Tailoring activated carbons as catalyst for catalytic wet air oxidation of phenol

Resum

Since the development of industrial activities in the whole world and specially nowadays, the access to clean water is more and more difficult. Over the past century our water consumption increased tenfold and, according to the World Health Organization, 1.1 thousand million people have no access to clean drinking water, while 2.4 thousand million lack proper sanitary provision. In addition, only 0.8 % of the world stock of water is actually fresh water, so that serious considerations on the availability of a constant supply of fresh water should be done. With this in mind, the need to have different techniques available to treat wastewater arises as a main concern, considering the great variety of chemical industries and activities than can produce contaminated effluents. Therefore, the detoxification of polluted water and its recycling when possible is mandatory for a sustainable and sustained development.For wastewater containing low to medium pollutant load, Catalytic Wet Air Oxidation (CWAO) yielded satisfactory results in laboratory studies, with the advantage that relatively mild operating conditions are needed, leading to lower investment and operation costs. Some activated carbons (AC) have been successfully used as catalyst in the CWAO of phenol in a trickle bed reactor (TBR) configuration. However, the reason for the catalytic activity of AC in CWAO is not yet completely understood. The objective of this work is to identify properties potentially responsible for the catalytic activity shown by some carbonaceous materials. With this goal, eight commercial activated carbons were tested as received and modified by several means (thermal treatment, acid wash and metal impregnation) and its catalytic performance evaluated in the oxidation of phenol. These modifications intended to highlight the effect of the surface oxygen functionalities, textural properties, i.e. surface area and pore volume, and iron (and other metals) content. Simultaneously, the activated carbons were physically and chemically characterised. The results confirm the hypothesis that mineral matter, specifically iron, is mainly responsible for the catalytic behaviour of activated carbons in the CWAO of phenol. However, having iron is not enough to be catalytically active in this reaction. It is necessary iron to be actually available in the carbonaceous matrix and in an appropriate state. Some final attempts for preparing tailored activated carbon from different raw sources still failed due to the lack of iron availability in the carbon matrix or the low mechanical stability of the material.The findings described in this PhD thesis open the doors for a better designing of tailored activated carbons from costless raw materials, allowing cheaper catalytic activated carbons to be available for wet oxidation of phenol. This fact should contribute to the wider implementation of CWAO for recovering polluted aqueous effluents that are reluctant to biotreatment.