Site Remediation Of An Industrial Waste Dump: Fenton Treatment Of PCB Contaminated Soil

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1 Site Remediation Of An Industrial Waste Dump: Fenton Treatment Of PCB Contaminated Soil Renato BACIOCCHI (1), Cesare CIOTTI (), Renato GAVASCI (3), Francesco LOMBARDI (4) (1),(),(3),(4) Dipartimento di Ingegneria Civile, Area di Ingegneria Sanitaria-Ambientale, Università di Roma "Tor Vergata" EXECUTIVE SUMMARY PCBs represent an important class of organic pollutants, due to their toxicity and ubiquity in contaminated sites. They are found in the environment mainly as a result of uncontrolled disposal of used oils and transformers. PCBs are refractory to most of current remediation technologies such that only thermal treatments can be effective in removing PCBs from contaminated soils. Advanced oxidation processes (AOPs) have become one of the most interesting and promising remediation techniques. Their operational principle is based on the idea of generating a pool of oxidizing species in the subsurface environment. The different AOP processes differ simply in the way this pool is produced. In Fenton s reaction hydroxyl radical (OH ) is generated by an iron-catalyzed reaction. The present work deals with the treatment of a PCB-contaminated soil, collected from a dump near Rome, by Fenton s reagent. The soil was a complex matrix characterized by a very high organic matter (TOC = 6.57%), with an average PCB concentration equal to 47 mg/kg (dry soil). All the experiments were carried out in batch reactors using a 1: soil:water ratio at hydrogen peroxide concentration of 6%, % and 0% wt/wt, and different catalysts (FeCl 3 or FeSO 4 ) and concentrations (mm, 8mM and 1mM). All experiments were carried out at acidic ph (between and 3). Hydrogen peroxide decomposition kinetics and residual PCBs concentrations after 8-hour treatment were measured. As far as H O decomposition kinetics are concerned, a linear correlation between H O decomposition rate and Iron/H O molar ratio was found when FeSO 4 catalyst was used. On the contrary such a relationship was not found when FeCl 3 catalyst was used. This different behaviour was ascribed to the different role played by Iron(III) ions with respect to Iron(II) ions within the Fenton s reaction. The maximum removal efficiency using FeCl 3 as catalyst was equal to 46% at [H O ] = % and [FeCl 3 ] = 8mM; The maximum removal efficiency using FeSO 4 as catalyst was equal to 38% at [H O ] = % and [FeSO 4 ] = mm. Based upon the obtained results, it can be concluded that Fenton s reagent may represent a valid process for treatment of PCB-contaminated soils. Clearly, optimisation of the operatives conditions is required in order to increase the removal efficiency and also to possibly reduce the oxidant requirements. CONTACT DETAILS Prof. Renato GAVASCI University of Rome Tor Vergata Department of Civil Engineering Via del Politecnico, Rome Phone (+39) gavasci@ing.uniroma.it Born in Rome on November 9th Degree in Civil Engineering at the University of Rome La Sapienza on March 5th Professor of Environmental and sanitary

