4.4 Novel bisphosphonate gelators (Paper IV)
4.4.4 Structural properties by X-ray diffraction
The synthesis products and xerogels of 5d-5f produced perceptibly similar XRD patterns which indicated moderate crystallinity of the samples (Figure 11 in Paper IV). Generally, an increase in aliphatic chain length could induce a one-dimensional change of the unit cell, since the long aliphatic chain favors a parallel packing motif due to the repetitive van der Waals interactions that are gradually enhanced by an increase in the alkyl chain length (Boczula et al. 2012, Gbabode et al. 2006, Moreno-Calvo et al. 2009, Peuronen et al. 2012, Zuniga & Chapuis 1983). Thus, this packing mechanism along with the capability to form hydrogen bonds between the phosphonic acid groups (and/or with solvent molecules;
water in this case) are presumably the prevailing packing forces that most likely guide the structures to form parallel-packed structure modifications, which in part may have contributed to the relatively high crystallinity of the synthesis products and the corresponding xerogels of compounds 5d-5f. Compound 7 and its corresponding xerogel, on the other hand, were poorly crystalline, which meant that their structural characteristics remained somewhat unclear.
5 Conclusions and future perspectives
In this thesis, novel applications related to BPs were discovered and investigated. The utilization of the metal chelation properties of BPs in the waste water purification as well as in the removal of metals from the soil by using a hyperaccumulative plant was investigated. In addition, a completely novel characteristic of BPs, the formation of organo- and hydrogels was discovered.
The purification of waste waters containing harmful metal ions is a current topic constantly discussed in the media. The protection of the environment from metals originating from mining and industry is a topic attracting more and more attention. It does seem that the purification methods currently available (Mukherjee et al. 2013), such as precipitation (Esmaeili, Mesdaghi & Vazirinejad 2005, Fabiani et al. 1997, Hintermeyer et al.
2008, Inglezakis, Loizidou & Grigoropoulou 2003, Kanagaraj, Chandra Babu & Mandal 2008, Mohan & Pittman Jr 2006, Wu et al. 2008),ion/exchange (Anirudhan & Radhakrishnan 2007, Cetin et al. 2012, Liu et al. 2001, Sahu et al. 2009),adsorption (Dahbi et al. 2002, Fahim et al. 2006, Fathima, Raghava & Nair 2012, Mahmoud et al. 2010, Mohan, Singh & Singh 2006, Rivera-Utrilla & Sanchez-Polo 2003, Tahir & Naseem 2007) and membrane processes (Barakat & Schmidt 2010, Fabiani et al. 1997, Gomes et al. 2010, Hintermeyer et al. 2008, Religa, Kowalik & Gierycz 2011) do not meet well enough the needs of industry. The novel method presented in this thesis using sparingly soluble BP for Cr3+ removal confers several benefits compared to the methods currently in use. Firstly, there is no need to use any extra material, such as resin, during the process, and the solid BP is readily filterable from the solution after the metal chelation. In addition, it lacks the need for a precipitation step which is mandatory for soluble chelating agents. Moreover, the production costs are low:
the syntheses of the BPs are straightforward with good yields and they are based on inexpensive starting materials. The regenerability of these compounds also diminishes the operation costs. Another advantage of these compounds is the wide pH range of efficient complexation of Cr3+. However, to further improve the usability of BPs in general in waste water purification, they could be conjugated to porous silicon material which has a large surface area leading to the maximal metal collection capacity of the material. In addition, a hybrid material of this kind would be better suitable for flow-through systems.
The discovery of the Cr3+ selective BP encourages for the search of other metal selective BPs to be utilized in this manner. First of all, the mechanism of this adsorbtion reaction would be worthwhile clarifying in the future e.g. by using single crystal and powder XRD, surface area measurements, SEM, transmission electron microscopy (TEM) and zeta potential. In addition, molecular modeling could be exploited for understanding the metal chelation process. However, the large size of the ligands forming the system might make the task challenging. Based on the studies related to this thesis, the long alkyl chain as a substituent in the BP structure is a favorable characteristic concerning both the metal chelation as well as the low solubility required for applicability. On the other hand, the presence of a phenyl group as the substituent was not beneficial for the metal binding.
