In this study, Bacillus subtilis biomodified PSAC, was using as a biosorbent for the removal of lead from aqueous solution. The biomodified PSAC was characterized for its surface area, point of zero charge and acidic surface group concentration. Batch adsorption experiments were carried out to study the effects of different initial metal ions concentration (from 30 mg/L to 300 mg/L) and initial solution pH (from pH 3 to pH 6). Percentage uptake of biomodified PSAC was found to decrease with an increase in initial lead concentration. This was due to limitation of the adsorbent mass where the binding site of adsorbent or solute ratio was smaller. The sorption capacity of biomodified PSAC increased with increasing in initial lead concentration and pH. This biosorbent showed high adsorption capacity for lead ions especially at pH 6 with an ultimate uptake of 71.43 mg/g due to the lower competition between protons and lead ions at higher pH. Biosorption of lead by biomodified PSAC was best described by the Langmuir adsorption isotherm model, in which the process can be concluded as monolayer sorption process of lead ions onto biomodified PSAC. The findings of this investigation suggest that biomodified PSAC can be used as an effective adsorbent for the removal of lead from wastewater.
Keywords: Adsorption; Lead; Palm Shell Carbon; Biomodification; Bacteria
INTRODUCTION
There is an increasing concern about heavy metal pollution in Malaysia due to the industrial and agricultural exploitation, increasing living standards and wide spheres of human activities. Heavy metal such as lead (Pb), copper (Cu), nickel (Ni), chromium (Cr), cadmium (Cd), mercury (Ag), aluminum (Al) and zinc (Zn) are commonly found to be disposed to the drainage system and flow straight to the river. Heavy metal ions are toxic, non-biodegradable and highly carcinogenic. In order to remove heavy metal ions from wastewater, many researches had been investigating effective water treatment technologies such as adsorption, chemical precipitation, reverse osmosis, solvent extraction and ion exchange. Among the various technologies available, adsorption by using activated carbon is the most popular and has been widely used for the removal of pollutants from wastewater in terms of the initial cost, simplicity of design, ease of operation and insensibility to toxic substances (Meshko, Markovska, Mincheva and Rodrigues, 2001). However, production and regeneration of commercial activated carbon is still expensive. Therefore, research attention has been focused on the industrial and agricultural wastes based activated carbons that are lower in cost. The conversion of natural organic wastes into environmentally-friendly porous materials is a technological alternative to their disposal.
In the year of 2006, Malaysia was the largest producer and exporter of palm oil in the world (Sumathi et al., 2008). With such a huge amount of palm oil production, the quantity of oil palm biomass produced is also high. In Malaysia, around 2 million tonnes of palm shell is generated annually (Chan, 1999). Therefore, application of palm shell activated carbon as adsorbent serves a double purpose by converting unwanted and surplus agricultural waste to useful, valuable material and provides an efficient adsorbent material for the removal of pollutants from water. Recently, several researchers have studied heavy metals removal from aqueous solution using PSAC (Issabaveya, 2005; Gueu,Yao, Adouby and Ado, 2006; Shilpi, Suparna and Padmaja, 2008; Chun, Mohamed and Wan Mohd Ashri, 2008).The researches indicated this material exhibited high adsorption capacity towards heavy metals. However, only few studies focused to the bacteria effect on heavy metals removal in granular activated carbon. Rivera-Utrilla, et al. (2001) investigated adsorption properties of Escherichia coli biomodification activated carbon toward lead aqueous solution. Kim, Chang, Jeong and Song (2010) studied capacity of iron-impregnated activated carbon to remove arsenic from aqueous solution in the presence of bacteria. The information on the biomodification of PSAC with bacteria is scarce.
