A.Vijaya Bhaskar Reddy et al., IJSID, 2012, 2 (1), 106-112
ISSN:2249-5347
IJSID International Journal of Science Innovations and Discoveries Research Article
An International peer Review Journal for Science
Available online through www.ijsidonline.info
DEGRADATION OF CHLORPYRIFOS IN AQUEOUS SOLUTIONS WITH CHITOSAN-STABILIZED Fe0 NANOAPARTICLES A. Vijaya Bhaskar Reddy, K. Gangadhara Reddy, V. Madhavi and G. Madhavi* Department of Chemistry, S.V. University, Tirupati, A.P, India
ABSTRACT
Received: 16.11.2011 Accepted: 14.02.2012
Chitosan-stabilized Fe0 nanoparticles (CS-Fe0) and unstabilized Fe0 nanoparticles synthesized in ethanol–water mixed system (EW-Fe0) and were tested for degradation of
*Corresponding Author
chlorpyrifos in aqueous solution. Fourier transform infrared (FTIR) study suggested that nitrogen and oxygen atoms are the binding sites for chitosan on iron which was accountable for the stability of Fe0 nanoparticles. The CS-Fe0 was still in zero valence state after exposure to air over 3-months period was observed by stability experiments. Batch experiments demonstrated that the maximum chlorpyrifos degradation rate for CS-Fe0 was extremely higher than EW-Fe0. Batch kinetic tests indicated that the rate constants kobs were 0.288h-1 and 0.703h-1 for WS-Fe0 and CS-Fe0, the degradation rate was increased with decreasing solution PH value upto 3. There after the removal efficiency decreased with decreasing PH. the degradation rate can be expressed by a pseudo-first order kinetics and the rate constants decrease with increasing the chlorpyrifos concentration. Due to the INTRODUCTION
Address:
fast reaction kinetics and good stability against oxidation in air, the chitosan-stabilized
Name: G. Madhavi Place: Associate Prof., Dept. of Chemistry SV. University, Tirupati, AP, India
E-mail:
[email protected]
Fe0 nanoparticles have the potential to become an effective agent for in situ subsurface environment remediation. Keywords: Chitosan, Rate constant, Chlorpyrifos, Effect of PH
INTRODUCTION
International Journal of Science Innovations and Discoveries, Volume 2, Issue 1, January-February 2012
106
A.Vijaya Bhaskar Reddy et al., IJSID, 2012, 2 (1), 106-112 INTRODUCTION Metal nano particles particularly iron offers many advantages for remediation of chlorinated and pesticide contamination sites [1]. In recent years nanoscale zerovalent iron has been used for the treatment of environmental pollution because of their small particle size, high specific surface area and excellent reduction reactivity [2-3]. It is very effective reducing agent for dehalogenation of chlorinated organic compounds including the persistent organic pollutants chlorohydrocarbons [4], chloroaromatics [5], chlorophenol [6], organochloride pesticides (OCPs) [7] and polybrominated diphenyl ethers (PBDEs) [8]. They are highly reactive because of their large surface area, which enables a more rapid degradation of contaminants, if compared to millimetric iron particles. Moreover, their small size facilitates the delivery of suspension close to the source of contamination [9]. The main disadvantage of the present types of nanoparticles is their quite fast surface oxidation in aqueous environment or in the air, which happens unfortunately already during the stage of their storage and application preparation. This oxidation causes not only the loss of a part of the reductive effect of nZVI, but due to the change of particle surface charge, the oxidized particles tend to a higher aggregation and to get stuck on the material whose pores they should be going through.
