Investigation of the sorption potential of rice husk, an agricultural waste, as an adsorbent was carried out. The rice husk was modified with orthophosphoric acid and was used for adsorption of lead (II) ions (Pb2+) from aqueous solution. Physicochemical properties of the modified rice husk were determined. Equilibrium sorption data were confirmed with Langmuir, Freundlich and Temkin adsorption isotherms. On the basis of adsorption isotherm graphs,
The pollution of water resources due to the disposal of heavy metal ions has been an increasing worldwide concern for the last few decades. It is well known that some metals are poisonous or otherwise toxic to human beings and ecological environments as reported by Abdel-Halim and coresearchers [
There are various methods of removing heavy metal ions, and they include chemical precipitation, membrane process, ion exchange, solvent extraction, electrodialysis, and reverse osmosis [
Rice is the second largest produced cereal in the world. Rice husk is the hard protecting covering of grains of rice. It is an agricultural waste material obtained from the threshing of the rice and constitutes about 20% of 650 million tons of rice produced annually in the world [
Most commonly used bioadsorbents are untreated/unmodified; however in this research, the potential of acid modified rice husk is investigated in the removal of toxic heavy metal ion such as lead from its aqueous solution namely, the study of the Langmuir, Freundlich, and Temkin adsorption isotherms.
The PRH was characterized by determining the following parameters: specific surface area, moisture content, loss of mass on ignition, pH, and bulk density using standard procedures.
Saers method has been used by a number of researchers [
5 g of the PRH was weighed into a crucible. This was placed in the oven and heated for 5 hrs at constant temperature of 105°C. The sample was then removed and put rapidly into a desiccator in order to prevent more moisture uptake from atmosphere. The sample was reweighed. This procedure was repeated several times until a constant weight was obtained. The difference in the mass constitutes the amount of moisture content of the adsorbent [ weight of crucible, initial weight of crucible with sample, final weight of crucible with sample.
This was done by weighing 10 g of the adsorbent and put inside furnace at constant temperature of 600°C for 2 hrs. After roasting, the sample became charred and was removed from the furnace then put in a desiccator for cooling. The residual product is then weighed, and the difference in mass represented the mass of organic material present in the sample. This operation was repeated four times.
pH of the samples was determined by weighing 1 g each of PRH, boiled in a beaker containing 100 mL of distilled water for 5 min; the solution was diluted to 200 mL with distilled water and cooled at room temperature. The pH of each was measured using a pH meter (model ATPH-6), and the readings were recorded [
Archimedes’ principle was used to simply determine the bulk density by weighing a 10 cm3 measuring cylinder before and after filling with the samples. The measuring cylinder was then dried, and the sample was packed inside the measuring cylinder, leveled, and weighed. The weight of the sample packed in the measuring cylinder was determined from the difference in weight of the filled and empty measuring cylinder. The volume of water in the container was determined by taking the difference in weight of the empty- and water filled-measuring cylinder. The bulk density was determined using the equation below [ weight of empty measuring cylinder, weight of cylinder filled with sample, volume of cylinder.
The preparation of adsorbate was carried out by preparing stock solution containing 1000 mg/L of Pb. 0.3998 g of Pb(NO3)2 in 250 cm3 of de-ionized water. Working concentration in the range of 10 mg/L–200 mg/L was prepared by serial dilution.
The equilibrium sorption of the Pb2+ ions unto PRH was carried out by contacting 0.1 g of the substrate with 100 cm3 of different concentrations from 10 mg/L–200 mg/L in 250 cm3 pyrex conical flask intermittently for 90 minutes. The mixture was filtered, and the residual concentration of the filtrate was analyzed using Atomic Absorption Spectrophotometer (2380 UNICAM AAS). The amount of adsorbed (mg/g) was calculated using the formulae reported by Vanderborght and Van Grieken [
The physicochemical parameters of the phosphoric acid modified rice husk (PRH) are shown in Table
Some physicochemical parameters of the phosphoric acid modified rice husk (PRH).
