Nanostructured Magnetite Coated with BiOI Semiconductor: Readiness Level in Advanced Solar Photocatalytic Applications for the Remediation of Phenolic Compounds in Wastewater from the Wine and Pisco Industry

Abstract

Heterogeneous photocatalysis is an advanced, efficient oxidation process that uses solar energy to be sustainable and low-cost compared to conventional wastewater treatments. This study synthesized BiOI/Fe₃O₄ using the solvothermal technique, evaluating stoichiometric ratios of Bi/Fe (2:1, 3:1, 5:1, and 7:1) under simulated solar irradiation to optimize the degradation of caffeic acid, a pollutant found in wastewater from the wine and pisco industry. The nanomaterial with a 5:1 ratio (BF-5) was the most effective, achieving a degradation of 77.2% in 180 min. Characterization by X-ray Diffraction (XRD), Transmission Electron Microscopy (TEM), Brunauer–Emmett–Teller (BET), Barrett–Joyner–Halenda (BJH), Fourier Transform Infrared Spectroscopy (FTIR), Diffuse Reflectance Spectroscopy (DRS), and Vibrating Sample Magnetometry (VSM) showed that BF-5 has a porous three-dimensional structure with BiOI nanosheets coating the Fe₃O₄ surface, while retaining the pristine BiOI properties. The magnetite provided magnetic properties that facilitated the recovery of the photocatalyst, reaching 89.4% recovery. These findings highlight the potential of BF-5 as an efficient and recoverable photocatalyst for industrial applications. The technical, economic, and environmental feasibility were also evaluated at the technological readiness level (TRL) to project solar photocatalysis in real applications.

1. Introduction

Water pollution is a critical challenge at a global level, affecting both developed and developing countries. This is especially true in the agri-food industry, such as with wine and pisco production [1]. The wine and pisco industry is one of Chile’s most important, representing 20% of agricultural exports and 0.5% of the gross domestic product (GDP) [2]. Pisco is made through wine distillation, and the annual production of these spirits is 44.4 ML [3] and 1103 ML [4], respectively. However, their production generates liquid industrial waste (LIW), specifically vinasse (VZ), which represents 70% of the total liquor [5].

These industries generate LIW, with a high organic load characterized as Chemical Oxygen Demand (COD) and Biological Oxygen Demand (BOD₅) [6]. This load mainly consists of high concentrations of difficult-to-degrade phenolic compounds [7]. For example, if discharged without adequate treatment, VZ’s composition represents a significant risk to the soil, surface, and underground water bodies, as well as to human health [6,8,9]. For these reasons, phenolic compounds such as gallic acid, caffeic acid, and gentisic acid must be removed from wastewater before it is discharged into the environment [10]. Among the pollutants mentioned, caffeic acid is one of the most refractory phenolic compounds due to its high toxicity and antimicrobial activity [9,11].

In the last decade, emphasis has been placed on developing innovative technologies to remove pollutants from water efficiently. Among these, Advanced Oxidation Processes (AOPs) have emerged as a promising option for treating wastewater with difficult-to-degrade organic compounds due to their high oxidizing capacity [12,13]. Heterogeneous photocatalysis, one of the most effective AOPs [14], stands out for its ability to degrade recalcitrant pollutants quickly, efficiently, and at a lower cost than conventional methods using solar radiation [15,16,17]. The development of advanced photocatalytic heterostructures, such as the BiOI semiconductor coupled to magnetite (Fe₃O₄), has proven highly effective in degrading persistent organic compounds under solar irradiation [18,19,20,21,22]. The solvothermal synthesis technique is particularly effective for manufacturing these materials, providing an accurate control over their structure and composition [23]. This technique takes advantage of high-temperature conditions above the solvent’s boiling point, and pressure to induce unique reaction pathways, resulting in nanomaterials with synergistic photocatalytic and magnetic properties [24,25]. The use of BiOI as a semiconductor is attractive due to its 1.9 eV bandgap, which can absorb 95% of sunlight within the visible range [26,27]. However, BiOI’s micrometric size, like other powdered photocatalysts, limits its recovery and separation in post-treatment. To overcome these limitations, the coupling of BiOI with magnetite provides magnetic properties that facilitate the separation and recovery of the photocatalyst, addressing a current problem for the applicability and sustainability of this technology on a large scale [28].

The potential applications of these nanomaterials are vast and range from wastewater treatment to improving industrial processes that require the removal of persistent and emerging pollutants [29]. Given the complexity of conventional and current systems, water treatment by solar photocatalysis is a process with high sustainability prospects, capable of degrading recalcitrant pollutants [30]. However, as far as we know, no studies guarantee this technology’s viability and sustainability using BiOI/Fe₃O₄ at a TRL higher than the validation under controlled laboratory conditions. Although photocatalysis offers significant advantages as a wastewater remediation method, its widespread adoption has faced obstacles, and studies often focus on the treatment’s technical success at a laboratory scale without evaluating its future scalability [31].

