Recycling of Pd(0) catalysts by magnetic nanocomposites—microbial extracellular polymeric substances@Fe₃O₄

Highlights

• Recycled Pd showed great catalytic reduction capacity to MB.

• EPS from Klebsiella pneumoniae J1 showed superior Pd(II) adsorption capacity.

• EPS@Fe₃O₄ showed superior adsorption capacity and high separation efficiency.

Abstract

Palladium (Pd) is extremely expensive due to its scarcity and excellent catalytic performance. Thus, the recovery of Pd has become increasingly important. Herein, microbial extracellular polymeric substances (EPS) and magnetic nanocomposite EPS@Fe₃O₄ were applied to recover Pd catalysts from Pd(II) wastewater. Results indicated that Pd(II) was reduced to Pd (0), which was then adsorbed by EPS (101.21 mg/g) and EPS@Fe₃O₄ (126.30 mg/(g EPS)). After adsorbing Pd, EPS@Fe₃O₄ could be collected by magnetic separation. The recovered Pd showed excellent catalytic activity in the reduction of methylene blue (MB). The pseudo-second-order kinetic model and Redlich-Peterson model best fit the adsorption results. According to spectral analysis, Pd(II) was reduced to Pd (0) by chemical groups in EPS and EPS@Fe₃O₄, and the hydroxyl had a chelating effect on adsorbed Pd. Therefore, EPS@Fe₃O₄ is an efficient adsorbent for recovering Pd from Pd(II) wastewater.

Introduction

Palladium (Pd), belonging to the precious platinum-group metals, has been widely applied in catalysis and other industries such as electroplating and precision manufacturing due to its high effectiveness and good stability (An et al., 2017; Le and Nishimura 2019; Yao et al., 2019). For example, Pd has been suggested to be an efficient catalyst for the reduction of toxic dyes to nontoxic or less toxic compounds (e.g., methylene blue (MB) to leuco-methylene blue (LMB)) (Hashemi Salehi et al., 2019; Ncube et al., 2015; Nguyen et al., 2018). However, Pd is expensive and extremely rare due to its excellent catalytic performance and its extremely scarce reserves (much less than gold) (Sharma and Rajesh 2016; Won and Yun 2013). The increasing demand for Pd has motivated the search for efficient methods to effectively recover and recycle Pd (Kim et al., 2017; Pat-Espadas et al., 2016; Wang et al., 2017).

Compared to conventional chemical materials, biological materials are renewable and do not introduce contamination (Beidokhti et al., 2019; Kim et al., 2020; Volesky 2007). A variety of microorganisms have been shown to be able to use enzymes to reduce soluble Pd(II) to insoluble Pd (0), thereby allowing the recovery of Pd from Pd(II) wastewater (Cui et al., 2017; Yates et al., 2013; You et al., 2019). However, considering appropriate temperature, oxygen, and nutriment requirements for microorganisms (Cordier et al., 2020; Jaroo et al., 2019), the recovery of Pd by microorganisms is to some extent restricted by the living conditions of microorganisms (Deplanche et al., 2014; Pat-Espadas et al., 2014).

Microbial extracellular polymeric substances (EPS) are mixtures extracted from microorganisms that mainly compose of proteins, polysaccharides and humic acids (Felz et al., 2019; Yan et al., 2019). EPS contains a wealth of functional groups (e.g., hydroxyl, carboxyl, and amino groups) that can adsorb metals through electrostatic interaction or complexation (Lai et al., 2018a; Palanivel et al., 2020; Yan et al., 2017). Moreover, EPS has been proven to demonstrate reducibility toward precious metals, such as Ag (Wei et al., 2017; Zhang et al., 2016). Compared with organisms, the resiliency of EPS compared to organisms allows for their continued function in some unforgiving conditions. Therefore, EPS could offer golden opportunities for the reduction and sequential adsorption of Pd(II) from Pd(II) wastewater, even though the mechanism of reduction and adsorption of Pd(II) by EPS has not been verified.

To the best of our knowledge, most research on Pd(II) recovery has focused on the adsorption of Pd but has ignored the collection and reuse of Pd (Chen et al., 2019; Zhou et al., 2016b). The separation and collection of Pd is difficult and time-consuming when the adsorbents are hard to separate from water. Magnetic separation technology has been increasingly studied for its convenient separation property (Theingi et al., 2019; Wei et al., 2017; Yang et al., 2020). In our previous work, EPS has been successfully combined with magnetic nanopowders, and a synthetic material EPS@Fe₃O₄ has been successfully used to reduce and adsorb Ag⁺ (Wei et al., 2017). Compared to common precious metals such as Ag⁺, the ionic forms of Pd(II) which play a significant role in Pd(II) recovery are more complicated. There are various of ionic forms of Pd(II) (e.g., PdCl₄²⁻, PdCl₃⁻, Pd(OH)⁺) and ion distribution of Pd(II) is dependent on pH conditions (Zhou et al., 2016a). The ionic forms of Pd(II) are important in its adsorption process. Therefore, further research on the recovery of Pd(II) based on its ionic forms is needed.

Furthermore, collecting Pd is not enough for reusing Pd because the recovered Pd may not function normally as a catalyst. The catalytic activity of Pd catalysts highly depend on their size, morphology and composition, therefore there are numerous studies on the synthesis of Pd catalysts (Hu et al., 2019; Wu et al., 2019; Zhang et al., 2020). However, most researchers who have studied the recovery of Pd rarely conduct a further investigation on the reuse of recovered Pd (Pat-Espadas et al., 2016; Zhou et al., 2016b). Therefore, the catalytic activity of recovered Pd also needs to be investigated after Pd is collected from Pd(II) wastewater. Among the numerous targeted contaminants of Pd catalysts, methylene blue (MB) is a model dye contaminant that is difficult to degrade but can be reduced to leuco-methylene blue (LMB) in the presence of a Pd catalyst (Hashemi Salehi et al., 2019; Nguyen et al., 2018).

In this work, EPS and a magnetic nanocomposite EPS@Fe₃O₄ were applied to recover and collect Pd catalysts from Pd(II) wastewater. The relationship between the Pd(II) ionic forms and the zeta potential of these two adsorbents were investigated. Reduction and adsorption mechanisms were investigated (adsorption kinetics, isotherms, and participating functional groups in the two adsorbents). The catalytic performance of the recovered Pd was tested by observing the reduction of methylene blue. This work is significant in regard to the recycling of Pd and other precious platinum-group metals.