Pollution of our water sources by inorganic pollutants with special reference
to the heavy metals (lead, mercury, zinc, cobalt, cadmium, nickel, chromium,
etc.) presents a massive threat to human health (Li et
al., 2003). Although heavy metals occur naturally, pollution by these
metals results to abnormally high concentrations (Parsons
and Jefferson, 2006). Heavy metals often find their way into aquatic ecosystems
from geochemical sources, agricultural materials, metallurgical industries and
battery industries (Parsons and Jefferson, 2006;
Aloway and Ayres, 1997). Once in the aquatic environment heavy metals can
be absorbed by living organisms and accumulate to acute concentrations as they
move up the food chain (Wan-Ngah and Hanafiah, 2008;
Rao et al., 2007). This leads to toxicity problems
making it essential to monitor and remove these heavy metals from water.
Several methods and materials have been proposed and used for the removal of
heavy metals from water namely: chemical precipitation, coagulation-flocculation,
membrane filtration, electrochemical techniques, ion exchange, chelating agents
and adsorption processes (Droste, 1997; Kurniawan
et al., 2006). However, some of these processes are often associated
with many problems which make their use ineffective. High operational costs
and the inability of these processes to remove the heavy metals to the acceptable
levels are the most common shortcomings of some of these processes (Kurniawan
et al., 2006). The adsorption processes are found to be the most
promising techniques for the removal of heavy metals from water with functionalized
carbon nanotubes as superior adsorbents.
These materials have shown good adsorption capacity for zinc, lead, copper,
nickel, chromium, cadmium and mercury (Li et al.,
2002, 2003; Lia et al.,
2003; Lu and Chiu, 2006; Lu
et al., 2008; Rao et al., 2007; Yang
et al., 2009).
Previous work in our laboratory showed that oxidized MWCNTs copolymerized with
CDS have the ability to remove organic compounds to ppb levels (Salipira
et al., 2007). In this study, oxidized MWCNT-CD polymer was tested
in order to evaluate its ability to remove lead and cobalt from synthetic water
solutions. Both lead and cobalt are documented as having devastating health
effects when they accumulate in the human body above the accepted maximum limits
(ATSDR, 2004, 2007). For example,
the maximum accepted levels for lead according to the South African and World
Health Organizations Drinking Water Standards are 20 and 10 ppb, respectively
(Mamba et al., 2008). Moreover, both metals have
also been labelled as possible carcinogens (ATSDR, 2004,
2007). Current water treatment technologies are specific
for either organic or inorganic pollutants whereas both organic and inorganic
pollutants coexist in the aquatic environment. Ideally this lead to the suggestion
of creating a material with dual applicability that would be capable of effectively
and simultaneously removing both organic and inorganic pollutants from water.
This study evaluates the ability of the polymer to quantitatively remove lead and cobalt as a function of initial metal concentration and contact time. In addition competitive adsorption studies were conducted and are presented in this article. Information from this latter study helped to evaluate the performance of the polymer in removing one metal while in the presence of the other.
MATERIALS AND METHODS
The work reported in this study was conducted between June 2009 and December
2009. All chemicals and reagents used in this study were of high purity and
were used without further purification. All solvents were dried and distilled
according to standard procedures and were stored under anhydrous conditions
(Armarego and Perrin, 2002). Reactions were carried out
in an inert atmosphere with either argon or nitrogen gas.
Preparation of adsorbents
Synthesis, purification and functionalization of MWCNTs: MWCNTs were synthesized
in our laboratory using the nebulized spray pyrolysis method as described in
literature (Vivekchand et al., 2004; Salipira
et al., 2008). Toluene was used as a carbon source with ferrocene
acting as a catalyst as well as an additional carbon source. Argon was used
as the carrier gas. The produced MWCNTs were then purified by removing amorphous
carbon and fullerenes following methodology outlined in literature (Ndzimandze,
2007). Purified MWCNTs were oxidized at 55°C for 24 h using a 3:1 H2SO4:HNO3
mixture. The oxidized MWCNTs were filtered and washed until a neutral pH was
achieved and then dried in an oven to remove the water (Ndzimandze,
Polymerization of oxidized MWCNTs with cyclodextrins: A literature procedure
was adopted for the polymerization reaction (Salipira et
al., 2008). Typically, β-CD (8 g) was polymerized with oxidized
MWCNTs (0.4 g) under the appropriate conditions using hexamethylene diisocyanate
(HMDI) as a linker. This gave a 5% MWCNT-CD polymer in terms of the mass of
the oxidized MWCNTs relative to the mass of the CDS.