Hence, the objectives of this study were to synthesize BiOI/Fe₃O₄ nanoparticles using the solvothermal method to evaluate the photocatalyst’s efficiency in remediating organic pollutants, such as low biodegradability phenolic compounds, in wastewater from the wine and pisco industry. Similarly, to assess sustainability through a scalability analysis that considers the technical, environmental, and economic feasibility to identify the technological readiness level, defined as a readiness integrity measure of a complex technology established in a series of sequential levels ranging from basic research to the product’s sale [32]. In this way, an effective transition from laboratory validation to its implementation on a larger scale is ensured, addressing a key challenge in applying heterogeneous photocatalysis in actual environments [33,34,35].

2. Materials and Methods

2.1. Materials

All reagents and solvents were obtained from Sigma–Aldrich (St. Louis, MO, USA) and Merck (Rahway, NJ, USA) with analytical and technical grades. The compounds were synthesized using ethylene glycol solvent without further purification.

2.2. Methods

2.2.1. Synthesis of Fe₃O₄ Nanoparticles

The magnetite synthesis was carried out using the solvothermal method, similar to that of Zheng et al. [20]. A total of 20 mmol of FeCl₃·6H₂O and 195 mmol of sodium acetate were mixed in 160 mL of ethylene glycol. The solution was then dispersed ultrasonically for 25 min with manual stirring every 5 min before being transferred to a Teflon-lined autoclave reactor and heated to 200 °C for 12 h. It was then allowed to cool to room temperature, and the product obtained (a black solid) was magnetically separated from the solution. Once recovered, the material was washed with absolute ethanol (three times), and finally it was dried at 60 °C in an oven for 12 h.

2.2.2. Synthesis of BiOI/Fe₃O₄ Nanoparticles

The synthesis of BiOI/Fe₃O₄ was carried out using the solvothermal method. A total of 3 mmol of bismuth nitrate pentahydrate (Bi(NO₃)₃·5H₂O, 99.0%) was dissolved in 80 mL of ethylene glycol. Once the solution was prepared, different proportions of magnetite (Fe) were added, and the solution was mechanically stirred (70 rpm) for 60 min. Without stopping the stirring, a solution of 3 mmol of potassium iodide (KI, 99.0%) in 80 mL of ethylene glycol was added drop by drop into the mixture (BiOI/Fe₃O₄). After adding the KI solution, it was left with mechanical stirring for 15 min at room temperature. Once the time was up, the mixture was transferred to a Teflon-lined autoclave reactor and heated to 160 °C for 8 h [20,36]. Four stoichiometric ratios between BiOI/Fe₃O₄ were evaluated in triplicate: 2:1, 3:1, 5:1, and 7:1. The response factor was the degradation rate (%) of the model compound.

2.2.3. Characterization of Nanomaterials

The crystallinity and phases of the synthesized material were identified using XRD in a dust diffractometer (Bruker^® D2 Phaser, Bruker Corporation, Billerica, MA, USA) operated with co-radiation and Fe Kß radiation filter in a 2Ɵ sweeping range of 10–90°. The structure and morphology were determined by TEM using a Hitachi HT7700 microscope (Hitachi, Marunouchi, Chiyoda-ku, Tokyo, Japan) operating at an acceleration voltage of 120.0 keV. The specific surfaces of the materials were studied by adsorption–desorption isotherms with nitrogen at 77 K using BET analysis in an Anton Paar Nova 600 model adsorption analyzer. The desorption isotherm and the BJH method determined the pore size distribution in an Anton Paar Nova 600 adsorption analyzer. The infrared spectra FT-IR were obtained using a Jasco FTIR-4600 spectrometer (Jasco, Ishikawa-cho, Hachioji, Tokyo) equipped with ATR PRO ONE (Jasco, Easton, MD, USA). By determining the energy bandgap (Ebg), the optical properties were obtained through DRS, using a Lambda 1050 Perkin Elmer UV-Vis-NIR spectrophotometer. The magnetic properties of the materials obtained were then measured using a Cryogenic 5T System physical properties measurement system (PPMS) equipped with a VSM.

2.2.4. Measurement of Photocatalytic Activity

The photocatalytic activity of the material obtained from BiOI/Fe₃O₄ was evaluated by the percentage of photocatalytic degradation of the pollutant (%η), caffeic acid, in water following Equation (1). The pollutant concentration was calculated through absorbance since its calibration curve demonstrates a linear relationship between the concentration and absorbance of caffeic acid [37].

$$\%\eta ={\displaystyle \frac{A_i-A_f}{A_i}}\ast 100$$

(1)

where %η: caffeic acid degradation (%); A_i: initial absorbance (nm); A_f: final absorbance (nm).

