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 and stored in a plastic container at room temperature for used throughout the experiment.
3.1.1 Key Microorganisms Used In This Study
Bacillus subtilis and Aspergillus niger that obtained from Universiti Tunku Abdul Rahman (UTAR) were used to biomodified PSAC in this study. All the glassware and material used in this study were sterilized by autoclaving at 121Â°C for 20 min to prevent interference by other microorganism. The spores of B. subtilis were isolated from the stock and cultured in the nutrient agar (NA) media (Merck) by using streak plate technique. The cultures were incubated at 37Â°C in standard size (85mm) plastic Petri dishes for 24-h. Aspergillus niger was cultured onto potato dextrose agar (PDA) plate from the stock culture and incubated at 37Â°C for one week before subcultured into liquid media.
3.1.2 Inoculums Preparation
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Three single colonies of 24-h B. subtilis were transferred to a flask with 100 mL of nutrient broth (Merck). The inoculums were incubated by agitation at 250 rpm, 30Â°C for 24-h. The suspension was centrifuged at 4Â°C and 5000 rpm for 15 min. The supernatant was removed and the pellet was washed two times with sterile deionized (SDI) water to prevent growth of the bacteria. Finally, 2 mL of cells were resuspended in 40 mL of SDI.
Fungal spores and conidia, grown onto PDA were suspended in 5 mL of SDI water. The biomass used for the biomodification process was prepared by incubating 2 mL of fungal spores' suspension in 100 mL of liquid medium. Medium for A. niger consisted of (g/ L): sucrose, 20; peptone, 10 and yeast extract, 3. Flasks containing liquid medium were autoclaved at 121Â°C for 15 minutes and incubated at 30Â°C, 250rpm for a week.
3.1.3 Biomodified of PSAC
A total of 40 mL of 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 27Â°C with slight agitation (45rpm) for 24-h (Rivera-Utrilla, et ac, 2001). After that, the biomodified PSAC was filtered and washed gently with SDI. It was dried in incubator (80Â°C) overnight and then cooled down in desiccators. The biosorbent were stored in a plastic container at room temperature for used throughout the experiment.
3.1.4 Brunauer-Emmet-Teller (BET) Method (Surface Area)
Nitrogen adsorption/ desorption isotherms at 77K were determined volumetrically using Sorptomatic 1990 Series analyzer, made by Thermo Finnigan Scientific Incorporation. Before the experiment began, the adsorbents were dgeassed at (10-4 mmHg) at 393K for at least 24 hours. The surface area of the samples were measured based on BET method and the Dubinin-Radushkevich (DR) equation was used to calculate the micropore volume, from which the micropore surface area was then determined. A Barrett-Joyner-Halenda (BJH) model and a Horvath & Kawazoe (HK) model were used for the determination of meso- and macropore size distribution and micropore size distribution respectively.
3.1.5 Boehm Titration of Adsorbent (Total Acidity)
Boehm Titration method is used to determine the acidity functions in the carbon. In this method, 1 g of PSAC was placed in a flask and 50 mL of fours bases of increasing strength (sodium bicarbonate [NaHCO3], sodium carbonate [Na2CO3], sodium hydroxide [NaOH] and sodium ethoxide [NaOC2H5], each 0.1N) were added to each flask, which afterwards placed on a shaker for 48 hours at room temperature. After that, 10 mL of the filtrate was taken from each flask and titrated with 0.1 N of hydrochloric acid. The quantity of neutralized base is given by:
C (meq/ g) = (3.1)
The concentration of each acidic surface group is given by:
Carboxyl group = [NaHCO3]
Lactones group = [Na2CO3] [NaHCO3]
Phenolic group = [NaOH] [Na2CO3]
Carbonyl group = [NaOC2H5] - [NaOH]
3.1.6 Determination of pH of Point of Zero Charge
A so-call pH drift method was used to establish pH of point of zero charge (Khormaei et al., 2007). 100 mL of 0.15 M NaNO3 solution was placed in a closed Erlenmeyer flask. The pH was adjusted to initial values of 2, 4, 6, 8, 10 and 12 by adding HCl or NaOH. Then, 250 mg of carbon was added into a flask of certain pH solution and left continuously mixed for 48 hours at room temperature. The stabilized final pH was measured and compared to the initial value. When the final pH of the solution was equal to the initial pH value is considered to be pH of PZC (point of zero charge).
