Characterizing the Photodegradation Ability of Pentachlorophenol in. β-cyclodextrin Soil Washing Solution

Transcription

1 Applied Mechanics and Materials Submitted: ISSN: , Vol. 768, pp Accepted: doi: / Online: Trans Tech Publications, Switzerland Characterizing the Photodegradation Ability of Pentachlorophenol in β-cyclodextrin Soil Washing Solution Zeyu ZHOU 1, Hongtao WANG 2 *, Wenjing LU 3 Department of Environmental Science and Engineering, Tsinghua University, Beijing , China 1 zy-zhou11@mails.tsinghua.edu.cn 2 htwang@tsinghua.edu.cn 3 luwenjing@tsinghua.edu.cn Keywords: Photodegradation; Pentachlorophenol; β-cyclodextrin soil washing solution; Photo irradiation. Abstract. The objective of this study was to characterize the photodegradation ability of pentachlorophenol (PCP) in β-cyclodextrin (β-cd) soil washing solution. After contaminated soil remediation by β-cd soil washing method, solution contained target contaminant was further treated with photodegradation technique. Degradation processes with Xe lamp to simulate natural sun light was investigated under different temperature, and various aeration rate. TiO 2 employed as photocatalyst was also tested in the soil washing system. The result demonstrated that photodegradation rate of PCP in soil washing solution was related to the aeration rate. The reaction could be slightly accelerated by increasing the temperature. TiO 2 adding will decrease the degradation speed, while using it together with H 2 O 2 will extremely increase the degradation rate of the target contaminant in this condition. Introduction pentachlorophenol (PCP), as a kind of Hydrophobic chlorinated aromatic pollutants, was among the most important biotoxic contaminant in the twentieth century [1]. The Stockholm Convention classified PCP as Persistent Organic Pollutants after the potential health hazardous and its global biota accumulation were gradually noticed in 2001 [2]. Although it has been banned or restricted since that time, millions of tons of this kind of contaminant still circulate in the environment due to its heavy utilization and high environmental persistence. Owing to the toxicity and the carcinogenicity of this PCP, the remediation of the PCP contaminated soil was extraordinarily needed [3]. Remediation of PCP polluted areas has recently drawn much attention, therefore physical, chemical, biological, and heterogeneous technologies have been used to remediate contaminated soil [4,5]. Among all the remediation technologies, soil washing technique with surfactant is quite suitable for this situation [6,7]. It has been well known that surfactant can remediate contaminated soil by dissolving the organic compounds and desorption of it from soil. To get rid of the problem that soil washing technique may lead new toxic contaminant into the soil, β-cyclodextrin (β-cd), a kind of burgeoning surfactant, has been proposed as an alternative surfactant to enhance the solubility of target contaminant [8]. β-cd has many advantages compares to other traditional surfactant: 1. It is not subject to precipitation or foam, which is beneficial for further treatment. 2. It is negligible adsorbing to solid phase so the surfactant remaining will be minimal. 3. It is non-toxic and highly biodegradable, thus leading no new contaminant to the ecosystem. All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of Trans Tech Publications, (ID: , Pennsylvania State University, University Park, USA-06/03/16,02:22:23)