The light source comprised a dark box, a 600 mL continuous-flow glass reactor, a 160-rpm propeller stirrer (DLab OS20-S, Beijing, China), a 1L/min flow rate peristaltic pump (longer pump BT300-2J), and an HI 6500K LED lamp (Figure 1). The photocatalytic tests were performed at room temperature, using a 10-ppm solution of the model pollutant (Figure 2) and a 400-ppm load of the magnetic photocatalyst. The suspensions were stirred in the dark for 40 min to reach the adsorption–desorption equilibrium before starting the irradiation.

After the dark time elapsed, each suspension was irradiated with the lamp for 180 min. Samples were then taken to analyze the degradation of caffeic acid at 40 min before the dark time and until the 180-min photocatalytic reaction was completed [−40, 0, 10, 20, 30, 60, 90, 120, 150, 180 min]. After irradiation, the magnetic photocatalyst was removed from each sample by stripping using a 6000 Gauss neodymium magnet. The samples were then analyzed using a spectrophotometer (QUIMIS, Q798ULTRA, Diadema, Brazil), which performed a sweep between 250 and 400 nm, taking as reference the highest point of the caffeic acid band at 312 nm.

2.2.5. Solar Photocatalysis Preparation Level Using BF-5

This study evaluated the sustainability of advanced photocatalytic applications using a scalability analysis focused on technical, environmental, and economic feasibility to determine the technological readiness level of the BiOI/Fe₃O₄ photocatalyst (BF-5). This evaluation was conducted in the context of the wine and pisco agribusiness, using data collected from the company Pisco Endémico, located in an arid area of Latin America, specifically in Coquimbo, Chile.

The technical feasibility included evaluating the photocatalytic process’ efficiency using BF-5 in the degradation of low-biodegradability phenolic compounds under simulated solar irradiation and the influent quality (VZ) before using heterogeneous photocatalysis [38]. The environmental feasibility considered the viability of the photocatalytic system for treating agro-industrial waters without generating toxic by-products or representing a significant risk to human health and the environment, complying with domestic and international regulations [39]. The solar irradiance potential was also identified in the study area [40]. Finally, a simple economic analysis was performed using the methodology adopted by Khairudin et al. [41], which focused on the photocatalytic system’s implementation costs and the solar reactor’s operation in a small pisco industry.

3. Results and Discussion

3.1. Physical and Optical Properties and Surface Chemistry of Fe₃O₄, BiOI, and BF-5

XRD analyzed the structures and phases of the obtained materials; the diffractograms are presented in Figure 3. The diffraction peaks of Fe₃O₄ 2θ = 30.1°, 35.4°, 43.0°, 53.4°, 56.9°, and 62.5°, attributable to the crystalline planes (JCPDS. 88-0315) (2 2 0), (3 1 1), (4 0 0), (4 2 2), (5 1 1), (4 4 0), showed a high purity of the magnetite obtained. The diffraction peaks of the individual BiOI were identified (Figure 3b), which appear at 2θ = 9.6°, 19.4°, 29.6°, 31.6°, 45.4°, 55.2°, and 66.2°, corresponding to the tetragonal phase of the semiconductor. According to the standard cards, (JCPDS.73-2062) and (JCPDS 88-0315), all diffraction peaks can be indexed to BiOI and Fe₃O₄ by their crystalline planes (0 0 1), (0 1 2), (1 1 0), (0 2 0), (1 1 2), (2 2 0) and (3 1 1), (4 4 0), respectively.

The results indicate a coexistence of BiOI and magnetite phases in the BF-5 material, observing an abundant BiOI phase with a high facet exposure (1 1 0) and a minority phase of Fe₃O_4, as reported by several authors, even with stoichiometric ratios higher than those presented in this study [18,22,41,42]. This can be seen in the planes (3 1 1) (4 4 0), visible in the magnetic material. In addition, there is no presence of pure bismuth (JCPDS 85-1329), evidencing that the materials were obtained without changing the original crystalline structure of BiOI.

To have greater clarity regarding the intensity and definition of the BF-5 signals, the average crystal sizes were estimated using XRD data from the samples synthesized by the Scherrer Equation (2), using the most intense peak widenings of the spectrum [43,44].

$$L={\displaystyle \frac{K\lambda }{\beta \cos \theta }}$$

(2)

where L represents the sizes of the nanoparticles, K represents the Scherrer constant equal to 0.9494; λ is the wavelength of the X-rays; β is the half-height width of the peak; and Ɵ is half of a diffraction peak [45,46]. An average BF-5 crystal size of 15.9 nm was obtained, and the estimate of the pure BiOI crystallite was 17.9 nm. These findings conclude that reasonable morphology control was achieved in the solvothermal synthesis and that there is no higher crystallinity between the sample obtained and the pure materials, considering BF-5 as a promising nanomaterial for photocatalysis. The higher peak intensity observed in Figure 3c could only be attributed to a greater presence of the (1 1 0) BiOI face on the surface of BF-5. Pan et al. [47] identified that the BiOI catalyst with the predominant facet (1 1 0) shows superior photocatalytic activity on the degradation of phenolic compounds compared to those with the predominant facet (0 0 1).

