Over the last couple of decades, metropolitan areas around the world have been engaged in a multitude of initiatives aimed at upgrading urban infrastructure and services, with a view to creating better environmental, social and economic conditions and enhancing cities' attractiveness and competitiveness. Reflecting these developments, many new categories of 'cities' have entered the policy discourse:
The adsorption of phosphonate inhibitors at the crystal surface of gypsum has been determined from the decrease in phosphonate concentration of the bulk solution. The phosphonate was therefore oxidized to orthophosphate and spectrophotometrically determined as a molybdovanadophosphonc acid complex. The adsorption isotherms of AMDP at pH 5 and HEDP at pH values of 5 and 7 were measured at 25°C. From a comparison with the inhibitor effectiveness of both compounds during suspension growth experiments under similar conditions the surface coverage needed for growth inhibition was concluded to be 4-5%. The growth process appears to be blocked when all active growth Sites provided by the steps upon the crystal surface, are occupied by adsorbed phosphonate ions. The phosphonate adsorption process appears to be irreversible and almost instantaneously completed. Differences in inhibitor performance of AMDP and HEDP at pH 5, or of HEDP at pH 5 and 7, can be explained from a different surface coverage of the gypsum crystals.
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