**1. Introduction**

Currently, since a greener environment is the hallmark of the scientific world, the contribution of naturally abundant clay minerals is notably visible. In recent decades, clay minerals based on Fuller's earth [1] have been used to modify the performance of photocatalytic reactions. Scientists are inspired by the outstanding characteristics of clay since it is abundant, cost-efficient, and benign to the environment and ecosystem [2]. Although Fuller's earth was first discovered in 1847, clay science is associated with prehistoric times. Its use in several applications can be traced back to 200 cultures, such as the ancient Egyptians, Amargosians, and South and North Americans [3,4]. Recently, Fuller's earth-based semiconductors have attracted great attention for their efficient performance in photocatalytic reactions for eliminating various contaminates from wastewater streams.

However, with the ever-increasing development of societies and the global industrialization revolution, the problems associated with environmental pollution are also tremendously increasing [5]. Massive amounts of organic dyes are discharged annually

**Citation:** Nour, M.M.; Tony, M.A.; Nabwey, H.A. Heterogeneous Fenton Oxidation with Natural Clay for Textile Levafix Dark Blue Dye Removal from Aqueous Effluent. *Appl. Sci.* **2023**, *13*, 8948. https:// doi.org/10.3390/app13158948

Academic Editors: Dae Sung Lee, Yolanda Patiño and Amanda Laca Pérez

Received: 19 May 2023 Revised: 21 July 2023 Accepted: 26 July 2023 Published: 3 August 2023

**Copyright:** © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

from various industries. Such industries include the printing and paper industry, photographic industries, tanning and leather industries, and textile dyeing industries [6]. The textile industry is considered the most polluting industry in the industrial sector [6]. This industry consumes substantial amounts of water in its processing and finishing. Thus, the result is a huge amount of wastewater contaminated with various dye species. Such waste causes persistent damage to the environment [7]. Most of the dyes included in this aqueous effluent are carcinogenic and cause severe damage to the ecosystem and its habitants. Hence, such contaminated wastewater must undergo treatment prior to its final disposal into the environment. When comparing photocatalytic reactions to other wastewater treatment technologies, the photocatalytic system is a superior candidate. This might be due to its complete mineralization tendency for contaminants in aqueous streams [8]. Among the photocatalytic reactions, the iron-based catalytic reaction that uses Fe2+ and Fe3+ as typical iron sources, the so-called Fenton reaction, has attracted scientists' attention for its unique photocatalytic activity [9]. Moreover, its use in contaminate elimination has increased due to the reaction being cost-efficient and the catalyst possessing optical properties [10]. Although the properties of this reaction are superior, there are three main defects that restrict its application. The obstructions are the chemical precursor's price, the need for an acidic pH medium for treatment, and the byproduct sludge after treatment that requires further handling prior to the final discharge [11]. From this concept, various trials have been developed to overcome these drawbacks of the Fenton process. For instance, introducing hetero-junction catalytic materials as the precursors of the Fenton reaction is an excellent strategy due to both its superior catalytic advances and its recovery ability [12]. Moreover, replacing Fe2+ or Fe3+ with non-iron metals, such as Cu2+, shows excellent results since this widens the acidic pH range. Furthermore, aluminum has been shown to be an excellent replacement for Fe2+ or Fe3+ in the reagent as anon-iron Fenton system [13]. Moreover, the aluminum that is present in natural-based materials [14] might be applied to initiate the iron-based Fenton system as a non-iron system [15].

