Highlighting inconsistencies regarding metal biosorption

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Highlighting inconsistencies regarding metal biosorption Artis Robalds a,∗ , Ghinwa Melodie Naja b , Maris Klavins a a b

Department of Environmental Science, University of Latvia, Raina Blvd. 19, Riga LV-1586, Latvia Science Department, Everglades Foundation, 18001 Old Cutler Road, Palmetto Bay, FL 33157, United States

h i g h l i g h t s

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• Different classification systems of metal biosorption mechanisms have been compared. • Mistakes made in previously published articles have been discussed. • A platform for future discussions among researchers investigating biosorbents.

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Article history: Received 14 March 2015 Received in revised form 12 October 2015 Accepted 20 October 2015 Available online xxx Keywords: Adsorption Biosorption Chemisorption Mechanisms Physisorption

a b s t r a c t Thousands of articles have been devoted to examine different types of biosorbents and their use in cleaning polluted waters. An important objective of some studies has been the identification of the biosorption mechanisms. This type of investigation is not always performed, as it can only be done if scientists are aware of all mechanisms that, at least theoretically, control the removal of the target substances. Mistakes are often made, even in highly cited review articles, where biosorption mechanisms are named and/or grouped. The aim of this article is to highlight errors and inaccuracies as well as to discuss different classification systems of the biosorption mechanisms. This article serves as a guide, as well as a platform for discussion among researchers involved in the investigation of biosorbents, in an effort to avoid reproducing errors in subsequent articles. © 2015 Elsevier B.V. All rights reserved.

1. Introduction While there is no doubt that biosorption is a widely studied topic, attention has been drawn to insufficient qualification of the researchers involved, the use of loose terminology, and the publication of low quality papers [1,2]. Deficiencies appear in some articles, where mechanisms of biosorption have been indicated and listed, as these lists are incomplete or even incorrect. Errors have appeared in recent studies as well as in highly cited review articles, thus it is necessary to inform the scientific community about those errors in order to avoid pitfalls. Some examples of mistakes

∗ Corresponding author. E-mail addresses: [email protected] (A. Robalds), [email protected] (G.M. Naja).

are discussed in this article without referring to the paper(s) where these mistakes have appeared. The aim of this article is to highlight the errors and to show different approaches for grouping biosorption mechanisms. A better understanding of the biosorption mechanisms responsible for heavy metal binding could assist in the optimization of performance of new biosorbents [3].

2. Mechanisms of metal biosorption and their classification systems Mechanisms of metal biosorption can be classified into various groups as presented in Figs. 1–3 . Those classification systems were identified after a careful investigation of the literature and to the best of our knowledge, they have never been shown grouped in one single source.

http://dx.doi.org/10.1016/j.jhazmat.2015.10.042 0304-3894/© 2015 Elsevier B.V. All rights reserved.

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Fig. 1. Classification of metal biosorption mechanisms according to Michalak et al. [4].

Fig. 2. Classification of metal biosorption mechanisms according to Naja and Volesky [5].

Fig. 3. Classification of metal biosorption mechanisms according to Srivastava and Goyal [6].

“Biosorption” is often used as an umbrella term to describe the experimental observations when the amount of sorbate in the solution decreases after a certain period of time due to the attractive forces between the sorbate and biosorbent (i.e., a material of biological origin that is used as an adsorbent) [7]. Using this general description, it is clear why microprecipitation as well as reduction and oxidation reactions leading to uptake on the sorbent material have been considered as part of the biosorption mechanisms (Fig. 3). If a researcher decides not to use the term “biosorption”, other terms (such as “removal”, “binding” or “uptake”) can be used, illustrating the biosorbent ability to reduce the concentration of pollutants in water or other environments. The term “adsorption” has been used as a synonym for “physical adsorption” by several authors [8–10]. To avoid any confusion, the term “physical adsorption” is proposed here to be used instead of “adsorption”, at least in the biosorption studies, since both chemical adsorption and physical adsorption can play a significant role in the

biosorption process. It also seems that some authors consider ion exchange mechanism as a chemisorption process (Figs. 1 and 3), however, other researchers hold a different view (Fig. 2). The use of the “microprecipitation” term (also written as “microprecipitation”) has brought a lot of confusion to readers, as the definition of this term has never been given. However, it seems that this term has been used to indicate precipitation taking place locally at the surface of the biosorbent (or within the pores of the biosorbent) due to local conditions [5], therefore a distinction between the terms “precipitation” and “microprecipitation” has been made. In addition, an alternate term “surface precipitation” has been used by other researchers to describe the same phenomenon [8,11]. Farooq et al. [12] stated the following: “Biosorption mechanism comprises a number of phenomena including adsorption, surface precipitation, ion-exchange and complexation”, showing that the terms “microprecipitation” and “surface precipitation” can be used as synonyms. Du et al. [13] reported that the removal of Pb, Cd and Zn ions from

