Kinetics of Lindane Dechlorination by Zerovalent Iron Microparticles

Subscriber access provided by Fudan University

Article

Kinetics of lindane dechlorination by zero valent iron microparticles: Effect of different salts and stability study. Carmen Maria Domínguez, Joana Parchão, Sergio Rodriguez, David Lorenzo, Arturo Romero, and Aurora Santos Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.6b03434 • Publication Date (Web): 29 Nov 2016 Downloaded from http://pubs.acs.org on December 1, 2016

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

Industrial & Engineering Chemistry Research is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 33

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Industrial & Engineering Chemistry Research

Kinetics of lindane dechlorination by zero valent iron microparticles: Effect of different salts and stability study.

Carmen M. Dominguez*, Joana Parchão, Sergio Rodriguez, David Lorenzo, Arturo Romero and Aurora Santos Dpto. Ingeniería Química, Facultad de Ciencias Químicas, Universidad Complutense Madrid. Ciudad Universitaria S/N. 28040, Madrid, Spain. *e-mail: [email protected] , tel. +34 913944106, fax: +34 913944171.

Abstract This report is focused on the dechlorination of lindane, a recalcitrant and refractory pollutant, by zero valent iron microparticles (ZVIM) in batch and continuous mode. Experimental variables such as initial lindane concentration, ZVIM dosage and temperature were studied. Batch experiments indicate that the lindane dechlorination is enhanced with the increase of ZVIM dosage and reaction temperature, and is maintained with increasing initial pollutant concentration. Kinetic analyses elucidated that lindane degradation followed a first order reaction for both pollutant and ZVIM concentration. The kinetic model can also accurately predict the results in continuous mode (more realistic conditions), where the high stability of ZVIM has been thoroughly demonstrated. Further studies indicated that co-existence of common ions can i) not affect (SO42-, Na+, Ca2+, Mg+) or ii) promote (HCO3-, Cl-) the lindane dechlorination process. The results implied that ZVIM is a potential approach for in situ remediation of soil and groundwater lindane contamination. Key

Words: zero

valent iron

microparticles,

lindane,

dechlorination, kinetics, salts.

1 ACS Paragon Plus Environment

pollutant reduction,


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 33

1. Introduction

Lindane (also known as γ-Hexachlorocyclohexane or γ-HCH) is a manmade organochlorine insecticide produced mainly after the Second World War until the 1990s 1

and widely used throughout the world for agricultural and public health purposes

during the last decades 2-4. The application of technical HCH, t-HCH (a mixture of α, β, γ, δ, and ε HCH isomers) and purified lindane (the only HCH isomer with insecticidal properties) has resulted in environmental contamination of global dimensions 5-7 making the HCHs one of the most frequently detected chlorinated contaminants in the environment 8. Concern for these pollutants has grown exponentially in the last years due to their toxicity and long persistence, resulting finally in lindane inclusion (along to α- and β-HCH) in the list of persistent organic pollutants (POPs) in the Stockholm Convention in 2009 9. Although its production and use has been banned or restricted in most countries

10,11

, large amounts of lindane still remain in the places where it was

produced and/or used

12

. These polluted sites represent nowadays one of the globe’s

largest hazardous organic waste challenges 9 and finding a solution for managing them must be a priority for the scientific community. Therefore, the development of costeffective, safe and environmentally technologies for lindane (and the rest of the HCH isomers) degradation is crucial. In this sense, several degradation methods for the elimination of lindane and its wastes (mainly α-, β- and δ-HCH isomers) have been tested, including conventional techniques (coagulation, flocculation, membrane separation, adsorption, etc.)

13

, incineration

12

,

biological treatments 14 and chemical redox reactions (both oxidation 15 and reduction 16 processes). Among them, reduction over zero valent metals is a promising alternative, being zero valent iron (ZVI) the most common metal used drawing on its activity, abundancy, low cost and benign environmental impact

3,10,17


. In the presence of ZVI,

Page 3 of 33

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


pollutants lose electrons, giving rise to less hazardous compounds, whereas iron accepts these electrons resulting in its oxidized form. Based on the iron particle size, ZVI can be used as nanoparticles (ZVIN) and microparticles (ZVIM), being the former much more studied in this process due to the high intrinsic activity of nanosized materials 2,4,10,11,1823