Figure 4c,d show the morphology of the BiOI semiconductor. This material has microspheres in the form of rosettes with an average diameter of 2.8 µm. Figure 4e,f show the BF-5 material synthesized in this study. The BiOI nanosheets coat the magnetite to form a three-dimensional structure with an approximate diameter between 200–450 nm, attached in the form of a cluster, as reported in the study by Zheng et al. [20]. It is observed that the Fe₃O₄ nanoparticles are embedded and coated by the BiOI nanosheets, forming a heterostructure that benefits the photocatalytic process by keeping the semiconductor surface exposed to pollutants in water [48]. It is relevant to note that when comparing the pristine BiOI with the BiOI/Fe₃O₄ heterostructure, a decrease in the particle size is observed, increasing the specific surface area of the synthesized catalyst. This reduction in particle size can be attributed to the strong interactions between Fe₃O₄ and BiOI.

Figure 4 shows the TEM of the obtained materials. Figure 4a,b identify the magnetite; its morphology is constituted by nanospheres, which have a homogeneous distribution and a uniform diameter between 50–200 nm; the size and morphology are similar to magnetite obtained and reported using similar synthesis methodology [20,49].

Adsorption/desorption isotherms of N₂ were performed to investigate the surface areas and pore size distributions of the obtained materials (Figure 5). The complete BET BiOI and BF-5 isotherm show an IVa-type isotherm with an H3 hysteresis loop, characteristic of mesoporous materials with high adsorption energy [50]. The Isotherm for Fe₃O₄ is of an IVa type with an H1 hysteresis loop, suggesting cylindrical uniform pores with monolayer adsorption [51]. In addition, the pore size graph confirms that the materials are located in the mesoporous range (2–50 nm).

Table 1. Textural and optical properties of pure BiOI, Fe₃O₄, and synthesized BiOI/Fe₃O₄ (BF-5).

Materials	BET (m2 g−1)	Pore Diameter (nm)	Pore Volume (cm3 g−1)	Eg (eV)
Pure BiOI	61	13.34	0.250	1.95
Fe3O4	16	4.42	0.032	2.04
BiOI/Fe3O4	20	3.95	0.056	1.97

shows the values of the optical and surface properties of the synthesized samples. The BET surface analysis indicates that the BF-5 nanomaterial has a specific surface area intermediate between the pure BiOI sample and the synthesized magnetite. The BET surface area, the mean BJH pore diameter, and the BJH pore volume of BiOI/Fe₃O₄ were 20.23 (m² g⁻¹), 3.95 (nm), and 0.056 (cm³ g⁻¹), respectively.

The improvement of the BET surface area can contribute to the generation of more active sites to improve the photocatalytic performance of BF-5 [52,53]. It has been pointed out in several studies that the surface area is an important factor in the generation of more active sites, improving the photocatalytic activity [54,55].

The optical properties of the synthesized materials are detailed in Table 1, presenting each material’s band gap energy values (Eg). These values were obtained using the Tauc Equation Equation (3), a method that provides a more accurate approximation than direct extrapolation of the UV-vis spectrum to determine the absorption start wavelength (start λ) [56,57].

$${\left(\alpha hv\right)}^n=A(hv-Eg)$$

(3)

In this equation, α represents the adsorption coefficient, A is the constant of proportionality, h is Planck’s constant, v is the frequency, Eg is the energy band gap, and the exponent n value denotes the electronic transition’s nature. The value of Eg can be obtained by extrapolating the slope of the curve to the x–axis intersection (see Figure 6). The graph (αE) 1/p vs. E can be plotted to obtain the band gap of the straight section of the resulting curve. The point of intersection of the straight line’s extension with the energy axis corresponds to the energy band gap [57]. For direct gap semiconductors, n is 2, and for indirect gap semiconductors, n is ½ [46,58]. The band gap of the BF-5 photocatalyst is 1.9797 eV, which allows it to absorb visible light in the solar spectrum region and makes it suitable for solar photocatalysis applications. It should be noted that the synthesis conditions applied in this study do not affect the catalyst band gap in the BiOI/magnetite system since the values of Eg are similar in the materials obtained.