Preparation of Pb2+ and CO2+ solutions: Stock solutions
(1000 ppm) of Pb2+ and Co2+ were prepared by dissolving
Pb (NO3)2 (1.599 g) and Co (NO3)2.
6H2O (4.938 g), respectively in 100 mL of deionised water in 1 L
volumetric flasks. The flasks were then made up to the mark using deionised
water. From each of the stock solutions, 2.5, 5, 7.5, 10 and 12.5 mL was pipetted
into 250 mL volumetric flasks then diluted to the mark to make solutions of
10, 20, 30, 40 and 50 ppm of Pb2+ and CO2+. In all the
adsorption experiments 0.05 g of the adsorbent and 30 mL of the metal solution
was used. The adsorption experiments were all done at room temperature with
the pH of the metal solutions kept between values 5 and 6.
Investigating the effect of the initial metal concentration: Metal solutions of concentrations 10-50 ppm were added into 100 mL glass bottles containing each of the adsorbents i.e., pristine MWCNTs, MWCNT-COOH and MWCNT-CD. The bottles were mounted on a Merx 261 orbital platform shaker and shaken at 140 rpm for 4 h. Solutions from each adsorbent were filtered using 0.45 μm membrane filters and then analysed.
Effect of contact time: To investigate the effect of contact time on the adsorption of each metal ion, a working concentration of 10 ppm was used. The bottles containing the metal solutions and the adsorbent were shaken from 20 to120 min at 20 min intervals.
Competitive adsorption: The ability of the three adsorbents to remove each of the metal ions in the presence of the other was investigated using binary solutions of the two metals. The metal concentrations were kept equal in the binary solutions. The effect of initial metal concentration and contact time were investigated.
Analytical instruments: An FT-IR spectrometer (MIDAC, model 400) was used to confirm the polymerization reaction and functionalization of the MWCNTs. To investigate the purity and surface morphologies of the adsorbents a scanning electron microscope (JEOL, model 5600 SEM) was used. The surface area analyses of the adsorbents were done on an automated gas adsorption analyzer (TriStar, model 3000). The concentrations of the metal ions were determined after adsorption using an atomic absorption spectrometer (Varian AAS, model SpectrAA 200).
Characterization of unfunctionalized and oxidized MWCNTs: The SEM image
of unfunctionalized MWCNTs is shown in Fig. 1 and it illustrates
relatively clean and well aligned MWCNTs. After oxidation (Fig.
2), the MWCNTs lost their well aligned structure. FT-IR spectrum of unfunctionalized
MWCNTs (Fig. 3) showed that the MWCNTs were relatively pure
while the spectrum of the oxidized MWCNTs (Fig. 4) showed
the presence of C = O and C-O groups, as a result of oxidation.
|| SEM image of pristine MWCNTs
|| SEM image of oxidized MWCNTs
From Table 1, the surface area of the unfunctionalized MWCNTs
was found to be smaller (39.24 m2 g-1) compared to the
oxidized MWCNTs (78.61 m2 g-1).
Characterization of MWCNT-CD polymer: SEM images (Fig.
5, 6) revealed that the polymer had a relatively non-uniform
surface. The FT-IR spectrum of HMDI (Fig. 7) at the start
of the reaction showed the isocyanate peak which decreased gradually with time
until it completely disappeared after 24 h (Fig. 8).
|| FT-IR spectrum of unfunctionalized MWCNTs
|| FT-IR spectrum of oxidized MWCNTs
||SEM image of the MWCNT-CD polymer showing spongy appearance
||SEM image of the MWCNT-CD polymer displaying a granular architecture
||FT-IR spectrum of HMDI showing the isocyanate peak at the
start of the reaction
|| FT-IR spectrum of the MWCNT-CD polymer
|| BET analysis of the adsorbents
||Effect of initial concentration on the adsorption of Pb2+
There was also emergence of peaks corresponding to amide functions, suggesting
that polymerization of the oxidized MWCNTs and cyclodextrins did take place.
The surface area of the polymer was found to be 5.46 m2 g-1
Adsorption experiments: The amount of each of the metals adsorbed was calculated using the following equation:
where, q is the amount of metal ions adsorbed (mg g-1), C0 is the initial metal concentration (mg L-1), Ct is the final metal concentration after a specific time interval (mg L-1), V is the volume of the metal solution used (L) and m is the mass of the adsorbent used (g).
