PubMed · 14485475
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M PERRAULT, J MODAI. 1961-09-10. [Chelation].. https://pubmed.ncbi.nlm.nih.gov/14485475/
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DFT calculations carried out on zinc acetate and zinc hydroxamates using the Hartree-Fock and B3LYP methods with the 6-311+G basis set give a series of stable pseudotetrahedral chelates (ZnL(2)) (L = OAc, FA, AA, NMeAA, GA, SA). Addition of a water molecule to these chelates gives the hydrates, ZnL(2).H(2)O, which in all cases are energetically more stable than the corresponding chelate. Hydrates formed from O,O coordinated hydroxamate species with a five-membered chelate ring contain water molecules occupying vacant coordination sites of the zinc atom. In contrast, those formed from zinc chelates with four-membered chelate rings contain a water molecule inserted into the chelate ring to give a six-membered ring in which one hydrogen of the water molecule is H-bonded to an oxygen atom of the zinc chelate with the water oxygen strongly bonded to the zinc. A slight lengthening of the H-bonded O-H bond suggests incipient hydroxide activation of water by zinc. In contrast, the O,O bonded hydroxamates do not incorporate water into the chelate ring nor activate the water in accordance with the ability of hydroxamic acids to inhibit zinc containing metalloenzymes.
In the present paper a process for removal of ions from wastewater or from washing water of contaminated soil by using the weakly basic water-soluble polymer polyethylenimine (PEI) as chelating agent and the Cu(2+) ion as model in combination with an ultrafiltration process was investigated. The complexing agent was preliminarily tested to establish the best operative conditions of the process. Next, ultrafiltration tests by using five different membranes were realised to check membrane performance like flux and rejection. Finally, the possibility for recovering and recycling the polymer was tested in order to obtain an economically sustainable process. Obtained results showed that complexation conditions depends on pH: indeed, at a pH>6 PEI-Cu(2+) complexes are formed, while at pH<3 the decomplexation reaction takes place. Saturation condition is 0.333 mg Cu(2+)/mg PEI, meaning a ratio PEI/Cu(2+)=3(w/w). UF tests showed good results using the PAN 40 kDa membrane reaching an average copper concentration in the permeate of 2 mg/l and a flux of 135.4 and 156.5l/h.m(2) at 2 and 4 bar, respectively. Metal rejection, permeate flow rate, and possibility to regenerating and recycling the polymer makes the polymer-assisted ultrafiltration process (PAUF) very interesting for metal ion removal from waters.
The purpose of this study was to evaluate the effect of step-back (SB) and crown-down (CD) techniques on crack development of stainless-steel Hedstrom files (H-files). Ninety-seven mesial root canals of freshly extracted human mandibular molars were selected. Six sets of H-files (#15, #20, and #25 ISO size each) were used with SB and CD techniques for the chemomechanical preparation of 8, 16, and 24 root canals using 2.5% NaOCl as an irrigant and EDTA gel as a chelating agent. Files were sterilized in an autoclave between successive uses. A seventh unused set served as control. After macroscopic examination all files were embedded in epoxy resin, ground, polished, and studied under a metallographic microscope. The maximum crack size of each file was measured by means of standard image-analysis procedures. Macroscopically the files presented no signs of plastic deformation, apart from a file #15, used with SB technique, which fractured after 16 root-canal preparations. The microscopic examination showed no cracks: on control files, on files used with the CD technique for 8 and 16 cycles, on file #20 used with the SB technique for 8 and 16 cycles, as well as on file 25 used with the SB technique for 8 cycles. Cracks were found on H-file #15 used with the SB technique for 8 and 16 cycles, on file 25 used with the SB technique for 16 cycles, and on all files used for 24 cycles with both SB and CD techniques. Under the conditions of this study it was concluded that the instrumentation technique is deeply implicated in the crack development. Crack propagation was accelerated when SB technique was used.