The composition and chemical bonding of the synthesized structures were studied using FT-IR. As shown in Figure 7, the FT-IR spectra of pure BiOI show specific IR bands assigned to the BiOI structure between 510 cm⁻¹ and 1300 cm⁻¹. This is how the characteristic peaks of the tetragonal crystalline bonds of BiOI are observed at 510 cm⁻¹ and at 766 cm⁻¹, which demonstrate the asymmetric stretching vibration of the Bi-O bond [59]. Similarly, the bands observed around 3700 cm⁻¹ at 1600 cm⁻¹ belong to the stretching and bending vibrations of the hydrogen-bonding -OH group, which are also visualized in the spectrum of BF-5 [60]. The IR of Fe₃O₄ shows two typical bands, one at 430 cm⁻¹ and another one at 563 cm⁻¹, which are attributed to the stretching vibration of the Fe-O bond in octahedral sites (Fe²⁺-O) and octahedral and tetrahedral sites (Fe³⁺-O) [61], respectively. These bands were shifted to higher wavenumbers, corresponding to 476 cm⁻¹ and 572 cm⁻¹, after the magnetite was coated with BiOI [62]. The vibration at 1395 cm⁻¹ corresponds to hydroxyl groups, which is also observed in the spectrum of BF-5 [63]. The infrared spectrum of BF-5 compared to the IR of the pure BiOI shows a difference in the region of 450 cm⁻¹ to 580 cm⁻¹, indicating the stretching vibration of the bonding bands due to the interaction that occurs by the adhesion of BiOI on the Fe₃O₄ surface [22]. In addition, specific absorbance bands assigned between 870 cm⁻¹ and 1290 cm⁻¹ are observed in both IR spectra (curves a and c), which demonstrate the asymmetric stretching vibration of the Bi-O [60].

3.2. Magnetism Test

Figure 8 shows the magnetic field response properties of the synthesized materials Fe₃O₄ and BF-5 through the typical hysteresis loops. The saturation magnetization value for Fe₃O₄ was 78 emu/g and decreased to 9 emu/g for BF-5 after the BIOI coated the magnetite particles, showing almost zero coercivity and remanence of the magnetization curve, findings that agree with those of Kim et al. [22]. The magnetic saturation value of BF-5 is sufficient to separate it from the aqueous solution with a magnet at 89.4% after the photocatalytic degradation of caffeic acid; the low magnetic remanence is desired because, when the external magnetic source is removed, the BF-5 is easily released.

3.3. Photocatalytic Removal Efficiency

Fe₃O₄ and BiOI/Fe₃O₄ were synthesized using the solvothermal method following the reported procedure. The synthesized magnetite is shown in Figure 9a, and the BiOI/Fe₃O₄ photocatalyst in Figure 9b.

There are several synthesis routes to manufacture this class of materials. However, the solvothermal synthesis that uses ethylene glycol as a solvent and equimolar amounts of Bismuth nitrate pentahydrate (Bi(NO₃)₃.5H₂O) and of KI has been used with positive results in obtaining BiOI/Fe₃O₄ photocatalysts with high photocatalytic activity and magnetic properties [20,64].

It is important to consider the molar ratio between Bi and Fe in the manufacture of the BiOI/Fe₃O₄ photocatalyst, as an optimal ratio favors the growth of BiOI sheets on the surface of the magnetic core, forming a core-layer type structure [18]. This structure prevents iron oxide (magnetite) corrosion in the aqueous environment and improves the material’s photocatalytic activity [18,20,22].

In addition, the ratio between Fe₃O₄ and BiOI influences the recombination process of photogenerated charge carriers, as the recombination rate of electrons and holes is reduced, to some extent, by the addition of Fe₃O₄ [18]. This means that the smaller the amount of magnetite, the higher the recombination rate will be (Figure 10). The research conducted by Y. Liu et al. [22], S. Gao et al. [21], and Qian et al. [18] indicate that a Bi/Fe ratio greater than 5:1 would reduce its photocatalytic activity due to an increased recombination of the electron-hole pairs.

Considering this background, four stoichiometric ratios between Bi/Fe: 2:1 (BF-2), 3:1 (BF-3), 5:1 (BF-5), and 7:1 (BF-7) were evaluated in this study (Figure 11), the results of which indicated that the most favorable stoichiometric ratio was the 5:1 ratio (Figure 10). Control experiments were also carried out to evaluate the photocatalytic activities of the pure compounds of BiOI and Fe₃O₄, as the degradation of caffeic acid without a catalyst in suspension was also measured under simulated visible light radiation using 25W LED lamps. This compares the photocatalytic performance of the BiOI/Fe₃O₄ heterostructures synthesized after 180 min of reaction.

The synthesized compounds BF-2, BF-3, BF-5, and BF-7 showed photocatalytic efficiencies in caffeic acid degradation of 59.1%, 65.0%, 77.2%, and 71.7%, respectively. An analysis of variance (ANOVA) was performed with a 95% confidence level to determine the significance of the differences in these performances. The p-value obtained (0.018) indicates statistically significant differences between the efficiencies of the different compounds, which suggests that variations in the composition and structure of photocatalysts significantly influence their ability to degrade phenolic compounds. This is consistent with the research conducted by Qian et al. [18], whose results indicate that the ratio between Bi and Fe influences the recombination process of photogenerated charge carriers as the recombination rate is reduced, to some extent, by the addition of magnetite. It was determined that the reaction kinetics for the degradation of the model contaminant using BF-5 are pseudo-first-order; other authors have obtained the same results [18,19,21].

