(545ad) Lead Removal from Water Using Insoluble Bacterial Carboxymethyl Cellulose

INTRODUCTION

Among
the most common water pollutants, heavy metals are highly disturbing due to
their non-biodegradability and their high toxicity.  For instance, lead (Pb) is
a potent neurotoxin which may cause critical diseases in human beings,
including cancer. Thus, many researches invest their efforts in the development
of systems that allow the elimination of lead from water to safe levels¹. In this context, many
cellulose-containing materials and cellulose derivatives have been reported as
useful for the adsorption of heavy metals². Both, the natural abundance
of cellulose, and the possibility of obtaining it not only from pulps derived
from wood but also from a number of alternative abundant and underutilized
cheaper cellulose sources, make this polysaccharide very attractive as raw
material. On the other hand, the separation processes of cellulose from plant
sources require the use of reagents that can have a negative environmental
impact³. Conversely, bacterial
cellulose (BC) has the same molecular formula as plant cellulose, but it has
the advantage of being produced free of lignin, hemicelluloses and pectin, thus
avoiding the need for chemical treatments devoted to the removal of these
compounds prior to cellulose derivatization.

Taking
into account the above discussion and considering that Pb (II) exists as a
positive ion, the use of insoluble carboxymethyl cellulose (CMC) as cation
exchanger could be a good alternative for lead removal from water. Water
solubility of CMC is determined by the degree of substitution⁴ (DS). Previously, some of us reported a
systematic study of reaction variables to tailor the DS of carboxymethylated BC
(CMBC)³. Based on our previous
experience, in this work, insoluble carboxymethyl cellulose was synthetized and
lead adsorption studies were carried out.

EXPERIMENTAL

BC and CMBC
were produced as we previously described^3,5. The DS of CMBC was determined by conductometric
titrations as it was described elsewhere⁴. FT-IR spectra of the samples were acquired on an IR Affinity-1
Shimadzu Fourier Transform Infrared Spectrophotometer in absorbance mode.

In order to
study the water solubility of CMBC, a film was made by drying the sample between
glasses at 110°C for 2 h. The film obtained was weighed before been placed in a
beaker with 50 mL of distilled water under static conditions for 24 h. Within
this interval, the film was periodically removed from water, dried at 110°C
during 1 h and finally weighed.

Lead concentration was
determined on an air-acetylene flame type atomic absorption spectrometer (FAAS)
(Model iCE 3000, Thermo Scientific).

Adsorption
Experiments

Adsorption
kinetics

Native BC
or CMBC (50 mg) was suspended in 100 mL of lead ion solution (100 mg.L^-1)
at pH 7 and the mixture was stirred at 400 rpm at room temperature (25°C). Samples
were taken at various time intervals and lead concentration was determined by
FAAS. As was expected, native BC shows a maximum lead adsorption capacity
almost ten times lower than CMBC.

Effect
of solutions pH values

The effect
of pH on the adsorption of lead ion was studied in a pH range of 2.5-7.0 at
25°C. 25 mL of an aqueous lead ion solution (100 mg.L^-1) was shaken
with 25 mg of CMBC at 400 rpm for 2 h. The pH was adjusted using HCl (0.01 M)
or NaOH (0.01 M). After filtration, lead concentration was determined by FAAS.

Adsorption
isotherms

The
adsorption isotherm experiments were conducted with initial lead ion
concentration in the range of 50-150 mg.L^-1. 25 mg of CMBC was
suspended in 25 mL of lead solutions with different concentration at pH 7 and 25°C.
The mixture was stirred at 400 rpm for 2 h. After filtration, lead
concentration was determined by FAAS.

Column
experiment: Breakthrough curve

Column
experiments were carried out using an acrylic column (I.D. 6 mm, L. 22 mm)
filled with CMBC (50 mg) and connected to a peristaltic pump by tygon tubes. Aqueous
lead ion solution (7 mg.L^-1) at pH 7 was sent into the column at a
flow rate of 2.5 mL.min^-1. Effluent samples were taken every 10 mL
and the concentration of lead ion in all samples were determined
by FAAS.

