Arch. Min. Sci., Vol. 56 (2011), No 1, p. 71–80

Arch. Min. Sci., Vol. 56 (2011), No 1, p. 71–80
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Electronic version (in color) of this paper is available: http://mining.archives.pl
AGNIESZKA GALA*, STANISŁAWA SANAK-RYDLEWSKA*
REMOVAL OF Pb2+ IONS FROM AQUEOUS SOLUTIONS ON PLUM STONES CRUSHED
TO PARTICLE SIZE BELOW 0.5 mm
USUWANIE JONÓW Pb2+ Z ROZTWORÓW WODNYCH NA FRAKCJI PESTEK ŚLIWEK
PONIŻEJ 0,5 mm
This article presents the results of the research on the Pb2+ sorption from aqueous solutions on plum
stones. For the used sorbent the depletion of Pb2+ in the solution was 47.2-81.3%. The effect of various
factors, such as the concentration of natural sorbent, the pH, and the temperature was studied. The process of Pb2+ ions sorption on plum stones was described by the Langmuir and Freundlich models. The
maximum sorption capacity of plum stones was for 21.2 mg/g.
Keywords: sorption, lead ions, plum stones
W niniejszym artykule zaprezentowano wyniki badań dotyczących sorpcji jonów Pb2+ z roztworów
wodnych na pestkach śliwek. Wykazano, że pestki śliwek mogą być wykorzystywane do usuwania jonów Pb2+ z roztworów wodnych. Dla badanego sorbenta redukcja zawartości jonów Pb2+ w roztworze
wyniosła 47,2-81,3%.
Zbadano również wpływ czynników, takich jak: stężenie sorbenta, pH roztworu i temperatura na
badany proces sorpcji. Optymalne stężenie sorbenta wynosi 5 g/dm3, powyżej tej wartości stopień redukcji
jonów Pb2+ nie zmienia się istotnie (rys. 4.1). Jest to prawdopodobnie związane ze zjawiskiem agregacji
cząstek sorbenta w roztworze, które może ograniczać dostęp jonów metalu do miejsc aktywnych obecnych
na powierzchni sorbenta. Do takiego samego wniosku doszedł w swojej pracy zespół Uluozulu (2008).
Bardzo ważnym parametrem wpływającym na sorpcję jest odczyn pH. Wykazano, że najlepszą sorpcję
na pestkach śliwek uzyskano przy pH 4,0±0,1 (rys. 4.2), w punkcie zerowego ładunku (tzw. pHzpc). W środowisku silnie kwaśnym ołów występuje głównie w postaci jonów Pb2+. Obniżenie sorpcji, występujące
przy pH poniżej 4,0 prawdopodobnie jest związane z konkurencyjnym oddziaływaniem jonów wodorowych
z jonami dwuwartościowego ołowiu oraz z dodatnio naładowaną powierzchnią sorbenta. Z kolei stopniowy
wzrost pH prowadzi do tworzenia jonów kompleksowych i ostatecznie strącania się jonów ołowiowych
w formie Pb(OH)2. Przy pH powyżej 5,0±0,1 prawdopodobnie tworzą się już jony hydroksylowe ołowiu
i może rozpoczynać się proces strącania wodorotlenku ołowiu (Meena i in., 2008).
*
UNIVERSITY OF SCIENCE AND TECHNOLOGY, FACULTY OF MINING AND GEOENGINEERING, AL. MICKIEWICZA 30,
30-059 KRAKOW, POLAND, CORRESPONDING AUTHOR: e-mail: [email protected]
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Wykazano również, że ze wzrostem temperatury w zakresie (293-303)K następuje stopniowy spadek
sorpcji (rys. 4.3). To potwierdza egzotermiczną naturę procesu sorpcji jonów Pb2+ na pestkach śliwek.
Adsorpcję jonów Pb2+ na pestkach śliwek opisano przy pomocy modelu Langmuira i Freundlicha.
Założenia obu modeli podano w tablicy 4.1.
Wartość parametru a izotermy Langmuira pozwala oszacować maksymalną pojemność sorpcyjną
sorbenta, czyli maksymalną ilość jonów metalu potrzebną do zapełnienia monowarstwy. Dla pestek śliwek
stała a przyjmuje wartość 21,2 mg/g. Drugi parametr izotermy Langmuira, stała b reprezentuje energię
adsorpcji. Im niższa wartość stałej b tym wyższe powinowactwo sorbentu w stosunku do jonów metalu.
