Magnetite CaF2 near-infrared photocatalysts fabricated with Ca-enriched ferrites derived from electroplating wastewater

https://doi.org/10.1016/j.cej.2020.124868Get rights and content

Highlights

  • CaSO4-ferrites were fabricated from electroplating wastewater.

  • CaF2-based FCT photocatalysts were synthesized by using the optimum CaSO4-ferrite.

  • The introduction of TiO2 largely increased the upconversion luminescence of FCT3.

  • FCT3 possessed the highest photodegradation rates of ciprofloxacin among the samples.

  • All the FCT photocatalysts are very stable and safe.

Abstract

The hydroxide precipitation with CaO or Ca(OH)2 is frequently applied in the ferrite formation process of electroplating wastewater, but the excess Ca2+ ions present often weaken the quality of the resultant mixed-ferrite (M−Fe3O4) precipitates. In this work, for resource utilization of the M−Fe3O4 and the Ca2+ sources introduced from Ca(OH)2 in the hydroxide precipitate process, the upconversion assisted magnetite near-infrared (NIR) photocatalysts of M−Fe3O4/Tm3+/Yb3+-doped CaF2/TiO2 (FCT) are synthesized. The M−Fe3O4 phases possess the advantages of magnetic separation, broadband spectrum absorption, and the heterostructure with TiO2, and absorb the strong upconversion luminescence emitted from Tm3+/Yb3+-CaF2 for photocatalysis. The TiO2 phase contributes to the large improvement of upconversion luminescence and photocatalytic activity. Because of the stronger UV–Vis-NIR absorption, upconversion luminescence, and electron-hole pair separation for the FCT sample with Ti: Ca molar ratio of 3: 1 (FCT3), higher yields of radical dotOH and O2radical dot radicals are generated, which enable FCT3 to possess much enhanced degradation rate (87.3%) of ciprofloxacin compared with those of other FCT samples under UV–Vis-NIR light irradiation. The fabrication of the upconversion magnetite NIR photocatalysts from electroplating wastewater is a promising strategy for its resource utilization and the environmental remediation.

Introduction

Electroplating wastewater containing heavy metals such as Cr, Cu, Zn, or Ni is often hazardous and harmful due to their toxicity, mutagenicity, and carcinogenicity [1], [2], [3], [4]. To recycle and reuse the heavy metals in electroplating wastewater, the ferrite formation strategy has been widely investigated [5], [6], [7], [8]. With the incorporation of heavy metal ions into the spinel ferrite structure, the mixed-ferrites (M−Fe3O4) consisted of Fe3O4, FeCr2O4, ZnFe2O4, CuFe2O4, or CoFe2O4 can be generated [9], [10], [11]. It should be noted that in the ferrite formation process, hydroxide precipitation is needed after the reduction of Cr6+ to Cr3+, and CaO or Ca(OH)2 is frequently used owing to its lower cost than those of other alkalis (e.g., NaOH or ammonia) [12], [13], as well as the easier post-treatment of Ca2+ ions compared with those of Na+ and NH4+ ions. Nevertheless, in the resultant ferrite precipitates, a large amount of impurity of CaSO4 is inevitably formed, which greatly reduces the quality of the obtained M−Fe3O4 products. To remove or separate CaSO4 from the ferrite precipitates is very difficult, and it is urgent to put forward a new effective method for the resource utilization of ferrite precipitates containing multiple Ca2+ ions.

