Efficient recovery of palladium nanoparticles from industrial wastewater and their catalytic activity toward reduction of 4-nitrophenol

Highlights

• Methodology for recovery of noble metals from wastewater has been proposed.

• Pd NPs was recovered from industrial wastewater using a pulsed laser process.

• Optimization of pulsed laser parameters for efficient recovery of Pd NPs.

• Effective recovery was achieved at laser 355 nm, 40 mJ/pulse for 30 min irradiation.

• Recovered Pd NPs showed highest catalytic activity for 4-nitrophenol reduction.

Abstract

Discharge of heavy metals from various sources of industrial wastewater poses significant environmental and health concerns. Thus, efficient recovery of precious metals from wastewater employing sustainable, rapid, and cost-effective treatment methods is highly desirable. In this work, palladium nanoparticles (Pd NPs) were successfully recovered from industrial wastewater using a pulsed laser process in the absence of additives or reducing agents. Notably, the developed approach is faster and more environmentally friendly than other conventional recovery methods. The recovered Pd NPs were characterized by X-ray powder diffraction (XRD), field emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), and inductively coupled plasma optical emission spectroscopy (ICP-OES). Various pulsed laser parameters (i.e., laser wavelength, power, and irradiation time) were optimized to obtain ideal conditions for the pulsed laser ablation process. Effective recovery of the Pd metal from industrial wastewater was achieved at a laser wavelength of 355 nm, power of 40 mJ/pulse, and irradiation time of 30 min. The Pd NPs exhibited excellent catalytic activity toward the reduction of 4-nitrophenol. Thus, the recovered materials showed remarkable potential for application in degradation of toxic aromatic nitro compounds in the environment.

Introduction

The development of uniform nanoparticles (NPs) has attracted significant attention due to the unique properties of these materials. In particular, considering their diverse applications in medicine, electronics, and catalysis, noble metal NPs have in recent years been extensively investigated (Parandhaman et al., 2017; Lee et al., 2020; Sharadaa et al., 2016; Theerthagiri et al., 2019, 2020). For instance, as a result of their remarkable characteristics, Pd NPs are of great interest in the fields of science and technology (Jung et al., 2018; Lee et al., 2019; Wolfson et al., 2020). The size and shape of Pd NPs affect their optical, magnetic, catalytic, and electronic properties (Jung et al., 2018). Thus, numerous studies have been conducted to develop new methods for synthesizing NPs displaying specific shapes and sizes. Among them, pulsed laser irradiation (PLI) in liquid has attracted considerable attention for utilization in a wide range of applications (Ibrahimkutty et al., 2015; Naik et al., 2020).

In recent years, the recovery of metals from industrial wastewater has been intensively investigated. Nonetheless, the majority of the studies are focused on the recovery of Cu, Ag, and Au, while research on the isolation of Pd has been limited (Behnamfard et al., 2013; Sharma et al., 2017; Kim et al., 2018). Zhang et al. (2014) developed a novel approach for the selective recovery of Pd from waste printed circuit boards without the use of corrosive acids and strong oxidants. Moreover, Sun et al. (2007) reported a simple synthetic reduction method to obtain rectangular Pd NPs in the absence of seeds or rigid templates. On the other hand, Wang et al. (2008) successfully prepared Pd NPs exhibiting different sizes by hydrogen adsorption and reduction cycles. Pd is a valuable metal due to its role as a heterogeneous catalyst in a wide range of reactions, including hydrogenation, oxidation, and dichlorination. In addition, Pd is also an important component of systems used in gas sensing and hydrogen storage (Klinkova et al., 2017; Bej et al., 2016; Yu et al., 2016). The catalytic reduction of highly toxic organic nitro compounds, such as nitrobenzene and 4-nitrophenol (4-NP), to the corresponding amines is an extensively utilized detoxification reaction. The inhalation or ingestion of nitro compounds causes headaches, drowsiness, nausea, cyanosis, and damage to the central nervous system, even at low concentrations (Zhang et al., 2019). 4-NP is predominantly used in the industry as an intermediate for the production of various pesticides, dyes, and pharmaceuticals (Singh et al., 2017). Consequently, it is discharged into the environment as part of the industrial waste, contaminating the atmosphere. However, the presence of 4-NP has been detected not only in industrial wastewater, but also in fresh and marine water. Based on their low biodegradability and toxicity, catalytic reduction of nitro compounds in presence of the metal NPs has been proposed as a viable approach for reducing the amount of the contaminant in the environment.

In the present work, pulsed laser ablation (PLA) in liquid was employed as the effective technique to recover Pd from industrial wastewater containing metal ions. In PLA in liquid, a laser exhibiting high power and energy is irradiated through a focusing lens into the waste solution containing metal ions, generating plasma plumes as a result of the change in the temperature and pressure. Despite involving a number of steps, the recovery and separation of Pd ions in the generated plasma plume is a rapid process. In this study, various pulsed laser parameters (i.e., laser wavelength, power, and irradiation time) were optimized to accomplish effective recovery of Pd from industrial wastewater. The power of the pulsed laser was adjusted to 10, 20, and 40 mJ/pulse by modifying the Q-switch and delay time. The wavelength of the laser was controlled at 1064, 532, and 355 nm using a wavelength adjusting device (BBO Crystal). The Pd NPs recovered from industrial wastewater were systematically characterized and their catalytic activity for 4-NP reduction was evaluated.