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Article

Recycling Industrial Waste: Ferritization Products for Zn2+ Removal from Wastewater

by
Dmitry Samchenko
1,
Gennadii Kochetov
1,
Shuwei Hao
2,
Yuliia Trach
3,4,*,
Roman Trach
3,4 and
Olena Hnes
4
1
Faculty of Engineering Systems and Ecology, Kyiv National University of Construction and Architecture, 03680 Kyiv, Ukraine
2
MIIT Key Laboratory of Critical Materials Technology for New Energy Conversion and Storage, School of Chemistry and Chemical Engineering, Harbin Institute of Technology, Harbin 150001, China
3
Institute of Civil Engineering, Warsaw University of Life Sciences, 02-776 Warsaw, Poland
4
Institute of Civil Engineering and Architecture, National University of Water and Environmental Engineering, 33028 Rivne, Ukraine
*
Author to whom correspondence should be addressed.
Sustainability 2025, 17(9), 4008; https://doi.org/10.3390/su17094008
Submission received: 10 March 2025 / Revised: 25 April 2025 / Accepted: 28 April 2025 / Published: 29 April 2025
(This article belongs to the Special Issue Innovative Materials and Technologies for Sustainable Water Treatment and Environmental Remediation)
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Abstract

This study presents a sustainable approach to recycling exhausted etching solutions through ferritization, using various activation methods and aeration rates. The process transforms industrial waste into valuable magnetic sorbents, supporting circular economy principles. Structural and chemical analysis of the ferritization products revealed the formation of ferromagnetic crystalline phases, including lepidocrocite (ɣ-FeOOH), ferrooxygite (δ-FeOOH), and magnetite (Fe3O4). Increasing the aeration rate and use of ultrasound treatment enhances Fe3O4 content and iron ion removal efficiency. The adsorption capacity of the recycled materials for Zn2+ removal was assessed under different pH conditions using mechanical mixing and ultrasound treatment. The highest level of Zn2+ removal (92.0%) was achieved at pH 8 with ultrasound-activated sorbents containing 61.3% δ-FeOOH and 38.7% Fe3O4. At pH 10, magnetite-based sorbents achieved over 98.9% Zn2+ removal, enabling the treated water’s reuse in industrial rinsing processes. Electron microscopy and X-ray fluorescence confirmed the presence of fine, spherical magnetite and zinc ferrite particles. These findings underscore the potential of ferritization-based recycling as an eco-friendly and efficient strategy for heavy metal removal from galvanic wastewater.

