Activation of peroxymonosulfate by magnetic catalysts derived from drinking water treatment residuals for the degradation of atrazine

Highlights

• MCs derived from WTRs were prepared and utilized in heterogeneous PMS process.

• Catalytic performance of the MCs was highly dependent on pyrolytic temperature.

• ^•OH and SO₄^•− predominated in the MC-600/PMS and MC-1000/PMS systems, respectively.

• Activation mechanisms and degradation pathways were elucidated and proposed.

Abstract

Magnetic catalysts (MCs) derived from iron-rich drinking water treatment residuals (WTRs) were prepared through pyrolysis treatment and introduced as activators of peroxymonosulfate (PMS) for refractory atrazine (ATZ) degradation. Comprehensive characterization analysis indicated that pyrolytic temperature could manipulate the crystalline structure evolution of the MCs and influence their physicochemical and catalytic properties. The catalytic performances of the as-prepared samples pyrolyzed at 600 °C (MC-600) and 1000 °C (MC-1000) were evaluated, and MC-1000 exhibited far more excellent catalytic reactivity than MC-600 in PMS oxidation system. Such difference was mainly attributed to that Fe₃O₄ and Fe° are the dominant active ingredients of MC-600 and MC-1000, respectively. The electron spin resonance (ESR) tests and radical quenching experiments revealed that hydroxyl radical (^•OH) and sulfate radical (SO₄^•−) predominated in the MC-600/PMS and MC-1000/PMS systems, respectively. The mechanisms of the MCs-mediated PMS activation process were elucidated, among which the role of iron mineral phase was emphatically explored. Furtherly, possible degradation by-products were identified by LC–MS, and potential degradation pathways were proposed. Ultimately, the effects of pivotal parameters (i.e. MC-1000 dosage, PMS concentration, initial pH, and water matrix species) on ATZ degradation were investigated to assess the applicability of the MC-1000/PMS system.

Introduction

Persulfate (PS)-based advanced oxidation processes have received extensive attention in the field of environmental remediation, especially in terms of the elimination of refractory organic contaminants. Generally, the oxidative degradation processes are dominated by highly reactive oxidizing species, i.e. ^•OH and SO₄^•−. Compared with ^•OH, SO₄^•− possesses advantages of higher redox potential, better pH-tolerance, and longer lifetime [1]. SO₄^•− can be generated by activating PS (peroxydisulfate (PDS) or peroxymonosulfate (PMS)) with base [2], thermal energy [3], ultraviolet radiation [4], carbon materials [5], transition metals [6], and organics [7,8]. Particularly, as a typical representative of transition-metal catalysts, non-toxic and abundant Fe(II) could induce the decomposition of PS to SO₄^•− through electron transfer reaction [9]. Although the homogeneous reaction is effective in degrading pollutants, harsh pH restriction, massive Fe(II) consumption, and slow conversion of Fe(III) to Fe(II) constrain its wide-span application [10].

Thus, interest in heterogeneous PS activation process has been increasing due to mild operation conditions and reduced metal leaching. Considerable efforts have been made to develop heterogeneous Fe-based catalysts, which have been extensively studied for PS activation to degrade various contaminants [[11], [12], [13]]. Ren et al. synthesized magnetic ferrospinel MFe₂O₄ (M = Co, Cu, Mn, and Zn) to activate PMS for di-n-butyl phthalate degradation and CoFe₂O₄ showed the best catalytic performance in PMS oxidation system [6]. However, the synthesis of heterogeneous Fe-based catalysts usually involves in the expensive precursor addition (i.e. iron-containing chemicals), massive energy input, and complex fabrication procedure, which impede their large-scale application.

More recently, natural minerals enriched in iron have been used as economical alternative PS activators for contaminations degradation [14] or bacterial inactivation [15]. Natural magnetic pyrrhotite was reported to be an environmentally friendly, easy available, and low-cost catalyst, which could effectively activate PS for water disinfection [16]. In addition, some iron-containing wastes such as red mud [17] and drinking water treatment residuals (WTRs) [10] were also directly applied to the field of environmental remediation. In our previous study, the WTRs/PMS system was constructed based on the hypothesis that Fe contained in WTRs had the potential of activating PMS and eliminating ATZ. However, because low catalytic Fe(III) rather than active Fe(II) was the main existing form of iron, WTRs showed limited PMS activation performance and moderate degradation efficiency. Besides, directly applying WTRs to environmental remediation may cause secondary pollution due to its complex components. In this regard, an effective strategy should be pursued that improving catalytic performance while reducing potential risks. It has been reported that Fe(III) has the potential of reacting with organic matters or carbon basal to convert to Fe(II), even Fe° during pyrolysis process [18,19]. Wang et al. successfully prepared magnetic porous carbon (MPC) through pyrolysis of sewage sludge conditioned by ferric salts, and the as-prepared MPC showed excellent PS activation performance and 2-Naphthol oxidation degradation [20]. During pyrolysis process, Fe(III) was orderly evolved into Fe₃O₄, Fe°, and Fe₃C with the elevated pyrolytic temperature. In view of theoretical basis and practical application, a new proposal was put forward that Fe-based catalysts was prepared via pyrolysis treatment of WTRs, which is byproduct generated during drinking water treatment and mainly consists of Al and Fe hydroxides as well as organic matters. It is speculated that redox reactions between iron and organic matters would induce the transformation of iron mineral phase, leading to the formation of effective Fe-based catalysts. Moreover, pyrolysis treatment could remove potential harmful substances to a certain extent and reduce the possibility of causing secondary pollution, making the pyrolyzed WTRs has the potential as environmentally friendly activator of PMS.

Therefore, in this work, pyrolysis treatment was innovatively applied to iron-rich WTRs, attempting to prepare magnetic catalysts (MCs) and construct the MCs/PMS system for the degradation of atrazine (ATZ), a refractory s-triazine herbicide frequently detected in different environmental matrices [21,22]. The physicochemical properties of the MCs were comprehensively analyzed and their catalytic performances were systematically evaluated. Dominant reactive species generated in the MCs/PMS system were identified and the underlying activation mechanisms were investigated. Particularly, the role of iron mineral phase in the MCs-mediated PMS activation process was emphatically explored. Additionally, the possible degradation pathways of ATZ were proposed based on the detected intermediates. Ultimately, the effects of influential parameters (MC-1000 dosage, PMS concentration, initial pH, and water matrix species) on ATZ degradation were systematically studied to assess the applicability of the MC-1000/PMS system. This work is expected to provide a promising utilization strategy for WTRs as high-efficiency, low-consumption, and environmental friendly catalyst for PMS activation.