The present study reports the chemical modification of agricultural waste (rice straw) with urea using microwave radiation and the efficiency evaluation of this modified rice straw for the adsorption of a toxic heavy metal, cadmium. The elemental analysis of urea modified rice straw affirmed urea grafting on rice straw, and FTIR spectra of chemically benign modified adsorbent showed the presence of hydroxyl, carbonyl, and amino functional groups. Effects of process parameters (adsorbent dosage, contact time, agitation speed, pH, and temperature) were studied in batch mode. Parameters were optimized for the equilibrium study, and adsorption mechanism was elucidated using five mathematical models (Langmuir, Freundlich, Temkin, Harkin-Jura, and Dubinin-Radushkevich). Binding of Cd(II) ions on modified adsorbent followed Langmuir model, and the maximum uptake capacity was found to be 20.70 mg g−1. Kinetic modeling was done using six different kinetic models. The process was considered physisorption according to the obtained activation energy value. Thermodynamic parameters confirmed the process to be favorable and feasible. Exothermic nature of adsorption of Cd(II) ions on urea modified rice straw was confirmed by the negative value of Δ
Rapid pace of industrialization has resulted in a number of problems among which water pollution is considered to be one of the serious problems. Industrial processes discharge huge amounts of untreated wastewater daily into the surrounding environment, leading to detrimental effects on aquatic, plant, and human life. Heavy metal such as lead, cadmium, chromium, and copper is regarded as major pollutants in wastewater. These contaminants are of major concern because they do not degrade naturally [
Cadmium has attracted wide attention of environmental chemists as one of the most toxic metals and has been categorized as a human carcinogen by USEPA (United States Environment Protection Agency), WHO (World Health Organization), and NTP (National Toxicology Program) [
It is important to treat contaminated waters on a continuous basis due to need of hour. A number of technologies are available with varying degree of success, and among them adsorption process is considered relatively better because of convenience, ease of operation, and simplicity of design [
Recently, many nonconventional, low-cost adsorbents including natural materials, (biosorbents) and waste materials have been proposed by several researchers. Cadmium has been reported to be removed and recovered from aqueous solutions by a number of biosorbents [
Rice (
The present study is based on the evaluation of effectiveness of a modified agricultural waste (rice straw) for the removal of toxic Cd (II) ions from aqueous solution in a batch process. Detailed equilibrium, kinetic, and thermodynamic studies elucidate the adsorption mechanism.
Dried rice straw (
The prepared material UMRS was characterized using FTIR, elemental analysis, and surface area. In order to study the presence of potential functional groups in UMRS, the FTIR spectrum was scanned in 4000–400 cm−1 using standard method with the help of FTIR spectrophotometer (Spectrum RX-1, Perkin Elmer). The elemental analysis was performed using elemental analyzer (EL III, Elementar, Vario) using corn gluten as a standard. The surface area was determined using Langmuir equilibrium model.
A concentration difference method was used to study the effect of various parameters on the biosorption of Cd(II) by UMRS. Cadmium (II) nitrate (Merck, Germany) was used to prepare the aqueous solution of Cd(II). Distilled water was used for all types of solution preparations and dilutions as per requirement. In all experiments, measuring/conical flasks (100 mL) were used containing Cd(II) solution (50 mg/L, 50 mL) of a known concentration at a specific pH. A known amount of UMRS (0.2–1.4 g) was added to the solution and then agitated on an orbital shaker (OSM-747, Vortex) at predefined speed (125 rpm). After a specific period of time, the contents were filtered, and the filtrate was analyzed using atomic absorption spectrophotometer (AAnalyst 100, Perkin Elmer) to determine the equilibrium Cd(II) ions concentrations. The difference of initial (
The effects of parameters like time of contact, pH, dose of UMRS, agitation speed, temperature, and concentration of Cd(II) ions on the biosorption process were studied in a similar way. Blank experiments were performed in order to study the adsorption of Cd(II) by the glassware. No detectable adsorption of Cd(II) was found by the glassware. All the graphs were prepared using Microsoft Excel 2003 software. Regression analyses have also been performed by calculating
The pH of the solution is probably the most important parameter as it affects the charges on biomass as well as metal speciation in the solution. The metal species are influenced by the solution pH. Cadmium ions are present as free Cd2+ species along the whole acidic pH range. As the pH is increased above pH 7.5, it starts to precipitate as Cd(OH)2, and thus it is no more “available” for biosorption. This narrows down the upper pH limit for the biosorption of Cd(II) by UMRS. So, during the study of the effect of pH, the range that should be scanned for the optimum pH is limited to an upper value of 7.5 [
On the other hand, in highly acidic pH, there are a greater number of H+ ions present in the solution. These H+ ions are readily sorbed on the sites of the biomass (UMRS) and thus protonate it before metal ions can attack these sites. This causes UMRS to behave as positive specie. Due to the electrostatic repulsive force present between two positive species, a limited number of Cd(II) ions are sorbed on UMRS, and thus there should be low
The protonation and deprotonation of other available functional groups can be explained on similar grounds. It can be predicted that at highly acidic pHs, Cd(II) binding with UMRS is reduced, and the binding increases with increase in pH because UMRS is negatively charged. This effect of pH of solution on biomass, thus, decides the lower limit of pH. Based on the previous discussion, the effect of pH was studied in pH range of 2–7.
