BATCH AND FIXED-BED COLUMN BIOSORPTION OF Cd2+ AND Cu2+
ONTO WHEAT STRAW
H. Muhamad, H. Doan, A. Lohi
Department of Chemical Engineering, Ryerson University
350 Victoria Street, Toronto, Ontario, Canada M5B 2K3
Abstract: Biosorption of Cd+2 and Cu2+ by wheat straw (Triticum Sativum) using a batch system and a continuous
upflow mode in a fixed bed column was studied. For the batch system, the effect of pH over a range from 3.0 to 7.0
and the temperature from 20 to 40 oC on the metal removal was investigated. Various initial metal concentrations
from 20 to 150 mg/L were used. Adsorption of metal ions was observed to increase with liquid pH and temperature.
Among the three widely used isotherms, namely the Langmuir, Freundlich, and Timken models, the experimental
data better fitted the Langmuir isotherm model.
For the continuous upflow mode in a fixed bed (4-inch diameter), experiments were performed over a range of flow
rate from 0.3-1.0 LPM and varied bed height of 0.5-2.0 m. The results obtained agree to the bed depth service time
(BDST) model well. In addition, for estimations of the parameters that are necessary for the design of a fixed bed
adsorber in practical applications, the experimental data were fitted to the Thomas, Adams-Bohart, and Yan models.
Thomas model appeared to better describe the experimental results.
Keywords: Biosorption ;Heavy metals; Isotherms Model; Fixed Bed Column; Break through curve.
1.
INTRODUCTION
Biosorption process is an innovative process that has proved itself an efficient and low cost process for removal of
heavy metals from waste water (Benguella and Benaissa, 2002). Nonliving microorganism has been as a bioadsorbent used to treat wastewater contaminated with heavy metals (Gadd, 1992). Biosorption technology can also
utilize naturally available raw materials as biomass, such as: sea weeds, fungi, and wheat straw .etc.., or some waste
materials. Availability is a major factor to be taken into account in selecting a biomass for biosorption purposes.
Biosorption mechanism is suggested analogues to phenomenon of adsorption (Wagner and Jula, 1981). Many other
mechanisms have been proposed in the past decades to understand the phenomenon of metals ions uptake by a
biomaterial. These proposed mechanisms are chemisorption, complexation, coordination, chelation of metals, ion
exchange, as well as adsorption and inorganic microprecipitation (Volesky and Naja, 2003).
The characteristics of the biosorption behavior are generally analyzed by means of equilibrium isotherms and
biosorption kinetics. Biosorption isotherms are also an important tool used to understand the biosorption mechanism
and the evaluation of the potential of a bio-adsorbent. Many models have been developed to relate the adsorbate
concentration in the bulk and the adsorbed amount on the adsorbent. Freudlich (1906), Langmuir, (1918), Temkin
and Pyzhev (1940) models are most commonly used to evaluate characteristics of the biosorption behavior.
Generally biosorption process can be performed either as batch or continuous manners. For a continuous process,
adsorption column can be classified in three categories: fixed bed biosorption column, fluidized bed column, and
completely mixed sorption column (Muhamad, 2008). The present study was on an investigation of the effectiveness
of wheat straw as a bio-adsorbent. The research was focused on the evaluation of the adsorption capacity of wheat
straw in batch and continuous fixed-bed column systems. The experimental results obtained from batch and
continuous studies were fitted with most commonly used models in literature. The influence of initial concentration,
temperature effect, pH value effect and co-adsorption effect was investigated for batch studies.
2.
