**2. Biosorption: a general overview**

Microorganisms, in particular bacteria and fungi, have been receiving particular attention in the area of environmental microbiology and biotechnology due to their ability not only to decompose a wide range of organic compounds, from natural and anthropogenic origin, but also to accomplish changes in the speciation and mobility of metals and radionuclides as well as other inorganic elements by oxidation-reduction and other changes, most of which are a direct consequence of metabolic properties of living organisms and microorganism [18].

agriculture or from industrial wastes; (iii) high efficiency; (iv) no additional nutrient requirement; (v) minimization of biological or chemical sludge; (vi) biosorbent regeneration; and (vii) the possibility of metal recovery. In addition to all this captivating features, biosorption processes can be accomplished in an extensive range of pH values (from pH 3 to pH 9) and tem-

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The first paper on biosorption was published in 1951, and since then, enormous efforts have been made to accomplish efficient, effective and economic biosorbents to be employed in wastewater treatment. Fundamental progresses have been accomplished over the past decades in order to understand the complex biosorption mechanisms, the methods for its quantification (equilibrium and kinetics) and the factors that influence efficiency and the rate of the process (**Figure 2**) [20]. Although the majority of the biosorption research conducted till now has been performed on microbial systems, mainly bacteria, microalgae and/or fungi, with metals and related substances (**Table 1**), the term is now being applied to all types of organic compounds and to particulates. The massive research concerning biosorption of metals is an unsurprising fact, taking into account not only the toxicity effect and increased discharge of these contaminants into the environment but also the nature of adsorption and ion exchange mechanisms. Nevertheless, it is also crucial to highlight that regardless the continuous increase in published research related to the biosorption of hazardous substances and/or elements, there has been little or no exploitation in a pilot and industrial scale and/or context [18] and on the decontamination of multicomponent solutions [3, 4, 7, 22, 23]. In fact, despite the biosorption process that has been discussed in literature for 60 years with over 13,000 scientific papers in peer-reviewed journals [19], so far most of this research was performed in a laboratory scale, using batch tank reactors or packed mini-column, and has not been extensively implemented in an industrial scale and in multicomponent solutions. The team of Professor Bohumil Volesky from McGill University, Canada, and his company BV Sorbex comprise the few researchers that have contributed to

perature values ranging from 4 to 90°C.

**Figure 2.** Biosorption process: a global overview (adapted from Refs. [2, 19]).

Although the term *biosorption* presents a multidimensional character, its definition is rather difficult and has been evolving over the past few decades, due to the diversity of the mechanisms that contribute to the overall process, depending on the sorbate and the biosorbent, on the environmental conditions and on the metabolic processes in the case of living organisms [18, 19].

Some publications [18, 20, 21] indicate that most researchers define biosorption as a passive and metabolically independent process that can be performed either by dead biomass or fragments of cells and tissues or by living cells as an active and metabolic-dependent process. It is important to highlight that both mechanisms can overlap adding additional confusion in the use of the terminology.

It is also important to highlight that (i) biosorption is a crucial part of many processes taking place in nature, including, for example, antigen-antibody immune reactions and adsorption to host cells, as the first stage in virus replication or sorption in soil and that (ii) numerous methodological approaches used in medicine, life sciences and biotechnology are, in fact, based on biosorption processes, for instance, the staining of microbial cells for electron microscopy and targeted therapies in cancer treatment. It is therefore possible to affirm that many life phenomena are in some way related to interactions between a sorbate and biological surfaces [18]. Basically, biosorption is a reversible and rapid process of binding of ions or neutral molecules from aqueous solutions onto functional groups that are present on the surface of biomass, independent on cellular metabolism, efficient and selective [21].

Presently, it is accepted that biosorption is a physico-chemical process, simply defined as the removal of substances from solution by biological material, and includes mechanisms such as absorption, adsorption, surface complexation ion exchange and precipitation [18]. The addition of the prefix *bio* to the term *sorption* denotes the involvement of a biological entity (living or death biomass, as well as their metabolites or synthesized products). Although the majority of biosorption research has been focused on metals and/or metalloid species, the substances to be sorbed can be from either organic or inorganic origin and presented in soluble or insoluble forms. Therefore, considering the diversity of sorbates and sorbents that can be used in all domains of life, it is plausible to use the term biosorption to describe any system where a sorbate (e.g. an atom, molecule, a molecular ion) interacts with a biosorbent (a solid surface of a biological matrix) resulting in an accumulation at the sorbate-biosorbent interface and therefore a reduction in the solution sorbate concentration.

