EDTA-functionalized adsorbents

Various materials from biomass to inorganic oxides have been functionalized with EDTA. The popularity of EDTA arises from its strong metal chelating ability, local availability, and low price. Table 6 shows the metal adsorption properties of EDTA-functionalized adsorbents.

Silica gel is an amorphous form of silicon dioxide where silicon atoms are linked together via oxygen atoms with siloxane bonds. Silica gel surface contains hydroxyl groups that can be further functionalized using a familiar silanization procedure [99,109]. Silica gel has various advantages as an inorganic support compared, for example, to organic polymeric resins. These include good selectivity, no swelling, rapid adsorption, and good mechanical stability. EDTA can be attached on the silica support by first using the amine group containing silylating agents ((3-aminopropyl)triethoxysilane, APTES) and then reacting surface bound amine groups with EDTA anhydride in a mixture of ethanol and acetic acid [110]. Table 6 shows relatively low adsorption capacities for metal ions by EDTA-silica gels, which can be attributed to the experimental conditions such as short contact time (1 h) and acidic media.

Chitosan can be prepared from chitin, one of the nature’s most abundant biopolymers.

Chitosan contains primary amino groups, which can be further functionalized with different organic ligands. In the synthesis of EDTA-chitosan, chitosan is reacted with EDTA anhydride in acetic acid/MeOH solution [111-113]. When EDTA is used as such, the formation of amide bonds between amino groups and EDTA carboxyl groups can be mediated using carbodiimides [114]. A complete functionalization of amino groups by EDTA leading to the adsorbents with very high adsorption capacities has been reported in the literature [111-113].

The same authors, who first prepared EDTA-functionalized chitosan, also tested a similar synthesis for polyallylamine. Adsorption properties of these two adsorbents were, however, quite different and they suggested that in the case of modified polyallylamine, carboxyl groups did not effectively participate in the chelate formation [113].

Silica polyamine composites have been functionalized with EDTA using a synthetic route similar to that of amino functionalized silica gel. A higher amount of available primary amino groups of composite materials, however, has enabled a better surface coverage of EDTA groups compared to that of silica gel [115].

Most of the commercial ion-exchange resins are based on the crosslinked polystyrene (PS). The high loading capacity (0.5–3 mmol/g) and low cost of PS make it a good support for solid phase synthesis [116]. Polystyrene based EDTA-resins were prepared by at first introducing ethylenediamine in styrene-divinylbenzene copolymer and then increasing the amount of carboxyl groups using chloroacetate [117,118]. The mechanical strength of the PS-EDTA resin was improved by wrapping it up by polyvinylalcohol (PVA) beads [119]. This material was an effective adsorbent for Zn(II).

Polyamidoamine (PAMAM) dendrimers are branched, well-defined synthetic nanoscale materials, consisting of globular macromolecules with three covalently bonded components: core, interior branch cells, and terminal branch cells. PAMAM dendrimers with amino terminal groups have previously been used for Cu(II) removal [120]. The difficult recycling process of pure dendrimers, however, has prompted researchers to design PAMAM based inorganic-organic hybrid materials. Mesoporous silica such as SBA-15 was used as a support for dendrimer structures prior to functionalization with EDTA [121].

Plenty of natural and low-cost materials have also been utilized as supports for EDTA functionalities. These are for example saw dust and sugarcane bagasse [122], mercerized cellulose and sugarcane bagasse [123], rice husk [124], maize husk [125], maize cob [126], biomass of baker’s yeast [127], and carbon cloth [128]. Some of these modified adsorbents have shown very high adsorption capacities, for example mercerized cellulose and sugarcane bagasse [123]. The authors stated that mercerization (treatment with 5 M NaOH) made hydroxyl groups of raw materials more susceptible to esterification with EDTA anhydride thus increasing the surface coverage of EDTA groups.

Finally, layered double hydroxide interchelated with EDTA (MgFe-LDH-EDTA) have been tested for Cu(II) and Pb(II) adsorption [129]. The high stability constant of Cu(II)EDTA chelate (Table 4) could explain a very high adsorption capacity obtained for Cu(II) (Table 6).

Table 6. EDTA-functionalized adsorbents.

Matrices functionalized with EDTA

composite 0.76

Co(II)

composite 0.56

Co(II)

*Competitive conditions, total adsorption capacity: 0.6 mmol/g

Table 6 shows that the type of the supporting material greatly affects adsorption performances of different materials although pH can play a significant role as well. Furthermore, the adsorption capacities of different metals do not follow any particular trend comparable to the results presented in Table 5. The order of stability constants of aqueous EDTA chelates is Cu>Ni>Pb>Co>Cd>Zn (Table 4) suggesting that Zn(II) should have the lowest capacity. The hydrated radius (R_H) on its behalf follows the order of Pb>Cd>Zn>Co>Cu>Ni and the primary hydration number the opposite order: Ni>Cu>Co>Zn>Cd>Pb [130]. Therefore, both the type of the supporting material and the metal ion properties affect the adsorption capacity of the immobilized EDTA. Almost all the results in Table 6, however, were obtained for one-component systems. Experiments performed in multi-metal solutions may provide better information about the true differences between different metals.

As with IDA/NTA-functionalized adsorbents, most of the authors do not give suggestions of exact structures of metal EDTA-chelates formed on the surface. The metal chelate formed on EDTA-silica gel was proposed to have an ideal octahedral structure without any convincing evidence. The authors only observed chelate formation between EDTA functionalized silica gel and Cu(II) from variations of diffuse reflectance spectra and electron spin resonance (ESR) analysis [110]. Pereira et al. [122] suggested a quinquedentate structure for Me(II)EDTA chelate immobilized on sawdust and sugarcane bagasse without any characterization of metal loaded adsorbent. Coordination of metals with EDTA immobilized on biomass was confirmed by XPS-analysis, which showed bonds between metal and nitrogen as well as carboxyl groups [127]. IR-measurements also suggested the coordination through both amino and carboxyl groups [117]

Finally, EDTA-functionalized mercerized cellulose and sugarcane bagasse reportedly adsorbed three metals per every two EDTA surface groups, but the binding mechanism was not discussed [123].