BATCH BIOSORPTION SYSTEM
2.1 Experimental Setup & Procedure
The experimental setup was basically a temperature-controlled variable-speed agitator water bath. Heavy metal
solution of known concentration was prepared by dissolving analytical grade Cadmium Sulfate Octahydrate
3CdH2SO4∙8H2O and Cupric Sulfate Pentahydrate CuSO4∙5H2O in distilled water. Stock solutions of 1000 mg/L
concentrations were prepared for both Cd2+ and Cu2+. Each individual ion or any combinations of ions solutions
were prepared in 200 ml volume at initial concentrations of Cd2+ and Cu2+ of 20, 50, 75, 100, and 150 mg/L in 300Page | 1
ml Erlenmeyer flasks. Wheat straw was cut into uniform lengths of 1cm, and 1 g (equivalent dose: 5g/L) of biomass
was pretreated and then put in each metal solution. The flask was placed in the shaker water bath and the
temperature of the water bath was set. The temperature effect on the adsorption of metal was studied over a
temperature range from 20 to 40 oC with 5 oC increments using an initial concentration of 100 mg/L and a shaker
speed of 120 rpm along with a pH range of 3 to 7. The solution pH was adjusted with 0.1 N H 2SO4 or 0.1 N NaOH.
Each run was performed for 6 hours and samples were taken at 5-minute intervals at the start of the experiment, and
then at 30-minute intervals towards the end of the experiment.
3.
RESULTS AND DISCUSSIONS
3.1 Effect of initial Cd2+and Cu2+concentration
The effect of initial concentration of Cd2+ and Cu2+ on the adsorption efficiency by wheat straw was systematically
investigated by varying the initial concentration between 20 and 150 mg/L. It was observed that the percentage of
metals ions removal reduced from 100 % to 53.5% and from 86% to 38% for Cd 2+ and Cu2+, respectively by
increasing initial concentration from 20 to 150 mg/L. The reason is availability of the same surface site of biomass
for the increasing initial concentration that resulted in the saturation of the sorption sites on the biomass. The same
trend of initial concentration dependency was observed by Malkoc (2006).
3.2 Effect of Contact Time
It was observed that for all initial metal concentrations used there was a rapid increasing trend of the metal uptake
during first 30 minutes of the experiment with an average of 80 −90 % of the overall uptake for both Cd 2+ and Cu2+.
However, the equilibrium time varied between 60−210 minutes for Cd2+ with increases in the initial concentration
and, 75−210 minutes for Cu2+.
3.3 Effect of Temperature
Effect of temperature on biosorption of metal ions by wheat straw was studied at varied temperatures of 20, 25, 30,
35 and 40 oC, the initial concentration of 100 mg/L and the pH of 6.0 and 5.5 for Cd2+ and Cu2+, respectively. It was
observed that there is a slight increase in metal uptake with increased temperature. The increase in biosorption with
temperature may be attributed to increase in the surface of the active sites available for biosorption on the adsorbent
and thus enhancing biomass uptake ability.
3.4 Effect of initial pH
The uptake of Cd2+ and Cu2+ as a function of the hydrogen ion concentration in the solution was examined over a pH
range of 3−7 for Cd2+ and 3−6.5 for Cu2+. The adsorption of both metal ions increased with pH. Cd2+ uptake, having
a maximum value of 14.4 mg/g at pH 7, showed a progressive decrease to 2.78 at a pH of 3. Similarly for Cu2+
uptake were 3.38 mg/g at pH 3 and 12.38 mg/g at pH 6.5. The main reason for this pH behavior could be that the
affinity of the proton (H+) to the binding sites of wheat straw was much greater than that of the heavy metals that
results no biosorption of the metal ions at pH below 2.2.
3.5 Isotherm models for biosorption of Cd2+ and Cu2+ on wheat straw
The experimental data for single ion solutions were fitted to the Langmuir, Freundlich and Temkin isotherm models
for both Cd2+ and Cu2+. The Langmuir, Freundlich and Temkin models can be expressed respectively as below:
(1)
(2)
(3)
Where qe is (mg/g dry weight) the amount of metal adsorbed, Ce (mg/L) is the equilibrium concentration of metal in
solution, qmax (mg/g) is the maximum adsorption capacity or the theoretical monolayer saturation capacity, b is the
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equilibrium constant in the Langmuir model. It was found that the Langmuir model gave the best fit with the
experimental results base on r2 values, thus indicating that wheat straw would provide monolayer and homogeneous
adsorption. Figure (1) shows the Langmuir biosorption isotherm of Cd2+ and Cu2+ by wheat straw.