The mesmerizing features of biosorption processes over traditional treatment methods comprise (i) economical operating costs; (ii) inexpensive biological materials, usually obtained from agriculture or from industrial wastes; (iii) high efficiency; (iv) no additional nutrient requirement; (v) minimization of biological or chemical sludge; (vi) biosorbent regeneration; and (vii) the possibility of metal recovery. In addition to all this captivating features, biosorption processes can be accomplished in an extensive range of pH values (from pH 3 to pH 9) and temperature values ranging from 4 to 90°C.

**2. Biosorption: a general overview**

54 Biosorption

use of the terminology.

Microorganisms, in particular bacteria and fungi, have been receiving particular attention in the area of environmental microbiology and biotechnology due to their ability not only to decompose a wide range of organic compounds, from natural and anthropogenic origin, but also to accomplish changes in the speciation and mobility of metals and radionuclides as well as other inorganic elements by oxidation-reduction and other changes, most of which are a direct consequence of metabolic properties of living organisms and microorganism [18].

Although the term *biosorption* presents a multidimensional character, its definition is rather difficult and has been evolving over the past few decades, due to the diversity of the mechanisms that contribute to the overall process, depending on the sorbate and the biosorbent, on the environmental conditions and on the metabolic processes in the case of living organisms [18, 19].

Some publications [18, 20, 21] indicate that most researchers define biosorption as a passive and metabolically independent process that can be performed either by dead biomass or fragments of cells and tissues or by living cells as an active and metabolic-dependent process. It is important to highlight that both mechanisms can overlap adding additional confusion in the

It is also important to highlight that (i) biosorption is a crucial part of many processes taking place in nature, including, for example, antigen-antibody immune reactions and adsorption to host cells, as the first stage in virus replication or sorption in soil and that (ii) numerous methodological approaches used in medicine, life sciences and biotechnology are, in fact, based on biosorption processes, for instance, the staining of microbial cells for electron microscopy and targeted therapies in cancer treatment. It is therefore possible to affirm that many life phenomena are in some way related to interactions between a sorbate and biological surfaces [18]. Basically, biosorption is a reversible and rapid process of binding of ions or neutral molecules from aqueous solutions onto functional groups that are present on the surface of biomass,

Presently, it is accepted that biosorption is a physico-chemical process, simply defined as the removal of substances from solution by biological material, and includes mechanisms such as absorption, adsorption, surface complexation ion exchange and precipitation [18]. The addition of the prefix *bio* to the term *sorption* denotes the involvement of a biological entity (living or death biomass, as well as their metabolites or synthesized products). Although the majority of biosorption research has been focused on metals and/or metalloid species, the substances to be sorbed can be from either organic or inorganic origin and presented in soluble or insoluble forms. Therefore, considering the diversity of sorbates and sorbents that can be used in all domains of life, it is plausible to use the term biosorption to describe any system where a sorbate (e.g. an atom, molecule, a molecular ion) interacts with a biosorbent (a solid surface of a biological matrix) resulting in an accumulation at the sorbate-biosorbent interface and

The mesmerizing features of biosorption processes over traditional treatment methods comprise (i) economical operating costs; (ii) inexpensive biological materials, usually obtained from

independent on cellular metabolism, efficient and selective [21].

therefore a reduction in the solution sorbate concentration.

The first paper on biosorption was published in 1951, and since then, enormous efforts have been made to accomplish efficient, effective and economic biosorbents to be employed in wastewater treatment. Fundamental progresses have been accomplished over the past decades in order to understand the complex biosorption mechanisms, the methods for its quantification (equilibrium and kinetics) and the factors that influence efficiency and the rate of the process (**Figure 2**) [20].

Although the majority of the biosorption research conducted till now has been performed on microbial systems, mainly bacteria, microalgae and/or fungi, with metals and related substances (**Table 1**), the term is now being applied to all types of organic compounds and to particulates.