1. Introduction

Environmental safety improvements through development of low-waste energy-efficient environmental protection technologies, in particular wastewater treatment ones, and introduction of resource circulation systems belong to main priority spheres of modern national economic development [1]. Water is one of the most important strategic resources for human life, industry and the environment. Estimates suggest that only 10% of the world’s available water is suitable for domestic use [2]. According to the World Health Organization and UNICEF, about 768 million people do not have access to fresh and clean water [3].
The condition of the environment is closely linked to the effective management of liquid industrial waste, including its treatment, processing, and disposal—issues that represent critical priorities in advancing sustainable development [4]. Wastewater flowing from electroplating production facilities is one of the most dangerous liquid industrial wastes [5]. This is due to their high volumes, as well as to their qualitative and quantitative chemical composition. In the whole range of different electroplating processes: copper, cadmium, chrome and brass plating, the largest volumes of coating production are associated with zinc plating 40 ÷ 50% and nickel plating 8 ÷ 10% [6].
Application of special methods and facilities for preliminary and final treatment of wastewater from electroplating industries is necessary for removal of heavy metals [7]. Wastewater is generated both during the process of removing scale and rust from metal surfaces before applying the coatings, and during subsequent washing of finished parts [8]. Depending on production capacity, 0.2 ÷ 5 m3 of technical water is consumed to wash 1 m2 of the coating surface, which must meet certain physical and chemical parameters to ensure the process quality and to prevent formation of defects on surfaces of metal products [9]. Rinsing wastewater after various technological operations has a concentration of heavy metals up to 150 mg/dm3. In addition, exhausted technological solutions are formed at electroplating plants: electrolytes, solutions of capture baths with total concentrations of heavy metal ions from 10 to 200 g/dm3 [10]. The vast majority of types of exhausted technological solutions are highly acidic (pH ≤ 3).
Given large volumes of toxic electroplating production wastewater, the specified problem is particularly acute. This is mainly due to on-site storage of waste residues from treatment of such wastewater at production facilities, resulting in non-compliance with technological requirements and even in emergencies, as well as in inevitable environmental pollution [11]. Such a waste contains a mix of heavy metals, namely Cr, Ni, Zn, Cd, Cu, Pb, etc. In addition, the waste may contain chemical reagents such as residues of coagulants (e.g., aluminum sulphate, ferric chloride), flocculants, acids and alkalis used to adjust pH [12].
In connection with the above, a current problem is the resource efficient treatment of zinc-containing rinsing wastewater with removal and subsequent utilization of heavy metals, in particular zinc and its compounds, and return of the treated water to production processes. Currently, a number of methods are used to treat wastewater for removal of heavy metals, based on various physicochemical processes [13]. The most common method is to convert heavy metals into insoluble hydroxides with alkalis NaOH or Ca(OH)2 [14]. As a result, voluminous chemically unstable hydroxide sludges are formed, which are difficult to dispose of. The use of the reagent method allows water to be purified to concentrations of heavy metals that are an order of magnitude higher than their maximum allowed concentrations (MACs) for discharge into water bodies [15]. As noted above, most rinsing solutions are used in electroplating surfaces of metal parts, so zinc is the dominant metal present in wastewater. It is known [16], that Zn2+ ions are most poorly precipitated in the hydroxide form and to achieve the desired effect pH values of the aqueous media must exceed 11–12 [17].
In recent years, major attention was paid to sorption methods of treatment of such wastewater for removal of heavy metals [18]. Mineral sorbents such as zeolite, volcanic tuffs, bentonite clay, kaolin and others natural materials are considered prospective for practical application [19,20]. Their use does not require significant resource and energy costs, as their preparation does not require any special production processes except grinding to a dispersed state. In addition, their advantage lies in the fact that they can be waste or by-products of extraction of mineral raw materials [21]. However, efficiency of wastewater treatment for removal of heavy metal ions by natural sorbents may decrease after several cycles of their use from 95 to 65% [22]. Improvement of the efficiency of sorbents is usually achieved by their modification (thermal, chemical), which is used to improve the specific surface area, increase surface porosity, thermal stability, and, therefore, to increase their sorption capacity [23].
Long-term use of sorbents leads to loss of their sorption capacity, and, therefore, their disposal or regeneration are necessary. There are many technological processes of varying complexity for sorbent regeneration [24]. These processes can cause environmental risks associated with accumulation of toxic waste after sorbent regeneration, and it is necessary to use specially prepared water for regeneration of sorbents (water with very low or high pH values) [25]. Taking into account the above considerations, scientists are still developing research studies to find methods for utilization of used sorbents. One of such methods is associated with their reuse, namely in production of new environmentally friendly materials from them [26,27].
A particularly significant application niche is occupied by sorbents with magnetic properties [28]. Their main attraction lies in their ease of application due to their ability to be easily separated from the liquid phase on magnetic filters. There are known studies on application of magnetic sorbents, such as magnetite Fe3O4, maghemite γ-Fe2O3, hematite α-Fe2O3 for removal of pollutants, in particular, heavy metal ions from industrial wastewater [29]. In addition, due to their unique magnetic, catalytic and other properties, these compounds began to find wide applications in various industries [30]. Synthesized iron-containing materials have higher sorption capacity (Table 1) compared to natural Fe-rich materials [31].
To obtain Fe-rich magnetic sorbents in laboratory conditions, the following methods are used: hydrothermal, solvothermal, ferritization, thermal decomposition, microemulsion, electrochemical, microwave [39]. Hydrophase ferritization, among the listed methods, has significant advantages due to ease of obtaining marketable products from treatment of Fe-rich wastewater [40]. The process of hydrophase ferritization occurs in a solution containing heavy metal ions, in particular, Fe2+, with addition of alkali (NaOH) and an oxidant (usually oxygen). As a result, heavy metal ferrites are formed [41]. The traditional hydrophase ferritization process occurs at temperatures above 75 °C, so its use requires a lot of energy [42]. As an alternative to the thermal activation of the reaction mix for the ferritization process, it is possible to use other energy efficient methods, in particular, treatment with electromagnetic pulse discharges and ultrasound [43]. By adjusting technological parameters of the ferritization process, it is possible to obtain Fe-rich materials of certain phase compositions from the liquid electroplating waste, which can be used as sorbents in the future.
The efficiency of sorption of heavy metals by magnetic Fe-rich compounds is influenced by the following factors: pH, contact time, temperature, sorbent doses and initial heavy metals concentrations [44]. The traditional method of sorption under static conditions is carried out by the mechanical mixing of the model liquids and sorbents at different speeds [45]. To increase the sorption capacity of Fe-rich materials, vibration, crushing and ultrasound can be used [46].
Thus, the use of Fe-rich sorbents derived from recycled industrial waste, combined with ultrasound activation, enables a resource-efficient and sustainable approach to wastewater treatment at electroplating facilities, meeting the standards required for the reuse of purified water in rinsing operations. This can ensure return of up to 95% of the treated water to circulating water supply systems, with simultaneous disposal of wastewater treatment waste.
The next practical significance of the obtained research results may be that after the sorption capacity is exhausted, the used sorbents can be disposed of as concrete components and as additives to powder paints [47,48], with subsequent production of coatings based on them, or as components in cements and concrete [49]. Cement is known to be able to encapsulate heavy metals and bind them into a geopolymer [50]. Such research contributes to the development of zero-waste technologies and the implementation of circular processes in industrial production.
Therefore, the aim of the work is to improve the process of production of Fe-rich containing materials of a given phase composition from etching wastewater in order to use them as sorbents of heavy metals. To achieve this goal, the following tasks were set:
  • − to study the structure of precipitates from ferritization treatment of etching solutions to confirm possibility of their use as sorbents;
  • − to study experimentally the sorption capacity of the Fe-rich materials obtained by the improved ferritization method to sorb Zn2+ ions;
  • − to determine the influence of sorption conditions on efficiency of zinc ion removal by these sorbents;
  • − to study structures of exhausted sorbents.

