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Article

Bacterial Inactivation and Organic Pollutant Degradation in Slaughterhouse Wastewater Using Ag2O/Ba/TiO2 Nanocomposite

by
Habib Ullah
1,†,
Izhar Elahi
2,†,
Sahar Saleem
1,
Rab Nawaz
3,
Shafi Ullah
2,
Samia Qadeer
4,
Bilal Kabeer
5,
Muzammil Anjum
2,*,
Yi Liu
1,
Asfandyar Shahab
6,
Abubakr M. Idris
7 and
Zepeng Rao
1,8,*
1
Innovation Center of Yangtze River Delta, Zhejiang University, Jiaxing 314000, China
2
Department of Environmental Sciences, Institute of Soil and Environmental Sciences, Pir Mehr Ali Shah (PMAS) Arid Agriculture University, Murree Rd, Rawalpindi 46300, Pakistan
3
Centre for Tropical Climate Change System, Institute of Climate Change, Universiti Kebangsaan Malaysia, Bangi 43600, Selangor, Malaysia
4
Department of Environmental Sciences, Allama Iqbal Open University, Islamabad 44000, Pakistan
5
Imam Turki bin Abdullah Royal Nature Reserve Development Authority, Riyadh 12512, Saudi Arabia
6
School of Environmental Science and Engineering, Hainan University, Haikou 570228, China
7
Department of Chemistry, College of Science, King Khalid University, Abha 62529, Saudi Arabia
8
Department of Environmental Science, Zhejiang University, Hangzhou 310058, China
*
Authors to whom correspondence should be addressed.
†
These authors contributed equally to this work.
Catalysts 2025, 15(5), 411; https://doi.org/10.3390/catal15050411
Submission received: 24 February 2025 / Revised: 28 March 2025 / Accepted: 10 April 2025 / Published: 23 April 2025
(This article belongs to the Special Issue Advances in Enhancement of Catalytic Performance for Wastewater Treatment)
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Abstract

:
Slaughterhouses generate a huge amount of highly polluted wastewater; if left untreated, this effluent could seriously threaten the environment and human health. In the present study, Ag2O/Ba/TiO2 nanocomposite was synthesized using the precipitation method, and its efficacy was investigated for the remediation of real slaughterhouse wastewater (SWW) under visible light. Its performance was assessed for the inactivation of bacterial strains identified in SWW and for the degradation of total organic solids, volatile solids, fixed solids, and heavy metals. The results indicated an excellent photocatalytic performance of the synthesized Ag2O/Ba/TiO2 nanocomposites, confirmed by 87.3% volatile solids, 30% total organic solids, and 40% fixed solids removal from SWW. The zone of inhibition runs from 4 to 9 mm, and the nanocomposites have demonstrated outstanding bacterial inactivation activity in this range. It has been shown that the synthetic Ag2O/Ba/TiO2 nanocomposites can function as an effective photocatalyst for the remediation of SWW and other waste products produced by various industries worldwide.

1. Introduction

Since the 1960s, the rapid expansion of the global meat industry [1] has brought a significant environmental challenge, mainly the management of slaughterhouse wastewater (SWW). This complex effluent contains a range of toxic and persistent organic pollutants, including recalcitrant and refractory compounds [2], along with high levels of organic solids, fats, proteins, detergents, blood cells, and pathogenic bacteria [3,4]. Consequently, effective SWW treatment is crucial for environmental protection and sustainability.
Numerous treatment methods for SWW remediation have been studied in the lab, such as membrane separation, advanced oxidation processes, electrocoagulation, anaerobic treatment, aerobic treatment, built wetlands, and biological processes [5,6,7,8]. However, only biological methods were frequently employed to treat SWW on an industrial scale. There are several important disadvantages to bioprocesses, including prolonged treatment times, sensitivity to temperature changes, a moderate removal of nutrients, and poor effluent quality even after the treatment [9]. Therefore, looking into alternative treatment options for SWW remediation is necessary.
Advanced oxidation processes (AOPs) have become promising substitutes for traditional treatments [10]. Several AOPs have been tested for slaughterhouse wastewater treatment, including ozonation, oxidation, gamma radiation, and UV/H2O2 [5,11]. AOPs produce highly reactive hydroxyl radicals (OH), which have the potential to completely mineralize organic compounds in wastewater [5]. One of the AOPs, called photocatalysis, has garnered much interest because it is effective and even more environmentally safe than other AOPs because it completely oxidizes organic compounds into water and carbon dioxide rather than turning pollutants into secondary byproducts [12,13,14,15,16]. No additional chemicals are required because the catalyst still appears to work as intended [11].
For the purpose of removing microbial contaminants, photocatalysis is frequently regarded as an effective and safe method [17,18,19]. In photooxidation reactions for SWW, various photocatalysts, including titanium dioxide, zinc oxide, cadmium oxide, or stannic oxide, have been used [20]. Although photocatalytic technology has some advantages, it still has some disadvantages and is not yet widely used in industry. These disadvantages include low energy efficiency, high cost, and possibly secondary heavy metal ion pollution. The drawback of titanium dioxide is that it can only be activated in UV irradiation [21] due to its inherent characteristics, particularly its wide bandgap energy (3.2 eV). Additionally, it only absorbs 5% of the solar energy. The rapid recombination rate of electron–hole pairs restricts the applications of TiO2 for wastewater treatment. To enhance visible light absorption and to prevent electron–hole recombination, TiO2 was modified with silver (Ag) [22], silver oxides [23,24], and barium (Ba) to improve its photocatalytic performance. Ag2O is a promising, low-cost p-type semiconductor with a narrow bandgap of 1.3 eV [25], making it suitable for coupling with TiO2. By promoting the interfacial electron transfer, Ag2O minimizes charge recombination, improving photocatalytic wastewater treatment and bacterial inactivation, as silver is known for its strong antibacterial activity [26]. Moreover, incorporating Ba in TiO2 has been reported to improve charge separation by introducing oxygen vacancies and causing significant lattice disorientation, thereby inducing polarization. As a result, the generation of reactive oxygen species (ROS) increases, leading to a better photocatalytic bacterial inactivation rate [27]. Although TiO2 has been coupled with Ag2O and Ba individually, the present study investigates the effect of coupling TiO2 with both simultaneously to evaluate its photocatalytic potential for wastewater treatment and bacterial inactivation. The simultaneous coupling of Ag2O and Ba with TiO2 enhances photocatalytic degradation of organic pollutants and bacterial inactivation due to their oxidation and corrosion resistance [21].
This study aimed to develop a highly active, visible-light-driven photocatalyst for the comprehensive remediation of SWW. The focus was on simultaneously removing heavy metals, organic pollutants, and bacterial pathogens. To achieve this, a novel Ag2O/Ba/TiO2 nanocomposite photocatalyst was synthesized via a precipitation method. This specific combination of materials was chosen to enhance the photocatalytic activity of TiO2 by extending its light absorption into the visible region (through adding Ag2O) and improving its surface properties and stability (via Ba doping). The efficacy of the synthesized Ag2O/Ba/TiO2 nanocomposite was rigorously evaluated by assessing its performance in (1) the inactivation of bacterial strains isolated and identified from real SWW samples and (2) the degradation of key SWW pollutants, including total organic solids (TOS), volatile solids (VS), fixed solids (FS), and representative heavy metals commonly found in this type of effluent. Specifically, fecal Escherichia coli (E. coli), a common indicator of fecal contamination and a relevant pathogen in wastewater, was employed as a model microorganism to quantify the photocatalytic antibacterial activity of the Ag2O/Ba/TiO2 nanocomposite. This multifaceted approach allowed for a thorough investigation of the nanocomposite’s potential for practical application in SWW treatment.

