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Article

Enhanced Photocatalytic Degradation of Cyanide in Mining Wastewater Using a ZnO-BiOI Heterojunction Catalyst

by
Darlington C. Ashiegbu
1,*,
Paballo Pilane
1,
John Moma
2,3 and
Herman Potgieter
1
1
Sustainable and Innovative Metals and Minerals Extraction Technology (SIMMET) Laboratory, School of Chemical and Metallurgical Engineering, University of the Witwatersrand, Private Bag X3, Wits, Johannesburg 2050, South Africa
2
Molecular Sciences Institute, School of Chemistry, University of the Witwatersrand, Private Bag 3, Wits, Johannesburg 2050, South Africa
3
DSI-NRF Center of Excellence in Strong Materials, University of the Witwatersrand, Private Bag 3, Wits, Johannesburg 2050, South Africa
*
Author to whom correspondence should be addressed.
Surfaces 2025, 8(1), 3; https://doi.org/10.3390/surfaces8010003
Submission received: 6 November 2024 / Revised: 28 December 2024 / Accepted: 3 January 2025 / Published: 5 January 2025
(This article belongs to the Special Issue Surface Modification and Coating to Improve Properties of Various Materials)
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Abstract

:
The mining industry often relies on the natural degradation of tailings dams to break down cyanide in wastewater. However, this method has drawbacks, including high costs due to significant water demand and variable effectiveness dependent on environmental conditions, and it is a time-consuming process. To address this issue, this study focused on preparing, characterizing, and applying a ZnO-BiOI heterostructure for cyanide removal in water. The heterojunction was thoroughly characterized using techniques such as SEM-EDX, X-ray diffraction, nitrogen adsorption–desorption isotherms, photoluminescence, and XPS scans. The photocatalytic efficacy was evaluated by degrading CN−-containing solutions across varying photocatalyst masses, temperatures, and initial cyanide concentrations. The results showed that 5 mg of the heterostructure completely eliminated 40 ppm of cyanide in 35 min. Increasing the catalyst mass to 15 mg significantly reduced the time for the complete degradation of 40 ppm cyanide, while 25 mg of the photocatalyst achieved cyanide removal in 35 min. The optimal temperature was found to be 50 °C, with complete cyanide removal occurring in 20 min within the temperature range of 25 °C to 70 °C. Moreover, when the cyanide concentration ranged from 40 ppm to 100 ppm, 15 mg of heterojunction catalyst achieved a 97% destruction efficiency in removing a 100 ppm cyanide solution within 35 min. These results strongly indicate that the synthesized heterojunction has the potential to serve as an effective and efficient photocatalyst for cyanide degradation in process effluents and wastewater.

