Oana Maria Paşkaa,
Cornelia Păcurariua and
Simona Gabriela Muntean
*b
aPolitehnica University of Timişoara, Faculty of Industrial Chemistry and Environmental Engineering, P-ţa Victoriei No. 2, 300006, Timişoara, Romania
bInstitute of Chemistry Timisoara of the Romanian Academy, 24 Mihai Viteazul, 300223, Timisoara, Romania. E-mail: sgmuntean@acad-icht.tm.edu.ro; Fax: +40-256-491824; Tel: +40-256-491818
First published on 12th November 2014
A low-cost waste biomass derived from corn plant (husk) was tested as an alternative to other expensive treatment options, for the removal of methylene blue (MB), from aqueous solutions. The effects of different experimental parameters, such as biosorbent dosage, dye concentration, contact time, and temperature, on the adsorption process were investigated. An optimum value of discoloration was observed at pH 6.0 and 2 g L−1 biomass dosage. The amount of dye removed per adsorbent unit decreased with increasing adsorbent dosage, temperature, and increased with increasing contact time, and concentration. Experimental data were modeled using first-order, pseudo-second-order, Elovich, and intraparticle diffusion kinetics models. The adsorption kinetics of MB could be described by the pseudo-second order reaction model. The experimental data were fitted to: Langmuir, Freundlich, Temkin, Redlich–Peterson, Toth, and Sips isotherm models and the best fitting was obtained with the Sips model. The thermodynamic parameters (ΔH°, ΔS° and ΔG°) obtained revealed that MB adsorption is a spontaneous, exothermic and physical process. The obtained results indicated that corn husk as a low-cost biomaterial is an attractive candidate for the removal of basic dye MB from aqueous solutions.
Simona Gabriela Muntean graduated in 1994 as chemical engineer at Simona University of Timisoara, Romania. In 1997 she obtained the Master of Science degree in chemistry at Politehnica University of Timişoara, In Organic Synthesis. From 1995 to 1999 she worked as Assistant Research, from 1999 to 2008 as Research Scientist and since 2008 as Senior Research Scientist, at the Institute of Chemistry Timisoara of Romanian Academy. From 2000 to 2008 she was Lecture at The “Victor Babes” University of Medicine and Pharmacy Timisoara, Faculty of Pharmacy. Simona Gabriela Muntean took her PhD in the field of Chemistry at The Romanian Academy, in 2009. The research interest of Dr Simona Gabriela Muntean is Synthesis, Characterization and Application of Dyes, in the topics of Dye Aggregation and Dyes Removal. Now she has published more than 45 papers in national and international journals, and one book. During the past 15 years she successfully completed many research projects and developed various processes. She was also supervisor of two Diploma (undergraduate) theses. Since 2000 is member of the Romanian Society of Chemistry. Dr Simona Gabriela Muntean is Editor-in-Chief of New Trends and Strategies in the Chemistry of Advances Materials (Timisoara). She was appointed as member in national committees and reviewer for several chemistry journals. |
Dyeing industry effluents present a high degree of coloration and the release into the environment without treatment may affect the ecosystem due to the toxicological impact and mutagenic character of dyes,4,5 and by reduction of sunlight penetration and photosynthetic activity.3 Also, dyes can be accumulating in sediment and soil, causing different problems to the ecological balance of the environment.6
Synthetic dyes present a complex aromatic structure with high stability to aerobic digestion, light, temperature, detergent and microbial attack,1,5 which makes them stable and resistant to biodegradation methods present in the environment, and to different chemical treatments.2,7
The conventional methods of dye removal involve the combination of physical and chemical processes such as adsorption, precipitation, sedimentation, ultrafiltration, oxidation processes, ozonation, coagulation/flocculation, ion exchange and reverse osmosis, and also biological degradation methods.2,5,8 However, these methods often have the disadvantage of a high operating cost, formation of hazardous by-products or intensive energy requirements.1,5,8 From these methods, adsorption has proved to be an effective alternative process, being an economic method that presents a simple operating design and highly efficiency for the removal of dyes from wastewater.2,7 For wastewater treatment, commercial activated carbon is currently the most widely used adsorbent due to its high adsorption capacity, surface area and degree of surface reactivity as well as a microporous structure. However, its use is limited by the operating cost and regeneration issues.1,2,9 Therefore, non-conventional low-cost alternatives have been searched in the last years, easy available, renewable and environmentally friendly materials that can successfully replace the classical adsorbents. For this purpose, different agricultural biomasses were examined for the removal of different classes of dyes: bean,10 kohlrabi peel,11 capsicum seeds,12 olive pomace,13,14 wheat waste,15,16 rice hull,17 sugar beet pulp,18 ginger,19 silk cotton hull,20 orange,1,21 pineapple leaf,22 coconut bunch,9 cones,3,5 sawdust;4,23,24 with or without chemically modifying.16,17,23 These materials are abundant in nature, by-products or waste from different industries, which involve a very low acquisition cost. Utilization of different by-products for the wastewater treatment could be helpful not only economically, but also to the environment by solving the solid waste disposal problem.25
The biggest problem when vegetable origin biosorbents are used is that they are only locally available and their transport over long distances would make the process economically unprofitable. Therefore, the aim of this study is to investigate the adsorbent properties of corn husk; corn is widely cultivated throughout the world, according to FAOSTAT organization (Food and Agriculture Organization of the United Nations), worldwide production was 844 × 106 tones in 2010, more than any other grain, adapting easily to different climate conditions, from which 9 × 106 tones represents the production on the Romanian territory.