2. Materials and Methods

To achieve the aim, at the first stage, precipitates were obtained by processing exhausted sulfuric acid etching solution by the hydrophase ferritization method. This solution was taken from a company manufacturing protective metal fences. The concentration of Fe2+ ions in the solution was 145.2 g/dm3, and the pH value was 1.45. The pH of the sulfuric acid solution was adjusted with 25% NaOH. The pH value of the reaction mixture during the study of the ferritization and sorption process was controlled on a PL—700AL pH metre (Zabrze, Poland).
Thermal, electromagnetic pulse and ultrasonic activation were used to carry out the ferritization process. Laboratory installations for the thermal activation method are described in detail in paper [51], and for electromagnetic pulse activation in [52]. The ferritization process during ultrasonic treatment was carried out in a laboratory tank with an ultrasonic bath (Figure 1). The technological parameters at which Fe-rich containing ferritization materials were obtained are given in Table 2.
Fe-rich containing materials were obtained by drying the precipitates in an electric drying oven at 105 °C for 2 h. As a result of our previous studies on the ferritization processing of etching solutions, we defined the influence of the pH of the reaction mixture, the concentration of Fe2+ ions in it and the duration of the process on the phase composition of the obtained precipitates was products of fertilization [52].
Phase analysis of Fe-rich materials, which were further tested as Zn2+ ion sorbents, was carried out by X-ray diffractometer (XRD) on an Ultima IV instrument (Rigaku, Tokyo, Japan) using Cu-Kα radiation. The analysis of the studied Fe-rich materials was carried out in the angle range 2θ 6 ÷ 65° with a scanning step of 0.05° and an exposure time at point 2c. The diffractograms were interpreted using the ICCD PDF-2 2003 database (The International Centre for Diffraction Data) and the Match! V.1.9a software (Crystal Impact, Materials Park, OH, USA).
To study capacity of the obtained Fe-rich materials to adsorb heavy metals, in particular, Zn2+, a model solution was prepared. A solution with a concentration of Zn2+ ions of 30.2 mg/dm3 and a pH value of 5.6 was obtained by dissolving ZnSO4 7H2O in distilled water. The indicated values correspond to rinsing water of industrial electroplating lines. The pH of the model zinc-containing solution was adjusted with 25% NaOH to values of 8.0 and 10.0 in sorption studies.
The process of sorption of Zn2+ ions by the obtained Fe-rich materials was carried out in a special tank with a volume of 1 dm3 with mechanical stirring 0.8 dm3 of the solution at a speed of 800 rpm, as well as under influence of ultrasound using the TUN-13 device (Jeken, Dongguan City, China) with a frequency of 40 kHz.
Concentrations of Zn2+ ions before and after contact with the samples were determined on a Hach DR3900 spectrophotometer (Hach Lange, Loveland, CO, USA) using standard reagent kits for zinc measurement No. 2429300 (ZincoVer). This reagent is used together with zinc as a chromophore reagent. ZincoVer forms a blue complex with Zn2+ ions in an alkaline medium, which allows for quantitative analysis of zinc in water. The optical density of the resulting solution was determined using a wavelength of 620 nm.
The degree of water purification from zinc ions was calculated by the formula:
α = C O − C i C O   ∗ 100 %
where CO and Ci—respectively, the initial and residual concentration of Zn2+ ions in the model solution, mg/dm3.
Determination of the total concentration of iron ions before and after adsorption was determined by the spectrophotometric method using standard reagent kits for iron determination No. 2105769.
After the ferritization process was completed, the condensed sorbent was separated under vacuum on a paper filter with a mesh size of 10 μm, using a vacuum flask and a Büchner funnel. The condensed precipitate was dried at a temperature of 18 ± 2 °C for 7 days. The dried condensed precipitate was ground in a porcelain mortar to a powdery state and washed with distilled water to remove residual sodium sulphate and dried for 24 h.
In sorption studies, the ratio of dry sorption powders to zinc ions Zn2+ in the solution was 300 mg to 30 mg/dm3, respectively. The contact time of Zn2+ ions with Fe-rich materials was 60 min. Based on results of the studies [32] this process duration is typical when using such magnetic sorbents.
The study of the surface structure of used Fe-rich materials after contact with Zn2+ ions was carried out on a scanning electron microscope (SEM) JSM-6510LV (JEOL, Tokyo, Japan). Samples for SEM weighing 1 mg were fixed on a special stand using double-sided carbon adhesive tape.
The mass content of elements in spent iron-containing sorbents was determined by X-ray fluorescence spectroscopy on an ElvaX Plus device (Elvatech, Kyiv, Ukraine).
The estimation of the dispersion and error limits of experimental studies of the determination of residual concentrations of Zn2+ ions after the sorption process was carried out according to the method [53] with a confidence probability of 0.95.

