**23. Low-cost sorbents**

In the literature, there are many examples of the alternative sorbents for noble metals removal produced from renewable and low-cost resources [143]. Among them, those based on bacteria, fungi and algae as well as agriculture and seafood wastes (coffee, green tea, tea, yuzu, aloe, wheat and barley straw, maize crop, coconut shell, rise husk, etc.) have been investigated. One of the low-cost sorbents is chitosan (CS) [144-155]. It is a kind of abundant natural polysac‐ charide. Chitosan is produced by the alkaline deacetylation of chitin, the most abundant biopolymer in nature after cellulose. It is extracted from shrimp and crab shells. Due to large availability of functional groups such as amino and hydroxyl ones, CS has been proved to be very efficient for the recovery of several toxic metal ions such as Cu(II), Cd(II) and Pb(II) and strategic metal ions such as Pt(IV) and Pd(II) [144]. It should be mentioned that sorption properties of CS are due to its composition and presence of active and functional groups. CS is characterized by its high percentage of nitrogen present in the form of amine groups, which are responsible for metal ion binding through chelation mechanisms. Due to the fact that it is protonated in acidic solutions, it is also capable of sorbing metal ions through anion exchange mechanisms. It should be mentioned that chitosan protonation in acidic solution causes the polymer to dissolve (except in sulphuric(VI) acid solutions). For the sorption of some metal ions (for example noble metals), sulphuric(VI) acid cannot be used for pH control due to reduction of sorption efficiency [145].

In the case of chitosan derivatives obtained by glutaraldehyde cross-linking (GA), poly(eth‐ yleneimine) grafting through glutaraldehyde linkage (PEI) or thiourea grafting (T), noble metal ions can be sorbed. The reasons for grafting new functional groups are (i) to increase the density of sorption sites, (ii) to change the pH range for metal ions sorption and (iii) to change the sorption sites and/or the uptake mechanism in order to increase sorption selectivity for the noble metals [146]. Such derivatives were used for palladium and platinum removal [147]. It was found that the maximum adsorption capacity occurred at pH 2.0 for both Pt(IV) and Pd(II) species. The material selectively adsorbs Pt(IV) and Pd(II) from binary mixtures with Cu(II), Pb(II), Cd(II), Zn(II), Ca(II) and Mg(II). The isotherm adsorption equilibrium was well described by the Langmuir isotherms with the maximum adsorption capacity of 129.9 mg/g for Pt(IV) and 112.4 mg/g for Pd(II), which was relatively high compared with the glycine chitosan derivative (122 mg/g for Pt(IV) and 120 mg/g for Pd(II)). The results show that 0.5 M EDTA-0.5 M H2SO4 solution can effectively desorb Pt(IV) and Pd(II) (>97%) metal ions from the adsorbent material. The high percentage of desorption obtained when the 0.5 M EDTA– 0.5 M H2SO4 solution was used and can be explained by both stable complexes and the electrostatic interactions between the Pt(IV) and Pd(II) species.

chloride solutions, the above components can partly form anions, which reduces the sorption capacity of weakly basic anion exchangers. The effect of the above-mentioned macrocompo‐ nents on decrease of sorption capacity towards platinum(IV) ions can be presented in the series: CuCl2 ≈ FeCl3 ≈ NiCl2 < AlCl3 < ZnCl2 [140, 141]. A similar series can be determined for the anion exchanger Duolite S 37, which contains secondary and tertiary functional groups added

In the literature, there are many examples of the alternative sorbents for noble metals removal produced from renewable and low-cost resources [143]. Among them, those based on bacteria, fungi and algae as well as agriculture and seafood wastes (coffee, green tea, tea, yuzu, aloe, wheat and barley straw, maize crop, coconut shell, rise husk, etc.) have been investigated. One of the low-cost sorbents is chitosan (CS) [144-155]. It is a kind of abundant natural polysac‐ charide. Chitosan is produced by the alkaline deacetylation of chitin, the most abundant biopolymer in nature after cellulose. It is extracted from shrimp and crab shells. Due to large availability of functional groups such as amino and hydroxyl ones, CS has been proved to be very efficient for the recovery of several toxic metal ions such as Cu(II), Cd(II) and Pb(II) and strategic metal ions such as Pt(IV) and Pd(II) [144]. It should be mentioned that sorption properties of CS are due to its composition and presence of active and functional groups. CS is characterized by its high percentage of nitrogen present in the form of amine groups, which are responsible for metal ion binding through chelation mechanisms. Due to the fact that it is protonated in acidic solutions, it is also capable of sorbing metal ions through anion exchange mechanisms. It should be mentioned that chitosan protonation in acidic solution causes the polymer to dissolve (except in sulphuric(VI) acid solutions). For the sorption of some metal ions (for example noble metals), sulphuric(VI) acid cannot be used for pH control due to