2 156 Selected Proceedings of the Ninth International Conference on Waste Management and Technology Although soil washing method with β-cd surfactant will extract the contaminant from soil phase successfully, the soil remediation does not complete. The target contaminants simply transfer from solid phase to liquid phase which need further treatment. The solvent contains target contaminants usually will be treated with photolysis technique, and the contaminant can be decomposed by photodegradation [9]. Application of photolysis method in degradation of organic contaminants received lots of attention in the past decades. To support the photolysis method, assorted types of photocatalysts have been investigated during the photodegradation procedure. OH can be formed from water molecules or other highly reactive solvents under irradiation, initiating the decomposition of the pollutants [10]. This study focused on the photolysis degradation of the soil washing solvent. Photodegradation ability of PCP in β-cd soil washing solution was investigated under various conditions. Xe lamp was operated as simulating natural sun light to serve the photodegradation energy. Under the same photo irradiation condition, the encouragement of the system temperature was examined. Result shows degradation rate will be increasing along with the temperature rising. Next, relationship between aeration rate and degradation rate was presented. Furthermore, additional TiO 2, H 2 O 2, and TiO 2 combined with H 2 O 2 were analyzed in this situation. Neither one of those two addition can accelerate the degradation of the target contaminant alone, while increasing the speed enormously if both TiO 2 and H 2 O 2 were added. Materials and methods Reagents and materials. β-cd was supplied by the Tianjin guangfu fine chemical research institute (China). Commercial P25 TiO 2 (80% anatase, 20% rutile) was supported by Degussa (Germany). H 2 O 2 was gotten from Xilong chemical co. ltd (China). Methanol (HPLC grade) was purchased from Fisher Scientific (USA). Water, which was used as a solvent, was obtained using a Milli-Q Water Purification System with the resistance value of 18.2 MΩ cm@25 C. Pentachlorophenol (PCP, purity > 98.0%) was purchased from the Sigma-Aldrich (USA). Photoirradiation procedure. β-cd surfactant solution was applied in PCP contaminated soil remediation procedure. After ex-situ soil washing method, the solvent with a concentration of 5 g L -1 β-cd contained approximate 10 mg L 1 target contaminant ( mm PCP). All photo irradiation parts of the experiments were conducted in a 500 ml glass reactor (length 310 mm, diameter 70 mm), which contained a sunlight simulating system, as shown in Fig. 1. The sunlight simulating system included a 400 W Xe lamp for photo irradiation procedure; a quartz well (length 300 mm, diameter 55 mm) equipped with a circulating water unit to maintain the system at a certain temperature; an aeration system for air explosion if necessary; and a magnetic stirrer to promote uniform mixing of solute in the solvent. The Xe lamp intensity at the reaction point was Lx, as measured by a Lux tester (Hioki, Japan). The radiation power of the Xe lamp at the reaction point was µw cm -2, as measured by a radiometer (Handy, China). Compared with the Lx, and µw cm -2, of the natural sunlight in the noon, 400 W Xe lamp could be the simulation of sunlight. Moreover, the photo energy distribution of sunlight at sea level and Xe lamp were similar in UV and visible light region. The different between them were Xe lamp offered more energy in infrared area, which was useless in photodegradation procedure.

3 Applied Mechanics and Materials Vol Fig. 1. Schematic experimental system for photo degradation experiments. Analytical methods. To follow the degradation process, 1 ml of mixture was extracted from the soil washing solution at the beginning, and at predetermined time-points after photo irradiation. The mixture was adequately shaken with the same volume of methanol if TiO 2 was added as photo catalyst and then filtered through a 0.45 µm pore size membrane filter. The concentrations of PCP in the β-cd soil washing solution were analyzed using high-performance liquid chromatography (HPLC, Agilent 1260) equipped with an Agilent TC-C18 reverse phase column. UV detection was performed at 249 and 254 nm for PCP and the by-products created during the photo irradiation procedure, respectively. A mixture of methanol and water was used as the mobile phase, with a gradient mixture and a flow rate of 1.0 ml min -1. The quantification of the target analytes was based on the calibration curve in the HPLC analysis, and the linear range was 0 mg L -1 to 10 mg L -1. Analytical results were verified by standard PCP solution with certain concentration. Results and discussion Photodegradation of PCP under different temperature. To explore the photodegradation rate of PCP in β-cd soil washing solution, variations in the concentrations of PCP at three different temperatures were investigated. The PCP photo activities at 5 C, 20 C, and 35 C with 400 W Xe light were compared in Fig. 2. Following the requirement that exploring the photo activities in the aqua phase, 5 C was chosen as the lowest temperature to avoid frozen. And to prevent the evaporation of the soil washing solution, 35 C was chosen as the highest temperature. The initial PCP concentration in the solution was 10 mg L -1 ( mm) and the solvent was β-cd soil washing solution with the concentration of 5 g L -1. It could be clearly seen that the photo activity of the target contaminant increased when the temperature increased. Although the acceleration was not much, the cooperative effect of temperature was proved.

4 158 Selected Proceedings of the Ninth International Conference on Waste Management and Technology Fig. 2. Photodegradation of PCP under simulate sunlight at different temperatures. The inset represents the logarithmic transform for each curve. The logarithm of the ratio between the initial concentration (C 0 ) of the target contaminant and its concentration (C) at a specific given time is shown in the inset of Fig. 2. The slope of the series of the points provided the apparent rate constant. All correlation coefficients (r 2 ) obtained were higher than 0.95, which indicated that all the photodegradation procedures followed the pseudo-first order kinetics. As shown in the figure, photodegradation of target contaminant at 35 C provided the highest photocatalytic rate constant, namely, min -1. The photocatalytic rate constant of PCP degradation at 20 C was a little lower at min -1. Photodegradation of target contaminant at 5 C provided the lowest photocatalytic rate constant, namely, min -1, exactly half of the one at 35 C, which is much lower than the other two. The results indicate that the temperature influence is higher at low temperature, and the effect will reduce with the rising temperature. OH will be formed easier at higher temperature in solution under photo irradiation, which can react with organic compound caused the degradation of target contaminant. Photodegradation of PCP under different aeration. To evaluate more influence of the photodegradation rate of PCP in β-cd soil washing solution, different aeration rate was also being considered. Degradations with three different aeration rates were compared with the one lacking aeration in Fig. 3. The effect of aeration was not obvious. The difference performed in the photodegradation contrast was limited, indicate that aeration do not play an important role in decomposition progress. In the experiment, the aeration grads were 0.15 L min -1, 0.45 L min -1, and 1.50 L min -1. Aeration was supplied by air pump with an aeration flow meter scaled from 0 to 1.50 L min -1. Air was inputted through a glass pipe to the bottom of the reactor. The inset part of the Fig. 3 indicated the logarithm of the ratio between the initial concentration (C 0 ) of PCP in β-cd soil washing solution and its concentration (C) for this four types of degradation process. The apparent rate constant slightly decreased with the increasing aeration rate. The photodegradation procedures