3. Results and Discussion

3.1. Results of Structural Analysis of Fe-Rich Materials Obtained by Ferritization of Etching Solutions

Figure 2 shows images of samples of ferritization precipitates of exhausted etching solutions obtained under different conditions (Table 2). The colour of the samples changes from light brown to black with increasing aeration rate of the reaction mixture. This indicates an increase in contents of the magnetite phase and a decrease in the iron oxohydroxide phases, which is confirmed by the data of Table 3.
The diffractograms of ferritization precipitates are shown in Figure 3, and their phase composition in Table 3.
The data of Figure 3 indicate that in all studied samples, exclusively ferromagnetic crystalline phases were identified. The chemically stable phase of magnetite Fe3O4 with a crystal lattice parameter (a) of 8.36 Å is present in all samples, except for the precipitate obtained by ferritization at room temperature and the minimum studied aeration rate Fe-1. In addition to formation of magnetite, the precipitates also contain other solid-phase products of the ferritization reaction, in particular, lepidocrocite ɣ-FeOOH and ferrooxygite δ-FeOOH with a value of a 3.85 and 2.95 Å, respectively. It should be noted that these oxohydroxide phases have low stability in alkaline media [54]. Unlike iron oxyhydroxides and hydroxides, magnetite is not able to dissolve not only in water, but also in dilute aqueous solutions of strong mineral acids and alkalis, which is due to the specific structure of its spinel-type crystal lattice. It should be noted that there is a narrow diffraction maximum at 2θ = 35.4° with the index (311), which corresponds to the Fe3O4 phase in the Fe-5 sample with thermal activation of ferritization. This indicates a higher degree of crystallinity of the structure of this sample compared to others, in which low peak intensity and a larger width of diffraction reflexes are observed.
Results of identification of quantitative and qualitative compositions of the obtained Fe-rich samples are presented in Table 3. Comparing the data of Table 2 and Table 3, we can conclude that with a gradual increase in the aeration rate of the reaction mixture to 0.06 dm3/s, the magnetite content increases due to the following phase transformation [55]:
ɣ-FeOOH → δ-FeOOH → Fe3O4
When using an aeration rate of 0.06 dm3/s, the ferritization Fe-rich materials contain only the magnetite phase.
It should be noted that results of X-ray phase analysis of the Fe-rich materials correlate with quality of ferritization treatment of exhausted etching solutions for removal of total iron ions by the ferritization method. Chemical analysis showed (Table 4) that the concentration of total iron in the solution after the ferritization process is in the range from 1.4 to 5.5 mg/dm3; the degree of extraction of this metal reaches 99.99%.
The highest degree of removal of total iron ions from the etching solution was achieved at the maximum aeration rate of the ferritization reaction mixture of 0.06 dm3/s, at which the sediments were obtained. This is obviously due to the formation of chemically stable iron oxide phases in the precipitate, in particular magnetite (sample Fe-5).

