Azael Fabregat

Towards advanced aqueous dye removal processes: a short review on the versatile role of activated carbon.

During the last decade, several physico-chemical and biological techniques have been developed to remove colour from textile wastewaters. Some of these techniques rely on and many will profit from activated carbon (AC). The role of AC is versatile: (1) it acts as a dye adsorbent, not only in straightforward adsorption processes but also in AC-enhanced coagulation and membrane filtration processes; (2) it generates strong oxidising agents (mostly, hydroxyl (OH) radicals) in electrochemical dye oxidation; (3) it catalyses OH production in advanced oxidation processes; (4) it catalyses anaerobic (azo) dye reduction and supports biofilm growth in microbial dye removal. This paper reviews the role of AC in dye decolourisation, evaluates the feasibility of each AC-amended decolourisation technique and discusses perspectives on future research.

Wet air oxidation of phenol using active carbon as catalyst

Catalytic wet air oxidation is a promising alternative for the treatment of phenolic waste water which cannot be treated in conventional sewage plants. Catalytic wet air oxidation of an aqueous phenol solution was conducted in a fixed bed reactor operating in trickle flow regime. Either active carbon or a commercial copper oxide supported over γ-alumina was used as catalyst. The performance of both materials was compared in terms of phenol conversion in 240 h tests. The results showed that the active carbon, without any active metal supported, gives the highest phenol conversion. The supported copper catalyst undergoes a rapid deactivation due to the dissolution of the metal active species in the acidic medium in which the reaction takes place. On the other hand, the active carbon maintains a higher activity throughout the test, although a decrease of the phenol conversion was also observed due to both the loss of active carbon by combustion and the reduction of its surface area. The phenol oxidation was proved to occur through a first order mechanism with respect to phenol. After the ten-day run, the catalytic activity of the active carbon was found to be about eight times higher than that of the commercial catalyst, also showing high selectivity to the production of carbon dioxide.

Catalytic removal of phenol from aqueous phase using oxygen or air as oxidant

Abstract A preliminary study of specific catalysts used to oxidize organic compounds which are hazardous to the environment is presented. The catalytic oxidation of phenol in an aqueous solution using commercially supported copper oxides was studied in a continuous trickle-bed reactor at three temperatures (120, 140 and 160°C) and under three partial pressures of oxygen (0.6, 0.9 and 1.2 MPa) using molecular oxygen or air as oxidation agents. The influence of various parameters is presented. Over the range of conditions used, the oxidation kinetics are first order with respect to phenol and COD (chemical oxygen demand) reduction and half order with respect to the partial oxygen pressure. The results are similar when using air or oxygen as an oxidizing agent. Activation energies for phenol oxidation and COD conversion were found to be 85 and 76 kJ/mol.

Bimetallic catalysts for continuous catalytic wet air oxidation of phenol

Catalytic wet oxidation has proved to be effective at eliminating hazardous organic compounds, such as phenol, from waste waters. However, the lack of active long-life oxidation catalysts which can perform in aqueous phase is its main drawback. This study explores the ability of bimetallic supported catalysts to oxidize aqueous phenol solutions using air as oxidant. Combinations of 2% of CoO, Fe2O3, MnO or ZnO with 10% CuO were supported on gamma-alumina by pore filling, calcined and later tested. The oxidation was carried out in a packed bed reactor operating in trickle flow regime at 140 degrees C and 900 kPa of oxygen partial pressure. Lifetime tests were conducted for 8 days. The pH of the feed solution was also varied. The results show that all the catalysts tested undergo severe deactivation during the first 2 days of operation. Later, the catalysts present steady activity until the end of the test. The highest residual phenol conversion was obtained for the ZnO-CuO, which was significantly higher than that obtained with the 10% CuO catalyst used as reference. The catalyst deactivation is related to the dissolution of the metal oxides from the catalyst surface due to the acidic reaction conditions. Generally, the performance of the catalysts was better when the pH of the feed solution was increased.

Kinetic modelling of catalytic wet air oxidation of phenol by simulated annealing

A detailed reaction network for the catalytic wet air oxidation (CWAO) of phenol on a CuO/-Al2O3 catalyst is proposed in this study. The reaction network proposed accounts for all detected intermediate products of phenol oxidation overcoming the usual lumping of compounds. The model is composed by several consecutive and parallel reactions. The parameters of the model were adjusted using experimental data obtained from a continuous trickle bed reactor using air as oxidant at different temperatures (120–160 ◦ C) and oxygen partial pressures (0.6–1.2 MPa). Simple power law as well as Langmuir–Hinshelwood (L–H) expressions accounting for the adsorption effects were checked in the modelling of the reaction network. A robust non-linear multiparameter estimation approach called simulated annealing was used to simultaneously evaluate the high number of model parameters (up to 38). Approach by simple power law only succeeded in fitting phenol disappearance. Instead, when L–H expressions are incorporated for the intermediate reaction steps, the model accurately describes all the experimental concentration profiles, giving mean deviations below 8%. In addition, all estimated parameters have physical meaning. In particular, activation energies mostly agree with those reported in the literature.

