Environ. Sci. Technol. 1994, 28, 222-230
Inorganic and Organic Byproducts of the Reactions between Chlorite, Activated Carbon, and Phenolic Compounds Nathalle Karpel Vel Leitner,t Joseph De Laat,*lt Marcel Dore,? Herve Suty,* and Michel Poulllot*
Laboratoire de Chimie de I'Eau et des Nuisances, ESIP, URA 1468,Universith de Poitiers, 40 Avenue du Recteur Pineau, 86022 Poitiers, Cedex, France, and Elf-Atochem, 95 Rue Danton, 92303 Levallois-Perret, Cedex, France The effect of phenolic compounds (phenol and p-nitrophenol) on the removal of chlorite in aqueous solution (50 mg/L; pH = 7.2; 20 "C) by filtration through granular activated carbon beds (3.0g of CECA 40 activated carbon) was examined. No reaction between chlorite and phenol or p-nitrophenol was observed in the absence of activated carbon. The presence of phenol or p-nitrophenol in solution or preadsorbed on carbon decreased the capacity of activated carbon to remove chlorite. High-performance liquid chromatography, total organic halogen, and gas chromatography/mass spectrometry analyses showed that many organic byproducts such as chlorophenols, p- benzoquinone, dimerization, and carboxylation products were formed on the surface of activated carbon. A mechanism for the formation of inorganic byproducts [chloride $1-1; chlorate (C103-)1 and of organic byproducts is discussed.
Introduction
Chlorine dioxide is used in many drinking water supply installations for chemical preoxidation and final disinfection. Compared with chlorine, the action of chlorine dioxide on organic matter produces far fewer organohalogen compounds and mutagenic compounds (1,2). When used for chemical preoxidation, it also reduces the concentration of organohalogen compound precursors prior to the final disinfection by chlorine (3). However, the oxidation of inorganic and organic compounds by chlorine dioxide is accompanied by a release of chlorite (0.6- 0.7 mg of ClOz-/mg of ClOz consumed), chloride, and traces of chlorate (4, 5 ) . Since these oxychlorine species are potentially toxic, current regulations governing concentrations of residual chlorine dioxide, chlorite, and chlorate in drinking water supplies are becoming increasingly strict. The World Health Organization has proposed a provisional guideline value of 200 pg L-' for the concentration of chlorite in drinking water (6).
Using chlorine dioxide for final disinfection does not usually cause any problem in meeting these standards owingto the low doses of chlorine dioxide necessary (10.20.3 mg/L). On the other hand, the use of chlorine dioxide for the chemical preoxidation of water with a high oxidant demand will require the concentration of oxychlorine species in the treatment system to be reduced. In recent years, several research programs have shown that chlorite ions could be reduced to chloride by treatment with activated carbon in powder form or by filtration on granular activated carbon (7-12). The results of this research show that the performance of the activated carbon was very variable (60-500 mg of ClOz-/g of GAC) owing to the various experimental procedures used. + Universitb de Poitiers. t
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Elf-Atochem. Environ. Sci. Technol., Voi. 28, No. 2, 1994
An earlier laboratory work showed that the interactions between the chlorite ions and the activated carbon in the absence of organic compounds lead to the formation of chloride, chlorate, and oxygen as final products and to the release of chlorine dioxide as an intermediate product (11). The relative yields of the different oxychlorine species and the performance of the different activated carbons depend on many parameters such as the initial concentrations of chlorite and activated carbon, the pH, and the nature and grain size distribution of the material. In a highly dilute aqueous medium ([ClOz-I < 1mg/L) with a pH close to neutral, the chloride ions are nevertheless the main final product of the decomposition of the chlorite ions (8). Reactions between the chlorite ions and the activated carbon could also lead to the formation of other byproducts in the presence of organic matter. Thus, Voudrias et al. (12)showed that chlorite reacts with vanillic acid in an aqueous medium only when activated carbon is present. Products identified by GC/MS analysis showed that decarboxylation and demethylation reactions were taking place in the breakdown of the chlorite by the activated carbon in the presence of vanillic acid at pH 6. However, no organochlorine compound was identified. This research was designed to examine the influence of organics on the efficiency of activated carbon to remove chlorite and to study the reactions between chlorite, activated carbon, and organic compounds. Phenol and p-nitrophenol were selected in this work because these compounds do not react with chlorite in neutral aqueous solution. Furthermore, phenol has already been used in previous works for the study of the reactions between chlorine (13,14),monochloramine (15),or chlorine dioxide (16) with organic compounds in the presence of activated carbon. Materials and Methods
Materials. The carbon selected for this study was a mineral-base granular activated carbon (CECA 40) produced by CECA, Parentis-En-Born,France, that has been also used in earlier works (8, 11). The characteristics of the raw material are listed in Table 1. All the experiments were carried out with the 0.4-0.5-mm size fraction which was prepared by grinding the GAC, sieving the particles, and washing them with ultrapure water to remove fines. The carbon was then dried a t 120 "C for 1week and kept a t ambient temperature in air-tight bottles. ~~~~
Table 1. Properties of CECA 40 Granular Activated Carbon origin carbon size (mesh) apparent density (g/cm3) pore vol (cm3ig) surface area (BET) (m2/g) iodine no. (mg of Iz/g) ash ( % ) 0013-936X/94/092S-0222$04.50/0
mineral 12-40 0.47 0.890 1400 1160 13 0 1994 American Chemical Sociefy
Autosampler and effluent storage
Table 3. Influent Composition of GAC Columns During Filtration
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Peristaltic pump Figure 1. Experimental setup for the GAC filtration studies.