In the case of chitosan derivatives obtained by glutaraldehyde cross-linking (GA), poly(eth‐ yleneimine) grafting through glutaraldehyde linkage (PEI) or thiourea grafting (T), noble metal ions can be sorbed. The reasons for grafting new functional groups are (i) to increase the density of sorption sites, (ii) to change the pH range for metal ions sorption and (iii) to change the sorption sites and/or the uptake mechanism in order to increase sorption selectivity for the noble metals [146]. Such derivatives were used for palladium and platinum removal [147]. It was found that the maximum adsorption capacity occurred at pH 2.0 for both Pt(IV) and Pd(II) species. The material selectively adsorbs Pt(IV) and Pd(II) from binary mixtures with Cu(II), Pb(II), Cd(II), Zn(II), Ca(II) and Mg(II). The isotherm adsorption equilibrium was well described by the Langmuir isotherms with the maximum adsorption capacity of 129.9 mg/g for Pt(IV) and 112.4 mg/g for Pd(II), which was relatively high compared with the glycine chitosan derivative (122 mg/g for Pt(IV) and 120 mg/g for Pd(II)). The results show that 0.5 M EDTA-0.5 M H2SO4 solution can effectively desorb Pt(IV) and Pd(II) (>97%) metal ions from the adsorbent material. The high percentage of desorption obtained when the 0.5 M EDTA–

to the phenol-formaldehyde skeleton [142].

reduction of sorption efficiency [145].

**23. Low-cost sorbents**

20 Ion Exchange - Studies and Applications

In the paper [143], rubeanic acid was grafted on chitosan through the reaction with glutaral‐ dehyde as the linker to obtain the sorbent for Au(III) with the thiol functional groups. It was found that the maximum sorption capacity was high and equal to 600 Au(III) mg/g. The speciation of gold in the chloride and hydroxide chloride systems appears to be a predominant parameter influencing the removal process. The optimum pH range was between 2 and 3 for glutaraldehyde cross-linked chitosan. However, the sorption capacity strongly decreases with the increasing pH. It was found that in the case of the grafting of sulfur compounds on chitosan derivatives, the partial change in the sorption mechanism occurs. In this case, metal ion chelation with sulfur compounds is weakly sensitive to the pH change [148]. Sorption capacity in the HCl system reaches 2 mmol/g (180 mg/g) and is slightly lower than for the chitosan derivatives obtained by grafting of pyridyl groups (6 mmol/g). Increasing chloride concentra‐ tion involves a significant decrease in sorption capacity.

The removal of Au(III), Pt(IV) and Pd(II) onto the glycine modified cross-linked chitosan resin was investigated in the paper by Ramesh et al. [146]. The results show that the optimum pH appeared to be 1.0–4.0, and the maximum percentage removal was obtained at pH 2.0 for Au(III), Pt(IV) and Pd(II). The pHZPC was found to be 5.1. At pH < pHPZC, the surface of modified chitosan resin is positively charged, whereas at a pH > pHPZC, the surface of modified chitosan resin is negatively charged. Due to the positive surface charge of sorbent at pH lower than pHPZC, it attracts the chlorocomplexes of platinum, palladium and gold, resulting in the greater amounts of adsorption at low pH. The authors proposed the following mechanism of the sorption process:

$$\begin{aligned} \text{R-NH}\_{2} &+ \text{HCl} \xleftarrow{\longrightarrow \text{RNH}\_{3}} \text{CNI}\_{3} \text{"} \text{Cl}^{\cdot} \\ \text{R-NH}\_{3} &+ \text{HCl} \xleftarrow{\longrightarrow \text{RNH}\_{3}} \text{RNH}\_{3} \text{"} \text{Cl}^{\cdot} + \text{H}^{+} \\ 2\text{R-NH}\_{3} \text{"} \text{Cl}^{\cdot} &+ \left[ \text{PtCl}\_{6} \right]^{2} \xleftarrow{\longrightarrow} \text{(RNH}\_{3} \text{}^{+} \text{)}\_{2} \left[ \text{PtCl}\_{6} \right]^{2} + 2 \text{Cl}^{\cdot} \\ 2\text{R-NH}\_{3} \text{"} \text{Cl}^{\cdot} + \left[ \text{PdCl}\_{4} \right]^{2} \xleftarrow{\longrightarrow} \text{(RNH}\_{3}{}^{+} \text{)}\_{2} \left[ \text{PdCl}\_{4} \right]^{2} + 2 \text{Cl}^{\cdot} \\ \text{R-NH}\_{3} \text{"} \text{Cl}^{\cdot} + \left[ \text{AuCl}\_{4} \right]^{-} \xleftarrow{\longrightarrow} \text{RNH}\_{3}{}^{+} \left[ \text{AuCl}\_{4} \right]^{+} \text{Cl}^{-} \end{aligned}$$

The results also demonstrated that the amount of adsorption was decreased with the increasing chloride ion concentration. This is because of strong interaction between the chloride ions and precious metal ions to form chlorocomplexes. The 0.7 M thiourea-2 M HCl solution was the most effective for the desorption of Au(III), Pt(IV) and Pd(II).

In the case of chitosan derivatives, noble metal ions are sorbed according to several kinetic models based on pure sorption, pure reduction and dual sorption-reduction mechanisms [149, 150]. Moreover, the optimum acid pH for noble metal ions sorption depends on the metal. For platinum and palladium, it was equal to 2. For CS cross-linked by glutaraldehyde (CS-GA) for

hydrochloric acid solutions of palladium at pH 2, sorption reached the same level as achieved at pH 1 (capacity strongly decreased) [151]. However, in the sulphuric(VI) acid solutions, the sorption capacity remains almost unchanged. It is well known that pH has a critical effect on the speciation of the metal in solution because the distribution of metal species depends on pH. Other parameters which affect the sorption efficiency are connected with the nature of the sorbent (ionic charge), chemistry of the metal ion: ionic charge, ability to be hydrolysed as well as metal concentration and the composition of the solution and the form of polynuclear species [151]. Sorption kinetics is controlled by particle size, cross-linking ratio and palladium concentration. In hydrochloric acid solutions, equilibrium is achieved at 24 h contact. For chitosan-cellulose fibres, it was found that incorporation of cellulose fibres improves the binding efficiency of chitosan towards Ag(I). The sorption capacity was close to 220 mg/g. This is much higher than for the pure chitosan (140 mg/g) [152]. The cellulose fibres contribute to dispersion of the chitosan chains that are more accessible and available for silver. It is also possible to modify the chitosan structure by introducing cross-linking structure, blending chitosan with synthetic polymers such as poly(vinyl alcohol) (PVA) – a non-toxic, watersoluble synthetic polymer with good physical and chemical properties and film-forming ability. It is also possible to apply the sol-gel process to develop organic–inorganic hybrid materials [153–155]. For this aim, clays and silicas are frequently used. Clays are composed of silicate layers which form three-dimensional structures after hydrated in water. They have negative charge and can interact with chitosan. Also silicas are characterized by several advantages which are among others surface stability in the acidic medium and highly developed surface, acceptable kinetics, thermal stability, resistance to microbial attack and low cost should be mentioned [156]. Chemically modified silicas (CMSs) with the functional groups covalently bound to the surface such as polyamines, particularly, linear polyhexamethylene guanidine (PHMG) with convenient amine group configurations and nitroso-R salt (NRS) were used in palladium sorption [157, 158]. Complex of palladium(II) with the ratio Pd:NRS = 1:2 formed the SiO2–PHMG–NRS. The other examples are presented in Table 2.


**Table 2.** Organofunctionalized silicas in pre-concentration of noble metal ions.