Dobry sorbent powinien charakteryzować się niską wartością współczynnika b i wysoką a.
Parametr K Freundlicha umożliwia ocenę zdolności sorpcyjnych sorbenta a stała n określa intensywność adsorpcji (Han i in., 2005). Obliczona wartość współczynnika n równa 1,31 dla pestek śliwek,
znajduje się w przedziale <1,10> co dowodzi, że proces adsorpcji na tym sorbencie jest „korzystny”. Im
wartość 1/n jest mniejsza tym większa jest niejednorodność energetyczna układu adsorpcyjnego (Bansal
i Goyal, 2009).
W przypadku pestek śliwek lepsze dopasowanie funkcji do danych pomiarowych uzyskano dla
izotermy Freundlicha, co świadczy o niejednorodności powierzchni (Atkins, 2007).
Słowa kluczowe: sorpcja, jony ołowiu(II), pestki śliw
1. Introduction
Among numerous pollutants present in natural waters, heavy metals are especially dangerous
for living organisms. The problem with heavy metals is not only their extreme toxicity, but also
their ability to accumulate in the organism. Lead is a heavy metal which gets into the environment with industrial effluents.
Among its other applications, lead is used to manufacture metal elements, pigments,
paints, ammunition; as a stabiliser in PVC production; and in the glass and ceramic industry.
For instance lead(II) chloride and sulphate(VI) are used to manufacture paints and pigments and
lead(II) nitrate(V) is used in photography and in the production of dyes and explosives. Organic
lead compounds are used as wood preservatives, paints, additives to lubricants, catalysts and
antibacterial substances, and tetraethyllead is an anti-knock additive to petrol. Thus the main
sources of lead pollution in the environment are emissions from combustion engines. Zinc-lead
ores smelting operations and solid and liquid products derived during production of non-ferrous
metals (e.g. copper, zinc and lead) by metallurgical methods are also significant factors in lead
pollution in the environment (Dojlido, 1995).
The concentration of lead in atmospheric water ranges from 1 to 50 μg/L, but on densely
inhabited and industrial areas it can reach 1000 μg/L (Moore & Ramamoorthy, 1984). Lead can
get into drinking water when it is in contact with lead pipes or when stored in containers coated
with paints containing this metal. The maximum lead concentration allowed in drinking water
is 50 μg/L (Dz.U.07.61.417).
The toxic effect of lead on living organisms manifests itself in the hematopoietic system,
in hem synthesis, the inhibition of haemoglobin synthesis and in shortening the lifespan of
erythrocytes, leading ultimately to anaemia (Seńczuk, 1999). Lead also has a toxic action on
the nervous and immune systems and impairs the functioning of the kidneys and the alimentary
system (Seńczuk, 1999; Ravi Raja at al., 2008). The degree of water toxicity varies depending
on the water quality characteristics, and also varies among organisms.
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The fact that heavy metals, including lead, are not biodegradable and show a tendency to
accumulate, poses one of the greatest risks to humans. That is why elimination of ionic forms of
lead from aqueous solutions is a serious challenge.
A number of research papers have been written on the removal of Pb2+ and other heavy
metals from aqueous solutions. A significant number of these papers discuss the use of the ability
of natural sorbents to remove heavy metals from aqueous solutions (Sanak-Rydlewska, 2007;
Gupta at al., 2009; White at al., 1995; Qi & Aldrich, 2008; Nadeema at al., 2008; Meena at al.,
2008; Han at al., 2005; Sari & Tuzen, 2008; Uluozulu at al., 2008).
In spite of a good deal of research in this field, the mechanism of binding and concentrating
heavy metals by sorbents of natural origin is not fully known. The process is supposed to proceed
through ion exchange owing to the presence of functional groups such as carboxyl, phenyl and
hydroxyl groups (Sanak-Rydlewska, 2007; Meena at al., 2008). Also, complexation reactions
are possible. In addition to chemisorption, physical adsorption, microprecipitation and redox
reactions can occur (Sanak-Rydlewska, 2007).
Important information concerning the mechanism of heavy metal ion binding could be the
form in which they occur in the solution, which depends on their concentration and the conditions
of the process environment. The sorption properties of the biomass are also important, which can
be enhanced by modifying its surface by physical or chemical methods (Nadeema at al., 2008).
