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New catalytic adsorption procedure for eliminating phenols from waste waters

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The research group on the Structure and Chemistry of Nanomaterials has developed a new procedure that enables phenols and other toxic compounds, such as nitrophenol and chlorophenol, to be eliminated through a combination of adsorption and catalysis techniques. The invention is especially suitable for the treatment of waste waters contaminated mostly with phenolic compounds.

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Description

This process takes place in mild reaction conditions and is of potential interest in the purification of waste waters with a high content in phenols, originating from refineries, paper-making, pharmaceuticals and resins manufacturing plants, among others. The group has also applied the same principles used in the elimination of phenols for the formation of carbon nanoparticles. Currently, the most-commonly employed process in the purification of such effluents is treatment by biological degradation.

Thus, since a great variety of compounds can be treated in this way, it has become the most appropriate technique for treating waste waters of diverse origins, such as urban waste waters, for example. However, the presence of particular highly toxic organic compounds, like phenols, drastically reduces its effectiveness, to the point that it ceases to be viable.

In such cases, the alternative is to employ substances like activated carbon, which is one of the most versatile adsorbents with good properties for adsorbing a wide range of organic contaminants. However, once the adsorbent is saturated, it loses its properties, and subsequent regeneration can be expensive.

Another factor is that, due to the presence of particular impurities, the adsorption of phenols increases notably, and it is observed that this adsorption is, to a large extent, irreversible. This is very important because, although the elimination yield is increased, it also makes it more difficult to eliminate the phenols irreversibly from the adsorbent. Effectively, it is impossible to reutilize the adsorbent, which in turn makes its industrial application also impossible.

With this situation as the point of departure, the research group has managed to develop a new water purification technology based on the irreversible catalytic adsorption of phenolic compounds, similar in performance to that employing activated carbon but resolving the problem of eliminating the polymerized phenols from the surface of the adsorbent by means of in-situ combustion. This allows the adsorbent to be reutilized.

Basically, in the group's new technology, a catalyzer with adsorbent characteristics is applied. It therefore involves a combination of processes: in the first phase the contaminants undergo a process of polymerization and are then adsorbed on the oxide; in the second phase, the catalyzer is subjected to calcination, which leads to the oxidation of the organic residue.

For this the group proposes to employ oxides of manganese and mixed systems, supported on materials such as SiO2, Al2O3, CeO2 and CeZrOx, or in the form of composites. Thus, the proposed mechanism comprises two steps:

  • Irreversible adsorption by polymerization on the surface of the catalyzer.
  • In-situ combustion, by calcination, at temperatures below 200 ºC.

The second step, calcination, is of fundamental importance since it produces the complete combustion of all the organic matter adsorbed on the surface of the catalyzer, but without affecting its properties. The complete regeneration of the adsorbent is thereby ensured.

This is made possible by the prior formation of spherical nanoparticles of manganese and of carbon (originating from the organic compounds). This takes place as follows: when the oxides of manganese are immersed in aqueous phenol solutions, part of the manganese dissolves and carbonous nanoparticles are generated; these are characterized by having a homogeneous diameter due to the polymerization of the phenols.

This represents added value for the technology, since the processes of synthesis of carbon nanospheres have potential applications in the separation of gases, as molecular sieves, as supports for catalyzers, and in electrodes in lithium ion batteries.

Advantages

  • This is an alternative to the biological methods used traditionally for the treatment of waste waters, which have serious limitations due to the toxicity of phenols.
  • The method allows the adsorbent to be regenerated.
  • It enables phenolic compounds to be completely eliminated from waste waters.
  • It takes place in mild reaction conditions, resulting from the combination of catalytic and adsorption techniques.
  • Homogeneous and spherical nanoparticles are generated, which have several useful industrial applications.
  • It is less costly than conventional treatment methods and offers a greater facility of operation.

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