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Physicochemical characterisation of the ubiquitous bracken fern as useful biomaterial for preconcentration of heavy metals

Posted Feb 02 2010 4:49am

1. Introduction
One simple and elegant approach to trace metal preconcentration
and/or speciation by solid phase extraction is the use of biological
substrates such as fungi, bacteria, algae, yeast, plant derived
materials, etc. These substrates have unique characteristics that
make them very attractive as an alternative to the existing preconcentration
methods. Among other advantages, biological substrates
offer a wide diversity of metal ion binding sites, they also are highly
selective for metal ions, less subject to interference from alkali and
alkaline earth metals than ion-exchange resins, and the accumulation
process does not necessarily require biological activity (Madrid
and Camara, 2000). On the other hand, as compared to
liquid–liquid extraction methods, they offer additional advantages
such as flexibility, higher enrichment factors, absence of emulsion
and more importantly, they are environmentally friendly. Biosorption
combines the effectiveness of the sorption processes in metal
uptake together with the use of low cost, in many cases industrial
by-products, dead biomass. It can be described as a passive process,
which can include ion exchange, coordination, complexation, chelation,
adsorption and microprecipitation. In biosorption, not necessarily
a single mechanism takes place and several of them can
be acting together, making difficult to distinguish individual steps
(Volesky, 2003).
Different sources of materials have been employed as metal
sequestering agents, as it can be found in books or reviews, see
for example (Bailey et al., 1999; Lodeiro et al., 2006b; Volesky,
2003) and references therein. The search of new sorbents with specific
properties for quantitative recovery of substances remains as a
topical task because most of the simple enrichment procedures
currently available rely essentially on adsorption of the analytes
of interest (Godlewska-Zylkiewicz, 2006).
Research studies of different biosorbents have shown that good
results are obtained when the material presents a large number of
chemical binding sites such as carboxyl, carbonyl, hydroxyl, amine
or amide groups. These chemical groups act as sequestering sites
when they interact with metals present in solution. Algae, chitinous
materials or cellulose containing biomass has been employed
owing to the occurrence of one or more of the mentioned chemical
groups in these materials. Studies with lignocellulosic materials, a
major component in the cell wall of plants, have determined the
presence of carboxylic and hydroxylic functional groups (Bouanda
et al., 2002).
Bracken fern, Pteridium aquilinum, is a ubiquitous plant wide
spread all around the world. Large amounts can be easily collected
and processed for a subsequent use as metal sorbent. Consequently,
a detailed physicochemical characterisation of cadmium
and lead uptake as an example of two different metals has been

done to determine fern potential use as sorbent for heavy metal
2. Experimental
2.1. Materials
Reagents used were NaOH pa. and HCl qp. from Panreac (Panreac
Química S.A., Barcelona, Spain). Cadmium and lead salts,
Cd(NO3)2 4H2O and Pb(NO3)2 pa. from Merck (Merck KGaA, Darmstadt,
Germany) were used to prepare metal solutions. NaNO3, KNO3
and KOH pa. were also from Merck. HNO3 Suprapur from Merck
was employed to adjust pH of solutions, and it was also used as
background electrolyte in anodic stripping voltammetry (ASV)
measurements. All solutions were prepared with MilliQ water
(MilliPore, Molsheim, France).
Dry bracken fern was collected at the end of summer season.
The fern was ground with an analytical mill (IKA A 10, IKA Werke
GmbH & Co. KG, Staufen, Germany) and sieved. Particles with size
ranged from 0.5 to 1 mm were collected and used for metal sorption
studies. A portion of these particles were acid washed to obtain
a fully protonated material for the acid–base titration studies.
Infrared spectra were obtained using a Bruker Vector 22
equipped with a Specac Golden Gate ATR device, which allows direct
determination of the infrared spectrum of solid materials
without further treatment or support.
2.2. Acid–base titrations
In order to determine the number of acidic groups present in
the sorbent material and the apparent pK of these groups, acid–
base titrations were carried out using fully protonated material.
Four grams of fern were placed in a conical flask and shaken with
200 mL of 0.1 mol L 1 hydrochloric acid for at least four hours.
