Huang Yuan-ying¹, Liu Fei², Shen Zhaoli²

1 National Research Center for GeoAnalysis, Beijing 100037, China;

2 China University of Geosciences(Beijing)/Beijing Key Laboratory of Water Resources&Environmental Engineering 100083, China)

Abstract:The research presented the effectiveness of nanoscale bimetallic particales( Ni/Fe and Cu/Fe) used to remove tetrachloroethene(PCE).Specific surface area of the laboratory synthesized nanoscale particles was approximately 52.61m²/g, which was tens of times larger than those of micrometer metal particles. PCE was tested by batch procedures in which 0.3g of Ni/Fe or Cu/Fe bimetallic particles was added to 120-mL vials. Batch studies demonstrated that bimetal systems were effective to the dechlorination of PCE. The degradation process appeared to be pseudo-first-order. A bimetallic structure was adopted in which a thin layer of catalyst(e.g., Ni, Cu) on the surface of a reductant (e.g., Fe).The catalyst reduced activation energy and increased the rate of dechlorination reactions. More importantly, production of chlorinated byproducts was less than 10%. The reaction rate constant (K_SA) was 4.283 mL/(m²·h) for nanoscale Ni/Fe. It was about 33.23, 11.59 times higher than microscale iron and Ni/Fe, respectively. K_SA for nanoscale Cu/Fe was 1.194 mL/(m²·h). It was about 9.26, 5.24 times higher than microscale iron and Cu/Fe, respectively. K_SA of nanoscale Ni/Fe was 3.59 times than that of nanoscale Cu/Fe under the same condition. Based on the rapid rate of degradation and no or less chlorinated byproducts, the nanoscale particles technology would offer great opportunities for remediation of contaminated ground water.

Keywords: nanoscale bimetallic particles; reaction rate constant K_SA; tetrachloroethene; dechlorination

1 Introduction

Because of many chlorinated solvents,such as tetrachloroethene(PCE),trichloroethene(TCE),dichloroethene(DCE) and vinyl chloride(VC),are known or potential threats to public health and the environment,so there is an urgent need to develop effective treatment methods. A survey of groundwater in a northern China city shows that TCE,followed by PCE ,was detected .And  44.44% of the samples have TCE concentration above the drinking water standards issued by the US EPA, and the highest concentration was almost 1000µg/L,200 times higher than the standard(Liu, 2006).

In the last two decades,Fe⁰ permeable reactive barriers technology has received much interest as an alternative to pump and treat systems. As contaminated water passed through the permeable wall of iron to form primarily benign compounds such as hydrocaron and chloride ion. In general,reactions between chlorinated organic compounds() and iron in aqueous solutions could be expressed by the following reaction(Zhang,1998):

    （1）

However, many challenges still exist for implementation of the zero-valent metal technology(Vidic et al,1996).Nanoscale metal particles, with diameter in the range 1-100nm, are characterized by high surface area to volume ratio, high level of stepped surface, and high surface energy. Nanoscale metal particles could be injected directly to contaminated soils,sediments, and aquifers for in situ remediation chlorinated hydrocanbons, instead of building permeable reactive barriers in the subsurface.The nanoscale particles can be anchored onto activated carbon and zerolite for ex situ treatment of contaminated waters and industrial effluents(Wang,1997). In the aqueous solution, the nanoscale iron particles could remain suspended under very gentle agitation.Within a bimetallic system, the dechlorination rates is the most rapid when Pd severs as catalyst.It is well known that its cost for application in engineering is expensive because Pd is a noble metal.We presented low-cost nickle and copper substitute for noble metal Pd. Using laboratory-synthesized nanoscale Ni/Fe or Cu/Fe remove PCE. To our knowledge, the report on dechlorination of chlorinated hydrocarbons by nanoscale Ni/Fe or Cu/Fe was not too much. It would have theory and actual signification if we work on this study.

2 Material and Methods

2.1 Synthesis of nanoscale iron particles

An appropriate amount of 1.0mol/L FeCl₃·6H₂O(v/v=30%ethanol) was placed in a conical flask, and purged with pure nitrogen for two hours. Adding 1.6mol/L NaBH₄ solution into the conical flask with 1.0mol/L FeCl₃·6H₂O(1:1 volume ratio ).The solution was mixed vigorously on the magnetic agitator under 22±1℃ for 5 minutes.Fe³⁺ was reduced by borohydride according to the following reaction:

       (2)

The nano-iron formed from the above reaction were rinsed with large volume of 0.5 mol/L HCl, de-oxygen water and ethanol for at least three times, respectively. Bimetals could be prepared by soaking freshly prepared nanoscale iron particles with ethanol solution containing 2% (wt.) NiCl₂ or 4% (wt.)CuCl₂. The mixture was stirred for 20 min and then filtered through a 0.2µm filter.This caused the reduction and subsequent deposition of Ni or Cu on the Fe surface:

       （3）

    （4）

Ni/Fe and Cu/Fe particles were washed three times with ethanol and drying them at 105℃ for 4h under a flow of N₂.Synthesizing nanoscale iron particles had average BET specific surface area around 52.61m²/g by nitrogen adsorption method (Autosorb-1 model SSA and porosity,USA). The micrographs  (Fig.1 and 2) showed that the synthetic particles were in the range of 20-60nm by scan electron microscopy(SEM) at 30,000 magnification.

