Co Qttotal
(3)
1000
Total removal percent of metal ion (column performance) with respect to flow volume can be also found from the ratio of total adsorbed quantity of metal ion (qtotal) to the total amount of metal ion sent to column (mtotal; Eq. (4)). Total Removal %
For continuous operation with granular activated carbon or immobilized biomass, the most convenient configuration is that of a packed column, much like that used for ion exchange. Continuous packed bed sorption has a number of process engineering advantages including high yield operations and relatively easy scaling up from a laboratory scale procedure. The stages in the separation protocol can also be automated and high degrees of purification can often be achieved in a single step process. A large volume of wastewater can be continuously treated using a defined quantity of sorbent in the column. Reuse of microorganism is also possible. After metal loading the metal may be concentrated in a small volume of solid material or desorbed into a small volume of eluant for recovery, disposal or containment [2,11,13,16,17,19,23,26]. Much of the information needed to evaluate column performance is contained in plots of adsorbed metal ion concentration (Cad /inlet metal ion concentration (Co) /outlet metal ion concentration (C )) or normalized concentration defined as the ratio of effluent metal ion concentration to inlet metal ion concentration (C /Co) as a function of flow time (t) or effluent volume (Veff) calculated from Eq. (1). Veff Qt total
mtotal
(1)
where ttotal and Q are the total flow time (min) and volumetric flow rate (ml min 1). The general position of the breakthrough curve along the volume axis depends on the capacity of the column with respect to the feed concentration and flow rate. The breakthrough curve would be a step function for favorable separations, i.e. there would be an instantaneous jump in the effluent concentration from zero to the feed concentration at the moment the column capacity is reached [2,23,27,29]. The area under the breakthrough curve (A ) can be obtained by integrating the adsorbed concentration (Cad) versus t plot. Total adsorbed metal ion quantity (qtotal) in the column for a given feed concentration and flow rate is calculated from Eq. (2):
177
qtotal mtotal
100
(4)
Adsorption is also a well-known equilibrium separation process for wastewater treatment. Equilibrium studies on adsorption give information about the capacity of the sorbent or the amount required to remove a unit mass of pollutant under the system conditions. Equilibrium metal uptake (qeq) in the column is defined by Eq. (5) as the total amount of metal ion sorbed (qtotal) per g of sorbent (X ) at the end of total flow time. qeq
qtotal
(5)
X
Unadsorbed metal ion concentration at equilibrium in the column can be defined by Eq. (6): Ceq
mtotal qtotal Veff
1000
(6)
The most widely used isotherm equation for modeling the equilibrium is the Langmuir equation which is valid for monolayer sorption on to a surface a finite number of identical sites [2,11,22,25,29]. The empirical Freundlich model based on sorption on a heterogeneous surface is also more widely used but provides no information on the monolayer adsorption capacity, in contrast to the Langmuir model [2,11,22,25,29].
3. Experimental 3.1. Microorganism The waste activated sludge collected from the wastewater treatment system of Meteksan Company/Ankara Paper and Board Mill, Turkey was used in this study. The harvested cells were washed thoroughly with sterile distilled water, centrifuged at 5000 rpm for 5 min, dried at 60 8C for 24 h and then powdered before immobilization.
t ttotal
qtotal
QA 1000
Q 1000
g
Cad dt
(2)
t 0
Total amount of metal ion sent to column (mtotal) is calculated from Eq. (3).
