The adsorption of aluminium(III) and iron(III) ions from their single and binary systems, by RHAC was investigated in a batch system. The activated carbon prepared from rice hulls was characterized by scanning electron microscopy and Fourier transformation infrared techniques. Batch adsorption experiments were performed under different operating conditions including pH (2 - 5), adsorbent dosage (0.5 - 2.0 g/l), initial ion concentration (5 - 100 mg/l), and contact time (30 - 240 min). The equilibrium time for maximum ions removal was found to be 180 min in single and binary ions systems. The kinetics of adsorption was evaluated using the pseudo-first order, pseudo-second order and Elovich kinetic models. The Langmuir, Freundlich and Temkin equilibrium models were applied to the adsorption experimental data. Real wastewater samples were collected from different locations to investigate the efficiency of rice hull activated carbon in treating real samples. The real wastewater samples were treated with the activated carbon prepared from rice hulls and a commercial activated carbon. The results showed that the activated carbon prepared in the present work was more efficient in the removal of aluminium and iron from real wastewater as compared to the commercial activated carbon which is more advantageous considering both economics and environmental parameters.
The presence of inorganic pollutants in water is a problematic environmental issue. The problem with metal ions pollution is that they are not biodegradable and are highly persistent in the environment [
Different methods have been used for the removal of metal ions from contaminated water. The commonly used procedures for removing metal ions from aquatic ecosystems include chemical precipitation, reverse osmosis and solvent extraction [
According to Ahmaruzzaman [
In Egypt, about 0.5 million tons of rice hulls are produced every year from the rice fields and rice milling process [
The aim of the present work is to prepare activated carbon from rice hulls (RHAC) and to characterize and investigate its adsorption efficiency towards the removal of iron and aluminium from their single and binary ion solutions in batch experiments considering all parameters affecting such processes as well as their kinetics and equilibrium.
Rice hulls used as a starting material for the preparation of activated carbon were obtained from the Egyptian Starch & Glucose Manufacturing Company (ESGC).
The activated carbons from rice hulls (RHAC) were prepared according to the procedure described by [
The prepared activated carbons were characterized by Fourier transformation infrared technique (FTIR) over the range of 500 - 4000 cm−1 using Thermo Nicolet Avatar 370 FTIR Spectrometer, Thermo scientific co. The surface characteristics of the adsorbents were also investigated by scanning electron microscope (SEM) using JEOL, JSM-6490LA Scanning Electron Microscope―JEOL USA, Inc.
Batch experiments were conducted to study the factors affecting the adsorption of Al(III) and Fe(III) onto RHAC. At the beginning of each experimental run, a known weight of RHAC was added to 50 ml solution containing a known concentration of single or binary aluminium(III) and iron(III) ions. The studied factors were pH (2 - 5), adsorbent dosage (0.5 - 2.0 g/l), initial ion concentration (5 - 100 mg/l), and contact time (30 - 240 min). The flasks were agitated in a shaking water bath at a 200 rpm constant shaking rate until equilibrium was reached. The mixture was then filtered and the remaining aluminium and/or iron concentrations were determined at 396.153 and 259.939 nm, respectively using atomic absorption spectrometer (Shimadzu, model AA-6300, Japan).
The amount of ions adsorbed at equilibrium (adsorption capacity), qe (mg/g), was calculated by the following equation:
whereas the ions removal percentage (R%) was calculated by the following equation:
where Ci and Ce are the ions concentrations at the initial time and at equilibrium (mg/l), respectively. V is the volume of the solution (l) and W is the mass of adsorbent used (g).
Seven water samples (S1, S2, S3, S4, S5, S6 and S7; three replicates each) were collected from different locations as given in
To compare the efficiency of RHAC and CAC for the removal of aluminium and iron from real wastewaters: 50 ml of each water sample were mixed with 0.1 g of RHAC or commercial activated carbon (CAC) obtained from Norit co. The samples were shaken for 240 min then they were separated from the adsorbent by filtration and finally the remaining concentrations of metal ions were determined.