Table 2. Parameters for GAC Column Studies column diameter (cm) GAC type carbon size (mm) GAC bed height (cm) GAC weight (9) flow rate (L/h) hydraulic loading (m/h) empty-bed contact time ( 8 ) influent pH (5 mM phosphate buffer) influent chloritea concn (mg/L) influent organic solute' concn (pM) a
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7.2 0 or 50 0 or 200
See text and Table 3.
Solutions of chlorite (50 mg/L), phenol, and p-nitrophenol (200 pM) were prepared in phosphate-buffered M phosphate buffer; pH = 7.2). ultrapure water (5 X All compounds used in this study were analytical grade and were used as received from the manufacturer (PROLABO, France). Column Studies. The experimental apparatus used in this study is schematically shown in Figure 1,and the parameters for the GAC columns are given in Table 2. The carbon beds (3.0 g) were secured between Pyrex glass wool plugs in 1.1-cm i.d. glass columns. The columns were thermostated a t 20.0 f 0.2 "C by the circulation of water. Solutions were pumped upwardly from 20-L glass carboys through 2-mm Isoversinic tubing using peristaltic pumps (Gilson Minipuls 2) at a rate of 360 mL/h through the columns. At the outlet of each column, a fraction of the effluent was continuously collected using a peristaltic pump (Gilson autosampler FC 220; 1sample/2 h). Glass carboys, GAC columns, and flasks containing effluent samples were covered with aluminium foil in order to prevent photochemical reactions. Two sets of experiments were done to study the interactions between phenol, chlorite, and activated carbon (fist set) or betweenp-nitrophenol,chlorite, and activated carbon (second set). In each set of experiments, three GAC columnswere operated simultaneously (columnsA-C for the first set and columns D-F for the second set). The influent composition for columns A, C, D, and F were changed during filtration. The bed volumes corresponding to the change in the influent composition are given in Table 3.
vol of influent (L) bed volumes 0-85 85-103.1 0-78.5 0-54.5 54.5-103.4 0-116.9 116.9-152.9 0-137.3 0-65.2 65.2-132.7
influent compositiona
0-10370 chlorite (50 mg/L) 10370-12570 phenol (200 pM) 0-9570 chlorite (50 mg/L) + phenol (200 pM) 0-6650 phenol (200 pM) 6650-12610 chlorite (50 mg/L) 0-14260 chlorite (50 mg/L) 14260-18650 pnitrophenol(200 pM) 0-16740 chlorite (50 mg/L) + p-nitrophenol(200 pM) 0-7950 p-nitrophenol(200 pM) 7950-16180 chlorite (50 mg/L)
All influents contained 5.0 mM phosphate buffer (pH = 7.2).