2.2 Batch experiments

Batch experiments were conducted to test reactivity of the laboratory synthesized nanoscale particles for dechlorination of PCE.Take 0.3g freshly-made nano-Ni/Fe(2%) or Cu/Fe(4%) particles into120-mL reaction bottle capped with a Teflon valve.The initial concentration of PCE was 21mg/L and volume was 50 mL.The bottle was mixed on a rotary shaker at a speed of 170 revolutions per minute at 20℃.Parallel experiments were also performed without metal particles(control sample). Analyses of organic mass in the control samples indicated that the total mass of parent chlorinated ethylenes in the batch bottles varied from 95 to 105% of the initial input when the experiment were completed. About 5% error of organic concentrations in the control samples was largely caused by volatilization of PCE, and the gas chromatograph(GC) analyses. Therefore, the bimetal particles resulted in the concentration of PCE in bimetal system degreasing largely.

Fig.1 SEM image of nano-Ni/Fe(2%)                 Fig.2 SEM image of nano-Cu/Fe(4%)

Note：BET data in Table １were supplied by the lab of Analytical Chemistry in China University of Geosciences(Beijing).

2.3 Methods of analysis

Periodically,50uL of the aqueous solution was withdrawn by a 100-uL gas-tight syringe,and spiked quickly into a 10 mL vial charged with 0.45mL purified water.Concentrations of chlorinated ethenes were measured by a HP 6890GC equipped with a HP-5 capillary column (30m×0.25mm×0.25µm)and an electron capture detector (ECD). The carrier gas was ultra-pure nitrogen.

GC HP-6890: inlet 160℃,column flow 1.0mL/min,oven 70℃,oven run time 10min,ECD 300℃.

Headspace sampler: vial 50℃,loop 60℃,transform line 70℃,vial equation time 10min, injection time 1min; shake time 5min;carrier flow 30 mL/min(Liu 2002).

Method detection limits are about 0.05µg/L which is below the standards for the drinking water by the US EPA.Calibration curves for each model compound were made initially daily before analysis.

2.4 Chemicals

Five halogenated hydrocarbons dissolved in the methanol were purchased from the Institute of Reference Materials belonging to the Environmental Monitoring General Station of China, including 200µg/mL CF,50µg/mLCT,200µg/mL TCE, 100µg/mL PCE and 200µg/mL bromoform(BF).DCEs was from National Center for Reference Materials with a concentration of 0.93µg/mL.VC was from Chemistry Service, PA,USA. FeCl₃·6H₂O、NaBH₄、NiCl₂、CuCl₂、HCl and anhydrous ethanol are analytical grade reagents.

3 Results and Discussion

3.1 Degradation of PCE with nanoscale Ni/Fe bimetallic particles

On the basis of forenamed method and processes, Fig. 3(a) showed the results of degradation of PCE by using nanoscale Ni/Fe bimetallic particles. No other product was detected with a reaction time(eg,TCE,DCEs and VC). ln(C/C₀)-t were shown in Fig. 3 (b).Linear regression equation （1）was ln(C/C₀)= -0.7008t, R²=0.7117(n=6).It indicated that the disappearance of PCE exhibited pseudo-first-order behavior.First-order rate constants K_obs=0.7008h^-1,half-life time t_1/2=ln2/ K_obs=0.99h.However,it took about 0.5h when reactive concentration C  was up to half of initial concentration C₀.According to reaction regression equation checkout, equation (1) was significative,but it distinctly deviated from the fact( Deng,1984). During the whole degradation process,it included two parts: firstly, in the first 2 hours, the concentration of PCE was reduced to 1.4mg/L from 21mg/L,that is, 93% of PCE was reduced using nanoscale Ni/Fe particles.As shown in Fig.3(c), linear regression equation（2）was C=21.07e^-1..352t(0≤t≤2h)，R²＝0.9974(n=4).It indicated that the disappearance of PCE exhibited pseudo-first-order behavior. First-order rate constants K_obs=1.352h^-1，half-life time t_1/2=ln2/K_obs=0.51h, which was close to 0.5h.So equation (2) accorded with the experimental result.When iron specific surface area constant Pa was more than 0.078 m²/mL(Gillham,1994), half-life time t_1/2 was inverse proportion to Pa while rate constants K_obs was direct proportion to Pa.In this experiment, Pa was 0.316m²/mL,so K_obs and t_1/2 was normalized per unit metal surface area, K_SA= K_obs/Pa=4.283 mL/(m²·h)，t_50-N=Pa×t_1/2=0.16h.Secondly, reaction regression equation achieved from the last 3 results in this experiment was（3）ln(C/C₀) = -0.2116t - 2.2074 (t≥2h)，R²= 0.9880(n=3)，which fits pseudo-first-order behavior.Within the first 2 hours, 93% of PCE was reduced using nanoscale Ni/Fe particles.The amount of ethane was about 83% of the total PCE. Maybe much ethane or hydrogen adhered to the surface of nanoscale Ni/Fe particles,which prevented PCE from arriving at the reactive situation.So reactive rate was reduced rapidly.On the other hand, nanoscale Ni/Fe reactivity was decreased over time, probably due to the precipitation on the surface of Ni/Fe.

