Enhanced adsorption of phenolic compounds, commonly encountered in olive mill wastewaters, on olive husk derived activated carbons
Introduction
Liquid and solid waste, are a huge and always increasing problem for the industries and the environment. One of the most popular ways of dealing with solid waste is its use as a raw material for the production of activated carbons that can be subsequently employed for the removal of pollutants from liquid waste.
As adsorbents, activated carbons bear extremely large surface area, with varying porous structure consisting of a network of interconnected macropores, mesopores and micropores. Their surface area also presents great diversity in terms of surface charge and surface groups, depending on the original raw material and the way of activation. For a raw material to be used for activated carbon production, it should have high carbon content. Agricultural waste is commonly exploited this way. An abundant and practically costless such material for the Mediterranean region is olive husk, which is produced in large quantities as a by-product of the production of olive oil. Activated carbons from olive kernels or olive cake have been prepared and presented interesting properties (Stavropoulos and Zabaniotou, 2005, Cimino et al., 2005, Martinez et al., 2006).
One of the most popular and extensively researched families of compounds that are adsorbed onto activated carbon is phenols. Much emphasis has been posed on phenol and its derivatives (Dabrowski et al., 2005, Furuya et al., 1997, Calleja et al., 1993), as they are highly toxic and are present in many industrial wastewaters. PAH’s, dyes, herbicides, surfactants, and pharmaceuticals have also received much attention (Mastral et al., 2003, Purkait et al., 2005, Gallardo-Moreno et al., 2004, Mall et al., 2006, Terzyk and Rychlicki, 2000, Ternes et al., 2002). Polyphenols are a class of compounds with well-established bioactivity and in particular, antioxidant properties. They are found mainly in by-products of the food industry, such as winery by-products, tomato juice production waste and olive mill wastewater. Especially for the Mediterranean area, olive mill wastewaters (omww) are a great problem, as enormous quantities are produced each year that are usually discarded without treatment. The polyphenol concentration in such waste is high enough to render them ecotoxical, with high values of BOD and COD (Moure et al., 2001, Niaounakis and Halvadakis, 2004, Obied et al., 2005).
To the authors’ knowledge, polyphenols have received little attention as far as their adsorption on activated carbon is concerned with the exception of the work of Garcia-Araya et al. (2003) who studied the adsorbance of three polyphenols on a commercial granular activated carbon with relatively small surface area. Also, the work of (Galiatsatou et al., 2002) addresses the issue of using activated carbon directly on omww for lowering the COD and BOD values of the waste but without focusing on the specific adsorbance of polyphenols.
The aim of this work was to explore further the production of high surface carbons from olive husk and to study in more detail the adsorption mechanism of selected polyphenols, in particular gallic acid, π-hydroxybenzoic acid, vanillic acid, caffeic acid and vanillin. An extensive characterization of selected activated carbons is presented. An attempt has also been made to elucidate the adsorption mechanism of a mixture of five polyphenols, considering their physicochemical properties, the interactions with the carbon surface and the effect of temperature on the adsorbing capacity. A thermodynamic interpretation of the results is also presented.
Section snippets
Materials
Gallic acid, π-hydroxybenzoic acid, vanillic acid, caffeic acid and vanillin were HPLC grade and were purchased from Fluka Chemie GmbH (Switzerland). H2O HPLC grade, Methanol HPLC grade and phosphoric acid 85% analytical grade were purchased from Merk (Germany).
Activated carbon preparation
The olive cake was supplied by local three-phase olive mills. The raw material was dried under air flow at 60 °C until constant weight was reached and was sieved to particle sizes of 75–250 μm. For the production of the char, pyrolysis of
Production of activated carbon
Different pyrolysis and activation conditions were employed for the production of activated carbons and their results were evaluated primarily based on the extent of the surface area produced. The results are presented in Table 2.
As far as activation at 800 °C is concerned, it appears that heating for 3 h (sample C2) results in a carbon with the highest surface area. Further increase of activation time did not result in an increase of the surface area, not even after the addition of more KOH
Conclusions
Using olive husk as a starting material and the chemical activation route (KOH), activated carbons with high surface areas and well developed porous structure were produced. The carbons with the largest surface area were compared against a commercial activated carbon and were found to posses almost double adsorptive capacity for certain polyphenols. It was concluded that microporosity controlled the extent of adsorption with the mechanism of micropore filling being the most plausible route of
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