DEGRADATION OF CHLORINATED ETHYLENES BY NANOSCALE ZEROVALENT IRON MODIFIED WITH SILICA COATING

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1 DEGRADATION OF CHLORINATED ETHYLENES BY NANOSCALE ZEROVALENT IRON MODIFIED WITH SILICA COATING Petra JANOUŠKOVCOVÁ, Lenka HONETSCHLÄGEROVÁ, Milica VELIMIROVIC, Leen BASTIAENS 1 Institute of Chemical Research in Prague, Department of Environmental Chemistry, Technická 5, Prague 6, The Czech Republic, address: petra.janouskovcova@vscht.cz 2 Flemish Institute for Technological Research (VITO), Boeretang 2, 24, Mol, Belgium Abstract We studied the degradation of different chlorinated ethylenes by silica coated nanoscale zerovalent iron particles nzvi. The silica coated nzvi has improved stability for its subsurface migration and can accelerate the degradation of studied contaminants. The reactivity of studied silica coated nzvi was compared to the uncoated nzvi and microscale zerovalent iron particles. To form the silica coating, the uncoated nanoiron particles in an aqueous suspension were modified by Stöber synthesis using tetraethylorthosilicate (TEOS). Lab-scale degradation experiments were performed in serum bottles containing a mixture of chlorinated ethylenes ( 5 mg.l -1 each) and a certain dose ( g.l -1 ) of silica coated nzvi, noncoated nzvi or microscale particles, to simulate conditions on contaminated sites. After five weeks, microscale particles degraded all three contaminants, but the degradation rate was up to two order lower than both nanoiron materials. In the presence of silica coated nzvi, the concentration of PCE, TCE and cis-dce dropped to 2 %, 39 % and 35 % of an initial concentration. The evaluated kinetic constants of pseudo-first-order kinetics confirmed an increase in the removal rate of PCE for silica coated nzvi. On the contrary, TCE and cis-dce were significantly fast degraded by nzvi. The change in the removal potential suggests that the silica coating influenced surface properties determining also degradation processes. Nevertheless, our study demonstrated that the silica (TEOS) coated nanoiron particles are suitable for the degradation of chlorinated ethylenes. Key words: nanoscale zerovalent iron, chlorinated ethylenes, remediation, coating, silica INTRODUCTION Chlorinated ethylenes are one of the main groups of harmful pollutants in groundwater. Transformation of the chlorinated ethylenes to benign nonchlorinated hydrocarbons is well provided by abiotic reductive dehalogenation [1] using the nanoscale particles of zerovalent iron (nzvi) as a result of the low reduction potential of metallic iron (-,477 V) and unique surface properties. Large surface area and large surface free energy (large reactivity) of the nzvi particles lead to the significantly faster and much effective transformation of the chlorinated ethylenes than by using microscale particles. The nzvi particles can be directly injected into the subsurface and transported with groundwater to contaminated plums. Nevertheless, the great surface free energy of the nzvi particles is spontaneously reduced by the formation of agglomerates and nonselective reactions with reducible abundant compounds in groundwater. The agglomeration causes problems with the application and subsurface migration of nzvi, and reduces nzvi reactivity, which can be 1-1 times smaller in the case of microscale agglomerates.[2]

2 The disintegration of agglomerates and the application of surface coatings can improve resistance against the re-agglomeration of individual nzvi particles, nzvi reactivity due to the reduced particle size and their chemical stability. The surface coating creates a barrier around a particle formed by physically or chemically bonded compounds on its surface. Depending on its structure and chemical composition, the barrier can be variously permeable for contaminants to the particle surface. Using a hydrophobic character or appropriate function groups of the coating, the affinity of chlorinated ethylenes can increase towards the coated particles. Thus the contaminants can get faster to the reaction with the particle surface. Polymers, electrolytes, emulsions and metal shells are often used to modify the surface of the nzvi particles.