1,721,062 research outputs found
Sequestration of Pd2+ by polyamino-polycarboxylic ligands
No full textThe increase of the worldwide demand of “Platinum group elements” (PGE) for application in several fields such as industry, medicine, jewellery and, especially, in catalyst converter production, caused a noticeable increasing of PGE concentration in the
environment. Though palladium, among the anthropogenic PGE, is not the most abundant one, it is the most hazardous since it undergoes easily and quickly oxidation to palladium(II) when in contact with soils, with a consequent increase of its mobility in the environment. The presence of complexing agents, which form soluble complex species with palladium(II), favours the mobility of the ion with an increase of its availability to plants, animals and humans. Among anthropogenic complexing molecules, an important role is played by synthetic aminopolycarboxylic chelating agents (usually called with the acronym APC) whose concentration in the environment is progressively increasing owing to their considerable use in several fields (agriculture, industry, medicine) and a low biodegradability of most of them. The interaction of these ligands with palladium(II) ion leads to the formation of soluble complex species whose stability influences strongly the availability of palladium(II) in the environment. With the aim to assess the strength of interaction of Pd2+ with aminopolycarboxylic ligands, here we report the results of a systematic study, , on the formation of palladium(II) complex species with five APCs [ethylenediamine-N,N,N’,N’-tetraacetate (EDTA), (S,S)-Ethylenediamine-N,N′-disuccinic acid (S,S-EDDS), Nitrilotriacetate (NTA) and diethylenetriamine-N,N,N’,N’’,N’’-pentaacetate (DTPA) and triethylenetetraamine-N,N,N’,N’’,N’’’,N’’’-hexaacetate (TTHA)]. Owing to the high stability of the Pd2+ - APC complex species, the calculation of their stability constants
was very difficult and was possible only by combining the results obtained from two series of ISE-H+ potentiometric titration (in NaNO3 and in mixed NaNO3 /NaI ionic medium) and from ISE-H+ potentiometric /spectrophotometric titrations (in NaClO4). As expected, the stability of Pd-APC complex species is function of the number of carboxylic and amino groups present in the ligand molecules (e.g., logKPd(APC) = 37.00, 36.31, 23.60, 23.07 and 17.82 for TTHA, DTPA, EDTA, S,S-EDDS and NTA in Na+ ionic media, at I = 0.1 mol L-1 and T = 25°C). Results obtained on the stability of species in the Pd- S,S-EDDS show that this ligand, which is the most biodegradable APC ligand, can be used successfully as environmental friendly
chelating agent in substitution of the other less degradable APCs in all their application fields. From the stability data of the Pd2+ - APCs the sequestration capacity [expressed as pL50, i.e. the –log (APC concentration] necessary to bind the 50% of the metal ion) of the ligands under investigation towards palladium(II) ion was determined in the pH range considered. The pL50 is easily correlated to important physico-chemical parameters (pH, ionic strength, temperature, etc.) as shown in the Figure, were the dependence on pH of pL50 of the APCs towards Pd2+ ion is reported. As can be seen, TTHA and DTPA show almost the same sequestering ability in the pH range 2 – 10, clearly higher than that of EDTA, S,S-EDDS and NTA where a minor number
of amino and carboxylic groups is present in the molecule
Interaction of Dioxouranium(VI) ion with aspartate and glutamate in NaClaq at different ionic strengths
No full textThe formation of complexes species of the dioxouranium(VI) ion with aspartic and glutamic acids was
studied in the pH range of 3 to 6 at 25 °C by potentiometric measurements (H+-glass electrode). Results
gave evidence for the formation of the following species: (UO2)A0, (UO2)AH+, and (UO2)2A(OH)2
0 (A2- ) a glutamic or aspartic ligand). Investigations were carried out in a NaCl ionic medium at I (0.1, 0.25, 0.5,