1. Introduction

The escalating trend of water contamination has led to numerous environmental challenges. The extensive annual global production of cyanide, estimated at 2 to 3 million tons, along with its wide range of use across multiple industries, has resulted in the significant contamination of water bodies with elevated cyanide concentrations [1,2,3,4]. The presence of cyanide in wastewater is particularly concerning due to its toxic effects [5]. The cyanide ion is composed of a carbon atom triple-bonded to a nitrogen atom. It has been applied across various industries including textiles, pharmaceuticals, coking, electroplating, mining, and more [3,6]. Cyanide occurs in several forms, including free cyanide (comprising hydrocyanic acid [HCN] and the cyanide ion [CN−]), simple inorganic salts (such as NaCN and KCN, which dissociate in water to release a cation and anion), metal cyanide complexes (denoted as M(CN)2), cyanates (OCN−), thiocyanates (SCN−), and nitriles (R-C≡N) [7]. The free cyanide forms HCN and CN− are considered the most toxic due to their significant potential for metabolic inhibition [8].
In the mining sector, gold extraction from ore predominantly relies on cyanidation leaching following the crushing process, with no viable alternative to cyanide currently available [9]. A typical composition of gold mining effluent includes high concentrations of heavy metals such as mercury, arsenic, lead, cadmium, and chromium, along with cyanide and thiocyanates formed during cyanide degradation. Additionally, the effluent contains suspended solids and may exhibit acidic or alkaline characteristics depending on the reagents used, such as lime or sulfuric acid. Sulfur compounds, including sulfates from sulfide mineral oxidation, are also present, as well as nutrients like ammonia and nitrates from explosive residues, and various organic compounds from residual reagents or hydrocarbons [10,11,12]. There are nearly 900 gold and silver extraction operations worldwide, approximately 460 of them utilize cyanide in their extraction processes; these operations account for more than half of the metallurgy industry [8]. Mining and metallurgical processes consume approximately 13% of the annual worldwide production of hydrogen cyanide, which amounts to nearly 1.1 million metric tons [9]. As reported by Pérez (2007), nearly 1.0 kg of cyanide (either NaCN or KCN) is required to recover just 1.5 g of gold [13]. Human activities predominantly influence the concentration of cyanide in the environment. In pristine streams and lake water, the typical cyanide levels range between 0.001 and 0.05 ppm. Industrial effluents generally have cyanide concentrations ranging from 0.01 to 10.00 ppm, but effluents from electroplating plants may contain significantly higher levels, reaching up to 100,000 ppm [14]. Hence, it is crucial to treat effluents from various industries to lower their cyanide concentration before releasing them into the environment.
The most used method for attenuating cyanide in process effluents in mines is natural degradation in tailings dams [3]. Over the last two decades, chemical, biological, and electrochemical alternatives have been developed. However, natural degradation is still the dominant approach to cyanide degradation in the mining industry [3]. Natural degradation tailings are the processes by which tailings, which are the waste materials left after the extraction of desired minerals from ore, undergo natural degradation or decomposition over time. However, the effectiveness of natural degradation processes in mitigating the environmental risks associated with tailings are dependent on factors such as the tailings composition, local environmental conditions, and the presence of reactive minerals or organic matter. A variety of methods have undergone extensive investigation for treating cyanide wastewater, including bioremediation, electrochemical processes, coagulation and flocculation, ion exchange, ozonation, Fenton oxidation, and photocatalytic degradation. However, many of these technologies have drawbacks, such as increased electricity demand and the formation of hazardous intermediate byproducts necessitating post-treatment, all of which contribute to increased remediation costs [15].
Advanced oxidation processes (AOPs) have been the focus of extensive research over the last 30 years and are now applied in contaminant remediation due to their financial viability, efficiency, and benign nature. AOPs facilitate the in situ generation of hydroxyl radicals (•OH), superoxide anions (O2•−), and electron–hole (e−/h+) pairs, which are instrumental in transforming organic pollutants into CO2, H2O, and other less harmful byproducts [16,17]. Among the various AOPs, including Fenton processes, UV/H2O2, O3/H2O2/UV, O3/UV, and semiconductor photocatalysis, photocatalysis has emerged as the most innovative, promising, and environmentally sustainable alternative. The photocatalytic degradation of contaminants is known for its rapidity, environmental friendliness, cost-effectiveness, and ability to completely degrade contaminants into harmless end products. Among various photocatalysts, ZnO stands out due to its non-toxic nature, excellent chemical stability, favorable optical and electronic properties, and affordability. However, ZnO’s ability to absorb visible light is limited due to its wide bandgap energy of 3.37 eV [18]. Additionally, the photocatalytic efficiency of ZnO is compromised by the rapid recombination of electron and hole pairs [19]. Consequently, modifying ZnO could significantly improve charge separation and light absorption, effectively addressing these key challenges and enabling more efficient photocatalytic processes.
To overcome this limitation, heterojunction materials have been developed by combining ZnO with other semiconductors. BiOI, a bismuth-based oxyhalide semiconductor, has gained interest due to its narrow bandgap, excellent visible light absorption, and unique layered structure that facilitates charge separation. However, as a standalone material, BiOI has a limited photocatalytic performance due to its weak photogenerated charge carrier mobility [20,21,22]. The combination of ZnO and BiOI into a heterojunction material offers a promising strategy to address the shortcomings of the individual components. The heterojunction structure promotes effective charge transfer and inhibits electron–hole recombination, resulting in an enhanced photocatalytic performance. Previous studies have reported the improved degradation of organic pollutants using ZnO-BiOI heterojunctions. For instance, ZnO-BiOI composites have demonstrated significant activity in degrading dyes, pharmaceuticals, and other persistent pollutants under UV–visible irradiation [23,24,25]. These findings underscore the potential of ZnO-BiOI heterojunctions in addressing environmental challenges.
Within this work, a heterojunction comprising the optimized etching of BiOI into a ZnO structure was fabricated and utilized for the photodestruction of cyanide under solar irradiation. The incorporation of BiOI acts as a photosensitizer, leveraging its visible light properties to augment the light absorption capabilities of ZnO, which primarily absorbs UV light [26]. By combining two or more semiconducting materials with complementary band structures, heterojunctions enable the efficient utilization of a broader spectrum of light, including visible light [27]. This mechanism is essential for mitigating the recombination of photogenerated charges, thus improving the photocatalytic activity [6]. The suitability of the prepared heterojunction photocatalyst for removing cyanide from effluents was investigated in this study, and the optimal removal efficiency was determined. The synergistic interaction between ZnO and BiOI enhances the photocatalytic efficiency, making it an advanced material for environmental remediation. The study also demonstrates the practical application of the synthesized heterojunction for effectively removing cyanide from mining wastewater. Cyanide is a persistent and toxic pollutant, and its removal using photocatalysis offers an environmentally friendly and sustainable solution, addressing a critical issue in mining waste management. This manuscript contributes to advancing sustainable wastewater treatment by demonstrating the superior performance of a ZnO-BiOI heterojunction catalyst for the enhanced photocatalytic degradation of cyanide in mining effluents.

2. Experimental

2.1. Materials

Throughout all the experiments, sodium cyanide (NaCN, 99%) sourced from the Associated Chemical Enterprises South Africa (ACE) served as the primary source of cyanide ions (CN−). Solutions were prepared by dissolving NaCN in deionized water and alkalizing it with sodium hydroxide (NaOH) to achieve a pH of 12. Silver nitrate (AgNO3, 99.95%) and 5,4 dimethylaminobenzylidene (rhodanine) (99%) were supplied by DRD Gold South Africa. Additionally, Sigma Aldrich (St. Louis, MO, USA) provided zinc oxide (ZnO, 99.9%), potassium iodide (KI, 99.5%), and bismuth nitrate pentahydrate (Bi(NO3)3•5H2O, 98%). The sodium hydroxide (NaOH) and ethanol utilized in the experiments were sourced from the laboratories of the School of Chemical and Metallurgical Engineering at the University of the Witwatersrand. The Millipore system (Hach, Johannesburg, South Africa) provided deionized water utilized in all preparation of aqueous solutions. The chemicals employed were of analytical reagent grade and were utilized without additional purification.

