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The abundance of urea in the natural environment is dictated by the fact that it is one of the major products of mammalian protein metabolism. Due to the extensive use of urea in many branches of industry, it is produced in large quantities. Urea enters into the environment not only with wastewater from the production plants but also by leaching from the fields, agro-breeding farms, and the effluents from the plants using it as a raw material. There are many methods of urea removal, but most of them are still being developed or are very new. The methods themselves differ in terms of physicochemical nature and technological ingenuity. Many wastewater treatment methods include processes such as hydrolysis, enzymatic hydrolysis, decomposition in the biological bed, decomposition by strong oxidants, adsorption, catalytic decomposition, and electrochemical oxidation. In this work, methods of urea removal from aqueous solutions have been reviewed. Particular attention was paid to electrochemical methods.

Investigations on the anodic decomposition of urea using Ti/Pt and Ti/(RuO 2 -TiO 2 ) 40:60 electrodes were carried out. The kinetics of the process were examined in a periodic electrolyser. The effect of anodic current density, initial urea concentration, and sodium chloride concentration on the effectiveness of the basic process (average rate of urea decomposition, current efficiency, and unit power consumption) is discussed. When a Ti/Pt electrode is applied for urea removal from aqueous solution urea is not decomposed directly at the surface of the electrode, but rather in the bulk of the solution by hypochlorite formed during the process. When the Ti/(RuO 2 -TiO 2 ) 40:60 electrode is used for the removal of urea from aqueous solutions, the reaction of urea with chlorine adsorbed at the electrode predominates. In both cases non-toxic products of urea decomposition (N 2 , CO 2 ,) are formed. Comparison of the effectiveness of anodic decomposition of urea for the Ti/Pt and Ti/(RuO 2 -TiO 2 ) 40:60 electrodes in the periodic electrolyser at optimum process parameters has revealed that the former electrode is more favorable.

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In this work, a review of the literature concerning the surface modification of implants composed of titanium and titanium alloys by plasma electrochemical oxidation (PEO), also known as micro-arc oxidation (MAO), is presented. The application of this process allows for the formation of oxide layers with different porosities on implants. Moreover, it is possible to enrich these oxide layers with species contained in solutions used for anodising, yielding suitable surface chemical properties. Anodising titanium implants in solutions containing compounds of calcium and phosphorous leads to the formation of bioactive layers and significantly reduces the time required for the osseointegration of implant to bone. Studies of the PEO process with respect to titanium implants have been conducted by a large number of research centres, and their results have been applied to the production of a new generation of titanium implants.