These relationships between the structure and the affinity for metal ions could be elucidated in the future by preparing new classes of BP structures and testing their metal chelation abilities for different groups of metal ions with variable oxidation states, starting from the transition elements, such as Mn(II), Cu(I)/(II), Zn(II), Co(II) and Ni(II).
Furthermore, the divergent behavior of the BP ligands in metal chelation regarding pH, temperature and contact time could be utilized for the selective recovery of the metal ions.
Another group of interest for selective chelators would be the rare earth elements, such as Sc(III), Eu(III), Y(III) and La(III) for which the extraction from the ore is challenging and requires novel methods.
Despite the extensive research in the field of herbicidal activity of BPs, the other beneficial properties of BPs, such as metal chelation, have not been previously considered to be utilized with plants. Phytoremediation, the use of plants to remove pollutants or make them harmless in soil, is a putative solution for the ever-increasing problems originating from water and soil contamination. Metal chelators, such as EDTA have been added into the soil in order to achieve increasing metal bioavailability, better uptake and improved translocation. (Ali, Khan & Sajad 2013, Vamerali, Bandiera & Mosca 2010, van der Ent et al.
2013, Vangronsveld et al. 2009) Thus, it was of interest to find out if BPs could act in the same way as effective metal chelators. Nonetheless, in our experiments, BPs did not increase the metal concentration in N. caerulescens. However, another beneficial effect was observed: by treatment with a sparingly soluble aminoBP which had a long alkyl chain in its structure (1h, see Table 3) the shoot biomass of the plant was increased, especially in the Ni-spiked soil thus, enhancing the shoot Ni removal from the soil. Since BPs have not been noted previously to have such an impact, it would be important to resolve the mechanism of this phenomenon. Firstly, the possible mode of transfer of the solid BP to the roots of the plant from the soil and further from roots to shoots needs to be clarified, for example by using a mass spectrometric method. In addition, the effects of the BP on FPPS expression could be investigated by molecular biological methods. The possible practical application of the elevated Ni removal by BP could be the purification of Ni contaminated soils. Hence, the efficacy of the method should be verified in Ni rich soil derived from Ni contaminated areas.
The discovery of BP gelator compounds especially in the case of hydrogelators can be considered of as a major advance, taking into account the importance of BPs in medicine and pharmacy. The chemical and physical nature of the formed gels were investigated using several analytical techniques, including liquid and solid state NMR and IR spectroscopy, scanning electron microscopy, powder X-ray diffraction, and thermal analysis. A detailed picture of the solid state structures of the synthesis products as well as the corresponding xerogels were drawn together with the thermal degradation and hydration levels of the compounds. The results showed that water plays an important role in the hydrogels formed by compounds 5d-5f (Table 3), however the function of the water was different in the case of compound 5e, in which the water molecules are more tightly bound within the gel network. The long alkyl chains combined with the ability of the phosphonate groups to form hydrogen bonds most likely guide the structures to form parallel-packed structure modifications, contributing to the high crystallinity found in compounds 5d-5f and their xerogels, which also showed potential liquid crystal properties.
In the future, to study the suitability of the gels for different purposes e.g. drug delivery related applications, the stability of the gel composition with additional substances (drug) and the release of drug from the gel will need to be tested.
6 Summary
The core of this thesis is summarized in Figure 12 where the central applications of BPs mentioned in the literature review as well as the novel applications developed in this study (Papers II-IV) are presented. The medical applications in the blue frames still clearly dominate the picture. However, the new applications related to this work (outlined in green) are on the other side with the non-medical applications, with hydrogelators placed in the middle for their still unknown possible uses. The location of the new applications highlights the versatility of BPs: Even though these compounds have been studied for decades, there is still a huge potential for their exploitation in different areas for revolutionary inventions to be made in the future.
Figure 12. Summary of applications related to bisphosphonates
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