The pH profile study is shown in Figure
Effect of change in pH on biosorption of Cd(II) on UMRS (
The contact time studies are very critical as these endow with the minimum time required to remove maximum amount of Cd(II) ions from the solution and thus help in scaling up the process. The optimum (equilibrium) time helps in studying the rate of biosorption process. With the help of kinetic data, the rate determining step of the transport mechanism and thus the modeling and design of the process can be described.
The effect of contact time on the removal of Cd(II) by UMRS is depicted in Figure
Effect of contact time on Cd(II) biosorption by UMRS (
The reaction kinetics was investigated by using a number of different available kinetic models. The experimental data obtained from the contact time studies was used for the purpose. The linear forms of the Elovich, first-order, pseudo-first-order, second-order, and pseudo-second-order kinetic models [
Kinetics of the biosorption of Cd(II) onto UMRS.
Model | Linear equation |
|
Model parameters | |
---|---|---|---|---|
Elovich model |
|
2.924 |
|
|
First order |
|
|
0.0452 | |
Pseudo-first order |
|
|
0.1074 | |
Second order |
|
|
0.0026 | |
Pseudo-second order |
|
|
4.6812 | |
Intraparticle diffusion |
|
|
0.0245 |
Kinetic models for the binding of Cd(II) onto UMRS (a) Elovich, (b) first-order, (c) pseudo-first-order, (d) second-order, (e) pseudo-second-order, and (f) intraparticle diffusion models.
The experimental data were used to study the kinetics of the process using Elovich model (Figure
The plots for first-order and pseudo-first-order kinetic models are shown in Figures
Comparison of capacity of UMRS with some other reported biosorbents.
Biomass | Biosorption capacity |
Equilibrium modela | Kinetic modelb | Reference |
---|---|---|---|---|
Sulfonated |
1.68 | — | PSO | [ |
|
2.80 | — | PSO | [ |
|
3.61 | — | [ | |
Coconut copra meal | 4.92 | L, RP | — | [ |
|
6.30 | L | PSO | [ |
|
11.56 | L | — | [ |
|
14.56 | L | PSO | [ |
Water Hyacinth | 14.67 | — | PSO | [ |
Papaya wood | 17.22 | L | PSO | [ |
Spent grain | 17.30 | L | — | [ |
Urea modified |
20.70 | L | PSO | Present work |
|
40.50 | L | — | [ |
The plots for second-order (
When the experimental data was used to draw the graph for pseudo-second-order kinetic model, straight plot for the whole set of data was observed. The comparison of calculated
It was found that Elovich, first-order, pseudo-first-order, and second-order models failed to explain the kinetics of Cd(II) biosorption by UMRS. In some cases, the experimental and calculated
In order to have an insight into the rate determining step, Weber and Morris model, that is, intraparticle diffusion (IPD) model, was employed [
where
As shown in Figure
The role of adsorption in the biosorption of Cd(II) ions by UMRS can be explained by the use of equilibrium modeling. The adsorption models indicate how Cd(II) ions distribute between the liquid and solid phases at equilibrium. A number of different models have been employed for the purpose in the present study. These models, namely, Langmuir equation (
Experimental data for the biosorption of Cd(II) ions by UMRS is plotted as
Equilibrium models studied at 303 K.
Model | Equation | Parameters | |
---|---|---|---|
Langmuir |
|
|
20.70 |
| |||
Freundlich |
|
|
1.2018 |
| |||
Temkin |
|
|
1.9022 |
| |||
Harkin-Jura |
|
|
2.4606 |
| |||
D-R |
|
|
5.8532 |
Equilibrium modeling of Cd(II) biosorption on UMRS.