3.2. Granular activated carbon A commercial, 18 /20 mesh (within the particle range of 0.84 /1.00 mm) granular activated carbon (Sigma) with a specific surface area of 600 /650 m2 g 1 was used
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for packed bed column studies. The particles were oven dried at 110 8C for 24 h and stored in a desiccator until used. 3.3. Preparation of Mowital† B30H resin immobilized activated sludge The method used for the immobilization is based on solvent evaporation. For each batch immobilization, 1 g Mowital† B30H (Hoechst) was dissolved in 8 ml of chloroform (Merck). When Mowital† B30H resin was completely dissolved in this solution, 1 g of dried and powdered activated sludge biomass was added to this medium mixed continuously by means of an agitator. 4 g of polyvinyl alcohol (Sigma) used as the stabilizator was solved in 500 ml of distilled water. After that, 0.3 g of sodium dodecyl sulphate (Sigma) used as the emulsifier was added to this mixture and mixed thoroughly. At the last step, the first prepared mixture was added to this mixture. The mixing operation was continued for 6 h at 750 rpm to obtain the porous, uniform and spherical immobilized particles and to evaporate chloroform. Finally, the particles were removed from the immobilization medium and washed with distilled water. These particles had 1.0 mm of mean diameter and contained 50% dried activated sludge by weight [30]. 3.4. Preparation of chromium(VI) solutions Chromium(VI) solutions were prepared by diluting 1 g l 1 of stock chromium(VI) solution which was obtained by dissolving exact quantity of K2Cr2O7 (Merck) in double distilled and de-ionized water. The range in concentrations of chromium(VI) prepared from stock solution varied between 50 and 500 mg l 1. Before the adsorption/biosorption in the continuous column system, the pH of each test solution was adjusted to the required value with diluted and concentrated H2SO4 and NaOH solutions.
uptake value (qeq) was determined as mg of sorbed chromium(VI) per g of sorbent. Continuous fixed bed column studies were performed in a fixed bed mini column reactor with an inside diameter of 0.96 cm, a bed depth of 6 cm and 3 g of immobilized cells containing 1.5 g of dried activated sludge biomass. For the adsorption of chromium(VI) to granular activated carbon, a fixed bed mini column reactor with an inside diameter of 0.96 cm, a bed depth of 10.4 cm and 4.2 g of granular activated carbon was used. The column was preconditioned to pH 1.0 for chromium(VI) (by eluting the column with 1 M H2SO4). The metal ion solution at a known concentration and flow rate was passed continuously through the stationary bed of sorbent. The flow rate was regulated with a variable speed pump by a Masterflex L/S digital drive and easy-load pump head. Samples were taken from the effluent at timed intervals and analyzed for chromium(VI) ions as described below. The experiment was continued until a constant concentration of metal ion was obtained. The experiments showed that no chromium(VI) was taken up by Mowital† B30H beads in the absence of entrapped biomass. For both the sorption systems, the studies were performed at a constant temperature of 25 8C to be representative of environmentally relevant conditions. All the experiments were carried out in duplicates and the average values were used for further calculations. 3.6. Analysis of chromium(VI) ions The concentration of free chromium(VI) ions in the effluent was determined spectrophotometrically by using diphenyl carbazide as the complexing agent. One milliliter of a 0.2% (w/v) of diphenyl carbazide solution prepared in 95% ethyl alcohol and 1 ml of 1/5 H2SO4 solution was added to the sample (1 ml) containing less than 100 mg l 1 of chromium(VI) ions and diluted to 100 ml with double-distilled water. The absorbance of the purple colored solution was read at 540 nm after 10 min [31].
3.5. Sorption studies in the batch and column systems The effect of initial pH on chromium(VI) adsorption/ biosorption by granular activated carbon and Mowital† B30H resin immobilized activated sludge were examined in a batch system. For the batch system studies, 0.05 g of granular activated carbon or immobilized particles containing 0.05 g of dried activated sludge were contacted with the known concentration of 100 ml metal-bearing solution in Erlenmayer flasks at the desired pH. The flasks were agitated on a shaker for 120 h which is more than ample time for sorption equilibrium. Three milliliter samples of solution were taken in definite intervals and analyzed for the residual chromium(VI) ions. The equilibrium chromium(VI)
4. Results and discussion 4.1. Effect of initial pH The most important single parameter influencing the sorption capacity is the pH of adsorption medium. The initial pH of adsorption medium is related to the adsorption mechanisms onto the adsorbent surface from water and reflects the nature of the physicochemical interaction of the species in solution and the adsorptive sites of adsorbent. The variation of equilibrium chromium(VI) uptake with initial pH obtained from batch system studies was given in Fig. 1. It was
Z. Aksu et al. / Process Biochemistry 38 (2002) 175 /186
179
Fig. 1. The effect of initial pH on the equilibrium chromium(VI) uptake in the batch system (Temperature, 25 8C; initial pH 1.0; Co, 100 mg l 1; X , 0.5 g l 1; agitating rate, 150 rpm).