Thereby a relatively significant loss of its transportability in saturated
environment occurs. A stabilizer can enhance dispersion (or reduce agglomeration) of nanoparticles through (a) electrostatic repulsion (i.e., adsorption of charged stabilizer molecules to the metal core results in an enhanced electrical double layer and, thus, increased Coulombic repulsion between the capped particles). To implement the in situ injection of Fe nanoparticles, an ideal stabilizer should (a) be able to specifically interact with the nanoparticles and hence suppress their growth, (b) be environmentally benign, (c) be cost-effective, and (d) be mobile in soils. NZVI particles can be covered in different polymers in order to increase the suspension stability and therefore particle mobility [10]. Compiled the following list of surface modifiers: polyacrylate [11] polyvinyl alcohol [12], guar gum [13], carboxymethyl cellulose, starch [14-15] etc. Recently, He and Zhao reported that a food-grade water soluble starch can improve both the dispersibility and the reactivity of Fe nanoparticles. Unfortunately, the starched Fe particles became less stable, as evidenced by the appearance of floc precipitates after 2 days, thereby limiting long-term storage and commercial application of these Fe nanoparticles. A better stabilizer with stronger interactions with the Fe particles needs to be developed that will allow longer-lasting effective stabilization and facilitate environmental applications of these nanoparticles. In recent years, chitosan has drawn significant attention as a surface coating agent due to its biocompatibility, biodegradability and amphiphilicity[16]. Chitosan represents an inert and biodegradable resource with promising applications in the area of environmental cleanup. The primary objective of this work was to prepare new class of chitosan stabilized Fe0-nanoparticles for the degradation of chlorpyrifos. Because of its wide usage chlorpyrifos was selected as the target compound. We also aimed to characterize rate of degradation by altering solution PH and initial chlorpyrifos concentration, degradation kinetics was measured by following pseudo-first order kinetics. MATERIALS AND METHODS Chemicals The following chemicals were used as received: Fe (ΙΙ) sulfate heptahydrate (FeSO4.7H2O), Sodium borohydride (NaBH4), anhydrous sodium sulfate (Na2SO4) and Chlorpyrifos (purity >99.9%) were purchased from Sigma-Aldrich (St.Louis, MO, U.S.A). Medium molecular weight chitosan (poly-(1,4-â-D-glucopyranosamine), 400 000 g/mol) was purchased from Fluka, and all compounds were used as received. Acetic acid (A. R.) was diluted to 1% aqueous solution before use. All aqueous solutions were made with ultra-high-purity water purified with an ultrapure water system Mill-Q Plus (Millipore Co.). International Journal of Science Innovations and Discoveries, Volume 2, Issue 1, January-February 2012
107
A.Vijaya Bhaskar Reddy et al., IJSID, 2012, 2 (1), 106-112 Synthesis of CS-Fe0 nanoparticles Chitosan was dissolved in 0.05 M HNO3 to make the final concentration of 0.5% by weight. Finally, chitosan solution was stirred overnight and was filtered through 0.2μm syringe filters to remove any suspensions. Chitosan-Fe0 was prepared in situ by reducing Fe (II) with NaBH4 in the presence of chitosan as a stabilizer. To ensure all the Fe (II) was reduced, excess of NaBH4 over the Fe (II) was used. The detailed procedure was as follows: 10 mL of solution containing 0.2978 g of FeSO4·7H2O was first mixed with 3 mL of 0.5% chitosan solution. The mixture was stirred for 30 min under nitrogen gas. Then, to the mixture, 5 ml of freshly prepared aqueous solution containing 0.3467 g of NaBH4 was added dropwise. At this stage, gas was evolved vigorously and black precipitate was formed. Again, the mixture was stirred for another 90 min. The resulted black precipitate was collected and washed by deoxygenated water for three times to get rid of the excess chemicals. The whole process was carried out in a nitrogen atmosphere. The prepared chitosan-Fe0 nanoparticle powder was stored at room temperature for further use. CS-Fe0 Nanoparticle characterization Morphological studies of the synthesized CS-Fe0 nanoaparticles were carried out by using scanning electron microscope (SEM) fitted with an Energy Dispersive X-ray Analysis (EDAX) System (model CARL-ZEISS EVO MA 15). The sample was observed at 10,000X magnification with an accelerating voltage of 20 kV. FT-IR analysis of the samples was carried out in the mid-IR range using a Perkin Elmer (USA) FT-IR Spectrum 2000 spectrometer. Batch experiments Batch experiments were conducted in 100 mL serum bottles containing 50 mL of aqueous chlorpyrifos solution with an initial concentration of 30mgL-1 and 1% w/v of CS-Fe0 and EW-Fe0 nanoparticles individually. The bottles were then fitted with Teflon Mininert valves and mixed on a rotary shaker (100 rpm) at 22°C. At selected time intervals, 1 mL of the aqueous sample was withdrawn then the sample was transferred into a 2 mL GC vial containing 1 mL of hexane for extraction of chlorpyrifos. Upon phase separation, the extract was analyzed for chlorpyrifos using GC-MS (JEOL GCMATE II GC-MS). Control experiments (without the addition of the nanoparticles) were carried out in parallel. The slight reduction (4%) in solvent extraction efficiency due to the use of biopolymer chitosan was corrected through blank tests. All experimental