Research article
Azo-dye degradation by Mn–Al powders

https://doi.org/10.1016/j.jenvman.2019.110012Get rights and content

Highlights

  • Manganese aluminum particles show high efficiency for azo-dye degradation.

  • High efficiency is related to particles composed of Mn-rich and Al-rich nanocrystals.

  • Particles nanostructure and composition provide good performance in alkaline waters.

  • Decolorization ability is assessed for various dyes and real textile wastewaters.

  • Reduction of toxicity is observed after the decolorization treatments.

Abstract

Manganese–Aluminum powders were recently reported to show high efficiency and fast reaction rates as decolorization materials for azo-dye aqueous solutions. This work presents a detailed study of different aspects of this material. Firstly, the influence of the crystalline phase and the microstructure was studied by comparing the efficiency of powders obtained by different production protocols. Secondly, the decolorization efficiency was investigated on various types of dyes, including real textile wastewater samples. The analysis of the treated water and the particles showed that the main reaction mechanism was the breaking of the azo-dye molecules, although important adsorption on the metallic surface was observed for some colorants. Finally, the reusability of the particles and the reduction of toxicity achieved during the treatments were assessed. The simple production and application methods, the high efficiency and the use of environmentally friendly metallic elements are the main advantages of Manganese–Aluminum powders compared to other high-efficient decolorizing metallic materials.

Introduction

The use of dyes in various processes of textile and other industries results in extensive amounts of water containing important loads of these substances. The treatment of these wastewaters consists of multiple steps, one of them is the decolorizing process. This step is needed in order to fulfill legal requirements as well as to enhance the effectivity of posterior biological and chemical treatments. Among other methodologies and materials (Weng et al., 2013) (Weng, 2017) (Liu et al., 2018b), a widely studied decolorizing technology is the use of metallic zero-valent metals (ZVM), which are able to activate the degradation of many types of dye molecules and other pollutants when put in contact with the dyed water (Qin et al., 2015) (Deng et al., 2018). This can be an important process in sequential biological-chemical methods, as the end products of the degradation reaction are usually less resistant than the original molecule and may be subjected to posterior treatments like aerobic biodegradation (Patel and Suresh, 2006). The most studied material for this application is zero-valent iron (ZVI), which shows good efficiency in the degradation of many different compounds in aqueous solutions (Zhang, 2003) (Weng et al., 2014) (Weng and Huang, 2015), among them azo-dyes (Nam and Tratnyek, 2000) (Raman and Kanmani, 2016) (Weng and Tao, 2018) (Liu et al., 2018a). These type of colorant compounds are the main family of dyes used in the textile industry and are frequently used as benchmark compounds to test the efficiency of decolorizing materials.

The effectivity of the decolorizing reaction by means of metallic particles depends on two fundamental aspects. Firstly, the chemical interaction between the metallic material, the dye molecules and the aqueous media, which is usually highly sensitive to temperature and pH conditions. Secondly, the specific physical parameters of the process such as the total surface of the decolorizing material in contact with the dyed water, the agitation or filtration protocols and the geometrical and dynamical parameters of the reactor. The research in new decolorizing metallic materials is focused on improving the effectiveness of the chemical mechanism under different water conditions as well as on designing new production methods to obtain the maximum active surface per gram of material (Deng et al., 2018).

One approach to improve the efficiency of the decolorizing reaction is to produce new intermetallic phases with metastable structures. The aim is to increase the reactivity of the materials by combining different metallic elements and by introducing defects in the metallic atomic-scale structure. Metastable phases like amorphous metals (metallic glasses) have been proven to show high reactivity (Zhang et al., 2010) (Wang et al., 2012) (Zhang et al., 2019) and improved decolorizing efficiency in comparison with more stable phases with the same chemical composition (Zhang et al., 2012a). Another approach is to increase the specific surface area, either by the production of nano-particles (Zhang, 2003) (Fan et al., 2009) (Bhakya et al., 2015) (Raman and Kanmani, 2016) or by the generation of nano-scale rough or porous metal surfaces (Luo et al., 2014). A simple method to produce metallic particles for decolorizing applications is the use of mechanical alloying in metallurgical ball mills (Suryanarayana, 2001). This classic metallurgical method introduces enough mechanical energy to produce new intermetallic or solid solution phases promoting the inter-diffusion of the initial raw components. Furthermore, the ball milled particles usually show highly metastable nanocrystalline or amorphous atomic-scale structures as well as small particles with rough surfaces.

Recent works reported the high efficiency of Manganese–Aluminum (Mn–Al) particles in the degradation of azo-dyes like Reactive Black 5 (RB5) (Ben Mbarek et al., 2017) and Orange II (AboliGhasemabadi et al., 2018). Both Mn and Al have the advantage of being biocompatible materials used in many environmental and biomedical applications (Hermawan et al., 2008). In this work, new results assessing different aspects of the Mn–Al particles will be presented, such as the effect of composition and structure in the degradation efficiency, the toxicity, the efficiency on different types of dye molecules and the reutilization capability. The aim is to provide enough information to environmental engineers and scientists to consider their possible application in water treatment processes.

Section snippets

Materials and methods

Metallic particles of Mn–Al were prepared by mechanical alloying in a mechanical mill SPEX 8000 under protective argon atmosphere. The pure, raw elements were introduced into a container together with hard material balls in order to produce powder by mechanical milling. Different milling times of 20, 30 and 60 h were used, applying intervals of 10 min of milling and 5 min of resting in order to avoid overheating and the formation of particle aggregates. An alternative route, with a previous

Efficiency of Mn–Al particles for azo-dye degradation

The effectiveness of Mn–Al particles was already reported in previous works (Ben Mbarek et al., 2017) (AboliGhasemabadi et al., 2018). The effect of temperature was studied determining an Arrhenius behavior of the reaction rate constants with relatively low activation energies (AboliGhasemabadi et al., 2018), similar to the ones found for metallic glass particles as discussed in (Zhang et al., 2019). Fig. 1 shows the evolution of the absorbance spectra (inset) and the corresponding values of

Conclusions

The production of Mn–Al powders was obtained by ball milling as well as by rapid solidification and posterior ball milling. In the first case the particles of the powder were composed of Mn-rich and Al-rich nanocrystals while in the latter by nanocrystals of a Mn(Al) solid solution. The Mn–Al powders showed a similar high efficiency and fast reaction rates in decolorization experiments of azo-dye solutions for both microstructures. In Reactive Black 5 and Orange II solutions the Mn–Al powders

Author contribution section

Mitra AboliGhasemabadi: Formal analysis, Investigation, Methodology, Writing - original draft Wael Ben Mbarek: Investigation; Methodology Andrea Cerrillo-Gil: Formal analysis, Investigation, Writing - original draft Helena Roca-Bisbe: Formal analysis, Investigation, Methodology Oriol Casabella: Formal analysis, Investigation, Methodology Paqui Blánquez: Methodology, Writing - review & editing Eloi Pineda: Supervision, Writing - original draft, Writing - review & editing Lluïsa Escoda:

Acknowledgements

E.P. acknowledges financial support from MINECO, Spain (grant FIS2017-82625-P) and Generalitat de Catalunya, Spain (grant 2017SGR0042).

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