observed that the removal of chromium(VI) ions from aqueous solution was more efficient with decreasing pH and was the greatest at pH 1.0. The equilibrium chromium(VI) uptakes were 72.2 and 61.0 mg g 1 for granular activated carbon and immobilized activated sludge, respectively, at this pH value. Packed bed column studies were also performed at the same pH. pH affects the surface properties of the sorbent, i.e., surface charge of the cells used as biosorbent. The isoelectric point of activated sludge, is usually between pH 1 and 3. From Fig. 1, as the pH is lowered, the overall surface charge on the cells will become positive and the interaction of metal ions such as chromium(VI), in anionic form, with the biosorbent will be primarily electrostatic in nature. At very low pH values, the surface of sorbent would also be surrounded by the hydronium ions which enhance the chromium(VI) interaction with binding sites of the biosorbent by greater attractive forces. As the pH increased, however, the overall surface charge on the cells became negative and biosorption decreased [2,4]. Chromium(VI) anions were bound to positively charged sites on activated carbon surface which is a collection of organic functional groups such as carboxyl, phenolic, alcoholic and quinone groups at pH 1. In addition, hydrogen bonding by the metal anion with the oxygen present on the carbon surface is also quite probable. The adsorption decreased with further increasing of pH due to repulsive forces between the negative surface charge of the activated carbon and anionic chromium(VI) ions [6,32].
4.2. Effect of flow rate In the first stage of removal studies in the continuousflow fixed column with granular activated carbon and Mowital† B30H resin immobilized activated sludge, the flow rate was changed from 0.8 to 3.2 ml min 1 while the inlet chromium(VI) concentration in the feed was held constant at 100 mg l 1 at pH 1.0. The plots of comparative normalized chromium(VI) concentration versus effluent volume at different flow rates are given in Fig. 2 for both sorbents. As the flow rate increased the effluent volume (or indirectly contact time) required for complete column saturation decreased for both sorbents due to less interaction between the metal ion and sorbent. The sorption data were evaluated for each sorbent and total amounts of chromium(VI) sent to column (mtotal), total sorbed quantities of chromium(VI) (qtotal), equilibrium chromium(VI) uptakes (qeq) and total removal percentages of chromium(VI) with respect to the volume of flow passed at different flow rates are presented in Table 1. In general mtotal values increased; qtotal, qeq and total chromium(VI) removal percentage values decreased with increasing flow rate and maximum values were obtained at 0.8 ml min 1 for each sorbent. When compared with the values of equilibrium uptake, total sorbed quantity and removal percentage of immobilized activated sludge beads, granular activated carbon particles showed higher uptake and yield values. The equilibrium uptakes and total chromium(VI) removal percentages were 96.5 mg g 1 and 42.2% and 16.4 mg g 1 and 22.4% by the activated carbon and
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180
Fig. 2. The breakthrough curves obtained at different flow rates for chromium(VI) biosorption/adsorption to Mowital† B30H resin immobilized activated sludge (open symbols) and granular activated carbon (filled symbols). (Temperature, 25 8C; initial pH, 1.0; Co, 100 mg l 1).
immobilized activated sludge beads, respectively, at 0.8 ml min 1 flow rate and 100 mg l 1 inlet chromium(VI) concentration. As may be seen from Fig. 2, the lowest flow rate gave a more gentle breakthrough curve for granular activated carbon. The breakthrough curves became steeper and the breakpoint time decreased with increasing flow rate. Early saturation and lower chromium(VI) removal were observed at the highest flow rate. The two main reasons for this behavior are the unsufficient adsorption equilibrium time in the column and the diffusion limitations of the solute into the pores of granular activated carbon.