Bioremoval of a pollutant using biosorbents is affected by several factors like the chemical nature of pollutant, specific surface properties of the biosorbent and environmental conditions.18
Methylene blue (MB) was selected as a basic dye model in order to evaluate the adsorption capacity from aqueous solutions of corn husk, an agro-waste renewable and without economic value. MB does not belong to the class of the most dangerous pollutants, but acute exposure causes difficult breathing, increased heart rate, nausea, vomiting, diarrhea, shock, jaundice, quadriplegia, and tissue necrosis in humans.4,13,15 Methylene blue can be used in different fields, including coloring paper, temporary hair colorant, dyeing cottons and wools.4 The investigations were performed also using activated carbon, in order to compare the adsorption capacity of corn husk with those of activated carbon. The effects of contact time, adsorbent dose, initial dye concentration and temperature on MB adsorption were evaluated. The kinetics, thermodynamic parameters and the factors controlling the adsorption process were also calculated and discussed.
Activated carbon was purchased from UTCHIM-ROMANIA. Like the corn husk, it was crushed and sieved; particles smaller than 630 μm were selected.
Methylene blue (basic dye, chemical formula: C16H18ClN3S; molecular weight 319.86 g mol−1) was supplied by CHEMICAL – ROMANIA and it was not purified prior to use. The chemical structure of MB is presented in Fig. 1.
Stock dye solution (1000 mg L−1) was prepared by dissolving in distilled water; desired concentrations were prepared by dilution of the stock solution.
The assessment of biosorption capacity was done by calculating the adsorption capacity (1), and the removal percentage (2) of the MB dye:
![]() | (1) |
![]() | (2) |
The broadband at 3200–3500 cm−1 is attributed to O–H stretching vibration in phenolic and aliphatic structures.27,28 The band at 2916.37 cm−1 is assigned to C–H stretching vibration in aromatic methoxyl groups and in methyl and methylene groups of side chains.28 The band at 1732.08 cm−1 indicates the presence of CO stretching of carbonyl group. The bands at 1633.71 cm−1, 1517.97 cm−1 and 1429.25 cm−1 are characteristic for the aromatic skeleton vibrations. The band at 1373.32 cm−1 can be assigned to aliphatic C–H bending vibrations and phenolic OH.27 The band at 1249.87 cm−1 can be assigned to C–O–C antisymmetric stretching vibration and the band at 1055.99 cm−1 can be attributed to C–O–C symmetric vibration plus C–OH in primary alcohols.27,29,30 The band at 1161.15 cm−1 and 1105.25 cm−1 can be assigned to C–OH stretching vibrations in phenol and respectively in secondary alcohols.27,30
This result is in accordance with the composition of lignocellulosic materials. The FTIR spectrum indicates that the corn husk presents different functional groups which may be potential biosorption sites for MB dye.
The results (Table 1) indicate that the quantity of dye adsorbed per unit of dry biosorbent decreased, and the uptake of the dye increase from 90.3% to 93.6% with increasing the biosorbent mass.