Supernatant solution was discarded and fern was washed again
with 200 mL of hydrochloric acid. A third wash of the fern was
done with 600 mL of hydrochloric acid. The material was then
rinsed with deionised water several times, and the conductivity
of water was measured. The washing with water was repeated until
the conductivity of the filtrate was below 10 lS cm 1. Protonated
fern was dried overnight at 70 C in a forced convection
oven. For titrations, 0.5 g of protonated fern were placed in a
thermostated cell together with 100 mL of a solution of KNO3
0.05 mol L 1 as supporting electrolyte and 0.001 mol L 1 of hydrochloric
acid which ensures that a large number of acidic groups in
the fern will be initially protonated. Titrations were done with KOH
0.05 mol L 1. A non-combination glass electrode (from Ingold, Wilmington,
USA) and a Ag/AgCl reference electrode (from Metrohm,
Herisau, Switzerland) were used with a GLP 22 pH-meter (from
Crison, Barcelona, Spain) to do the measurements. Titrant additions
were done with an automatic burette microBU 2031 from Crison.
Data acquisition and burette control were done with a home made
program. Titrations and calibrations of the glass electrode were
done at 25 C in a nitrogen atmosphere. A full description of
the calibration and titration procedure can be found elsewhere
(Rey-Castro et al., 2003).
2.3. Sorption experiments
Kinetic studies were monitored voltammetrically and potentiometrically.
For potentiometry, a cadmium ion selective electrode
(Model 9448 cadmium half-cell electrode from Orion Research
Inc., Beverly, USA) was used together with a non-combination glass
electrode to follow pH evolution. A Ag/AgCl electrode was used as
reference electrode for these measurements. pH was adjusted and
maintained constant at the desired value by addition of small aliquots
of 0.3 mol L 1 NaOH. NaNO3 was added to adjust ionic
strength to 0.05 mol L 1. Potentiometric readings were recorded
using a home made program; this program also controlled additions
of base to the biomass suspended solution. Kinetic experiments
were done in a thermostated cell where ca. 0.25 g of
sorbent were placed together with 100 mL of metal solution (i.e.
the same sorbent dose used in equilibrium studies, 2.5 g L 1). A
magnetic stirrer was used to shake the solutions and high purity
(99.9995%) nitrogen was bubbled through the solution to avoid
interferences with atmospheric CO2. Small aliquots were taken
periodically and metal concentration in solutions was measured
by ASV using a 747 Computrace polarograph (from Metrohm), with
a conventional system of three electrodes: hanging mercury drop
electrode as working electrode, a platinum auxiliary electrode
and 3 mol L 1 Ag/AgCl reference electrode. ASV differential pulse
analysis was done under the following conditions: 20 s equilibrium
time, 0.8 V deposition potential, 180 s deposition time, 20 s equilibrium
time, 0.01 V/s sweep rate, 0.002 V voltage step, 0.05 V
pulse amplitude and 40 ms pulse time. Start potential was 0.8 V
and end potential was 0.4 V. This was the procedure for cadmium
ASV determination. Similar conditions were used for lead determination
with small changes in the deposition potential ( 0.6 V), the
start potential ( 0.6 V) and the end potential ( 0.2 V). As it will be
discussed later, lead measurements were done only by ASV.
pH dependence studies were done placing accurate amounts of
sorbent material (ca. 0.1 g) in conical flasks, which contained
40 mL of metal solution. Equilibrium isotherm studies were done
with an identical sorbent concentration (0.1 g in 40 mL, that is
2.5 g of sorbentper litre of solution),andseveralmetal concentrations
(from 10 to 500 mg L 1 for cadmium and from 10 to 600 mg L 1
for lead). pH was monitored periodically and adjusted to the desired
value by addition of small volumes of concentrated HNO3
Suprapur, or NaOH pa. (0.3 mol L 1) when it was necessary. An
orbital shaker (Edmund Bühler GmbH, Hechingen, Germany) was
used to improve contact between sorbent and solution. Agitation
speed of 175 rpm was experimentally observed to be enough to
achieve a good mixing of the sorbent material with the solution.
When equilibrium was reached, solutions were filtered with Albet
(Barcelona, Spain) cellulose nitrate membrane filters (pore diameter
0.45 lm), and metal remaining in solutions was determined by
All the experiments were done in duplicate at 25 C.
3. Results and discussion
3.1. Sorbent characterisations
The infrared spectrum (not shown) obtained for bracken fern
showed a very wide absorption band at 3330 cm 1 that can be assigned
to both aromatic and aliphatic hydroxyl groups and also to
amine groups (Bouanda et al., 2002). Intense bands at 2918 and
2850 cm 1 can be attributed to the C–H stretching vibration in
methyl and methylene groups and methoxyl group respectively
(Bouanda et al., 2002). 1730 cm 1 band corresponds to C@O
stretching of the carbonyl groups of carboxylic acids. 1650 cm 1
band is associated to C@O stretching of conjugated carbonyl
groups (Suhas et al., 2007). 1605 and 1511 cm 1 bands can be assigned
to C–C stretching in aromatic skeletons (Suhas et al., 2007).