2. Materials and Methods

2.1. Chemicals and Reagents

Analytical grade commercial titanium oxide (P25, TiO2), barium chloride (BaCl2), ferrous ammonium sulfate (Fe(NH4)2(SO4)2.6H2O), potassium dichromate (K2Cr2O7), mercury sulfate (HgSO4), silver nitrate (AgNO3), sodium hydroxide (NaOH), silver sulfate (Ag2SO4), sulfuric acid (H2SO4), and nitric acid (HNO3) were purchased from Sigma-Aldrich (Darmstadt, Germany). All the chemicals were analytical grade and used as received. Deionized water was used throughout the experiments.

2.2. SWW Sampling

The integrated slaughterhouse processing plant in Sihala, Islamabad, Pakistan, provided the SWW from the slaughterhouses used in the current study. This particular slaughterhouse was chosen because it is one of the largest in the city and slaughters 500 to 600 animals daily. Random SWW samples were taken from various parts of the slaughterhouse and mixed to make it homogeneous. To avoid aeration of the sample, which could alter the SWW’s characteristics, the samples were collected in pre-sterilized plastic bottles tightly sealed with lids. The samples were kept in the dark at 4 °C in a cold storage. Before further analysis, the samples were filtered through a Whatman Filter 0.45 μm (CYTIVA, Darmstadt, Germany) to remove meat and bone fragments.

2.3. Synthesis of Ag2O Nanoparticles

The scheme of synthesis of Ag2O nanoparticles Ag2O/Ba/TiO2 nanocomposite is shown in Figure 1. Ag2O nanoparticles were prepared via the precipitation method. First, 4.5632 g of AgNO3 was added into a flask containing 400 mL of deionized water and vigorously stirred. Then, 2.5 molar NaOH solution was added to the AgNO3 solution and stirred for about an hour. The solution was kept for 24 h at room temperature to let the particles settle down. Then, the particles were recovered from the solution and washed several times with double distilled water. The particles were also washed with diluted acetone (v/v, 70:30) to remove residual impurities. The particles were recovered using centrifugation and dried at 70 °C for 12 h in an oven. The sample was ground into fine powder for further processing and analysis.

2.4. Synthesis of Ag2O/Ba/TiO2 Nanocomposite

The synthesized Ag2O and analytic grade TiO2 were mixed in 1:1 (0.432 g each) and transferred into 50 mL of BaCl2 solution (0.1 molar). The solution was directly calcined in a muffle furnace at 550 °C for h. The particles were recovered and washed several times with diluted acetone (v/v, 70/30). The particles were dried in an oven at 80 °C for 6 h and ground into a fine powder using a mortar and pestle. Ag2O/Ba/TiO2 was selected due to its high stability, good electrical and optical properties, cost-effectiveness, and nontoxicity of TiO2 [28]. Ag2O was chosen because of its low band gap (1.3 eV) and strong antibacterial properties [25,26]. Additionally incorporating Ba into Ag2O and TiO2 enhances ROS generation by inducing oxygen vacancies and polarization [27], improving the overall performance of the photocatalyst.

