INFLUENCE OF VARIOUS TREATMENTS ON OXYGEN ATED SURFACE GROUPS AND GRANULAR ACTIVATED CARBON POTENTIAL

Granular activated carbons (GAC) are commonly used for the purificatio n of water contaminated with chloroform or aromatic compounds. Experim ents were performed to determine if the adsorption/desorption capaciti es of the carbons could be changed by application of an electric poten tial on the carbons. In this study, various pretreatments were applied to GAC before electrochemical treatment, and their influence on the o xygenated surface groups' contents was examined. Three granular activa ted carbons were selected, and their main properties are reported in T able 1. These samples were further submitted to: a chemical treatment: The granular activated carbon was dispersed in a boiling solution of nitric acid (32%) for 1 hour and then purified by water extraction. a thermal treatment: the carbon was heated at 800-degrees-C for 12 hours , under N2. an electrochemical treatment: the granular activated carbo n was submitted to either an anodic or a cathodic (+/-2V/SCE) potentia l. The percolating solution was made of ultra pure water (P = 18MOMEGA cm) containing a conducting salt (KNO3 or K2SO4, 0.1 mol.L-1) and occa sionally a phosphate buffer delivering a pH equal to 7. In that case, AgNO3 was added to avoid bacterial proliferation. The electrochemical cell is shown in Figure 1. It consists of 2 thermostated concentric tu bes. Before introduction, the granular activated carbon was dried at 1 05-degrees-C. A peristaltic pump (P) circulates the solution through t he filter (F) (pores' diameter = 0.45 mum), the stopcock (R), and the column containing the GAC. The lateral tube allowed the air evacuation during the water admission. The potentiometer (Tacussel PRT 10-05) al lowed control of the potential, using 3 electrodes: a working electrod e (ET) made of a platinum wire in contact with the carbon a reference saturated calomel electrode (SCE) connected to the cell by an agar-aga r bridge. an auxiliary platinum electrode (EA). The potential differen ces between EA and ET and the electrical current crossing EA were meas ured with 2 viewmeters (AOIP MN 5126). The surface groups analyses wer e performed with Boehm's method[4). Table 2 indicates the classificati on of the surface groups as well as the corresponding reagents. The re sults of the electrochemical treatments are presented in Table 3 and F igures 2, 3, and 4. Experiments 2b and 2c were performed on nonbuffere d solutions, whereas all other experiments were made at pH 7. The supp ort electrolyte was mostly KNO3, except for runs 4b, 4c, 6b, and 6c, w here K2SO4 was used. The following main remarks can be made: The total number of acidic groups greatly exceeds the number of basic groups on raw carbons. Carbonyl groups are predominant. The electrochemical tre atment clearly influences the yield of surface groups. For the positiv e value + 2V/SCE, the creation of groups I, II, and III is favored. Fo r the negative value -2V/SCE, group IV and basic groups are obtained. The results corresponding to chemical and thermal treatments of the ac tivated carbon B are shown in Table 4, and Figure 5 shows the repartit ion of the surface groups after treatments. Previous conclusions were confirmed, and the influence of the chemical pretreatments on the elec trochemical behaviour of the samples is very pronounced. A potentiomet ric study of granular activated carbons has been performed: the electr ical evolution of aqueous granular activated carbon suspensions was re corded (Figure 6) and the influence of pH, conducting electrolyte, ori gin of carbons, and redox properties of solutions was determined. The differences are explained using Garten[3], Huang[22], or Zarrouki[32] approaches. In conclusion, chemical, thermal, and electrochemical meth ods appear well suited for the surface modification of granular activa ted carbons. Such modifications will in turn influence the adsorption capacities of the carbons.