The species used in our experiments is Bacillus subtilis, a gram positive aerobic species commonly found in wastewater system. Most of the bacteria are negatively charged because the anionic group present within the cell wall (Sukdeb, Joardar and Joon, 2006). Therefore, change of the surface potential of PSAC to be positive is expected better bacteria adsorption efficiency. Bacterial cell walls contain a variety of surface organic functional groups, including amino, carboxylic, hydroxyl and phosphate sites (Jeremy, Christopher, Nathan and Thomas, 1997). B. subtilis is rod-shaped and typically 5.0 µm long and1.0 µm wide. Kim et al. (2010) indicated that the bacterial attachment could be restricted mostly to the external surfaces of granular activated carbon due to their smaller size relative to bacterial sizes. The waste microorganisms had been used as alternative adsorbent for the treatment of heavy metal containing wastewater. Bacteria attach on PSAC will change the properties and structure of adsorbent, in turn effect the adsorption properties of adsorbent. It was show that Bacillus sp. effectively cleans 21mg/g of copper ions (Lo et al., 2003) and remove 68.5 mg/g of mercury (Xue et al., 2010)
In this research project, a study has been carried out to examine the adsorption capacity of untreated and pretreatment of PSAC with Bacillus subtilis toward Pb(II), Cd(II), Zn(II) and Cu(II) present in aqueous solution. Both of the adsorbent were characterized by different physio-chemical methods. The effect of initial adsorbate concentration, solution pH and contact time on the adsorption of heavy metals were also evaluated.
MATERIALS AND METHODS
Preparation of Adsorbate. Lead (Pb), Zinc (Zn), Cadmium (Cd) and Copper (Cu) were selected as the key sorbates in this study. 0.15 M of NaNO3 from Merck was used as the background electrolyte while 0.1 M of Pb(NO3)2, Zn(NO3)2, Cd(NO3)2, and Cu(NO3)2 were used as the heavy metals ions source. Stock solutions of the four heavy metal solution with concentration of 0.1 M were prepared by dissolving with background solution. The test solutions were prepared by diluting the stock solution to the desired heavy metals concentrations.
Preparation of Adsorbent. The commercial granular palm shell based activated carbon (PSAC) was provided by a local manufacturer "Bravo Green" Sdn Bhd in Kuching, Sarawak, Malaysia. The PSAC was produced by physical activation process with steam as the activating agent. It was crushed and sieved to produce fix particles of sizes ranging from 0.6 to 1.2 mm for used throughout the experiment. Bacillus subtilis, gram positive bacteria that obtained from Universiti Tunku Abdul Rahman (UTAR) was used to biomodified PSAC in this study. A total of 40 mL of bacteria cell suspension was added to a 50 mL centrifuge tube which containing 1.2 g of PSAC. The contents of the centrifuge tube were stirred with a vortex mixer for 2 min and the tube was then placed in an orbital shaker. The tubes were maintained at 27°C with slight agitation (45rpm) for 24-h (Rivera-Utrilla, et ac, 2001). After that, the biomodified PSAC was filtered and washed gently with sterile deionizer water. It was dried in incubator (80°C) overnight and then cooled down in desiccators.
Adsorption Studies. In this study, the adsorption experiments were carried out in an orbital shaker (LabTech) at a constant speed of 220 rpm at 27°C using 250 mL conical flask. The flasks contain 250 mg of adsorbent in 100 mL heavy metal solution with concentration range from 30 to 300 mg/L were placed in the shaker for 24-h unless otherwise stated. At the end of the experiments, the adsorbent was removed by filtration through ''Double-ring'' No. 102 filter paper (Xinhua Papermaking Ltd Co., Hangzhou, China). The remaining heavy metal concentrations in the solution were estimated by using Optima 7000 DV Inductively Coupled Plasma-Optical Emission Spectrometer (ICP-OES) (Perkin Elmer, USA) according to standard procedures. All the experiments were carried out in duplicates and the results are presented in the percentage uptake and sorption capacity.
The percentage removal of heavy metal in this bio-sorption experiment was calculated by:
(1)
where Ci and Ce (mg/L)are initial and equilibrium heavy metal concentration on PSAC. The amount of heavy metal adsorbed on the PSAC at equilibrium was calculated from the mass balance of the equation as given below:
(2)
where qe (mg/g) is equilibrium heavy metal concentration on PSAC at any time, M (g) is the mass of the PSAC used and V (L) is volume of the heavy metal solution.