2.2. Heterojunction Synthesis

Employing the approach previously reported by Ashiegbu et al., 2022 [18], the heterojunction synthesis commenced by pouring ZnO in DI, while bismuth nitrate pentahydrate was introduced into ethanol in a separate vessel under vigorous stirring. The resulting bismuth nitrate pentahydrate and ethanol mixture were gradually added to the ZnO suspension with continuous stirring facilitated by a magnetic stirrer. Subsequently, potassium iodide was solubilized and incrementally added to the mixture under vigorous and continuous agitation. The mixture was stirred for three hours before being subjected to centrifugation using a Hettich ROTOFIX Benchtop Centrifuge at 4000 rpm to isolate the solid from the liquid phase. Following decantation of the liquid, the solid material was air-dried for a period of 18 h under ambient conditions. Subsequently, the dried product underwent grinding, using an agate mortar to yield the heterojunction photocatalyst in powder form.

2.3. Heterojunction Characterization

A Bruker D2 diffractometer (Bruker Corporation, Billerica, MA, USA) was used to examine the diffraction patterns of both the synthesized composite and the pristine materials. Morphological characterization of the heterojunction was performed through scanning electron microscopy (SEM; Carl Zeiss Sigma FE-SEM), combined with Oxford X-act energy dispersive X-ray spectroscopy (EDS; Oxford Instruments, Oxfordshire, UK) for elemental analysis. Optical assessments and photodestruction evaluations were carried out employing a UV 1800 Shimadzu UV–Vis Spectrophotometer (Shimadzu Corporation, Kyoto, Japan). Steady-state photoluminescence (PL) spectra were recorded at room temperature using a Horiba QM8000 spectrofluorometer (HORIBA Advanced Techno, Co., Ltd. Kyoto Japan), with a Xe lamp as the excitation source. The textural properties were assessed using a Micrometrics TriStar 3000 instrument (Micrometrics Instrument, Norcross, GA, USA). Additionally, an AM 1.5G 100 mW/cm2 (Newport Corporation, Irvine, CA, USA) light source was employed for this investigation.

2.4. Photocatalytic Degradation

The activity of ZnO-[10%]BiOI was assessed by studying the degradation of a CN− solution under various photocatalyst mass, temperature, and initial cyanide concentration conditions. These experiments were conducted utilizing a solar simulator positioned 20 cm from the quartz glass vessel during the experiments. In a typical trial, a 100 mL solution of CN− was prepared at the desired concentration, typically 40 ppm, and the cyanide solution pH was adjusted by gradual dropping of 1 M NaOH solution, with pH measurements conducted using an OHAUS Starter 3100 pH meter (OHAUS Corporation, Parsippany, NJ, USA). The pH was maintained above 11 to ensure the presence of free cyanide in its ionic form and to avert the generation of toxic hydrogen cyanide gas. To isolate any influence of light on the degradation process, a photolysis aliquot (without catalyst) was initially sampled after 10 min of solar simulator exposure. Subsequently, the solar simulator was turned off, and the requisite amount of ZnO-[10%]BiOI catalyst was introduced into the cyanide solution and magnetically stirred in darkness for 30 min (catalyst mass preweighed). To ensure the uniform distribution of the photocatalyst active sites throughout the solution, continuous magnetic stirring was used throughout the experiments. Following the dark experiment, the solar simulator was reactivated to irradiate the cyanide solution containing the catalyst for 35 min, and the reaction mixture was extracted at intervals of 5 min using a sterile syringe attached to a 0.22 µm filter to remove the catalyst.
All experiments were repeated twice, and all the samples were promptly refrigerated and shielded from light to halt further reactions. The degradation efficiency (%) of cyanide was computed using Equation (1):
Cyanide   degradation = C 0 − C t C 0   ×   100

Analytical Methods

The free cyanide concentration in the collected samples was analyzed using silver nitrate titration (SNT), a titrimetric technique involving the use of a standard silver nitrate solution (0.0192 M) as a titrant, with rhodanine acting as an indicator. In this method, silver forms stable complexes with cyanide (Equation (2)), and once all the free cyanide complexes are combined with silver, any excess silver added begins to react with the indicator, resulting in a shift in color in the solution from orange to light purple, indicating the end of the reaction [28]. Titrations were performed in duplicate to enhance the accuracy of the results.
Ag+ + 2CN− → Ag(CN)2−

