Functionalization of adsorbents with different organic groups may change the adsorbent properties into the desirable direction. Functional group addition may increase the adsorption capacity, make adsorbent more selective, or increase the stability of the adsorbent. In the case of resins functionalized with chelating agents, the main applications are the removal of heavy metals from contaminated waters, separation of different metals, or preconcentration of metal ions prior to their analysis.
1.3.1 Removal of metals from different solution matrices
In principle, most of the synthesized adsorbents are first tested in pure solutions containing only one target metal. Real waters to be treated, however, contain various metals as well as salts, acids, and different organic species. Therefore, the adsorption performance of novel adsorbents should also be tested in different solution matrices.
The benefits of APCA-functionalized adsorbents are their selectivity toward heavy metals due to their high affinity surface groups. Therefore, they can show high adsorption efficiencies in the presence of various interfering species. Wood sawdust and sugarcane bagasse modified with EDTA were able to remove 89-92% of Zn(II) from real electroplating wastewater [122]. Three different commercial IDA-bearing chelating resins were used to remove Ni(II) from the acid leached fly ash samples [133]. In this study, however, precipitation of Al(III) and Fe(III) was conducted before a successful Ni(II) adsorption (adsorption capacity 1.3 mmol/g). IDA-functionalized glycidyl methacrylate effectively adsorbed Cr(III), Cu(II), Cd(II), and Pb(II) in the presence of high content of Ca(II) and Mg(II) as well as NTA as aqueous organic species [134].
Furthermore, in a recent study IDA-functionalized Purolite S-920 resin was used to remove
metals complexed by 1-hydroxyethylene-1,1-diphosphonic acid (HEDP) from industrial effluents [135].
One of the most important organic compounds found in water matrices is EDTA. EDTA containing wastewaters are problematic since the metals chelated by it do not effectively adsorb on the adsorption resins and also EDTA enhances the transportation of metals from disposal sites in soils. Ion-exchange has been presented as the most promising solution for the metal removal in the presence of EDTA [136]. Most of the studies are based on anion-exchange, however, when the whole metal EDTA chelate is bound on the adsorbent surface [136,137]. Due to the possible leaching of metal EDTA chelates a better way would be the use of strong cation-exchangers that are able to capture metal ion from its aqueous chelate. Only a few cation-exchangers, however, have been applied for this purposes and evidence of the actual separation of metal and EDTA have not been reported [138,139]. From APCA-functionalized adsorbents material with surface group that forms more stable chelates with metals than EDTA would be preferred. Based on the stability constants of aqueous species (Table 4), DTPA-functionalized adsorbents could provide the solution, but none of the materials presented in Table 7 have not been tested for this application.
1.3.2 Separation of metals by chelating adsorbents
The reuse of metals collected from wastewater requires their separation from each other.
Conventional separation methods for metals are precipitation and liquid phase extraction.
Compared to those, however, solid-phase extraction using chelating resins offers several advantages such as high recovery and enrichment factors, low consumption of organic solvents, and repeated uses [140].
Particularly at low pH range metal ions are separated by many of the APCA-functionalized adsorbents [110,113]. Different commercial IDA-APCA-functionalized resins were found to be selective sorbents for Ni(II) and Co(II) in the simulated pressure acid leach liquor containing Al(III), Fe(III), Zn(II), Mn(II), Mg(II), and Cu(II) [141]. Using EDTA- and DTPA-functionalized chitosans, Co(II) (0.2 mM) could be separated from the solution containing 93 mM of Al(III) and Ni(II) from the solution containing the same amount of Ni(II) and Co(II) (1.7 mM) [113]. The separation of Co(II) from Ni(II) is important for example in hydrometallurgy
[142]. In addition, the separation of Ni(II) from the excess of Al(III) by modified chitosans [111]
and from Fe(II), Co(II), and Zn(II) by EDTA-functionalized silica-poly(allylamine) composite material [115] has been reported. Shiraishi et al. [110] showed the separation of Cu(II), Ni(II), VO2+, Zn(II), Co(II), and Mn(II) using EDTA-silica gel.
1.3.3 Preconcentration of trace amounts of metals using chelating adsorbents
Determination of trace amounts of metals from environmental samples is highly important. The analysis equipments such as the inductively coupled plasma mass spectrometry (ICP-MS), however, are very sensitive for matrix effects and high concentrations of salts, for example, considerably decrease their detection limits. Preconcentrating metals by chelating resins is the most promising technique to obtain reliable and repeatable results in metal analysis.
Prior to ICP-MS analysis, commercial IDA-functionalized Muromac A-1 chelating resin was successfully used to preconcentrate 15 rare earth elements from seawater [143]. For similar equipment, over 85% recovery of most of the trace metals (10 different) from three kinds of seawater samples was obtained using Chelex 100 (also IDA-functionalized) packed minicolumn as a preconcentration unit [144]. The benefit of the APCA-functionalized chelating adsorbents is that they can selectively bind trace amounts of heavy metals in the presence of high amounts of salts. For NTA Superflow resin almost 100% recovery was obtained for Cu(II) and Fe(III) at pH range 2-6 from 100 µm metal solutions. For Co(II), Ni(II), Zn(II), and Cd(II), however, pH above 5 was required for their total recovery [103].
2 OBJECTIVES AND STRUCTURE OF THE WORK
Study of heavy metal adsorption by EDTA/DTPA-functionalized adsorbents
The first aim of this work was to study general adsorption properties of EDTA/DTPA-functionalized silica gel and chitosan materials from pure metal solutions in different experimental conditions (Papers I-III). The effect of pH, contact time, and metal concentration as well as regenerability of the spent adsorbents was investigated. Moreover, one of the objectives was to suggest reaction mechanisms describing the adsorption processes.
Modeling of adsorption isotherms and kinetics
The aim of the modeling part was to find the isotherm and kinetic equations that could describe the obtained experimental data. Furthermore, it was important to verify the applicability of the model by comparing the properties of adsorbents with the theories behind the models. Besides work presented in Papers I-V, some additional modeling was conducted to compare all the isotherm models presented in sections 1.1.5 and 1.1.6 as well as investigate the effect of error function. Linear and non-linear regression was also compared. Paper IV in this work is almost purely theoretical and presents the fitting results of one- and two-component isotherms to very extensive experimental data range.
Application study of EDTA/DTPA-functionalized adsorbents
In application testing the main interest was to test DTPA-functionalized silica gel and chitosan in capturing Co(II) from its aqueous EDTA-chelate (Paper V). In the same study the effect of various solution matrices was investigated. Separation of metals was studied in two-component (Papers II and IV) and multimetal systems (Paper III). Finally, some previously unpublished data about the applicability of EDTA- and DTPA-silica gel as preconcentrating adsorbents is presented.