. Recently, main efforts are focus in improving nanoparticles stability (the bottleneck

of these materials) by adding stabilizers 18,23 or supporting iron nanoparticles in order to avoid their agglomeration 2, which highly increments the cost of the process and hinders its application. The relatively lower activity of iron microparticles can be deeply offset by its higher stability (no agglomeration), since this aspect is essential for the implementation of this technology. ZVIM can be used for in situ environmental remediation techniques, such as in situ chemical reduction (ISCR), as permeable reactive barriers (PRBs) or directly injected into contaminated soil, depending on its size, the source of contamination (plume, groundwater, etc.) and soil characteristics 24. Anions and cations present in the groundwater can influence the reactivity of granular iron (Fe0)

25

and precipitates from salts may passivate reactive surfaces by blocking

electron-transfer sites and thus, reducing the iron reactivity 26. While there are very few studies on this topic (none for iron microparticles, to the best of our knowledge) and some controversy about the results, the influence of different salts in lindane (or HCH in general) degradation using ZVIM is an aspect worthy to be investigated. Moreover, the viability of ZVIM for in situ applications (injection or PRBs) in lindane degradation in soil and groundwater will be a suitable and cost effective option depending on the long-term integrity and effectivity of the microparticles. Hence, the long-term performance of ZVIM is also a topic of noteworthy practical importance. However, there is a lack of studies in this field to date.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

In the current work, the attention is focused on the potential application of ZVIM in lindane reduction. With this purpose, the effect of the main operating conditions, initial pollutant concentration, ZVIM concentration and reaction temperature, has been investigated, as well as the influence of different inorganic salts usually present in soil and groundwater. Special attention has been paid in ZVIM stability, which determines the feasibility of the process. A kinetic model for lindane degradation with reaction time in the presence of ZVIM has been proposed for the first time, which includes the temperature dependence.

2. Materials and Methods 2.1. Reagents Synthetic wastewaters of different concentrations were prepared by dissolving γ− hexachlorocyclohexane (Fluka) in acetone (Sigma-Aldrich) (10 g/L) and further

diluted in milli-Q water. Working standard solutions of γ− hexachlorocyclohexane, benzene (Sigma-Aldrich), and NaCl (Sigma-Aldrich) were prepared for CG-MS, HPLC and IC calibration respectively. To study the effect of different anions and cations in dechlorination reaction, NaHCO3 (Panreac), Na2SO4 (Sigma-Aldrich), CaSO4 (Riser) and MgSO4·7H2O (PROBUS) were used. Other reagents used were C2H3N (Scharlau), H2SO4 (Fisher), Na2CO3 (Panreac), C6H10O (Sigma-Aldrich). Reagents were of analytical grade. All the solutions were prepared with milli-Q water.

2.2. Zero-valent iron microparticles Commercial zero-valent iron microparticles (ZVIM) were supplied by Höganäs and used as received. This material was selected among five types of commercial ZVIM tested in a previous work in the dechlorination of a mixture of HCH isomers (α-, β-, γ-


Page 4 of 33

Page 5 of 33

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


and δ-HCH) since it showed the highest activity 27. ZVIM are composed mainly by iron in zero state (>99%) and only traces of C (0.003%), O (0.12%), S (0.01%), P (0.008%) and Mn (0.18%) could be identified. The SBET is 0.035 m2/g and the particle diameter 70 µm. Characterization of ZVIM (N2 adsorption/desorption isotherm and X-ray diffractogram can be found in Supporting Information as S.I.1 and S.I.2).

2.3. Dechlorination experiments 2.3.1. Batch experiments The batch experiments were carried out in 26 mL glass reactors shaken in a constanttemperature bath (10, 20 and 30 ºC) at an equivalent stirring velocity of 100 rpm. 20 mL of synthetic wastewater (concentration between 0.5 and 6 mg/L of γ-HCH) at natural pH (≈ 6.3) were added to the reactor containing 20-100-200 mg of ZVIM (CZVIM = 1-510 g/L), which represents the beginning of the reaction. With the aim of analyzing the effect of different anions and cations in dechlorination reaction, five salts (NaHCO3, Na2SO4, NaCl, CaSO4 and MgSO4·7H2O) in different concentrations were added to the initial lindane solution. In order to follow the evolution of the reaction, several reactors were prepared (one for each reaction time), and placed simultaneously in the temperature bath. Liquid phases were sampled from the reactor through a needle (for minimize the loss of volatile compounds, such as benzene), separated from iron microparticles by filtration (0.45 µm Nylon filter) and immediately analyzed. The possible decrease of lindane concentration by volatility (without ZVIM), as well as lindane adsorption over ZVIM, were previously discarded.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 6 of 33