[3] In many nanomaterial applications silica is applied to coat their surface because of its special properties. The nzvi particles are also coated by silica species to maintain their physical stability in an aqueous solution. The principle of formation of Si-nZVI is that the molecules of silica and polymerized forms of silica adsorb to the oxidized outer layer of the nzvi particle to form the structure of an iron core-silica shell. [4] The permeability of the silica shell depends on the density of the adsorbed silica on the nzvi surface and the morphology of the silica shell. In aqueous silicate solution, adsorbed silica on nzvi particles inhibited or supported the corrosion process of metal iron depending on environmental conditions. Therefore the silica can influence redox reactions with contaminants and thus slow or accelerate their degradation rate.[5],[6] We focused on the effect of silica coating on reactivity of nzvi particles to remove mixture of selected chlorinated ethylenes from an aqueous solution. The silica coating was prepared by the hydrolysis and condensation of tetraethylortohosilicate (TEOS) on nzvi particles, which were temporary stabilized by polyvinylpirolidone (PVP). Only limited amount of studies are focused on the effect of silicaalkoxydes on the nzvi reactivity. In our study, the reactivity of silica coated nanoparticles Si-nZVI was also compared to reactivity of microscale nanoparticles. EXPERIMENTAL SECTION Chemicals and particles Cis-dichlorethylene (cis-dce), trichloroethylene (TCE), tetrachlorethylene (PCE, 99+ %, extra pure, stabilized, Acros Organics), CaCl 2.2H 2 O (p.a. Merc), MgCl 2.6H 2 O (p.a. Merc), NaHCO 3 (p.a. Merc), KHCO 3 (p.a. Merc), hydrochloric acid (32% HCl, for analysis, Merc), MiliQ Water (Milipore, Symplicity UV System). The commercial available suspension of nanoscale zerovalent iron particles (nzvi) NANOFER 25 (NANOIRON Ltd.), the suspension of silica coated particles NANOFER 25 (Si-nZVI) and the powder of microscale zerovalent iron particles (mzvi) BASF HQ were used for a degradation experiment. The coating of nzvi was performed according to the Stöber synthesis when tetraethylorthosilicate (TEOS) as a precursor of silica was hydrolyzed in the mixture of ethanol, hydroxide ammonium and the nzvi suspension at mild heating to precipitate the silica on the surface of the particles. Polyvinylpirolidone (PVP) was used to maintain the individual nzvi particles before the coating and to anchor the silica to the particles. The Si-nZVI mixture was directly added to the degradation experiment. Particles characterization Specific surface area (SSA) of the particles was measured using N 2 -BET. Dry samples for the SSA measurement were obtained by the lyophilization of studied suspensions. The amount of Fe was determined from the volume of hydrogen after the acidific mineralization of the particles in 1M HCl. Degradation experiment and analysis The mixture of selected chlorinated ethylenes (PCE, TCE, cis-dce) in an aqueous solution was degraded in 16 ml serum bottles capped by Teflon lined aluminium crimp caps at anaerobic conditions. In an anaerobic chamber, deoxygenated MiliQ Water enriched with.5 mm Cl - and.5 mm HCO 3- was spiked by pure phases of PCE, TCE and cis-dce. Before spiking, the ph of the anaerobic solution was modified to value 7.

3 The bottles were filled by the volume of nzvi suspension containing.212 g Fe or by dry mzvi particles (2.5 g) containing 2.2 g Fe. The initial concentration of each contaminant was 5 mg.l -1 in 1 ml of the degradation mixture. The bottles were intensively shaken (11 rpm) at 12 ± 1 C in darkness. Control bottles without the nzvi particles demonstrated loses of the contaminants (max 12 % per 6 weeks), which were mainly due to sampling. The average of triplicate measurements was used to determine the concentration profiles of individual degradation processes. Chlorinated ethylenes were analysed via headspace measurements using a Varian GC-FID (CP-38) (Varian CP_38 with CTC-autosampler) equipped with RT-U-PLOT/ DB-1 GC/ RT-X-52.2 capillary columns. Quantification was performed according to the one point calibration with an external standard. Hydrogen evolved from the Fe dissolution was quantified using Trace GC MPT-1286 equipped with packed columns Heyesep Q and Carbosphere and TCD detector. A calibration curve was made from standards ranging % (vol/vol) hydrogen in N 2 -headspace. Experimental data were evaluated by the fitting of the pseudo-order kinetics equation in the program ERA 3.. The parameters of initial concentrations C and observed kinetic constants K obs were optimized. The constants Kobs were normalized per the mass concentration of Fe in particles to obtain constants Km Fe. We decided to evaluate Km Fe because the excess of