3.2. Study of the Ability to Adsorb Zn2+ Ions by the Studied Fe-Rich Samples

Samples of Fe-rich materials obtained at different technological parameters of the ferritization process were analyzed for their capacity to adsorb heavy metals, in particular zinc ions. Figure 4a,b present the results of the initial experimental studies on the adsorption of Zn2+ ions in the modes of mechanical stirring and ultrasound, respectively. As shown by the comparison of the obtained results, the use of ultrasound increases the degree of purification of the solution from Zn2+ ions by 20–40%.
Analysis of the experimental results, as presented at Figure 4a,b, shows that with an increase in the pH value of the model solution from 5.6 to 8.0, the degree of removal of Zn2+ ions increases. Among the studied samples, the higher adsorption capacity was shown by the Fe-3 sample, the phase composition of which is 61.3% δ-FeOOH and 38.7% Fe3O4. The use of this material as a sorbent during ultrasonic treatment of the solution reduces the concentration of zinc ions to 2.4 mg/dm3, which provides a high degree of their removal of 92.0%. The obtained data can be explained by the fact that the Fe-3 sample is dominated by the ferroxygite phase δ-FeOOH, which is unstable in alkaline media [56]. Under the influence of ultrasound, ferrooxygite is destroyed, and in the presence of zinc ions in the solution it can transform into other, more stable phases of zinc ferrite (ZnFe2O4) and magnetite (Fe3O4). However, the above-mentioned quality of wastewater treatment is insufficient for its reuse in electroplating production and discharge into water bodies. Acceptable levels of Zn2+ concentration for water reuse in the electroplating industry must be ≤1.5 mg/dm3 [57] or ≤5.0 mg/dm3 for discharge into the water bodies of Ukraine [58]. The analysis of Figure 4 revealed that only for samples of sorbents Fe-3, Fe-4 and Fe-5 at pH 8.0 and ultrasonic treatment a concentration values below 5 mg/dm3 for Zn2+ ions are achieved.
To use a two-phase sample of iron-containing material Fe-3 (Table 3) as a sorbent with predetermined properties, it is necessary to keep a certain quantitative ratio of phases. The obstacle to this is that the solutions from which the sorbents are obtained always have different concentrations of iron ions Fe2+.
Analysis of the obtained data, namely qualitative and quantitative composition of the mineral phases of ferritization products, comparison of efficiency of their use for adsorption of Zn2+ ions, as well as the experimental results of other authors, which are presented in [55,59] showed that it is advisable to continue further research in the direction of a thorough study of adsorption capacity of Fe-rich materials that contain exclusively the magnetite phase. Magnetite can be obtained as a result of the processing of exhausted etching solutions of various compositions [60]. It is a chemically stable and environmentally safe substance. Table 5 shows results of the study of Zn2+ ions removal by Fe-5 sample, which contains magnetite only.
As can be seen from Table 5, the higher level of removal of Zn2+ ions from the solution is 98.9% at pH 10 and using ultrasound; the lowest is at pH 5.6 and mechanical mixing. The analysis of Figure 4 revealed that only for samples of sorbents Fe-3 and Fe-4 at pH 8.0 and ultrasonic treatment a concentration values below 5 mg/dm3 for Zn2+ ions are achieved. The low degree of purification from zinc ions can be explained by the fact that the studied solutions, in addition to Zn2+ ions, also contains Fe2+ and Fe3+ ions, which are capable of leaching from the studied sample Fe-5 and can be sorbet, competing with zinc ions. This assumption is confirmed by measuring of iron ions concentration in the solution after sorption studies. The concentrations of total iron were 0.08 and 0.19 mg/dm3 for experiments using mechanical stirring and ultrasound, respectively. At pH values of 8.0 and 10 after corresponding sorption experiments, iron ions were not detected in concentrations available for measurement. This indicates high stability of the sorbent at pH values ≥ 8.0.
Experimental studies of magnetite (Fe-5) capacity to adsorb zinc ions showed different efficiency. The lowest efficiency of Zn2+ removal was in the solution at the initial pH of 5.6, and the highest one at pH 10. This can be explained by the fact that magnetite partially decomposes into hematite (Fe2O3) and Fe2+ ions at pH below 6.5 [61]. Also, the results presented in this article allow us to state that at pH of the solution higher 6.0 the structure of magnetite becomes more stable and strong [62].
The authors of paper [63] suggest that the point of zero charge (PZC) is the pH value at which the net surface charge of the adsorbent is zero. The surface of the adsorbent is positively charged at pH < pHPZC and negatively charged at pH > pHPZC. However, the magnetite studied in this work, which was synthesized using a precise solvothermal method, demonstrated the ability to adsorb both anionic (Cr2O72−) and cationic (Pb2+) pollutants despite having a pHPZC value of 2.1.
In another paper [64], studies on the adsorption of heavy metals by magnetite, in particular Zn2+, were carried out at a solution pH value higher than the PZC value (pHPZC = 3.8). Other research studies claim that the experimental PZC values for different magnetite samples are in the pH range from 3.8 to 9.9 [65,66,67]. Thus, analysis of results of studies on determination of pHPZC of magnetite, suggest that this value is not constant and unchanging. It is not possible to predict in what range of pH values of the solution the efficiency of adsorption of cationic or anionic contaminants by magnetite is possible and requires separate additional studies.