Fig. 1: Biosorption isotherms of Cd2+ and Cu2+ based on the Langmuir model at (25oC)
4.
CONTINUOUS BIOSORPTION SYSTEM
4.1 Experimental Setup & Procedure
The experimental setup for the continuous fixed-bed column system mainly consisted of a set of four equilength
columns, a solution holding tank, a centrifugal pump for solution supply to the columns, and a flow meter. The
holding tank had a capacity of 1000 liters. Solutions of Cd2+ and Cu2+ with an initial concentration of 100mg/L were
prepared in the tank. All experiments for the continuous study were performed at room temperature (25 oC) with
initial pH of 6.0 for Cd2+ and 5.5 for Cu2+. The initial pH was decided based on the precipitation properties of Cd2+
and Cu2+. The effect of the length of the pack bed was studied by using one, two, three or all column sections
available. The equivalent column heights were 0.5, 1.0, 1.5 and 2.0 meters. Flow rates of 0.3, 0.5, 0.8 and 1.0 L/min
were used for both the Cd2+ and Cu2+ solutions.
Dry wheat straw was cut to one-inch lengths and packed into the columns with synthetic fiber meshes placed at both
the top and the bottom to hold the wheat straw packing. To distribute the flow of contaminated water, a liquid
distributor was placed on the fiber mesh of each column. In order to prepare packing of the same porosity each time,
the column was packed at various heights using a density of wheat straw packing of 68.54 g/L that was based on the
packing density in a 0.5 m high column. The four columns were connected in series, and the liquid was pumped into
the bottom of each column.
During each run, samples were taken from the sample points at 5-minute intervals at the start of experiment and then
at 30-minute intervals until the experiment were completed. The experimental time for each run was approximately
eight hours in order to assure the formation of a breakthrough curve. The number of samples collected from each
column for each run was 20. The residual metal concentration in the water samples was measured using an atomic
adsorption spectrophotometer. Total quantity of metal adsorbed in the column Mads (mg) was calculated from
breakthrough curves. The area above the breakthrough curve was obtained from the plot of outlet metal
concentrations (mg/L) versus time (min) multiplied by the flow rate (LPM). The uptake capacity was calculated by
dividing the metal adsorbed (Mads) by the total weight of the biomass in the column (g). The total amount of metal
ions that entered the column was calculated from the following equation:
Mtotal=CoF
(4)
The removal (%) of heavy metal ions was calculated from the ratio of metal mass adsorbed by wheat straw in the
column (Mads) to the total amount of ions that entered the column (Mtotal). The expression employed is given as:
(5)
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4.2 Effect of Bed Height in Continuous Biosorption Process
Effect of bed height was investigated at various flow rates of 0.3, 0.5, 0.8, and 1.0 LPM, for Cd2+ and Cu2+
respectively, along with the determination of breakthrough curves. Figures (2a-d) shows the breakthrough curves for
various bed depths at specified flow rates. Empty Bed Contact Time (EBCT) was determined from the following
equation:
(6)
a
b
c
d
Fig. 2: Effect of the bed depth on adsorption of Cd2+at a flow rate of (a) 0.3 LPM (b) 0.5 LPM (c) 0.8 LPM (d) 1.0
LPM and an concentration of 100 mg/L
From the results at various bed depths with different flow rates it was found that the EBCT, the volume treated (Vb),
and the breakthrough time (tb) increase with increasing bed depth while the shape and gradient of the breakthrough
curves are significantly different for various bed depths as shown in Figures (2a-d). The slope of the breakthrough
curves of the longer bed (2.0 m) tended to be more gradual. This effect was more pronounced at a higher flow rate
of 1.0 LPM. As expected, an increased bed height resulted in a larger volume of the metal solution treated and a
higher percentage removal of Cd2+ while the uptake capacity of Cd2+ per unit mass of wheat straw remained almost
identical for different bed heights since the adsorbed amount was strongly dependent on the amount of sorbent
available for sorption. As can be seen from all breakthrough curves the slope of S–shape from tb to ts (saturation
time) decreased as the bed height increased from 0.5–2.0 m. For Cu2+, a similar trend was observed.