RESULTS AND DISCUSSION

Characterization
of the CMBC

The DS of CMBC determined by
conductometric titration showed to be 0.17, and the insolubility of the sample
was confirmed by no significant change in sample’s weight after being 24 h in
water. Comparison of FT-IR
spectra of BC and CMBC samples also provided evidence of etherification (Fig.
1). In the CMBC spectrum, apart from the typical bands of cellulose⁶, the band characteristic of the stretching vibration
of the carboxylate groups at 1607 cm^-1 was observed, confirming that
the carboxymethylation had occurred.

Fig.1. FT-IR
spectra of BC and CMBC.

Adsorption
kinetics studies

The uptake of
lead by CMBC increased rapidly in the first 20 min and then the sorption slowed
down significantly (Fig. 2).

Fig.2.
Amount adsorbed of  Pb²⁺ on CMBC at different adsorption time  (initial
metal ion 100 mg.L^-1, pH 7).

To study
the controlling mechanism of the adsorption process, experimental data were adjusted
to pseudo-first-order and pseudo-second-order models (eq. 1 and 2,
respectively).

       eq.1

                          eq.2

Results
undoubtedly showed that the pseudo-second-order kinetic equation more
appropriately described the lead adsorption on CMBC (Table 1). Therefore, as it
was reported elsewhere, the adsorption mechanism is kinetically controlled
mainly by chemical interactions^7,8.  Moreover, the excellent concordance between the
experimental data and those calculated by means of the pseudo-second-order
kinetic equation, is evidenced in Figure 2.

Table 1. Comparison of the adsorption kinetic models

Effect
of pH

Figure 3
shows the strong pH dependence of lead adsorption on CMBC. The critical role of
the pH on the adsorption capacity of the material was in concordance with that
expected considering that the pka values of the carboxylic acids is around 5.

Fig.3.
Effect of pH on removal of lead: adsorbent dose, 50 mg/25 mL; Pb²⁺ concentration,
100 mg.L^-1

Adsorption
isotherms

Experimental
isotherm data were fitted to different models of sorption isotherms: Langmuir,
Freunlich, Dubinin-Radushkevich and Frumkin. As can be seen in Table 2, the Langmuir
equation gives the best fit of our data. The experimental data of Pb²⁺
sorption on CMBC as well as the isotherm calculated by the Langmuir model are
shown in Figure 4.

Table 2. Adsorption
isotherm model parameters.

On the
other hand, the separation factor R_L indicates the type of the
isotherm to be either unfavorable (R_L>1), linear (R_L=1),
favorable (0<R_L<1) or irreversible (R_L=0). In our
case, the values of R_L factor were always between 0 and 1 for any
value of initial concentration (C₀), indicating a favorable
adsorption of lead onto CMBC^9,10.

Fig.4.
Adsorption isotherm of lead on CMBC (pH 7, 25 °C)

Column experiment: Breakthrough curve

In order to
obtain the characteristic parameters for fixed-bed systems, experiments using a
column packed with CMBC under continuous flow were conducted. The performance
of this experiment can be described in terms of time vs. effluent concentration
(breakthrough curve) (Fig. 5).

Several
simple mathematical models have been developed to predict the dynamic behavior
of the column. Among them, Bohart-Adams, Clark and modified dose-response
models are recommended because of the usefulness of the designed parameters
that they provide¹¹. Thus, those models were used here to estimate and
analyze the column adsorption performance. In order to evaluate the
goodness-of-fit of the equation to the experimental data, the correlation
coefficient (R²) as well as the residual root mean square error
(RMSE) and the chi-square test (χ²) are
presented.

As can be
seen, the Bohart-Adams model gives the best fit to the experimental data (Table
3).

Fig.5.
Breakthrough curve.

Table 3.
Breakthrough curve model parameters.

CONCLUSIONS

In conclusion, water insoluble CMBC
was synthesized and its adsorption capacity of lead ions was investigated. CMBC
showed good adsorption performance in the pH range of 4.5-7.0. The adsorption
was a fast process following the pseudo-second-order kinetic model, indicating
the importance of chemical interactions in the process. The experimental
adsorption isotherm data and the breakthrough curve were well fitted with the Langmuir
and Bohart-Adams models respectively. 

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