In the conducted experimental studies of adsorption capacity of magnetite, which is obtained by the method of hydrophase ferritization, to adsorb zinc ions, ultrasound was used. The application of ultrasound, as a relatively new technology, can lead to mixing of magnetite particles in the solution, and also helps to control morphology of magnetite particles. Aggregated magnetite particles can be dispersed to nano-range sizes [68,69,70].
Cavitation bubbles formed during ultrasonic treatment in the system enhance removal of contaminants of various origin. High energy generation due to ultrasonic treatment shifts the adsorption–desorption equilibrium. Thus, the increase in adsorption efficiency is explained by a change in the equilibrium position and an improvement in adsorption kinetics. High-speed microcurrents and high-pressure shock waves formed due to the formation of cavitation bubbles disrupt the sorbent structure, which leads to an increase in its adsorption capacity [71]. Furthermore, in all the study adsorption processes, the overall initial values of adsorption rates are relatively large after the solution is treated with ultrasound. These results confirm the role of ultrasound in accelerating the adsorption kinetics, and the obtained results are consistent with previous studies [72].
The higher effect of sorption of Zn2+ ions during ultrasound, compared to conventional mechanical mixing, can be explained by the fact that the cavitation effect can clean the magnetite surface, and turbulence can reduce the thickness of the liquid film and, as a result, accelerate mass transfer between the sorbent and pollutants [73].
Another study [74] reported that ultrasound can increase the activity of magnetite during the process of ozonation of the solution, i.e., when saturating water with oxygen without compromising the stability of this iron compound. Thus, the combination of ultrasound and saturation of water with oxygen has a synergistic effect [75]. Compared to systems that use either oxygen/ozone or ultrasound alone, the combined process allows for increased initiation of the formation of •OH radicals, which can be generated in the pore phase of cavitation bubbles during ultrasound.
It should be noted that the pH value of the solution during the adsorption process decreases from 10.0 to 7.5. This can be explained by the fact that in wastewater in the presence of dissolved oxygen and under the action of ultrasound, substances with the properties of weak acids can be formed, in particular, hydrogen peroxide, which reduces the pH of the treated solution (Table 5) [75]. Thus, the treated water pH value meets the requirements for its reuse in electroplating production in etching operations and its discharge into municipal sewers for further treatment at municipal wastewater treatment facilities.
Structural studies of exhausted magnetite-based adsorbents (samples Fe 5) were conducted. Adsorption of Zn2+ by these sorbents was carried out at pH 10. Under the influence of ultrasound in alkaline media, in our opinion, magnetite is destroyed with the formation of a zinc ferrite phase in the zinc-containing solution. According to the results of X-ray phase analysis (Figure 5), only one magnetite phase was identified in diffractograms of the samples. But this does not contradict to our assumption, since magnetite and zinc ferrite have identical inverted spinel crystal lattices; therefore, it is not possible to confirm immobilization of a small amount of zinc in the form of ions, as well as zinc ferrite, into the magnetite structure using this method.
In addition, the SEM results (Figure 6 and Figure 7) indicate that both samples are characterized by presence of structures with different morphology of particles and aggregates, among which crystalline formations of magnetite and zinc ferrite of spherical shape stand out. The action of ultrasound contributes to the destruction of agglomerates of different sizes and magnetite particles (Figure 6), and, therefore, to a significant increase in the dispersion of the sorbent (Figure 7). Thus, the use of ultrasound increases the ability of the sorbent to remove zinc ions from the solution.
The above is confirmed by the data of X-ray fluorescence analysis of samples, which indicate an almost twice as high content of zinc ions in samples of exhausted adsorbents during ultrasonic treatment (Figure 8a) compared to mechanical mixing of the solution (Figure 8b): 24.7 and 13.7 wt. %, respectively.
Thus, the data on residual contents of Zn2+ in the treated water (Table 4) correlate with results of structural studies of exhausted sorbents (Figure 5, Figure 6, Figure 7 and Figure 8). The developed process for using such adsorbent for treatment of Zn-rich solution with magnetite and ultrasound has demonstrated its high efficiency, and, therefore, it has prospects of implementation in electroplating production for treatment of such wastewater.
Considering the peculiarities of organization of electroplating production, wastewater treatment technologies, and the utilization of the formed waste for further practical use, we can recommend a combined system for processing liquid and solid production waste for treatment of concentrated wastewater from etching solutions with Zn2+ to obtain ferromagnetic materials. Such technology can be a sustainable approach to wastewater treatment at electroplating facilities.