The removal of Al(III) and Fe(III) from their single ion solutions as a function of contact time at different initial ions concentration is represented in
Sample | Location | pH | Temperature | Metal concentration |
---|---|---|---|---|
S1 | Fostat station | 7.45 | 20.8˚C | Al (0.583 mg/l) |
S2 | Sludge | 7.39 | 19.2˚C | Al (0.456 mg/l) |
S3 | Shoubra El-khima station | 7.62 | 21.8˚C | Al (0.363 mg/l) |
S4 | Iron and steel factory | 6.84 | 20.9˚C | Fe (3.478 mg/l) |
S5 | Iron and steel factory (cooling area) | 6.57 | 21.8˚C | Fe (0.726 mg/l) |
S6 | From Shoubra El-khima station | 7.95 | 19.8˚C | Fe (0.477 mg/l) |
S7 | Nile during the rains | 8.35 | 20.5˚C | Fe (2.908 mg/l) |
showed that although the equilibrium time for maximum ions removal was the same in single and binary ions systems, but the removal percentages attained was higher in single than in binary ions systems. The maximum removal of Al(III) in its single ion solution was 82% and decreased to 54% in the binary aluminum-iron solution. Similarly, the maximum Fe(III) removal was 67% in the single iron solution and decreased to 62% in binary aluminum-iron solution system. Similar findings were obtained by Lugo-Lugo et al. [
In order to analyze the adsorption kinetics of Al(III) and Fe(III) adsorption by RHAC in single and binary systems, pseudo-first order, pseudo-second order and Elovich kinetic models were applied to the experimental data.
The pseudo-first order model is represented by:
The kinetic constants and the determination coefficients for the three tested models are summarized in
The results indicated that in both single and binary systems, the adsorption of Al(III) and Fe(III) was found to be best described by the pseudo-second order model as well as the Elovich’s model. On the other hand, the pseudo first order model showed the least fit to the data. The adequacy of the pseudo second order model to fit the adsorption data suggests that the rate-limiting step is a chemical sorption or chemisorptions involving valence forces through sharing or exchange of electrons between adsorbate and adsorbent [
The results also showed that there was a noticeable reduction in RHAC adsorption capacity values towards both studied ions in the binary system as compared to the single systems which seems logic referring statistical vacant active adsorbent sites capacity of RHAC.
Based on previously published studies, the effect of pH on the removal of aluminum and iron ions by RHAC
Pseudo-first order | Pseudo-first order | Elovich model | qe (exp.) | |||||||
---|---|---|---|---|---|---|---|---|---|---|
k1 (L/min) | qe, calc. (mg/g) | R2 | k2 (g/mg.min) | qe, calc. (mg/g) | R2 | α (mg/g.min) | β (g/mg) | R2 | ||
Single ion system | ||||||||||
Al(III) | 0.015 | 30.88 | 0.93 | 6.20 × 10−3 | 21.74 | 0.96 | 9.17 | 26.82 | 0.95 | 22.07 |
Fe(III) | 0.017 | 27.94 | 0.90 | 8.13 × 10−4 | 22.73 | 0.99 | 7.74 | 20.52 | 0.96 | 20.61 |
Binary ions system | ||||||||||
Al(III) | 0.016 | 19.12 | 0.98 | 1.17 × 10−3 | 19.60 | 0.99 | 6.02 | 17.50 | 0.98 | 14.20 |
Fe(III) | 0.012 | 15.20 | 0.95 | 1.36 × 10−3 | 18.87 | 0.99 | 5.78 | 14.11 | 0.98 | 16.86 |
was studied in the range between pH 2 and 5 to avoid metal ions hydroxide precipitation at higher pH values [
It is clear that the removal of both metal ions was low at pH 2 and increased by increasing the pH value of the metal ions solution. It is generally agreed that at very low pH values the metal ions removal is weak due to the competition between the positively charged hydronium ions present in solution and the metal ions to occupy the adsorbent’s active sites. As the solution pH is raised less hydronium ions are present and thus the opportunity of the positively charged metal ions to occupy the adsorbent’s surface increases. The maximum removal of aluminum and iron ions due to adsorption was obtained at pH 5 and pH 3, respectively (
In agreement with our results; Cayllahua and Torem [
The adsorption of Al(III) and Fe(III) by RHAC was analyzed by well documented Langmuir (Equation (4)) [
where: Ceq: equilibrium concentration in mg/l; qe: equilibrium capacity in mg/g; qmax: maximum capacity in mg/g; b (l/ mg) is Langmuir constant; kf and n are Freundlich constants; bt and at are Temkin isotherm constants; qm is the theoretical saturation capacity (mg/g), β is a constant related to the mean free energy of adsorption per mole of the adsorbate (mg2/J2), and ε is the Polanyi potential.
The adsorption free energy (E; kJ/mol) can be calculated from the D-R model as follows: E = 1/√−2β.