3. Results and Discussion

3.1. Structural, Morphological, and Compositional Analyses

The structural characterization of the synthesized materials, including the ZnO nanoparticles, BiOI photocatalyst, and ZnO-BiOI heterostructure, was carried out using X-ray diffraction (XRD) analysis (Figure 1a). The XRD pattern of the prepared ZnO nanoparticles displayed sharp and intense peaks at 2θ values of 31.7°, 34.4°, 36.1°, 47.3°, 56.3°, 62.6°, 66.3°, 67.9°, and 69.1°, corresponding to the (100), (002), (101), (102), (110), (103), (200), (112), and (201) crystal planes of hexagonal ZnO. These intense and narrow peaks indicate the high crystalline nature of the ZnO nanoparticles, consistent with the hexagonal wurtzite phase indexed on JCPDS card number 01-075-6445. The absence of impurity peaks further confirms the complete decomposition of the precursors and the purity of the sample. The high crystallinity of ZnO is attributed to the elevated calcination temperature used during synthesis. Using the Debye–Scherrer formula applied to the most intense peaks, the average crystallite size of the ZnO nanoparticles was determined to be 37.6 nm, confirming their nanoscale dimensions. The XRD pattern of the prepared BiOI photocatalyst revealed intense peaks, characteristic of the tetragonal phase of BiOI, indexed on JCPDS card number 00-010-0445. The peaks observed at 10°, 30°, 34°, 46°, and 56° correspond to the (001), (102), (110), (200), and (212) diffraction planes, respectively. Additionally, the sample exhibited mixed phases, including Bi5O7I and Bi7O9I3, attributed to the synthesis method involving a long hydrothermal reaction in a Teflon-lined autoclave. The sharp and well-defined peaks confirmed the high crystallinity of the BiOI photocatalyst. Using the Debye–Scherrer equation, the average crystallite size of BiOI was calculated as 40 nm, further validating its nanoscale structure. The ZnO-BiOI heterostructure exhibited typical diffraction peaks corresponding to both the ZnO and BiOI phases. The ZnO diffraction peaks, observed at 2θ values of 31.7°, 34.4°, 36.1°, 47.3°, 56.3°, 62.6°, 66.3°, 67.9°, and 69.1°, correspond to the (100), (002), (101), (102), (110), (103), (200), (112), and (201) planes of hexagonal ZnO. In addition, a peak at 29° was assigned to the (102) diffraction plane of BiOI, confirming the successful integration of BiOI into the ZnO structure. The absence of impurity peaks highlights the effective synthesis and purity of the ZnO-BiOI heterostructure. The pronounced diffraction peaks also indicate the high crystallinity of the heterostructure. Using the Debye–Scherrer formula, the average crystallite size of the ZnO-BiOI composite was calculated as 31 nm, further establishing the nanoscale dimensions of the material.
Morphological studies of the heterojunction are presented in Figure 1. Figure 1b provides a visual representation of the heterojunction, revealing the presence of nanoparticles with diverse shapes, including rod-like, spherical, and irregular structures. The images captured by SEM illustrate the non-uniformity of the surfaces, agglomeration, and absence of a specific orientation, highlighting the complex and heterogeneous nature of the heterojunction. These observations offer important insight into the structural properties of photocatalytic materials, which can influence their ability to catalyze chemical reactions and degrade pollutants.
The EDS analysis revealed that zinc was the major element in the heterojunction, accounting for 68.5 wt% of the total composition. Oxygen was the second most abundant element, present at 16.2 wt%, followed by bismuth (11.2 wt%) and iodine (4.1 wt%). These results are summarized in Supplementary Table S1. The EDS analysis confirmed that the target elements were present in the synthesized catalyst in the desired stoichiometry, and no impurities were detected within the catalyst. This indicates that the synthesis process was successful in producing a high-purity heterostructure with the expected composition.
The synthesized heterojunction’s oxidation states and surface chemical compositions were analyzed utilizing X-ray Photoelectron Spectroscopy (XPS). As illustrated in Figure 1c, the XPS spectrum displays distinct peaks associated with the elements Zn (2p), Bi (4f), O (1s), and I (3d), as well as a notable carbon peak, which is attributed to the instrumentation itself. The peaks located at 158.46 eV and 163.76 eV correspond to Bi 4f5/2 and Bi 4f3/2, respectively, with an energy difference of 5.3 eV, suggesting that the predominant valence state of Bi within the heterojunction is +3. Furthermore, the prominent peaks at 1021 eV and 1044 eV are identified as Zn 2p1/2 and Zn 2p3/2, respectively, with a peak separation of 23 eV, indicating the presence of Zn2+ cations in the ZnO-BiOI heterostructure. The peaks at 619 eV and 630.5 eV are associated with I 3d5/2 and I 3d3/2, respectively, which result from the spin–orbit splitting of the I 3d levels. The peak difference of 11.5 eV confirms the existence of I- anions. Additionally, the O 1s spectrum reveals a single type of lattice oxygen in the heterojunction at 530 eV, attributed to the weakening of Zn-O bonds due to the formation of Zn-I bonds and the dispersion of ZnO. Doping with ZnO leads to an increase in the interlamellar spacing between Bi2O2 slabs in the BiOI phase, which results in a decrease in the outer-shell electron density of O. Overall, the comprehensive XPS analysis validates the dual coupling between the phases within the ZnO and BiOI heterostructure, aligning with findings from previous studies [29,30,31].
The surface area and textural characteristics of the photocatalysts were evaluated through nitrogen (N2) adsorption–desorption isotherms, complemented by Barrett–Joyner–Halenda (BJH) pore distribution curves. The findings from the N2 isotherm analysis were aligned with the IUPAC classification system. The specific surface area of the ZnO-[10%]BiOI heterostructure was measured at 19.79 m2/g. The N2 adsorption–desorption isotherm for this heterostructure exhibited a Type IV profile, characterized by the absence of noticeable hysteresis, as shown in Supplementary Information S1a. Further analysis of the BJH curve revealed the existence of both mesopores and macropores, with a predominance of macropores, as illustrated in Supplementary Information S1b.
Steady-state photoluminescence (PL) spectra were recorded at room temperature using a Horiba QM8000 spectrofluorometer, employing a Xe lamp as the excitation source. A 320 nm long-pass filter was utilized to eliminate higher-order reflections from the excitation line at 300 nm. The emission spectra were intensity-corrected to account for the throughput of the emission spectrograph and the efficiency of the detector. Under the same measurement conditions, the heterojunction sample’s PL intensity was approximately ten times weaker than that of the undoped ZnO starting material. This significant reduction in the PL intensity indicates that, in the BiOI-doped sample, the valence electrons excited by the incident light at 300 nm are primarily engaged in non-radiative processes—such as electrochemical or catalytic reactions—rather than returning to the valence band and emitting photons at 380 nm, which corresponds to the bandgap energy at room temperature. This phenomenon confirms a charge transfer from the BiOI layer to the ZnO within the heterojunction, leading to a decrease in electron–hole recombination and demonstrating improved photoinduced charge separation. The narrow peak at 380 nm, as depicted in Figure 1d represents the band-to-band luminescence transition related to the bandgap. In the 10% BiOI-loaded ZnO sample, a broad orange-red emission peak is observed with a maximum at approximately 630 nm. In contrast, the ZnO sample exhibits a weaker violet band centered at 442 nm, accompanied by a broader orange-red band around 600 nm. These broad orange-red emissions are attributed to various intrinsic defects within ZnO, such as the positive charge state of oxygen vacancies.

