Enhanced degradation of bisphenol A by mixed ZIF derived CoZn oxide encapsulated N-doped carbon via peroxymonosulfate activation: The importance of N doping amount

Highlights

• CoZnO-PC composites are prepared using PVP to enrich the nitrogen content.

• CoZnO-PC/PMS system exhibits prominent catalytic performance for BPA removal.

• Radical and non-radical pathways are simultaneously existed in the system.

• CoZnO-PC are magnetically separable with low Co²⁺ leaching.

• Non-radical process is significantly enhanced by PVP encapsulation.

Abstract

In this study, mixed metal cobalt zinc oxide embedded nitrogen enriched porous carbon composites (CoZnO-PC) were prepared via pyrolyzing polyvinylpyrrolidone (PVP) encapsulated Co, Zn-bimetal centered zeolitic imidazolate frameworks (ZIF). The prepared composites were then used to activate peroxymonosulfate (PMS) for bisphenol A (BPA) removal in water. When mole ratio of Co/Zn was 2/1, the resulted Co₂Zn₁O-PC possessed spinel structure with prominent degradation capability, in which the introduction of Zn accelerated the PMS activation performance of Co through establishing bimetal synergistic interactions. Both radical and non-radical activation pathways were existed in the Co₂Zn₁O-PC/PMS system, in which Co₂Zn₁O dominated the radical pathway whereas PC dominated the non-radical way. Since PVP contained abundant nitrogen atoms and could form strong coordination interactions with the ZIF precursor, the introduction of PVP in the ZIF precursor prevented pore collapsing during pyrolysis process, as well as enhancing the nitrogen content in the pyrolzed composites, which significantly promoted the generation of singlet oxygen. With combined pathways, the Co₂Zn₁O-PC/PMS system showed a wide pH application range with promising mineralization rate. Meanwhile, the spinel-structured Co₂Zn₁O-PC was magnetically separable with desirable recyclability. This study presents a novel composite with remarkable performance for the removal of refractory organic pollutants in municipal wastewater.

Introduction

With the development of human society and industry, synthetic organic chemicals are emerging at an incredible speed every year, many of which are discharged into the water environment through a variety of ways (Hsu et al., 2019, Xu et al., 2020a). Most synthetic organic chemicals have stable structure, high carcinogenicity, teratogenicity and mutagenicity, which are not easily removed from the water using traditional wastewater treatment approaches such as active sludge treatment and biofloculation (Gong et al., 2020). Thus, it is of great significance to develop effective approaches which can in-depth degrade these refractory organic chemicals (ROCs) in water (Yang et al., 2020, Liang et al., 2021). Amongst the newly emerged techniques, the sulfate radical (SO₄^-•) based advanced oxidation technology has received tremendous research interest due to the merits of higher redox potential (2.5–3.1 V), longer half-life period (30–40 μs), and wider pH application range (Gan et al., 2019). Theoretically, the SO₄^-• can mineralized most organic compounds in water (Liu et al., 2020a). It has been reported that various ROCs such as organic dyes, antibiotics, pesticides, endocrine-disrupting chemicals could be effectively and promptly degraded by SO₄^-• (Rao et al., 2019). Heterogeneous cobalt activated peroxymonosulfate (PMS) system is considered as a promising way to generate SO₄^-• under ambient temperature without introducing external energies, due to the high redox potential of cobalt (Co³⁺/Co²⁺, 1.82 V). compared with other transition metal elements (Wan et al., 2019). Besides, since nano-scaled catalyst can provide more active sites, a variety of cobalt based heterogeneous nano-catalysts have been developed to activate PMS for the generation of SO₄^-• (Yao et al., 2015). However, the leaching of the Co²⁺ during the PMS activation process is inevitable, which might cause secondary pollution to the water. Besides, the aggregations of the nanoparticles could also reduce the activation efficiency of the cobalt catalysts (Al-Anazi et al., 2018). These defects seriously restrict the application of Co/PMS system for ROCs degradations.

Metal organic frameworks (MOFs) derived metal oxide/porous carbon composite materials have recently received enormous interest as the catalyst for PMS activation (Zhang et al., 2018). Since MOFs have periodic network structure with high porosity and surface area, the metal oxide nanoparticle can uniformly disperse within the porous carbon skeleton without aggregations (Wang et al., 2019b). Moreover, the ‘nanocages’ built by the porous carbon can also prevent the leaching of the metal ions into the water (Abdul Nasir Khan et al., 2019). Zeolite imidazolate frameworks (ZIFs) are a constitution family of the MOFs which have desirable features of exceptional chemical stability, rich structural diversity and porosity (Liao et al., 2019). Due to this reason, the ZIFs family have been frequently selected as the precursor to produce metal oxide incorporated porous carbon composites for various applications, such as supercapacitor, gas separator, as well as high-active catalyst (Xu et al., 2018). Moreover, the existence of imidazole ligand in the ZIFs could also provide nitrogen atoms to the pyrolzed graphitic carbon (Bhadra et al., 2018). It has been reported that the PMS activation capability of a catalyst could be significantly boosted by nitrogen doped carbon, which makes ZIFs a promising template for preparing high-performance PMS activator based on metal oxide-nitrogen co-doped porous carbon composites (Liu et al., 2019).

Cobalt centered ZIF-67(Co) is a typical ZIFs crystal with desirable porous structures, which could be easily prepared through room temperature co-precipitation method. Although ZIF-67(Co) pyrolyzed composites have been already used for the activation of PMS, the leaching of Co ions was still inevitable (Wang et al., 2020a). It has been reported that through introducing second metal with Co and forming spinel structured bimetal oxide, the leaching of Co ions could be effectively avoided (Wu et al., 2020). Moreover, another concern which restricts the application of ZIF-67(Co) pyrolyzed composites is that the nitrogen concentration in the resulted carbon materials is relatively low since most nitrogen atoms are evaporated during the pyrolysis process (Zhang et al., 2020). Many researchers encapsulated polyvinyl pyrrolidone (PVP) in the ZIFs, which not only provided more nitrogen to the resulted carbon materials, but also prevented the porous structure from collapsing during the pyrolysis process (Liang et al., 2018).

Hence, we prepared CoZnO incorporated porous carbon composites (CoZnO-PC) from PVP assisted Co-Zn bimetal based ZIFs to activate PMS for ROCs degradation in water. Specifically, Zn was used to bond with Co to prevent the Co leaching, and PVP was introduced to increase the nitrogen doping amount and support the pore structure of the composites. By this means, Zn could accelerate the PMS activation performance of Co through establishing bimetal synergistic interactions with Co. Meanwhile, enriched nitrogen content in the resulted CoZnO-PC could provide more active sites to active PMS via non-radical pathways for ROCs degradation (Oh and Lim, 2019, Wan et al., 2020). Bisphenol A (BPA) was selected as the target pollutant due to its endorphine disrupting nature and abundant existing. The impact of PVP doping on the catalytic performance of the resulted CoZnO-PC composites, as well as the degradation mechanisms were studied in detail.