2.3.2. Continuous experiments Continuous experiments were performed in a fixed bed reactor of Teflon (16 cm length and 0.4 cm diameter) with a ZVIM loading of 5 g, supported by glass wool layers (bulk density of the iron microparticles is about 2.99 g/cm3). The lindane solution was stored in a glass bottle submerged in a constant water bath (20 ºC) and conducted upwards through the column by action of a peristaltic pump. Samples were periodically extracted through a needle using a three-way valve. Different relations W/QL (ranged between 10 and 2778 g·h/L) were selected in order to evaluate the ZVIM stability for representative lindane conversions. Once column achieved the equilibrium for a given flow, the pollutant liquid flow (QL) was maintained during a time on stream of 10 days and reaction samples were periodically collected. After that, the liquid flow was modified and a new experiment performed, maintaining ZVIM in the fixed bed reactor without any treatment. All the experiments (batch and column) were performed by duplicate being the standard deviation always less than 10% (error bars have been included in Figures). Table 1 summarizes the dechlorination reaction runs carried out both in batch and column experiments, as well as the operating conditions selected for each experiment.

Table 1. Operating conditions of the dechlorination runs. Run 1 2 3 4 3 5 6 3 7 8 9

Reactor type Batch

Clindane,0 mg/L 0.5 3 6

CZVIM* g/L

Tª (ºC)

W/QL (g·h/L)

tos** (days)

Ion type

Cion (mM)

5

20

-

-

-

-

Batch

6

1 5 10

20

-

-

-

-

Batch

6

5

10 20 30

-

-

-

-

Batch

6

5

20

-

-

HCO3-

2 10


Page 7 of 33

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


10 20 11 0.2 12 Batch 6 5 20 SO420.5 13 2 5 14 15 5 20 15 Batch 6 Cl16 50 17 1 18 Batch 6 5 20 Ca2+ 5 19 10 20 0.1 21 Batch 6 5 20 Mg2+ 1 22 5 11 0.2 12 Batch 6 5 20 Na+ 0.5 13 2 23 42 Column 6 20 24 167 10 25 2778 * concentration of zero valent iron microparticles ** time on stream (column experiments)

2.5. Analytical methods The progress of the reaction was followed by analyzing liquid samples at different reaction times. Lindane was identified and quantified by means of a HP6890 Gas Chromatograph coupled with a HP5973 Mass Spectrometric Detector (MSD) using a CTC CombyPAL (GC sampler 80). The organic species were extracted by solid phase microextraction (SPME) using Polyacrylate (PA) coating fiber (3600 s at 38 ºC) and desorption was conducted in the injector at 270 ºC (180 s) in splitless mode. A SPB-624 fused-silica-capillary column (30 m × 0.25 mm ID and 1.40 µm the thickness) was used as stationary phase, 4-methylcyclohexanone as internal standard (ISTD) and Helium as carrier gas (1 mL/min). Benzene was identified and quantified by High Performance Liquid Chromatography (HPLC Agilent, mod. 1100) using an Agilent poroshell 120 SB-C18 column as the stationary phase and a mixture of 4 mM H2SO4 aqueous solution and acetonitrile (40-60% respectively) at 0.5 mL/min as mobile phase. A diode array detector (G1315A) at a wavelength of 210 was used. The dechlorination degree was evaluated in terms of chlorine ions formed, which were determined by IC (Metrohm 7 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

761 Compact IC) with anionic chemical suppression using a conductivity detector. A Metrosep ASUPP5 column (5 cm length, 4 mm diameter) was used as stationary phase and 1 mL/min of an aqueous solution 3.2 mM of Na2CO3 and 1 mM of NaHCO3 as mobile phase. The pH evolution was measured using a Basic 20-CRISON pH electrode.

3. Results and discussion The effect of several operating conditions such as initial pollutant concentration, iron microparticles concentration and temperature reaction were investigated following the evolution of lindane concentration with reaction time as well as the formation of benzene generated and chorines released to the liquid medium from lindane dechlorination. Results obtained were analyzed to obtain a kinetic model for lindane degradation. The influence of different salts solved in lindane solution and ZVIM stability were also checked.