Natural sorbents are also a feedstock for the production of activated carbons (Baccar at al., 2009;
Tajar at al., 2009; Kazemipour at al., 2008).
2. Goal of the study
The goal of the study was to determine the sorption properties of plum stones (Prunus domestica L) toward Pb2+ and the effect of selected factors, such as sorbent concentration, and pH
and temperature of the solution on Pb2+ sorption.
3. Methods of the study
3.1. Preparation of sorbents
Plum stones (Prunus domestica L) without the kernels were used for the study as natural
sorbents. Washed and dried material were crushed to particle size below 0.5 mm. Before the
sorption, the shells and the stones were treated with 0.001 mol/L nitric acid to remove surface
impurities, and then washed with reverse osmosis water until the pH attained that of pure reverse
osmosis water (about 5.90), and next dried at temperatures of up to 323K.
3.2. Sorption process
The study of the sorption process was based on the placing a known mass of sorbent in the
beakers with solutions about the initial Pb2+ concentration ranging from 6.0 to 110 mg/L. Metal
was introduced into the solution in the form of salt Pb(NO3)2. The experiments were performed
for 0,3; 0,5; 1,0; 1,5; 2,0 g of the plum stones. The volume of the solutions in the beakers
was 100 mL. The pH of the studied solutions ranged from 2.0 to 5.0 at ionic strength equal to
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0.02 mol/L. The pH was controlled with a 0.02 mol/L solution of nitric acid, while the ionic
strength was controlled with a 0.04 mol/L solution of potassium nitrate(V). All solutions were
prepared using reverse osmosis water.
Next, the content of the beakers was continuously stirred with a mechanical stirrer at 120 rpm
at the constant temperature ranged from 293 to 303±0.5 K. The experiments lasted for one hour
(in this time the system reached equilibrium).
The lead(II) ions content of the solutions after adsorption was determined by flow-through
coulometry using EcaFlow 150 GLP apparatus manufactured by POL-EKO. Before measurements,
the solutions were passed through a filter paper to remove solid particles. Three measurements
were performed for each sample.
The amount of lead(II) ions adsorbed on the sorbent’s surface was calculated from this
formula:
Q=
where:
Q
V
co and ck
m
—
—
—
—
V (c0 - ck )
m
(1)
amount of the metal ions per gram of the sorbent (mg/g);
is the volume of the solution in the flask (L);
are the initial and final concentrations of lead(II) ions (mg/L);
is the quantity of dry mass of the adsorbent (g).
4. Discussion of the results
4.1. The effect of the sorbent concentration on the sorption of
Pb2+ ions from aqueous solutions
The study of the sorbent’s adsorption as a function of its concentration is shown in Fig. 4.1.
The curves show that that the effectiveness of Pb2+ removal from the solution steeply increases
with sorbent concentration up to 5 g/L. At that point, the reduction reaches 69,5%. Further increasing of sorbent concentration does not significantly modify the degree of reduction of Pb2+ ions.
This is most likely due to the phenomenon of aggregation of sorbent’s particles in the solution,
which can block the access of metal ions to functional groups on the sorbent surface (Uluozulu
at al., 2008). Consequently, further study was continued at a sorbent concentration of 5 g/L.
4.2. The effect of pH on the sorption of Pb2+ ions from aqueous
solutions
The pH of the solution is one of the most important factors controlling the removal of heavy
metals from aqueous solutions. The effect of this factor on Pb2+ sorption on plum stones was
studied in very acidic solutions (pH 2.0-5.0).
The results obtained, shown in Fig. 4.2, confirm the close relationship between the pH of
the solutions undergoing treatment and the efficiency of Pb2+ removal on the sorbent studied.
For plum stones, the sorption increases in pH range from 2.0 to 4.0, to reach the maximum at
pH 4.0 ±0.1. For solution with a pH of 5.0±0.1, the sorption was lower.
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Fig. 4.1. The effect of the sorbent concentration on the sorption of Pb2+ ions on plum stones (initial Pb2+
concentration 15.6 mg/L; pH of 4.0±0.1; sorbent’s particle size class < 0.5 mm; ionic strength 0.02 mol/L;
temperature (298±0.5)K; time of adsorption 1 h; mixing rate 120 rpm.).