2.5. Characterization of Nanomaterials

The synthesized nanoparticles were characterized by different analytical techniques. FT-IR analysis of the dried AgO2/Ba/TiO2 was carried out using the KBr pellet method, and the presence of the various vibrational modes in the synthesized nanoparticles was investigated. The phase structure and identification of AgO2/Ba/TiO2 material were studied by X-ray diffractometer (Theta-Theta S/N 65022, STOE Germany, Darmstadt, Germany). The microstructure and morphology of the synthesized photocatalyst Ag2O/Ba/TiO2 composite were analyzed using SEM (Carl Zeiss, EVO LS, Jena, Germany). The SEM micrographs were acquired at different magnifications to examine the shape and distribution. The optical properties of the prepared Ag2O/Ba/TiO2 composite photocatalyst were analyzed by Perkin Elmer UV–vis diffuse reflection spectrometer (Waltham, MA, USA). The absorbance spectra were recorded in the 200 to 800 nm wavelength range to observe its bandgap and light absorbance properties.

2.6. Analytical Procedures

2.6.1. COD Determination

The COD of the SWW was determined using the SemiAutomated Colorimetry technique. The colorimetric determination was performed manually. Before their first use, all the sample tubes were screw capped with 20% H2SO4 to avoid contamination. To ensure quality control and prevent traces of contamination, all the tubes were ignited for one hour at 500 °C in a muffle furnace. The SWW was pipetted into 16 × 100 mm tubes in a volume of 2.5 mL. Then, 250 mL of reagent water was diluted to 500 mL with 1.5 mL of the digestion solution (K2Cr2O7, H2SO4, and HgSO4), and the mixture was thoroughly mixed. The mixture in the tubes was then spiked with 3.5 mL of a catalyst solution made of silver sulfate. The tubes were kept inside a block digester for 120 min at 150 °C. The tubes were taken out of the digester and cooled down. A Perkin Elmer Lambda 35 UV-vis spectrophotometer was used to measure the COD of the solution.

2.6.2. Solids Determination

The SWW was analyzed for its organic solids, including total solids (DS), volatile solids (VS), and fixed solids (FS). For this purpose, a crucible was weighed before SWW was added, and the data were recorded. The 18 mL of SWW was added to the crucible and placed inside in an oven at 105 °C for two hours to remove the moisture content. The crucible was weighed again after drying. The data were recorded, and the concentration of the solids was calculated using the following Equations (1)–(3):
T S   % = A − B C − B × 100 %
V S   % = A − D A − B × 100 %
F S   % = D − B A − B × 100 %
where A is the weight in mg of dried residue and crucible, B is the weight of crucible, C is the weight of wet sample and crucible, the weight of wet sample and crucible, and D is the weight of residue and crucible after ignition.
For determination of TS, the filter was first weighed and then 18 mL of the SWW was passed through the filter paper. The filter paper was dried in an oven at 70 °C for 2 h, and the filter, along with the residue, was again weighed. The data were recorded, and the TS was estimated according to Equation (4).
T D S = W i e g h t   a f t e r   d r y i n g − w i e g h t   o f   t h e   f i l t e r   p a p e r T o t a l   v o l u m e   o f   t h e   s a m p l e
The pH of samples SWW was measured by the glass electrode method. The process involved taking 10 mL of sample in the test tube and measuring the pH by dipping the probe into the sample and then noting the pH readings.

2.7. Colony Forming Unit Tests for Screening

The antibacterial activity of the synthetic Ag2O/Ba/TiO2 was evaluated for E. coli inactivation (ATCC 25922). Three different broth media were made for this purpose in the following ways: The first batch of broth media contained 1.0 mL of untreated SWW, the second contained 1.0 mL of SWW that was treated with photolysis using only light, and the third contained 1.0 mL of SWW that was treated with photocatalysis using Ag2O/Ba/TiO2 as the photocatalyst. These three-broth media were incubated for 24 h at 28 °C to check colony-forming units. The colony forming (CFU) was estimated using Equation (5).
C F U = N o . o f   c o l o n i e s × D i l u t i o n   f a c t o r V o l u m e   o f   c u l t u r e   p l a t e d

2.8. Antibacterial Activity Testing

Zone of Inhibition Assessment

The antibacterial activities of as-synthesized Ag2O/Ba/TiO2 nanocomposite were examined via E. coli ATCC 25922 inactivation. Before the experiment, the culture medium and the glassware were sterilized in an autoclave at 120 °C for 20 min, and all the tests were performed under sterile conditions. The zone of inhibition was evaluated via an agar well diffusion method. For the zone inhibition assay, sterile nutrient solid agar plates were first made separately. The E. coli suspension with 107 CFU/mL was used for inoculation. A sample of 1 g/L synthesized material was loaded into each well and incubated at 28 °C for 24 h. The zone of inhibition was measured by measuring the mean diameter around the AgO2/Ba/TiO2 sample in mm (2 mm-sized wells). These plates were then moved into an incubator for dark reactions and visible light.

2.9. Photocatalytic Experiments

2.9.1. Wastewater Degradation

The photoactivities of the synthesized AgO2/Ba/TiO2 sample prepared at different pHs were investigated through the COD removal of slaughterhouse wastewater under solar light. Firstly, the pH is maintained at 5, and then the sample is kept in the dark for 30 min so that the catalyst becomes attached to the pollutants. Then, it was kept in the solar light for 7 h under continuous stirring and aeration. Constantly, after every one hour, the sample was taken and stored in the sampling bottle, and all the experiment was completed in 7 h; after that, the samples were analyzed by using volatile solid and fixed solid and COD.

