Organic solvent free iron oxide nanomaterial used for lead removal was synthesized by co-precipitation method. Fourier transform infrared spectroscopy (FT-IR), scanning electron microscopic with energy dispersive X-ray analysis (SEM-EDX), X-ray diffraction (XRD) and thermo gravimetric-differential thermal (TG-DTA) analysis were used to determine the surface characteristics and analysis of iron oxide. Optimization of solution pH, adsorbent dosage, contact time, agitation speed and initial lead ion concentration were conducted for further adsorption isotherm, kinetics, thermodynamics and desorption study. Langmuir sorption isotherm model fits the adsorption data better than Freundlich, Dubinin-Radushkevich (D-RK) and Flory-Huggins (FH) models. The mean adsorption energy and free energy obtained from D-RK and FH models guides that the mechanism was under control of physical adsorption and actuality of spontaneous reaction, respectively. From kinetics of adsorption pseudo second (PSO) model fits well than pseudo first (PFO) and Elovich adsorption-reaction models. And to test whether the reaction is under control of adsorption-diffusion or not the intra particle diffusion (IPD) model was tested, but it fails to pass through the origin. This indicates that the reaction mechanism only under control of adsorption-reaction. The maximum adsorption capacity (qmax) of the adsorbent was 70.422 mg/g.
Heavy metals are the major cause of water pollution. Among many hazardous toxic heavy metals which cause significant health risks for human, lead stands on the second [
Heavy metals remediation has been efficiently accomplished in the past by several nano metal oxides, including: ferric oxides, manganese oxides, aluminum oxides, titanium oxides, magnesium oxides and cerium oxides [
The synthesis routes of metal oxides nano materials include gas phase deposition, electron beam lithography, pulsed laser ablation, laser induced pyrolysis, powder ball milling, aerosol from physical methods and reverse micelle (or micro-emulsion), sol-gel, co-precipitation, hydrothermal, electrochemical deposition, sonochemical, thermal decomposition, gas-phase reduction from chemical methods [
For this work co-precipitation has been used for synthesis of the adsorbent by using only distilled water as a solvent. The main objective of this study was to synthesize and characterize the as-synthesized materials using different analytical techniques (FT-IR, XRD, SEM-EDX and TGA-DTA) and to evaluate its adsorption efficiency for the removal of lead ions from aqueous solution. For this study the mechanism of the adsorption was evaluated with the help of Langmuir, Freundlich, D-RK and FH adsorption isotherm models. Moreover to check the kinetics of adsorption PFO, PSO and Elovich models were used for adsorption-reaction and IPD model were used for adsorption-diffusion.
All the chemical/reagent used for the current investigation was analytical grade and only distilled water was used as a solvent. Nano-sized Fe oxide was prepared by co-precipitation method using iron (III) nitrate Nonahydrate [Fe (NO3)3∙9H2O] salt as precursor and cetyltrimethyl ammonium bromide (CTAB) as a precipitating agent. The respective ratio used during mixing of precursor and precipitating agent was 8:1 with continuous stirring on magnetic stirrer. Sodium borohydride was then added as reducing agent. The pH of the solution was adjusted to 12 by drop wise addition of acid and base. The solution was refluxed at 100˚C for 2 hours with constant stirring. The obtained precipitate was then separated from solution using centrifuge. After washing the precipitate was washed with alcohol and water, it was dried in oven over night at a temperature of 105˚C and finally the dry powder was calcined at 450˚C for two hour.
The sorption test was conducted in 50 mL erlenmeyer flasks containing adsorbent to solution ratio of 0.5:300 [0.05 g of iron oxide: 30 mL of 45 mg/L of Pb(NO3)3]. The pH of the solution was adjusted with the help of dilute HCl and NaOH solution. Equilibration of the experiments was done on a rotary shaker. The amount of lead adsorbed was known by the differences in equilibrium and initial lead concentration. Atomic adsorption spectrometer (AAS) was used for quantitative estimation of lead. The samples containing more lead ion than the highest concentration of detection limit of the instrument were diluted and the dilution factor was compensated during final calculation. The pH, adsorbent dose, agitation speed, contact time and initial lead ion concentration were optimized during experimentation [
From mass balance for the adsorbate in the glass ware is:
m ( q − q e ) = ( C o − C e ) V (1)
From which a relationship between value of C and the corresponding equilibrium value of q can be established. To determine equilibrium relationship qo become equal to zero (qo = 0),
q e = ( C o − C e ) × V / m (2)
The percent of adsorption (%) were calculated using equation:
% = ( C o − C e ) / C o × 100 (3)
where: Co = initial concentrations (mg/L) and Ce = equilibrium concentrations (mg/L) of lead ion, qe = adsorption capacity of adsorbent (mg/g), V = volume of reaction mixture (L), m = mass of adsorbent (g).
To optimize the effect of pH on the lead sorption, 0.1 g of the sorbent was added into 50 mL erlenmeyer flask containing 30 mL of 45 mg/L lead ions, then by varying the pH of the solutions from 1 - 11 with two increments and keeping the other conditions at constant value (rotation speed of 120 rpm and contact time 130 min.) the optima were obtained.