Uncertainty of the sorption capacity: 0,39; 0,21; 0,63; 0,47; 0,36%
100
Sorption (%)
90
80
70
60
50
40
1,0
2,0
3,0
4,0
5,0
6,0
pH
Fig. 4.2. The effect of pH on the sorption of Pb2+ ions on plum stones (initial Pb2+ concentration 15.6 mg/L;
sorbent concentration 5 g/L; sorbent’s particle size class < 0.5 mm; ionic strength 0.02 mol/L;
temperature (298±0.5)K; time of adsorption 1 h; mixing rate 120 rpm.).
Uncertainty of the sorption capacity: 0,13; 0,43; 0,21; 0,50%
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The observed effect of pH can be explained on the basis of sorbent surface charge. The pH
of the solution and the functional groups present on the sorbent surface (e.g., carboxyl, phenyl,
hydroxyl and amide groups) determine the sign and the value of this surface charge (Bansal &
Goyal, 2009). The pH for which the maximum sorption capacity was noted (in this case at a pH
of 4.0±0.1) is the so-called pHpzc (point of zero charge) (Meena at al., 2008). In solutions with pH
below pHpzc, the sorbent surface is positively charged, and above pHpzc, negatively. The positive
charge can be attributed, among other things, to the presence of alkaline functional groups and
excessive protonation of the surface (Bansal & Goyal, 2009). The thus formed barrier of positive
charge hinders the access of metal ions to the sorbent surface, which results in lower sorption. At
higher pH the sorbent surface is negatively charged due to the ionization of acidic groups (mainly
carboxyl, phenyl and hydroxyl groups). This results in electrostatic attraction between metal ions
and negative sites on the sorbent surface which decreases sorption. Also, with an increase in pH,
the solution contains less hydrogen ions which compete with Pb2+ for sorption, thus increasing
the efficiency of Pb2+ sorption.
In addition to surface charge, pH also determines the type and concentration of ion forms
in the solution. In highly acidic solutions, lead is present mainly in the form of Pb2+. A gradual
increase in pH leads to the formation of complex ions and precipitation of lead ions in the form of
Pb(OH)2. The reduction in sorption, which in this case occurs at pH 5.0±0.1, is probably caused
by the fact that lead hydroxyl ions are already forming (Meena at al., 2008). Considering that the
exponent of the Pb(OH)2 solubility product is 20, it was calculated that in a solution with a lead
concentration of 15.6 mg/L, lead hydroxide will begin to precipitate at about pH 6.0 (Mizerski,
2008). For this reason the effect of pH on the sorption process was studied only in the pH range
from 2.0 to 5.0.
4.3. The effect of temperature on the sorption of Pb2+ ions
from aqueous solution
In order to study the effect of temperature on Pb2+ sorption on plum stones, sorption experiments were performed in the range from 293 to 303K. Other conditions remained the same as
in previous experiments.
It was noted that as the temperature was increased in the range from 293 to 303K, sorption
decreased (Fig. 4.3). Such a result confirms the exothermic nature of Pb2+ sorption on plum
stones. The decrease in sorption properties in a temperature range of 293 to 303K may result
from damage to active sites on the sorbent surface or the shift of the process equilibrium towards
desorption of metal ions from the surface to the solution (Uluozulu at al., 2008).
4.4. Isotherm of Pb2+ adsorption on plum stones
Sorption of Pb2+ on the studied sorbents was described using the two most common adsorption models – the Langmuir and Freundlich isotherms. A characteristics of these models is
given in Table 4.1
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100
Sorption (%)
90
80
70
60
50
290
294
298
302
306
Temperature (K)
Fig. 4.3. The effect of temperature on the sorption of Pb2+ ions on plum stones (initial Pb2+
concentration 15.6 mg/L; sorbent concentration 5 g/L; sorbent’s particle size class < 0.5 mm;
ionic strength 0.02 mol/L; pH of 4.0±0,1; time of adsorption 1 h; mixing rate 120 rpm.).
Uncertainty of the sorption capacity: 0,12; 0,21; 0,30; 0,59%
TABLE 4.1
Characteristics of adsorption isotherms (Meena at al., 2008; Atkins, 2007)
Isotherm
Langmuir
Freundlich
Assumptions
Monolayer adsorption on a uniform
surface
Adsorption on a non-uniform surface;
empirical
Q=
Equation
Linear form
where:
ab × c k
(1 + b × c k )
æ
1
1 æ1
=
× çç + bçç
Q
ab è ck
è
Q
ck
a and b
K and n
—
—
—
—
(5)
(6)
Q = K × ck
log Q = log K +
1/ n
1
× log ck
n
(7)
(8)
amount of the metal ions per gram of the sorbent (mg/g);
the final concentrations of lead(II) ions (mg/L);
Langmuir coefficients;
Freundlich coefficients
The initial concentration of Pb2+ ions on the sorbents studied changed from about 6.0 to
110 mg/L. The results of the study described with the Langmuir and Freundlich equations are
shown in Fig. 4.4.
The values of coefficients a and b in the Langmuir equation, and K and n in the Freundlich
equation were determined based on the linear form of these equations (Table 4.1) using the effective regression method (Szydłowski, 1996). All the parameters and their uncertainties were
calculated using Microsoft EXCEL. The calculated values of adsorption isotherm coefficients
and the correlation coefficients R are presented in Tables 4.2.
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Fig. 4.4. Langmuir and Freundlich isotherms for Pb2+ adsorption on plum stones (sorbent concentration 5 g/L;
sorbent’s particle size class < 0.5 mm; ionic strength 0.02 mol/L; pH of 4.0±0.1; temperature (298±0.5)K;
time of adsorption 1 h; mixing rate 120 rpm.). Uncertainty of the sorption capacity: 0,00; 0,03; 0,06;
0,12; 0,17; 0,24; 0,28; 0,34; 0,40 mg/g
TABLE 4.2
Langmuir and Freundlich isotherms coefficients and their uncertainties obtained for plum stones
Langmuir isotherm
Freundlich isotherm
a (mg/g)
∆a (mg/g)
b (1/mg)
∆b (1/mg)
R
21.2
K
(L/mg)
0.78
0.47
0.03346
0.00005
0.940
∆K
n
∆n
R
0.02
1.31
0.01
0.993
Langmuir and Freundlich isotherms provide important information about the sorbent studied
and the process of Pb2+ sorption from aqueous solutions. Knowledge of the parameter a in the
Langmuir isotherm allows the assessment of the maximum sorption capacity of the sorbent, i.e.
the maximum amount of metal ions needed to form a complete monolayer (Han at al., 2005).
For plum stones, the constant has a value of 21.2 mg/g.
The other parameter of the Langmuir isotherm, the constant b, represents the energy of
adsorption (Meena at al., 2008). The lower the value of the constant b the higher the affinity of
the sorbent towards metal ions. In general, a good sorbent should be characterised by a low value
of b and a high value of a (Gupta at al., 2009).
The constants in the Freundlich isotherm also provide valuable information about the adsorption process. The parameter K in the Freundlich isotherm allows, among other things, the
assessment of the sorption capacity of the sorbent, while the constant n defines the intensity of
adsorption (Han at al., 2005). The calculated value of the coefficient n is 1.31 for plum stones;
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the fact that this parameter takes values in the interval <1,10> demonstrates that adsorption on
these sorbents is “advantageous”. The lower the 1/n value, the greater the adsorption system’s
non-uniformity in terms of energy (Bansal & Goyal, 2009). In contrast to the Langmuir model,
adsorption described by the Freundlich isotherm is not limited to a monolayer.
In the case of plum stones a better fit of experimental data was noted for the Freundlich
isotherm, which confirms heterogeneity of the surface (Atkins, 2007).
5. Summary and conclusions
1. Plum stones can be used to remove Pb2+ ions from aqueous solutions. For this sorbent
the depletion of Pb2+ in the solution was 47.2-81.3%.
2. Sorption of Pb2+ on plum stones depends on the sorbent concentration, the pH of the
solution and the temperature. The optimum sorbent concentration is 5 g/L. The best sorption on
plum stones was noted at pH 4.0±0.1. The study also showed that as the temperature increases
(from 293 to 303K) sorption gradually decreases.
3. The maximum sorption capacity of plum stones was for 21.2 mg/g.
4. For plum stones, a better fit of the function to the measurement data was obtained for
the Freundlich isotherm.
5. The reason for the practical use of tested sorbents should be their regeneration, by which
we obtain more concentrated solution of removed ions and sorbent can be reuse again. Research
in this direction are in preliminary stage.
The study was carried out as part of the AGH research programme number 11.11.100.196.
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Received: 21 May 2010