4.3 Effect of Flow Rate in Continuous Biosorption Process
In order to investigate the effect of liquid flow rate on biosorption of Cd2+ and Cu2+ by wheat straw, experiments
were performed with four different flow rates of 0.3, 0.5, 0.8 and 1.0 LPM. The flow rate effect was studied along
with the effect of the column height at an initial concentration of 100 mg/L. In Figure (3) the concentration of Cd2+
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at the bed outlet is plotted against time for different flow rates with a column height of 2.0 meters. The percentage
uptake of heavy metal appeared to be strongly dependent on flowrate. In addition, the breakthrough curve shifted
towards lower time scale with flowrates from 0.3 to 1.0 LPM. This indicates a shorter active life of the column. The
same trend was observed for all column heights with a slight difference in the percentage metal removal. The
sensitive behavior of column adsorption to flow rate can be explained by the fact that the liquid residence time in the
column is critical for the biosorption process (Netpradit et al., 2004). When the flow rate is increased, the liquid
residence time in the column decreases; and hence, the percentage metal removal is decreased.
Fig. 3: Effect of the flow rates on adsorption of Cd2+ at a 2.0 m bed depth and a concentration of 100 mg/L.
4.4 Bed depth service time model (BDST)
Data collected during laboratory and pilot plant tests serve as the basic for the design of a full–scale adsorption
column. Figure (4) shows the plot of the service time versus the bed height at different flow rates with an initial
concentration of 100 mg/L and at 10% breakthrough point (i.e. Cout=10 mg/L) for Cd2+ ions. The equations were
obtained by linear regression with r2 above 0.99 indicating the suitability of the BDST model in representing the
adsorption in a fixed bed in the present study. The sorption capacity of the bed per unit volume (N o) was calculated
from slope of the line in Figure (4) utilizing the following equation. (Kumar and Bandyopadhyay, 2006):
Fig. 4 BDST curve at 10 % breakthrough for biosorption of Cd2+on wheat straw
(7)
The rate constant (K) of the BDST model calculated from the intercept of the plot, characterizes the rate of solute
transfer from the liquid phase to the solid phase. The minimum bed depth (Zmin) was also calculated using following
equation:
(8)
It was observed that the adsorption capacity (No) of Cd2+ was slightly reduced with increasing the flow rate; this
indicates that the adsorption capacity may increase with decreasing flow rate or increasing contact time (EBCT).
The minimum bed depth (Zmin) was higher when the flowrate was increased because the adsorption zone needed be
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increased for the removal of metal ions. Similar observation was reported by Netpradit et al., (2004) in their study
using metal hydroxide sludge for reactive dye adsorption in a fixed–bed column. The advantage of the BDST model
is that any experimental test can be reliably scaled up to get the design parameters without further experimental data
and analysis. Although the model used values of intercept and slope at a given flow rate, in order to obtain higher
accuracy more data should be obtained to check any deviation in the slopes of the straight lines.
4.5 Column Biosorption Data Description
The experimental results obtained from column biosorption (continuous process) were fitted with three most
commonly used models to describe column biosorption data. The models employed were: 1). Thomas Model, 2)
Adams-Bohart Model and 3) Yan Model. Equations (9-10) shown below are for the Thomas, Adams-Bohart and
Yan models, respectively.
(9)
(10)
(11)
It was found that the Thomas model was best fitted with experimental Cd2+ and Cu2+ uptake data, based on r2 values.
The biosorption process showed very effective uptake percentage at low flow rates (0.3 LPM) and tall columns
(Z=2.0 m). The BDST model was used to predict the relationship between the bed depth and the service time. The
minimum bed height was estimated to be 0.4-1.55 m for flow rate 0.3-1.0 LPM, respectively. Application of other
mathematical models to fixed bed biosorption systems was also studied. The Thomas, Adams-Bohart and Yan
models were used to fit the experimental data. The Thomas model was best fitted with experimental results.
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