and 1.0) mol L-1. The dependence on ionic strength of the formation constants was analyzed by the specific
ion interaction theory (SIT) model. The formation constants at infinite dilution, obtained using this model,
are log â110 ) 8.53 ( 0.03, 8.37 ( 0.05; log â111 ) 13.60 ( 0.05, 13.42 ( 0.02; and log â21-2 ) 3.31 ( 0.05,
2.98 ( 0.03 for glutamate and aspartate, respectively, where indexes for the overall formation constant
âpqr refer to the equilibrium pUO22+ + qA2- + rH+ ) (UO2)p(A)qHr(2p - 2q + r). The specific interaction
coefficients are also reported. A mononuclear hydroxo species (UO2)A(OH)-, of great interest for very
low naturally occurring concentrations of uranium(VI), was hypothesized, with equilibrium constant log
K [(UO2)A0 + H2O ) (UO2)A(OH)-])-5.4 at t ) 25 °C and I ) 0 mol L-1. Speciation profiles for different
concentrations of UO22+ in the presence of both amino acids are discussed
Speciation of dimethyltin(IV) – and trimethyltin(IV) –carbocysteinate and – glutamate systems in aqueous media
No full textThe formation of complex species in the dimethyltin(IV) and trimethyltin(IV)-carboxymethyl-L-cysteinate (carbocysteinate) systems in NaClaq, at different ionic strengths, and in a multicomponent Na+, K+,
Ca2+, Mg2+, Cl- and SO42- medium representative of the seawater major composition, is discussed. Experimental results give evidence for the formation of the following species (L1⁄4carbocysteinate):
[(CH3)2Sn(L)]0, [(CH3)2Sn(HL)]+, [(CH3)2Sn(OH)(L)]-, [(CH3)2Sn(OH)2(L)]2- in the DMT–CCYS system, and [(CH3)3Sn(HL)]0, [(CH3)3Sn(L)]- and [(CH3)3Sn(OH)(L)]2- in the TMT-CCYS system. The ionic strength dependence of formation constants was taken into account by an extended Debye Huckel type equation and by the SIT (Specific ion Interaction Theory). Measurements were carried out also on the dimethyltin(IV)-glutamate and trimethyltin(IV)-glutamate systems in NaClaq, owing the strict similarity of glutamate and carbocysteinate. Results obtained show the formation of complex species having the same stoichiometry as those formed in the DMT- and TMT-carbocysteinate systems, with very
similar stability, confirming that carbocysteinate behaves as a dicarboxylic amino acid without involving the sulfur-bridge potential binding site in metal coordination
Capacità sequestrante di acido alginico e fulvico nei confronti di composti di triorganostagno(IV)
No full textSequestering ability of aminopolycarboxylic (APCs) and aminopolyphosphonic (APPs) ligands toward palladium(II) in aqueous solution
No full textThe binding capacity of three aminopolycarboxylates [nitrilotriacetic acid (NTA), ethylene-glycol-bis(2-aminoethyl ether)-N,N,N,N-tetraacetic acid (EGTA), and diethylenetriamine-N,N,N,NN-pentaacetic acid (DTPA)] and two aminopolyphosphonates {(1-hydroxyethane-1,1-diyl)bis(phosphonic acid) (HEDP) and [[(phosphonomethyl)imino]bis[2,1-ethanediylnitrilobis(methylene)]] tetrakis-phosphonic acid (DTPP)} toward palladium(II) ion was studied by potentiometric and spectrophotometric titrations at different temperatures (283.15 ≤ T/K ≤ 318.15) and ionic strengths (0.1 ≤ I/mol·dm -3 ≤ 1.0) in NaClO4. The hydrolysis of Pd2+ and the protonation of ligands were always taken into account in the speciation models of Pd2+/L systems investigated. Equilibrium reaching experiments were performed to check and confirm the reaching of the equilibrium state. Owing to the high stability of the PdL species (K > 1020), for EGTA, HEDP, and DTPP it was determined using exchange measurements with auxiliary ligands, such as iodide (I-) and ammonia (NH3). For the other ligands the stability of the PdL species was reported in the literature. The general speciation scheme consisted of mononuclear differently protonated species with general formula PdHiL and only in the case of the NTA ligand the formation of the PdL2 species was found. The stability of the PdL species is high: as an example we have log KML = 17.82, 22.60, 36.31, 23.49, and 27.27 for NTA, EGTA, DTPA, HEDP, and DTPP, respectively. Among the ligands, DTPA shows the highest formation constants and sequestering ability, evaluated using the pL0.5 parameter, as well. The complex formation reaction is always exothermic and in general the entropic contribution to the stability is dominant. Some empirical relationships were found to model the dependence of the formation constants on the number of protons and of the sequestering ability on pH