Adsorbent dosage was evaluated and optimized by taking various amounts of adsorbent dose as, 0.01, 0.03, 0.05, 0.1, 0.2 and 0.4 g, while the other parameters were kept at constant value and pH was maintained at the optimized value of 9.
The effect of agitation speed was optimized by varying the speed of rotary shaker to 30, 90, 120, 150, 180 and 200 rpm. During here during optimization dosage and pH were kept at optimized value and initial lead concentration at constant value.
The degree of affinity of the adsorbate towards adsorbent is quantified using adsorption isotherms. A number of isotherms have been developed to describe equilibrium relationships. Here Langmuir, Freundlich, D-RK and FH models were engaged to describe the experimental results of lead sorption. Sorption isotherms experiment was done again by keeping all parameters at optimized values and by varying lead ion concentration as: 25, 45, 65, 85, 105 and 125 mg/L in separate 50 mL erlenmeyer flask. And the study of kinetics was conducted by taking various contact time 10, 50, 90, 130, 170, 210 and 240 minutes under optimized values of pH, adsorbent dose, agitation speed and lead ion concentration.
In order to determine the effect of temperature on sorption phenomenon, all predetermined and optimized values of parameters were used and the temperature established during sorption was varied from 25˚C to 55˚C with increment of 10˚C.
Lead ions desorption studies were done using lead ions loaded powder sample which is obtained after adsorption of lead ions on powder using all optimized values. 0.05 g of iron oxides of lead loaded powder was added into flasks containing 30 mL of double deionized water. 0.1 M NaOH and 0.1 M HCl solutions were used to adjust pH of the solution to: 1, 3, 5, 7, 9 and 11. After agitation the filtered solution were analyzed for desorbed lead ions concentration.
The as-synthesized iron oxide nanoparticle which is examined by XRD technique was given in
SEM (Model ZEISS EVO 18 with INCA software for quantitative Analysis) is another useful tool for the morphological analysis of the surface of solids. The images of as-synthesized nano material with different magnification are as shown in
The FTIR (Bruker IFS120 M Perkin Elmer) spectrum (
The result of stability test of the as-synthesized iron oxide nanomaterial which is done by TG-DTA (DTG-60H) instrument is shown in
The effect of pH, adsorbent dose, speed of agitation and contact time on the lead ion sorption efficiency of nanosorbent is presented in the
On pH optimization it was observed that greatest lead ion adsorption occurred under basic condition and the optimum pH value for sorption was found to be 9 as revealed by pH verses % of sorption plot (
point since the adsorbent sites almost occupied it becomes constant. Short adsorption time for iron oxide based nano-adsorbents may possibly due to porosity of the adsorbent.
The effect of initial concentration on lead ion uptake shows almost constant flow up to optimum value (45 mg/L) (
The linear Langmuir equation is as given below:
C e / q e = 1 / b q max + C e / q max (4)
The significant characteristics of a Langmuir isotherm can be expressed in terms of a constant separation factor or equilibrium parameter, RL:
R L = 1 / ( 1 + b C o ) (5)
The linear Freundlich equation expressed as:
Log qe = log Kf + 1/n log Ce (6)
The linear Dubinin-Radushkevich equation:
ln q e = ln q s − β ε 2 , where ε = R T ln ( 1 + 1 C e ) and E = 1 2 β (7)
The linear Flory-Huggins equation:
ln ( θ C o ) = ln k F H + n ln ( 1 − n ) and Δ G o = R T ln ( k F H ) (8)
where, Co = initial adsorbate concentration in solution (mg/L), Ce = adsorbate equilibrium concentration in solution, qo = initial amount of adsorbate per unit mass of adsorbent (mg/g), qe =amount of adsorbate accumulated per gram of the adsorbent material, qmax = maximum uptake corresponding to the site saturation, b = ratio of adsorption and desorption rates, Kf = distribution coefficient and represents the quantity of adsorbate adsorbed onto adsorbent for unit equilibrium concentration, 1/n = an empirical constant related to the magnitude of the adsorption or surface heterogeneity, θ ( 1 − C e / C o ) is Fractional coverage, ε = Dubinin - Radushkevich isotherm constant and qs is saturation capacity (mg/g), β is the constant related to free energy, n is the number of ions occupying adsorption sites, T absolute temperature (k) and R is the universal gas constant (8.314 J/molK). From sorption isothermal studies, the obtained sorption parameters given in
From D-RK model the calculated mean adsorption energy, E was 1.89 KJ/mol, being its value less than 8 KJ/mol specifies the domination of physical interaction.