Three-phase reactors for environmental remediation: catalytic wet oxidation of phenol using active carbon

Wet oxidation of phenol aqueous solutions was carried out in a fixed bed reactor operating in trickle flow regime. Mild conditions of temperature (1408C) and oxygen partial pressure (1‐9 bar) were used. Three active carbons and one commercially available supported copper catalyst were tested as catalytic material. Previous studies demonstrated that active carbon gives higher phenol conversion than conventional oxidation catalysts, although significant loss of active carbon due to combustion was also found. In the present study, the combustion of the active carbon during the process is highly reduced by lowering the oxygen partial pressure from 9 to 2 bar, maintaining an acceptable phenol conversion. The comparison of the performance of three different active carbons shows that their physical and chemical characteristics largely influence on the phenol conversion achieved. # 1999 Elsevier Science B.V. All rights reserved.

AQUEOUS PHASE CATALYTIC OXIDATION OF PHENOL IN A TRICKLE BED REACTOR: EFFECT OF THE pH

The catalytic oxidation of organic compounds in aqueous phase is a promising technique for waste water treatment. Obtaining efficient and durable catalysts and determining the optimal process conditions are the key to successfully implementing this treatment. Copper-based catalysts supported over either silica or γ-alumina were prepared for this purpose. This research studies the influence of the pH on the performance of these catalysts. Activity tests were conducted for nine days in a trickle bed reactor operating at 140°C using air as oxidant. The results show that the silica supported catalyst is very sensitive to the acidic medium which leads to very short lifetimes. At first, the alumina supported catalyst also quickly losses activity but subsequently it stabilises with a residual phenol conversion several times higher than that of the silica supported catalyst. For both catalysts, a higher pH reduces the rate of catalyst deactivation by preventing the leaching of the copper oxides and lengthening their lifetime. The atmosphere (air or nitrogen) during calcination does not change their performance.

Supported Cu(II) polymer catalysts for aqueous phenol oxidation

Supported Cu(II) polymer catalysts were used for the catalytic oxidation of phenol at 30 degrees C and atmospheric pressure using air and H(2)O(2) as oxidants. Heterogenisation of homogeneous Cu(II) catalysts was achieved by adsorption of Cu(II) salts onto polymeric matrices (poly(4-vinylpyridine), Chitosan). The catalytic active sites were represented by Cu(II) ions and showed to conserve their oxidative activity in heterogeneous catalysis as well as in homogeneous systems. The catalytic deactivation was evaluated by quantifying released Cu(II) ions in solution during oxidation, from where Cu-PVP(25) showed the best leaching levels no more than 5 mg L(-1). Results also indicated that Cu-PVP(25) had a catalytic activity (56% of phenol conversion when initial Cu(II) catalytic content was 200 mg L(Reaction)(-1)) comparable to that of commercial catalysts (59% of phenol conversion). Finally, the balance between activity and copper leaching was better represented by Cu-PVP(25) due to the heterogeneous catalytic activity had 86% performance in the heterogeneous phase, and the rest on the homogeneous phase, while Cu-PVP(2) had 59% and CuO/gamma-Al(2)O(3) 68%.

Nonlinear kinetic parameter estimation using simulated annealing

The performance of simulated annealing (S-A) in nonlinear kinetic parameter estimation was studied and compared with the classical Levenberg � /Marquardt (L � /M) algorithm. Both methods were tested in the estimation of kinetic parameters using a set of three kinetic models of progressively higher complexity. The models describe the catalytic wet air oxidation of phenol carried out in a small-scale trickle bed reactor. The first model only considered the phenol disappearance reaction, while the other two included oxidation intermediate compounds. The number of model parameters involved increased from 3 to 23 and 38, respectively, for the three models. Both algorithms gave good results for the first model, although the L � /M was superior in terms of computation time. In the second case the algorithms achieved convergence, but S-A resulted in a better criterion and kinetic parameters with physical meaning. In the more complex model, only S-A was capable of achieving convergence, whereas the L � /M failed. For the second and third model the solution of S-A could be further improved, when used as an initial guess for the L � /M algorithm. # 2002 Elsevier Science Ltd. All rights reserved.

Immobilisation of horseradish peroxidase on Eupergit®C for the enzymatic elimination of phenol

In this study, three different approaches for the covalent immobilisation of the horseradish peroxidase (HRP) onto epoxy-activated acrylic polymers (EupergitC) were explored for the first time, direct HRP binding to the polymers via their oxirane groups, HRP binding to the polymers via a spacer made from adipic dihydrazide, and HRP binding to hydrazido polymer surfaces through the enzyme carbohydrate moiety previously modified by periodate oxidation. The periodate-mediated covalent immobilisation of the HRP on hydrazido EupergitC was found to be the most effective method for the preparation of biocatalysts. In this case, a maximum value of the immobilised enzyme activity of 127 U/g(support) was found using an enzyme loading on the support of 35.2mg/g(support). The free and the immobilised HRP were used to study the elimination of phenol in two batch reactors. As expected, the activity of the immobilised enzyme was lower than the activity of the free enzyme. Around 85% of enzyme activity is lost during the immobilisation. However, the reaction using immobilised enzyme showed that it was possible to reach high degrees of phenol removal (around 50%) using about one hundredth of the enzyme used in the soluble form.