For all flow rates studied, after the breakpoint, chromium(VI) removal was maintained over a long period showing approximately constant uptake. Apparently, the intraparticular uptake was still in progress long after the breakpoint. Fig. 2. also illustrated that breakthrough for granular activated carbon occurred much later because of its higher chromium(VI) uptake capacity and, therefore, the saturation time of activated carbon was also longer than that of immobilized biomass. As indicated in Fig. 2, much sharper breakthrough curves were obtained by immobilized activated sludge at
Table 1 The effect of flow rate on total amount of chromium(VI) sent to column (mtotal), total adsorbed quantity of chromium(VI) (qtotal), equilibrium chromium(VI) uptake (qeq) and total removal percentage of chromium(VI) for chromium(VI) adsorption to Mowital† B30H resin immobilized activated sludge and granular activated carbon Q (ml min 1)
t
total
(min)
Veff (ml)
mtotal (mg)
Mowital† B30H resin immobilized activated sludge 0.8 1800 1440 145.8 1.6 660 1056 107.5 3.2 210 672 69.3 Granular activated carbon 0.8 11700 1.6 4950 3.2 1065
9360 7920 3408
961.1 801.7 351.3
Total chromium(VI) removal (%)
qeq (mg g 1)
32.7 20.3 9.6
22.4 18.9 22.4
16.4 10.1 4.8
405.4 221.8 79.2
42.2 27.7 22.5
96.5 52.8 18.9
qtotal (mg)
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all studied flow rates. The breakpoint time and total adsorbed chromium(VI) quantity also decreased with increasing flow rate. Unlike for chromium(VI) adsorption on granular activated carbon, saturation was reached within 210/1800 min at different flow rates from 0.8 to 3.2 ml min 1 and prolonged exposure time did not increase removal of chromium(VI) by Mowital† B30H resin immobilized activated sludge. At the lowest flow rate of 0.8 ml min 1, relatively higher uptake values were observed for chromium(VI) biosorption to the immobilized activated sludge at the beginning of column operation could be due to the magnitude of concentration driving force. But, as solution continued to flow, the concentration of chromium(VI) in the effluent rapidly increased, the bed became saturated with chromium(VI) and the concentration of solute in the effluent suddenly rose to inlet chromium(VI) concentration. This behavior can be explained that chromium(VI) biosorption by activated sludge biomass is also affected by insufficient residence time of the solute in the column, the diffusion of the solute into the pores of biosorbent and limited number of active sites and ionic groups of biomass for biosorption in the matrix. 4.3. Effect of inlet chromium(VI) concentration In the sorption of chromium(VI) to both the sorbents, a change in inlet chromium(VI) concentration affected the operating characteristics of the packed column. The sorption breakthrough curves obtained by changing inlet chromium(VI) concentration from 50 to 500 mg l 1 at 0.8 ml min 1 flow rate for granular activated carbon and Mowital† B30H resin immobilized activated sludge are given in Fig. 3. The breakpoint time decreased with increasing inlet chromium(VI) concentration as the binding sites became more quickly saturated in both the systems. The equilibrium chromium(VI) uptakes, total sorbed chromium(VI) quantities and chromium(VI) removal percentages of both systems related to the feed chromium(VI) concentration are also compared in Table 2. Although the equilibrium chromium(VI) uptake and amount of total sorbed chromium(VI) increased with increasing inlet chromium(VI) concentration from 50 to 500 mg l 1, the total chromium(VI) removal percentages for both the sorbents showed opposite trend. The removal efficiency was higher at low influent chromium(VI) concentration. Table 2. shows that for the inlet chromium(VI) concentrations of 50 and 500 mg l 1, 55.7 and 30.3 and 25.8 and 17.0% of total chromium(VI) applied to the column was adsorbed by the granular activated carbon and Mowital† B30H resin immobilized activated sludge beads, respectively. The equilibrium chromium(VI) uptake increased from 71.8 to 135.8 mg g 1 for activated carbon and from 15.3 to 18.5 mg g 1 for the immobilized biomass with an increase of inlet chro-
181