Initial dye concentration (mg L−1) | Mass of biosorbent (g L−1) | Temperature (K) | |||||||||
---|---|---|---|---|---|---|---|---|---|---|---|
20 | 30 | 50 | 70 | 100 | 1 | 2 | 3 | 298 | 313 | 333 | |
qe (mg g−1) | 9.34 | 13.86 | 22.44 | 30.14 | 41.55 | 18.06 | 9.34 | 6.24 | 9.34 | 9.26 | 8.85 |
η (%) | 93.40 | 92.39 | 89.74 | 86.12 | 83.10 | 90.30 | 93.40 | 93.60 | 93.40 | 92.55 | 88.54 |
te (min) | 34 | 42 | 53 | 65 | 85 | 60 | 34 | 26 | 34 | 28 | 16 |
By increasing the biosorbent dosage the number of active sites available for adsorption increased, facilitating the adsorption of dyes, which explain the increase of removal percentage. The decrease of adsorption capacity by increasing the biosorbent dosage (higher weight of biosorbent per dye ratio) is maybe due to the unsaturation of biosorbent sites, during the adsorption process. Similar results have been reported by other authors.15,31,32,35 The further studies were carried out using 2 g L−1 biosorbent mass.
![]() | ||
Fig. 4 Influence of the initial concentration and contact time (2 g L−1, 298 K, pH 6) onto the MB dye removal. |
The obtained data presented in Table 1 show the increase of the amount of MB adsorbed at equilibrium, and the decrease of the percentage of dye removal with increasing the initial dye concentration.33,34,36 It can be also noted the increase of equilibrium time with increasing the initial dye concentration,37 due to the fact that adsorption can occur both at the surface and in the pores of the biosorbent, and the diffusion to the internal adsorption sites requires a longer time.
It is evident that the increase of the amount of MB adsorbed is fast in the first hour of the process, and becomes much slower near the equilibrium. This can be explained by the large number of vacant sites available at the initial stage which gradually are occupied in time as a result of sorption process. These results indicating that the dye removal is concentration dependent.
Corn husk presents a good adsorption capacity of 41.55 mg g−1, indicating that it could be considered a promising material for the removal of MB dye from aqueous solution.
Watching the process in time, it can be observed that the necessary time for reaching equilibrium decreases from 80 minutes to 45 minutes with the increase of temperature, most of the dye being adsorbed in the first hour of the process. The adsorption capacity and the percentage of MB removal decreases with the increase of temperature suggesting that biosorption of MB is an exothermic process (Table 1). Similar results have been obtained for MB removal by other researchers.38–40,62
These results are economically advantageous because the MB removal can be conducted at environmental temperature (25 °C) without additional costs for power generation.
![]() | ||
Fig. 6 The amount of MB removed using corn husk, respectively activated carbon as adsorbents (C0 20 mg L−1, 298 K, pH 6, biosorbent mass 2 g L−1). |
It can be noticed that the amount of MB removed by corn husk (9.34 mg g−1) is comparable to those removed by activated carbon (9.83 mg g−1). The very close removal capacity for the two adsorbents and the notable economic advantages of using corn husk, recommend it as a viable alternative for the MB removal.
ln(qe − qt) = ln![]() | (3) |
![]() | (4) |
![]() | (5) |
qt = kit0.5 + l | (6) |
The initial biosorption rate, h (mg g−1 min−1), can be defined as:
h = k2qe2 | (7) |
The correlation coefficients were used to determine the best fitting kinetic model. The comparison of the experimental adsorption capacity values (obtained at different temperatures) with the computed results estimated from eqn (3)–(6), are presented in Table 2.
Kinetic model | Kinetic parameters | Temperature | ||
---|---|---|---|---|
298 | 313 | 333 | ||
qe,exp (mg g−1) | 9.34 | 9.26 | 8.85 | |
First-order kinetic model | qe,calc (mg g−1) | 12.17 | 3.01 | 0.31 |
k1 × 102 (min−1) | 11.98 | 9.73 | 7.36 | |
R2 | 0.9853 | 0.9367 | 0.6525 | |
SD | 0.1909 | 0.3302 | 0.5516 | |
Pseudo-second-order kinetic model | qe,calc (mg g−1) | 9.48 | 9.19 | 8.78 |
k2 × 102 (g mg−1min−1) | 3.93 | 6.87 | 15.24 | |
h (mg g−1min−1) | 3.53 | 5.79 | 11.75 | |
R2 | 0.9998 | 0.9999 | 0.9999 | |
SD | 0.0175 | 0.0240 | 0.0405 | |
Elovich model | Α | 180.61 | 18.23 × 103 | 30.38 × 1011 |
Β | 1.03 | 1.61 | 3.67 | |
R2 | 0.9320 | 0.9016 | 0.8824 | |
SD | 0.2967 | 0.2322 | 0.1121 | |
Intrapart diffusion | ki | 0.88 | 0.67 | 0.30 |
c | 4.43 | 5.66 | 7.20 | |
R2 | 0.9651 | 0.9826 | 0.9873 | |
SD | 0.2656 | 0.1421 | 0.0544 |
The correlation coefficient close to unity, low standard deviation, and experimental values for qe similar to the calculated ones (Table 2) indicate that MB biosorption process is described by the pseudo-second-order model (Fig. 7).
