XX century was characterized with the significant technological progress, which has improved the life of people. Numerous products and services became available, labor and domestic life became easier and more comfortable. However, waste problem is the other side of the technological progress. The solid, liquid and gas waste are formed, and thus land, natural water and air quality degrade. Therefore, in XXI century the research efforts are dedicated to development of methods for sustainable use of water, gas, and solid resources.
Wastewater polluted with dye residuals originate from textile, leather and food industry. These industries produce goods that are used by all population, and thus volume of waste is vast. The present situation with water quality indicates that the effective, inexpensive and sustainable wastewater methods are necessary. The effectiveness of water treatment method is measured by the residual (or equilibrium) concentration of the dye. The inexpensive method uses cheap reagents and consumes minimum energy. Sustainability refers to absence of the secondary pollution and use of non-hazardous reagents. Among the methods used to remove the low concentrations of dye, adsorption takes a specific place since it is easy to perform. The high cost of the activated carbon initiated the search for the inexpensive feedstock for the adsorbent. Numerous options have been found and are being tested for efficiency in pilot and industrial experiments.
Theory of Adsorption
Adsorption is a physico-chemical process of concentrating the substance (adsorptive) from the volume of water in the surface layer of the adsorbent. Adsorbent is a solid porous material with the excess surface energy. When the adsorptive is concentrated on the surface of the adsorbent, adsorbate is formed (the conjugate of the adsorptive and adsorbent surface). Thus, adsorption is a surface process (Masel, 1996).
The adsorption takes place on the adsorptive centres. There are micro-, meso-, and macro porous adsorptive materials, depending on the size of the adsorptive centres. When the adsorption of dye is studied, size is an important factor. The dyes molecules are large, and incapable of interaction with microporous adsorbents (Masel, 1996).
There are two types of adsorption: physical and chemical. Physical adsorption is a process that involves only physical forces (adhesion and surface tension). During the chemical adsorption, the chemical reaction takes place between the adsorptive and the adsorbent. Thus, physical adsorption is reversible, while chemical is irreversible. The reverse adsorption, or desorption takes place when the environment changes, for example when pH decreases, or when the adsorbent is treated with eluate (the specific solution capable of destroying the surface complexes) (Masel, 1996).
The adsorption is probability-related process. The adsorption is a process for removal of the residual concentration form wastewater. Typically, the maximal concentration of the pollutant to be removed by adsorption does not exceed 1000 mg/l. Therefore, the adsorbate molecules have to migrate to the surface of the adsorbent, and this is probability-ruled process. In addition, at the equal concentrations, each of the ions in the solution have equal probability of being adsorbed; the molecules or atoms with higher concentration are more likely to be adsorbed. Hence, the atoms of the background electrolyte are more likely to be adsorbed, comparing to the atoms of with the residual concentrations. This leads to the decision that the adsorption is retarded in presence of substances with high concentrations, and the competitive adsorption takes place. Typically, the adsorption of the dye in presence of high concentrations of salts is retarted (Masel, 1996).
Industrial Design for Adsorption Process
There are two methods of industrial adsorption treatment: batch and packed column adsorption. The batch adsorption is realized when a dose of adsorbent is mixed with wastewater, and then separated by precipitation, filtration or centrifugation. In case the residual concentration is higher than the maximum allowable, then the process is repeated, and the multi-stage adsorption is realized. The batch adsorption is characterized with low dosage of the adsorbent applied. The sorbent is used as a powder or small granules, since powder form is characterized with the highest active surface area. However, the adsorption capacity is not fully used in this method. It provides good results for adsorption from multicomponent solutions since the adsorption capacity is partially used for other components removal (Masel, 1996).
The packed column process is realized through filtration of wastewater through the column packed with the adsorbent. The adsorbent is used in granular forms, and the stability of granules is an important factor. In case the granules are instable, the column gets blocked, and the process stops. The packed column takes longer than the batch method, yet it provides low residual concentrations because the water is filtered through fresh surface of the adsorbent (Masel, 1996). Although the packed column method is more efficient, the majority of the published research is dedicated to batch adsorption. The adsorbents are first tested in batch experiments, since they allow determining the main adsorption characteristics (Geçgel, Özcan, & Gürpınar, 2013). After this, the adsorbents are tested in packed columns.
