Pentachlorophenol reduction by Pd/Fe bimetallic nanoparticles: Effects of copper, nickel, and ferric cations

Abstract

Bimetallic nanoparticles have been used for effective reduction of chlorinated compounds; however, the study of cation effect on degradation is limited. This study examined the effect of three selected cations normally co-present in soil and groundwater contamination sites on the degradation kinetics and removal efficiency of pentachlorophenol (PCP) by Pd/Fe nanoparticles. Degradation of PCP by Pd/Fe nanoparticles was carried out in aqueous solutions containing different cations in sulfate form, Na₂SO₄, CuSO₄, NiSO₄, and Fe₂(SO₄)₃, respectively. The observed inhibitory effect of Na₂SO₄ on degradation of PCP was contributed to the existence of SO₄²⁻ ions. Overcoming the inhibitory effect of SO₄²⁻ ions, Cu²⁺, Ni²⁺, and Fe³⁺ could facilitate the degradation kinetics and efficiencies of PCP by Pd/Fe nanoparticles. XANES absorption spectra were performed to characterize their valences. The enhancement effect of Cu²⁺ and Ni²⁺ ions result from the presence of reduced forms of copper and nickel on Pd/Fe surfaces. The presence of reduced forms of copper and nickel on Pd/Fe nanoparticles were confirmed by ICP–MS analysis. The addition of Fe³⁺ ions caused a decrease in pH and can reasonably account for the enhancement seen in the PCP degradation process. These observations lead to a better understanding of PCP degradation with Pd/Fe nanoparticles and can facilitate the remediation design and prediction of treatment efficiency of PCP at remediation sites.

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⁺.