5 Applied Mechanics and Materials Vol of PCP in β-cd soil washing solution without aeration followed the pseudo-first order kinetics. The appaent rate constant of each procedure with air input was higher at the beginning of the reaction and then went down, which revealed that air addition was helpful to the OH forming in the solution under photo irradiation. Dissolved oxygen in β-cd soil washing solution could be react with water molecules and produce OH which could accelerate the photodegradation reaction. While the effect of this was relatively low, and was not beneficial by increasing air loaded. By contrary, the bubbles created in the solution would reflect the light caused the input photo energy deceased to the system. That explained the phenomenon degradation rate decreased when aeration rate increased. Fig. 3. Photodegradation of PCP under simulate sunlight with different aerations. The inset represents the logarithmic transform for each curve. Photodegradation of PCP with additional TiO 2 and H 2 O 2. To enhance the photodegrade ability of the target contaminant in solution, TiO 2 usually be used as photocatalyst in photodegradation method, and H 2 O 2 was also used as the reagent in some degradation procedure. In this study, 1 ml H 2 O 2 was added in the reactor at the beginning of the photo irradiation to explore the influence of H 2 O 2 in the reaction. As shown in Fig. 4, the degradation speed almost remained the same when H 2 O 2 was added in the system. Although H 2 O 2 addition was positive correlation with the OH creation, the ph of the soil washing solution was decreased. Original ph of H 2 O 2 (30%) was tested at 3.1. Additional 1 ml H 2 O 2 in the 500 ml β-cd soil washing solution changed the ph of system from neutral to weak acid, which would unconducive to the OH forming. In that case the enhancement and the reduction of the H 2 O 2 addition caused the degradation rate maintained similar.

6 160 Selected Proceedings of the Ninth International Conference on Waste Management and Technology Fig. 4. Photodegradation of PCP under simulate sunlight with different additions. 400 mg L -1 TiO 2 was added as the photocatalyst in this study to observe the relationship between degradation rate and catalyst addition. The photodegradation rate of the target contaminant in the β-cd soil washing solution was not enhanced as the simple compound photodegradation as described in the references. On the opposite, photodegradation rate of PCP was decreased much when TiO 2 was added in. The possible explanations were deduced as following. In the β-cd soil washing solution, most PCP compound was contained in the β-cd molecular structure, which caused the exposed PCP quite limited and the TiO 2 was mostly contact with β-cd molecules. Besides, β-cd molecule contains lots of the C O bond, the energy of which was lower than the C Cl bond of the PCP compound. So TiO 2 prefer to react with β-cd under photo irradiation and the photo energy was consumed by degradation of the solvent. Furthermore, TiO 2 catalyst was suspended in the soil washing solution, and much of the photo energy was absorbed by it, so the photons which could be directly affected on the solution were blocked. Because the soil washing solution was equilibrated with the photocatalyst prior to irradiation, part of the target contaminant adsorbed on the catalyst surface, and it was easily to be decomposed when photo irradiation began. So the photocatalytic rate constant was higher at the beginning than the later time. Photo irradiation with additional TiO 2 and 1.50 L min -1 air explosion was also examined. For the same reason, photodegradation rate constant was decreased.

7 Applied Mechanics and Materials Vol Fig. 5. Photodegradation compare between pure irradiation and additional TiO 2 with H 2 O 2. The inset represents the logarithmic transform for each curve. Photo irradiation procedure with additional 400 mg L -1 TiO 2 and 2 g L -1 H 2 O 2 was demonstrated in Fig. 5. The photo degradation rate was extremely increased when TiO 2 with H 2 O 2 was added in the soil washing solution. In this situation, when TiO 2 absorbed photo energy, reaction was priority to H 2 O 2 and lots of OH was created in the solution. The target contaminant reacted with the produced OH and the degradation was accelerated.