Adsorption of Pb2+ ions using pristine MWCNTs, oxidized MWCNTs
and MWCNT-CD polymer: Figure 9-11 show
the performance of the three adsorbents for the removal of Pb2+ ions
from synthetic solutions. The oxidized MWCNTs showed superior adsorption capacity
as the concentration increased, followed by the polymer and lastly the pristine
MWCNTs (Fig. 9). The oxidized MWCNTs and the polymer showed
lightly increasing adsorption capacity as the time increased (Fig.
10). The polymer recorded a maximum % Pb2+ removal of 68.0% at
10 ppm (Fig. 11) while it reaching a maximum % removal of
68.5% after 60 min (Fig. 12).
|| Effect of contact time on the adsorption of Pb2+
||Effect of initial concentration on the % removal of Pb2+
Adsorption of Co2+ ions by pristine MWCNTs, oxidized MWCNTs and
MWCNT-CD polymer: The polymer showed a higher adsorption capacity at 10
ppm dropping remarkably as the concentration increased (Fig.
13). A similar adsorption pattern to that observed for Pb2+ ions
as a function of contact time, by the polymer, was observed for CO2+
ions (Fig. 14). The oxidized MWCNTs recorded a higher % removal
of CO2+ ions followed by the MWCNTs-CD polymer and lastly the pristine
MWCNTs (Fig. 15, 16).
|| Effect of contact time on the % removal of Pb2+
||Effect of initial concentration on the adsorption of CO2+
|| Effect of contact time on the adsorption of CO2+
||Effect of initial concentration on the % removal of CO2+
|| Effect of contact time on the % removal of CO2+
Competitive adsorption of Pb2+ and CO2+ ions:
The competitive adsorption studies revealed that the adsorption capacity of
all three adsorbents for both metal ions is lower in competitive adsorption
compared to the single metal ion adsorption (Fig. 17, 18).
The % removal of both metals by the three adsorbents was found to be lower in
the competitive adsorption compared to the single metal ion adsorption (Fig.
19, 20). The polymer recorded a maximum % removal of
59.1% for Pb2+ ions and 55.7% for Co2+ at 10 ppm (Fig.
Isotherm models: Adsorption of the two metal ions by the three adsorbents was studied using the Langmuir and Freundlich isotherm models. The following linearized Langmuir (Eq. 2) and Freundlich (Eq. 3) equations, respectively, were used:
||Effect of initial concentration on the competitive adsorption
of Pb2+ and CO2+
||Effect of contact time on the competitive adsorption of Pb2+
where, Ct is the metal concentration after some time (mg L-1), qm is the maximum sorption capacity (mg g-1), KL is the Langmuir sorption constant (L mg-1) and KF, n are Freundlich constants related to the sorption capacity of the adsorbent and sorption energy, respectively.
The Langmuir and Freundlich parameters were calculated and presented in Table
2. The oxidized MWCNTs were found to have a maximum sorption capacity (qm)
of 54.38 and 49.94 mg g-1 for Pb2+ ions and CO2+
|| Calculated Langmuir and Freundlich constants for the adsorption
of Pb2+ and CO2+
|MWCNT = Pristine MWCNTs
||Effect of initial concentration of the % removal of Pb2+
||Effect of contact time on the % removal of Pb2+
The polymer displayed a maximum sorption capacity of 28.86 and 21.44 mg g-1
for Pb2+ ions and CO2+ ions, respectively.
Characterization of unfunctionalized and functionalized MWCNTs: From
the SEM image Fig. 1, the MWCNTs appear as straight bundles
of uniform lengths. Each of the bundles consists of numerous closely packed
MWCNTs ropes with no indication of surface modifications (Vivekchand
et al., 2004). After oxidation (Fig. 2), the MWCNTs
lost their uniform and straight orientation. The oxidized MWCNTs appear highly
distorted and surface modification is evident (see arrow insert).
The FT-IR spectrum of the pristine MWCNTs (Fig. 3) does not
show the presence of any distinctive functional groups. The peak at 1623 cm-1
corresponds to the C = C of the carbon nanotube skeleton and the peak at 3436
cm-1 is due to moisture. On completion of oxidation (Fig.