2.9.2. Bacterial Inactivation

The antibacterial activity of the synthesized AgO2/Ba/TiO2 composite was assessed using two different methods: vis well diffusion and spectrophotometric methods. The good diffusion method was used for antibacterial activity. Broth media were prepared to grow microorganisms found in the effluent from slaughterhouses. A wastewater sample of 1 mL was added to 300 mL of broth media, which was then incubated at 28 °C for 24 h. For the zone inhibition assay of freshly produced compounds, sterile nutrient solid agar plates were made separately. Prior to that, a prepared new culture of wastewater from a slaughterhouse was diluted ten times. Using a sterile cotton swab, the 10−6 times diluted solution was evenly dispersed across the entire nutritional agar plate. Using a sterile borer, 2 mm-sized wells on nutrient agar media on plates were created to observe the impact of catalyst materials on bacterial growth in nutritional agar plates. These plates were then moved into an incubator for dark reactions and visible light.
In the photocatalytic inactivation test, 4 mL of a diluted E. coli suspension containing 107 CFU/mL was added to the Pyrex reactor along with 396 mL of sterile saline water, and the temperature was raised to 37 °C to optimize the bacterial growth conditions. A nanocomposite concentration of 1 g/L was then added to the reactor. After that, the mixture was magnetically agitated in the dark for 30 min before exposure to solar light. The bacteria were separated, washed, and treated with sterile saline solution at predetermined intervals. Following sample dilution, 0.5 mL of the diluted suspension was pipetted onto the NA plates, spread out, and the plates were incubated for 24 h at 37 °C. Finally, bacterial colonies started growing on the NA plates.

2.10. Statistical Analysis

All the results are presented as average values followed by their standard deviation. Additionally, one-way analysis of variance (ANOVA), coupled with the Tukey test, was applied wherever applicable using Origin 8 (Origin Lab Corporation, Northampton, MA, USA) to evaluate significant differences (p < 0.05) among the results.

3. Results and Discussion

3.1. Structural and Spectroscopic Characteristics of Ag2O/Ba/TiO2 Nanocomposite

XRD patterns were used to assess the structure and crystallite size of the synthesized Ag2O/Ba/TiO2 nanocomposite (Figure 2a). The XRD of Ag2O shows various peaks at 2(θ°) values of 38.13°, 44.32°, 64.49°, and 77.40°, corresponding to the (111), (200), (220), and (311) planes, which are consistent with the previous findings [29]. XRD results revealed the face-centered cubic structure of Ag2O (JCPDS No. 04-0783). These peaks could also be seen in the Ag2O/Ba/TiO2 XRD patterns, indicating that the crystal structure of Ag2O was intact after combining with TiO2 and Ba. The peak observed in the composite at 25.96° corresponds to the anatase phase, specifically associated with the (101) crystal planes [30,31]. However, peaks found at 44.51°, 56.60°, and 68.85°, correspond to rutile phase, particularly associated with the (210), (220), and (301) crystal planes, respectively [32,33]. These results indicate that the XRD pattern confirms the presence of TiO2 mix containing both anatase and rutile phases. The XRD patterns also showed the formation of barium titanate (BaTiO3) with characteristic peaks at 22.07°, 31.62°, 38.90°, and 74.63°. These peaks correspond to the (001), (110), (111), and (103) planes, which is consistent with previous findings [34]. The formation of BaTiO3 is likely due to calcination at 550 °C, as high temperature facilitates diffusion of Ba into the TiO2 lattice [35]. Ag2O, BaTiO3, and TiO2 were validated by the XRD diffractogram, indicating that the solution approach was successfully employed for producing the nanocomposite.
To further substantiate the development of the Ag2O/Ba/TiO2 and Ag-O, stretching and bending vibration modes are responsible for the three different absorption bands identified at 889, 693, and 540 cm−1. The FTIR spectra of Ag2O/Ba/TiO2 and Ag2O photocatalysts is illustrated in Figure 2b. The adsorption bands, which are located at 572, 801, 1070, and 1342 cm−1, respectively, display the broadening and twisting Ag-O vibration modes. In the range of 700–500 cm−1, a very broad band is seen due to the Ti-O-Ti stretching vibration linked with Ti-O bond. Prominent absorption bands in Ag2O were observed mainly in the region of the lower wavenumber, possibly related to the stretching vibrations of metal–oxygen. The FTIR spectrum of Ag2O/Ba/TiO2 showed complex features, which particularly showed interactions between TiO2, Ag2O, and Ba. The additional absorption peaks observed in the Ag2O/Ba/TiO2 spectra suggested successful amalgamation of TiO2 and Ba, thus leading to the structural modifications and potential bond establishment. Furthermore, the intensity variations and shift in peaks endorse the functional groups and lattice structure alterations.

3.2. Morphology and Optical Properties of Ag2O/Ba/TiO2 Nanocomposite

The morphology of Ag2O/Ba/TiO2 is shown in Figure S1 at different magnification scales. The microstructures of Ag2O/Ba/TiO2 resemble a particulate or a granular-like morphology. The material presents a uniform clustered configuration, suggesting aggregated particles. The SEM revealed irregularly shaped particles with uneven surfaces and porous structures at the higher resolution. The overall heterogeneous textured particulate structures characteristically due to the different phases of Ag2O, Ba, and TiO2 and interactions among the components.
The UV-Vis absorbance spectra of synthesized materials are illustrated in Figure S2. The Ag2O spectrum expresses absorption in the visible region, representing its characteristic bandgap and light absorption wavelength. In the case of Ag2O/Ba/TiO2, the photocatalyst reveals a modified light absorbance profile, showing a potential shift towards the visible region, possibly due to the interaction of Ag2O/Ba with UV active TiO2. The Ag2O/Ba/TiO2 photocatalyst showed an absorbance edge at 422 nm, and thereafter, a higher absorbance was observed in the visible region. These variations indicate altered electronic characteristics and enhance visible light absorbance due to the formation of composite, thus, benefit in use of Ag2O/Ba/TiO2 photocatalyst to harvest solar energy.