3.1.7 Ash Content
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About 2 g of adsorbent was weighted and charred at 650Â°in the vertical tube furnace (Lenton) for overnight. The samples were transferred to desiccators and weighted after cooling. The ash content for each sample was calculated based on the following formula:
Ash Content (%) = (3.2)
m1 = weight of the sample after calcinations
m2 = weight of the sample after calcinations
3.1.8 Scanning Electron Microscope (Surface Morphology)
3.1.9 X-Ray Diffraction (XRD) Analysis
XRD analysis was carried out using Ni-Cu (KÎ±) radiation operating at 40 kV and 30 mA on a Shimadzu XRD-6000 diffractometer to indicate the inorganic surface functional group and percentage of amorphous structure of the samples. The scan speed was 0.5Ëš Î¸/2Î¸ per minute and step size was 0.02ËšÎ¸/2Î¸. XRD Patterns were recorded from 10 to 70ËšÎ¸/2Î¸. The dried carbons were ground carefully with an agate mortar and pestle into powder form before the measurement.
3.1.10 Fourier Transform Infrared (FT-IR) Spectroscopy Technique
In this study, Fourier Transform Infra Red (FT-IR) Spectroscopy (Perkin-Elmer Inc., United States of America) was used to observe the changes in functional groups of the natural PSAC, and bio-modified PSAC before and after absorption. The dried carbons were first ground in an agate mortar and pestle into powder form, and mixed with KBr powder by the ratio of 1:10 in the sample disk. The mixture was pressed at 20 tonnes for 3 to 4 minutes to form the pellets. The background obtained from scan of pure KBr was automatically subtracted from the sample spectra. All the spectra obtained were plotted using the same scale on the absorbance axis in the range of 400-4000 cm-1.
3.2 Preparation of Adsorbate Solutions
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 deionizer water. Information of the four heavy metals was given in Table 2. The test solutions were prepared by diluting the stock solution to the desired heavy metals concentrations.
3.2.1 Batch Adsorption Experiment
In this study, the adsorption experiments were carried out in an orbital shaker (LabTech) at a constant speed of 220 rpm at 27Â°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., 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 conical flasks were previously washed with detergent, rinsed with distilled water, soaked in 0.1 M nitric acid to avoid any binding of metal on the surface of the glassware, rinsed with distilled water and oven-dried.
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 biosorption experiment was calculated using Equation 3.3:
% Removal = x 100% (3.3)
Ci = initial heavy metal concentration on PSAC (mg/g) and
Ce = equilibrium heavy metal concentration on PSAC (mg/g)
The amount of heavy metal adsorbed on the PSAC at equilibrium was calculated from the mass balance of the equation as given below:
qe = equilibrium heavy metal concentration on PSAC at any time (mg/g),
M = mass of the PSAC used (g), and
V = volume of the heavy metal solution (L).
3.2.2 Effect of Initial Heavy Metal Concentration
Metal solutions with concentrations varying from 30 to 300 mg/L were prepared by diluting the stock solution (0.1 M) with 0.15 M of NaNO3. Sorption experiments were performed by adding 250 mg of original or biomodified PSAC to 100 mL of metal solution. The flasks were agitated in the orbital shaker at 220 rpm and 27Â°C for 24-h. The activated carbon and treated solutions were then separated by filtration. The initial and final metal concentration in solution was determined by ICP analysis.
3.2.3 Effect of Initial pH
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The effect of different initial solution pH was investigated by adjusting pH (from pH 3 to pH 6) of the synthetic solution with 0.1 M HCl and/or 0.1 M of NaOH solutions. The pH of the solution was measured by pH meter (pH 510, Eutech Instrument).The adsorption tests were conducted on a shaking incubator at 220 rpm, 27Â°C for 24-h. The samples were then separated by filtration. The initial and final metal concentration in solution was determined by ICP analysis.
Effect of tempearture experiment was performed under ambient conditions at 303, 313 and 323 K, respectively at 220 rpm for 24-h. The flasks contain 250 mg of adsorbent in 100 mL heavy metal solution with concentration range from 30 to 300 mg/L. KÂ° can be obtained from the Langmuir's equation by the plots of versus Ce (Chen et al., 2007),
Ce = equilibrium concentration (mg/ L),
qe = amount of heavy metal absorbed (mg/g) at equilibrium,
QÂ° = maximum adsorption (Âµmol/g) and
KÂ° = binding constant.
âˆ†GÂ° can be obtained using the following equations:
The âˆ†Hâ-¦ values are calculated from the slopes of the linear variation of lnK versus 1/T.
The values of âˆ†SÂ° were calculated from:
âˆ†Hâ-¦ = enthalpy change (kJ molâˆ’1)
âˆ†Sâ-¦ = entropy change (kJ molâˆ’1 Kâˆ’1)
âˆ†Gâ-¦ = Gibbs free energy (kJ molâˆ’1)
R = ideal gas constant (8.3145 J molâˆ’1 Kâˆ’1) and
T = temperature (K)
3.4 Adsorption Isotherm Analysis
The equilibrium adsorption isotherm is fundamentally important in the design of absorption systems. Equilibrium studies in adsorption give the capacity of adsorbent. It is described by adsorption isotherm characterised by certain constants whose values express the surface properties and affinity of the adsorbent. Equilibrium relationships between adsorbent and adsorbate are described by adsorption isotherms, usually the ratio between the quantity adsorbed and that remaining in the solution at a fixed temperature at equilibrium (Han, Zhang & Zou, 2005). 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 (as shown in Equation 3.9) was used to characterise the adsorption process of heavy metals onto activated carbon.
= + (3.9)
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 shown in Equation 3.10:
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).
3.5 Adsorption Kinetics Studies
The study of kinetics of adsorption is quite significant in wastewater treatment as it describes the solute uptake rate, which in turn controls the residence time of adsorbate uptake at the solid solution interface. The kinetic adsorption data were processed to understand the dynamics of adsorption process in terms of the order of rate constant (Ahmad, Hameed & Aziz, 2007). Kinetic adsorption data of heavy metal by activated carbon at various initial metal concentrations and pH values were treated with pseudo-first-order (Equation 3.11) and pseudo-second-order kinetic models (Equation 3.12).
A linear form of pseudo-first-order model is:
qe = amount of metal adsorbed on the carbon (mg/g) at the equilibrium,
q = amount of metal uptake (mg/g) at time t (min),
k1 = rate constant of pseudo-first-order biosorption, (min-1).
A linear form of pseudo-second-order model (Ho & McKay, 2000) is:
k2 = equilibrium rate constant pseudo-second-order (g/ mg min)
h (k2qe2) = the initial sorption rate (mg/ g min)
From the plots of (log qe - qt) versus t and t/q versus t, the constant value for both first-order and second-order can be calculated respectively. The values of qe, k2 and h (k2qe2) against Co in the corresponding linear plots of the pseudo-second-order equation were regressed to obtain expressions for these parameter values in terms of the initial metal concentration. Each of this parameter can be expressed as a function of Co for four heavy metals as reported by Ho and Mckay (2000).
Aq, Bq, Ak, Bk, Ah, and Bh are constant related to the respective equations.
3.6 Speciation Diagrams of Heavy Metals
It is important to outline how aqueous and solid phases are generally considered and presented by VMinteq software. Speciation diagrams for each metal as affected by changes in pH were generated using VMINTEQ 2.50 software (Gustafsson, 2006). VMINTEQ 2.50 is a chemical equilibrium model for determination of metal speciation and solubility equilibria for natural waters. In this study, the concentrations of elements involved in the batch adsorption, namely, metal ions (Pb2+, Zn2+, Cu2+ or Cd2+), Na2+ and NO3- were inputed into VMINTEQ 2.50 to generate concentration data for all species in the solution at a particular pH. The concentration data were then converted to percentages to relative to the total metal species in separate aqueous solutions for pH 1 to 14.