The massive research concerning biosorption of metals is an unsurprising fact, taking into account not only the toxicity effect and increased discharge of these contaminants into the environment but also the nature of adsorption and ion exchange mechanisms. Nevertheless, it is also crucial to highlight that regardless the continuous increase in published research related to the biosorption of hazardous substances and/or elements, there has been little or no exploitation in a pilot and industrial scale and/or context [18] and on the decontamination of multicomponent solutions [3, 4, 7, 22, 23]. In fact, despite the biosorption process that has been discussed in literature for 60 years with over 13,000 scientific papers in peer-reviewed journals [19], so far most of this research was performed in a laboratory scale, using batch tank reactors or packed mini-column, and has not been extensively implemented in an industrial scale and in multicomponent solutions. The team of Professor Bohumil Volesky from McGill University, Canada, and his company BV Sorbex comprise the few researchers that have contributed to

**Figure 2.** Biosorption process: a global overview (adapted from Refs. [2, 19]).


of the organic contaminant have also been poorly investigated. For these reasons, the authors opted to review the state of the art of biosorption from multicomponent solutions, from a

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Costa and Tavares [3] studied the ability of two fungi and one bacteria (*Penicillium* sp., *Alternaria* sp. and *Streptococcus equisimilis*) to simultaneously treat tertiary solutions containing diethylketone, Cd(II) and Ni(II), and they determined the influence of the initial concentration of metal on (a) the microbial growth, (b) the biosorption capacity of these pollutants and (c) the biological activity after exposure. The results obtained regarding the tertiary solutions allowed to infer that *S. equisimilis* presented the best performance in terms of uptake, for all the conditions tested and that an increase in the initial concentration of metal promoted an increase in the uptake. For the same experimental conditions, the biosorption data obtained for the three microorganisms showed (i) a higher affinity of the biosorbents towards Ni(II) and (ii) a strong and detrimental effect of the metals either in the biosorption process or in the microbial growth. These results may be explained by the fact that not only Ni(II) can be used by the cells as a cofactor, competing actively and passively with Cd(III) but is also less toxic than Cd(II).

More complex systems were further evaluated [4] with a suspended bacterial culture of *Streptococcus equisimilis* with different initial concentrations of Ni(II) (5–450 mg/L) and Cd(II) (5–100 mg/L) in single-component solutions compared to vermiculite to decontaminate singlecomponent solutions composed either by diethylketone, Cd(II) or Ni(II) and binary-component solutions composed either by diethylketone and Cd(II) or diethylketone and Ni(II). A *S. equisimilis* biofilm supported on vermiculite to decontaminate binary solutions composed either by diethylketone and Cd(II) or diethylketone and Ni(II) was also evaluated. The principal aim of this research was the characterization of the interactions between the different concentrations of sorbates and the biosorbents used, when employed in single or binary solutions. For the first set of experiments (*S. equisimilis* and different concentrations of Ni(II) or Cd(II), it was observed that the uptake and percentage of influent Ni(II) sorbed depended on the initial concentration of the sorbate. No significant pH changes had occurred, and the uptake suffered a 30-fold increase with the increase of the initial concentration between 5 and 80 mg/L. Nevertheless, there were no significant changes (<10%) in terms of biosorption percentage for the same initial concentration. The biosorption of Cd(II) was also found to depend on the initial concentration, suffering fluctuations lower than 18%. In these assays, there was an increase in terms of pH (from 6.05 to 6.98). This increase resulted in an increase in the hydroxyl and other anionic functional groups, which made the bacterial surface more negative increasing the number of electrostatic interactions. For the second set of experiments, it was observed that the presence of Cd(II) decreases significantly the sorption percentage of diethylketone, but the presence of this organic compound increased Cd(II) sorption percentage. The presence of Ni(II) has a synergetic effect on diethylketone biosorption. For the third set of experiments, it was established that the presence of the biofilm is an advantage, obtaining promising results, specially taking into account not only the concentrations employed but also the toxicity of the metals. In these experiments, a common increase in terms of sorption efficiency was observed, and this may be explained by the functional groups present on the biofilm that can implement the substrate molecule adsorption and eventually promote the biodegradation of diethylketone and by the

laboratory scale to a pilot and/or industrial scale.

increase of the available sites for sorption.

**Table 1.** The top 10 publications in the ISI Web of Science database (Web of Science Core Collection) for 'all years' (1970–2016) with 'biosorption' in the topic.

the scale-up of sorption process field, from a laboratory scale to a pilot or industrial scale [21]. The team of the Centre of Biological Engineering from the University of Minho, Portugal, has also been contributing to the study and understanding of (i) the scale-up of biosorption processes from a laboratory scale to a pilot scale and (ii) the increase of solution complexity to be decontaminated, evolving from single-component solutions to multicomponent solutions, mixing organic and inorganic compounds [3, 4, 7, 9, 15, 24], the main subject of this chapter.
