Caracterizacion por difraccion de rayos-X, analisis termico y espectroscopia Raman de los cristales mixtos Li(NH4)(1-x)KxSO4

En este trabajo se estudia la preparaci�� n y caracterizaci�� n de las fases de los cristales mixtos de f ��rmula Li(NH4)1-xKxSO4. La caracterizac��i n se ha efectuado por an�� lisis t�� rmico con DSC y ATD, difracci ��n de rayos-X sobre polvo cristalino con temperatura variable, difracci n de rayos-X sobre muestra monocristalina a temperatura variable a fin de determina su estructura cristalina y por espectroscop"a Raman a temperatura variable. Se han obtenido dos tipos de fases. Una soluci n s lida con 0.94 < x < 1 que presenta los mismos tipos estructurales que el LiKSO4, pero seg n el m todo de cristalizaci n pueden aparecer nuevas fases que no presenta el LiKSO4. El segundo tipo de compuesto tiene por f rmula Li(NH4)0.53K0.47SO4, el cual presenta una estructura hexagonal con par metro a �� 3 aLiKSO4. Este compuesto tiene por encima de la temperatura ambiente una sola transici n a 463K.

Resposta de les esp��cies algals a canvis en la disponibilitat de nutrients

L'objectiu general del projecte de tesi ��s el d'estudiar la resposta de les esp��cies algals a canvis en la disponibilitat de nutrients (i.e. NH4+/NO3-/PO43-). La hip��tesi de partida ��s que les alteracions en les concentracions absolutes i relatives de nutrients provoquen canvis en l'activitat de les esp��cies algals que seran m��s o menys intensos en funci�� de l���esp��cie i que s��n fruit de les caracter��stiques eco-evolutives intr��nseques de cadascuna. Els objectius concrets es diferencien per l���escala temporal a la que s���observa la resposta, que va des de minuts fins a setmanes: Objectiu 1. Analitzar canvis a curt termini en l���activitat de les esp��cies degut a augments puntuals en la concentraci�� absoluta i relativa de nutrients (NH4+/NO3-/ PO43-). S'ha vist que algunes esp��cies redueixen l���activitat fotosint��tica a curt termini per destinar tots els recursos a l���assimilaci�� de nutrients (Elrifi i Turpin 1986). S���estudiar�� si aquesta pauta es troba de forma generalitzada i si respon a patrons evolutius, funcionals (mida, forma...) o ecol��gics (disponibilitat de nutrients). Objectiu 2. Analitzar, a mig termini, l���activitat de les esp��cies algals a diferents concentracions de NH4+/NO3-/ PO43-. Les respostes s���intentaran modelar, de manera que els par��metres es puguin considerar trets funcionals caracter��stics de les esp��cies. S'estudiar�� si la variabilitat en aquests trets respon a patrons evolutius o a d���altres trets funcionals com per exemple la mida. Objectiu 3. Analitzar com afecta, a llarg termini, la disponibilitat de nutrients en la composici�� d���esp��cies. Es detectar�� quines esp��cies s��n m��s sensibles a les condicions de NH4+:NO3- i N:P del medi, i com es relaciona aix�� amb les respostes espec��fiques trobades en els objectius anteriors.

X-ray diffraction, thermal analysis, and Raman scattering study of K2BeF4 and comparation to other member of the (beta)-K2SO4 family with ferroelectric -paraelectric transition

Thermal analysis, powder diffraction, and Raman scattering as a function of the temperature were carried out on K2BeF4. Moreover, the crystal structure was determined at 293 K from powder diffraction. The compound shows a transition from Pna21 to Pnam space group at 921 K with a transition enthalpy of 5 kJ/mol. The transition is assumed to be first order because the compound shows metastability. Structurally and spectroscopically the transition is similar to those observed in (NH4)2SO4, which suggests that the low-temperature phase is ferroelectric. In order to confirm it, the spontaneous polarization has been computed using an ionic model.

Mineralization of the recalcitrant oxalic and oxamic acids by electrochemical advanced oxidation processes using a boron-doped diamond anode

Oxalic and oxamic acids are the ultimate and more persistent by-products of the degradation of N-aromatics by electrochemical advanced oxidation processes (EAOPs). In this paper, the kinetics and oxidative paths of these acids have been studied for several EAOPs using a boron-doped diamond (BDD) anode and a stainless steel or an air-diffusion cathode. Anodic oxidation (AO-BDD) in the presence of Fe2+ (AO-BDD-Fe2+) and under UVA irradiation (AO-BDD-Fe2+-UVA), along with electro-Fenton (EF-BDD), was tested. The oxidation of both acids and their iron complexes on BDD was clarified by cyclic voltammetry. AO-BDD allowed the overall mineralization of oxalic acid, but oxamic acid was removed much more slowly. Each acid underwent a similar decay in AO-BDD-Fe2+ and EFBDD, as expected if its iron complexes were not attacked by hydroxyl radicals in the bulk. The faster and total mineralization of both acids was achieved in AO-BDD-Fe2+-UVA due to the high photoactivity of their Fe(III) complexes that were continuously regenerated by oxidation of their Fe(II) complexes. Oxamic acid always released a larger proportion of NH4 + than NO3- ion, as well as volatile NOx species. Both acids were independently oxidized at the anode in AO-BDD, but in AO-BDD-Fe2+-UVA oxamic acid was more slowlydegraded as its content decreased, without significant effect on oxalic acid decay. The increase in current density enhanced the oxidation power of the latter method, with loss of efficiency. High Fe2+ contents inhibited the oxidation of Fe(II) complexes by the competitive oxidation of Fe2+ to Fe3+. Low current densities and Fe2+ contents are preferable to remove more efficiently these acids by the most potent AO-BDD-Fe2+-UVA method.