Similar results were reported for MB adsorption on meranti sawdust,4 olive pomace and charcoal,13 scolymus hispanicus,35 and banana stalk waste.44 As the temperature increases, the k2 constant increases indicating that the necessary time for reaching the equilibrium decreases with increasing temperature, and that the adsorption of MB on corn husk is an exothermic process.
In order to explain the diffusion mechanism the intraparticle diffusion model was used. The dye adsorption process involves several steps: dye diffusion through solution to the outer surface of the adsorbent (film diffusion), dye adsorption on the outer surface of the adsorbent, dye diffusion from the surface into the adsorbent interior (intraparticle diffusion) and, dye adsorption onto the active centres in the interior surface of the adsorbent. From the plot of qt versus t0.5 the values of intraparticle diffusion rate constant (ki) and the effect of boundary layer thickness (l) were calculated (Table 2). As can be seen from Fig. 8, the plots are not linear over the whole time range which means that the intraparticle diffusion is not the rate determining step of the biosorption mechanism of MB onto corn husk. The boundary layer diffusion was also significant.16 Maximum is the intercept length (l), adsorption is more boundary layer controlled.
The second-order rate constants listed in Table 2 were used to calculate the activation energy for MB biosorption on corn husk, using Arrhenius equation:
![]() | (8) |
The activation energy was calculated from the slope of linear fitted function of ln(k2) versus 1/T and was found to be 32.02 kJ mol−1, which indicates that the MB biosorption is a physical process. This value is of the same order of magnitude with the value in the literature.15,23,34,45
Langmuir isotherm model represented by the eqn (9) is based on the assumptions:5,48 finite number of identical sites, homogeneously distributed over the adsorbent surface; monolayer coverage of adsorbate over the adsorbent surface; no interaction between the adsorbent molecules and the heat of adsorption is independent of the coverage of adsorbent surface.
![]() | (9) |
The essential characteristic of Langmuir isotherm can be expressed using a dimensionless constant, the separation factor RL:5,49
![]() | (10) |
The Freundlich empirical isotherm is applicable to the adsorption on heterogeneous surfaces with interaction between adsorbed molecules. In this case, the heat of adsorption exponentially decreases with the coverage of adsorbent surface.5 The Freundlich isotherm is expressed as:48
qe = KFC1/ne | (11) |
The Temkin isotherm assumes the linear decrease of the heat of adsorption with the coverage of the adsorbent surface due to some adsorbate–adsorbent interaction.6,48 The Temkin isotherm is expressed as:35,49
![]() | (12) |
The Redlich–Peterson isotherm is an empirical with three parameters equation. This isotherm is valid over a wide range of concentrations and combines elements from Langmuir and Freundlich isotherms. The Redlich–Peterson isotherm is expressed as:50,51
![]() | (13) |
Toth isotherm is derived from Langmuir equation with the purpose of reducing errors arising from experimental data. This model presumes a quasi-Gaussian distribution of energy. This is a three parameters isotherm, fits multilayer adsorption processes and is expressed by the equation:35,51
![]() | (14) |
The Sips isotherm is a three-parameter empirical equation, a combination of the Langmuir and Freundlich models. This equation is based on the Freundlich equation assumption, where the amount of adsorbed dye increases with the increase of initial concentration, but Sips equation presumes that the adsorption capacity has a finite limit when the concentration is sufficiently high. This model is represented as:35,51
![]() | (15) |
The analysis of the experimental data and determination of isotherms parameters were performed using the non-linear regression analysis from ORIGIN 6.1 software package. The main statistical criteria were the squared multiple regression coefficient (R2), and the chi-square analysis (χ2) (16).