Bands at 1025 and 1360 cm 1 are characteristic of carbohydrate
units (Bouanda et al., 2002). Band association of the FTIR spectrum
suggests that the main composition of the bracken fern corresponds
to a carbonaceous material with aromatic, hydroxyl, methoxyl
and carbonyl functionalities. The similarity of IR spectrum
with the spectrum of the lignocellulosic substrate reported by

Bouanda et al. (2002) verifies what it could be expected, the main
component in the dried bracken fern used as sorbent is most likely
lignocellulosic material, the main constituent in the cell wall.
Scanning electron microscopy (image not shown) of the biomass
showed large three-dimensional particles with a spongy aspect
and fibrous protuberances. Much smaller particles could
also be observed; they were either originated during the grinding
process or came off from the larger particles.
Dry fern particles are initially slightly hydrophobic and do not
get wet fast, therefore special care must be taken into account to
ensure that the sorbent is in good contact with the solution during
the sorption experiments, especially in the kinetic studies.
3.2. Acid–base studies
According to Bouanda et al. (2002) who performed acid–base
titrations of lignocellulosic substrate extracted from wheat bran,
the most likely acidic functional groups present in their material
were carboxylic and phenolic sites. These authors pointed out that
it is not possible to distinguish phenolic groups from potentiometric
titrations in aqueous media. Ephraim et al. (1986) indicated the
same observation in the study of fulvic acid equilibria. In fact, these
authors suggested that non-aqueous titrations of fulvic acid allowed
distinguishing two types of acidic groups. They assigned
these two acidic groups to carboxylic and hydroxyl functionalities.
The combination of total carboxylic and hydroxyl functionalities
calculated from the non-aqueous titrations agreed well with total
acidic capacity obtained from the equivalence point of aqueous
titrations. A small excess in the total acidity in non-aqueous media
was observed for some of the substrates. Ephraim et al. (1986) concluded
that most likely the total acidity observed in aqueous media
corresponded to the contribution of carboxylic moieties and an
acidic alcohol (enol). The excess in acidity observed in non-aqueous
media was attributed to the presence of phenolic functionality
not measurable in aqueous media.
Fern titrations were done very slowly (each titration took
approximately 12 h) owing to the long equilibration times required
to acquire potentiometric data, especially when the titration was
close to the equivalence point (Naja et al., 2005; Rey-Castro et al.,
2003). From the equivalence point of the titrations, and the concentrations
of the acid and the base used, the number of acidic functional
groups present in the material was estimated. An average
value of 0.432 ± 0.006 mmol g 1 acidic groups was calculated for
the fern used in the present study. Bouanda et al. reported a higher
value (0.54) for the carboxylic content of lignocellulosic substrate
extracted from wheat bran. Olive pomace biomass titrations
(Pagnanelli et al., 2008) provide a value of 0.17 mmol g 1 carboxylic
groups, meanwhile 0.39 carboxylic groups have been found in citrus
peels (Schiewer and Patil, 2008). Potentiometric titrations of activated
carbons obtained from lignocellulosic material (Puziy et al.,
2007) have shown a wide distribution in the amount of strong-medium
acidic sites (between 0.93 and 0.24 mmol g 1 for strong acidic
sites and 0.50–0.08 for medium ones) depending on the chemical
and temperature conditions followed to obtain the activated carbon.
These examples show that the amount of acidic sites present
in the biomass employed can change significantly owing to the high
heterogeneity in the sources of the materials employed. In fact,
other study done with organic matter extracted from wheat bran
(Ravat et al., 2000) report a much lower amount of carboxylic
groups, 0.08 mmol g 1, than the value reported by Bouanda et al.
The total acidic content of fern is significantly small compared to
other materials, such as fulvic acids (Ephraim et al., 1986) with a
content in acidic groups above 5 mmol g 1 or some dry algae which
contain more than 2 mmol g 1 (Herrero et al., 2006; Lodeiro et al.,
2005). This is likely the reason of the lower uptake observed for cadmium
or lead by fern if it is compared with those algae.
