As we have already discussed in section 4.3, one of the mechanisms involved in the sorption of positively charged metal species is ion-exchange. Vegetal biomaterials (constituted principally by lignin and cellulose as major constituents and by a non negligible portion of fatty acid, bearing functional groups such as alcohol, ketone and carboxylic groups that can be involved in complexation reactions with metallic cations) can be viewed as natural ion — exchange materials. These materials primarily contain weak acid and basic groups on the surface, whose ionization degree strongly depends on the pH of the solution. Several authors have performed potentiometric titrations to investigate acid-base properties on the surface of biosorbents and to determine the number of active sites for metal ion sorption.

The strong pH dependence of the sorption parameters can depend on several factors, which can be simplified as follows:

1. behaviour and speciation of metal ions;

2. dependence of the acid-base characteristics of the adsorbing material on the pH;

3. dependence of the interaction metal ion-sorbent on the pH.

As far as point 1 is concerned, we report a statement made by Baes and Mesmer, 1976, in their classical book on the hydrolysis of cations: "soluble hydrolysis products are important when cation concentrations are very low and can profoundly affect the chemical behaviour of the metals; the formulas and charges of the hydrolysis products formed in such systems can control such important aspects of chemical behaviour as:

a. sorption of the dissolved metals in mineral and soil particles;

b. tendency of metal species to coagulate colloidal particles;

c. solubility of the hydroxide (or oxide) of the metals;

d. extent to which the metals can be complexed in solution or extracted from solution by natural agents;

e. oxydizability or reducibility of the metals to another valence state."

Based on these considerations, we demonstrate the influence of pH on sorption taking as an example the behaviour of one of the most important toxic metal ion, lead, in presence of different coordinating groups. Firstly we take into account the hydrolysis of this metal ion at two different concentrations, 100 mg/L and 0.05 mg/L, i. e. at concentration in strong polluted water and at concentration equal to EU recommended value for drinking water (Fig. 2). At 100 mg L-1, the species Pb(OH)+ (pH> 6) and the polynuclear species Pb3(OH)42+ and Pb6(OH)84+(pH >7) are formed before hydroxide precipitation occurs at pH~9.5; at 50 pg L-1, Pb2+ do not form precipitates and only the mononuclear species are formed instead of the polynuclear ones observed at 100 mg L-1. Metal ion hydrolysis equilibria, as well as hydroxide precipitation, can help explain the dependence of metal ion sorption on the pH. In most cases, the observed pH dependence lies in a range in which the metal ion is completely insensitive to the acidity of the medium. In metal ion sorption, pH effects are commonly accounted for by charge variations on the sorbent surface: protonation of basic sites or dissociation of acidic groups. According to the majority of authors a negative charge favours metal ion sorption by an ionic exchange mechanism or by electrostatic interactions, i. e. the sorption is completely determined by the acid-base behaviour of the functional groups on the surface of the adsorbing material.

The real behaviour is certainly far more complex and can be rationalised in terms of metal ion coordination by surface binding groups. The presence of phenolic, carboxylic, catecholic, amino, and mercapto groups on the surface is well known. As a working hypothesis we can imagine that the different binding groups on the solid particles, dispersed in the metal ion solution, behave as different ligands. With this simplifying assumption, we can consider our system as set of solution equilibria. In this assumption we can treat our system as solution equilibria between various ligands competing for a metal ion or for various metal ions. For example, a carboxylic group near a phenolic group on the surface can be assumed to behave as a salicylate ligand, limited to form only 1:1 chelates being anchored to a solid surface.

In the example showed in Fig. 3, we took into consideration three different coordinating groups as possible ligands for lead: COOH, hard, NH2, intermediate, and SH, soft donors. Furthermore, we also considered all the possible combination of them to obtain bidentate ligands, COOH-COOH; COOH-NH2, COOH-SH, NH2-NH2, NH2-SH, and SH-SH.

pH

Fig. 2. Species distribution diagrams for Pb2+ hydrolysis at two different total concentration 100 mg/L (solid lines) and 0.05 mg/L (dashed lines).

Starting from the distribution curves, obtained using the literature constants for lead complexes with different ligand bearing the above mentioned coordinating groups, some conclusions can be drawn. The soft metal Pb2+ ion prefers the soft SH group, which became completely coordinated in 4-6 pH range. No data is available in literature for a single NH2- Pb interaction. The carboxylic group forms a weak complex in the pH range corresponding to its deprotonation. The addition of a second group (COOH or SH) to the starting SH favours lead coordination, while the addition of a NH2 group has an adverse effect. Two vicinal COOH groups allow lead complexation at low pH values and act much better than a single COOH group, even if the per cent of complex formation is still much lower than that reached by SH groups. Regarding the coordinating properties related to the amino group, the complex formation, taking place at basic pH > 7, does not prevent the hydroxide formation.

2. Conclusion

The numerous studies on metal sorption by biomass are extremely spread: the investigation of the mechanism involved in metal ion sorption is performed by different techniques, methods and approaches that are related to the equipment availability in the researcher’s laboratories and to the researcher education. The use of highly sophisticated and extremely expensive techniques, as mentioned in the above sections, enables one to obtain structural information on the sorbent morphology and indirect knowledge of the implied sorption mechanisms, by comparing some physical properties of the material before and after metal sorption. Even if little importance is given to the classical chemical methods, such as potentiometry and alkaline and alkaline-earth metal ion release, these on the contrary offer several advantages, such as the easy availability in all laboratories, the fact that they are fast, cheap, and friendly-used. The main benefit of these methods is the attainment of quantitative results, which allow the evaluation of the amount and the kind of functional groups involved and the amount of exchanged metal ions.

We hope that the achievements obtained from this enormous quantity of research works can lead in the coming years to a real outlet of practical applications, even if a lack of protocol or systematic approach in this kind of studies has to be remarked. Furthermore, the reached level of knowledge acquired should allow the classification of biomass on the basis of structural coordinating groups on its surface, essential to forecast their behavior toward the different toxic metal ions. Thank to this information, it will be possible to depict the strength of interaction and the pH range more useful for metal removal.

The application of biosorption for effluent detoxification will have a strong ecological impact, joining the advantage of recycling waste biomass and of purifying contaminated waters from toxic metal ions.