Sorbent | Langmuir model | Freundlich model | Dubinin-Radushkevich | Flory-Huggins | |||||
---|---|---|---|---|---|---|---|---|---|
Fe-Oxide | qmax(mg/g) | b | RL | Kf | 1/n | E (kJ/mol) | β | n | ∆G (kJ/mol) |
70.42 | 0.57 | 0.038 | 26.06 | 0.35 | 1.89 | −0.14 | −1.28 | −21.36 |
And from FH model the obtained values for free energy, ΔG was −21.36 (kJ/mol), this shows that the spontaneity of the reaction [
The result of kinetics of sorption study obtained after optimization of all parameters is shown in Figures 4(a)-(d). And the results of kinetics study and kinetics constants values are given in
PFO:
log ( q e − q t ) = log q e − K 1 t / 2.303 (9)
PSO:
t / q t = 1 / K 2 q e 2 + t / q e (10)
Elovich:
Metal | Pseudo-1st-order | Pseudo-2nd-order | Intra-particle diffusion | Elovich | ||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|
q e * (mg/g) | K1 | q e * * (mg/g) | R2 | K2 | q e * * (mg/g) | R2 | ki | R2 | C | β | R2 | |
Pb(II) | 26.68 | 0.019 | 5.32 | 0.9835 | 0.01 | 27.17 | 0.999 | 0.37 | 0.887 | 21.61 | 0.65 | 0.983 |
*calculated value and **experimental value.
q t = ( 1 / β ) ln ( α β ) + ( 1 / β ) ln t (11)
Weber-Morris IPD model:
q t = k i t 1 / 2 + C (12)
where, qe and qt are the amounts of adsorbate adsorbed on the adsorbent at equilibrium and at various times t (mg/g), respectively, k1 is the rate constant of the FSO model for the adsorption process (min−1), k2 is the rate constant for the PSO model (mg/gmin, α is the initial sorption rate (mol/g∙min) and β is the desorption constant (g/mol)), ki is the IPD rate constant (mg/g∙min1/2).
The plot of log (qe − qt) verses t from PFO (
Depending on R2 values relatively, PSO models adequately described the kinetics of sorption of lead ion better than others model, and its theoretical equilibrium capacity of 26.68 mg/g fit well with experimental data value of 27.17 mg/g, but not for PFO. Elovich model which is useful model in describing the chemisorption behavior of adsorbate-adsorbent interaction gives comparatively less R2 values (
The effects of temperature on lead ion sorption are shown in
Δ G = – R T ln K c (13)
ln K c = − Δ H o / R T + Δ S o / R (14)
Δ G o = Δ H o − T Δ S o (15)
where R (8.314 J/mol∙K) is the gas constant, T (K) is the absolute temperature and Kc is the standard thermodynamic equilibrium constant which is defined by qe/Ce.
By plotting the graph of lnKc versus T−1, the value of ∆H˚ and ∆S˚ can be estimated from the slopes and intercept. The obtained values of ΔG˚, ∆H˚ and ∆S˚ different temperature were given in
The reusability of the adsorbent after its repeated usage has been a crucial factor for economic compatibility. From the reproducibility/desorption studies (
T (K) | ΔG (kJ/mol) | ΔH (kJ/mol) | ΔS (J/molK) |
---|---|---|---|
298 | −9.612 | 19.19 | 99.07 |
308 | −10.603 | ||
318 | −11.593 | ||
328 | −12.584 |
the amount of desorption of lead ion decreases as the pH of solution increases. 83.94% of Pb(II) ions was removed in the first cycle. Desirability was obtained using equation:
% Desorption Efficiency = D/A × 100 (16)
where, D = Desorbed is the concentration of lead ion after the desorption process and A = Adsorbed is (Co - Ce) for each recovery process.
Co-precipitation method has been successfully carried out to synthesize nano sized iron oxide at a relatively low temperature. XRD pattern confirmed the rhombohedral (hexagonal) structure of α-Fe2O3. The crystalline size of iron oxides particles was found to be 16.55 nm. The as-synthesized iron oxide nanoparticles was examined for morphological details by SEM which confirmed almost round shape and nano-powders were found to be less agglomerate. The observed peaks in EDX spectrum confirmed the presence of Fe & O and the absence of impurities in the prepared Fe2O3. The presence of Fe-O bond and its stretching vibration mode was confirmed by FTIR data. Langmuir isotherm model describes the adsorption data well compared to Freundlich, D-RK and FH models. From PFO, PSO, Elovich adsorption-reaction models, PSO fits well and IPD adsorption-diffusion models fail to fit and the line also will not pass through the origins. This means that the reaction is under control of adsorption-reaction model. The obtained maximum sorption capacity for the sorbent was 70.422 mg/g and 83.94% of Pb(II) was liberated during desorption studies.
Authors are great full to the management of Adama Science and Technology University for providing financial support towards this research work. The author acknowledges Mr. Yilkal Dessie for software support and Dr. Senthilkumar Subramanian for characterization support.
The authors declare no conflicts of interest regarding the publication of this paper.
Abebe, B. and Murthy, H.C.A. (2018) Fe-Oxide Nanomaterial: Synthesis, Characterization and Lead Removal. Journal of Encapsulation and Adsorption Sciences, 8, 195-209. https://doi.org/10.4236/jeas.2018.84010