8 162 Selected Proceedings of the Ninth International Conference on Waste Management and Technology Fig. 6. Photodegradation rate constants compare of different treating method. The photodegradation rate constants of pure irradiation, H 2 O 2 added, TiO 2 added, and TiO 2 added with H 2 O 2 were compared in Fig. 6. The rate constant of PCP degradation in β-cd soil washing solution was min -1. While the H 2 O 2 added one was similar at min -1. The rate constant was decreased to min -1 when TiO 2 was added alone as the photocatalyst. And when the TiO 2 was added with H 2 O 2, the rate constant rose to min -1. Conclusions The character of the photodegradation ability of PCP in β-cd soil washing solution was researched in this study. The relationship between various influencing factors and the degradation rate was explored. Photodegradation rate under simulate sunlight in the natural condition was reasonable, and could be increased by rising the temperature. The relationship between aeration and PCP degradation was not much. Escalating the aeration rate was not helpful for the reaction. The situation that additional of catalyst and reagent in the solution was complex. H 2 O 2 addition was not benefit for degradation process. TiO 2 added along would degrade the β-cd in the solvent instead of the target contaminant, which caused the photodegradation rate of PCP decreased. Addition of TiO 2 with H 2 O 2 would enhance the reaction hugely and cut down the remediation time. For the soil washing solution disposing purpose, temperature rising and TiO 2 added with H 2 O 2 could be chosen. References [1] X. Quan, X. Zhao, S. Chen, H. Zhao, J. Chen, Y. Zhao, Chemosphere 60 (2005) [2] B. O. Clarke, N. A. Porter, P. J. Marriott, J. R. Blackbeard, Environ. Int. 36 (2010) [3] G. N. Lu, X. Q. Tao, W. Huang, Z. Dang, Z. Li, C. Q. Liu, Sci. Total Environ. 408 (2010) [4] T. Sato, T. Todoroki, K. Shimoda, A. Terada, M. Hosomi, Chemosphere 80 (2010)

9 Applied Mechanics and Materials Vol [5] E. Eek, G. Cornelissen, A. Kibsgaard, G. D. Breedveld, Chemosphere 71 (2008) [6] Q. Zeng, S. Peng, M. Liu, Z. Song, X. Wang, X. Zhang, S. Hong, Chem. Eng. J. 230 (2013) [7] M.A. Sanchez-Trujillo, E. Morillo, J. Villaverde, S. Lacorte, Environ. Pollut. 178 (2013) [8] S. Ehsan, S.O. Prasher, W.D. Marshall, Chemosphere 68 (2007) [9] X. Miao, S. Chu, X. Xu, Chemosphere 39 (1999) [10] E. Arany, R. K. Szabo, L. Apati, T. Alapi, I. Ilisz, P. Mazellier, A. Dombi, K. Gajda-Schrantz, J. Hazard. Mater. 262 (2013)

10 Selected Proceedings of the Ninth International Conference on Waste Management and Technology / Characterizing the Photodegradation Ability of Pentachlorophenol in β-cyclodextrin Soil Washing Solution / DOI References [1] X. Quan, X. Zhao, S. Chen, H. Zhao, J. Chen, Y. Zhao, Chemosphere 60 (2005) /j.chemosphere [2] B. O. Clarke, N. A. Porter, P. J. Marriott, J. R. Blackbeard, Environ. Int. 36 (2010) /j.envint [3] G. N. Lu, X. Q. Tao, W. Huang, Z. Dang, Z. Li, C. Q. Liu, Sci. Total Environ. 408 (2010) /j.scitotenv [4] T. Sato, T. Todoroki, K. Shimoda, A. Terada, M. Hosomi, Chemosphere 80 (2010) /j.chemosphere [5] E. Eek, G. Cornelissen, A. Kibsgaard, G. D. Breedveld, Chemosphere 71 (2008) /j.chemosphere [6] Q. Zeng, S. Peng, M. Liu, Z. Song, X. Wang, X. Zhang, S. Hong, Chem. Eng. J. 230 (2013) /j.cej [7] M.A. Sanchez-Trujillo, E. Morillo, J. Villaverde, S. Lacorte, Environ. Pollut. 178 (2013) /j.envpol [8] S. Ehsan, S.O. Prasher, W.D. Marshall, Chemosphere 68 (2007) /j.chemosphere [9] X. Miao, S. Chu, X. Xu, Chemosphere 39 (1999) /S (99) [10] E. Arany, R. K. Szabo, L. Apati, T. Alapi, I. Ilisz, P. Mazellier, A. Dombi, K. Gajda-Schrantz, J. Hazard. Mater. 262 (2013) /j.jhazmat