Pentachlorophenol reduction by Pd/Fe bimetallic nanoparticles: Effects of copper, nickel, and ferric cations
Graphical abstract
Highlights
► Pd/Fe bimetallic nanoparticles effectively degrade PCP. ► Sulfate ions inhibit on degradation of PCP with Pd/Fe nanoparticles. ► Cu2+, Ni2+, and Fe3+ ions enhance the degradation of PCP by Pd/Fe nanoparticles. ► The reduced form of copper and nickel on Pd/Fe surfaces facilitates the degradation.
Introduction
Pentachlorophenol (PCP) is a manufactured chlorinated organic compound which has been widely used as a herbicide, pesticide, wood preservative and in a variety of other industrial applications. Broad use and environmental stability of PCP have led to the extensive contamination of soil, surface water, and groundwater aquifers [1], [2]. It has been reported that adverse effects of PCP on the environment and humans might last for a long time [3], [4]. Chronic exposure to PCP can damage the liver, kidney, blood, and nervous systems [5]. Furthermore, extremely toxic organic compounds such as PCDD/F may be generated from PCP by photochemical reactions [6]. Owing to its high toxic risk for humans and the environment, PCP is listed as one of the priority pollutants by the US Environmental Protection Agency, European Union [7], and Taiwan. Although PCP has been severely restricted since 1984, PCP still remains in the environment because of improper disposal of industrial wastes and its persistence in the environment. Especially, one heavily PCP contaminated site in an old shutdown PCP factory was found in Taiwan. Therefore, it is essential to find ways to remove PCP from the environment effectively.
Among the current methods for removal of chlorinated organic compounds, the chemical reduction method has attracted a lot of attention due to the short treatment time required [8]. Zerovalent metal serves as a donor of electrons (reducing agent) that is capable of promoting the reductive dechlorination of chlorinated organic compounds, as expressed by the following reaction [9], [10]:M0 + RCl + H+ → M2+ + RH + Cl−
In recent years, zerovalent metals have been widely applied to the treatment of halogenated organic pollutants such as polychlorinated biphenyls, chlorophenols, trichloroethylene, and polybrominated diphenyl ethers [11], [12]. However, the dechlorination of PCP by zerovalent metals was not effective due to slow dechlorination rates, high adsorption proportion, and incomplete dechlorination [13], [14], [15]. Various methods were developed to improve the efficiency of dechlorination of PCP by zerovalent metals. For example, the dechlorination of PCP by Fe0 coupled with hydrogen peroxide [16] and microwave [17], Ag/Fe0 coupled with ultrasound [18], as well as Ag/Fe0 or Pd/Mg0 coupled with subcritical water [19] and supercritical carbon dioxide [20], [21] has been reported.
In the last decade, bimetallic particles have been exploited to dechlorinate chlorinated organic compounds due to their high efficiencies compared with the primary metal alone. It has been suggested that supplying a second catalytic metal such as Pd, Pt, Ag, Ni, or Cu on primary zerovalent metal could prevent toxic byproduct formation by dechlorinating chlorinated pollutants via hydrogen reduction rather than via electron transfer [22], [23]. The dechlorination of PCP by microscale bimetallic particles including Pd/Fe, Pt/Fe, Ni/Fe and Cu/Fe [24], Pd/Mg and Pd/Fe [14], and Ag/Mg and Pd/Mg [25] have been reported.
Furthermore, researchers found that nanoscale zerovalent iron (NZVI), which has high specific surface area, showed much more reactivity for the transformation of halogenated organic compounds than commercial iron powder [9], [26], [27], [28]. On the other hand, nanoscale bimetallic technology offers new potential in treatment of organic and inorganic pollutants in the environment. Nanoscale palladium/iron particles are highly reactive remediation agents for chemical reduction of halogenated compounds. The use of nanoscale palladium/iron on groundwater remediation of chlorinated solvents such as trichloroethylene has been well studied [29]. An efficient degradation of hexachlorobenzene [30] and PCP [31] by Pd/Fe bimetallic nanoparticles has been demonstrated in our previous work.
However, studies on the effect of cations on degradation of PCP, a polychlorinated compound, by nanoscale metals are scarce. Common cations in water, soil and groundwater will affect the degradation of halogenated compounds by metals. And the mixed contamination of heavy metal ions and chlorinated compounds has been found in many sites. The cation Cu2+ can enhance the dechlorination of carbon tetrachloride by iron nanoparticles and led to the production of more benign products such as CH4 [32]. The tetrachloride reduction rates with green rust (GR) were greatly increased for systems amended with Cu+, Au3+, and Ag+ relative to GR alone [33]. In order to assess the applicability of Pd/Fe nanoparticles for remediation of PCP contaminated soil and groundwater, this study examined the effect of three selected cations normally present in soil and groundwater on the removal kinetics and efficiency of PCP by Pd/Fe nanoparticles. Salts are generally used to introduce the heavy metal cations. They usually accompany anions such as SO42−, Cl−, HCO3−, HPO42−, and NO3−. The effect of the anions on the reaction of the NZVI has been investigated in literature. Fan et al. [34] reported that SO42− had a negative effect on the decolorization of azo dye methyl orange with NZVI. Lim and Zhu [35] concluded that SO42− anions have no influence on dechlorination of trichlorobenzene by Pd/Fe. A huge controversy over the effect of anions still exists. In this work, sulfate was added in the solutions that introduced cations such as Cu2+, Ni2+ and Fe3+.
Section snippets
Chemicals and standards
Pentachlorophenol was purchased from Sigma. Methanol and n-hexane were obtained from J. T. Baker. Ferrous sulfate, sodium borohydride, hydrochloride acid, sodium hydroxide, sodium sulfate, cupric sulfate, nickel sulfate and ferric sulfate were purchased from Riedel-deHaën. All aqueous solutions were made in water purified with a Milli-Q system (18.2 MΩ/cm).
Preparation of nanoscale Pd/Fe particles
NZVI particles were produced by adding a NaBH4 aqueous solution to a flask containing FeSO4 7 H2O aqueous solution at ambient temperature.
Effect of sodium sulfate on the degradation kinetics of PCP by nanoscale Pd/Fe particles
The degradation kinetics of PCP by Pd/Fe nanoparticles without electrolytes was studied as the basis of PCP degradation and for later comparison (Fig. 2). In the absence of salts, the rate constant and degradation efficiency within 100 min of PCP by Pd/Fe nanoparticles were 0.083 min−1 and 97%, respectively (Table 1). The effect of different concentrations of Na2SO4 on the degradation kinetics of PCP by Pd/Fe nanoparticles is also shown in Fig. 2. As the Na2SO4 concentration increased from 2.5 to
Conclusions
The removal of PCP by synthesized Pd/Fe nanoparticles in the presence of different types and concentrations of cations was examined. Rapid degradation kinetics of PCP by synthesized Pd/Fe nanoparticles was observed. Na2SO4 was found to reduce degradation performance. Since Na+ ion cannot affect degradation kinetics and removal efficiencies of PCP due to its high reduction potential, SO42− ions are the primary contributors to the inhibition of degradation. As compared with the degradation
Acknowledgment
The authors gratefully acknowledge the financial support of the National Science Council of Taiwan, ROC (Contract NSC 97-2313-B-002-048-MY3).
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