Palladium(II) sequestration by natural and synthetic chelating agents
No full textPalladium belongs to the so called “Platinum group elements” (PGE), and it is normally present at very low concentration in the environment. In the last twenty years, there has been an increasing PGE demand in different fields such as medicine, electronics, jewellery and car industry. Moreover, palladium (and other PGE) is broadly used as catalyst in a wide number of reactions. The high catalytic properties of PGE favoured their use in the production of catalytic converters. Unfortunately, this caused a noticeable increasing of PGE concentration in the environment, mainly constituted by platinum, with a particular accumulation in urban areas. However, since 1993 platinum has been increasingly replaced by palladium as the predominant substance in catalysts. Now the release of palladium in automotive catalysts is becoming just a critical problem as that of platinum. Furthermore, whilst the emission, the environmental distribution and the possible health effects of platinum have been widely investigated, there is a clear lack of information concerning palladium. Palladium also represents an environmental risk higher than platinum, because it easily and quickly undergoes oxidation processes when put in contact with soils. If the oxidation of palladium may represent a serious environmental problem, at the same time it could represent an advantage for soil and water remediation, if an appropriate chelating agent for the metal ion removal is found. Since the sequestering ability of a chelating agent is strictly related to the stability of the complex species formed with the metal ion to be removed, the speciation studies are of great importance, and represent a fundamental step for the understanding and the optimization of the whole sequestration process. To this end, we focused our attention on the sequestering ability of a natural polyphosphate chelating agent, the so called phytic acid [myo-Inositol hexakis(dihydrogen phosphate)] and of three synthetic chelating agents: the ethylenediamine-N,N,N’,N’-tetraacetate (EDTA), the diethylenetriamine-N,N,N’,N’’,N’’-pentaacetate (DTPA) and the triethylenetetraamine-N,N,N’,N’’,N’’’,N’’’-hexaacetate (TTHA). In this contribution, a series of ISE-H+ potentiometric titrations were carried out in aqueous solutions containing different amounts of the ligands used and of palladium(II) ion, in NaNO3aq at I = 0.1 mol L-1 and t = 25°C. In some cases, some measurements were also performed in NaClaq. The use of two different supporting electrolytes was useful for the evaluation of the effect of the ionic medium on the speciation of Pd2+, with particular reference to the influence of chloride ion. In fact, palladium(II) ion forms stable complex species with this anion, whilst nitrate is weakly interacting with Pd2+. Due to the great tendency of palladium(II) to hydrolyze even at low pH, some ISE-H+ titrations were also carried out to study the acid-base behaviour of Pd2+ ion in the same experimental conditions of Pd2+-ligand systems investigated. Though the literature on the hydrolysis of palladium(II) ion in different ionic media and ionic strengths is abundant, this check was necessary owing to the evident discrepancies noticed in terms of both the number of hydrolytic species and the hydrolysis constant values reported by the different authors. The sequestering ability of the investigated ligands toward Pd2+ was then evaluated by the calculation of various pL50 values in different conditions. Finally, the dependence of pL50 on pH was modelled by simple empirical relationships
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