Figure 11 shows that caffeic acid degrades minimally when exposed to visible light, indicating that it is not degraded by photolysis and that pure magnetite has a minimal effect on the removal rate of the model compound. It is relevant to note that BF-5 shows a photocatalytic performance comparable to that of pristine BiOI (80.5%) with the additional advantage of possessing magnetic properties that facilitate separation and post-treatment recovery processes.

3.4. Future Projections Based on the Readiness Level of Solar Photocatalysis Using BF-5 Area of Study

Worldwide, the growing water scarcity has driven the search for new water sources, and the reuse of treated wastewater is presented as a challenge for different governments and international committees [65]. It is projected that by 2060 the global demand for freshwater will reach 6906 km³, but only 60% could be covered [66]. Changes in the hydrological cycle are evident in Latin America [67] and Chile is one of the Latin American countries most affected by water scarcity (see Figure 12) [68].

The Coquimbo region comprises a hyper-arid, arid, and semi-arid zone from north to south is located in a transition zone between the Mediterranean area of Chile to the south and the Atacama Desert to the north [69]. This region has 94% of the pisco production vines in Chile [3] and faces a significant water crisis [70,71], where the LIW the wine and pisco industry generated could potentially be reused. In this sense, the reuse of treated wastewater can help face future problems of water scarcity if subjected to adequate treatment so that it can be used in diverse productive activities in the agricultural, mining, and industrial sectors [72]. The pisco company that provided the information for this evaluation is Pisco Endémico, located at Parcela 18, Fundo Titón, Valle de Elqui, Chile (Figure 12).

3.5. Technical Feasibility

The technical feasibility was evaluated based on the ability of the heterogeneous photocatalysis to improve the quality of the influent according to the water quality physicochemical parameters and the input biodegradability index (BOD₅/COD), following the criteria proposed in

Table 2. Characterization of the Pisco Endémico tributary (measured in February 2024).

Parameter	Unit	Value
Chemical oxygen demand (COD)	mg/L	11978
Biochemical oxygen demand (BOD5)	mg/L	2400
BOD5/COD	-	0.20
pH	-	3.73
Turbidity	NTU	309
Electrical conductivity (EC)	mS/cm	2.97
Total suspended solids (TSS)	mg/L	733.30

[73].

The selection of a technology for LIW treatment depends on the properties of the pollutants found in the wastewater. For this, the BOD₅/COD relationship was applied Equation (4) by providing the characteristics of the input wastewater, which can define the performance of Wastewater Treatment Plants (WWTPs), where phenolic compounds are the main contributors to this relationship [74]. If the influent has a complex biodegradability index and persistent pollutants that are difficult to remove by a conventional WWTP, it is feasible from the technical field to implement AOPs.

$$Bi={\displaystyle \frac{{BOD}_5}{COD}}$$

(4)

where, $Bi$ is the Biodegradability index, BOD₅ in mg/L, and COD in mg/L. Values higher than 0.6 indicate biodegradable LIW, the range between 0.3 and 0.6 shows moderately biodegradable LIW, and values lower than 0.3 show LIW with low biodegradability [75].

The physicochemical parameters of BOD₅ and COD of Pisco Endémico’s LIW were measured under standardized methods proposed by APHA [76]. The results showed values of 2400 mg/L and 11978 mg/L, respectively (see Table 2). The biodegradability index value is 0.20, indicating that using BF-5 in a photocatalytic system after an effective primary treatment to decrease the values of TSS and Turbidity is technically feasible in this company.

3.6. Cost Analysis of a Medium-Scale Photocatalytic System

The economic viability of phenolic compound degradation was simulated using a magnetically recoverable BiOI/ Fe₃O₄ photocatalyst. The Composite Parabolic Solar Concentrator (CPC)-type photocatalytic system was selected as a good option to achieve high percentages of photodegradation due to its reflective geometry [77], based on parabolic and involute sections to increase their efficiency (Figure 13). The CPC has great potential to be implemented in Pisco Endémico, given that this small industry generates a maximum amount of 1167 L/month of wastewater in its pisco production.

The CPC collector will purify 40 L/day with a useful ground area of 3 m² in 180 min, data that are corroborated by Muscetta et al. [39]. The heterogeneous photocatalysis system implies being a unitary primary post-treatment operation to improve the water quality physicochemical parameters and biodegradability indices, so that the treated effluent is compatible with a subsequent biological treatment [78].

The photocatalytic system has a hydraulic pump with corrosion resistance of 70 L/min and 220 V, and an anodized aluminum CPC solar collector molded according to parabola and involute angles, allowing the installation of five 50 mm borosilicate glass tubes connected using PVC pipes, as shown in Figure 13a. It also has a magnetic separation unit. It considers adding 400 ppm of BF-5 to the continuous system for 180 min, which will be activated by solar radiation. In this way, a simple cost analysis was performed and tabulated in

Table 3. Costs associated with BiOI/Fe₃O₄ pilot scaling for use in wastewater remediation from a small Pisco industry. It is considered to operate 0.04 m³ of LIW in the system from 12 to 3 p.m. (180 min), where the greatest solar irradiance is concentrated.