Experimental Design
The basic experimental procedures for the adsorption experiments:
dye calibration preparation;
adsorption experiments;
determination of the residual concentration of the dye.
The dye concentration is determined by the standard analytical procedure for UV-vis spectroscopy (Martins & Nunes, 2015).
The typical procedure of adsorption experiment is as follow. The dose of the adsorbent (approximately 10 mg) is placed in 100 ml flask, then 80 mg of the solution is added. The system is immersed into the water bath at 30 ºC to reach the stable temperature condition. The solutions are stirred and kept for the time required to reach equilibrium (1-2 hours). The adsorbent is separated from the solution by filtration or centrifugation. Then, the residual concentrations are found basing on the calibration line. The equilibrium adsorption capacity is measured by the decrease in the dye concentration after the contact with the adsorbent (Martins & Nunes, 2015).
The experiments are performed at various conditions: synthetic solutions with different hardness levels (80 and 200 mg/l of CaCO3 equivalent), pH, adsorbent: water ratio. The efficiency of the adsorption process is assessed with the dye percentage removal, equilibrium concentration, and adsorption capacity of the adsorbent (Geçgel, Özcan, & Gürpınar, 2013).
The relation between the equilibrium concentration and the adsorption capacity is used to plot isotherms. The isotherms are typically the hyperbolic curves. Traditionally, they are linearized to determine the main adsorption parameters (Martins & Nunes, 2015). However, the linearization causes the hidden change in the errors structure. Nowadays, the application of software allows determination of the main adsorption characteristics without linearization.
The isotherm analysis includes obtaining the isotherm constants (maximum adsorption capacity, adsorption constant), and the goodness-of-fit characteristics (the determination coefficient, R2, or other characteristics (mean absolute error, standardized errors, etc.), which describe how well the adsorption line fits the experimental data parameters (Martins & Nunes, 2015).
The convenient research practice includes matching the adsorption characteristics with textural and structural properties of the adsorbent: porous volume, surface area, XRD and IR data, microphotographs, etc. (Ahmad, Puad, & Bello, 2014, Sun et al., 2013).
Dyes Used for the Experiments
Dyes exhibit various chemical properties (Hunger, 2003), and therefore the adsorption behaviour may significantly differ. The comprehensive research papers study the adsorption of mordant blue (a cationic synthetic azo dye, representing a group of synthetic dyes (Martins & Nunes, 2015)), methylene blue (Geçgel, Özcan, & Gürpınar, 2013, Rahman, Amin, & Shafiqul Alam, 2012, Tan, Ahmad, & Hameed, 2008), remazol reactive yellow, black, and red (Al-Degs et al., 2007, Sun et al., 2013), brilliant (Ahmad, Puad, & Bello, 2014), acid blue 350 (Demiral et al., 2008), maxilon blue and direct yellow (Aljeboree, Alshirifi, & Alkaim, 2014), malachite green, crystal violet, rhodamine-B (Baseri, Palanisamy, & Kumar, 2012). The study of adsorption performance during industrial wastewater treatment with mixture of red, blue, and yellow reactive, and blue, red, and yellow dispersive (Mohammed, Farhood, & Al-Mas'udi, 2007) is of particular practical interest.
Activated Carbon Types
The adsorption uptake depends on the porous structure and on the chemical composition of the activated carbon. The activated carbon has surface oxygen groups are vital in the adsorption process, and the type of the oxygen group determines the acid or base character of the carbon (Martins & Nunes, 2015).
The commercial samples of the carbons are applied for dye removal: Panreac and Fagron (Martins & Nunes, 2015), Filtrasorb, unactivated (Al-Degs et al., 2007), activated carbon mixed with aluminum in packed column (Mohammed, Farhood, & Al-Mas'udi, 2007), activated carbon and sulphuric acid activated (Sun et al., 2013, Aljeboree, Alshirifi, & Alkaim, 2014).
The alternate sources of activated carbons are tested in adsorption experiments. They originate from pea shells activated with ZnCl2 solution (Geçgel, Özcan, & Gürpınar, 2013), rice and coconut husk (Rahman, Amin, & Shafiqul Alam, 2012, Tan, Ahmad, & Hameed, 2008), hazelnut bagasse activated by ZnCl2 (Demiral et al., 2008), pomegranate peel microwave and KOH activated (Ahmad, Puad, & Bello, 2014), tree Thevetia peruviana (Baseri, Palanisamy, & Kumar, 2012).
