Effect of oxidant treatment of date pit activated carbons application to the treatment of waters

Three activated carbons have been prepared from dates stones, by carbonization without adjuvant and by chemical activation with ZnCl2 and KOH. According to the nature of the adjuvant, the activated carbons present different porosities. Chemical activation of the date stones with KOH produces an activated carbon having mésoporosity as well developed as in the ZnCl2 activation. This last process led also to develop the microporosity more than KOH activation. The surface area, pore texture, and dosage of Boehm, of the activated carbons have been studied. Adsorption at 298 K of phenol, p-cresol and o-chlorophenol from aqueous solutions on activated carbons has been studied. The adsorption capacity of the activated carbons depended on the surface area and porosity of the carbon, the solubility of the phenolic compounds, and the hydrophobicity of the substituent. The modification of the porous texture and the chemical nature of the surface of activated carbons have been realized by oxidization with nitric acid. This oxidization lead to a considerable decrease of the surface area SN 2 and of the microporeux volume W0, fixing a large amount of oxygen surface groups who influences negatively to the properties of adsorption of the activated carbons. A heat treatment of these activated carbons leads to a reduction of the concentration of the oxygen groups and therefore an amelioration of the adsorption capacity.

Date pits have a high potential as starting for activated carbons showing good performances as adsorbent for the depollution of waters containing phénolic compounds. The characteristics of the activated carbons obtained depend on the activation process and the nature of the chemical reagent. Chemical activation of the date pits with ZnCl2 or KOH produces an activated carbon have a high specific surface area reaching about 880 and 1030 m2/g for DZ and DK carbon, respectively. The adsorption capacities of the activated carbon for the phenols studied depended on the degree of activation of the activated carbon, the solubility of the phenol compound in water, and the hydrophobicity of the substituents of the phenols. Thus, the adsorption capacity rose with increased development of carbon surface area and porosity, with decreased water solubility of the phenolic compound, and with increased hydrophobicity of its substituent. The relative affinity of the phenolic compounds toward the surface of the activated carbon was related to the electron donor-acceptor complexes formed between the basic sites on the carbon surface (basic surface oxygen complexes and/or π electron-rich regions at the basal planes) and the aromatic ring of the phenols. Thus, the relative affinity increases with the basicity of the carbon surface and with the electron-withdrawing property of the substituent of the phenol. The oxidization of activated carbons DK and DZ with nitric acid provoked a considerable increase of the acidic functions (carboxylique, lactonique and phenolique) and decreased the capacity of adsorption of the phenolic compounds. The functions oxygenated of surface, implanted during oxidization with nitric acidic cover a part of the pores which becomes inaccessible to the molecules of the phenol and the o-cresol. The heat treatment to different temperatures under nitrogen of the oxidized activated carbons (DKN and DZN) decreased strongly the acidic functions and increased the capacity of adsorption of the activated carbons. Indeed, the thermal treatment decreased the content of the oxygenated functions that decreased the access of the molecules phenolic to the surface of activated carbons by their considerably sterique clutter.