Description	Cost * (USD/h) Solvothermal	Cost * (USD/h) Hydrothermal
A photocatalytic system with a solar CPC reactor, includes a hydraulic pump (USD 6807 a, service life 15 years)	0.218	0.218
High gradient magnetic separator, operates 1 h/day (USD 68.98, service life 15 years)	0.002	0.002
Wastewater feeding system, considering hydraulic passage pump and PVC pipes. (USD 96.26, service life—5 years)	0.009	0.009
Energy cost (step pumps and CPC system)	0.004	0.004
Labor b, considers system operation and maintenance.	2.900	2.900
Raw material c	13.044	1.100
Total (USD/hour)	16.178	4.234

*: Calculation based on the estimated cost for 8 h/day, 5 days/week, and 52 weeks/year. a: Chilean peso to US dollar conversion (accessed on 23 August 2024). b: Cost estimation based on the labor rate of technical operators, Chile 2024. c: Estimation of the cost of materials for synthesizing BiOI/Fe₃O₄. Based on the price of quotations for 2024 in Chilean companies.

, following the methodology of Khairudin et al. [41].

According to the analysis, the treatment cost for removing pollutants with low biodegradability using magnetically recoverable BF-5 with a load of 400 mg/L is estimated at USD 16,178/h. In this case, the raw material for the manufacture of the photocatalyst contributed 80.67% of the total cost. Therefore, reducing these costs, such as replacing the solvent used in the synthesis of BF-5, will generate a significant fluctuation in the operating price in the near future. Cheaper syntheses should be investigated using the hydrothermal method (water and ethanol) to reduce operating costs to USD 4,234/h.

By way of comparison, according to several studies [6,79,80,81], one of the most effective methods to remove a high organic load with low biodegradability from vinasse wastewater is anaerobic membrane bioreactor (AnMBR) digestion. According to Moreira et al. [79], removing the BOD₅ and COD parameters in LIW comprising Vinasse using AnMBR would have a total cost of USD 271,405/h, higher than the USD 16,178/h to implement a photocatalytic system using BF-5. The high value of AnMBR can be attributed to what is indicated by An et al. [82]. The authors argue that the main challenge lies in the high cost of the membranes and the substantial energy consumption associated with mitigating the membrane damage, despite the inherent energy advantages of anaerobic systems. Considering current systems’ complexity and high costs, heterogeneous photocatalysis represents a promising solution for eliminating low-biodegradability organic compounds.

3.7. Environmental Feasibility

It is necessary to consider factors that may limit the applicability of certain processes, such as the availability and characteristics of the terrain, as well as environmental and climatic aspects. These analyses will make it possible to guarantee that the proposed photocatalytic system is viable from a sustainable and safe point of view for the environment where it will be implemented [83]. The application of magnetite-based photocatalysis to treat persistent wastewater is very promising for safeguarding environmental integrity [84]. The BF-5 proposed in this study is distinguished by its magnetic properties, high photocatalytic activity against pollutants in the wine and pisco industry, and its narrow band gap that allows it to be activated with solar irradiation and biocompatibility. According to what Abdel Maksoud et al. [85] suggest, one key advantage of Fe₃O₄-based photocatalysts is their ease of recovery, significantly reducing the generation of secondary pollutants during treatment processes.

In this study, the integration of solar energy was evaluated by the availability of solar irradiation in Pisco Endémico throughout the year (Figure 14) and the classification of the photocatalyst as a substance, so that its use does not represent an environmental or health risk according to domestic and international legislation [86], essential information for the viability of projects seeking environmental sustainability.

The light that actually reaches ground level depends on different parameters, such as weather conditions, latitude, and time of day. At the study site, located in the north of Chile, the solar exposure has the highest solar energy potential in the world [87], mainly between 12 and 3 pm (GTM -4). The CPC reactor (Figure 13) will be installed on a platform inclined at 30°, at the latitude of the Coquimbo region, forming an acceptance angle of 90° to capture incident solar irradiance on the reactor surface for maximum efficiency. The main advantage of CPC is the ability to concentrate both the direct and a large part of the diffuse radiation that impinges on the acceptance angle (θ) [88].

Regarding the dangerousness of the substances used in the synthesis of BF-5, bismuth (Bi) is an element of great interest in the synthesis of advanced photocatalysts due to its semiconductor properties, photocatalytic activity under visible light, and the fact it is biocompatible and non-toxic at low concentrations [89]. These characteristics exclude it from the category of environmentally hazardous substances, making it an ideal choice for applications in industrial wastewater remediation [90]. On the other hand, magnetite (Fe₃O₄) is a non-toxic, low-cost substance with a mass-production capacity [91], widely used in coupling with other photocatalysts to improve recoverability by magnetic separation [92]. According to Jogaiah et al. [93], the toxicity of nanocatalysts is highly dependent on the exposure dose in the environment, and the system’s magnetic recovery can significantly mitigate the risks [94]. In addition, it has been shown that coated Fe₃O₄ nanoparticles do not present ecotoxicity in plants [95]. Overall, bismuth-based nanoparticles have essential attributes that have been evaluated in the cosmetic and pharmaceutical industry over time, standing out for their minimal or no zero toxicity, stability, abundance in the Earth’s crust, and affordability [96,97].

