Paper C-010

Heat-activated persulfate was tested for destruction of polychlorinated dibenzo-p-dioxins/furans (PCDD/Fs) in the laboratory. Combined technology of in situ chemical oxidation (ISCO) and in situ thermal remediation (ISTR) using heat-activated persulfate can be applied for destruction of the PCDD/Fs polluted soil and the PCDD/Fs containing fly ash from incineration process. To explore the treatability of PCDD/Fs, we determined the kinetics and products of the various congeners of PCDD/Fs oxidation using heat-activated persulfate. To simulate the soil environments, PCDD/Fs were absorbed to various sizes of glass beads as the absorbent. As a results, the kinetics of lower chlorinated (0~3 chlorines) congeners disappearance were followed to pseudo-firstorder, but higher chlorinated congeners (4~8 Chlorines) were inhibited and not followed to pseudo-first-order. As a general, the kinetics of PCDD/Fs decreased as the numbers of chlorines increased. The effects of the size of medium and the dosage of persulfate are discussed. INTRODUCTION Polychlorinated aromatic compounds are among the most problematic pollutants due to their chemical inertness, lipid solubility and toxicity. In particular, polychlorinated dibenzo-p-dioxins (PCDDs) and polychlorinated dibenzofurans (PCDFs) are known to be most toxic because they induce developmental toxicity, cancers and endocrine disruption, and are known recalcitrant pollutants. PCDDs/Fs are essentially planar chlorinated tricyclic aromatic compounds that are characterized by extremely low water solubility (19.3 ng/L) and low vapor pressure (7.4×10-10 Torr at 25°C) (Shiu et al., 1988). These ubiquitous pollutants are released into the environment as by-products from sources such as waste incinerators and chlorinated chemical industries or forest fires (Kim et al., 2003). The fate of PCDD/Fs is finally to the particle e.g. soil or sediments, because they are strongly bound to the surface due to their low solubility. There are extensive studies to treat PCDD/Fs on particulate matter, such as thermal destruction, Fenton process for AOP, bioremediation including anaerobic and aerobic microorganisms. Fenton oxidation process to the degradation of PCDDs/Fs showed that pretreatment with Fenton’s reagent before biological degradation is more effective than direct biodegradation of PCDD contaminated soils (Kao et al., 2000). The persulfate free radical (SO4, ORP = 2.6 V), which can be generated from boiling persufate, is roughly equivalent to the hydroxyl free radical in reactivity. Heat activated persulfate (HAP) using combined technology of in situ chemical oxidation (ISCO) and in situ thermal remediation (ISTR) was applied for destruction of PCE, TCE and the other toxic organics (Liang et al., 2004; Waldemer et al., 2007). The majors of HAP process are (1) direct transport of HAP to PCDD/Fs contaminated site though water, (2) HAP doesn’t need any catalyst for activation compared to Fenton oxidation process. To explore the treatability of PCDD/Fs on particulate matter, several PCDD/Fs congeners were tested to examine the degradation kinetics by HAP. To simulate the particulate matters containing PCDD/Fs, various size of glass beads were used as the absorbent for PCDD/Fs. Also the dosages of persulfate to treat PCDD/Fs were examined. MATERIALS AND METHODS Batch Experiments. Glass beads (3.0mm and 0.15 mm size, Biospec Products, Inc.) were washed with 0.1 N HCl before experiments. 10 g of the washed beads were put into the 40mL reaction vials and 1 μ mol of PCDD/Fs native congeners (dibenzofuran(DF), dibenzo-p-dioxin(DD), 1-monochlorinated dibenzo-p-dioxin (1-MCDD), 1,2-DiCDD, 1,2,3-Trichlorinated dibenzo-p-dioxin (1,2,3-TriCDD), 1,2,3,4-tetrachlorinated dibenzop-dioxin (1,2,3,4-TCDD) and octachlorinated dibenzo-p-dioxin (OCDD), Accustandard) in acetone were spiked to the surface of glass bead. Then 2mL of acetone were added to the vials and was removed under N2 stream to dope the PCDD/Fs on glass bead homogenously and to remove the effect of solvent on reaction of PCDD/Fs. Then 10mL of distilled water to the vials and the vials were heated to 70°C. After temperature equilibration time 10 ~ 100 μ mol of persulfate (Sigma, >98%, prepared at 0°C) was added to the vials to initiated the reaction. The vials were mixed on the orbital mixer (150 rpm) operated at 70°C. At designed sampling time, the vials were cooled in ice to quench the reaction. Extraction and Analysis. After cooling, the solution and the glass bead were separated and extracted respectively. 