Columns A and D were fed by a solution of chlorite (50 mg/L). When the chlorite concentration in the effluent was 140 mg/L, 50 mL of phosphate-buffered ultrapure water (pH = 7.2) was passed through the columnstoremove chlorite, and then a solution of phenol (column A) or p-nitrophenol (column D) was applied. Columns B and E received as influent a solution containing both chlorite (50 mg/L) and phenol (200 pM) (columnB) or chlorite andp-nitrophenol(200 pM) (column
E). GAC in columns C and F was preloaded with phenol (column C) or with p-nitrophenol (column F) by passing aqueous solutions of phenol or p-nitrophenol (200 pM) through the GAC columns before a chlorite solution (50 mg/L) was pumped to the columns. Analytical Methods. Inorganic species were analyzed by the methods developped in our laboratory. Chlorite was analyzed by using a SpectraPhysics IsoChrom HPLC pump, an automatic Waters WISP 712 sample injection system (100-pLloop), a Spherisorb reverse-phase column (RP-18, 5 pm, 4.6 mm i.d. X 25 cm), and a LDC SpectroMonitor 3100 UV detector set at 260 nm. The mobile phase was methanol/water in the ratio 15:85 containing ammonium hydrogen phosphate (0.4 g/L) and n-octylamine (0.7 mL/L) and was adjusted to pH 6.8 with concentrated phosphoric acid. The detection limit was 0.05 mg of C102-/L. Chlorate and chloride were analyzed by ion chromatography. The HPLC system consisted of a Waters Chromatography pump (Model 510), a Rheodyne 7010 injector (100-pL loop), a Waters IC PAK anion column (10pm, 4.6 mm X 50 mm) equipped with a Millipore Guard Pak precolumn, and a Waters Model 430 conductimetric detector. The mobile phase was acetonitrile/water in the ratio 12:88 containing 160 mg of sodium gluconate/L, 180 mg of boric acid/L, 66 mg of sodium triphosphate, 12 HzO/ L, and 2.5 mL of glycerol/L. The flow rate was 1.2 mL/ min. Under these conditions, the detection limit was 0.5 mg of C l O d L and 0.2 mg of C1-/L. A Tracor 995 Isochromatographic pump module equipped with a Rheodyne 7010 injection valve, a Nucleosil (2-18reverse-phase column (5 pm, 4.6 mm X 25 cm), and a LDC SpectroMonitor I11 variable wavelength UV detector was used for the analysis of phenol and p-nitrophenol. The mobile phases were methanol/water (6040) and methanol/water (75:25) adjusted at pH 4.0 with phosphoric acid, for phenol and p-nitrophenol, respecEnviron. Scl. Technol., Vol. 28, No. 2, 1994 223
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obtained from the column that received chlorlte (column D), a mixture of chlorite and pnitrophenol (column E), or pnltrophenol followed by chlorite (column F).
tively. The UV detector was set at 275 nm for phenol and at 315 nm for p-nitrophenol. Total Organic Halogen (TOX). The total organic halogen (TOX) adsorbed on the carbon was determined by combustingcarbon that had been washed with ultrapure water. The halides liberated during the combustion were measured with a microcoulometer and expressed as microgram equivalent of chloride per gram of GAC (pg of Cl/g of GAC). All TOX measurements were made with a Dohrmann DX20 TOX analyzer including a pyrolysis furnace, a digital MC 3 microcoulometer, and a T 620 titration cell. Extraction Procedures, GC and MS Analyses. At the conclusion of the column runs, carbon was removed from the GAC columns and carefully transferred to 100mL flasks with 2 mL of methanol and 20 mL of methylene
chloride as described by Snceyink et al. (I7). After a 48-h contact time, the samples were Soxhlet extracted for 24 h with 60 mL of methylene chloride. The extracts were dried with sodium sulfate and then concentrated to 10 mL by rotary vacuum evaporation, followed by the passage of a stream of high-purity nitrogen over the extracts. Half of the extracts (5 mL) were methylated with diazomethane. A total of 2 p L of each extract was injected into a DB5, 30 m X 0.25 mm i.d. fused-silica column (Chrompack) using a Chrompack 9100 gas chromatograph operated in the split injection mode (141),programed from 40 "C (with a 5-min hold) to 240 OC at 5 OC/min and equipped with a flame ionization detector. GC/MS analyses were performed with a quadripolar hyperbolic filter system CPV/MY. Compounds were tentatively identified by the interpretation of mass spectra,
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Environ. Sci. Technol., Vol. 28, No. 2, 1994
Table 4. Cumulative Values for Chlorite Removal and for Chloride and Chlorate Productions column B
column C
85 545 20.4 8.57 6.33 0.80 7.13
0 78.5 673 19.2 5.73 4.35