PVP and TEOS in the Si-nZVI suspension influenced the weight of material after drying. The evaluation of K SA kinetic constants normalized per mass of the particles and specific surface area was only evaluated for the mzvi and nzvi particles because a reaction is mediated by the iron particle surface and not the silica surface. A standard deviation of the kinetic parameters was determined from the triplicate concentration profiles. RESULTS AND DISCUSION Particle characterisation The specific surface area of the non-coated nzvi particles determined by BET surface analysis was 28.7 m 2 g -1. The specific surface area of the Si-nZVI nanoparticles was 29.9 m 2 g -1. A small increase in the specific area of the Si-nZVI particles may be caused by the external morphology of the silica coating. The specific surface area of the mzvi particles (BASF HQ) was significantly smaller than both types of the nzvi particles and it was.95 m 2 g -1. The mass fraction of Fe (solid reagent) for the nzvi was 72. ± 3.3 % and for the Si-nZVI particles was 56.6 ± 1.6 %. The mzvi particles had 87.7 ±.1 % of Fe. Particles/solid reagent in the suspension of nzvi and Si-nZVI was 24.3 ± 1.4 % and 11.1 ±.1 %, respectively. Oxidation of iron metal was not considered within drying of the suspensions. Effect of the silica coating on contaminant removal Natural attenuation on contaminated sites causes that chlorinated ethylenes and their intermediates are presented in contaminated groundwater in a mixture with various composition. TCE and cis-dce are intermediates of a higher chlorinated ethylene which form by minor hydrolysis (PCE TCE cis-dce) during the reaction with zerovalent iron.[7] After the injection of nzvi suspension, all the chlorinated ethylenes are then removed from an aqueous solution together at the same time. In our experiment, the mixture of PCE, TCE and cis-dce (Fig. 1 A, B, C) was tested to investigate the effect of a silica coating on the ability of non-coated nzvi particles to remove the chlorinated ethylenes, furthermore, the removal ability the non coated nzvi and microscale mzvi particles was also tested and compared to the Si-nZVI particles. In the presence of the nzvi particles, the concentration of PCE, TCE and cis-dce decreased to 94 %, 78 % and 89 % of an initial concentration after 1 day. In 36 days of experiment, TCE and cis-dce were degraded completely. At that time PCE was removed with 92% efficiency. In the case of the Si-nZVI particles, PCE was removed with 17 % efficiency, but TCE and cis-dce only with 4 % and 11 % after 1 day of the reaction. Only 2 % of the PCE concentration left in degradation system after 36 days experiment, while TCE and cis-dce concentration decreased to 88 % and 89 % of an initial concentration.

4 cis-dce (mg.l -1 ) TCE (mg.l -1 ) PCE (mg.l -1 ) , Brno, Czech Republic, EU In the mzvi degradation system (22.1 g.l -1 of Fe ), PCE, TCE and cis-dce were removed with the efficiency of 96 %, 9 % and 22 % after 36 days. According to evaluated constants Km Fe (table I), the silica coating exhibited a positive effect on the nzvi removal ability towards PCE. The removal rate of PCE was slightly (4 %) faster by the Si-nZVI particles than by the nzvi particles. On the contrary, the Si-nZVI particles reduced the removal rate of TCE and cis-dce which was 4.5 times and 8.6 times slower in comparison with the nzvi particles. It seems that the concentration trend of TCE is almost linear in contrast with the exponential trend of PCE and cis- DCE (figure 1B). The linearity might indicate the change of reaction kinetics to zero order of the reaction with the kinetic constant k obs =.77 mol.d -1 (R 2 =.9528). As shown in the Table 1, the removal rate of all three contaminants by mzvi particles was significantly slower than in the Si-nZVI system; moreover, the removal rate of TCE and cis-dce was slower up to two orders of magnitude. This effect was probably caused by huge difference in the specific area of the mzvi and Si-nZVI particles. R 2 = Surface area - normalized rate constants K SA of the C nzvi particles was 7.44 E-4 Lm -2 d -1 for PCE, 2.59E-3 Lm -2 d -1 for TCE and 1.56E-3 Lm -2 d -1 for cis-dce. In the case of the mzvi particles, K SA values were 3.25E- 3 Lm -2 d -1, 1.13E-4 Lm -2 d -1 and 2.86E-4 Lm -2 d -1 for PCE, TCE and cis-dce. The fast removal rate of PCE R 2 =.9999 R 2 =.9881 R using the mzvi particles may be due to its adsorption =.9914 on carbon impurities on particle surface because Time (days) mzvi BASF HQ is made of Fe(CO) 5. Another explanation is interspecies competition, which can be Fig. 1: Concentration trends of PCE, TCE and significant for the mixture of parent compounds and its cis-dce within removal by 2.1 g/l of Fe in nzvi intermediates degraded by mzvi particles.