2 Engineering at the University of Rome Tor Vergata. He has authored or co-authored over 80 technical publications concerning: wastewater treatments, municipal solid waste, hazardous solid waste, water purification, air pollution control. He is registered Civil Engineer in Rome and an active member of numerous national and international professional associations. His research interests are in the areas of biofilm processes, treatment processes for sewage with high concentration of pollutants, ground water remediation, interaction between sanitary landfill leachate and soils. INTRODUCTION Advanced oxidation processes (AOPs) have become one of the most interesting and promising remediation techniques for soils contaminated by a wide range of organic pollutants. One of the typical advanced oxidation process (AOPs) is based on the property of H O to generate hydroxyl radicals by reacting with ferrous ions in the well known Fenton s reaction. The possibility of applying this process to contaminated soils was first demonstrated by Watts et al. (1991) in batch lab-scale experiments and later by Ravikumar and Gurol (1994) with sand-packed column tests and by Kakarla et al. (1997) with soil-packed column tests. One of the main drawbacks of in situ Fenton treatment relies in the instability of H O, when it gets in touch with inorganic compounds, such as iron oxyhydroxides and manganese oxyhydroxides catalysts or with organic compounds such as catalase or peroxidase enzymes, that are widespread in surface soils (Watts et al., 1991). This instability may dramatically reduce the concentration of H O at increasing soil depths and consequently the total amount of hydroxyl radicals available for oxidation of pollutants; in fact, during Fenton processes, the species that are directly involved in the oxidation are hydroxyl radicals and other oxygen transient species like radical superoxide anion (O - ) or radical hydroperoxide anion (HO ) and not H O. The decomposition of H O in subsurface environments was also studied, even if in model systems (Miller and Valentine, 1995). Recent studies of Baciocchi et al. (003; 004) have demonstrated the tight correlation between H O lifetime and oxidation efficiency of Fenton s and Fenton-like systems. That means a more simple preliminary screening phase of operating conditions, allowing to reduce the number of cases to be tested completely. In fact, the selection of the more appropriate operating conditions for an in situ treatment, based on the Fenton s or Fenton-like process, is usually accomplished through a lab-scale oxidation experiments, that require monitoring the concentration of the pollutant(s), with often time-expensive and cumbersome extraction/analytical procedures. This article summarizes the results of a lab-scale feasibility study of a Fenton s process applied to a dump soil contaminated by an high concentration (47 mg/kg (dry soil) ) of Polychlorinated Biphenyls (PCBs). The study was performed in two sequential phases: first, a series of preliminary tests was performed in order to evaluate the best configuration for the experimental batch reactions. This step was aimed to obtain a proper mixing and especially to achieve an effective dissipation of the heat developed during the reaction. This last aspect may play a special role when elevated H O concentrations are used in tests. The second phase consisted in carrying out different experiments in order to test the influence of different concentrations of H O and iron salt (Iron(II)sulphate and Iron(III)chloride) on the PCB removal efficiency. Finally, the results were fitted with a statistical approach and iso-response curves were built in order to obtain indication on the selection of optimal operating conditions of the Fenton s process.

3 MATERIAL AND METHODS Reagents Hydrogen peroxide (30% wt/wt), Iron(II)sulphate, Iron(III)chloride, Sulphuric acid (96%) were purchased from Fluka Riedel-de Haën. PCB-mix and Phenanthrene (used respectively as calibration and internal standard) were purchased from AccuStandard. All reagents used in this research were ACS or HPLC grade. Characterization of soil samples The soil, whose main properties are summarized in Table 1, was collected from a dump near Rome. Prior to testing, it was air dried, homogenised with a sample divider Retsch PT0 and sieved at mm with a sieve shaker Retsch AS00 Control g. Parameter Value Humidity 5% ph 6.98 Total Organic Carbon (TOC) 6.57% Fe (mg/kg (dry soil) ) Mn (mg/kg (dry soil) ) Easily Exchangeable Bound to Carbonates Bound to Oxides Bound to Organic Matter Residual Fraction Total Table 1: Properties of tested dump soil Batch hydrogen peroxide decomposition tests (Preliminary tests) Kinetics of H O degradation were studied through batch experiments performed in 50 ml and 00 ml beakers, kept in continuous agitation (400 rpm) on a magnetic stirrer, supplied by VELP Scientifica (Italy). The temperature of the batch was continuously monitored with a thermocouple during all the preliminary phase. A 15 g soil sample was added to the beaker together with a 0 ml solution of distilled water and H O (6%, % and 0% (wt/wt)). The ph of the soil slurry was measured with a phmeter HI 8314 (Hanna Instruments); the ph was kept constantly between and 3 with few drops of sulphuric acid. The experiment was started when H O was added to the slurry. The reaction was stopped adding sulphuric acid (until ph 1) to the sample immediately after its collection from the soil slurry. Then, the sample was centrifuged at 4000 rpm for minutes in a PK 1 centrifuge supplied by ALC (Italy). After centrifugation, the supernatant was analyzed for H O, as described below. Repeating the same batch experiment by sampling at different reaction times allowed to obtain the kinetics of hydrogen peroxide decomposition. It is worth pointing out that no external iron was added in this tests, so that the soil was the only iron source in these preliminary tests. Besides, relatively high concentration of hydrogen peroxide were used (at least 1.8M versus - M typically employed in aqueous solution) because hydrophobic contaminants like PCBs, are strongly adsorbed on the soil surface or even present like NAPL. This condition may cause a reduced reactivity with