Isotherm Studies
The isotherm studies help to determine the maximum equilibrium adsorption capacity, which is the maximum amount of the adsorbate that can be formed on the surface of the adsorbent. The adsorption isotherms indicate how the dye is distributed between the solid and the liquid phase. Typically, the adsorption isotherms are interpreted on the Langmuir and Freundlich isotherms. Langmuir isotherm is developed for the monolayer adsorption; Freundlich assumes that the adsorbent surface is heterogeneous, or with different levels of adsorption. It is assumed that if the experimental data fit well the isotherm model, then the adsorption follows the mechanism represented. In the study of Geçgel, Özcan, & Gürpınar, (2013) Langmuir fitted better.
Al-Degs et al. (2007) studied isotherms for single, binary and ternary mixtures, and the results correlated modified Langmuir and Freundlich (for competitive adsorption), and Redlich-Peterson models. Although the adsorption capacity from binary and ternary mixtures was lower than from the single solution, yet the measure of decrease was not proportional. This is explained by different affinities expressed by the dyes. The multi-solute mixtures are better described with Langmuir and Redlich-Peterson, whereas Freundlich is applicable only for single component adsorption. From practical prospective, the adsorption from the binary and ternary solution is more efficient.
Langmuir and Freundlich models are the most common, yet there are numerous other isotherms applied to the experimental data: Temkin, Dubinin-Radushkevitch, etc. (Sivakumar, Muthirulan, & Sundaram, 2014).
The isotherm constants have physical sense: Langmuir constant indicates the affinity between the adsorbent surface and the dye (Martins & Nunes, 2015).
Effect of pH
The pH dependence is related to electrostatic attraction and the chemical properties of adsorbent and dye (Demiral et al., 2008). The pH of the solution determines the surface charge of the activated carbon. At point of zero charge pHpzc, an overall surface charge is zero; if pH > pHpzc, the carbon is capable of removing cations, while at pH < pHpzc, the anions are removed (Martins & Nunes, 2015).
The effect of pH is typically investigated in the concentration range 2-11.5. For the pea shells, the pHpzc was found 5.86. At pH > pHpzc, the adsorbent surface is negatively charged, and the interaction with cationic dye increases. This trend is confirmed for pea shells; however, the increased uptake is observed at low pH, which is explained by the replacement of the surface H+ ions with the ions of cationic dye (Martins & Nunes, 2015).
Mohammed, Farhood, & Al-Mas'udi (2007) stated for packed column experiments that adsorption is better at lower pH values, and they observed almost instant gradual breakthrough at pH = 8. On the contrary, Rahman, Amin, & Shafiqul Alam (2012) reported increase in removal efficiency from 40% at pH = 4 to 95% at pH = 10, and this is typical situation for adsorption.
For anionic dye, the higher removal efficiency was observed at pH=2 (Demiral et al., 2008). Thus, the various pH-dependence patterns are observed due to different molecular structure of dyes and their different chemical properties.
Competitive Adsorption and Influence of Other Ions
The rise of ionic strength promotes the interaction between dye molecules and carbon surface in the work of Martins & Nunes, (2015). This is a sign that the surfaces of Panreac and Fagron carbons are selective for the Mordant Blue.
Martins & Nunes (2015) studied the effect of surfactants on methylene blue adsorption. The surfactants are commonly present in wastewaters, and thus may be competitors for the active adsorptive centres. The anionic surfactant improves the electrostatic interaction, while the cationic causes the repulsion, and the non-ionic improves the hydrophobic forces between dye and surfactants.
Kinetics Studies and Modelling
Kinetic studies are performed to find the equilibrium time and thus the time necessary for batch adsorption. The time set for the experiment depends on the researcher: Al-Degs et al. (2007) performed experiment for 1-5 weeks, while the experiment in Geçgel, Özcan, & Gürpınar (2013) took only 3 hours.
The general pattern of the kinetics experiments is as follows: the adsorption capacity increases with the increase of time; as the time approaches the equilibrium, the increase rate significantly decreases, and at equilibrium time the capacity reaches maximum.