3.1.1. Effect of initial pollutant concentration The effect of initial lindane concentration in the range 0.5-6 mg/L, which covers the interval commonly found in groundwater for this pollutant, was analyzed at room temperature, neutral pH and 5 g/L of ZVIM concentration. Normalized concentration of lindane (Clindane/Clindane,0) and chloride balance (expressed as the relative amount of Clmeasured in the bulk normalized by the chlorine content of the initial amount of lindane treated) resulting from runs 1-3 are represented in Figure 1a and 1b, respectively. First of all, the relationship between lindane degradation and chloride ions formation with reaction time highlights that dechlorination of lindane is complete and occurs also simultaneously (lindane is degraded releasing 6 chlorines to the reaction medium).


Page 8 of 33

Page 9 of 33

Secondly, the rate of lindane disappearance is independent of the initial lindane concentration, obtaining very similar profiles for the three concentrations tested (0.5, 3 and 6 mg/L). This aspect indicates that lindane degradation in the presence of ZVIM follows a first-order reaction with respect to the pollutant concentration, which is pretty common in systems in which ZVI nanoparticles

10,19,28-30

and microparticles

3,31

are

involved. Accordingly to these results and taking into account that lindane is reduced via dechlorination, the chloride concentration increases with reaction time and is the expected one for the lindane conversion obtained in each case. The results of Cl-/Cl0 corresponding to the lower lindane concentration tested, Clindane,0=0.5 mg/L, are not included in Figure 1b because of experimental limitations associated to chloride measure when working at such a low concentration (the theoretically chloride concentration for the complete depletion of lindane in this case is 0.36 mg/L, pretty low to measure accurately).

1.0

1.0

0.0

b) Ln(Clindane/Clindane,0)

a)

0.6

-1.5

0.8 -3.0

0.6

-4.5

-

Clindane/Clindane,0

0.8

Cl /Cl0

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


-1

2

-1

2

kapp1 = 0.0071 ± 0.0001 h (R =0.996) kapp5 = 0.0334 ± 0.0010 h (R =0.984)

-6.0

-1

2

kapp10 = 0.0664 ± 0.0029 h (R =0.991)

0.4 0

20

40

0.4

60

80

100

t(h)

Clindane=0.5 mg/L

0.2

Clindane=3 mg/L

0.2

Clindane=6 mg/L

0.0 0

20

40

60

80

100

20

40

t (h)

60

80

0.0 100

t (h)

Figure 1. Normalized lindane concentration (a) and Cl- balance (b) in the presence of ZVIM (T=20 ºC, pH0=6.3, CZVIM=5 g/L) at different initial concentrations of lindane. Experimental data (symbols) and predicted values (lines) from Eq.8. Inset in Figure 1a:



apparent rate constant (kapp) at different ZVIM loadings (T=20 ºC, pH0=6.3, Clindane,0=6 mg/L). The concentration of benzene measured for the three experiments was always increasing with reaction time up to around 72 hours, remaining practically constant from this time, and was also proportional to the initial lindane concentration (data not shown). Taking into account the reaction stoichiometry (C6H6Cl6+ 3 Fe0 → C6H6 + 3 Fe2+ + 6 Cl-) and the complete dechlorination degree achieved at the end of the reaction (Figure 1b) a value around 33% of the theoretic amount expected for complete lindane reduction was measured in all cases at 96 hours (in the liquid phase), being the remaining benzene in the gas phase, due to the high volatility of this compound

11,23

. Otherwise, no

differences in terms of pH were found during dechlorination reaction, showing the effluents values between 6.4 and 6.7.

3.1.2. Effect of ZVIM concentration The effect of ZVIM loading was analyzed at room temperature, and neutral pH. In order to facilitate the products identification and quantification, these experiments were performed at the higher concentration of pollutant tested, 6 mg/L (runs 3-5). 1.0

b)

a)

0.8

0.8

0.6

0.6

0.4

0.4

-

Clindane/Clindane,0

1.0

Cl /Cl0

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 10 of 33

0.2

CZVIM=1 g/L

0.2

CZVIM=5 g/L CZVIM=10 g/L

0.0 0

20

40

t (h)

60

80

100

20

40

60

t (h)


80

0.0 100

Page 11 of 33

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 2. Normalized lindane concentration evolution (a) and Cl- balance (b) (T=20 ºC, pH0=6.3, Clindane,0=6 mg/L) at different ZVIM concentrations. Experimental data (symbols) and predicted values (lines) from Eq.8. As expected, the lindane degradation rate increases with increasing the iron microparticles dose from 1 to 10 g/L (Figure 2a), as a result of a higher amount of reductant sites available for lindane reduction, since the degradation of halogenated compounds by ZVI takes usually place on the iron-water interface

