Kinetic, isotherm and thermodynamic studies on biosorption of chromium(VI) by using activated carbon from leaves of Ficus nitida
© The Author(s) 2016
Received: 4 January 2016
Accepted: 17 May 2016
Published: 31 May 2016
Kinetics, thermodynamics and equilibrium of the removal of chromium(VI) ions from aqueous solutions by using chemically activated leaves of Ficus nitida were investigated. Adsorption runs were performed as a function of pH, mass of biosorbent, contact time, initial concentration of chromium(VI) ions and temperature.
The optimum conditions for maximum removal of chromium(VI) ion from aqueous solutions (about 99 %) were found to be 0.80 g of chemically activated leaves of F. nitida, 25 min, 50.0 mg/L of initial concentration of chromium(VI). Values of thermodynamic activation parameters proved that the biosorption process is spontaneous and endothermic. Results were analyzed by using Langmuir, Freundlich and Temkin models.
In the recent years the activities of industrial sectors has showed a considerable spread and development, but concurrently the natural environment has been contaminated. Heavy metals are one of the most widespread pollutants which contaminate the environment and cause serious damage to the ecosystem and also may be a reason for various dangerous diseases suffered by animals and human beings . A number of industries are causing heavy metal pollution e.g. battery manufacturing processes, mining and metallurgical engineering, dyeing operations, electroplating, nuclear power plants, tanning, production of paints and pigments . Heavy metals that may be considered as risky environmental pollutants are Cd, Hg, Pb, As, Cr, Hg, Ni and Cu. Comparing with organic pollutants, heavy metals are normally refractory and cannot be degraded or easily detoxified .
Chromium(VI) is one of the most poisonous contaminants which cause severe diseases and very harmful environmental complications. When chromium(VI) accumulates at high levels, it may lead to serious problems and even be fatal when concentrations reach 0.10 mg/g of body mass . Chromium(VI) is more toxic than chromium(III) and as such receives more attention. Strong exposure to chromium(VI) has been linked to various types of cancer and may cause epigastric pain, nausea, vomiting, severe diarrhea and hemorrhage .
The removal of toxic metals from wastewater has been achieved using various methods like ion electro dialysis , sedimentation , ion exchange [8, 9], biological operations , coagulation/flocculation , nanofiltration technology , solid phase extraction , adsorption by chemical substances [14, 15] and electrokinetic remediation . All these techniques suffer from multiple drawbacks such as high capital and operational costs and disposal of residual metal sludge . In contrast, the bio-sorption method has become one of the most favored ways to remove heavy metals because it is environmentally friendly, highly efficient and has low associated costs. Various parts of plants are commonly used as biomass adsorbent for Cr(VI) adsorption from drinking water and wastewater. These include Syzygium jambolanum nut , Sophora japonica pods powder , rice bran , neem bark, neem leaves, rice straw and rice husk , gooseberry seeds , husk of Bengal gram , Cupressus lusitanica Bark  and Azadirachta indica .
Activated carbons are more effective in the removal of heavy metals ions because of some specific characteristics that augment the use of activated carbon for the removal of pollutants including heavy metals from water supplies and wastewater . The ability of activated carbon to remove Cr(VI) by adsorption was reported many times. Activated carbon derived from procumbens , oil palm shell charcoal , groundnut hull , Sweet lime fruit skin and bagasse  were used for removal of Cr(VI) from aqueous solutions.
The aim of this study was to prepare activated carbons derived from leaves of Ficus nitida (AFNL) by chemical activation using H2SO4 and to use this activated carbon in removal of Cr(VI) ions from aqueous solutions.
Preparation of biomass adsorbent
Leaves of F. nitida were collected from the main campus of King Khalid University, Abha, Saudi Arabia in September 2015. Leaves were thoroughly washed with distilled and deionized water, dried at room temperature for 3 days. The dried leaves were ground in an electric mill and then mixed with concentrated sulfuric acid in a mass ratio of 1:1.8 biomass:acid , then the mixture was filtrated and the obtained activated carbon was rinsed thoroughly with deionized water to remove the acid residue and dried for 6 h at 105 °C.
Preparation of Cr(VI) solutions
Stock solution of potassium dichromate of 1000 mg/L concentration was prepared by dissolving the appropriate weight in 1.0 L of deionized water. The required concentrations were then prepared by taking adequate volumes from the stock solution.
Batch bio-sorption study
pH measurements were carried out by using pH meter Hanna 211. Equilibrium concentrations were measured by using flame atomic absorption photometer (Spectra AA 20) in an air-acetylene flame. Chromium hollow cathode lamp was used as the radiation source with lamp current of 7 mA, wavelength of 357.9 nm and slit width of 0.2 nm. The specific surface area was measured using a SA-9601 analyzer.
