Copper Loaded Hydroxyapatite Nanoparticles as eco‐friendly Fenton-like catalyst to Effectively Remove Organic Dyes

Highlights

• Natural mesoporous hydroxyapatite was successfully synthesized by dissolution-precipitation of phosphate rock.

• Copper loaded natural hydroxyapatite (Cu/NHAp) have been prepared via ion-exchange method.

• The obtained Cu/NHAp catalyst can efficiently activate H₂O₂ for degradation of a toxic organic compound.

• Possible mechanism for H₂O₂ activation by Cu/NHAp catalyst was discussed.

• The catalyst showed good stability of catalytic activity, even after four-cycles of usage.

Abstract

In this study, natural hydroxyapatite (NHAp) was synthesized by the dissolution of phosphate rock in acidic medium and precipitated via basic solution. Then, a series of copper loaded NHAp (Cu/NHAp) were prepared using ion exchange method and employed as a novel heterogeneous Fenton-like catalyst for degradation of a toxic organic compound. The Cu/NHAp materials have been characterized by different physico-chemical techniques including XRD, XPS, FT-IR, Raman & UV–vis spectroscopies, NMR ³¹P, SEM, TEM and nitrogen adsorption-desorption methods. The materials exhibit a high catalytic activity for the degradation of methylene blue in the presence of H₂O₂ as green oxidant under basic medium. Moreover, the effects of different parameters, including pH, H₂O₂ concentration, catalyst loading, initial dye concentration, and temperature on the degradation kinetics were investigated and discussed in detail. Furthermore, based on the results obtained by radical quenching experiments and electron paramagnetic resonance (EPR), it was shown that hydroxyl radicals were the main reactive species responsible for MB degradation. Finally, TOC analysis, catalyst recyclability and stability confirmed that copper loaded hydroxyapatite might be an alternative Fenton-like catalyst towards organic pollutants degradation and environmental remediation.

Introduction

In recent years, the challenges related to environmental problems have received increasing attention. These problems include the contamination of wastewater by industrial organic substances, such as paper, cosmetics, plastics, food, synthetic detergents, and textile [1]. This contamination has become a serious problem, since the release of dyes into the environment by different human and industrial activities can, without proper treatment, cause a severe threat to the aquatic environment and human health [2], [3]. This threat is related to the genotoxic and mutagenic properties of these pollutants (even at trace concentrations) when they are bio-accumulated and transported in the food chain, hence the need of pre-treatment [4]. In particular, methylene blue is one of the important dyes, which is widely used as a colorant in the textile industry. It may present eco-toxic hazards and may eventually affect public health through the food chain [5]. Therefore, it becomes essential to remove this dye from wastewater, or to treat it to reduce its impact on the environment and public health. Various treatment methods have been employed for the treatment of wastewater containing organic dyes, such as physical methods (filtration, adsorption, and coagulation-flocculation) [6], [7], [8], [9], biological methods (microorganisms and enzymes), and chemical methods (oxidation, electrolysis and ozonation) [10], [11]. Unfortunately, these techniques have been shown to be either non-destructive or inefficient for the removal of dyes [12]. Hence, advanced oxidation processes (AOPs), such as Fenton and photo-Fenton processes, which involve a homogeneous solution of iron (Fe²⁺) and hydrogen peroxide for the generation of hydroxyl radicals (^•OH), are believed to be a good alternatives for treating and eliminating dyes since they are capable of completely decolorizing and mineralizing the textile dyes in a short period of time [13], [14], [15]. However, the Fenton process suffers from several disadvantages, such as the narrow pH reaction range (2.5–3.5), and the non-recyclability of iron, which is considered a secondary pollutant that can influence water quality [16], [17], [18], [19]. In order to address these issues, many efforts have been directed towards the development of heterogeneous Fenton catalysts due to their advantage of degrading pollutants in a wide pH range with less metal loss; moreover, the catalysts can be easily recovered and reused for another cycle [20]. Recently, various heterogeneous catalysts such as copper-based catalyst have been used to activate H₂O₂ to remove organic compounds. For example, magnetic material include CuFe₂O₄ [21], [22], CuWO₄ [23]_, Mesoporous silica-based material Cu-MCM4 [24], perovskite-like oxides LaBO₃ (B = Cu, Fe, Co, Ni, Mn), metal oxide CuO [25] and mixed oxide CoO-CuO [26].

In the last few years, hydroxyapatite has attracted much attention for its use in a several applications such as dental implants [27], drug delivery vector [28], [29], adsorbent [30], [31], [32], [33], catalyst [34], [35], [36] and photo-catalyst [37], [38], [39], [40]. Hydroxyapatites are usually synthesized from compounds containing calcium and phosphorus by different methods including co-precipitation, sol-gel and hydrothermal [41], [42], [43], [44]. However, hydroxyapatite can also be naturally obtained from waste by-products showing environmental advantages and interesting properties making them excellent candidates for their use as catalytic systems or as supports for the elaboration of catalysts [45], [46], [47]. Piccirillo et al. [48] have reported the synthesis method of HAp from fish bones a method HAp synthesis in which bones of waste by-product. In the other hand, El Asri et al. [49] demonstrated the advantage of using phosphate rock as precursors of Ca²⁺ and PO₄^3- ions to prepare hydroxyapatite via dissolution-precipitation reaction. The obtained hydroxyapatite has shown better performance in terms of sorption capacity for heavy metal removal.

Moroccan phosphate rock (MPR) has the advantage of very low cost and large abundance but exhibits a limited surface area and porosity. MPR is normally used for the preparation of phosphoric acid and different types of fertilizers. In this work, we report the synthesis of nano-hydroxyapatite (NHAp) based material from Moroccan phosphate rock. Thereafter, copper was loaded on natural hydroxyapatite (Cu/NHAp) via ion exchange method. All materials were characterized by different techniques such as XRD, FT-IR, Raman, XPS, ³¹P NMR UV–vis, SEM, TEM and nitrogen adsorption-desorption. The as-prepared (Cu/NHAp) was then evaluated for degradation of dyes in wastewater (MB was chosen as model organic pollutant). The effects of various parameters such as time, solution pH, H₂O₂ dosage and catalyst loading were investigated. The mechanism of MB degradation using Cu/NHAp catalyst was also proposed.