ABSTRACT
Chemically activated carbons were prepared from apricot stones. Phosphoric acid [25 - 75 wt%] was used as activating agent at 400 - 600 °C. Zinc chloride - activated carbons were also prepared at 600 °C and at 700 °C using zinc chloride [50 - 200 wt%] as activating agent. The textural parameters were determined from nitrogen adsorption data at 77 K. The chemistry of the carbon surfaces was determined from the FTIR spectrograms and from the base and acid neutralization capacities. The adsorption of phenol, p -nitrophenol and 3,5-dinitrophenol was carried out at 298 K for all the prepared carbons and also at 308 and 317 K for some selected carbons. Chemically - activated carbons exhibited high adsorption capacity towards phenols. Phenols adsorption increased with the increase of the molecular weight of the phenol and with the decrease of its solubility in water. The adsorption of phenols increased also with the decrease of the surface density of carbon - oxygen group existing. Adsorption of phenols proceeds via physical adsorption on oxygen - free surface and via chemisorption on oxygen contaminated carbon surfaces. Activated carbons [AC's] are widely used as adsorbents in decontamination process because of their extended surface area, microporous structure high adsorption capacity and special surface reactivity[1-4]. The surface of AC is considered heterogeneous from the point of view of both porosity and surface chemistry. The heterogeneity of the pore structure is a result of the existence of a wide range of size from a few to a few hundred angstroms[5]. The heterogeneity of the chemistry of AC is attributed to the existence of heteroatoms such as oxygen, nitrogen, hydrogen and phosphorous[6-7]. The heteroatoms, particularly oxygen, consist of organic functional groups at the edges of carbon crystallites. Carbon-oxygen groups include carboxyls, phenols, ethers, lactones and carbonyl[8]. There has been an increasing interest in the study of adsorption of organic molecules on AC. This is due to the importance of the removal of organic pollutants from water streams and the atmosphere[9]. Adsorption of aromatic compounds from solution, particularly phenols, has been studied extensively[10-14]. Phenolic compounds exist widely in the industrial effluents such as those from oil refineries, cool tar, leather, paint, pharmaceutical and steel industries. Since they are highly toxic and, in general, not amenable to biological degradation, methods of treatment are continuously modified and developed at all treatments. Among adsorbents, granular or powder AC proved to be the most efficient[15-19]. Regardless of the enormous studies undertaken concerning the adsorption of phenols on carbon, it can be seen that more intensive investigations still need to be undertaken to find convincing answer for some problems still remaining on phenol uptake by AC. The objective of this paper is to study the effect of surface chemistry and surface area of carbons on their adsorption of phenols. The samples are different from the point of view of the activating agent used [Zinc chloride and phosphoric acid]. An important feature common for all the samples is their acidic surface pH. Under such condition phenols exist in their molecular form. Three phenols were studied, namely phenol [P], paranitrophenol [PNP] and dinitrophenol [DNP]