3.2 Degradation of PCE with nanoscale Cu/Fe bimetallic particles

Fig.4(a) presented the results of degradation of PCE by using nanoscale Cu/Fe bimetallic particles.Unlike using nanoscale Ni/Fe where no chlorinated intermediates or final products, significant amounts of TCE were detected in the solution in runs.Within a 6-h, 90% of PCE was reduced using nanoscale Cu/Fe particles. ln(C/C₀)-t were shown in Fig. 4 (b).Linear regression equation was （2）C=21.07e^-0.377t, R²=0.9944(n=6).It indicated that the disappearance of PCE exhibited pseudo-first-order behavior.First-order rate constants K_obs=0.7008h^-1,half-life time t_1/2=ln2/K_obs=1.84h.Because there Pa was0.316m²/mL,K_SA=K_obs/Pa=1.194mL/(m²·h),t_50-N=Pa×t_1/2=0.58h. The results obtained by different reactive materials were listed in Table 2.Compared with micrometer grade iron,bimetallic Ni/Fe and Cu/Fe system, nanoscale bimetal for PCE exhibited higher activity, especially nanoscale Ni/Fe particles.The reaction rate constant (K_SA) was 4.283 mL/(m²·h)for nanoscale Ni/Fe. It was about 33.23, 11.59 times higher than micrometer iron and Ni/Fe, respectively.K_SA for nanoscale Cu/Fe is 1.194 mL/(m²·h). It was about 9.26, 5.24 times higher than micrometer iron and Cu/Fe, respectively.K_SA of nanoscale Ni/Fe was 3.59 times than that of nanoscale Cu/Fe under the similar conditions.The concentration of Ni²⁺ ion in solution, determined by ICP-MS, was less than 10µg/L when the reaction completed, which suggested that Ni likely served as catalyst within a bemetallic system. At present, there are different opinions on mechanisms of the dechlorination in  a bemetallic system.Zhang(1998) and Mallat et al(1991) thought that presence of the noble metal on the surface of iron certainly creates many galvanic cells thus promoting the electron release reactions of iron corrision.Quan et al(1998) studied the characters on the degradation of TCM,CT and TCE  in water using different bimetallic systems (Cu/Fe,Zn/Fe,Pd/Fe,Ni/Fe).The results showed hydrogenation catalyst such as Pd and Ni acted as important function during the transfer of H₂.In addition, Pd and Ni as transition metal had vacant orbits,which could form transition compounds with theelectron pair or bond of chlorine atom in chlorinated hydrocarbons, and decreased the reactive energy.

Fig.3  Degradation curve of PCE using nanoscale Ni/Fe

          (a)

         (b)

          (c)

Fig.4 Degradation curve of PCE using nanoscale Cu/Fe

         (a)

         (b)

Table 2  Comparison on the reaction rate constant and half life time between different materials for dechlorination of PCE

Note:1）Mass ratio of nanoscale bimetals was 2％or 4% and metal to solution ratio was 0.30g/50mL；Mass ratio of  micrometer grade Ni/Fe and Cu/Fe was 0.048% and 0.040%, respectively, and metal to solution ratio was 5.0g/50mL；Metal to solution ratio for micrometer grade iron was also 5.0g/50mL.

2）^①experimental data see also Table 3 in this article by the author, He Xiaojuan(2003).

3）* value listed were K_SA ratio of using different materials to millimeter grade Fe under similar conditions.

4 Conclusions

In summary, nanoscale Ni/Fe and Cu/Fe particles had indicated distinct dechlorination for PCE. The degradation process appeared to be pseudo-first-order. Many advantages of a nanoscale bimetallic particles for treatment of PCE include:1)high spectific surface area and high surface reactivity, the nanoscale iron particles were observed to suspend in the aqueous solution, which favored to increase the dechlorination rate.Compared with micrometer grade iron,bimetallic Ni/Fe and Cu/Fe system, nanoscale bimetal for PCE exhibited higher activity, especially K_SA of nanoscale Ni/Fe particles were improved 10~35 times.2) Due to the presence of catalyst Ni or Cu on the surface of iron,decreased reaction energy,increased the dechloriation rate,importantly reduced the production of byproducts. No other product was detected within a reaction time(eg,TCE,DCEs and VC) for the degradation of PCE with nanoscale Ni/Fe while significant amounts of TCE were detected in the solution with nanoscale Cu/Fe in runs.

Aknowledgement

This work was funded by the NSFC (40372109) and the project of Study on Remediation Technology of Soil Contaminated by Oil from the Beijing Education Commitee.The analytical data were supplied by the Lab of Water Resources and Environmental Engineering in China University of Geosciences (Beijing).

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