All the parameters calculated from the studied equilibrium models along with the correlation coefficients (R2) are given in
The coefficients of determination (R2) for the Langmuir plots (
Langmuir model | Freundlich model | Temkin model | Dubinin-Radushkevich (D-R) | ||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|---|
qmax (mg/g) | b (l/mg) | R2 | kf | n | R2 | at | bt | R2 | qm (mg/g) | β (mg2/J2) | E (KJ/mol) | R2 | |
Single ion system | |||||||||||||
Al(III) | 34.48 | 0.05 | 0.98 | 1.16 | 0.47 | 0.97 | 51.85 | 32.05 | 0.78 | 91.83 | 2 × 10−6 | 13.50 | 0.98 |
Fe(III) | 45.45 | 0.03 | 0.99 | 1.07 | 0.68 | 0.97 | 32.27 | 28.36 | 0.79 | 126.09 | 2 × 10−5 | 8.85 | 0.98 |
Binary ions system | |||||||||||||
Al(III) | 21.28 | 0.08 | 0.99 | 1.65 | 0.54 | 0.96 | 39.63 | 37.77 | 0.71 | 73.33 | 4 × 10−6 | 9.54 | 0.97 |
Fe(III) | 26.32 | 0.02 | 0.98 | 2.92 | 0.62 | 0.96 | 28.92 | 37.76 | 0.77 | 117.21 | 2 × 10−5 | 8.2 | 0.97 |
Al(III) and Fe(III) onto RHAC is a monolayer adsorption taking place at the surface groups binding sites of the adsorbent. The maximum adsorption capacities calculated from the Langmuir model were found to be 34.48 and 45.45 mg/g for aluminium and iron in their single solutions and 21.28 and 26.32 mg/g for aluminium and iron in their binary solutions, respectively. It is clear that the maximum capacity of the metal ions in their single solutions was higher than the maximum capacity values recorded in the binary ions solution. A comparison between the maximum Langmuir capacity values obtained in the present study with those previously reported in literature for aluminium and iron adsorption onto various adsorbents is given in
The Freudlich model plots obtained for the experimental data are given in
The plots of Dubinin-Radushkevich (D-R) isotherm are shown in
On the other hands the Temkin isotherm model (
Based on the R2 values which is a measure of the goodness of model’s fit [
The results also show that the maximum capacity of the metal ions in their single solutions was higher than the maximum capacity values recorded in the binary ions solution.
Adsorbent/metal ion | Qmax (mg/g) | Reference |
---|---|---|
C. vulgaris/Fe(Ill) | 24.491 | [ |
R. arrhizus/Fe(Ill) | 34.733 | [ |
Raw clinoptilolite/Fe(III) | 98.00 | [ |
Geobacillus thermodenitrificans/Fe(III) | 79.9 | [ |
SCB/Fe(III) | 331.1 | [ |
Orange peel/Fe(III) | 9.4308 | [ |
Date-pit/Al(III) | 5.831 | [ |
BDH activated carbon/Al(III) | 6.562 | [ |
R. opacus/Al(III) at 25˚C | 41.584 | [ |
RHAC/Fe(III) | 45.45 | Present study |
RHAC/Al(III) | 34.48 | Present study |
The activated carbon prepared from rice hull was characterized by FTIR and SEM techniques in order to identify the surface functional groups and surface morphology of the adsorbent.
The FT-IR spectrum of (RHAC) is shown in
The scanning electron microscopy micrograph of RHAC is shown in
Thus by considering the FTIR surface bands and the surface morphological characteristics of RHAC, it can be inferred that RHAC possess a surface capable of adsorbing metal ions.
The results of real water samples treatment using RHAC and CAC are presented in
Metal ion | Concentration before adsorption (mg/L) | Rice hull activated carbon (RHAC) | Commercial activated carbon (CAC) | ||
---|---|---|---|---|---|
Concentration after adsorption (mg/L) | Removal percentage (%) | Concentration after adsorption (mg/L) | Removal percentage (%) | ||
Al(III) | 0.583 | 0.061 | 89.474 | 0.137 | 78.204 |
0.456 | 0.015 | 96.635 | 0.150 | 67.154 | |
0.363 | 0.044 | 87.960 | 0.143 | 60.478 | |
Fe(III) | 3.478 | 0.106 | 96.952 | 0.557 | 83.985 |
0.726 | 0.056 | 92.287 | 0.145 | 80.028 | |
0.477 | 0.047 | 90.147 | 0.142 | 70.231 | |
2.908 | 0.722 | 75.172 | 0.108 | 96.286 |
commercial activated carbon. It is worth to note that from an economic point of view, the use of RHAC will be more advantageous than the commercial activated carbon as RHAC is cost effective natural adsorbent.
In the present study, rice hulls were successfully used as a starting material for the preparation of a cost effective activated carbon. The prepared activated carbon showed good adsorption capacity for removing both aluminium and iron ions from their single and mixed ion solutions. The adsorption equilibrium was reached in 180 minutes. The adsorption of aluminium and iron was found to be well fitted to the pseudo-second order kinetic model and the Langmuir equilibrium model. The activated carbon prepared from rice hulls showed high aluminium and iron removal efficiency from real wastewater samples. It can be concluded that rice hulls could be effectively used for the production of cost effective activated carbons that could be applied for the removal of metal ions from wastewater.