3.2. Photocatalytic Activity

The impact of various photodegradation conditions on the removal of cyanide CN− from aqueous solution was investigated by varying the photocatalyst mass, temperature, and initial cyanide concentration under simulated solar light irradiation. The photocatalytic plot and degradation efficiency of CN− by the synthesized ZnO-[10%]BiOI heterojunction are shown in Figure 2a,b. The effects of photolysis and adsorption–desorption experiments were negligible, as observed from Figure 2a. The results indicate that 5 mg of the heterojunction was able to degrade 40 ppm of CN− within 35 min, demonstrating its high efficiency and improved photocatalytic activity compared to those of bare/undoped photocatalysts.
The quick and enhanced degradation of CN− by the synthesized catalyst can be attributed to its narrow bandgap, which improves light absorption and increases the formation of hydroxyl radicals, enhancing the number of photocatalytic active sites [20]. Additionally, the inhibition of charge carrier recombination in the heterojunction further contributes to its photocatalytic activity. These factors make the synthesized ZnO-[10%]BiOI heterojunction a promising candidate for the effective elimination of cyanide from wastewater. In comparison to a study by Mediavilla and coworkers (2019), in which commercial undoped titania (TiO2) was used as a catalyst and cyanide was wholly degraded after 5 h under simulated solar light, the synthesized heterojunction showed a significantly quicker degradation of cyanide [32]. This highlights the potential of the synthesized catalyst for practical application in the treatment of cyanide-containing wastewater.

3.3. Impact of the Photocatalyst Mass on the Degradation of Cyanide

Figure 3a,b presented below illustrate the influence of the photocatalyst mass on the abatement of cyanide. As the mass of the photocatalyst was raised from 5 mg to 15 mg, a corresponding increase in the degradation percentage of cyanide was observed, as depicted in Figure 3b. Utilizing 15 mg of the catalyst led to the complete degradation of cyanide within 15 min. The escalation in the degradation percentage with a higher catalyst mass can be attributed to the enhanced active sites on ZnO-[10%]BiOI, resulting in the enhanced generation of reactive oxygen species [33].
However, when 25 mg of catalyst was applied, the photocatalytic degradation process took longer to achieve complete cyanide degradation. This delay is ascribed to catalyst overloading, which induces increased turbidity in the solution, subsequently reducing light absorption and diminishing photocatalytic activity [18]. The efficiency of cyanide degradation is evidently impacted by the mass of the catalyst, mirroring the findings of Lubis and colleagues (2019), who also observed improved degradation efficiency with increased catalyst mass in their study [34].