[8] Small and silica stabilized Si-nZVI, and 22.1 g/l of Fe differences between K SA for the nzvi and mzvi in mzvi. Solid lines represent model fitting. particles are because big surface area of the small amount of nzvi particles is amended by huge amount of mzvi particles. Nevertheless higher efficiency of nzvi particles was observed. Beneficially effected removal of PCE in the Si-nZVI degradation system can be explained by the presence of PVP polymer [9] and/or ethanol. The ethanol is a precursor of the silica coating and is also formed by TEOS hydrolysis. Ethanol can increase solubility of chlorinated ethylenes in water. PCE is the most hydrophobic from chlorinated ethylenes (Kow, PCE = 2.6, log Kow, TCE = 2.29, log Kow, cisdce = 1.59) [1], therefore, the fast removal rate of PCE might be well observed as its affinity to ethanol and PVP in the suspension. According to the study [11], more than 15 % of ethanol increases the solubility of PCE in water. Only 3 % (vol) of the ethanol was contained in the Si-nZVI degradation system. Therefore, we assumed the negligible effect of the ethanol on the removal rate of PCE. We rather suppose the effect of adsorption of the chlorinated ethylenes on the PVP polymer which was adsorbed on the Si-nZVI particles. As a secondary effect, PVP might intensify contact of the chlorinated ethylenes with the surface of the coated particles R 2 =.9958 nzvi Si-nZVI (TEOS) mzvi R 2 =.9926 R 2 =.8745 R 2 =.9999 R 2 = A B

5 Fe in suspension (%) , Brno, Czech Republic, EU Tab. 1: Evaluated kinetic parameters of pseudofirstorder kinetics and experimental conditions. material particles/solid Fe contaminant C Kobs Km Fe reagent (g.l -1 ) (g.l -1 ) (mg.l -1 ) (d -1 ) (L.g -1.d -1 ) nzvi/ NANOFER 25 Si-nZVI/ Si NANOFER 25 mzvi/ BASF HQ PCE 7.85 ±.7.632± TCE 4.89 ± ± cis-dce 5.1 ± ± PCE 7.2 ±.14.88± TCE 5.59 ±.3.253± cis-dce 5. ± ± PCE 6.79 ± ± TCE 4.29 ±.9.25±.9.1 cis-dce 4.35 ±.13.68±.5.3 The silica coating in the solid-water interface could affect the interaction of all the three chlorinated ethylenes with the silica coated particle surface due to several possible factors: 1) The silica coating might limit the transport of the contaminants from the bulk of the solution to the particle surface. [11] 2) The adsorbed components of the surface coating reduced available surface area and a number of reactive sites which are convenient for the direct reduction of the chlorinated. The lack of the reactive sites could slow the degradation of the contaminants. [12] 3) According to the study [12], the adsorption of silica can change the morphology of the surface of ZVI particles. The changes of the surface properties and the blocking of the reactive sites could influence the distribution of products due to the reaction with other sites with different reactive intensity. Incomplete dechlorination and the accumulation of less chlorinated intermediates (TCE and cis-dce in our experiments) can occur. The accumulation of the intermediates could change reaction kinetics from pseudofirst order to zero order [13]. Similar kinetic changes in the reaction order were observed within the degradation of high TCE concentration by nzvi. 4) Using the assumption of the study, the small reactivity of Si-nZVI towards TCE and cis-dce may be due to an interspecies competition at the particle surface.[12] As we reported in the recent study [14], the removal rate of chlorinated ethylenes by nzvi (NANOFER 25) is DCE>TCE>PCE. Therefore, we suppose that the removal rate of chlorinated ethylenes by non-stabilized nzvi can be only slightly inhibited by the reactants and intermediates. All factors together can additively cause a decrease or increase in the removal rate of the chlorinated ethylenes. Effect of the silica coating on particle storage nzvi Si-nZVI Time (days) Fig. 2: Changes of Fe amount in the nzvi and Si-nZVI suspensions during storage. Figure 2 shows only a slight decrease of Fe amount in both the storage suspensions of the non-coated nzvi and the Si-nZVI particles during two months from the beginning of the experiment at ideal conditions (darkness, 4 C and in anaerobic conditions). The slightly variable values of Fe in the nzvi suspension are given by the heterogeneity of agglomerated nanoparticles. The stable values of the Si-nZVI suspension show that the preparation of Si-nZVI produced deaglomerated particles with a quite high degree of homogeneity. The slope of trend lines determined for the nzvi and Si-nZVI suspension was.18 and.1. According to the almost constant trend of % Fe in Si-nZVI