2,19,21,32,33

. Thus, the

required time for achieving total depletion of lindane working with 10 g/L of iron concentration is around 48 h, while only a decrease of 37% is obtained at 1 g/L. Consequently, the concentration of chlorines measured in the reaction medium (Figure 2b) increases as long as the iron microparticles concentration increases. Only when working with the lower ZVIM load, chloride balance (Cl-/Cl0) is not accomplished, due to the incomplete degradation of lindane (Xlindane=76% at 96 h). Accordingly, the same tendency was obtained for benzene concentration (the higher ZVIM load, the faster benzene generation). Regarding the pH, no differences were found when ZVIM load is modified. 3.1.3. Effect of temperature Figure 3 presents lindane concentration upon reaction time at different temperatures, 10, 20 and 30 ºC (runs 6, 3 and 7, Table 1), dealing with 6 mg/L of initial pollutant concentration, 5 g/L of ZVIM and neutral pH. As can be seen, the rate of lindane disappearance increases substantially with the temperature within the 20 ºC range tested, as expected taking into account the Arrhenius law (Eq. 10). Complete pollutant degradation was achieved at 30 ºC after 48 h reaction time vs around 50% at 10 ºC, showing the important effect of this variable in dechlorination rate. Accordingly,



benzene concentration also increases with temperature up to reach approximately 33% of the theoretic amount expected for complete lindane reduction.

6

º º º

10 C 20 C 30 C

5

Clindane (mg/L)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 12 of 33

4 3 2 1 0 0

20

40

60

80

100

t (h) Figure 3. Normalized lindane concentration evolution in the presence of ZVIM (pH0=6.3, Clindane,0=6 mg/L, CZVIM=5 g/L) at different temperatures. Experimental data (symbols) and predicted values (lines) from Eq.8. 3.2. Kinetics of lindane degradation Considering that the degradation of lindane in the presence of ZVIM is a heterogeneous reaction that takes place over microparticles surface, the disappearance rate for lindane degradation can be expressed by a simple power law-rate model:

− = · (

·

)

Eq.1

Where, -Rlindane is the rate of lindane disappearance, kW, the rate constant related to ZVIM mass (L/gZVIM·h), Clindane, the lindane concentration (mg/L), and m, the reaction order with respect to lindane concentration. 12 ACS Paragon Plus Environment

Page 13 of 33

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


From the results obtained (Xlindane ≈ Cl-/Cl0) it can be assumed that, in the presence of ZVIM, lindane is degraded in three rapid dechloroelimination steps to benzene releasing Cl- to the liquid phase. Then, the global reaction scheme for lindane dechlorination by ZVIM can be described as follows: C6H6Cl6 + 3 Fe0 → C6H6 + 3 Fe2+ + 6 ClTherefore, taking into account the reaction stoichiometry, = −6 ·

Eq.2

Considering the mass balance for lindane degradation in a stirred batch reactor, with constant reactor volume, Eq. 1 can be expressed as follows: −

= · ·

Eq.3

where CZVIM is the zero valent iron microparticles concentration (g/L). As the reductant (Fe0) is in highly excess in relation to the pollutant (lindane), ZVIM concentration can be assumed constant during reaction time, obtaining that:

= ·

Eq.4

Thus, the expression for the lindane degradation rate [Eq. 3] results in: −

= ·

Eq.5

Assuming that the disappearance rate of lindane can be described by pseudo-first order reaction kinetics for pollutant concentration, m is equal to 1 (Section 3.1.1., Figure 1a). The integration of [Eq. 5] results in the following expression: = exp (− · ) ,