The Langmuir model is one of the most frequently used equilibrium models and is employed to determine the maximum capacity of the biosorbent to bind the metal ions. It assumes that the uptake/binding of Cd(II) ions occurs on the homogenous surface by monolayer adsorption without any interaction between adsorbed ions. The nonlinear form of Langmuir model is used to explain the behavior of the Cd(II)-UMRS biosorption process. It can be observed that the Langmuir equilibrium curve significantly overlaps the experimental data (Figure
The
The feasibility of Langmuir model is usually expressed by a dimensionless constant separation factor or equilibrium parameter
The Freundlich model assumes the nonzero interactions between the adsorbate particles and is based on multilayer adsorption on heterogeneous surface. The nonlinear plot for the model is shown in Figure
Temkin model provides information about the heat of adsorption and the adsorbent-adsorbate interaction on the surfaces. Harkin-Jura model indicates the multilayer adsorption. The parameters for the both equilibrium models are shown in Table
The physical or chemical nature of biosorption of Cd(II) onto UMRS can be assessed by determining the energy of sorption (
where
The effect of change in temperature on the Cd(II)-UMRS sorption system was studied to resolve the thermodynamic parameters and to investigate the nature/feasibility of the process. It was observed that the sorption capacity increased with increase in temperature. The
The experimental data were used to determine the thermodynamic parameters like changes in standard free energy (
The thermodynamic parameters given in Table
Values of thermodynamic parameters for Cd(II)-UMRS system.
Temperature |
Δ |
Δ |
Δ |
---|---|---|---|
293.16 |
|
117.81 | 409.08 |
303.16 |
|
||
313.16 |
|
(a) Thermodynamic modeling and (b) activation energy for the Cd(II) onto UMRS under optimum conditions.
Activation energy (
where
Dose of biomass is very important in determining the minimum amount required to treat a solution of given metal concentration. By increasing the amount of biomass, the number of available sites is also increased. The effect of dose of UMRS on percentage adsorption of Cd(II) ions was studied at an initial concentration of 50 mg L−1 by varying the amount from 0.2 to 1.4 g per 50 mL of Cd(II) solution. At optimum conditions, it was observed that by increasing the dose, the
Agitation speed is considered as one of the important process parameter which significantly affects the biosorption of Cd(II) onto UMRS. When UMRS is made to come into contact with Cd(II) bearing solution, the metal ions present close to it are readily attached. This generates a concentration gradient in the metal-biomass system. By agitating the metal-biomass system, the effect of such a concentration gradient is minimized, and the metal ions present in the solution are distributed evenly in the solution. Moreover, agitation distributes the biomass in the solution more evenly as compared to the situation when there is no agitation. On the other hand, agitation also causes desorption of the loosely bound metal ions from the surface of biomass. So an optimum speed of agitation is very much essential for the efficient removal of metal ions from the solution.
The effect of agitation speed was monitored on the biosorption of Cd(II) by UMRS by varying the speed from 50 to 250 rpm at optimum conditions (figure not shown). As the agitation speed was increased,
Elemental analysis and FTIR analysis were carried out using powdered, dried urea modified rice straw. The characterization revealed the information regarding adsorption sites in terms of functional groups. Simple rice straw consists of cellulose (32.24%), hemicellulose (21.34%), lignin (21.44%), and mineral ash (15.05%) [
Characterization of UMRS.
Elemental analysis | C (%) |
49.98 |
| ||
FTIR analysis (cm−1) |
3819.1; 3287.4; 2349.4; 1697.9; 1429.1; 1048.8; |
The FTIR is an important technique to identify potential functional groups that may participate in the binding of metal ions. The characteristic FTIR bands for UMRS (Figure
FTIR of UMRS.
Proposed attachment/binding sites of Cd(II) ions onto the UMRS biosorbent (
The present study was based on the efficiency evaluation of a low cost urea modified agricultural waste material for the adsorption of Cd(II) ions from water. Characterization of the modified adsorbent using FTIR and elemental analysis affirmed the urea modification by showing peaks of amide group and a high nitrogen content, respectively. Process parameters were optimized for equilibrium study. According to the results maximum adsorption was observed when 0.8 g per 50 mL of modified adsorbent remained in contact with 50 mg L−1 Cd(II) ions solution for 10 min at pH 6 keeping agitation speed 125 rpm at temperature 303 K. Five different adsorption isotherms (Langmuir, Freundlich, Temkin, Harkin-Jura, and Dubinin-Radushkevich) were used for the adsorption modeling and equilibrium study. It was observed that Langmuir model better fitted to the equilibrium data, the maximum uptake capacity was found to be 20.70 mg g−1, and