Properties | PRH |
---|---|
pH | 6.30–6.50 |
% Moisture content | 12 |
% Loss of mass on ignition | 0.9 |
Bulk density (g/cm3) | 0.386 |
Particle size | 300 |
Surface area (m2/g) | 198 |
The physicochemical parameters of the modified rice husk are, namely, pH, % moisture content, % loss of mass on ignition, bulk density (g/cm3), particle sizes, and surface area (m2/g). The values reported are in the range with those reported in the literature [
The equilibrium sorption of the Pb2+ ions was carried out by contacting 0.1 g of the PRH with 100 cm3 of 1000 mg/L of different concentrations from 10 mg/L–200 mg/L in 250 cm3 Pyrex conical flask intermittently for 90 minutes on the orbital shaker. The mixture was filtered, and the filtrate was analyzed for metal ions concentration using AAS. The data were fitted into the following isotherms: Langmuir, Freundlich, and Temkin.
The adsorption data obtained with the adsorbent correlates well with Langmuir, Freundlich, and Temkin adsorption models and were illustrated in Figures the equilibrium concentration of adsorbate (mg/L−1), the amount of metal adsorbed per gram of the adsorbent at equilibrium (mg/g), Langmuir constants related to adsorption capacity (mg/g), rate of adsorption (L/mg). The values of initial concentration, the constant related to the energy of adsorption (Langmuir constant),
where
Langmuir adsorption isotherm of Pb(II) ion unto PRH.
Freundlich adsorption isotherm of Pb(II) ion unto PRH.
Temkin adsorption isotherm of Pb(II) ion unto PRH.
Freundlich model was chosen to estimate the adsorption intensity of the sorbent towards the sorbate [
Freundlich adsorption is commonly used to describe the adsorption characteristics for the heterogeneous surface [
The constant
Temkin isotherm model was also chosen for the study of this equilibrium sorption. This was done by plotting the quantity sorbed Temkin isotherm equilibrium binding constant (L/g), Temkin isotherm constant, universal gas constant (8.314J/moL/K), Temperature at 298 K.
Table
Parameters for plotting Langmuir, Freundlich, and Temkin adsorption isotherms of Pb(II) ion unto PRH.
S/N |
|
|
Log |
Ln |
|
Log |
|
% Sorption |
---|---|---|---|---|---|---|---|---|
1 | 0.00 | 0.00 | 0.00 | 0.00 | 0.00 | 0.00 | 0.00 | 0.00 |
2 | 10.00 | 0.00 | 0.00 | 0.00 | 10.00 | 1.00 | 0.00 | 100 |
3 | 50.00 | 0.00 | 0.00 | 0.00 | 50.00 | 1.70 | 0.00 | 100 |
4 | 100.00 | 6.70 | 0.826 | 1.902 | 93.30 | 1.970 | 0.072 | 93.30 |
5 | 150.00 | 26.94 | 1.430 | 3.293 | 123.06 | 2.09 | 0.22 | 82.04 |
6 | 200.00 | 62.03 | 1.793 | 4.128 | 137.97 | 2.140 | 0.45 | 68.99 |
Langmuir, Freundlich, and Temkin constants for the adsorption of Pb(II) unto PRH.
Metal | Langmuir | Freundlich | Temkin | |||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|
Pb2+ |
|
|
|
|
|
|
|
|
|
|
|
|
138.89 | 0.699 | 0.0141 | 0.99 | 0.59 | 1.02 | 3.88 | 0.80 | 3.38 | 92.14 | 26.89 | 0.916 |
For the past few years, there is an increasing interest in the preparation of low-cost adsorbent as an alternative to biosorption of lead(II) ions. In this research, rice husk has shown its potential to be an active bioabsorbent material in solving waste water pollution as a cost-effective adsorbent. The usage of the rice husk might help to overcome part of the excessive agricultural wastes in some part of the world. This research proved that rice husks possess different physical characteristics. Chemical modification using phosphoric acid improves the adsorption capacity of active binding sites. Hence, modified rice husk is a potent and low-cost alternative adsorbent for the treatment of lead-polluted waste water.
The authors are grateful to the management of Landmark University for the timely supply of the necessary equipments into the Department of Sciences for core research work.