Eq.6

Thus, the apparent rate constants at different ZVIM loadings, viz 1, 5 and 10 g/L (named kapp1, kapp5 and kapp10) can be calculated from the representation of Ln (Clindane/Clindane,0) vs reaction time (h) (inset in Figure 1a, kapp1=0.0071 h-1, kapp5=0.0334 13 ACS Paragon Plus Environment


h-1 and kapp10= 0.0664 h-1). For obtaining kapp,5 value, the three experiments carried out at different initial lindane concentration (runs 1-3) have been used. In order to verify whether lindane dechlorination over ZVIM is a heterogeneous reaction (assumption made previously), the obtained apparent rate constants were represented vs ZVIM concentration (Eq. 4), obtaining a kW value of 0.066 L/g·h from the slope of the line, and an intercept of the regression line negligible (Figure 4). This fact confirms our assumption, indicating that there is not homogeneous contribution or mass transfer problems in the experimental conditions tested. Thus, lindane degradation can be also considered as a first order reaction with respect to the ZVIM concentration. This fact also suggests that the heterogeneous active sites of iron surface are not covered or poisoned during reaction. The resulting rate constant, normalized by the iron microparticles surface (0.035 m2/g), kW´ (0.194 L/m2·h) is of the same order of magnitude to that obtained by Wang (2009)

3

when working with commercial ZVIM

obtained from Fisher Scientific Co (SBET=0.1582 m2/g) for lindane dechlorination and significantly higher than the corresponding to another commercial ZVIM purchased from Beijing Chemical Factory (SBET=0.234 m2/g) used also for lindane treatment 31.

0.08 kW = 0.066 ± 0.005 L/gh (R2=1)

0.06 -1

kapp (h )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

0.04

0.02

0.00 0

2

4

6

8

CZVIM (g/L) 14 ACS Paragon Plus Environment

10

Page 14 of 33

Page 15 of 33

Figure 4. Linear relationship between apparent rate constants and ZVIM concentration (T=20 ºC, pH0=6.3, Clindane,0=6 mg/L). The rate constant (kW) is temperature dependent and it was assumed according to the Arrhenius equation:

= # exp (

−$% ) ·&

Eq.7

where A is the Arrhenius constant, Ea is the activation energy (kJ/mol), R is the universal gas constant (kJ/mol·K), and T is the absolute reaction temperature (in K). The apparent rate constant corresponding to 10, 20 and 30 ºC (kapp10ºC, kapp20ºC and kapp30ºC), working with CZVIM=5 g/L has been calculated from Figure 5.

0 -1

Ln(Clindane/Clindane,0)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


-2 -3 -4 -5 kapp10ºC = 0.0147 ± 0.0004 h-1 (R2=0.991) kapp20ºC = 0.0334 ± 0.0010 h-1 (R2=0.984)

-6

kapp30ºC = 0.067 ± 0.0026 h-1 (R2=0.992)

-7 0

20

40

60

80

100

t (h)

Figure 5. Apparent rate constant (kapp) at different temperatures (pH0=6.3, Clindane,0=6 mg/L, CZVIM=5 g/L).



As expected, the apparent reaction rate increased rapidly with increasing temperature (Figure 5). Using the obtained apparent rate constants (kapp10 ºC, kapp20 ºC and kapp30 ºC), and taking into account that kW=kapp/CZVIM [Eq. 4], the value of activation energy and pre-exponential factor were determined from the corresponding Arrhenius plot (Figure 6). The fitting of Ln(k) vs 1/T yields a straight line (R2 = 0.999) throughout the temperature range studied. Accordingly, the slope of the straight line gives an activation energy (Ea) of 54.15 kJ/mol, and the Arrhenius constant (A) was calculated to be 2.91·107 from the intercept of the line. Dechlorination reactions controlled by mass transfer have generally low activation energy values, typically between 10 and 20 kJ/mol

21,34

, whereas the surface controlled

reactions are characterized by higher activation energies, usually above 29 kJ/mol 35. As the Ea value was found to be 54.15 kJ/mol, it can be inferred that lindane degradation kinetics by ZVIM is controlled by the surface chemical reaction, which is in accordance to the results previously obtained (Figure 4).

-4.0

-4.5

Ln (kW)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

-5.0

-5.5 2

Ea = 54.15 ± 0.02 kJ/mol (R =0.999)

-6.0 0.0033

0.0034

0.0035 -1

1/T (K )


Page 16 of 33

Page 17 of 33

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 6. Linearized Arrhenius plot of kW as function of 1/T (pH0=6.3, Clindane,0=6 mg/L, CZVIM=5 g/L).

Based on the above results, after substituting the values for lindane reaction partial order (m) and the activation parameters (A, Ea) into [Eq. 1], the rate expression upon the power-law model now becomes: − = 2.91$7 · ,

-../-±1.12 3·4

· (

56 789%9, ) 6 :;

Kinetics of Lindane Dechlorination by Zerovalent Iron Microparticles

Recommend Documents