mium(VI) concentration from 50 to 500 mg l 1. The equilibrium chromium(VI) uptakes, total sorbed chromium(VI) quantities and adsorption yields were highest for the granular activated carbon, which was expected, because of the greater specific surface area and the porous structure of granular activated carbon. The biosorption of chromium(VI) to the immobilized activated sludge appeared to be significantly lower compared with the adsorption to granular activated carbon. The relatively low chromium(VI) retention for immobilized activated sludge can be attributed to the difference in the surface morphology. Immobilized activated sludge particles do not have many micro or macropores, so its low surface area also results in lower sorption capacity. As seen from Fig. 3 at lower inlet chromium(VI) concentrations breakthrough curves were dispersed, breakthrough occurred very late and the surface of the adsorbent was saturated with chromium(VI) after a longer time for granular activated carbon. At 50 mg l 1 of inlet chromium(VI) concentration and at the lowest flow rate of 0.8 ml min 1, the sorption, although continuous with time, was very efficient in the initial steps of the process. This fact is probably associated to the availability of reactional sites around or inside the granular activated carbon particles, able to capture chromium(VI) and sufficient retention time. In a second stage, with the gradual occupancy of these sites, the uptake become less effective. Even after the breakthrough occurred, the column was still capable of accumulating chromium(VI), although at a progressively lower efficiency. In continuous-flow column, equilibrium chromium(VI) uptake level was higher than the batch uptake value at 100 mg l 1 inlet chromium(VI) concentration and at 0.8 ml min 1 flow rate. The sorption behavior of immobilized cells for chromium(VI) was different from that of chromium(VI) adsorption by granular activated carbon. The beads adsorbed chromium(VI) very rapidly at the beginning of the biosorption but as the experiment continued, the concentration of chromium(VI) in the effluent increased rapidly, the bed became saturated with chromium(VI) and the concentration of solute in the effluent rose to the inlet chromium(VI) concentration Fig. 3 shows that much sharper breakthrough curves so very low chromium(VI) uptakes and removal yields were obtained especially at higher inlet chromium(VI) concentrations for the chromium(VI) biosorption to immobilized activated sludge. At 100 mg l 1 inlet chromium(VI) concentration when compared with the batch system biosorption, it was observed that equilibrium metal uptake level decreased from 61.0 to 16.4 mg g 1 in continuous packed bed studies at 0.8 ml min 1.
Z. Aksu et al. / Process Biochemistry 38 (2002) 175 /186
182
Fig. 3. The breakthrough curves obtained at different inlet chromium(VI) ion concentrations for chromium(VI) biosorption/adsorption to Mowital† B30H resin immobilized activated sludge (open symbols) and granular activated carbon (filled symbols) (Temperature, 25 8C; initial pH 1.0; Q , 0.8 ml min 1).
4.4. The application of the Langmuir and Freundlich models Analysis of the isotherm data is important in order to develop an equation which accurately represents the results of the column and which could be used for
column design purposes. Two kinds of several isotherm equations have been applied for this study, Langmuir and Freundlich isotherms. The linearized Freundlich and Langmuir adsorption isotherms of chromium(VI) obtained at 25 8C were shown in Figs. 4 and 5 for both sorbents. The Freundlich and Langmuir adsorption
Table 2 Total amounts of chromium(VI) sent to column (mtotal), total adsorbed quantities of chromium(VI) (qtotal), equilibrium chromium(VI) uptake (qeq) and total removal percentages of chromium(VI) for chromium(VI) adsorption to Mowital† B30H resin immobilized activated sludge and granular activated carbon at different inlet chromium(VI) concentrations Q (ml min 1)
t
total
(min)
Veff (ml)
Mowital† B30H resin immobilized activated sludge 51.4 2880 2304 101.2 1800 1440 251.5 840 672 503.1 540 432 Granular activated carbon 51.2 13200 102.7 11700 251.5 7800 502.3 4680
10560 9360 6240 3744
mtotal (mg)
qtotal (mg)
Total chromium(VI) removal (%)
qeq (mg g 1)
118.4 145.8 169.0 217.3
30.6 32.7 34.4 36.9
25.8 22.4 20.4 17.0
15.3 16.4 17.2 18.5
541.0 961.1 1569.6 1880.6
301.6 405.3 527.9 570.2
55.7 42.2 33.6 30.3
71.8 96.5 125.7 135.8
Z. Aksu et al. / Process Biochemistry 38 (2002) 175 /186
183
Fig. 4. The linearized Langmuir adsorption isotherms for chromium(VI) biosorption/adsorption to Mowital† B30H resin immobilized activated sludge and granular activated carbon.
