**3.2. Correlation between metal concentration and bioremoval inhibition**

It is acknowledged that the bioremoval of an organic pollutant decreases as the concentration of bioavailable metal increases in co-contaminated systems (**Figure 3**). However, this pattern is not always observed. Two other additional patterns describing the effect of metals on the bioremoval of organic pollutants have been shown.

Low metal concentration enhances bioremoval of organic pollutant; high metal concentrations inhibit it—additional pattern 1: diverse studies showed a pattern of metal toxicity in which low metal concentrations enhance bioremoval activity, till the maximum level of stimulation is reached. After this point, an increase in metal concentration will lead to an increase in metal toxicity (**Figure 3**, Line 2). Sustaining this pattern is the result obtained by Capone et al. [29] showing that methanogenesis was enhanced by the addition of some metals.

Bioremoval inhibition of organic pollutants is due to low metal concentration; lower bioremoval inhibition of organic pollutants is due to high metal concentration—additional pattern 2: several studies suggested that low concentrations of metal strongly inhibit bioremoval activity, until a maximum of inhibition is achieved (**Figure 3**, Line 3). After this point, an

results suggest a dependence of metal bioremoval by *N. muscorum* on the metals and their concentration combination in the multi-metal solution. It was also observable that the metals' uptake depended upon their concentration combination in solution and the bioremoval order observed was Pb(II) > Cu(II) > Cd(II) > Zn(II). In this study, Pb(II) showed not only a better bioremoval efficiency compared with the other three metals but also that its bioremoval was unaffected by the presence of the three other metals. However, the presence of Pb(II) exerted a strong negative effect on the bioremoval of all other metals. These results may be explained by taking into consideration the Pb(II) strong interaction with the functional groups present on the biomass and because Pb(II) presents the smallest radius among the four metals tested in

Biosorption of Multicomponent Solutions: A State of the Art of the Understudy Case

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61

these assays (the smaller the hydrated radius, the higher is the affinity of its binding).

Three aspects related to the influence of anions on the biosorption processes are usually considered in the available literature: (i) the influence that the anion has on the maximum biosorption capacity of the sorbent, in single-metal solutions [39]; (ii) the influence of anion concentration on the biosorption of several metal ions, in multi-metal solutions [37–41]; and (iii) the nature of the biosorbent that can influence significantly the effect of the anion on the

The biosorption of four metals—Cr(VI), Co(II), Ni(II) and Zn(II)—by the *Aspergillus niger* fun-

3− by the brown macroalga *Ascophyllum nodosum* and concluded that the presence of these anions did not reveal any influence on the biosorption performance, as opposite to the pres-

was observed in the biosorption of Zn(II) by the cyanobacterium *Oscillatoria angustissima* [41],

The degree of inhibition for the biosorption of La(III), Cd(II), Pb(II) and Ag(I) cations, by the

As referred previously, the influence of the anion on the biosorption capacity will vary depending on the metal ion oxidation state, as it was observed for the biosorption of Cr(III)

Considering the limited number of active sites present on the biosorbent surface, it is accepted that the biosorption capacity of the biosorbent towards a specific pollutant (metal or not) in a multicomponent solution is inferior to the one in single-component solutions; therefore, the

anions, that strongly inhibited the biosorption process. The opposite situation

the biosorption performance of the four metals, whereas the presence of Cl<sup>−</sup>

affect the biosorption performance of the four metals in multi-metal solutions.

Kuyuca and Volesky [42] studied the biosorption of Co(II) ions in the presence of SO4

2−, NO3 − and Cl<sup>−</sup> − and SO4

2− did not significantly affect

had the following biosorption inhibi-

−

2− > Cl<sup>−</sup> ≈ NO<sup>3</sup>

2− > Cl<sup>−</sup> > PO4

and NO3

did negatively

2− and

3− > glu-

<sup>−</sup> > SO4 2−.

*3.3.1. Effect of anions*

biosorption capacity [21].

−

<sup>2</sup> <sup>−</sup> .

*3.3.2. Effect of the ionic concentration*

and it was stated that the presence of SO4

2− > Cl<sup>−</sup> > NO3

PO4

ence of NO3

tion order SO4

tamate > CO3

gus [40] revealed that the presence of anions such as NO3

− .

*Rhizopus arrhizus* fungus [43], usually followed the order EDTA > SO4

contaminants will compete for the active sites, available for sorption [44].

and CR(VI) ions [44], with the following inhibitory orders SO4

**Figure 3.** Metal concentration impact on bioremoval inhibition pattern of organic pollutants, assuming (1) a direct or linear relationship, (2) additional pattern 1 and (3) additional pattern 2.

increase in metal concentration will lead to a decrease of metal toxicity. An example is the work conducted by Said and Lewis [26] where an increase in metal concentration was responsible for a decrease in 2,4-DME bioremoval.

Briefly, the existence of different patterns of responses of organic pollutants towards metals is possible to assume and that this variety of responses makes the understanding and prediction of metal toxicity in the environment more difficult, since these elements may influence both the ecology and physiology of the pollutant-degrading microorganisms.

Unless the models used to predict the influence of metals on the bioremoval of organic pollutants incorporate both the ecologic and physiologic effects of metals towards the pollutantdegrading microorganisms, they may fail their main purpose.

#### **3.3. Biosorption in multi-metal solutions**

As previously mentioned, despite the research concerning biosorption processes has been well documented in the literature, biosorption of different metal ions by different types of biological materials has been mainly conducted in single-metal solutions [21]. Information concerning biosorption studies in binary- [30–34], tertiary- [31–35] and quaternary-component solutions [36] is very scarce. Moreover, the use of different evaluation methodologies makes any attempt to draw any meaningful and universal conclusion very difficult and, on the other hand [37], the influence that anions may exert on the biosorption process of metal cations has been somehow neglected.

*Nostoc muscorum*, a cyanobacterium indigenous from coal mining sites, was employed as biosorbent to decontaminate aqueous solutions containing Cd(II), Cu(II), Pb(II) and Zn(II) (5 or 10 mg/L) [38]. The results obtained in these experiments showed a maximum bioremoval of both Pb(II) (96.3%) and Cu(II) (96.4%) followed by Cd(II) (80.0%) and Zn(II) (71.3%) after 60 h of culture period. The bioremoval of Cd(II), Cu(II) and Pb(II) was maximum at 5 mg/L, whereas Zn(II) bioremoval has a maximum when all the four heavy metals were set at 5 mg/L. These results suggest a dependence of metal bioremoval by *N. muscorum* on the metals and their concentration combination in the multi-metal solution. It was also observable that the metals' uptake depended upon their concentration combination in solution and the bioremoval order observed was Pb(II) > Cu(II) > Cd(II) > Zn(II). In this study, Pb(II) showed not only a better bioremoval efficiency compared with the other three metals but also that its bioremoval was unaffected by the presence of the three other metals. However, the presence of Pb(II) exerted a strong negative effect on the bioremoval of all other metals. These results may be explained by taking into consideration the Pb(II) strong interaction with the functional groups present on the biomass and because Pb(II) presents the smallest radius among the four metals tested in these assays (the smaller the hydrated radius, the higher is the affinity of its binding).