RESULTS AND DISCUSSION
Characteristic of Activated Carbon Samples
Table 1: Properties of different type of untreated and biomodified PSAC
Untreated PSAC
B. subtilis treated PSAC
BET specific surface area (m2/ g)
716.89
688.88
Maximum pore volume (cm3/g)
0.11
Monolayer volume (cm3/g)
164.90
Porosity: Micro (<2nm)
Meso (2-50nm)
23% micro
77% meso
pH point of zero charge
9.25
8.50
Carboxylic groups (meq/g)
3.20
3.00
Untreated PSAC
Effect of Initial Concentration
Effect of pH
The parameters of Langmuir and Freundlich equations for the adsorption of lead ions on untreated PSAC
Langmuir Constant
Freundlich Constant
qmax (mg/g)
b (1/mg)
R2
Kf
1/n
R2
pH 3
39.68
0.04
0.992
10.78
0.28
0.964
pH 4
53.76
0.22
0.991
15.51
0.29
0.914
pH 5
59.88
0.24
0.997
21.07
0.24
0.993
pH 6
64.52
0.28
0.995
34.17
0.13
0.976
B. subtilis treated PSAC
Effect of Initial Concentration
Effect of pH
The parameters of Langmuir and Freundlich equations for the adsorption of lead ions on biomodified PSAC
Langmuir Constant
Freundlich Constant
qmax (mg/g)
b (1/mg)
R2
Kf
1/n
R2
pH 3
29.67
0.05
0.994
4.45
0.37
0.981
pH 4
59.53
0.27
0.993
17.68
0.28
0.994
pH 5
68.49
0.11
0.990
23.65
0.21
0.999
pH 6
71.43
0.12
0.996
25.62
0.20
0.990
Adsorption isotherms. In this study, both Langmuir's and Freundlich's absorption isotherm equilibrium models were used for the analysis of the carbon-metal sorption system. The linearised form of Langmuir isotherm was used to characterise the adsorption process of heavy metals onto activated carbon. The equation can be linearized as
(3)
Where,
qe = Amount of metal absorbed (mg/g) at equilibrium,
qmax = Maximum of Langmuir monolayer adsorption capacity (mg/g),
b = Langmuir or dissociation constant (L/mg), and
Ce = Equilibrium concentration of the metal in the solution (mg/L).
The Freundlich isotherm (Freundlich, 1906) is the earliest known relationship describing the sorption equation. The linearised form of Freundlich isotherm is given as
(4)
Where,
qe = Amount of metal absorbed (mg/g) at equilibrium,
Kf = Freundlich constant (mg/g (1/mol)1/n),
1/n = exponential constant,
Ce = equilibrium concentration of the metal in the solution (mg/L).
large decrease in the pHPZC (pH at the point of zero charge) of the samples and a large increase in the total negative surface charge. Attractive electrostatic interactions predominate between the positive surface charge and the negative charge of the anions at solution pH=5. This behavior may result from the large uptake of TA by ACC, which can produce some pore
blockage, thereby decreasing the accessibility of Cd(II) species to the carbon surface.
The speciations of arsenious acid [H3AsO3, As(III)] and arsenic acid [H3AsO4, As(V)] are affected by the solution pH. For As(III), H3AsO3 dominates at pH 2.0−9.2, while H2AsO−3
dominates at pH 9.2−12.7. For As(V), H2AsO−4 is dominant at pH 2.7−6.8, while HAsO2− 4 is dominant at pH 6.8−11.6.[27] Under our experimental conditions (pH = 7.5), H3AsO3 (uncharged form) and HAsO2− 4 (divalent anion) were the dominant forms of As(III) and As(V), respectively. Adsorption of As(V) to bacteria can be attributed to functional groups on bacterial surfaces. Functional groups such as carboxyl (COO−), hydroxyl (OH−), and phosphate (PO3− 4 ) are abundantly present on bacterial cell walls. Due to the presence of these negatively-charged groups, bacteria have net negative surface charges around the circumneutral pH, so their surfaces can actively bind metal cations.[42,43]
However, positively-charged amine groups (NH+3 ) are also present on bacterial surfaces,[44] so anionic species can adsorb to bacteria via electrostatic attraction or hydrogen bonding.[43]. At a reaction time of 12 hr, the percentage of As(III) removal in the presence of bacteria was 61.3%, which was slightly lower than that in the absence of bacteria (65.2%). The percentage of As(V) removal in the presence of bacteria (64.4%) was also slightly lower than that in the absence of bacteria (69.9%). At 72 hr, the percentage of As(III) removal in the presence of bacteriawas 61.0%,while it was 61.5% in the absence of bacteria. The percentages of As(V) removal in the presence and absence of bacteria were 61.0% and 62.5%, respectively. This indicated that the influence of bacteria on arsenic removal in Fe-GAC was not eminent in our experimental conditions.
CONCLUSION