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Fig. 4. A new classification of the metal biosorption mechanisms.

solution by surface precipitation led to crystal growth, that is crystallization. Interestingly, some authors considered crystallization as one of the biosorption mechanisms [14] and others do not [15]. A new schematic diagram (Fig. 4) has been proposed here summarizing and classifying the sorption mechanisms while assuming that biosorption refers to an adsorption mechanism to materials of biological origin [4,16] It is worth noticing that in the present study, chelation was considered as a special case of complexation following Manahan [17]. 3. Where mistakes have been made? Most of the errors have been made because researchers are not generally familiar with the above mentioned classification systems. Most of the common deficiencies can be arranged into several groups. First of all, researchers are not aware of the fact that several terms, for example, “coordination” and “complexation” are overlapping. An article published in 2009 (cited more than 300 times in both Web of Science (WoS) and Scopus databases) states the following: “In another sense, it can also be defined as a collective term for a number of passive accumulation processes which in any particular case may include ion exchange, coordination, complexation, chelation, adsorption and microprecipitation”. To be accurate, the sentence should be rephrased and written as: “Biosorption may include ion exchange, complexation (including coordination and/or chelation), physisorption, or microprecipitation”. It is also known that physical adsorption results from the electrostatic and van der Waals interactions (forces) [18,19] as reported in Yang and Volesky [20]: “The metal ion binding in biosorption could be attributed to several mechanisms such as ion exchange, complexation, electrostatic attraction and microprecipitation”. Unawareness of this fact can lead to some misunderstanding and confusion. In addition, other researchers have chosen to use the term

“electrostatic adsorption” instead of/in addition to “electrostatic attraction” [21]. Secondly, it is often ignored that some of the mechanisms are “umbrella terms”, therefore a distinction should be made between lower and higher hierarchical levels. In a review paper on low-cost natural adsorbents published in 2009 and cited approximately 200 times (both WoS and Scopus), it has been reported that: “This term [biosorption] is adopted to indicate a number of metabolism independent processes (physical and chemical adsorption, ion exchange, complexation, chelation, and micro-precipitation) . . .”. It is proposed that the sentence should be rephrased and written as: “This term [biosorption] is adopted to indicate a number of metabolism independent processes: physical adsorption, ion exchange, chemisorption (including complexation), and microprecipitation”. Thirdly, sometimes not all of the biosorption mechanisms are listed. In a recent review paper on biosorption of heavy metals by algal biomass published in 2014 the authors stated: “Biosorption is a complicated process involving ion-exchange, complexation and coordination”. Physisorption, chemisorption and microprecipitation were not mentioned as part of the biosorption mechanisms. Finally, when citing other papers, authors should also be extremely careful to ensure that their work carries the same original idea. For example, authors of a review article discussing metals and biosorption (published in 2007 and cited more than 100 times both in Scopus and WoS) reported that ion exchange is one of the physical sorption mechanisms, although the original reference [22] clearly indicates that ion exchange is one of the chemisorption mechanisms. 4. Conclusions Miscommunication among researchers about biosorption mechanisms has led to some confusion, which in turn have contributed to errors in papers where these mechanisms have been

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discussed. This article could be a cornerstone for the development of an accurate system to categorize the biosorption mechanisms into appropriate groups. This in turn will help researchers to achieve one of the main goals in the biosorption field; identification of the binding mechanism(s). Finally, this article should be considered as a platform for future discussions among researchers investigating biosorbents, as there is a need for an in-depth summary of the biosorption mechanisms. Acknowledgement This work was supported by the European Social Fund (ESF) under Grant 2014/0009/1DP/1.1.1.2.0/13/APIA/VIAA/044. References [1] C. Tien, Remarks on adsorption manuscripts revised and declined: an editorial, J. Hazard. Mater. 150 (2008) 2–3. [2] M. Fomina, G.M. Gadd, Biosorption: current perspectives on concept, definition and application, Bioresour. Technol. 160 (2014) 3–14. [3] B. Volesky, Z.R. Holan, Biosorption of heavy metals, Biotechnol. Progr. 11 (1995) 235–250. [4] I. Michalak, K. Chojnacka, A. Witek-Krowiak, State of the art for the biosorption process—a review, Appl. Biochem. Biotechnol. 170 (2013) 1389–1416. [5] G. Naja, B. Volesky, The mechanism of metal cation and anion biosorption, in: P. Kotrba, M. Mackova, T. Macek (Eds.), Microbial Biosorption of Metals, Springer, Dordrecht, 2011, pp. pp. 19–58. [6] S. Srivastava, P. Goyal, Novel Biomaterials: Decontamination of Toxic Metals from Wastewater, Springer, New York, 2010. [7] M. Salman, M. Athar, U. Farooq, Biosorption of heavy metals from aqueous solutions using indigenous and modified lignocellulosic materials, Rev. Environ. Sci. Biotechnol. 14 (2015) 211–228.

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