3.3. Antibacterial Activity of Ag2O/Ba/TiO2 Nanocomposite

The activity of Ag2O/Ba/TiO2 nanocomposite for inactivation of E. coli was investigated. The zone of inhibition (retention of activity) of Ag2O/Ba/TiO2 nanocomposite for E. coli inactivation is displayed in Figure 3. According to Figure 3c, under visible light irradiation, 0.1 M Ag2O/Ba/TiO2 has the biggest zone of inhibition with a diameter of 4 to 9 nm, followed by 0.5 M Ag2O/Ba/TiO2 nanocomposite with a diameter of 2 to 6 nm (Figure 3a). The smallest zone of inhibition for Ag2O can be shown in Figure 3d, which indicates that silver oxide is only marginally effective at inactivating E. coli. When exposed to visible light, the nanocomposite outperformed individual photocatalysts regarding bacterial inactivation. It should be noted, nonetheless, that 0.5 M Ag2O/Ba/TiO2 performed poorly in comparison to 0.1 M Ag2O/Ba/TiO2, indicating that the concentration of the dopant material is crucial in determining the final product’s antibacterial properties. The synthesized materials’ antibacterial efficacy was also assessed in dark conditions.
As shown from Figure 4a, the Ag2O/Ba/TiO2 nanocomposite successfully inactivated E. coli even in the absence of light, indicating that the nanocomposite is suitable for bacterial inactivation in SWW. The E. coli grew well in the presence of Ag2O and Ag2O/TiO2 in the dark, demonstrating how adding Ba to the Ag2O/TiO2 altered the material’s antibacterial activity. The Ag2O/Ba/TiO2 nanocomposite performed better than all other photocatalysts tested in the current study in both light and dark conditions.
This may be mostly the result of the Ag2O/Ba/TiO2 nanocomposite particularly due to its intense light absorption and more effective photogenerated charge carriers’ separation, which may provide a high supply of ROS (such as O2•, H2O2, and •OH) that can be used to inactivate E. coli [36]. According to Shukla et al. [37], ions released by nanoparticles interact with thiol groups in proteins found on the bacterial cell membrane. Such proteins enable the transport of nutrients across the cell membrane of bacteria by protruding on the bacterial cell surface.
Rajeshkumar et al. [38] reports that microorganisms transmit a positive charge. This treats the cell membrane and induces an “electromagnetic” attraction between the bacteria. The current investigation shows that the E. coli is completely susceptible to the bactericidal effects of Ag2O/Ba/TiO2 nanocomposite. The Ag2O/Ba/TiO2 nanocomposite was used successfully to prevent the growth of the E. coli found in SWW. Bacterial inactivation was assessed using spectrophotometric method. The growth media was added to a tube and supplemented with a specific amount of different nanocomposite materials and inoculated with 105 CFU/mL E. coli in each tube. Following 24, 48, and 72 h, the optical density was assessed at 600 nm using a spectrophotometer microplate reader, as shown in Figure 5a,b, respectively. Among the nanocomposites, the optical density of the CFU treated with 0.1 M Ag2O/TiO2 was the lowest in both dark conditions and under visible light irradiation demonstrating that this composite has inhibited the growth of E. coli. The optical density obtained in all the treatment applied was significantly different compared to control under both dark and visible light conditions (p < 0.05).