constants evaluated from the isotherms with the correlation coefficients were also presented in Table 3. In view of the values of linear regression coefficients in the table, the Langmuir model exhibited a little better fit to the sorption data of both the chromium(VI)-sorbent systems than the Freundlich model in the studied concentration ranges. However, the Freundlich model also seemed to agree well with the experimental data of
chromium(VI) considering that obtained linear regression coefficients are greater than 0.972. An adsorption isotherm is characterized by certain constants the values of which express the surface properties and affinity of the sorbent and can also be used to compare the sorptive capacity of sorbents for the chromium(VI) in the fixed column. KF and n , the Freundlich constants, are indicators of adsorption
Fig. 5. The linearized Freundlich adsorption isotherms for chromium(VI) biosorption/adsorption to Mowital† B30H resin immobilized activated sludge and granular activated carbon.
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Z. Aksu et al. / Process Biochemistry 38 (2002) 175 /186
Table 3 The comparison of the Langmuir and Freundlich adsorption constants evaluated from the Langmuir and Freundlich isotherms for chromium(VI) adsorption to Mowital† B30H resin immobilized activated sludge and granular activated carbon Freundlich model KF [(mg g Immobilized activated sludge Granular activated carbon
1
)(mg l
Langmuir model 1 n
) ]
11.6 35.4
capacity and adsorption intensity, respectively. The closer the n value of Freundlich is to zero the more heterogeneous is the system. From Table 3, the high values of KF showed easy uptake of chromium(VI) ions from wastewater with high adsorptive capacities of these sorbents. Table 3 also indicates that n is greater than unity, indicating that chromium(VI) ions are favorably adsorbed by the sorbents. The obtained data in Table 3 also pointed out that granular activated carbon had a higher biosorption capacity than that of Mowital† B30H resin immobilized activated sludge and represented a greater heterogeneity than immobilized activated sludge. The Langmuir constants, Qo and b have been determined from Ceq/qeq versus Ceq plots obtained for each sorbent and the results are also tabulated in Table 3. The maximum capacity Qo defines the total capacity of each sorbent for chromium(VI) ions. The maximum sorption capacities of granular activated carbon and immobilized biomass were determined as 147.1 and 18.9 mg g 1, respectively, in the packed bed column. The value of Qo appears to be significantly higher for the chromium(VI)-granular activated carbon system in comparison with the uptake of chromium(VI) on immobilized activated sludge. But a large value of b , related to binding energy, implied strong bonding of chromium(VI) on to Mowital† B30H resin immobilized activated sludge. The Langmuir model makes several assumptions, such as monolayer coverage and constant adsorption energy while the Freundlich equation deals with heterogeneous surface adsorption. The applicability of both Langmuir and Freundlich isotherms to the chromium(VI)-granular activated carbon and immobilized activated sludge systems implies that both monolayer adsorption and heterogeneous surface conditions exist under the experimental conditions used. It was shown that Mowital† B30H resin immobilized dried activated sludge can be used as an adsorbent in the same way as granular activated carbon in a continuous packed bed column to separate chromium(VI) ions from aqueous solutions. The immobilized biomass was preferable over native biomass in packed column operations as the dried, powdered biomass was not rigid and strong enough to use in downtlow packed bed column operations and presents an unacceptable pressure drop
2
n
R
13.2 4.2
0.988 0.972
Qo (mg g 1)
b (l mg 1)
R2
18.9 147.1
0.0799 0.0370
0.999 0.999
to flow. Diffusion limitations causing a decrease in biosorption capacity is one of the major disadvantages of immobilized systems. Considerable research has been focused on reducing diffusion limitations, but no generally satisfactory solution has been found. The major requirement is to develop immobilization systems which are more open to hydraulic flow but still provide large contact areas and which have good mechanical properties for large-scale systems. The beads must be mechanically strong, chemically resistant, inexpensive and easy to regenerate and they must not swell and contract during use as do many ion exchange resins. The production of the beads must be examined in the hope of increasing the yield in the desired size range, improving porosity, and increasing capacity. The performance of polymer, which have been used to immobilize activated sludge must be significantly improved by modifying bead fabrication procedure to improve surface and/or interior properties. Mowital† B30H resin immobilized activated sludge can be used in continuous packed bed columns for chromium(VI) removal from waste waters because they satisfy most of the specifications defined above. They can be hardened, formed into beads and used highly efficiently in packed-bed bioreactors that resemble the column processes used with ion exchange resins and activated carbon systems. This suggests that if the porosity of the beads could be increased by modification of the biosorbent preparation method, then the contact time necessary for maximal biosorption could be significantly reduced.