4 respect to chemical or biological treatments. Watts et al. assert that, if [H O ] 0.3M, removal of adsorbed pollutants proceed more faster than natural desorption rate: in this conditions Fenton process may accelerate the desorption of hydrophobic compounds in order to make available the desorbed compounds to hydroxyl radical attack. The oxygen transient species like radical anion superoxide (O - ) or radical hydroperoxide (HO ), are considered as responsible of the process. Moreover these species can promote reducing pathways, helping in the degradation of compounds in oxidized form. Batch hydrogen peroxide decomposition tests In this phase 00 ml beakers were selected and all the batch experiments were carried out in a thermostatic bath to minimize the exothermic effect of the Fenton reaction at high hydrogen peroxide concentrations. The experiments were carried out at increasing concentration of hydrogen peroxide (6, and 0% wt/wt) and Iron(II) sulphate or Iron(III) chloride (,8 and 1 mm). Slurry was kept at acidic condition with few drops of sulphuric acid (5% v/v) (ph = 3). The rest of the procedure was the same of the preliminary tests. The choice of Iron(III) instead of Iron(II) seems to be in disagreement with Fenton chemistry, that is based on the folowing reaction: Fe HO Fe + OH + OH Iron participates to a redox cycle in which the reduced species can be generated from at least other two reactions: Fe O 3+ + H O + Fe 3+ Fe Fe H + O + + HO Using Iron(III) in place of Iron(II) may help in obtaining a higher removal efficiency and in minimizing the hydrogen peroxide demand from part of system. Batch PCBs degradation tests The PCBs oxidation experiments were performed in the same operating conditions, with an initial contaminant concentration of 47 mg/kg (dry soil). In this case, the slurry were analyzed for PCBs as described below. Analytical methods Extraction of PCBs from the soil samples was performed following EPA method Namely, PCBs were extracted from the soil using n-hexane as solvent, by means of Dionex ASE 00. The extract was then concentrated by solvent evaporation, obtained by means of a nitrogen flux and then purified by flowing it through a column packed with Florisil 60/0 mesh. The extract, obtained as above described was analyzed by the GC/MS technique, following EPA method 680. Phenanthrene was used as internal standard in order to minimize the systematic errors. The samples were analyzed with a GC/MS AUTO/HRGC/MS VG QUATTRO (Carlo Erba Instruments) equipped with a capillary column DB-5 (30m x 0.5mm x 0.5m) purchased from Supelco. Determination of hydrogen peroxide was performed by the iodometric method.

5 RESULTS PCB determination As shown in Figure 1, the soil contamination is characterized by an abundance of tri, tetra, penta, esa and epta-chlorinated congeners. So it s reasonable to suppose that this soil was contaminated by a mixture of Aroclor 154 (chlorine = 54% wt/wt) % % 14% mg(pcb)/kg(terr) % 3-CB 4-CB 5-CB 6-CB 5 7-CB 0 1-CB -CB 3-CB 4-CB 5-CB 6-CB 7-CB 8-CB -CB 58% Figure 1: Composition of the PCB mix present in soil before treatment in weight (left) and in percentage (right) Preliminary tests Tests performed at an hydrogen peroxide concentration of 6% and % did not show remarkable thermal effects in all the batch configurations. Namely, during the first 0 minutes of test duration, the temperature raised slightly with a maximum of about -15 C above the initial temperature, and then decreased back to ambient values in hours. On the contrary, as show in Figure, tests performed at hydrogen peroxide concentration of 0% have been characterized by a strong heat development that increased the slurry temperature almost to the boiling point Temperature ( C) Time (min) Figure : Temperature of the slurry with ( ) and without ( ) thermostatic bath ([H O ] = 0% (wt/wt)) This violent heat development can be attributed to the simultaneous presence of more factor such as: Oxidation of the organic matter that was present in high concentration in the tested soil; Disproportion of hydrogen peroxide that is an exothermic reaction.