3.4. Organics Removal from SSW Using Ag2O/Ba/TiO2 Nanocomposite

The ability of the synthesized Ag2O/Ba/TiO2 nanocomposite to remove organics from SWW, including COD, total solids, volatile solids, and fixed solids, was evaluated. Figure 6 depicts the photocatalytic removal of total solids from SWW. In 7 h of reaction in the dark, the Ag2O/Ba/TiO2 nanocomposite removed 38% of the total solids from SWW, demonstrating the outstanding adsorption performance of the synthesized photocatalysts.
The total solids removal rose to 73.07% when the Ag2O/Ba/TiO2 nanocomposite was exposed to visible light for 7 h, which is equivalent to the total solids removal efficiency (82.2%) using a hybrid system comprising a constructed wetland and UV lamp [39]. On the other hand, the addition of the nanocomposite with visible light did not result in a noticeably higher removal efficiency for volatile solids as illustrated in Figure 6. In both instances, employing photolysis and photocatalysts with the Ag2O/Ba/TiO2 nanocomposite present, the removal efficiency of volatile solids was over 87% suggesting that photolysis and photocatalysis are both potential methods for the removal of volatile solids from SWW; however, it has a poor service life due to the catalysts being poisoned by intermediates [40]., making it expensive According to a different study by Cervantes-Aviles et al. [41], the production of biogas caused the chemical breakdown of volatile solids within an 84-day period in the UASB process for both control and TiO2. It is significant to note that, as indicated in Figure 6, the removal of fixed solids using either photolysis or photocatalysis was the lowest of all solids. Only a 40% reduction in the fixed solids concentration was achieved. The removal of total solids was the highest; this can be attributed to the fact that total solids are made up of both solid particles like silt and plankton as well as dissolved salts like sodium chloride.
In order to further assess the synthesized nanocomposite’s viability for SWW remediation, the photoactivity of the material was tested for the removal of COD from SWW. Figure 7a,b illustrates the performance of the composite for COD removal from SWW in both light and dark conditions. As shown in Figure 7a, COD concentration significantly reduced by 96.84% from 219 mg/L to only 6.9 mg/L. The COD removal obtained by synthesized nanocomposite during photolysis was significantly different compared to photolysis (p < 0.05). Figure 7c shows information on pH levels affected by nanoparticles over time. Both photocatalysis and photolysis changed the pH of the SWW effluent with time. The pH during photocatalysis started off at 9.38, fell somewhat over time, and was then raised to 8.45 at the conclusion of the experiment. The pH value during photolysis was 8.2 at 0 h and 8.8 at 6 h. Our findings are consistent with those of Padmavathy et al. [42], who also found that the effectiveness of magnetic nanoparticles at removing trash decreased as PH increased.
Comparison of different Ag and Barium based photo catalyst for bacterial inactivation and degradation of organic compounds is given in Table 1. Most of the studies either focus on bacterial inactivation or degradation of organic compounds individually. However, very few studies have studied them simultaneously. Photocatalysis using a silver oxide-based catalyst can result in the leaching of Ag, as Ag is soluble in water. Ng et al. [43] reported that after three continuous cycles of Ag/TiO2, the Ag content decreased from 0.5 wt% to 0.28 wt%, confirming the leaching of Ag during photocatalysis. The leaching of Ag from the photocatalyst increases the band gap and reduces the overall photocatalytic efficiency. Likewise, a reduction in Ag content was detected in the TiO2NT/Ag photocatalyst due to leaching of Ag nanoparticles from surface of TiO2NT [44]. The leached Ag, when released into water bodies without recovery, results in secondary pollution and Ag ion toxicity, negatively affecting microorganisms, vertebrates, invertebrates, and the marine community. This results in various diseases and disorders [45,46]. Therefore, it is important to recover leached Ag prior to its disposal in the environment.

3.5. Kinetic Modeling

Figure 8 depict kinetic plots of the photodegradation of VS and COD. The degradation of VS and COD in the presence of Ag2O/Ba/TiO2 was examined using the zero order and pseudo-first-order kinetics using Equations (6) and (7), respectively [54,55].
C o − C = K a p p t
− l n C o C = K a p p t  
where Co and C are the final and initial concentration of VS and COD, respectively. Kapp is the rate constant (min−1). The fitting results of VS and FS degradation indicate that it follows pseudo first order kinetics as shown in Figure 8b. However, it was observed that degradation of TS followed zero order kinetics. On the contrary, the degradation of COD followed zero order kinetics with R2 of 0.98 as depicted in Figure 8c.
The rate constant of VS degradation for pseudo first order kinetics was 0.252 min−1, which was 6 times higher than the rate constant of 0.039 min−1 obtained for zero order kinetics. Moreover, the rate constant for TS degradation using photocatalyst was 0.05124 min−1, which is 2 times faster than degradation without photocatalysis. On the other hand, the constant of COD degradation for zero order kinetics was 0.543 min−1, which was 1.25 times higher than rate constant of 0.433 min−1 obtained for Pseudo first order kinetics.
Conventional wastewater treatment technologies, such as biological (constructed wetlands), chemical (coagulation, flocculation) and physical (adsorption) methods, are applied for organic removal but are less commonly used today [56]. These conventional treatment methods have many drawbacks, including high operational cost, slow process, complexity, and low removal efficiency [57]. Biological treatment approaches, while effective to a certain extent, are insufficient for the removal of COD or total organic carbon (TOC) [58]. Additionally, the efficiency of biological methods decreases with a change in environmental factors. Chemical treatment processes, on the other hand, require large amounts of chemicals, increasing the overall cost. Similarly, physical treatment methods only transfer the pollutants from one phase to another, leaving the pollutants in the environment without eliminating or degrading them completely [59]. For the removal of pathogens (bacteria and viruses) in wastewater, advanced oxidation processes (chlorination, ozonation), membrane technologies (filtration), adsorption and microbial purification are used. However, these methods face various challenges, such as unacceptable taste and odor, reduced disinfection efficiency, membrane fouling, high energy consumption, and solid waste generation [60].
To overcome these challenges, photocatalysis is a highly promising method for treating organic pollutants and achieving microbial inactivation [61,62]. This technique results in complete degradation of organic compounds into harmless byproducts, such as water (H2O) and carbon dioxide (CO2). Furthermore, it is an environmentally friendly technology that is less toxic, cost-effective, and requires only solar light, artificial light, or UV light along with a photoactive material to function efficiently. Herrera Melián et al. [63] studied phenol removal using TiO2 photocatalysis and constructed wetland. It was found that phenol removal was three to four times faster with TiO2 photocatalysis compared with constructed wetland. In another study, the photocatalytic inactivation of Clostridium perfringens spores on TiO2 electrodes was evaluated. It was reported that using a Degussa-Ti alloy electrode, 99.7% inactivation of 3 × 104 spores cm−3 was achieved in 2 h [64]. Notably, Clostridium perfringens spores are resistant to chlorine inactivation at concentrations commonly used in portable water supplies. Despite these promising benefits and extensive research, photocatalysis has not yet moved from lab-scale to pilot-scale application [65]. This limitation is mainly due to the high cost associated with reactor designing and the increased residence time and space velocities needed to effectively remove microbes and organic pollutants [66]. Therefore, further research is needed to overcome the above-mentioned limitation and facilitate the upscaling of this technology.