The common TiO2-based photocatalysts have attracted considerable attention in the photodegradation of environmental pollutants, but one of their major disadvantages is the low utilization efficiency of sunlight [14], [15]. To extend the light absorption ability to visible or Near-infrared (NIR) light ranges, the TiO2 photocatalysts modified with dopants or narrow bandgap semiconductors have been widely reported [16], [17], [18]. Additionally, in recent years, there has been increasing interest in the development of the NIR driven upconversion photocatalysts with narrow bandgap energies [10], [19], [20], [21], [22], [23], [24], where the NIR light can not only be directly absorbed, but can also be upconverted to ultraviolet (UV) and shorter-wavelength visible or NIR light. Because of the broadband absorption ability, these upconverted light emissions can be reutilized for photocatalysis. The upconversion luminescence is mainly originated from the lanthanide ions, such as the activators of Er3+ and Tm3+ ions and the sensitizers of Yb3+ ions, while the host matrix is very important for upconversion efficiency [25], [26], [27]. The fluorides are favored by many researchers due to their low phonon energy and high stability [28], [29]. Among them, CaF2 is one of the most promising host matrices because the Ca2+ source is abundant and very cheap [30]. Up to now, the lanthanide ions doped CaF2 upconversion phosphors have been intensively studied, and the Ca2+ precursors of Ca(NO3)2 [31], [32], CaCl2 [33], [34], and Ca(CH3COO)2 [35], [36] are mainly used for the formation of CaF2. In our previous researches, to synthesize CaF2 in a multi-stage process, other kinds of Ca2+ sources, such as CaWO4 [37] and CaTiO3 [38] were also selected. Therefore, for the purpose of resource utilization, the ferrite precipitates fabricated from electroplating wastewater may be utilized as the Ca2+ precursor, because there is a large amount of CaSO4.

We have synthesized the M−Fe3O4 products from electroplating wastewater [9], in which NaOH was chosen for the hydroxide precipitation reaction. In the practical treatment, because many Ca2+ ions (from CaO or Ca(OH)2) often remain in the resultant ferrites, we also simulated the introduction of Ca2+ ions into the ferrite precipitates by using Ca(NO3)2 [10]. In this work, to completely follow the actual ferrite formation process, Ca(OH)2 was directly used instead of NaOH in the hydroxide precipitation process. In the crystal phases of the obtained ferrite precipitates, except for the M−Fe3O4, a large amount of CaSO4 was appeared. Following the addition of the F ions, TiO2, and the lanthanide ions of Tm3+ and Yb3+, all the CaSO4 phases were disappeared and a series of the upconversion assisted magnetite NIR photocatalysts of M−Fe3O4/Tm3+/Yb3+-doped CaF2/TiO2 (FCT) were synthesized. The structures, morphologies, and optical properties of the FCT samples were investigated in detail. To evaluate their photocatalytic activities under light irradiations with different wavelengths, a widely used antibiotic of ciprofloxacin was chosen, since ciprofloxacin is difficult to be decomposed by general chemical methods or natural microorganisms [39], [40].

Section snippets

Chemicals and materials

The chemical reagents of titanium isopropoxide (TTIP, C12H28O4Ti, CAS: 546-68-9), ferrous sulfate heptahydrate (FeSO4·7H2O, CAS: 7782-63-0, analytical grade), ytterbium nitrate pentahydrate (Yb(NO3)3·5H2O, CAS: 35725-34-9), thulium nitrate hexahydrate (Tm(NO3)3·6H2O, CAS: 36548-87-5), and calcium hydroxide (Ca(OH)2, CAS: 1305-62-0) were all purchased from Aldrich and used as received. The electroplating wastewater was kindly provided by an electroplating plant, and it mainly contained

Optimization of the ferrite precipitates formation

In the fabrication process of the ferrite precipitates, sufficient FeSO4·7H2O was firstly added into electroplating wastewater to reduce Cr6+ ions and supply Fe2+ ions for ferrite formation. Then, different amounts of Ca(OH)2 were added for the hydroxide precipitation reactions. The obtained ferrite precipitates are characterized by XRD. As shown in Fig. 2a, in the XRD pattern with the mass ratio of 1: 1, most of the diffraction peaks are attributed to CaSO4·2H2O (JCPDS Card No. 70-0982), while

Conclusions

In conclusion, the novel upconversion assisted magnetite NIR photocatalysts of the FCT samples were synthesized based on the ferrite precipitates consisted of the crystal phases of CaSO4·2H2O and M−Fe3O4. All the FCT samples contained the crystal phases of (Ca0.8Yb0.2)F2.2, anatase TiO2, and M−Fe3O4. With the combination of narrow bandgap semiconductors and the NIR-driven upconversion property, excellent photocatalytic activities were exhibited in the degradations of ciprofloxacin, and FCT3

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (21607101) and the Postgraduate Research & Practice Innovation Program of Jiangsu Province (SJCX19_0748).

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