points were duplicated. Conditions for chlorpyrifos analysis A GC-MS (JEOL GCMATE II GC-MS) was used for the detection of intermediate products of chlorpyrifos during the degradation .The GC was equipped with a HP-5MS capillary column (30 m x 0.025 mm i.d) in helium carrier gas (1 mL per min) and with splitless injection system. The column was initially maintained at 900C for 5 min, and then increased to 290oC at a rate of 8oC per min and hold at 290oC for 5 min. The injector and interface temperature were kept at 280o C and the source temperature at 250O C. The injection volume was 1µL. Mass spectra were obtained by the electron impact (EI) at 70 eV using SIM mode. RESULTS AND DISCUSSION Characterization of nanoaparticles Scanning electron microscopy (SEM) images of both chitosan stabilized and unstabilized Fe0-nanoparticles were shown in fig.1. Fig 1.a shows that Fe0 nanoparticles formed much larger dendritic flocks without chitosan. Fig.1.b shows that Fe0 nanoparticles and chitosan polymers are arranged in a homocentric layered structure with both Fe0 nanoparticles and chitosan polymers associated each other. Excess chitosan in the solution may form polymer nanoparticles in different International Journal of Science Innovations and Discoveries, Volume 2, Issue 1, January-February 2012
108
A.Vijaya Bhaskar Reddy et al., IJSID, 2012, 2 (1), 106-112 morphology. The presence of chitosan effectively prevents agglomeration of the iron nanoparticles and thus maintains high surface area and great reactivity. FTIR analyses of chitosan-stabilized samples did not show any changes, especially, in the strongly absorbing bands corresponding to the wide peak at 3449 cm−1 stretching vibration of hydroxyl, amino and amide groups, which indicated there is no chemical interaction between these groups and Fe0. The FT-IR spectrum of Chitosan itself showed some features of amide groups: amide I and amide II bands at ≈1640 cm−1 (Fig. 2). These data clearly indicated that there were no chemical interactions between the Fe0 nanoparticles and chitosan. The expected stabilization of nanoparticles was achieved probably due to electrostatic interactions between the two constituents, resulting into surface passivation. Such a property of the stabilizing agent could be highly desirable [17-18].
Figure.1 A) SEM image of unstabilized Fe0 B) SEM image of CS-Fe0 nanoparticles.
Figure.2 FT-IR analysis of A)Chitosan and B) Chitosan stabilized Fe0 nanoparticles. Degradation of chlorpyrifos In this study, it is found that degradation of chlorpyrifos was preceded in two steps. During the first 30 min of the experiment, decrease of chlorpyrifos is much dramatic. After the first 30 min till the end of the experiment the reaction can be expressed with pseudo-first order kinetics. The reduction of chlorpyrifos with unstabilized Fe0 nanoparticles exhibits slow reduction. d[cpp]/dt = -kobs [cpp]
(1)
Where [cpp] is the concentration of chlorpyrifos in the aqueous solution (mg/L) at time t (min), Kobs is the pseudofirst-order rate constant (min-1). The reaction kinetics of chlorpyrifos degradation was modeled with a pseudo-first order rate equation. Plot of lnC versus time through linear regression analysis was used to obtain first order rate constants (Kobs). Disappearance of chlorpyrifos with CS-Fe0 and WE-Fe0 nanoparticles was plotted in the form of ln(C/C0) as a function of time International Journal of Science Innovations and Discoveries, Volume 2, Issue 1, January-February 2012
109
A.Vijaya Bhaskar Reddy et al., IJSID, 2012, 2 (1), 106-112 is shown in Fig. 3.a. It shows that the chlorpyrifos degradation rate was extremely slow when unstabilized Fe0 nanoparticles were applied. However, the degradation rate was clearly improved when the chitosan was applied to yield more dispersed iron nanoparticles. The rate constants (Kobs) can then be determined by fitting the pseudo-first rate expression from eq. 1 to the experimental data. The value of Kobs was 0.703h-1 for chitosan-stabilized Fe0 and 0.288h-1 for unstabilized Fe0 particles. The blank experiments (without nanoiron particles) did not show any appreciable degradation of chlorpyrifos during the course of the experiment (4hours).
Figure.3 a) Time course degradation of Chlorpyrifos with1% w/v of nanoparticles
Figure.3. b) First-order kinetic model for chlorpyrifos degradation. Effect of solution PH The effect of solution pH on chlorpyrifos degradation was observed by keeping all other parameters constant and varying the initial solution pH values from 7 to 2. The degradation rate of chlorpyrifos was increased with solution PH upto 3.0 then the degradation efficiency started to decrease, indicating an optimum PH is around 3.0 (Fig.4). Degradation rates were 89%, 92%, 94%,96%, 99% and 86% for pH 7,6,5,4,3 and 2 respectively (with CS-Fe0). Slower degradation with increasing pH has previously been observed on treatment of metolachlor [19], several factors may explain this trend. But the possible reason is the formation of iron oxides on the surface of Fe0 passivates the iron surface, this hinders the surface reactivity of nano particles. Low pH would remove these passivating layers from iron surface and render it free for reaction with the
International Journal of Science Innovations and Discoveries, Volume 2, Issue 1, January-February 2012
110
A.Vijaya Bhaskar Reddy et al., IJSID, 2012, 2 (1), 106-112 halogenated molecules [19-20]. Moreover, at very low PH (