During titration, the charge developed in the sorbent material
has to fulfil the electroneutrality balance:
½A ¼ ½Hþ þ ½Kþ ½Cl ½OH ð1Þ
where the proton concentration, [H+], can be obtained from the
potentiometric readings after glass electrode calibration, hydroxide
concentration, [OH ], is obtained from proton concentration and
the equilibrium constant of water, Kw, and potassium and chloride
concentrations, [K+] and [Cl ], are calculated from the concentrations
of base and acid used and their respectively volumes added
to the solution. [A ] is the concentration of ionised acidic groups
in the sorbent material at each step of the titration.The degree of
dissociation, a, of the acidic groups present in the sorbent can be
calculated as:
a ¼
½A tot
C0:V0=ðV0 þ VKOHÞ
where C0 is the total concentration of the acidic groups, obtained
from the equivalence point of the titration. V0 is the initial volume
of the solution and VKOH is the volume of the potassium hydroxide
solution added at each titration point.
According to Katchalsky formalism, the titration curve of a polyacid
can be empirically described as (Katchalsky and Spitnik,
pH ¼ pK n0 log
1 a
a ð3Þ
where pK is the apparent pK of the polyacid, and n0 is a constant
with a value larger than one. The representation of pH versus
log 1 a
a should be a straight line with a slope equal to n’ and the value
of the apparent pK of the polyacid is calculated at the intercept
of the fitting (i.e. when a = 0.5). The mean value calculated for the
apparent pK from the fern titration was 4.37 ± 0.13, which could
be ascribed to a carboxylic functionality. Bouanda et al. (2002) have
reported a higher intrinsic vale of carboxylic pK (5.51) meanwhile
Ravat et al. (2000) indicated a lower value (3.6) for lignocellulosic
substrate obtained from wheat bran.
3.3. Kinetic studies
Kinetic experiments allow obtaining fundamental information
about the rate at which sorption process takes place. Fig. 1 shows
the kinetic data recorded for cadmium uptake using potentiometric
readings. A preliminary study showed that maximum cadmium
uptake occurred at pH close to 7, so that pH was chosen to do the
kinetic studies for cadmium sorption. As it can be seen in Fig. 1,
after 200 min the concentration of cadmium measured in solution
continues decreasing although the slope of the graph for times
above 200 min is much smaller than the corresponding slope observed
during the first 100 min. Small aliquots of the supernatant
solution were collected during the potentiometric experiments,
and cadmium concentration was determined using ASV. Fig. 1 also
shows qt, the mmoles of cadmium sorbed per gram of fern, determined
by potentiometry and voltammetry, calculated as follows:
qt ¼
V:ðCi CtÞ
m ð4Þ
where V is the volume of solution (L), Ci is the initial metal concentration
in solution (mmol L 1), Ct is the concentration of metal in
solution at time t, and m is the sorbent mass (g).
As it can be seen in Fig. 1, measurements made by ASV provided
smaller sorption values than potentiometric results. These observations
indicate that for times above 50 min, the amount of complexing
organic matter released into the solution from the
sorbent particles starts to increase significantly. Cadmium will be
present in solution as free metal ion and organic matter bound

ion. Ion selective electrode responds only to free cadmium ion.
Cadmium bound to organic matter present in solution is not measured
by the ion selective electrode, and therefore, potentiometric
readings indicate lower concentrations of cadmium, hence, higher
metal uptake values. ASV measurements were made in acidic media.
Protons compete with cadmium ions for the organic matter; in
acidic media the equilibrium is shifted towards the release of free
metal ion, and therefore ASV measurements determine the total
cadmium in solution. Since the concentration of cadmium determined
by ASV will be larger than by potentiometry, uptake values
calculated by ASV will be smaller than potentiometric results. After
120 min (see Fig. 1) cadmium uptake determined by ASV reaches a
maximum value, which will indicate the maximum uptake of the
solid particles of biomass. However from the potentiometric readings,
uptake apparently continues increasing even at times above
250 min which indicates that as shaking proceeds more and more
organic matter is released from the particles to the solution and it
binds to free cadmium ions.
Since organic matter was expected to react with lead in a similar
manner than cadmium, kinetic experiments for lead were only
followed by ASV. Results obtained for lead are shown in Fig. 1. In
this case, a preliminary study indicated that optimum lead sorption
took place at pH 4.5–5, which was the pH chosen for these
experiments. As it can be seen in Fig. 1, lead uptake by fern is
slower than cadmium uptake. No significant increase is observed
for time above 250 min.
