**4. Critical assessment concerning the biosorption research on multicomponent solutions**

As previously mentioned, in the past few decades, there has been an intense study and research concerning biosorption processes to treat contaminated environmental matrices and wastewaters. However, it is doubtful whether such a remarkable rise in published output has significantly enhanced the knowledge about biosorption process, or aided any industrial exploitation, which so often is the primary underlying principle for such investment and work [18, 19, 21]. Despite the incontestable progress made over decades of research, most of the biosorption studies are still conducted at a laboratory scale and involve (i) the characterization of a selected sorbent, which will sorb a given contaminant from solution, (ii) the study of the effect of physico-chemical parameters may have on biosorption and (iii) the use of metals. Considering that the majority of elements present in the periodic table are classified as metals, the potential number of 'original' research is most likely beyond comprehension, especially if coupled with the gigantic number of microbial species, strains and metabolites/derived substances. It is therefore expected that the output of publications related to biosorption shows no sign of decreasing and will be increased due to the continuing number of new journals, including those that are web based [18, 19].

This is the case of the amount of Cr(VI) biosorbed per unit weight of *Rhizopus arrhizus* that

Fagundes-Klen et al. [47] observed that the amount of Zn(II) biosorbed by *S. filipendula* in the presence of high concentrations of Cd(II) decreased significantly (56.8 %) when comparing the biosorption results achieved in single-metal solution. These results are easily explained by the reduced number of coordination, the ionic radius and the higher ionization potential

It is therefore worth noting that as the ionic concentrations become higher, there is a growing force able to overcome the mass resistance transfer of metal ions through the biosorption process. The published data [48] showed that even though lead ions (Pb2+) have higher affinity than copper (Cu2+) to be biosorbed by an algae belonging to the genera *Gelidium* uptake, Cu2+ uptake was higher than Pb2+ uptakes due to the higher initial concentration of Cu2+. Similar results described the biosorption of Pb2+ and Cu2+ by pine cone shells [49]. When binary solutions were tested, the uptake of both metals was significantly inhibited, revealing an antagonistic effect.

In multi-metal solution, the electronegativity and atomic weight of metals can also have an important role in the biosorption process and efficiency. Biosorption experiments showed that when mixed, Ni(II) and Zn(II) sorption by wheat straw presented different performances, revealing a competition between both metals for the actives sites present on the biosorbent surface and a higher preference for Zn(II) rather than Ni(II) [50]. These results are easily justified taking into consideration the more appealing physical characteristics of Zn(II): lower electronegativity and higher atomic weight of Zn(II). The oxygen-containing group present on the wheat straw (negative sites) repels Ni(II) more than Zn(II), making it more difficult to be sorbed.

As previously mentioned (see Section 2, **Figure 2**), temperature also plays an important role on the biosorption processes, as well as on all biological and physico-chemical processes. The biosorption of Cr(III), Cu(II) and Zn(II) by wine-processing waste sludge (WPWS) in a ternary system was found to be significantly affected by temperature. At normal conditions, the biosorption of these three metals in a mixture by WPWS followed the trend Cr(III) > Cu(II) > Zn(II). However, when the temperature decreases to 10°C, the biosorption of Cr(III) was inferior than Cu(II) [50].

As previously mentioned, in the past few decades, there has been an intense study and research concerning biosorption processes to treat contaminated environmental matrices and wastewaters. However, it is doubtful whether such a remarkable rise in published output has significantly enhanced the knowledge about biosorption process, or aided any industrial

**4. Critical assessment concerning the biosorption research on** 

decreased with the increase of Fe(III) concentration as an antagonistic effect [45, 46].

*3.3.3. Effect of electronegativity and atomic weigh of metals*

of Zn(II).

62 Biosorption

*3.3.4. Effect of temperature*

**multicomponent solutions**

It is also logical to infer that several technical and scientific issues should be solved in order to meet the industrial demands and bring the biosorption technology into commercialization. Based on this, several future perspectives can be made:

