ReviewMistakes and inconsistencies regarding adsorption of contaminants from aqueous solutions: A critical review
Introduction
Nowadays, the number of publications in international scientific journals has increasingly become a standard for the assessment of scientists (Geckeis and Rabung, 2004). As the number of scientific publications increase, mistakes and misconceptions enter scientific literature and some can also be repeated in subsequent publications. Once errors and mistakes enter the literature, it is difficult to eradicate them. The entrance of mistakes in scientific publications, as well as their propagation, can result from the subjectivity of the authors and/or objective reasons.
A typical study on the adsorption of organic and inorganic contaminants comprises several sections, such as the effects of solution pH and ionic strength, studies of adsorption kinetics, isotherms, thermodynamics, desorption, and regeneration. Adsorption processes can be conducted using either batch or column techniques. It has been found that many scientific articles in the field of adsorption science and technology contain several mistakes and misconceptions including the use of technical terms, application of quantities, determining the roles of some constants like pKa, performing kinetic studies and modeling kinetic adsorption data, finding a suitable isotherm model to fit equilibrium data, calculating thermodynamic parameters, correct interpretation of adsorption mechanisms, and measuring the specific surface area of an adsorbent. Moreover, a survey of the scientific publications related to the adsorption of contaminants in aqueous solutions reveals that some less obvious inconsistencies and mistakes have unavoidably slipped the attention of authors and were repeated in subsequent publications.
The observation of these inconsistencies and mistakes, regardless of their origin, along with comments and open discussions can prevent their propagation in scientific knowledge transfer. Therefore, the presentation of a comprehensive review on common mistakes in adsorption studies, besides introducing the most correct approaches to use and giving a source of up-to-date literature on this issue, seems to be valuable to both readers and researchers in this field.
The main purpose of this review is to identify mistakes in past publications and prevent the propagation of such mistakes in future scientific literature. Herein, we present some common mistakes in various aspects of adsorption studies and analyze the reasons for their entrance into the literature. This was accomplished by discussing the published papers in the field of adsorption of contaminants from aqueous solutions, commonly including “comments on”, “reply for comments”, “response to”, “authors' response to comments on”, “comments on the authors” response to comments on”, “comments on comment on”, “in reaction to”, “critical review”, and “a note on the comments” published works. In addition to our comprehensive review on the mistakes existing in publications, some correct approaches are suggested where necessary to avoid the propagation of such mistakes in the future scientific literature. This paper primarily focuses on three basic constituents of adsorption theory: adsorption equilibria, kinetics, and thermodynamics.
Section snippets
Technical terms used in the study of adsorption
Adsorption processes have their own vernacular. Therefore, correctly understandings the technical terms used in adsorption technology can prevent the introduction of several unexpected ambiguities and discrepancies. Some basic adsorption terms are summarized in Fig. 1.
To some extent, a thorough understanding of the properties of adsorbents might prevent common mistakes made in interpreting adsorption processes and adsorption mechanisms. Numerous techniques can be used to characterize an
Quantities for measuring adsorption performance
In the field of adsorption, adsorption performance can be expressed as the amount of adsorbate adsorbed at equilibrium or the percentage of removed adsorbate. The amount of adsorbate adsorbed at equilibrium (qe; mg/g) is often calculated using the material balance of an adsorption system; the adsorbate, which has disappeared from the solution, must be in the adsorbent (Volesky, 2007).where C0 (mg/L) and Ce (mg/L) are the initial and equilibrium adsorbate concentration in solution,
Incorrect assumptions regarding pKa
A mistake regarding the fundamentals of chemistry was commented on by Rayne (2013). The commented paper reported the adsorption of n-perfluorooctanesulfonamide (PFOSA) onto multiwalled carbon nanotubes with differing oxygen content. However, PFOSA was mistakenly assumed to be an “organic base”, resulting in the following invalid interpretation: “for PFOSA (pKa = 6.52), when pH < pKa, protonation occurs on the amino group, and the decreased protonation leads to the increased adsorption, but when
The important role of initial contact time
In the study of adsorption kinetics, Azizian proposed that by the observation of breaking point or deviating from linearity at initial regions of t/qt versus t plots, means presence of at least two different sites for adsorption Azizian (2006) or contribution of diffusion on the rate of adsorption (Haerifar and Azizian, 2013) and therefore fitting with the PSO model should be avoided. Adsorption kinetics data in the initial contact time periods play an important role in performing accurate
Adsorption equilibrium
Milonjić (2009a) and Milonjić (2010) suggested that the amount of adsorbate adsorbed onto an adsorbent depended on the equilibrium concentrations of metal ions, equilibrium solution pH, and temperature. An adsorption isotherm should be given as the equilibrium adsorbed amounts versus the equilibrium ion concentrations for a constant equilibrium solution pH and temperature. Thus, environmental parameters in the sorption system (especially solution pH) must be carefully controlled at the given
Principles of adsorption thermodynamics
Thermodynamic studies are an indispensable component of predicting adsorption mechanisms (e.g., physical and chemical). The key distinctions between physical and chemical adsorption were summarized in detail in a recent report (Tran et al., 2016). The thermodynamic parameters can be computed according to the laws of thermodynamics using the following equations:
The relationship between ΔG° and ΔH° and ΔS° is described as follows:
The well-known van't Hoff equation is
Inconsistent data points in experimental data and model fitting
Significant differences between the number of data points in an experiment and those used for model fitting should be avoided. For example, Azizian (2008a) made the following critical comments about the paper of Karadag et al. (2007): there were six experimental data points regarding the adsorption kinetics (plots of qt versus t), but only five experimental data points were used for fitting of the experimental data to the PSO model (plot of t/qt versus t). This omission resulted in inaccurate
Nonlinear-optimization technique
To calculate the parameters of kinetic and isotherm models accurately in both batch and column experiments, application of the nonlinear method instead of the linear method has been recommended by many researchers (Porter, 1985, Ho, 2004b, Ho et al., 2005, Bolster and Hornberger, 2007, Han et al., 2007, Kumar, 2007, El-Khaiary et al., 2010, Chowdhury and Das Saha, 2011, Lima et al., 2015, Tran et al., 2015, Tran et al., 2017d).
Furthermore, to identify the best-fit model, calculation of the
Conclusions
In scientific publications, the correct use of technical terms and accurate calculations are essential. According to the viewpoints discussed in this review, we put forward the following key conclusions and perspectives:
- i.
Adsorption performance should be expressed as qe (mg/g), while the use of the unit of %removal should be avoided or used cautiously.
- ii.
The use of accurate citations and correct mathematical expressions in original works is also recommended.
- iii.
Measurements of adsorption kinetics should
Acknowledgements
The first author would like to thank Chung Yuan Christian University for the Distinguished International Graduate Students (DIGS) scholarship to pursue his doctoral studies. The authors gratefully acknowledge the editor and anonymous reviewers for their invaluable insight and helpful suggestion.
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