X-ray diffraction, thermal analysis, and Raman scattering study of K2BeF4 and comparation to other member of the (beta)-K2SO4 family with ferroelectric -paraelectric transition

Thermal analysis, powder diffraction, and Raman scattering as a function of the temperature were carried out on K2BeF4. Moreover, the crystal structure was determined at 293 K from powder diffraction. The compound shows a transition from Pna21 to Pnam space group at 921 K with a transition enthalpy of 5 kJ/mol. The transition is assumed to be first order because the compound shows metastability. Structurally and spectroscopically the transition is similar to those observed in (NH4)2SO4, which suggests that the low-temperature phase is ferroelectric. In order to confirm it, the spontaneous polarization has been computed using an ionic model.

A general approach to the in situ energy butget of Eudendrium racemosum (Cnidaria, Hydrozoa) in the Western Mediterranean

An in situ energy budget of the hydropolyp Eudendrium racemosum (Cavolini, 1785) is presented. Ingestion and respiration rates and ammonium excretion were studied over two 24 h cycles, with two-hour sample intervals. The species ingested as much as 25.9% of its own biomass per day (minimum rate). Respiration was 1.62 ml O2 g-1 d w h-1 while excretion was 13.6 mM NH4 g-1dw h-1. We estimated that the species increased its biomass at a rate of 9.6% per day (Growth + Reproduction). This value is higher than those previously reported for other cnidarians. We can assume that the capacity of E. racemosum to survive - albeit for a limited period of the year - in the highly-competitive shallow-water communities is based on its high growth rate.

Effects of riparian vegetation removal on nutrient retention in a Mediterranean stream

We examined the effects of riparian vegetation removal on algal dynamics and stream nutrient retention efficiency by comparing NH4-N and PO4-P uptake lengths from a logged and an unlogged reach in Riera Major, a forested Mediterranean stream in northeastern Spain. From June to September 1995, we executed 6 short-term additions of N (as NH4Cl) and P (as Na2HPO4) in a 200-m section to measure nutrient uptake lengths. The study site included 2 clearly differentiated reaches in terms of canopy cover by riparian trees: the first 100 m were completely logged (i.e., the logged reach) and the remaining 100 m were left intact (i.e., the shaded reach). Trees were removed from the banks of the logged reach in the winter previous to our sampling. In the shaded reach, riparian vegetation was dominated by alders (Alnus glutinosa). The study was conducted during summer and fall months when differences in light availability between the 2 reaches were greatest because of forest canopy conditions. Algal biomass and % of stream surface covered by algae were higher in the logged than in the shaded reach, indicating that logging had a stimulatory effect on algae in the stream. Overall, nutrient retention efficiency was higher (i.e., shorter uptake lengths) in the logged than in the shaded reach, especially for PO4-P. Despite a greater increase in PO4-P retention efficiency relative to that of NH4-N following logging, retention efficiency for NH4-N was higher than for PO4-P in both study reaches. The PO4-P mass-transfer coefficient was correlated with primary production in both study reaches, indicating that algal activity plays an important role in controlling PO4-P dynamics in this stream. In contrast, the NH4-N mass-transfer coefficient showed a positive relation-ship only with % of algal coverage in the logged reach, and was not correlated with any algal-related parameter in the shaded reach. The lack of correlation with algal production suggests that mechanisms other than algal activity (i.e., microbial heterotrophic processes or abiotic mechanisms) may also influence NH4-N retention in this stream. Overall, this study shows that logging disturbances in small shaded streams may alter in-stream ecological features that lead to changes in stream nutrient retention efficiency. Moreover, it emphasizes that alteration of the tight linkage between the stream channel and the adjacent riparian zone may directly and indirectly impact biogeochemical processes with implications for stream ecosystem functioning.

Efecto de la sustituci��n cati��nica y ani��nica en las transiciones de fase del LiNH4SO4

El �� L��NH4SO4 (�� LAS) presenta dos transiciones de fase, una alrededor de 10 ��C y la otra a 186 ��C. La fase intermedia es ferroel��ctrica. En este trabajo se estudia el efecto que produce la sustituci��n parcial del cati��n amonio por rubidio y del ani��n sulfato por seleniato en dichas transiciones de fase. Se establece la zona de existencia de las soluciones s��lidas Li(NH4)-^_^Rb^S04 y LiNH4(SO^)^_^(Se04)^ en las que se mantiene la estructura del �� LAS. La presencia de aniones seleniato o de cationes rubidio afecta a las transiciones de fase de alta y baja temperatura, por lo que, tanto los tetraedros sulfato como el cati��n amonio intervienen en dichas transiciones. Hay un efecto cooperativo entre el desorden de los grupos sulfato y las distorsiones de los tetraedros amonio. El mecanismo que puede explicar estas transiciones es del tipo orden-desorden.