1,2,3,4-tetrachlorinated naphthalene was spiked as recovery standard. The solution was extracted by 3mL of toluene and the glass bead was separated by 3mL acetone at first and by 3mL of toluene at second. All extracts was kept in refrigerator before analysis to prevent the sulfate radical generation. The identification of each congeners was performed by gas chromatography (Column: DB5-MS, 30 m × 0.25 mm, 0.25 m) with ion-trap mass spectrometry (IT-MS; Finnigan, Model: Polaris Q). The quantification was performed by GC-FID (HP6890N, DB5, 30 m × 0.25 mm, 0.25 m). The approximate detection limits were 10 picomoles with IT-MS and 100 picomoles with GC-FID. RESULTS AND DISSCUSION Figure 1 shows the degradation results of DF and OCDD by HAP. Non-chlorinated compounds, DF was degraded fast within 2 hours, however, fully chlorinated compounds, OCDD was degraded slowly and only about 33% of OCDD was removed when OCDD was adsorbed on 3mm glass bead medium and 100 times persulfate dosage. The kinetics of lower chlorinated (0~3 chlorines) congeners disappearance were followed to pseudofirst-order, but higher chlorinated congeners (4~8 Chlorines) were inhibited and not followed to pseudo-first-order. All experimental results of persulfate reaction for each PCDD/Fs congeners, each type of glass beads medium and persulfate dosages are listed on Table 1. FIGURE 1. Degradation profiles of A) DF and B) OCDD by persulfate injection. TABLE 1. PCDD/Fs removal rates after 1 hr and 6 hr reaction. 1:10 (PCDD/Fs : persulfate) 1:100 (PCDD/Fs : persufate) 3 mm 0.15 mm 3 mm 0.15 mm Congeners Removal @ 1H (%) Removal @ 6H (%) Removal @ 1H (%) Removal @ 6H (%) Removal @ 1H (%) Removal @ 6H (%) Removal @ 1H (%) Removal @ 1H (%) DF 85 100 87 100 91 100 96 100 DD 78 98 84 100 88 100 90 100 1-MCDD 61 91 74 94 62 100 78 100 1,2,3TriCDD 48 60 45 62 46 61 43 59 1,2,3,4TCDD 21 36 33 34 29 31 62 61 OCDD 6 5 31 32 31 33 48 59 Effect of Chlorination. Figure 2 shows the relationship between the degrees of chlorination of the selected PCDDs and their removal rate by HAP. As a general, the kinetics of PCDD/Fs decreased as the numbers of chlorines increased. One possible reason for the effects of chlorination may be steric hindrance between the sulfate free radical and chlorines in PCDD/Fs molecules. This phenomenon is also observed in microorganism and advanced oxidation process such as Fenton’s reaction, which are utilizing hydroxyl free radical as oxidant (Kao et al., 2000; Nam et al, 2006). Degree of chlorination 0 2 4 6 8 10 R em ov al ra te a t 6 h r ( % ) 0 20 40 60 80 100 120 FIGURE 2. Relationship between the degrees of chlorination of the selected PCDDs and their removal rate by heat-activated persulfate. Effects of Medium Size. The removal rate of PCDD/Fs on 0.15 mm glass bead was about 8% higher than that on 3 mm glass bead as Table 1 shown. Experiments for various sized medium of absorbents should be carried out further to elucidate the relationship between the reaction area and removal rate. However we can postulate that the increased area of reaction site might be the reason for the PCDD/Fs removal enhancement. On the analogy of this finding, treatment of PCDD/Fs polluted soil or sediment by HAP could have the advantage, because the levels of PCDD/Fs in environmental particulate matrices tends to increase as the particle sized decreased (Lee et al., 2006). Effects of Persulfate Dosage. In case of lower chlorinated compounds (0~3), the removal rate was not enhanced by the increase of persulfate dosage. However, the removal rate of higher chlorinated compounds increased as persulfate dosage increased as Figure 3 shown. With this finding, the dosage of persulfate should be determined from not only the total concentration of PCDD/Fs, but also from the homologue pattern of PCDD/Fs in HAP application.