Kinetics of 2-chlorobiphenyl Reductive Dechlorination by Pd-fe0 Nanoparticles

Kinetics of 2-chlorobiphenyl (2-Cl BP) catalytic reductive dechlorination by Pd-Fe nanoparticles were investigated. Experimental results showed that ultrafine bimetallic Pd-Fee nanoparticles were synthesized in the presence of 40 kHz ultrasound in order to enhance disparity and avoid agglomeration. The application of ultrasonic irradiation during the synthesis of Pd-Fe nanoparticles further accelerated the dechlorinated removal ratio of 2-Cl BP. Up to 95.0% of 2-Cl BP was removed after 300 min reaction with the following experimental conditions: initial 2-Cl BP concentration 10 mg L, Pd content 0.8 wt. %, bimetallic Pd-Fe nanoparticles prepared in the presence of ultrasound available dosage 7 g L, initial pH value in aqueous solution 3.0, and reaction temperature 25 . The catalytic reductive dechlorination of 2-Cl BP followed pseudo-first-order kinetics and the apparent pseudo-first-order kinetics constant was 0.0143 min.


Introduction
Historical and continued releases of polychlorinated biphenyls (PCBs) to the environment have resulted in polluted water, soil, and sediment [1].These contaminated soils, sludges and groundwater need to be effectively remediated at ambient temperatures using either ex situ or in situ methods.Methods for PCBs dechlorination include chemical reduction, catalyzed hydrodechlorination, and electrolytic dechlorination.Most of these techniques use highly active reducing hydrogen from different sources to facilitate the dechlorination through the use of noble metal catalysts such as palladium (Pd) and rhodium (Rh), etc [2][3][4].
Pd-Fe 0 bimetallic nanoparticles have been improved to completely dechlorinate chloroaromatics and thoroughly purify a wide range of chlorinated organic compounds without leaving chlorinated side products [2][3][4][5][6].As one of the noble metal, Pd can utilize the produced H 2 from zero-valent iron (ZVI) corrosion and improve the rates and efficiencies of dechlorination reaction [1,4].In addition, the presence of Pd not only reduces the accumulation of toxic byproducts, but also inhibits particle oxidation in air [5].Pd-Fe 0 bimetallic nanoparticles, when compared to the conventional large particles have some advantages with possessing large specific surface area and high surface reactivity [6].In order to obtain the stabilized and high reactive Pd-Fe 0 bimetallic nanoparticles, ultrasound is applied to the preparation of Pd-Fe 0 nanoparticles because acoustic cavitation can enhance the surface area and surface properties of the reactive solids by causing particles to rupture [7].
In the present work, Pd-Fe 0 bimetallic nanoparticles were prepared in the presence of 20 kHz ultrasound and 2-chlorobiphenyl (2-Cl BP) dechlorination kinetic by Pd-Fe 0 bimetallic nanoparticles in aqueous solution was investigated.Meanwhile, other influential factors contributing to 2-Cl BP reductive dechlorination, such as Pd-Fe 0 nanoparticles availability, Pd content over Fe 0 , initial 2-Cl BP concentration, reaction temperature and the initial pH values, were also studied .Pd-Fe 0 nanoparticles were synthesized immediately before use.

Experimental procedures
In a 500 mL three-necked flask, Pd-Fe 0 bimetallic nanoparticles were prepared in the presence of 40 kHz and 150 W ultrasound under nitrogen gas.

Methods of analysis
All fresh prepared synthesized bimetallic Pd-Fe 0 nanoparticles (with Pd content 0.3 wt.%) were immersed in absolute ethyl alcohol and dispersed by an ultrasonator.Transmission electron microscope (TEM) images were obtained through a microscope (JEOL JEM 200CX JEOL Electronics Co., JP).X-ray diffraction (XRD) analysis was performed by using X'Pert Pro advanced X-ray diffractometer (Ȝ = 1.5418AÛ).Brunauer-Emmett-Teller (BET) specific surface area of all synthesized bimetallic Pd-Fe 0 nanoparticles were measured using nitrogen adsorption method with a surface analyzer (ASAR2020M+, Micromeritics Instrument Corp., US).Before the analysis, the particles were dried in vacuum at 25 for 24 h and then hydrogen flow at 260 for further 4 h.Organic compounds such as 2-chlorobiphenyl and BP were analyzed by Thermofish Trace 1310 Gas Chromatography.ECD and FID detector, Thermofish TR-5 Column (30 m×0.32 mm, 1.0 ȝm), inlet temperature 300 , detector temperature 250 , no split injection, injection volume 1 ȝL, carrier gas nitrogen (purity 99.99%), column flow 1.5 mL/min (constant current), program temperature: column temperature 100 , holding 0.5min, 25 /min was raised to 300 , maintained 2 min.