![]() | (16) |
The data obtained for the fitted models are presented in Table 3, and the comparison between experimental data and the best fitted sorption isotherm curves is presented in Fig. 9.
Parameters | Isotherm | |||||
---|---|---|---|---|---|---|
Langmuir | Freundlich | Temkin | Redlich–Peterson | Toth | Sips | |
K | 0.19 | 14.58 | 3.11 | 8.24 | 1.12 | 0.16 |
qm | 47.95 | — | — | — | 78.05 | 50.69 |
RL | 0.025–0.173 | — | — | — | — | — |
n | — | 3.76 | — | — | — | 1.19 |
b | — | — | 294.14 | — | — | — |
αR | — | — | — | 0.16 | — | — |
β | — | — | — | 1.01 | — | — |
t | — | — | — | — | 0.35 | — |
R2 | 0.9944 | 0.9565 | 0.9815 | 0.9929 | 0.9841 | 0.9956 |
χ2 | 1.75 | 13.85 | 4.22 | 2.57 | 5.79 | 1.58 |
![]() | ||
Fig. 9 Correlations between experimental data and sips adsorption isotherm for MB biosorption on corn husk. |
Comparing the correlation coefficients of the analyzed isotherms, it follows that the Sips model yields a better fit for the experimental equilibrium data than the other isotherms. These results suggest that the adsorption process of the MB dye is following a combined Freundlich and Langmuir model: diffused adsorption at low dye concentration, and a monomolecular adsorption with a saturation value – at high dye concentrations. Similar results were reported for the adsorption of MB on yellow passion fruit waste.50,52
The maximum adsorption capacity of the corn husk determined from the Sips sorption isotherm curves was 50.69 mg g−1, and the values are higher or comparable with the data reported before (Table 4). This data indicates that corn husk can be considered a promising material for the removal of MB dye from aqueous solution.
Adsorbent | Adsorption capacity (mg g−1) | Ref. |
---|---|---|
Neem leaf | 8.76–19.61 | 54 |
Palm-trees waste | 8.4 | 30 |
Data stones | 8.8 | 30 |
Fly ash | 13.42 | 55 |
Yellow passion fruit | 14 | 28 |
Wheat shells | 16.56–21.50 | 14 |
Oak sawdust | 29.94 | 56 |
Cherry sawdust | 39.84 | 56 |
Indian rosewood sawdust | 11.8–51.4 | 57 |
Rice husk | 40.58 | 58 |
Corn husk | 18.06–41.55 | This work |
Coconut bunch | 30.42–65.55 | 9 |
In order to establish if the biosorption process is favorable or not, the RL factor was determined from the Langmuir isotherm model.
The obtained values for RL parameter were in the range of 0.025–0.173 (Table 3), quite close to zero, indicating that the MB biosorption process on corn husk is favorable, and it is a relatively irreversible reaction.9,44,49,53
ΔG° = −RT![]() ![]() | (17) |
![]() | (18) |
![]() | (19) |
The enthalpy ΔH° and entropy ΔS° of biosorption process were estimated from the slope and intercept of the plot of lnK versus 1/T (figure not show).
The negative values of ΔG° (Table 5), calculated using K, indicate that the biosorption of MB onto corn husk is thermodynamically possible and it is a spontaneous process.
T (K) | K | ΔG° (kJ mol−1) | ΔH° (kJ mol−1) | ΔS° (J mol−1 K−1) |
---|---|---|---|---|
298 | 14.1515 | −6.5651 | ||
313 | 12.5135 | −6.5755 | −4.3792 | 15.9408 |
333 | 7.6957 | −5.6497 |
The negative value of ΔH° confirms the exothermic nature of biosorption process. The value of ΔH° for the present study is less than 20 kJ mol−1 (Table 5), indicating that MB biosorption on corn husk is likely a physical process, which is in agreement with the results obtained from activation energy.34,59,60
The positive value of ΔS° reflects the increased randomness at the solid-solution interface during the biosorption of MB on corn husk.49,61
Taking into consideration the presented results, it can be concluded that corn husk can be an alternative adsorbent reported to other more expensive adsorbents used for coloured wastewater treatment.
Footnote |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4ra10504d |
This journal is © The Royal Society of Chemistry 2014 |