6 With smaller beakers this violent effect was much more important; thus, a 00 ml beaker was used for all the tests. Besides, in order to attenuate the heat development, all the next experiments were carried out in a thermostatic bath (T bath = 0 C), thus allowing to keep the reaction temperature of the slurry constant throughout the experiment (see Figure ). The results obtained in the Fenton-like tests (see Table b) performed without any iron amendment show an almost linear correlation between hydrogen peroxide initial concentration and PCB removal efficiency. Besides, it is worth noting that, the hydrogen peroxide decomposition proceeds approximately always at the same rate. As clearly shown in Table a the pseudo first-order kinetic constant was almost constant at the tested operating conditions, within the experimental variability essentially due to the soil heterogeneity. [H O ] (%) K (H O ) (min -1 ) 6% % % (a) [H O ] (%) (b) PCB Removal (%) 6% 14.6 %.8 0% 36.7 Table : (a) Hydrogen peroxide decomposition and (b) PCBs removal efficiency of Fenton-like tests. No external iron amendment Fenton process catalyzed by Iron(II) As clearly shown in Table 3, during these tests an increase of hydrogen peroxide decomposition rate at increasing Iron(II) amendment was observed. Namely, a linear correlation between the H O decomposition kinetic constant and the FeSO 4 /H O molar ratio was observed, as shown in Figure 3. It is worth pointing out that the intercept value of the regression line is approximately equal to the value estimated in Fenton-like experiments, performed without iron amendment (Table a). [H O ] K (H O ) (min -1 ) [Iron(II)]=mM [Iron(II)]=8mM [Iron(II)]=1mM 6% % Table 3: Pseudo first-order kinetic constant of the hydrogen peroxide decomposition reaction with Iron(II) amendment [H O ] PCB Removal (%) [Iron(II)]=mM [Iron(II)]=8mM [Iron(II)]=1mM 6% % Table 4: PCBs removal through Fenton s reaction performed with Iron(II) amendment

7 0,0500 0,0450 0,0400 0,0350 K cin (min -1 ) 0,0300 0,050 0,000 0,0150 0,00 0,0050 y = 4,8351x + 0,0111 R = 0,899 0,0000 0,0000 0,00 0,000 0,0030 0,0040 0,0050 0,0060 0,0070 0,0080 Fe/H O Figure 3: Kinetic rate versus molar ratio Iron(II)/H O Meanwhile, as shown in Table 4 a decrease of PCB removal efficiency was also observed. This result is in agreement with previous observations (Baciocchi et al., 003, 004), where it was concluded that an higher removal efficiency of contaminants is obtained for operating conditions characterized by an higher stability of H O. Finally, the experimental removal efficiency were fitted by means of a polynomial quadratic regression: R = [ H + FeSO O ] [ H O ] 0.83[ FeSO 4 ] 0.06[ H O ][ FeSO 4 ] 0.05[ 4 ] The results were plotted in Figure 4 in terms of iso-response curves using a commercial software (SURFER ). The shape of the iso-response curves clearly indicate that the role played by Fe(II) concentration was very weak, i.e. large increase in Iron(II) amendment did not cause major changes in PCB removal. Besides, the shape of these curves are such that it is very difficult to select or even predict an optimal operating condition in terms of H O and Fe(II) concentration HO (%) FeSO4 (mm) Figure 4: Iso-response curve for Fenton process catalyzed by Iron(II) Fenton process catalyzed by Iron(III) The hydrogen peroxide decomposition tests performed with Iron(III) amendment, whose results are reported in Table 5 show that there is not a linear correlation beyween the Iron(III)/H O ratio and the value of the first-order kinetic constant. Namely, the values of the kinetic constants were all the same, within the experimental uncertainity and taking in account the soil heterogenity. Besides, it is worth pointing out that they were also very near to these found when Fenton-like conditions with no iron amendment was tested (see Table a). Despite any change in terms of H O and/or Iron(III) concentration did not notably affect the hydrogen peroxide decomposition kinetics, the influence on PCB degradation efficiency was much more evident, as clearly shown in Table 6. Namely, the obtained