5. Conclusion The breakthrough curves for column sorption of chromium(VI) from dilute solutions using granular activated carbon and Mowital† B30H resin immobilized activated sludge have been measured at 25 8C. The obtained results showed that the sorption of chromium(VI) is dependent on the flow rate, the inlet chromium(VI) concentration and time and these parameters affect the saturation capacity of both sorbents directly. The breakpoint time and total sorbed chromium(VI) quantity and chromium(VI) removal yield decreased with increasing flow rate and chromium(VI) concentration in both systems. Although granular
Z. Aksu et al. / Process Biochemistry 38 (2002) 175 /186
activated carbon had a higher adsorption capacity for chromium(VI) (147.1 mg g 1), the experimental results indicated that dried activated sludge has also a considerable potential for the removal of chromium(VI) over a wide range of chromium(VI) concentration (18.9 mg g 1). Consequently, it can be said that for a given type of immobilized biosorbent, the saturation capacity is greater under conditions of lower concentrations of chromium(VI) and lower flow rates. By adjusting the operating characteristics of the packed column, for example the flow rate, inlet chromium(VI) concentration, particle size, biomass quantity very rapid and efficient chromium(VI) uptake can be achieved for the system. The Freundlich and Langmuir adsorption models were used for the mathematical description of the adsorption of chromium(VI) ions to granular activated carbon and Mowital† B30H resin immobilized activated sludge in a continuous packed bed column and the isotherm constants were determined to compare the adsorption capacity of granular activated carbon and Mowital† B30H resin immobilized activated sludge for chromium(VI) ions. It was seen that the adsorption equilibrium data fitted very well to Langmuir adsorption model. It seems that although the immobilization process decreased the metal biosorption properties of the biomass, the use of Mowital† B30H immobilized activated sludge technology for wastewater treatment will offer an attractive alternative for various industries. If processing time is important, recycle or linked-column series may be also employed in order to obtain higher chromium(VI) removal yields at higher flow rates and higher inlet chromium(VI) concentrations. If the quantity of biomass per unit volume of the column is increased (decrease of the void fraction of the column) chromium(VI) uptake also will be higher for concentrated chromium(VI) solutions because of the increasing of active adsorption sides.
Appendix A: Nomenclature Co
inlet (feed) chromium(VI) concentration in the column (mg l 1) C effluent chromium(VI) concentration in the column (mg l 1) Cad( /Co / adsorbed chromium(VI) concentration in C) the column (mg l 1) Ceq nonadsorbed chromium(VI) concentration at equilibrium in the column (mg l 1) mtotal total amount of chromium(VI) ions sent to column (mg) qeq equilibrium chromium(VI) uptake in the column or in the batch system (mg g 1)
qtotal Q t ttotal Veff X
185
total adsorbed quantity of chromium(VI) in the column (mg) flow rate ( ml min 1) flow time (min) total flow time (min) effluent volume (ml) amount of sorbent in the column (g) or sorbent concentration in the batch system (g l 1)
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