MO (VI), V(V), NB(V) eta TA(V)-aren zenbait hidrolisiri buruzko azterketak

Lan honetan aztertu da Mo(VI), V(V), Nb(V) eta Ta(V) hidrolisia, hau da, metal horiek uretan aurkituz gero ematen dituzten espezieen banaketa bai kontzentrazioaren arabera eta bereziki pH-aren eta indar ionikoaren arabera.

Metal horiek aleazio berezi batzuetan agertzen dira ezaugarri gehigarriak eman ahal izateko. Metalen hondarrak tratatu behar direnean bide hezeen bitartez, funtsezkoa da hidrolisiari dagozkion xehetasunak ezagutzea metal horien errekuperazioa eta berrerabilpena bideratzeko.

Mo(VI)-aren eta V(V)-aren hidrolisia neurri handi batez aurretiaz aztertuta izan denez, abiapuntu nagusia hidrolisiaren espeziazioa modelizatzea izan da, hau da oreka guztien formazio-konstante termodinamikoen balioak ematea eta indar ionikoarekiko mendetasuna koantitatiboki azaltzea.

Horretarako, gure lan-taldean garatutako Bromley-en eredu berriztatua izan da gakoa ereduak landu ahal izateko. Eredu horren bitartez, NaClO4tan eta NaCl-tan bildutako hidrolisiaren oreka konstante guztiak batu eta landu dira 1 < pH < 14 bitartean. Horrenbestez, oreka bakoitzaren formazio-konstante termodinamikoa eta espezie bakoitzaren elkarrakzio-parametroen balioak kalkulatu dira.

Horrez gain, Mo(VI)-aren hidrolisia eta konplexazioa HClO4 eta HCl giro azido kontzentratuan (0.1 M < Cazido < 6 M) aztertu da espektrofotometrikoki. Lan hau aurrera eramateko, indar ionikoa aldagarria izan da halabeharrez beraz, ohiko baliabide grafiko eta numerikoak ebazpen partziala eskaini dituzte hidrolisiaren oreka-konstanteen balioak emateko orduan.

Ebazpen numeriko zehatza lortu ahal izateko SPECA programa garatu behar izan da eta horren xehetasunak tesi honetan ematen dira. Izan ere, SPECA programak datu espektrofotometrikoak tratatzen ditu kontuan izanik absorbatzen duten espezie guztiak, espezie horien masa-balantzeen ekuazioen ebazpena eta ebazpen horretarako baldintza bakoitzari dagokion formazio-konstante estekiometrikoa eta elkarrakzio-parametroen balioak. Kalkuluak iteratiboki hiru maila horietan (absorbantziak, masa-balantzeak, aktibitatea) egiten ditu Levenberg-Marquard-Nash algortimoa erabiliz. Mo(VI)-aren hidrolisi azidoari dagokionez, programa honen bitartez bi espezie kationikoren eta beste bi klorokonplexu kationikoren formazioa zehaztu da.

Azkenik, Nb(V) eta Ta(V) hidrolisia modu potentziometrikoa aztertu izan da. Izan ere, kontuan izanda metal horien hidrolisa oso geldoa dela eta hauspeakinarik gabeko tartea oso estuan aurkitzen zela (10< pH< 12 Nb(V)-ren kasua) erabilitako metodoa balorazio potentziometrikoa-coulombimetriko automaziatua izan da.

In this work it is decribed the hydrolysis of Mo(VI), V(V), Nb(V) and Ta(V), i.e. the especiation of these metals in aqueous solution at different total concentration and, especially, at different pH values and ionic strenghts.

These metals are included in many different alloys to provide special physical and chemical features to the metallic alloy. This information is key to develop hydrometallurgical processes when metal scraps containing any of these metals are present and when the content of these metals is recycled.

The hydrolysis of Mo(VI) and V(V) has been thoroughly described in the literature so the main objetive of this work was to model the aqueous speciation. This means to account for all the thermodynamic formation constants of the different species and the effect of the variation of the ionic strength on the activity coefficients.

In our research group we have developed the modified Bromley model to explain the variation of the of the activity coefficents. In this particular case, the hydrolysis of these two metals in NaClO4 and NaCl and within 1 < pH < 14 has been accomplished and all the thermodynamic formation-constants as well as all the interaction parameters have been estimated.

In addition to these two theoretical works, the complexation of Mo(VI) in concentrated HClO4 and HCl (0.1 M < Cazido < 6 M) media was studied spectrophotometrically. In order to perform this work theionic strength changed from one measurement to other so the classical graphical and numerical procedures only provided partial solutions regarding the formation constants.

In order to accomplish a full numerical resolution of the mass-balance equations the program SPECA was built and the main features are provided in this Thesis. In fact, SPECA programs takes into account the distribution of all absorbing species, the resolution of all mass-balance equations of the chemical system considering the correspoding stoichiometric formation constants, and the variation of the activity coefficient as a consequence of the variation of the ionic strength. All the calculations are carried out iteratively in the mentioned three levels (absorbance, mass-balance, activity) making use of the Levenberg-Marquard-Nash algorithm. In the case of the Mo(VI) hydrolysis, we can conclude the formation of two cationic species of Mo(VI) and two more chlorocomplexes.

Finally, the hydrolysis of Nb(VI) and Ta(V) was studied potentiometrically. Taking into account the slow hydrolysis equilibria of these two metals and the narrow range of pH in which these species are found in solution (10 < pH < 12, for Nb) the experimental measurements were carried out with an automatic potentiometric and coulombimetric system.