8 results indicated a positive influence of Iron(III) amendment far lower H O concentration (6 and %); on the contrary, an increase of Iron(III) amendment negatively affected the process performance in terms of PCB removal at 0% H O concentration. [H O ] K (H O ) (min -1 ) [Iron(III)]=mM [Iron(III)]=8mM [Iron(III)]=1mM 6% % % Table 5: Pseudo first-order kinetic constant of the hydrogen peroxide decomposition reaction with Iron(III) amendment [H O ] PCB Removal (%) [Iron(III)]=mM [Iron(III)]=8mM [Iron(III)]=1mM 6% % % Table 6: PCBs removal through Fenton s reaction performed with Iron(III) amendment This behaviour observed at such a high H O concentration may be possibly attributed to: High H O concentrations may lead to iron colloid precipitates. These compounds consume the catalyst and acceleratying the unproductive decomposition of H O ; Cl - consumes the OH through the reaction: Cl + OH ClOH High hydrogen peroxide concentration cause the formation of hydroxyl radicals mainly in the bulk of the solution, far away from the adsorbed pollutants. Finally, it is worth pointing out that, differently from tests performed with Iron(II) catalyst, those performed with Iron(III) did not show a clear correlation between hydrogen peroxide decomposition behaviour and PCB removal efficiency. These indications are confirmed by looking at the iso-response curves in the ([H O ]/Fe(III)) plane, that were built by fitting thee results reported in Table 6 by means of a polynomial quadratic equation curve using a commercial software (SURFER ). R = [ H + FeCl O ] 0.48[ H O ] [ FeCl 3 ] 0.3[ H O ][ FeCl 3 ] 0.05[ 3 ] The shape of iso-response curves indicates clearly the presence of an optimal operating condition that is probably positioned inside the optimum area that has been tentatively indicated in Figure 5.

9 HO (%) FeCl3 (mm) CONCLUSIONS Figure 5: Iso-response curve for Fenton process catalyzed by Iron(III) In this work the results of a feasibility study of Fenton oxidation for the treatment of a dump soil contaminated by PCBs have been presented and discussed.the choice of the operating conditions, as well as the process efficiency in terms of PCB degradation, were strongly influenced by the soil properties, namely by its high TOC amount (6.57%); for the above reasons, high reagents concentration have been applied in all experiments, with up to 0% H O and 1 mm Iron(II) or Iron(III) concentrations. Despite so severe operating conditions, the removal efficiency did not allow to reduce the PCBs concentration below the italian limit for industrial soil, set equal to 5 mg/kg (dry soil). The best removal efficiency were 38% and 46% for Iron(II) and Iron(III) catalyzed Fenton s process, respectevely mg (PCB) /kg (TERR) 0 15 mg(pcb)/kg(terr) Before Treatment HO %; FeSO4 mm 1-CB -CB 3-CB 4-CB 5-CB 6-CB 7-CB 8-CB -CB 5 0 Before Treatment HO %; FeCl3 8mM 1-CB -CB 3-CB 4-CB 5-CB 6-CB 7-CB 8-CB -CB Figure 6: Comparison between PCB congeners concentration in soil before and after treatment. It is worth pointing out that for both type of catalysts used, the best performances in terms of PCB removal were not achieved at maximum hydrogen peroxide (0%), but rather at %. This suggests that at too high H O concentration, H O may undergo undesidered decomposition reaction, as well as termination reactions of hydroxyl radicals with hydrogen peroxide may become more important. Finally, as shown in Figure 6, it is worth pointing out that, the process was almost equally effective for all PCBs congeners.

10 REFERENCES Baciocchi R., Boni M.R., D'Aprile L. (004), Application of Hydrogen peroxide lifetime as an indicator of TCE Fenton-like oxidation in soils, Journal of Hazardous Materials, B7 (004), 97-. Baciocchi R., Boni M.R., D'Aprile L. (003), Hydrogen peroxide lifetime as an indicator of Fenton's and Fenton-like 3-chlorophenol oxidation in soils, Journal of Hazardous Materials Kakarla P.K.C., Watts R.J. (1997), Depth of Fenton-like Oxidation in remediation of Surface Soils, Journal of Environmental Engineering, 13, Miller C.M., Valentine R.L.,(1995), "Oxidation behaviour of Aqueous Contaminants in the Presence of Hydrogen peroxide and Filter Media", Journal of Hazardous Materials 41, Ravikumar J.X., Gurol M.D., (1994), "Chemical oxidation of Chlorinated Organics by Hydrogen peroxide in the Presence of Sand", Environmental Science and Technology, 5, Watts R.J., Foget M.K., Kong, S., Teel A.L. (1999) Hydrogen peroxide decomposition in model subsurface systems, Journal of Hazardous Materials B 69, Watts R.J., Udell M.D., Rauch P.A. (1991), "Treatment of Pentachlorophenol-Contaminated Soils Using Fenton's Reagent", Hazardous Waste & Hazardous Materials, 7,