4. Conclusions

The processing of exhausted etching solutions by ferritization with the use of different activation methods and aeration rates of the reaction mixture was studied. The chemical composition and structure of the obtained samples of ferritization sediments were identified. Ferromagnetic crystalline phases of lepidocrocite ɣ-FeOOH and ferrooxygite δ-FeOOH, as well as a chemically stable phase of magnetite Fe3O4 were identified in the products of ferritization. Its content, as well as the degree of treatment of the solution for removal of iron ions, increases with increasing aeration rate.
Adsorption properties of the study samples for Zn2+ removal in modes of mechanical mixing and ultrasound at different pH values of the solution were studied. The higher efficiency of Zn2+ removal of 92.0% was achieved at pH = 8 with use of ultrasound and the samples containing 61.3 and 38.7% of the δ-FeOOH and Fe3O4 phases, respectively. Increasing the pH of the solution to 10 allows to use a chemically stable adsorbent containing exclusively the magnetite phase and reaching a degree of water treatment for removal of Zn2+ ions of more than 98.9%. This quality of water treatment is acceptable for its reuse in industrial production in rinsing operations [57].
Data from electron microscopy and X-ray fluorescence analysis of Fe-rich sorbents of different phase composition confirmed presence of structures with different morphology of highly dispersed particles and their spherical aggregates, mainly magnetite and zinc ferrite.
Analysis and generalization of the obtained data confirms the prospects of using effective and environmentally safe magnetic sorbents for treatment of rinsing wastewater of electroplating industries to remove Zn2+ ions. Under formation conditions of stable solid phases of magnetite or zinc ferrite, the exhausted adsorbent is suitable for further utilization, particularly in construction materials.

Author Contributions

Conceptualization, D.S., G.K. and Y.T.; methodology, D.S., G.K. and Y.T.; software, R.T. and O.H.; validation, D.S., G.K. and Y.T.; formal analysis, D.S., S.H. and Y.T.; investigation, Y.T. and R.T.; resources, Y.T., S.H. and R.T.; data curation, D.S.; writing—original draft preparation, D.S., G.K. and Y.T.; writing—review and editing, Y.T. and R.T.; visualization, O.H. and S.H.; supervision, G.K.; project administration, Y.T.; funding acquisition, D.S. and G.K. All authors have read and agreed to the published version of the manuscript.

Funding

Project with the assistance of the Ministry of National Education and Science, state registration number: 0123U101948.