Reliability of results
A calibration curve was obtained using 0.5–4 mg/L concentration range of Cr(VI) ions. Linearity was calculated in order to investigate the reliability of results. Limit of detection LOD and limit of quantification LOQ were determined by reported method . Precision was verified by determination of relative standard deviation RSD and accuracy was checked by recovery study.
Results and discussion
Reliability of results
A number of parameters i.e., linearity, LOD, LOQ, RSD were determined in order to check the reliability of results.
The linearity of the calibration curve was evaluated by plotting the absorbance of standard solutions of Cr(VI) against the concentration. A straight line with regression coefficient (R2) of 0.997 was obtained indicating good linearity.
LOD and LOQ
Sensitivity was evaluated by determination of limit of detection (LOD) and limit of quantitation (LOQ). (LOD) and (LOQ), were determined by measuring 10 blank samples. By using the relationships 3.3SD/b and 10SD/b, it was found that LOD = 0.02 mg/L and LOQ = 0.06 mg/L, respectively.
The relative standard deviation (RSD) usually expresses precision of measurements. Practically, precision is determined by evaluating the reproducibility of the results. Ten blank samples were measured at the same conditions and the obtained RSD value was 7.05 % which is in the acceptable limit .
Usually recovery studies are carried out in order to check the accuracy. Recovery studies were performed by spiking technique. The recovery value, determined as 93.2 %, is within the acceptable range .
Surface area of AFNL
The BET surface area analysis revealed that AFNL has a specific surface area of 1230 m2 g−1 indicating that AFNL may have good metal uptake capacity.
Effect of pH
Effect of biomass weight
Effect of contact time
The impact of contact time on the removal of 50 mg/L of Cr(VI) ions from aqueous solutions was also investigated. Results revealed that the metal ions removal increases linearly with time up to 25 min and then remains at the same level. The rate of metal ion removal is higher in the beginning because of the large surface area of the adsorbent available for the adsorption of the Cr(VI). Furthermore, no major changes were observed in the removal of Cr(VI) ions from the aqueous solution after 24 h of equilibration.
Effect of interfering ions
An aqueous solution containing 50 mg/L of Cr(VI) ions, 5 mg/L of Pb(II) ions, 5 mg/L of Cd(II) ions and 5 mg/L of Ni(II) ions was used to study the effect of interfering ions on the efficiency of AFNL on removal of Cr(VI) ions. Results showed that after 30 min of shaking time, 96 % of Cr(VI) ions were removed from the aqueous solution indicating that the interfering ions have almost no effect on the efficiency of AFNL to remove Cr(VI) ions. Furthermore very small quantities of the interfering ions were removed demonstrating that AFNL may be used as selective bio-sorbent for Cr(VI) ions. This may be attributed to the fact that the experiment was carried out at the optimal conditions for Cr(VI) removal.
Effect of Cr(VI) concentration
The effect of initial concentrations of Cr(VI) ions on its adsorption on the ALFN was investigated by varying the initial concentration from 50 to 200 mg/L. Results revealed that the removal percentage is inversely proportional to the initial Cr(VI) concentration. This may be attributed to coverage of active sites of adsorbent as the concentration of Cr(VI) increases.
Adsorption of Cr(VI) ions onto ALFN was studied using three models of adsorption isotherm: Langmuir, Freundlich and Temkin isotherms. The aim of adsorption isotherms is to explain the relation between the remaining concentration of the adsorbate and the adsorbed quantity on the sorbent surface.
Constants of different adsorption isotherm models
Kf, mg/g (L/mg)1/n
Thermodynamic parameters of the biosorption of Cr(VI) ions onto ALFN
ΔSo (J/mol K)
Comparison of ALFN with other sorbents
Comparison of maximum uptake capacity for various bio-sorbents
Metal uptake capacity, mg/g
Activated carbon from Ficus nitida leaves
Activated carbon from Rosmarinus officinalis leaves
Mangifera indica bark
Syzygium cumini bark
Ground nut shell
Biosorption of Cr(VI) ions onto activated carbon prepared from leaves of F. nitida was investigated and found to be dependent on pH value of solution, adsorbent mass, contact time, temperature and initial Cr(VI) concentration.
Data of biosorption of Cr(VI) on ALFN were applied to three adsorption isotherm models. The maximum adsorption capacity was determined from the Langmuir isotherm as 21.0 mg/g. The n value obtained from the Freundlich isotherm indicates that the sorption of Cr(VI) ions onto ALFN is favorable. Adsorption process of Cr(VI) ions onto ALFN was found to obey the second-order kinetic equation. Thermodynamic parameters proved that the adsorption process is spontaneous and endothermic.
IHA carried out the design of the study; all batch biosorption studies, analysis of data and writing the manuscript. HMA carried out the collection of leaves of Ficus nitida, preparation of the activated carbon and measurements of Cr(VI) concentration by using atomic absorption spectrometer and helped in data analysis. Both authors read and approved the final manuscript.
The authors declare that they have no competing interests.
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