3.6. Upscaling and Environmental Application

The experimental study on removing COD from real slaughterhouse wastewater using Ag2O/Ba/TiO2 demonstrated excellent results, suggesting its potential suitability for large scale applications. Based on the study’s findings, a photocatalytic reactor with a working volume of 1 m3 containing slaughterhouse wastewater could remove 262 kg/m3/year of COD. The total amount of photocatalyst required for this process would be 1080 kg/m3/year. However, these values may vary under real environmental conditions, as they are based on the lab scale experimental data. Variations in the characteristics of slaughterhouse wastewater, concentration of COD, solar light intensity, pH, and other physicochemical parameters can significantly affect the results in real-world applications.
SWW contains high organic matter (BOD5 and COD), TS, and inorganic nutrients. The discharge of untreated SWW, rich in organic content and microorganisms (including pathogens), has a significant negative impact on the aquatic ecosystem [67]. It depletes dissolved oxygen in water bodies, leading to the loss of aquatic species [8]. Additionally, the presence of compounds such as ammonium nitrogen and chromium in SWW poses a direct threat to aquatic life [68]. Another important source of pollution is the addition of surfactants during cleaning processes in the meat processing industry. These surfactants cause both short-term and long-term environmental damage, affecting vegetation, fish, and humans. The environmental impacts of SWW are further characterized by the presence of pathogens, which persist in the soil and grow over time. Pathogens in SWW can be transmitted to humans through exposure to polluted water bodies, making the water unsafe for human use [69]. Therefore, effective treatment of slaughterhouse wastewater is essential to overcome these environmental impacts. The treatment of organics from real SWW in the present study using a visible light photocatalyst depicts environmental benefits. Further research should focus on pilot-scale using real wastewater to comprehensively evaluate the efficiency of the process.

4. Conclusions

The current study investigated the photocatalytic treatment of effluent from SWW using Ag2O/Ba/TiO2 nanocomposite. The nanocomposite was prepared using the precipitation method and characterized using XRD and FTIR. The nanocomposite showed excellent performance in terms of total solids, volatile solids, and fixed solids removal from SWW. The nanocomposite also had an impact on the removal of COD and bacterial strains from the SWW. When exposed to visible light, the nanocomposite outperformed individual photocatalysts regarding bacterial inactivation. The Ag2O/Ba/TiO2 nanocomposite also successfully inactivated E. coli even without light, indicating that the nanocomposite is suitable for bacterial inactivation in SWW.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/catal15050411/s1, Figure S1: Scanning electron microscopic images of Ag2O/Ba/TiO2; Figure S2: UV-visible spectra of Ag2O/Ba/TiO2 and Ag2O.

Author Contributions

Conceptualization, Methodology, Investigation, Formal analysis, Writing—original draft, H.U.; Methodology, Investigation, Formal analysis, Writing—original draft, I.E.; Writing—Review and Editing, S.S.; Conceptualization, Methodology, Writing—Review, R.N.; Methodology, S.U.; Methodology, Validation, Resources, S.Q.; Validation, Writing—Review and Editing, B.K.; Project administration, Writing—Review and Editing, M.A.; Methodology, Writing—Review and Editing, Y.L.; Formal analysis, Writing—Review and Editing, A.S.; Validation, Data curation, Writing—Review and Editing, A.M.I.; Supervision, Project administration, Funding acquisition, Writing—Review and Editing, Z.R. All authors have read and agreed to the published version of the manuscript.

Funding

This research was supported by the Key Research and Development Program of Zhejiang Province, China (2024C03125). The authors acknowledge the partial support provided by the Office of Research, Innovation and Commercialization (ORIC), PMAS Arid Agriculture University, Rawalpindi, Pakistan. The authors also extend their appreciation to the Deanship of Scientific Research at King Khalid University for funding this work through Large Groups Project under grant number RGP.2/25/46.

Data Availability Statement

Data is contained within the article and Supplementary Materials.