3.4. Impact of Temperature on the Degradation of Cyanide

In previous research conducted by Chen and Hsu (2021), it was noted that the temperature range of 20–80 °C is optimal for effective photocatalysis [35]. Therefore, temperature variations were carried out within this range. The temperature was adjusted using a digital magnetic stirrer with a heating plate. As illustrated in Figure 4b below, the peak photoactivity and reaction rate were achieved at 50 °C, followed by 60 °C and 70 °C. Notably, temperatures of 50 °C, 60 °C, and 70 °C showed superior photocatalytic activity compared to the lower temperatures of 25 °C and 40 °C. The decline in activity at 25 °C and 40 °C is ascribed to the reduced kinetic energy of the reactant molecules at lower temperatures, resulting in a decline in the efficiency of the photocatalytic reaction.
Elevating the temperature within the range of 50 °C to 70 °C resulted in a subsequent increase in photocatalytic activity. This enhancement can be attributed to the heightened activity and mobility of photoelectron–hole pairs at elevated temperatures, which boosts the photocatalytic performance [36]. However, at lower temperatures (25 °C and 40 °C), a decrease in photocatalytic activity was observed compared to that at higher temperatures. This is because at lower temperatures, the reactant molecules have less energy available to overcome the activation energy required for the reaction to occur, resulting in slower reaction rates [37]. Studies have shown that elevated temperatures can influence the formation of free radicals and other reactive intermediates, which play crucial roles in photochemical mechanisms [38,39]. Therefore, selecting the appropriate temperature range is crucial for optimizing the photocatalytic reaction efficiency.

3.5. Impact of Concentration on the Degradation of Cyanide

To investigate the impact of the initial cyanide concentration on the degradation process, a series of experiments were conducted using the optimal catalyst mass of 15 mg at the optimal temperature of 50 °C. Four distinct initial cyanide concentrations of 40, 60, 80, and 100 ppm were employed. As depicted in Figure 5b, after a reaction time of 5 min, more than 85% degradation efficiency was achieved for 40 ppm cyanide, while 100% degradation was achieved within 20 min. The excellent performance of the heterojunction was evident in achieving a 97% degradation of 100 ppm cyanide concentration within 35 min under optimized conditions. These results suggest that the degradation efficiency decreases with an increasing initial concentration of cyanide.
Lubis and colleagues (2019) observed similar results, noting that the degradation efficiency declined with higher initial concentrations of the contaminant [34]. The decrease in the degradation efficiency at higher cyanide concentrations within a reaction time of 35 min can be ascribed to the overwhelming coverage of cyanide ions on the surface of the photocatalyst, leading to a reduction in photogenerated charge carriers. Additionally, the decreased production of reactive oxygen species also contributes to the decline in degradation efficiency. This phenomenon occurs because the higher levels of cyanide ions on the photocatalyst surface hinder the generation and migration of charge carriers, causing a decline in the production of reactive oxygen species (ROS) and, consequently, a reduction in the degradation efficiency [14]. Therefore, optimizing the initial cyanide concentration is crucial for achieving efficient degradation via photocatalytic processes.

3.6. Kinetic Study

The photodegradation reaction data obtained from the experiment using 5 mg of the heterojunction mass for 40 ppm CN treatment were analyzed by fitting them to the pseudo first-order and pseudo second-order kinetic models. The purpose of this analysis was to determine the most suitable model for describing the photocatalytic degradation of CN− by a heterojunction photocatalyst.
As depicted in Figure 6a,b, both pseudo first-order and second-order kinetic models were applied to the photocatalytic degradation of CN− by the heterojunction photocatalyst. Upon closer examination, it was observed that the pseudo first-order kinetic model provided a better fit for the reaction. This observation suggested that the photodegradation of CN− by the heterojunction photocatalyst was better described by the pseudo first-order kinetic model, which assumes that the reaction rate is directly proportional to the concentration of the reactant. Further analysis revealed that the concentration of CN− was halved to its initial level within approximately 12 min when utilizing the ZnO-[10%]BiOI heterojunction photocatalyst.
Table 1 shows a comparison of the photocatalytic performances of various photocatalysts and heterojunctions with varying initial concentrations, reported in previous studies. The pseudo first-order kinetics for the effect of the initial cyanide concentration using 15 mg of the synthesized heterojunction are presented in Supplementary Information S2a–d. All the photocatalytic systems presented in Table 1 demonstrated the ability to remove cyanide within a range of 52–100% over durations of 20–600 min. Notably, the synthesized ZnO-[10%]BiOI heterojunction exhibited superior cyanide degradation performance compared to that of numerous reported studies [5,6,15,32,40,41]. It is worth highlighting that the ZnO-[10%]BiOI heterojunction exhibited a faster degradation rate for cyanide than most studies documented in the literature.
The ZnO-[10%]BiOI heterojunction consists of ZnO and BiOI nanoparticles, which are intimately connected to form a heterojunction interface. This interface facilitates the separation and transfer of photogenerated charge carriers, inhibiting recombination and enhancing the photocatalytic activity. Moreover, the presence of BiOI in the heterojunction provides a narrower bandgap than ZnO, allowing improved visible light absorption and increased photocatalytic activity.