4) peaks corresponding to C = O and C-O are observed at 1724 and 1062 cm-1,
respectively. This is an indication of the presence of carboxylic groups on
the MWCNTs. The C-H peak also emerges at 2919 cm-1 indicating a change
of hybridization state from sp2 to sp3, as a direct result
from the attachment of the carboxylic groups. This clearly confirms that oxidation
of the MWCNTs took place (Lu et al., 2008).
Characterization of MWCNT-CD polymer: The SEM images show tshat the
polymer has an uneven surface which appears to be porous or spongy. The spongy
appearance is more evident at low magnification (Fig. 5) while
at higher magnification the polymer appears to consist of numerous granular
aggregates (Fig. 6).
Physically, the polymer also displays a non-uniform appearance as some of it is powdery while the other part of it is granular. The granular appearance could probably be as a result of a highly cross-linked structure of the polymer.
Characterization of the MWCNT-CD polymer: The polymerization of the functionalized MWCNTs and CDS was assessed by monitoring the disappearance of the isocyanate peak (2273 cm-1) of the bifunctional linker (Fig. 7). After 24 h, the isocyanate peak had disappeared, marking the completeness of the reaction.
From the FT-IR spectrum of the MWCNT-CD polymer (Fig. 8),
the peak at 3425 cm-1 is due to OH group while the two peaks at 2932
and 2850 cm-1 correspond to the C-H asymmetric and symmetric stretches
respectively. The peaks at 1705 and 1640 cm-1 can be assigned to
the C = O stretching vibrations of the carbonyl skeleton and the amide functions,
respectively. The peak at 1558 cm-1 can be associated with the N-H
bending vibrations of secondary acyclic amides (Silverstein
et al., 2005). The presence of these peaks gives an indication of
the amide functions and confirms that polymerization took place. The C-N stretching
vibration of the amide can be found at 1261 cm-1 while the C-O stretching
vibration is observed at 1040 cm-1 (Yu et
Surface area analysis of the adsorbents: The oxidized MWCNTs possess the largest surface area, followed by the pristine MWCNTs and lastly the MWCNT-CD polymer. The surface area of the MWCNTs increased remarkably after oxidation. This might be caused by the action of the acid by untying and separating the bundles of MWCNTs resulting in loose strands and therefore increasing the surface area of the oxidized MWCNTs. The polymer was found to have the lowest surface area but the highest pore diameter. The lower surface area of the polymer could probably be as a result of its granular nature.
Adsorption of Pb2+ ions: The absorption capacity of MWCNT-CD
polymer for Pb2+ gradually decreased as the concentration increased
from 10-50 ppm (Fig. 9). The highest amount of lead adsorbed
was 4.08 mg g-1 at 10 ppm, dropping to a low 1.57 mg g-1
at 50 ppm. This could probably be due to the smaller surface area of the polymer
which quickly becomes saturated by the adsorbate ions.
The highest % removal of Pb2+ was 68.0% at 10 ppm, dropping to 9.98% at 50 ppm (Fig. 11). An investigation into the effect of contact time on the adsorption capacity of the MWCNT-CD polymer indicated no significant change in the amount of Pb2+ adsorbed as the time increased (Fig. 10). The amount of Pb2+ adsorbed reached a maximum of 3.81 mg g-1 (68.5% removal) after 120 min (Fig. 12). The oxidized MWCNTs showed an increasing adsorption capacity of 4.94 mg g-1 at 10 ppm to 22.62 mg g-1 at 50 ppm. A maximum adsorption capacity of 5.05 mg g-1 (91.9% removal) was recorded after 120 min. As expected the pristine MWCNTs showed very little affinity for the metal and the lowest adsorption capacity was recorded.
Adsorption of CO2+ ions: Figure 13 and
14 show the effect of concentration and contact time, respectively,
on the adsorption of CO2+. The % removal of CO2+ at different
concentrations and times is shown in Fig. 15 and 16,
respectively. A similar trend to Pb2+ adsorption was observed for
the CO2+ uptake by the MWCNT-CD polymer. A maximum uptake of 3.89
mg g-1 at 10 ppm was reached, dropping to 1.85 mg g-1
at 50 ppm.
The highest % removal of CO2+ was 64.5% at 10 ppm. As a factor of time, there was a slight increase in the amount of CO2+ adsorbed in the first 40 min, dropping after 60 min and increased again and reached maximum adsorption after 100 min. The oxidized MWCNTs showed an increasing adsorption capacity from 5.64 mg g-1 at 10 ppm to 18.32 mg g-1 at 50 ppm, with a maximum % removal of 87.4% at 20 ppm. The amount of CO2+ adsorbed with time by oxidized MWCNTs increased slightly from 2.99 mg g-1 to a maximum of 3.86 mg g-1. The pristine MWCNTs displayed a similar performance to Pb2+ uptake and showed very low adsorption capacity for CO2+.
