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

Granular activated carbons (GAC) are commonly used for the purification of water contaminated with chloroform or aromatic compounds. Experiments were performed to determine if the adsorption/desorption capacities of the carbons could be changed by application of an electric potential on the carbons. In this study, various pretreatments were applied to GAC before electrochemical treatment, and their influence on the oxygenated surface groups' contents was examined. Three granular activated carbons were selected, and their main properties are reported in Table 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 carbon was submitted to either an anodic or a cathodic (+/-2V/SCE) potential. The percolating solution was made of ultra pure water (P = 18MOMEGAcm) containing a conducting salt (KNO3 or K2SO4, 0.1 mol.L-1) and occasionally 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 tubes. Before introduction, the granular activated carbon was dried at 105-degrees-C. A peristaltic pump (P) circulates the solution through the 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) allowed control of the potential, using 3 electrodes: a working electrode (ET) made of a platinum wire in contact with the carbon a reference saturated calomel electrode (SCE) connected to the cell by an agar-agar bridge. an auxiliary platinum electrode (EA). The potential differences between EA and ET and the electrical current crossing EA were measured with 2 viewmeters (AOIP MN 5126). The surface groups analyses were performed with Boehm's method[4). Table 2 indicates the classification of the surface groups as well as the corresponding reagents. The results of the electrochemical treatments are presented in Table 3 and Figures 2, 3, and 4. Experiments 2b and 2c were performed on nonbuffered solutions, whereas all other experiments were made at pH 7. The support electrolyte was mostly KNO3, except for runs 4b, 4c, 6b, and 6c, where 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 treatment clearly influences the yield of surface groups. For the positive value + 2V/SCE, the creation of groups I, II, and III is favored. For the negative value -2V/SCE, group IV and basic groups are obtained. The results corresponding to chemical and thermal treatments of the activated carbon B are shown in Table 4, and Figure 5 shows the repartition of the surface groups after treatments. Previous conclusions were confirmed, and the influence of the chemical pretreatments on the electrochemical behaviour of the samples is very pronounced. A potentiometric study of granular activated carbons has been performed: the electrical evolution of aqueous granular activated carbon suspensions was recorded (Figure 6) and the influence of pH, conducting electrolyte, origin 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 methods appear well suited for the surface modification of granular activated carbons. Such modifications will in turn influence the adsorption capacities of the carbons.