3.7. Mechanisms Behind the Enhanced Photocatalytic Activity of the ZnO-BiOI Heterojunction

The enhanced photocatalytic activity of the ZnO-BiOI heterojunction can be attributed to several synergistic mechanisms that improve charge separation, light absorption, and redox potential. Below are the possible mechanisms:
  • The ZnO-BiOI system forms a Type-II heterojunction, which facilitates efficient charge separation. In this structure, the following points apply:
    • BiOI has a narrow bandgap that allows it to absorb visible light.
    • ZnO, with its wider bandgap, absorbs UV light. Upon irradiation, electrons in the conduction band (CB) of BiOI can migrate to the conduction band of ZnO due to the band alignment, while holes remain in the valence band (VB) of BiOI. This spatial separation reduces the recombination rate of photogenerated electron–hole pairs, enhancing the photocatalytic efficiency.
  • The presence of BiOI expands the absorption spectrum of ZnO from the UV to the visible region due to the narrow bandgap of BiOI. This allows the heterojunction to utilize a broader range of the solar spectrum, increasing the generation of electron–hole pairs under visible light irradiation.
  • The heterojunction interface promotes efficient charge transfer across ZnO and BiOI, thereby delaying recombination. The migration of electrons from BiOI to ZnO enables the simultaneous accumulation of holes on BiOI’s VB, where oxidation reactions can occur, and electrons in ZnO’s CB, where reduction reactions proceed [44].
  • The heterojunction may introduce oxygen vacancies and defects, especially on the ZnO surface. These vacancies act as charge traps that suppress recombination and enhance the lifetime of photogenerated carriers, contributing to the higher photocatalytic activity [45,46].
  • Strong oxidation and reduction potential. The CB of ZnO has a highly negative potential, which facilitates the reduction of molecular oxygen to reactive oxygen species (e.g., superoxide radicals, •O2−). The VB of BiOI, on the other hand, has a relatively positive potential, enabling the generation of hydroxyl radicals (•OH) through water oxidation (Ashiegbu et al., 2022). These reactive oxygen species (ROS) play a crucial role in breaking down organic pollutants and other contaminants in the water [47].
  • The coupling of ZnO and BiOI creates a favorable energy band alignment that enhances the generation of photogenerated electron–hole pairs under light exposure [44], providing sustained photocatalytic activity over time.
The enhanced photocatalytic performance of the ZnO-BiOI heterojunction results from the synergistic effects of improved charge separation, extended light absorption into the visible region, increased surface area, and the generation of reactive oxygen species, all of which contribute to efficient pollutant degradation [48].

3.8. Suggested Cyanide Degradation Pathway

Cyanide degradation under photocatalytic conditions typically follows a series of oxidative steps facilitated by reactive oxygen species (ROS) such as hydroxyl radicals (OH) and superoxide radicals (O2−). These species are generated through the activation of the ZnO-BiOI heterojunction under light irradiation. The proposed pathways can be summarized as follows:
  • Initial Oxidation of Cyanide (CN−): cyanide reacts with hydroxyl radicals, leading to the formation of cyanate (CNO−) as a primary intermediate:
CN− + *OH → CNO− + H+
  • Further Degradation of Cyanate: cyanate undergoes further oxidation to produce bicarbonate (HCO3−), ammonia (NH3), and other small nitrogen-containing species such as nitrate (NO3−):
CNO− + 2*OH → HCO3− + NH3
  • Formation of Nitrate (NO3−): ammonia (NH3) can be subsequently oxidized to nitrate under prolonged irradiation, completing the mineralization process:
NH3 + 3O2 → NO3− + H2O
These pathways suggest that the cyanide is progressively degraded into non-toxic end products, such as bicarbonate and nitrate, making the process environmentally benign. To further support these proposed pathways, spectroscopic or chromatographic methods (e.g., HPLC and LC-MS) can be used to identify and quantify intermediates in future investigations.

4. Conclusions

This study reports the preparation, characterization, and subsequent application of a ZnO-BiOI heterostructure for the treatment of cyanide in water. The synthesized heterojunction photocatalyst exhibited the accelerated degradation of cyanide compared to that in previous studies. Adjusting the process parameters revealed that increasing the photocatalyst mass from 5 mg to 15 mg resulted in quicker cyanide degradation. However, employing 25 mg of catalyst slightly prolonged the degradation process due to catalyst overloading, causing increased turbidity. The optimal conditions were identified, with 50 °C being the most effective temperature and 15 mg of the heterojunction mass achieving the complete degradation of 80 ppm cyanide in 35 min. Further analysis indicated that the concentration of CN− halved within approximately 12 min when utilizing the ZnO-[10%]BiOI heterojunction photocatalyst. Both pseudo first-order and second-order kinetic models were applied to the photocatalytic degradation of CN− by the heterojunction photocatalyst, with the pseudo first-order model offering a better fit. The findings showed that ZnO-[10%]BiOI has the potential to be a highly effective photocatalyst for cyanide degradation in industrial effluents and wastewater treatment processes.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/surfaces8010003/s1, Figure S1: (a) N2 adsorption-desorption isotherm (b) pore size distribution; Figure S2: Pseudo first order kinetic models for impact of CN- concentration (a) 40 ppm (b) 60 ppm (c) 80 ppm (d) 100 ppm; Table S1: SEM (EDS) elemental analysis of the synthesized ZnO-[10%]BiOI heterojunction.

Author Contributions

Conceptualization, D.C.A., J.M. and H.P.; methodology, D.C.A. and H.P.; validation, D.C.A.; formal analysis, P.P. and D.C.A.; investigation, P.P. and D.C.A.; resources, H.P. and J.M.; data curation, D.C.A. and P.P.; writing—original draft preparation, D.C.A. and P.P.; writing—review and editing, D.C.A. and H.P.; visualization, D.C.A., J.M. and H.P.; supervision, D.C.A. and H.P.; funding acquisition, H.P. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Data Availability Statement

The datasets used and/or analyzed during the current study are available from the corresponding author upon reasonable request.