Competitive adsorption of Pb2+ and CO2+ ions: From Fig. 17 it can be observed that the MWCNT-CD polymer adsorbed Pb2+ better than CO2+ from the binary solution with a maximum adsorption of 3.82 mg g-1 compared to 3.05 mg g-1 for CO2+ at 10 ppm. The polymer recorded a maximum % Pb2+ removal of 59.1% compared to 50.7% recorded for CO2+ (Fig. 19). As a factor of contact time (Fig. 18) the maximum absorption of Pb2+ by the polymer was found to be 2.84 mg g-1 after 60 min while for CO2+ was found to be 2.66 mg g-1 after 80 min. These values correspond to 60.9 and 55.5% Pb2+ and CO2+ removal, respectively (Fig. 20). The oxidized MWCNTs displayed an increasing adsorption capacity for both metals with increasing concentration. Pb2+ was adsorbed better than CO2+ with a maximum adsorption of 14.06 mg g-1 (80.5%) compared to 10.46 mg g-1 (74.7%) for CO2+. A similar adsorption pattern was recorded for the pristine MWCNTs and as expected they displayed the lowest adsorption capacity for the metals.
The amount of both Pb2+ and CO2+ adsorbed by the polymer was lower in the competitive adsorption compared to the single ion adsorption. The same adsorption pattern was observed for the other adsorbents. This observation suggests a possible competition between the two metal ions for the adsorbents. The maximum % removal of both metal ions was also lower in the competitive adsorption than in the single ion adsorption. In the presence of the competing metal ions, the polymer managed to remove more than 55% of each of the metal ions.
Isotherm models: From Table 2, the qm values
for Pb2+ and CO2+ by the MWCNT-CD polymer were calculated
as 28.86 and 21.44 mg g-1, respectively. This is in agreement with
the observed adsorption pattern of the two metals, where Pb2+ was
adsorbed better than CO2+. For the oxidized MWCNTs, qm
values were found to be 54.38 and 49.94 mg g-1 for Pb2+
and CO2+, respectively, indicating a superior adsorption capacity
compared to the other two adsorbents. Pristine MWCNTs showed qm values
of 8.73 and 10.92 mg g-1 for Pb2+ and CO2+,
respectively. The Kf values were also higher for the adsorption of
Pb2+ than CO2+ which indicated better adsorption of Pb2+
compared to CO2+. Except for the pristine MWCNTs the values
for n were all close to the range of 2-10 which is an indication of good adsorption
(Jiang et al., 2009). For the pristine MWCNTs
the values for n were below 1 which indicated poor adsorption and this observation
was in line with earlier observations. The values for R2 were close
to 1; indicating good correlation and based on these values it was observed
that the adsorption of Pb2+ and CO2+ by the three adsorbents
can be better explained by the Langmuir model.
The results obtained from this study indicated that the MWCNT-CD polymer had a smaller surface area when compared to the oxidized MWCNTs and the pristine MWCNTs. The polymer showed better adsorption at 10 ppm for both metals and the adsorption capacity dropped drastically with increasing concentration owing probably to the smaller surface area of the polymer. In terms of the competitive adsorption studies, Pb2+ was found to be adsorbed better than CO2+. Additionally, the adsorption capacity of the polymer as well as the other adsorbents for Pb2+ and CO2+ was found to be lower in the competitive adsorption than in the single ion adsorption, which suggests competition between the two metal ions. The oxidized MWCNTs showed a superior adsorption capacity for the two metals compared to the other two adsorbents used. The adsorption isotherms indicated that adsorption of the two metals by the three adsorbents can be explained by the Langmuir model, as evident from the larger r2 values for the Langmuir isotherms compared to the Freundlich isotherms.
Although, the polymer did not display a higher adsorption capacity for the metals it can still be used as a dual water purification system for the removal of both organic and inorganic pollutants, especially at low concentrations. The polymer shows a greater applicability for metal concentrations around 10 ppm and below.
Our gratitude goes to the National Research Foundation (NRF), Nanotechnology Innovation Centre (NIC) and the University of Johannesburg (UJ) for their immeasurable support in the project.