6 (.5 % Fe ), we suppose that the silica coating made nzvi chemical resist towards the medium of suspension. Moreover, alkaline ph of the suspension (ph 11.23) helped to preserve Fe in the Si-nZVI particles. CONCLUSION We demonstrated the efficient removal of the mixture of chlorinated ethylenes by the silica coated commercial available suspension of nanoscale zerovalent particles to improve the migration of nzvi particles in contaminated ground water. Resulting in the beneficial contribution of the Si-nZVI particles, the PCE removal rate was faster than using the non-coated nzvi particles. On the contrary, the particles with the adsorbed layer of the silica on the surface showed the lack of reactivity towards TCE and cis-dce. We observed the change from pseudofirst to zero order reaction for the removal of TCE. The change in kinetic parameters suggests that the silica coating influenced surface properties determining also degradation processes. The Si-nZVI particles showed higher efficiency to remove chlorinated ethylenes compare to mzvi particles. Therefore, we suggest that Si-nZVI is well competent for chlorinated ethylenes. It seems that the silica coating contributed to the preservation of zerovalent iron within the storage of the suspension. Furthermore, polyvinylpyrolidone and ethanol from surface coating can be used as the source of carbon for microorganisms which can participate in the biologic cleaning of contaminated sites. ACKNOWLEDGEMENTS This work was financially supported from specific university research MSMT no. 21/212. This work was supported by VITO Belgium using their equipment, laboratories and knowledge. LITERATURE [1] VOGEL T. M., at al. Transformations of halogenated aliphatic compounds. Environ. Sci. Technol., 1987, 21, pp [2] QUINN, J., et al., Use of nanoscale iron and bimetallic particles for environmental remediation: A review of field-scale applications. Environmental applications of nanoscale and microscale reactive metal particles, Copyright American Chemical Society, 29, Chapter 15, pp [3] PHENRAT, T., et al., Adsorbed Polyelectrolyte Coatings Decrease Fe Nanoparticle Reactivity with TCE in Water: Conceptual Model and Mechanisms. Environ. Sci. Technol., 29, 43 (5), p [4] BERGNA, H. E.; et al. Dense silica coatings on micro and nanoparticles by deposition of monosilicic acid. Colloidal silica fundamentals and applications; CRC Press: Boca Raton, 26, Chapter 53, pp [5] FUJITA N., MATSUURA C., ISHIGURE K., The effect of silica on hydrogen evolution and corrosion of iron in high-temperature water. Corrosion, 199, 46, pp [6] POWEL R. M., PULS R. W. Proton generation by dissolution of intrinsic or augmented aluminosilicate minerals for in situ contaminant remediation by zero-valence-state iron. Eviron. Sci. Technol, 1997, 31, pp [7] ARNOLD W. A., ROBERTS A. L., Pathways and kinetics of chlorinated ethylene and chlorinated acethylene reaction with Fe() particles, Environ. Sci. Technol., 2, 34(9), p [8] SHÄFER R. at al, Competing TCE and cis-dce degradation kinetics by zero-valent iron experimental results and numerical simulation, Journal of Contaminant Hydrology, 23, 65, pp [9] SAKULCHAICHAROEN, N., et al. Enhanced stability and dechlorination activity of pre-synthesis stabilized nanoscale FePd particles, J. Contam. Hydrol., 21, doi:1.116/j.jconhyd [1] DRIES, J et al., Competition for sorption and degradation of chlorinated ethenes in batch zero-valent iron systems. Environ. Sci. Technol., 24, 38 (1), p [11] LADAA T. I. at al. Cosolvent effects of alcohols on the Henry s constant and aqueous solubility of tetrachloroethylene (PCE). Chemosphere, 21, 44, pp [12] KOHN T., ROBERTS A. L., The effect of silica on the degradation of organohalides in granular iron columns. Journal of contaminant hydrology, 26, 83, pp [13] LIU Y. at al. TCE Dechlorination Rates, Pathways, and Efficiency of Nanoscale Iron Particles with Different Properties. Environ. Sci. Technol., 25, 39 (5), pp [14] JANOUŠKOVCOVÁ P., HONETSCHLAGEROVA L., KOCHÁNKOVÁ L., Effect of silica stabilization on the degradation ability of zerovalent iron nanoparticles. NANOCON 211 3th international conference, Brno, The Czech Republic, September 211, p. 6.