Being technological processes, sorptton and copreclpitation are Important in chemical manufacturing plants. They also have an important bearing on the purification of natural water and mine water involving removal of organic and inorganic impurities by aluminum and iron oxyhydrates (hydroxides).At the Mining Institute, Siberian Branch, Academy of Sciences of the USSR comparative investigations have been performed on removal of finely dispersed organic and inorganic impurities by reagent coagulation and electrocoagulation. It has been experimentally established that iron hydroxide precipitates obtained electrochemically have very high adsorption properties. Table 1 gives an example of water purification; it will be seen that electrocoagulation treatment of water leads to more complete removal of both finely dispersed mechanical impurities and impurities of the salt type.The use of electrocoagulation in industry is so effective that in certain technological water preparation schemes for special technological purposes, distillation and the use of chemical reagents have been eliminated. However, the adsorption properties of the precipitates have been assessed largely from the difference between the initial and final impurity concentrations in the water;, the nature of the adsorption processes and the cause of the marked difference between the results of water processing by chemical and electrochemical procedures have therefore remained obscure.The sorption parameters of hydroxide precipitates depend primarily on their chemical nature and structure. A comparison of the structural properties of precipitates obtained by chemical and electrochemical procedures is therefore of considerable theoretical and practical Interest.Sorption systems involving hydroxide precipitates are extremely complex. On the one hand the indefinite structure of the precipitates at the moment of formation causes considerable difficulties in the investigation and consistent description of the phenomena occurring in these systems; on the other hand, in our opinion, in this indefiniteness we have a clue to the development of more effective inorganic sorbents.The structure of pure iron hydroxide precipitates obtained by chemical reactions has been fairly thoroughly investigated. For example, Glemser [1] assumes that the Fe203--H20 system may contain the four following modifications (forms): c~-FeOOH (goethite), ~-FeOOH, ~-FeOOH (lepidocrocite), and ferromagnetic 6-FeOOH. It is also known that the highest adsorptlvity is exhibited by hydroxide precipitates at the moment of their formation. Aging of precipitates, which is assumed to be a combination of sequential and independently parallel processes, depends on the aging time, the pH of the medium, the temperature, and, to a considerable degree, the presence of impurities which can retard or accelerate this process. Ryzhak et al., [2] showed that even freshly precipitated hydroxide, obtained by mixing 1 N solutions of FeC13 and NaOH, is already slightly crystallized after 10 min and consists predomi...
In the ore mining industry, to purify effluent from mechanical impurities wide use is made of coagulation by aluminum and iron oxyhydrates. It is important to improve this method and to increase its efficiency.It is known that in most cases aluminum oxyhydrate freshly precipitated by the reagent method remains amorphous to x rays for a long time and is an amorphous phase containing admixtures of polymorphic modifications --boehmite, bayerite, and hydrargillite. Spontaneous aging of the oxyhydrate leads to the appearance of the end products --the crystalline monohydrate (boehmite) and trihydrates (5ayerlte and hydrargilllte) [I].The capacities for adsorption and chemical and catalytic activity in the various modifications of aluminum oxyhydrate are in the order boehmite > bayerite > hydrargillite.It is also known that when polymorphlc modifications of chemical aluminum oxyhydrate react with ions of salt solutions, the changes in the structure of the oxyhydrate (formation of a sorption complex) occur most markedly in boehmite, less so in bayerite, and have not been detected at all in hydrargillite.The latter modifications (5ayerlte and hydrargillite) are similar inasmuch as most of the OH groups lie in the inner sphere of the aquohydroxocomplexes. Hydrargillite, which has the lowest chemical (including sorptive) activity, has the most stable crystal lattice --a sign of stability of the modification. Boehmite, which has the greatest sorptive activity, changes during aging to a more stable modification --bayerite or hydrargillite.It has been observed that electrochemically synthesized A1 and Fe oxyhydrates have greater sorptlon capacity for water impurities than oxyhydrates obtained by hydrolysis of salts of these metals. The cause of this phenomenon is unknown. However, considering the results [2-3] of a study of certain structural properties of oxyhydrates of Fe (III), obtained chemically and electrochemically, by analogy we can advance the hypothesis that the main cause of the high activity must be sought primarily in the chemical structures of these compounds.In this article we investigate certain structural features of A1 oxyhydrates obtained chemically and electrochemically.Precipitates I were obtained by mixing a 0.3 M solution of chemically pure AICIs and a 1.2 M solution of NaOH in distilled water at 18-20~To the resulting aluminate solution we added a 0.3 M solution of HCI. The oxyhydrate precipitate formed according to the reaction equationsPrecipitates II were obtained by anodic solution of chemically pure aluminum in a field of dc current. The electrolyte was distilled water with added NaC1. The amount of additive was calculated from the amount of chlorine ions added to the distilled water in the composition of AICIs and HCI in obtaining precipitates I. In both cases, the amount of precipitate, its accumulation, the precipitation pH, and the temperature were kept constant. To remove impurity ions, the precipitated oxyhydrates were washed in thrice their volume of distilled water and dried at room tempera...
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