Concerning the disposal of the spent catalyst, the EPA [98] classifies waste as hazardous or non-hazardous under the Resource Conservation and Recovery Act, USA., RCRA 94-580, 1976 [99]. Waste is considered hazardous if it has any of the following characteristics: ignitability, corrosivity, reactivity, or toxicity. The BiOI/magnetite photocatalyst, under the conditions studied, does not show properties that classify it as ignitable, corrosive, reactive, or toxic. In the European Union, waste classification is governed by the Waste Framework Directive (2008/98/EC, 2008) [100], which establishes the criteria for determining whether waste is hazardous. The regulations evaluate aspects such as acute toxicity, ecotoxicity, and persistence in the environment. BiOI/magnetite does not contain substances listed as hazardous in the European classification of hazardous waste (LWP 2014/955/EU) [101] and also does not qualify as a hazardous substance in the Classification, Labeling, and Packaging of Substances and Mixtures-CLP Regulation [102].

In the context of where Pisco Endémico is located, and according to the Chilean Government’s Supreme Decree SD. 148, 2004 [103], waste or a mixture thereof is considered hazardous if it presents a risk to public health and/or adverse environmental effects, either directly or as a result of its current or planned management. In the case of the BiOI/magnetite photocatalyst used, none of its constituent substances are classified as hazardous according to Chilean legislation RE. 777, 2021 [104], which makes it possible to rule out its dangerousness both in its application as an active substance and in its management as waste.

It is important to note that although BiOI/magnetite can be considered non-hazardous waste under the regulations mentioned above, its handling must follow the best waste management practices to minimize any potential environmental impact. Hence, the proposed advanced technology offers an environmentally feasible system, both in waste management and in implementing the photocatalytic process using solar radiation, representing a potential alternative in environmental sustainability for managing water resources in arid zones. It also considers that better management of water and sanitation services contributes to addressing the serious problem of water availability, as well as to reducing health risks and promoting environmental protection [105].

3.8. Scalability of Heterogeneous Photocatalysis Using BF-5

To evaluate the level of technological preparation of the BiOI/Fe₃O₄ material in photocatalysis (Figure 15), its manufacture and efficiency in degrading pollutants in the wine and pisco industry were considered. The geographical characteristics, climate of the study area, and economic viability of the operation were also reviewed.

The scalability was evaluated based on the technological preparation achieved in this study, which presents a TRL 3 by demonstrating effectiveness in the process at a laboratory scale and high efficiency in the degradation of caffeic acid under controlled conditions [39]. Optimizing synthesis and operational parameters is essential for moving from TRL 4 to TRL 6 since they are unique for each process and will depend on the characteristics of the synthesized material (BF-5) and the nature of the pollutants to be removed. The transition to a pilot stage must focus on improving reactor parameters such as flow rates, residence and hydraulic retention times, and photocatalyst loading [106]. This sequence will allow validating the system in conditions closer to operational ones, increasing its scalability, and preparing its future industrial implementation.

4. Conclusions

This study demonstrated that the BiOI/Fe₃O₄ heterostructure, synthesized by the solvothermal technique with a 5:1 molar ratio of Bi/Fe, is an efficient photocatalyst for degrading phenolic compounds in wastewater under simulated solar irradiation. Combining the photocatalytic properties of BiOI with the magnetic properties of Fe₃O₄ allowed an efficient recovery of the photocatalyst using only low-cost magnetic equipment. The three-dimensional porous structure and the high photocatalytic activity under simulated solar radiation conditions position it as a viable solution to advance technological preparation.

Implementing heterogeneous photocatalysis using BF-5 presents an innovative and sustainable approach to wastewater remediation in the wine and pisco industry, particularly in arid areas such as the Coquimbo region, Chile. The evaluation conducted confirms that the technology has reached TRL 3 under controlled laboratory conditions. It demonstrated high efficiency in the degradation of persistent pollutants such as caffeic acid, making it a relevant technology to be implemented in LIW with complex biodegradability indices. The cost associated with the photocatalytic system’s operation is also lower than other current systems that achieve similar removal efficiencies; the study sector has a high availability of solar irradiation, and the raw materials used in the manufacture of BF-5 do not present significant risks to the environment and human health.

According to the analysis carried out, to move towards a larger-scale implementation, it is essential to optimize the synthesis and operational parameters and conduct pilot tests that allow the system to be validated in real conditions. This transition will guarantee scalability and economic sustainability in a more limited time, addressing the critical challenges in managing water resources in areas affected by scarcity.