Data Availability Statement

The raw data supporting the conclusions of this article will be made available by the authors on request.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Laboratory installation for carrying out the ferritization process with ultrasonic activation: 1—stand, 2—container with treated solution, 3—ultrasonic bath, 4—aerator, 5—compressor.
Figure 1. Laboratory installation for carrying out the ferritization process with ultrasonic activation: 1—stand, 2—container with treated solution, 3—ultrasonic bath, 4—aerator, 5—compressor.
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Figure 2. General view of the studied iron-containing materials obtained by ferritization.
Figure 2. General view of the studied iron-containing materials obtained by ferritization.
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Figure 3. Diffraction patterns of precipitates obtained by ferritization of exhausted etching solutions at different initial technological parameters.
Figure 3. Diffraction patterns of precipitates obtained by ferritization of exhausted etching solutions at different initial technological parameters.
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Figure 4. Efficiency of sorption of Zn2+ ions by the studied samples: (a) mechanical mixing; (b) ultrasound action.
Figure 4. Efficiency of sorption of Zn2+ ions by the studied samples: (a) mechanical mixing; (b) ultrasound action.
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Figure 5. Diffractograms of exhausted sorbents: (a) mechanical mixing; (b) ultrasonic impact.
Figure 5. Diffractograms of exhausted sorbents: (a) mechanical mixing; (b) ultrasonic impact.
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Figure 6. SEM images of magnetite aggregates and particles with entry of zinc ions into their structure during adsorption with mechanical mixing, magnification: (a) 250×; (b) 5000×; (c) 20,000×.
Figure 6. SEM images of magnetite aggregates and particles with entry of zinc ions into their structure during adsorption with mechanical mixing, magnification: (a) 250×; (b) 5000×; (c) 20,000×.
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Figure 7. SEM images of aggregates and magnetite particles with zinc ions entering their structure during adsorption with ultrasound treatment: magnification (a) 250×; (b) 5000×; (c) 20,000×.
Figure 7. SEM images of aggregates and magnetite particles with zinc ions entering their structure during adsorption with ultrasound treatment: magnification (a) 250×; (b) 5000×; (c) 20,000×.
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Figure 8. Contents of chemical elements in the composition of exhausted sorbents; adsorption conditions: (a) mixing; (b) the action of ultrasound.
Figure 8. Contents of chemical elements in the composition of exhausted sorbents; adsorption conditions: (a) mixing; (b) the action of ultrasound.
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Table 1. Comparison of the adsorption capacity for synthesized Fe-rich materials.
Table 1. Comparison of the adsorption capacity for synthesized Fe-rich materials.
№Magnetic SorbentsAdsorption Capacity, mg/gAdsorption Duration, minpHTemperature °CLiterature
1γ-Fe2O3111.10-625[32]
2nano-tubes Fe3O4107.2760625[33]
3nano-tubes γ-Fe2O386.95---[34]
4Fe3O4-SiO281.60605-[35]
5Fe3O4-SiO2-TiO2137.030--[36]
6Fe3O4-MnO2100.24--25[37]
7MnFe2O4454.40120625[38]
8CoFe2O4384.60120625[38]
Table 2. Technological parameters for obtaining Fe-rich materials from exhausted etching solutions.
Table 2. Technological parameters for obtaining Fe-rich materials from exhausted etching solutions.
IDMethod of Activating the Mix in the Ferritization ProcessAir Oxygen Aeration Rate, dm3/spH of LiquidConcentration Fe2+ in Liquid, g/dm3Duration of the Ferritization Process, min
Fe-1Temperature (20 °C)0.0210.514.530
Fe-2Ultrasonic0.02
Fe-3Electromagnetic pulse0.04
Fe-4Electromagnetic pulse0.05
Fe-5Thermal (75 °C)0.06
Table 3. Phase composition of products of ferritization.
Table 3. Phase composition of products of ferritization.
Study SamplsPhase Content, %
ɣ-FeOOHδ-FeOOHFe3O4
Fe-125.974.1-
Fe-230.240.129.6
Fe-3-61.338.7
Fe-4-31.768.3
Fe-5--100
Table 4. Efficiency of removal of iron ions from exhausted etching solutions.
Table 4. Efficiency of removal of iron ions from exhausted etching solutions.
Study SamplesResidual Concentration of Total Iron in Solution, mg/dm3Removal of
Total Iron, %
Fe-15.4799.96
Fe-23.5899.97
Fe-32.8299.98
Fe-41.9199.98
Fe-51.4099.99
Table 5. Results of adsorption of Zn2+ ions by the studied Fe-5 sample.
Table 5. Results of adsorption of Zn2+ ions by the studied Fe-5 sample.
Adsorption ConditionspH of SolutionResidual Concentration Zn2+ in Solution, mg/dm3Efficiency of Zn2+Adsorption, %
Before AdsorptionAfter Adsorption
Mechanical
mixing		5.65.616.8443.8
8.07.814.5251.6
10.09.63.7887.4
Ultrasound5.65.515.4348.5
8.06.25.3182.3
10.07.50.3198.9
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Samchenko, D.; Kochetov, G.; Hao, S.; Trach, Y.; Trach, R.; Hnes, O. Recycling Industrial Waste: Ferritization Products for Zn2+ Removal from Wastewater. Sustainability 2025, 17, 4008. https://doi.org/10.3390/su17094008

AMA Style

Samchenko D, Kochetov G, Hao S, Trach Y, Trach R, Hnes O. Recycling Industrial Waste: Ferritization Products for Zn2+ Removal from Wastewater. Sustainability. 2025; 17(9):4008. https://doi.org/10.3390/su17094008

Chicago/Turabian Style

Samchenko, Dmitry, Gennadii Kochetov, Shuwei Hao, Yuliia Trach, Roman Trach, and Olena Hnes. 2025. "Recycling Industrial Waste: Ferritization Products for Zn2+ Removal from Wastewater" Sustainability 17, no. 9: 4008. https://doi.org/10.3390/su17094008

APA Style

Samchenko, D., Kochetov, G., Hao, S., Trach, Y., Trach, R., & Hnes, O. (2025). Recycling Industrial Waste: Ferritization Products for Zn2+ Removal from Wastewater. Sustainability, 17(9), 4008. https://doi.org/10.3390/su17094008

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