Scientists' crucial goal is identifying environmentally benign materials. In this regard, the naturally abundant Fuller's earth is a suitable candidate. Fuller's earth is the main component of clay minerals that comprise silicon, calcium, and aluminum oxides with a dominant fraction of Al2O3. The elementary molecular structure is an aluminum octahedral structure [3]. Due to its environmental benignity, Fuller's earth is applied in the fields of drug delivery and wastewater treatment. The recently published volume of literature associated with Fuller's earth and its applications indicates an interest in using Fuller's earth in various applications, especially in wastewater treatment. Such wastewater management applications include adsorption techniques or augmentation with semiconductors to act as a photocatalyst. For instance, Safwat et al. [5] used Fuller's earth augmented with kaolin for the elimination of phenolic compounds from an aqueous stream via adsorption methodology. Moreover, Shah and his co-workers [9] used a modified form of Fuller's earth in combination with a surfactant to improve its adsorption capacity for eliminating acid red 17 dye from wastewater. However, to the best of the authors' knowledge, it has not been applied in its solo form as a photocatalyst for dye removal, especially as a source of the Fenton reaction. Catalysts from raw clay, such as Fuller's earth, could represent environmentally friendly and environmentally benign catalysts that possess many advantages. Such materials are cost-efficient and naturally abundant, in addition to being non-toxic to the environment, and they also possess excellent properties, such as a high surface area.

Fuller's earth clay is critical for creating ˙OH radical species, and this is achieved by using the elements in Fuller's earth, such as aluminum and iron ions [14]. Such ions react with hydrogen peroxide, and the reaction is initiated by ultraviolet light and then produces hydroxyl radicals. Iron and aluminum ions are formed and react with hydrogen peroxide to form further hydroxyl radicals and elemental ions (Equations (1) and (2)). ˙OH radicals are categorized as non-selective species that attack pollutant molecules and strongly oxidize them. Aluminum might initiate the Fenton reaction via the acyclic reaction [15]. A general trend of the aluminum-based Fenton reaction mechanism is the formation of the Al3+ superoxide complex (Equation (3)) [16]. In this reaction, the aluminum is able to stabilize a superoxide radical (O2 <sup>−</sup> anion), and thus, the formed Al3+ superoxide complex (Equation (4)) is capable of reducing Fe3+ to Fe2+. Hence, Fe2+ could enhance the production of ˙OH radicals through the Fenton reaction (Equation (5)) [17,18].

$$\text{Fe}^{2+} + \text{H}\_2\text{O}\_2 \rightarrow \text{Fe}^{3+} + \text{OH}^- + \text{OH}^\cdot \tag{1}$$

$$\text{Fe}^{3+} + \text{H}\_2\text{O}\_2 \rightarrow \text{Fe}^{2+} + \text{OH}\_2^{\cdot} + \text{H}^+ \tag{2}$$

$$\rm Al(H\_2O)\_6^{3+} + O\_2^{\cdot -} \rightarrow Al(O\_2)(H\_2O)\_5^{2+} + H\_2O \tag{3}$$

$$\text{Al}(\text{O}\_2^{\cdot})(\text{H}\_2\text{O})\_5^{2+} + \text{O}\_2^{\cdot-} \rightarrow \text{Al}(\text{O}\_2^{\cdot})(\text{OH})(\text{H}\_2\text{O})\_4^{+} + \text{H}\_2\text{O} \tag{4}$$

$$\rm{Fe^{3+} + AlO\_2^{2+} \rightarrow Fe^{2+} + AlO\_2^{3+}}$$

To the best of the authors' knowledge, according to the cited literature, "Fuller's earth" has not yet been applied as an oxidation source for pollutant remediation. Traditionally, clay sources are applied as adsorbent materials. Therefore, this investigative study introduces the novel application of Fuller's earth as the elemental source of Fenton oxidation. The goal of the current work is based on altering the traditional Fenton source with the environmentally benign, naturally available, and abundant clay "Fuller's earth", which comprises various metals. Such metals lead to Fenton reaction oxidation. The system is applied to mineralize Levafix Dark Blue aqueous effluent as a simulation of textile-effluent-polluted wastewater. The influence of various operating variables, i.e., Fuller's earth and the hydrogen peroxide reagent concentration, the pH of the medium, dye loading, and the temperature of the wastewater are assessed in order to meet the real application requirements.