Experimental data (qt versus time) were fitted to the pseudo
second order sorption process proposed by Ho et al. (1996):
dt ¼ kðqe qtÞ2 ð5Þ
where k is the pseudo second order sorption constant and qe is the
metal uptake at equilibrium (calculated with Eq. (4) when Ct = Ce
concentration at equilibrium). If boundary conditions are considered,
qt = 0 at t = 0 and qt at time t, Eq. (5) can be integrated
qt ¼
1 þ qekt ð6Þ
Pseudo second order integrated equation has been used to fit
data obtained by ASV measurements. Results are collected in Table
1. Fitting of the experimental values to the pseudo second order
model is good (r2P0.994). Kinetic constant is higher for cadmium
as it could be expected from the observed behaviour (i.e. maximum
uptake was attained faster for cadmium than for lead).
3.4. pH dependence studies
The influence of pH of solution upon metal uptake is very
strong (Wase and Forster, 1997). This fact can be explained taking
into account the competition established between protons and
metal ions for active sites of the sorbent material. Appropriate tuning
of the pH of solution can render the percentage of metal removed
to very high values, close to 100% elimination of metal
from solution, or hinder metal sorption capability of the material
used (Lodeiro et al., 2004; Volesky, 2003; Wase and Forster,
1997). A detailed analysis of the influence of pH upon the sorption
process is therefore required if best performance of the sorbent
used is desired.
pH dependence for cadmium was analysed for an initial metal
concentration of 500 mg L 1. Cadmium uptake by fern was analysed
for pH values ranging from 1 to 8. pH was not increased further
to avoid precipitation of metal as a hydroxide. Fig. 2 shows the
results obtained. For very low pH (<2) cadmium sorption is negligible.
As pH is increased further, sorption starts to increase, and
for pH values higher than 6, sorption reaches a stable value. These
results support the preliminary pH studies done with the material.
Further sorption analysis studies were done taking into consideration
these observations. The shape of the curve can also be correlated
with the acid–base titrations of the material. Fig. 2 shows the

degree of dissociation of the acidic functional groups associated to
the biomass calculated from the Katchalsky analysis of the potentiometric
titrations (dashed line). As it can be seen, there is a close
agreement between the percentage of dissociated acidic groups
(most likely carboxylic functionalities) and cadmium uptake. This
fact suggests that at low pH values, the acidic groups of biomass
are protonated and they can not act as ligands for the free cadmium
ion. When the pH starts to increase, some of the acidic
groups start to dissociate, and they bind to a fraction of the cadmium
ions present in solution. As the degree of dissociation increases,
more cadmium ions get bound. Once all the acidic
groups are fully deprotonated and bound to cadmium ions, biomaterial
becomes saturated of cadmium and no further uptake can be
Analysis has also been done for lead sorption by fern. An initial
lead concentration of 900 mg L 1 was used, which corresponds
with a molar concentration of 4.34 mmol L 1, similar to the cadmium
concentration used. In the case of lead, precipitation occurs
at lower pH values than cadmium; therefore the pH analysis was
performed only up to pH 6. The results obtained are shown in
Fig. 2. Owing to the precipitation of lead hydroxide at low pH values,
it becomes difficult to analyse pure sorption process, separated
from precipitation even at pH 5 (see in Fig. 2 the sharp
increase above this pH).The pH dependence in lead uptake for an
initial metal concentration of 450 mg L 1 (i.e. 2.17 mmol L 1) is
also shown in Fig. 2. In this case the curve shape suggests that at
a pH 4.5 lead uptake is reaching a plateau and precipitation is
not taking place, hence this was the compromise pH chosen for
the analysis of metal concentration in order to obtain an isotherm
for lead uptake by fern without the interference of metal hydroxide
precipitation. The trend in lead uptake shown in Fig. 2 at low pH
follows a similar behaviour as cadmium, with no lead elimination
at pH<2, and a progressive increase as pH increases. However in
the case of lead, the occurrence of precipitation at lower pH values
compared to cadmium makes difficult to establish a direct relationship
between the degree of dissociation values of the acidic
groups of the biomass and the lead uptake. As is could be expected,
metal uptake process is influenced by both the dissociation state of
the biomaterial employed and the chemical speciation of the metal
3.5. Effect of the initial metal concentration
Solutions containing different metal concentration were used to
study the effect of initial concentration upon metal uptake once
equilibrium was attained. Initial cadmium concentration was studied
in the range of 10–500 mg L 1 and lead was studied for concentrations
between 10 and 600 mg L 1. The biomass dosage
employed was the same used in all the previous experiments,
2.5 g L 1. Cadmium study was done at pH = 7, and lead equilibrium
data were measured at pH = 4.5. Fig. 3 shows the results obtained.
