limination of phenolic aqueous wastes using new chemical oxidation processes
Despite of a growing concern about the problems of wastes elimination during the previous years, there is still a lack of processes in order to treat industrial aqueous wastes. Organic aqueous wastes and specially phenolic wastes, that can be either nonbiodegradable or toxic, give rise to one of the main problems. Landfilling disposal and related methods are a priori rejected as they appear to leaving the legacy of a problem we have net been able to solve rather than to considering our environment as being borrowed from the future mankind. Various oxidation techniques are suited for the elimination of this class of wastes. But, because of the environmental and economical drawbacks of incineration, it seems that liquid phase oxidation techniques should be preferred. The paper reviews : two liquid phase purification techniques using the chemical oxidation route; phenol being used as a test compound. The first technique is adapted from the wet air oxidation (WAO) process and uses molecular oxygen as the oxidizing agent. In the meantime, hydrogen peroxide is added at a low dosage and promotes the radicle reactions. Thus, the reaction temperature and pressure can be set at lower values (typically 160 °C, 25 bar) than usually. In this way, the conventional WAO process, which is very capital intensive because of temperature and pressure constraints is turned into a more affordable process. The second technique uses hydrogen peroxide as the oxidizer. it is associated to a ferrous salt as in the Fenton's reagent but it is run out under temperature (about 120 °C) so that a very important total organic carbon (TOC) removal efficiency con be obtained. This technique was named wet peroxide oxidation (WPO) process. As opposed to WAO, WPO needs only limited capital but generates higher running colts. Yet, both techniques can be regarded as efficient and economically satisfying in order to treat organic aqueous wastes containing fair amounts of phenol or phenolic compounds. The test compound was selected considering the frequent occurrence of phenol within the wastewaters of refineries, steel works and chemical industries. Their biological treatment is still very difficult for high concentrations despite of an important research activity. Treatment times and efficiencies of physicochemical methods are not but seldom satisfactory. Then, liquid phase oxidation methods have their whole interest. As it was reported that phenolic compounds (methylphenols, chloro-phenols) oxidation proceeds in a similar way than for phenol, the last molecule was considered for assessing the efficiency of both oxidation methods. The first method (WAO) was tested using a completely mixed batch reactor (stirred autoclave): The cold reactor was loaded with a phenol (2100 mg. 1-1) and ferrous sulfate (10 mg. l-11) solution al the convenient pH value (3.5). After heating at the rated temperature, the run was started by injecting instantaneously a large amount of oxygen (10 times the amount necessary). At the same time, a dosing pump was started and fed continuously hydrogen peroxide within the reactor all along the run (90 minutes). The total amount injected was usually 10 % of the amount necessary for a stoechiometric oxidation. The promoting effect of hydrogen peroxide on molecular oxygen is evidenced on figure 2 where the initiating period is shortened and on figure 3 where the oxidation efficiency actually obtained (curve 3) is greater than expected by adding the efficiencies of molecular oxygen and hydrogen peroxide oxidations if separated (curve 2). WAO promoted with hydrogen peroxide gave after 90 minutes better oxidation efficiencies at 160 °C than conventional WAO at 220 °C, then turning into a medium pressure process a high pressure one. The promoting effect of the peroxide is more marked at 160 °C than above 200 °C where a rapid decomposition occurs; dosages greater 15 % do not significantly increase the efficiency and dosages as small as 0,2 % have already a significant affect (see figure 5). Various compounds have been identified and the oxidation sequence is as follows : phenol -> dihydroxy-benzenes -> maleic acid -> oxalic, formic, acetic acids. Most of the remaining chemical oxygen demand (COD) of the oxidized solutions is acetic acid. Only more drastic experimental conditions allow its total removal. The WPO runs (second oxidation method) were conducted into a similar reactor. It was batch loaded with the phenol (2300 mg. 1-1) and ferrous sulfate (30 mg. l-1) solution at pH 3.5. After heating at 120 °C, the run was started and hydrogen peroxide was continuously fed using a dosing pump. The total amount injected all along the run (60 minutes) was the amount necessary for a stoechiometric oxidation. A similar oxidation sequence than reported hereon was observed; pyrocatechol, bydroquinone and oxalic acid were evidenced (figure 9) but, in this case, only very limited amounts of formic and acetic acids were detected. For the two processes, tables 2 and 3 summarize the material balances of the various products as a function of the oxidation time. A 90 % COD removal efficiency and a 70 % total organic carbon (TOC) removal efficiency is reported on figure 10. This result has to be compared with the TOC removal efficiencies (< 25 %) reported for the usual Fenton’s reagent at room temperature. The changes of the pH value and of the COD/TOC ratio (figure 11) during the run are easily explained by considering that oxalic acid is quite the sole product remaining after oxidation contrarily to promoted WAO where acetic acid is the major remaining product. Besides the production of radicles that bring on the oxidation process, a side-reaction decomposes hydrogen peroxide into molecular oxygen which is net active at such a low temperature. The competition between the two reactions makes optimum operating conditions to exist and to lead to a maximum efficiency of the process. Both processes bring on new methods in order to treat fairly concentrated phenolic solutions with a typical 90 % COD removal efficiency. The products remaining after oxidation (mainly acetic acid or oxalic acid) should not be regarded as a drawback of these processes. In actual fact, such compounds can be easily treated by adding a biological post-treatment unit to the chemical oxidation.