Hydrolysis product speciation

Hydrolysis reactions of Am(in) and Pu(VI) ions in CO2-free solutions of 0.1 M NaC104 were studied by means of solubility experiments using the oxide or hydroxide of 241 Am and 238Pu. The pH of solutions was varied from 3 to 13.5. All experiments were carried out under an argon atmosphere. The speciation of dissolved species was determined as far as possible by spectrophotometry. Various ultrafiltration membranes were applied to examine the proper phase separation. Stability constants of all possible hydrolysis products are presented and compared with literature data. 

Platinum speciation using IPC-ICP-MS has been achieved by Zhao et al. [50], An ODS C18 column and 1-heptanesulfonate ion-pairing reagent at pH 2.6 were used to separate the metabolites of cisplatin and cisplatin hydrolysis products. The low pH was required in order to retain thiol containing complexes. All complexes were resolved and urine and blood samples were analyzed by the speciation method. 

Tetravalent. The hydrolysis of tetravalent actinide ions can begin to occur in solutions with pH levels < 2. Under dilute conditions, species of the form An(OH) " (n = 1 4) are predicted however, most hydrolysis studies have only been able to identily the first hydrolysis product, An(OH) +. It should be noted that in all of these compounds the remainder of the coordination sphere is made up of bound H2O molecules. The end member of the speciation is the neutral An(OH)4 or An02-2H20. This complex has low solubihty but has been postulated to exist in solutions from solubihty experiments when using the isolated solid as the starting material. Under more concentrated conditions, polymeric materials have been postulated. In modeling the hydrolysis of thorium at concentrations greater than mM, polynuclear species of the form Th2(OH)2 +, Th2(OH)4 +, Th4(OH)g +, Th6(OH)i4 +, and so on, have been included. 

Hexavalent. As with most reactions, the hydrolysis of U02 + is the best studied of the hexavalent actinides. The hydrolysis of U02 + begins at pH 3, while the onset for the hydrolysis of Np02 + and Pu02 + each occur at a higher pH. The monomeric hydrolysis products of the uranyl ion, U02(0H) n = 1, 2) can be studied in solutions with uranimn concentrations less than 10 M. For solutions with higher uranium concentrations, multinuclear cationic species dominate the speciation, for example, (U02)2(0H)22+, (U02)3(0H)42+, and (U02)3(OH)s+. These cations have been crystallized from solutions with the formulas (U02)2(at2-OH)2(OH2)6 + and (U02)3(M3-0)(/x2-0H)3(0H2)6+ (21). For Np and Pu, the dimer of the first hydrolysis product, (An02)2(OH)2 + (22), has also been identified but not fully stracturally characterized. 

Another example of aqueous speciation that includes redox can be shown with the arsenic pe-pH diagram shown in Figure 1. Arsenic can exist in several oxidation states including As(-lll) as in arsine gas (ASH3), As(0) as in elemental arsenic, As(ll) as in realgar (AsS), As(lll) as in orpiment (AS2S3) and dissolved arsenite, and As(V) as in dissolved arsenate. Figure 1 shows the dominant dissolved species, arsenate and arsenite, and their hydrolysis products as a function of redox potential and pH based on the thermodynamic evaluation of Nordstrom and Archer (2003). These results show the dominance of hydrolysis for arsenate species, but it is of minor consequence for the arsenite species. 

Here we assume that the hydrolysis products are exclusively mononuclear and that the solution concentrations do not allow precipitation to occur (see Table Al). Speciation diagrams can thus be constructed from knowledge of global hydrolysis constants P which govern the formation of mononuclear hydroxides. For example, if a metallic cation M forms several hydroxide species M(OH) - M(OH) "-, M(OH), - , . . . , M(OH),/- " , the corresponding... 

Actinides have particular spectroscopic properties which are characterized primarily by the / - / transitions within the partially filled 5f shell [42] and thus by a number of relatively weak but very sharp absorption bands. The optical spectra of actinides are characteristic for their oxidation states, and to a lesser degree dependent upon the chemical environment of the ion [43]. Thus spectroscopic investigation provides information on the oxidation state of an actinide element [42] and also serves to characterize the chemical states, such as hydrolysis products [44], various complexes [37, 45, 46] and colloids [29, 40]. Hence, laser-induced photoacoustic spectroscopy (LPAS) with its high sensitivity can be conveniently used for the speciation of aqueous actinides in very dilute concentrations [17-28]. 