As it can be seen, the lead shows a very sharp increase in sorption
for low initial concentration values. The nearly vertical slope of the
lead sorption graph for low metal concentrations indicates a high
affinity of the biomaterial employed for this metal. Therefore, very
high efficiencies for lead removal can be expected when fern is
used under these conditions (pH and lead concentration). Cadmium
sorption shows a more gradual increase, with no initial
sharp trend. Even though, the maximum uptake is very similar
for both metals.
3.6. Isotherm models
Experimental data of several metal concentrations at equilibrium
with the sorbent allow the determination of the isotherm of
the sorption process. One of the models most commonly used is
the Langmuir isotherm:
qe ¼
qmax b Ce
1 þ b Ce
where qmax represents the maximum sorption capacity attained by
the sorbent biomaterial, and b is the equilibrium constant of the
sorption process. Table 2 lists the results obtained for the Langmuir
fitting of the cadmium data. The sharp increase of lead sorption
makes the fitting of data to the Langmuir isotherm less convenient.
With so sharp increase in data trend other models provide better results
(Barriada et al., 2007). The Langmuir–Freundlich isotherm was
used in the fitting of lead data. Eq. (8) shows the function corresponding
to this isotherm:
qe ¼
qmax ðb CeÞ1=n
1 þ ðb CeÞ1=n ð8Þ
Table 2 also lists the fitting results obtained for lead using the
Langmuir–Freunlich isotherm model. As it can be seen in this table,
maximum sorption capacity of fern for both metals is the same,
0.41 mmol of metal per gram of fern. This value is very similar to
the total number of acidic functionalities found for the biomaterial
employed, which was 0.432. This result supports the hypothesis of
chemical interaction of the fern acidic groups with the metal ions
(as it was stated, most likely carboxylic functionalities with possible
hydroxylic group interactions), maintaining the same mole ratio
for both metals. In the case of lead, b value is one order of
magnitude higher than the value for cadmium, in agreement with
the sharper shape of the isotherm. This value indicates that the
affinity for lead by fern is higher than for cadmium. The formation
constants of lead and cadmium with simple carboxylates follow
also a similar trend (Martell and Smith, 1977). Differences for these
simple carboxylic acids are also in general about one order of mag-

nitude. Following these results, the fern biomass presents a higher
selectivity for lead if it is compared to cadmium. Cystoseira baccata,
a marine brown macroalga, has also shown a similar behaviour in
the affinity constants found for both metals (Lodeiro et al., 2006a),
with differences even slightly larger between the two constants
Table 3 lists different maximum sorption capacities of several
biosorbents collected from the literature. Some substrates, mainly
different kinds of algae, have shown a higher capacity for the metals
studied than bracken fern. However, fern still constitutes an
attractive alternative as sorption material, with an efficiency higher
than other materials reported in literature. The low cost of fern,
the ease to process in order to obtain the sorbent particles, and the
acceptable sorption capacity provide good justified reasons to
claim the use of fern as a sequestering material for preconcentration
of heavy metals.
4. Conclusions
The study of fundamental physicochemical parameters regarding
to sorption capacity of new materials provides useful information
for future applications of these materials in sorption based
experiments. In the present study, the analysis of the behaviour
of bracken fern indicates that it could be used as a sequestering
agent for cadmium or lead enrichment in solid phase separation
studies if the appropriate conditions are chosen. Removal of cadmium
from solution takes place at an optimum pH of 7 meanwhile
for lead sorption a pH value of 4.5 allows a high metal uptake while
hydroxide precipitation is avoided. The maximum sorption of the
material is reached within two hours for cadmium uptake, but lead
uptake requires longer contact times and four hours are needed to
attain the maximum lead removal. Although the rate of metal uptake
significantly differs for both metals, the maximum metal sorption
obtained was 0.41 mmol g 1 for both of them. This value is
very close to the total acidic groups determined for this material,
0.432 mmol g 1 suggesting a 1:1 interaction between the metal
and the acidic groups of the biomass. The IR spectrum of the material
also identifies the main constituents as lignocellulosic materials
with a high content of hydroxylic groups which could also
interact with the metals bound to the biomass.

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