Conflicts of Interest

The authors declare no conflict of interest.

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Surfaces 08 00003 g001aSurfaces 08 00003 g001b
Figure 1. (a) XRD pattern of the ZnO-[10%]BiOI heterostructure, ZnO, and BiOI. (b) SEM image. (c) XPS spectrum. (d) PL spectrum.
Figure 1. (a) XRD pattern of the ZnO-[10%]BiOI heterostructure, ZnO, and BiOI. (b) SEM image. (c) XPS spectrum. (d) PL spectrum.
Surfaces 08 00003 g001aSurfaces 08 00003 g001b
Surfaces 08 00003 g002
Figure 2. (a) Photodegradation plot of 40 ppm CN− with 5 mg of heterostructure. (b) Corresponding degradation efficiency plot.
Figure 2. (a) Photodegradation plot of 40 ppm CN− with 5 mg of heterostructure. (b) Corresponding degradation efficiency plot.
Surfaces 08 00003 g002
Surfaces 08 00003 g003
Figure 3. (a) Photodegradation plot for impact of catalyst mass. (b) Corresponding degradation efficiency plot.
Figure 3. (a) Photodegradation plot for impact of catalyst mass. (b) Corresponding degradation efficiency plot.
Surfaces 08 00003 g003
Surfaces 08 00003 g004
Figure 4. (a) Photodegradation plot for impact of temperature. (b) Corresponding degradation efficiency plot.
Figure 4. (a) Photodegradation plot for impact of temperature. (b) Corresponding degradation efficiency plot.
Surfaces 08 00003 g004
Surfaces 08 00003 g005
Figure 5. (a) Photodegradation plot for impact of CN− concentration. (b) Corresponding degradation efficiency plot.
Figure 5. (a) Photodegradation plot for impact of CN− concentration. (b) Corresponding degradation efficiency plot.
Surfaces 08 00003 g005
Surfaces 08 00003 g006
Figure 6. (a) Pseudo first-order kinetic plot for photodegradation of CN− concentration. (b) Second-order kinetic plot for photodegradation of CN− concentration.
Figure 6. (a) Pseudo first-order kinetic plot for photodegradation of CN− concentration. (b) Second-order kinetic plot for photodegradation of CN− concentration.
Surfaces 08 00003 g006
Table 1. Comparative analysis of the cyanide degradation efficiency of various photocatalyst systems.
Table 1. Comparative analysis of the cyanide degradation efficiency of various photocatalyst systems.
CatalystCN− Concentration (ppm)Removal Efficiency (%)Time (mins)R2(k) min−1Reference
K2La2Ti3O10—KLTO100903000.7620.0078[5]
TiO2-Pd-HAP-Fe-TCPP nanocomposite100901200.9000.4520[40]
TiO2-Pd-HAP nanocomposite100521200.9900.0420[40]
ZnO-[10%]BiOI40100200.9510.0889This work
ZnO-[10%]BiOI60100250.9990.1005This work
ZnO-[10%]BiOI80100350.9880.0592This work
ZnO-[10%]BiOI10097350.9370.0600This work
TiO2/Fe2O375062120--[42]
TiO2105100600--[32]
Cu-TiO260100180--[41]
GO/TiO2/ZSM-572931800.9880.0128[6]
Fe-TCPP-S-TiO2@rGO (PUF-immobilized system)100911200.9880.0196[15]
Fe-TCPP-S-TiO2@rGO (suspended system)100751200.9840.0113[15]
TiO2/Fe2O3/PAC300971700.9840.0144[43]
TiO2/Fe2O3/Zeolite200891600.9870.0186[43]
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Ashiegbu, D.C.; Pilane, P.; Moma, J.; Potgieter, H. Enhanced Photocatalytic Degradation of Cyanide in Mining Wastewater Using a ZnO-BiOI Heterojunction Catalyst. Surfaces 2025, 8, 3. https://doi.org/10.3390/surfaces8010003

AMA Style

Ashiegbu DC, Pilane P, Moma J, Potgieter H. Enhanced Photocatalytic Degradation of Cyanide in Mining Wastewater Using a ZnO-BiOI Heterojunction Catalyst. Surfaces. 2025; 8(1):3. https://doi.org/10.3390/surfaces8010003

Chicago/Turabian Style

Ashiegbu, Darlington C., Paballo Pilane, John Moma, and Herman Potgieter. 2025. "Enhanced Photocatalytic Degradation of Cyanide in Mining Wastewater Using a ZnO-BiOI Heterojunction Catalyst" Surfaces 8, no. 1: 3. https://doi.org/10.3390/surfaces8010003

APA Style

Ashiegbu, D. C., Pilane, P., Moma, J., & Potgieter, H. (2025). Enhanced Photocatalytic Degradation of Cyanide in Mining Wastewater Using a ZnO-BiOI Heterojunction Catalyst. Surfaces, 8(1), 3. https://doi.org/10.3390/surfaces8010003

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