In the companion paper (Hanna and Sarac 1977b) the oxidation of benzilic acid is reported with emphasis on learning the mechanistic imphcations with respect to Ce(TV). As is generally observed, the rates are much slower in sulfuric acid solutions and the existence of a stable precursor complex is doubtful. The authors calculate the equihbrium speciation of Ce among the first two hydrolysis products, free metal ion, and 1 1, 1 2, 1 3 Ce-S04 complexes and find that the rate of reaction is most strongly correlated with CeSO ". In the presence of sulfuric acid a precursor complex of the form CeSO HL is proposed. In perchlorate solutions there is some evidence for a reactive CeHL species and an unreactive Ce(HL) " species. The precursor complex stability constant and first-order oxidation rate parameter for the former intermediate are in excellent agreement with those reported by Amjad et al. (1977) for mandelic acid. 

A recent example that demonstrates the power of HPLC to speciate metabolites of a "Tc radiopharmaceutical is provided by a detailed study of the Tc(I) complex [ c(CNC(CH3>2COOCH3 ] and its hydrolysis products [28]. Internal surface reversed phase HPLC also promises to be a very powerful tool in the analysis of biological samples [23,29]. Some of the potential pitfalls of HPLC analyses of "Tc radiopharmaceuticals have been noted [23] these largely stem from the difficulty in maintaining Tc complexes in defined oxidation states when the chemical concentrations of these complexes are in the order of 10" ° M. This problem is especially apparent in the HPLC analysis of bone-seeking "T c radiopharmaceuticals that incorporate Sn(II) [30,31]. 

Iron hydrolysis and solubility revisited Observations and comments on iron hydrolyses characterizations. Marine Chem. 70 23—38 Byrne, R.H. Kester, D.R. (1976) Solubility of hydrous ferric oxide and iron speciation in seawater. Marine Chem. 4 255—274 Byrne, R.H. Luo,Y.-R. (2000) Direct observations of nonintegral hydreno ferric oxide solubility products K Sq = [Fe ][H ] Geo-chim. Cosmochim. Acta 64 1873-1877 Cabrera, F. de Arambarri, P. Madrid, L. ... 

Eventually this process forms the neutral species Fe(H2O)3(OH)30, which precipitates as amorphous iron hydroxide, which may settles out of the water column. Figure 3 shows the predicted effect of pH on the relative concentrations of the various iron hydrolysis species with and without considering the iron hydroxide solid, which dominates the speciation above pH 3.0 at 1 dM total iron. The log of the solubility product of this solid is -38.8, indicating that iron is very insoluble at natural pH values. Over time, this metastable amorphous material converts to more thermodynamically... 

Oxidation-reduction (redox) reactions, along with hydrolysis and acid-base reactions, account for the vast majority of chemical reactions that occur in aquatic environmental systems. Factors that affect redox kinetics include environmental redox conditions, ionic strength, pH-value, temperature, speciation, and sorption (Tratnyek and Macalady, 2000). Sediment and particulate matter in water bodies may influence greatly the efficacy of abiotic transformations by altering the truly dissolved (i.e., non-sorbed) fraction of the compounds — the only fraction available for reactions (Weber and Wolfe, 1987). Among the possible abiotic transformation pathways, hydrolysis has received the most attention, though only some compound classes are potentially hydrolyzable (e.g., alkyl halides, amides, amines, carbamates, esters, epoxides, and nitriles [Harris, 1990 Peijnenburg, 1991]). Current efforts to incorporate reaction kinetics and pathways for reductive transformations into environmental exposure models are due to the fact that many of them result in reaction products that may be of more concern than the parent compounds (Tratnyek et al., 2003). 

Further hydrolysis of FeOH, followed by nucleation, yields the product Fe(OH)3(s). Radical species, HO2 and OH , and hydrogen peroxide, H2O2, are reaction intermediates in the proposed mechanism. The importance of specia-tion in kinetics goes beyond reactants and products in the overall reaction. Some of the features of the experimental rate law are accounted for if steps ii and iii are slow. See Wehrli (1990) for a thorough account of the role of reactant speciation in the rates of metal ion oxygenations. 

Studies carried out to evaluate the uptake of Fe by phytoplankton showed that only the dissolved metal is bioavailable and that a thermal or photochemical treatment is necessary for the colloidal Fe to become bioavailable (163). Moreover, the chemical form in which Fe is present can also affect its availability for plankton. The distribution of Fe(II) in the euphotic layer of the equatorial Pacific Ocean was examined by O Sullivan et al. (164). Its concentration is regulated by the balance between production and removal Fe(II) can be produced by microbial and chemical reduction, while the loss in surface water is controlled by biological uptake and by oxidation to Fe(III), subsequent hydrolysis, ageing and settling. The results showed maximum concentration near the surface and at the depths with higher chlorophyll a levels, the concentration ranging between 0.12 and 0.53 nM. Laboratory experiments carried out by the same authors showed that photoreduction can be an important source of Fe(II). Considering the different chemical speciation observed at various depths, different bioavailability can be expected in the examined zone. 

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