The kinetic experiments are fitted using the pseudo-first-, pseudo-second-order models, chemisorption, intraparticle diffusion (Aljeboree, Alshirifi, & Alkaim, 2014, Tan, Ahmad, & Hameed, 2008), Avrami equation (Ahmad, Puad, & Bello, 2014), and Elovich model (Baseri, Palanisamy, & Kumar, 2012).
The experiments are performed at different concentrations, and the profiles are similar for all the concentration ranges (Geçgel, Özcan, & Gürpınar, 2013).
Packed Column Experiments
The effect of particle size, amount of adsorbent, initial concentration, and flow rate was studied (Rahman, Amin, & Shafiqul Alam, 2012).
The dyes with higher molecular weights were characterized with better adsorption, which was attributed to lower solubility and higher affinity to adsorbent surface. The process from multisolute solution caused more rapid breakthrough (Mohammed, Farhood, & Al-Mas'udi, 2007).
Rahman, Amin, & Shafiqul Alam (2012) proposed the mechanism for adsorption of the cationic dyes on rise husk. At pH > pHpzc, the surface of the adsorbent is negatively charged, and thus the cationic substances dye adsorption is favoured.
References
Ahmad, M. A., Puad, N.A., & Bello, O. S. (2014). Kinetic, equilibrium and thermodynamic studies of synthetic dye removal using pomegranate peel activated carbon prepared by microwave-induced KOH activation. Water Resources and Industry, 6; 18-35.
Aljeboree, A., Alshirifi, A., & Alkaim, A. (2014). Kinetics and equilibrium study for the adsorption of textile dyes on coconut shell activated carbon. Arabian Journal of Chemistry. In Press.
Al-Degs, Y., Khraisheh, M.A.M., Ahmad, M.N.N., & Walker, G. (2007). Competitive adsorption of reactive dyes from solution: Equilibrium isotherm studies in single and multisolute systems. Chemical Engineering Journal, 128, 163-167.
Baseri, J. R., Palanisamy, P. N., & Kumar, P. S. (2012). Adsorption of basic dyes from synthetic textile effluent by activated carbon prepared from Thevetia peruviana. Indian Journal of Chemical Technology, 19, 311-321.
Demiral, H., Demiral, I., Karabacakoğlu, B., & Tümsek, F. (2008). Adsorption of textile dye onto activated carbon prepared from industrial waste by ZnCl2 activation. Journal of International Environmental Application & Science, 3(5), 381-389.
Geçgel, Ü., Özcan, G., & Gürpınar, G. Ç, (2013). Removal of methylene blue from aqueous solution by activated carbon prepared from pea shells (Pisum sativum). Hindawi Publishing Corporation Journal of Chemistry, 1-9.
Hunger, K. (2003). Industrial dyes: Chemistry, properties, applications. Weinheim: Wiley-VCH.
Martins, A. & Nunes, N. (2015). Adsorption of a textile dye on commercial activated carbon: a simple experiment to explore the role of surface chemistry and ionic strength. Journal of Chemical Education, 92, 143-147.
Masel, R. I. (1996). Principles of adsorption and reaction on solid surfaces. New York: Wiley.
Mohammed, W. T., Farhood, H. F., & Al-Mas'udi, A. H. B. (2007). Removal of dyes from wastewater of textile industries using activated carbon and activated alumina. Iraqi Journal of Chemical and Petroleum Engineering, 9(4), 43-52.
Rahman, M. A., Amin, R. S. M., & Shafiqul Alam, A. M. (2012). Removal of methylene blue from waste water using activated carbon prepared from rice husk. Dhaka University Journal of Science, 60(2), 185-189.
Sivakumar, S., Muthirulan, P., & Sundaram, M. (2014). Adsorption kinetic and isotherm studies of Azure A on various activated carbons derived from agricultural wastes. Arabian Journal of Chemistry. In Press.
Sun, D., Zhang, Z., Wang, M., Wu, Y. (2013). Adsorption of reactive dyes on activated carbon developed from Enteromorpha prolifera. American Journal of Analytical Chemistry, 4, 17-26.
Tan, I. A. W., Ahmad, A.L., & Hameed, B.H. (2008). Adsorption of basic dye on high-surface-area activated carbon prepared from coconut husk: Equilibrium, kinetic and thermodynamic studies. Journal of Hazardous Materials, 154(1-3), 337-346.