Pd-Fe 0 nanoparticles Characterization
Fig. 1 (a, b) shows TEM images of the freshly synthesized bimetallic Pd-Fe 0 nanoparticles in the presence and absence of ultrasound.The freshly prepared Pd-Fe 0 nanoparticles in the absence of ultrasound, were spherical in shape with particle size ranging from 20 to 100 nm, and appeared to aggregate together (Fig. 1(a)).However, Pd/Fe nanoparticles synthesized in the presence of ultrasound were also spherical, but the particle size ranges from 10 to 100 nm, mainly foused on 30 to 75 nm, and appeared to be smaller particles diameter and better dispersion (Fig. 1(b)).The sizes and specific surface area of most particles synthesized in the presence of ultrasound were obviously modified.The ultrafine and highly dispersion nanoparticles were obtained.
Fig. 2(a, b) shows the XRD patterns of the fresh and the 300 min reacted bimetallic Pd-Fe 0 nanoparticles synthesized in the presence of ultrasound.The XRD pattern for the fresh sample presents a strong peak 44.66 ƕ which corresponds to the body-centered cubic N-Fe 0 at the (110) plane.The peak in the XRD pattern of the reacted sample shows evidence of iron oxides, possibly Fe 3 O 4 or Fe 2 O 3 , or their mixture.This is consistent with the previous work [8].
In addition, BET specific surface area of the synthesized Pd-Fe 0 nanoparticles in the presence and absence of 40 kHz ultrasonic irradiation were 41.68 m 2 g -1 and 22.39 m 2 g -1 , respectively.Therefore, bimetallic Pd-Fe 0 nanoparticles were synthesized by using ultrasound strengthened liquid phase reductive method in the following experiments.Figure 3 shows the hypothetical reductive dechlorated pathways of 2-chlorobiphenyl (2-Cl BP) by Pd-Fe 0 nanoparticles prepared in different methods.Biphenyl (BP) is the sole final organic product.In addition, ased on the previous studies, the catalytic reductive dechlorination of chloroaromatics by bimetallic Pd-Fe 0 nanoparticles involves three main steps [2,4,[9][10][11][12][13]: (1) chloroaromatics in solution diffuse to Pd-Fe 0 nanoparticles surface and adsorb in the surface of catalyst; (2) the adsorbed chloroaromatics takes reduction reaction under the effect of Pd-Fe 0 nanoparticles; (3) the products of reaction desorbs from Pd-Fe 0 nanoparticles and diffuses to the bulk solution.It is well established that the pseudo-first-order kinetics could be applied in the reductive dechlorination of chloroaromatics if Pd-Fe 0 nanoparticles availability is excessive in the reaction.Therefore, the pseudo-first order reaction kinetics was adopted to model the 2-Cl BP dechlorination reaction by Pd-Fe 0 nanoparticles.Therefore, it is assumed that 2-Cl BP was hydrodechlorinated according to the following sequence of steps.
Where Į represents the molar fraction of the subscript organic compound to the initial concentration of 2-chlorobiphenyl.Since a fraction of 2-Cl BP and BP were absorbed on the larger surface of Pd-Fe 0 nanoparticles, the actual concentration of the 2-Cl BP and BP in aqueous phase has to be amended.As the production of BP was step wise, equilibrium time for organic compounds absorbed onto Pd-Fe 0 nanoparticles was little, and the total molar fraction of 2-Cl BP and BP did not change, the ratio of BP in the aqueous phase can be seen as invariable, as a result, Eq. ( 5) can be revised as follows: ' (1 ) Where Į ' represents the molar fraction of the organic compounds which were adsorbed onto bimetallic Pd-Fe 0 nanoparticles to the initial concentration of 2chlorobiphenyl.Then k values were derived from fitting the experimental data into Eq.( 6) according to the nonlinear least-square regression.The influential factors, such as the preparation method of bimetallic Pd-Fe 0 nanoparticles, Pd content over Fe 0 , bimetallic Pd-Fe 0 nanoparticles dosage, reaction temperature, initial pH values and initial 2-Cl BP concentration on the 2-Cl BP dechlorination efficiency were investigated in the following sequences.Kinetic constant k values in different experimental conditions were listed in Table 1.
The experimental results showed that k values obviously increased from 0.0018 to 0.0143 min -1 under bimetallic Pd/Fe nanoparticles prepared in the absence and presence of ultrasound, respectively.They increased from 0.0119 min í1 to 0.0203 min í1 as the Pd content over Fe 0 was increased from 0.70 wt.% to 0.90 wt.%, and they increased from 0.0098 min í1 to 0.0309 min í1 as the bimetallic Pd-Fe 0 nanoparticles available dosage was increased from 5 g L -1 to 9 g L -1 .In short, k values increased with the increasing Pd content over Fe 0 , Pd/Fe nanoparticles available dosage and reaction temperature, with the decrease of initial pH values.
Firstly, nanoscale zero-valent iron (nZVI) particles were synthesized by drop wise addition of stoichiometric amounts of NaBH 4 aqueous solution into a flask containing FeSO 4 •7H 2 O aqueous solution simultaneously with mechanical stirring at 25 .nZVI particles were then rinsed several times with deoxygenated deionized water.nZVI particles were then rinsed several times with deoxygenated deionized water.Subsequently, Pd-Fe 0 nanoparticles were prepared by reacting with the wet nZVI particles in an aqueous solution of potassium hexachloropalladate under mechanical stirring.Batch experiments of 2-Cl BP catalytic reductive dechlorination were performed in the same flask into which nanoscale bimetallic Pd-Fe 0 particles were added.2-Cl BP stock solutions and a certain amount of deoxygenated deionized water and methanol solution (50:50,V/V) were added into the flask containing fresh prepared nanoscale Pd-Fe 0 particles into 500 mL of total reaction volume.The reaction solution was stirred under nitrogen flow to simulate anaerobic environment at 25 .Aliquots of samples were periodically collected with glass syringes and the reaction was quenched by passing through 0.22 m P polyether sulphone (PES) membrane filters.

Figure. 1 . 2 DOIFigure. 3 .
Figure. 1.(a) TEM image of fresh Pd/Fe nanoparticles prepared in the absence of ultrasound, (b) TEM image of fresh Pd/Fe nanoparticles prepared in the presence of 40 kHz ultrasound The corresponding reaction rate equations for the disappearance of 2-Cl BP and the accumulation of BP in the batch system are shown as follows:

Table 1 k
values in different experimental conditions