Conflicts of Interest

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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Catalysts 15 00411 g001
Figure 1. Schematic illustration of the synthesis of AgO2/Ba/TiO2 nanocomposite.
Figure 1. Schematic illustration of the synthesis of AgO2/Ba/TiO2 nanocomposite.
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Figure 2. (a) XRD patterns and (b) FTIR spectra of the synthesized Ag2O and Ag2O/Ba/TiO2 nanocomposite.
Figure 2. (a) XRD patterns and (b) FTIR spectra of the synthesized Ag2O and Ag2O/Ba/TiO2 nanocomposite.
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Figure 3. Zone of inhibition for E. coli inactivation under visible light using (a) 0.1 M Ag2O/Ba/TiO2, (b) 0.5 M Ag2O/Ba/TiO2, (c) Ag2O/TiO2, and (d) Ag2O.
Figure 3. Zone of inhibition for E. coli inactivation under visible light using (a) 0.1 M Ag2O/Ba/TiO2, (b) 0.5 M Ag2O/Ba/TiO2, (c) Ag2O/TiO2, and (d) Ag2O.
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Figure 4. Zone of inhibition for E. coli inactivation under dark conditions using (a) 0.1 M Ag2O/Ba/TiO2, (b) 0.5 M Ag2O/Ba/TiO2, (c) Ag2O/TiO2 and (d) Ag2O.
Figure 4. Zone of inhibition for E. coli inactivation under dark conditions using (a) 0.1 M Ag2O/Ba/TiO2, (b) 0.5 M Ag2O/Ba/TiO2, (c) Ag2O/TiO2 and (d) Ag2O.
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Figure 5. Optical density of E. coli in presence of Ag2O/TiO2 under dark (a) and visible light (b) condition.
Figure 5. Optical density of E. coli in presence of Ag2O/TiO2 under dark (a) and visible light (b) condition.
Catalysts 15 00411 g005
Catalysts 15 00411 g006
Figure 6. Solids removal from the SWW via photolysis (P) and photocatalysis (PC) using Ag2O/Ba/TiO2.
Solids removal from the SWW via photolysis (P) and photocatalysis (PC) using Ag2O/Ba/TiO2.
Figure 6. Solids removal from the SWW via photolysis (P) and photocatalysis (PC) using Ag2O/Ba/TiO2.
Solids removal from the SWW via photolysis (P) and photocatalysis (PC) using Ag2O/Ba/TiO2.
Catalysts 15 00411 g006
Catalysts 15 00411 g007
Figure 7. (a) COD concentration in SSW after the treatment, (b) COD removal from SSW and (c) pH of the SSW after treatment via photolysis and photocatalysis.
Figure 7. (a) COD concentration in SSW after the treatment, (b) COD removal from SSW and (c) pH of the SSW after treatment via photolysis and photocatalysis.
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Catalysts 15 00411 g008
Figure 8. Kinetic of solids degradation via photolysis (P) and photocatalysis (PC); zero order (a) and pseudo first order kinetics (b) and kinetic of COD degradation; zero order (c), and pseudo first order kinetics (d).
Figure 8. Kinetic of solids degradation via photolysis (P) and photocatalysis (PC); zero order (a) and pseudo first order kinetics (b) and kinetic of COD degradation; zero order (c), and pseudo first order kinetics (d).
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Table 1. Comparison of different Ag and Ba based photocatalyst for bacterial inactivation and organics degradation.
Table 1. Comparison of different Ag and Ba based photocatalyst for bacterial inactivation and organics degradation.
PhotocatalystPhotocatalyst Concentration (mg/mL)Bacterial Species/Organic PollutantLight IntensityOrganic Removal Bacterial Inactivation (%)Reference
Ag/TiO2 NCs1 Escherichia coli5 mWcm−2-100[47]
TiO2/TiO21
0.4		E. coli (ATCC 15597)
Methylene blue		10 mW/cm2
300 W ***		100100[48]
Ag/SCA/TiO2NTAs0.14 E. coli93.5 W/m2-2.99 × 102 *[49]
TiO2: Ba0.4
1		E. coli
Rhodamine B		-
450 W ***		100100[27]
Ag/TiO2-N3Acinetobacter baumannii300 W ***-100[50]
Ag NPs on FBP1E. coli--100[51]
Ag2O/TiO2-Zeolite0.5Norfloxacin6.7 mW·cm−298.7-[52]
TiO2 embedded AgO/Ag2O1Reactive Blue 220125W ***100-[53]
Ba-TiO20.2Methylene blueSolar light75-[35]
Ag2O/Ba/TiO21E. coli
COD		Solar light98.74 to 9 ** nmPresent study
* Bacterial inactivation in (CFU/(g·min)), ** Zone of inhibition in nm and *** represent power of bulb used to provide light.
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Ullah, H.; Elahi, I.; Saleem, S.; Nawaz, R.; Ullah, S.; Qadeer, S.; Kabeer, B.; Anjum, M.; Liu, Y.; Shahab, A.; et al. Bacterial Inactivation and Organic Pollutant Degradation in Slaughterhouse Wastewater Using Ag2O/Ba/TiO2 Nanocomposite. Catalysts 2025, 15, 411. https://doi.org/10.3390/catal15050411

AMA Style

Ullah H, Elahi I, Saleem S, Nawaz R, Ullah S, Qadeer S, Kabeer B, Anjum M, Liu Y, Shahab A, et al. Bacterial Inactivation and Organic Pollutant Degradation in Slaughterhouse Wastewater Using Ag2O/Ba/TiO2 Nanocomposite. Catalysts. 2025; 15(5):411. https://doi.org/10.3390/catal15050411

Chicago/Turabian Style

Ullah, Habib, Izhar Elahi, Sahar Saleem, Rab Nawaz, Shafi Ullah, Samia Qadeer, Bilal Kabeer, Muzammil Anjum, Yi Liu, Asfandyar Shahab, and et al. 2025. "Bacterial Inactivation and Organic Pollutant Degradation in Slaughterhouse Wastewater Using Ag2O/Ba/TiO2 Nanocomposite" Catalysts 15, no. 5: 411. https://doi.org/10.3390/catal15050411

APA Style

Ullah, H., Elahi, I., Saleem, S., Nawaz, R., Ullah, S., Qadeer, S., Kabeer, B., Anjum, M., Liu, Y., Shahab, A., Idris, A. M., & Rao, Z. (2025). Bacterial Inactivation and Organic Pollutant Degradation in Slaughterhouse Wastewater Using Ag2O/Ba/TiO2 Nanocomposite. Catalysts, 15(5), 411. https://doi.org/10.3390/catal15050411

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