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Magnesium is generally considered to be one of the most attractive base materials for biodegradable implants, and many magnesium alloys have been optimized to adjust implant degradation. Delayed degradation, however, generates prolonged presence in the organism with the risk of foreign body reactions. While most studies so far have only ranged from several weeks up to 12months, the present study provides data for complete implant degradation and bone regeneration until 24months, for two magnesium alloys (ZX50, WZ21) with different degradation characteristics. μCT monitoring, histological staining and LA-ICP-MS illustrate the distribution of the elements in the neighboring bony tissues during implant degradation, and reveal in particular high concentrations of the rare-earth element Yttrium.

A sample of goethite was mixed for periods which ranged from 2 hours to 8 weeks with solutions of dilute nitrate salts of Pb, Hg, Cd, Zn, Cu, Ni, Co, Mn, Cr and Al. The amount of sorption after each period was measured for an appropriate pH range for each metal. The sorption behaviour was characterized both by using characteristics of the sorption curves such as the pH at which half of the added metal was sorbed (pH 50 ) and by fitting a model in which sorption was mainly characterized by an affinity constant and by a diffusion constant. Initial sorption, whether characterized by the pH 50 or by the affinity constant, was closely correlated with the appropriate dissociation constants of the metals. The greater the affinity of the metals for hydroxide ions, the greater their affinity for the goethite surface. The metals differed in the rate at which they continued to react with the goethite. Lead had the slowest continuing reaction, cobalt the fastest. The continuing reaction was due to diffusion into the particles. It was characterized by the fitted diffusion constant and by the change with time in the pH 50 . For seven of the eight divalent metals, these were correlated with the ionic radius of the metals: the larger the radius, the slower the reaction. For Al and Cr, rates were slower than would be expected from the ionic radii and we suggest this shows that these ions react as the larger M(OH) 2þ ions. The behaviour of Ni was consistent with oxidation of the surface species and diffusion of Ni(OH) 2þ ions. The continuing reaction was similar to that observed when metal ions react with soils and suggests that their reaction with iron oxides is important in soils. The results also show that studies in which sorption is measured at only one period of reaction are incomplete and the application of equilibrium models to such results is misleading.

A multidomain goethite was analysed with atomic force microscopy (AFM) to obtain information about the size and shape of pores on the surface, which are important for sorption processes of cations and anions. Wedge-shaped pores > 100 nm long at the surface occurred at the domain boundaries. They were 20-30 nm wide, becoming narrower towards the interior of the crystals to 2 nm and less. The AFM showed these pores to be at least 4-6 nm deep. The shape of the pores suggests that smaller pores continue much further into the interior of the crystals. Besides the irregularities between domains, randomly distributed steps of < 1 nm occurred. Hydrothermal treatment did not change crystal size, but made the surfaces smoother and reduced both the size and number of the pores and also the BET surface area (from 75.3 to 35.1 m2 g-'). The existence of meso-(2-30 nm width) and micropores ( < 2 nm width) in goethite crystals explains the slow diffusion of heavy metals, phosphate and other ions into goethite particles. Therefore, the external and internal surfaces of goethite crystals and their spatial heterogeneity have to be taken into account in models of ion adsorption by goethite.