Dye Degradation and Antibacterial Activity Studies of Green Synthesized CaO Nano Particles from Mint Leaves Forming Composites with Barium Strontium Titanate

Herbal plants were considered as one of the greatest gifts from nature that human beings could receive. About 80% of these plants have medicinal uses. For thousands of years, people across different cultures have relied on herbs as natural remedies to maintain health and treat illnesses. Long before modern medicine, herbal plants formed the foundation of traditional healing systems such as Ayurveda, Traditional Chinese medicine, and Indigenous folk medicine. These plants contain bioactive compounds like antioxidants, essential oils, and alkaloids that can support the body in various ways, from boosting immunity to aiding digestion and reducing inflammation. In today’s world, herbal plants continue to gain popularity as people seek natural and sustainable alternatives to synthetic drugs [1, 2, 3]. When used correctly and responsibly, herbal plants can play an important role in promoting overall health and well-being. Herbal plants have several useful applications in water purification, especially in traditional, rural and eco-friendly water treatment methods [4, 5, 6]. Their natural compounds can help remove impurities, reduce microbes and improve water quality. Many herbal plants contain antibacterial and antifungal compounds that help kill or inhibit harmful microorganisms in water. Mentha arvensis, commonly known as field mint, wild mint or Japanese mint, is an important aromatic and medicinal plant belonging to the family Lamiaceae. It is one of the most widely cultivated mint species, especially in Asian countries like India, China, and Japan, mainly due to its high menthol content. In traditional medicine, Mentha arvensis, commonly known as mint, has many applications. The plant holds great significance in traditional medicine, modern pharmaceuticals, flavouring industries, and organic chemistry because of its rich essential oil composition [7, 8, 9]. It is a perennial herb that grows to a height of about 30–60 cm. It has a creeping underground stem (rhizome) that allows vegetative propagation and rapid spread. The stem is erect, quadrangular, and usually covered with fine hairs, which is a characteristic feature of plants in the Lamiaceae family. The leaves are opposite, ovate to lanceolate in shape, with serrated margins and a strong minty aroma when crushed. The plant produces small purple to pale lilac flowers, arranged in axillary whorls. Flowering generally occurs during the summer season. The strong fragrance of the plant is due to the presence of essential oils stored in specialized glandular trichomes on the leaves and stems. Mentha arvensis grows well in moist, fertile soils and prefers a cool climate with adequate sunlight. In India, it is extensively cultivated in states like Uttar Pradesh, Punjab, and Bihar. It has been used for centuries in traditional systems of medicine such as Ayurveda, Unani, and folk medicine. The therapeutic value of Mentha arvensis is mainly attributed to its essential oil, which is rich in menthol and other bioactive compounds. Due to its wide range of pharmacological actions, the plant plays an important role in both traditional and modern healthcare. One of the most important medicinal uses of Mentha arvensis is in the treatment of digestive disorders. The leaves and essential oil act as a carminative, helping to relieve flatulence, indigestion, and abdominal discomfort. Menthol relaxes the smooth muscles of the gastrointestinal tract, reducing spasms and improving digestion. Decoctions and infusions prepared from the leaves are commonly used to treat stomach pain, nausea, and loss of appetite. It is also is widely used in the management of respiratory ailments such as cold, cough, bronchitis, and nasal congestion [10]. Menthol produces a cooling and soothing effect on the respiratory tract and acts as a mild expectorant, helping to clear mucus from the airways. Mint oil is a key ingredient in inhalers, vapor rubs, and cough syrups, where it provides relief from congestion and irritation of the throat. The essential oil of Mentha arvensis exhibits significant analgesic and anti-inflammatory properties. When applied externally, menthol stimulates cold receptors in the skin, producing a cooling sensation that helps reduce pain. This property makes it useful in the treatment of headaches, muscle pain, joint pain, and minor injuries. Mint oil is commonly used in balms and ointments for relief from muscular aches and rheumatic pain. The aroma of Mentha arvensis has a calming and refreshing effect on the nervous system. Inhalation of mint oil is known to reduce mental fatigue, improve alertness, and relieve stress. It is also used to treat headaches and migraines, either by inhalation or by topical application on the forehead and temples. The mild sedative action of the plant helps in reducing anxiety and promoting relaxation. Due to its cooling, soothing, and antiseptic properties, Mentha arvensis is widely used in skin care preparations. It helps relieve itching, irritation, and inflammation caused by insect bites, rashes, and mild burns. The plant extracts are also used in cosmetic products to refresh the skin and control excess oil production. Mentha arvensis possesses antimicrobial and antiseptic properties due to the presence of menthol, menthone, and other terpenoid compounds. Extracts of the plant have been shown to inhibit the growth of several bacteria and fungi. Because of this activity, mint oil is used in oral hygiene products such as toothpaste and mouthwashes to prevent bad breath, dental infections, and gum diseases. It is also used in minor skin infections and wounds to prevent microbial growth [11, 12]. The present study, the mint leaf extract has been used to synthesis nanoparticles using the mint leaf extract as a biosource for the extraction of CaO nanoparticles. Calcium oxide (CaO) nanoparticles are also considered to be safe for human use. Environmental pollution, particularly water contamination by industrial dyes and microbial pathogens, is a global concern. Traditional remediation methods often involve harsh chemicals and generate secondary pollutants. Nanotechnology, specifically metal oxide nanoparticles, offers a promising eco-friendly alternative. Calcium oxide, commonly known as quicklime or burnt lime, is an important inorganic compound widely used in industries, construction, agriculture, and environmental applications. It is a white, caustic, alkaline solid obtained by the thermal decomposition of calcium carbonate. Due to its high reactivity, especially with water, calcium oxide plays a vital role in many chemical and industrial processes. Calcium oxide (CaO) nanoparticles are critically important in wastewater treatment due to their multi-functional roles as photocatalysts, adsorbents, and antimicrobial agents. Their high surface-area-to-volume ratio and unique physicochemical properties allow them to effectively degrade persistent organic pollutants and remove toxic inorganic substances that conventional methods struggle to eliminate [13,14, 15, 16]. A primary application of CaO nanoparticles in wastewater treatment is the degradation of synthetic organic dyes used in the textile, leather, and tanning industries. These dyes are often non-biodegradable and hazardous to aquatic ecosystems [17]. Research shows that CaO nanoparticles can achieve up to 98% removal of Methylene Blue and significantly degrade other dyes like Indigo Carmine and Bromocresol Green [18,19]. CaO nanoparticles serve as effective adsorbents for removing heavy metals from contaminated water, which is vital because these metals are non-biodegradable and can bioaccumulate [20]. The nanoparticles provide a massive surface area for metal ions to adhere to, facilitating their extraction from industrial effluents. Studies highlight their effectiveness in removing Chromium (Cr), Mercury (Hg), and Lead (Pb) from aqueous solutions. CaO nanoparticles are essential for the purification of drinking water by acting as a powerful antimicrobial tool against fecal indicators and waterborne pathogens. They are highly effective against bacteria such as E. coli (a primary pathogen of the human gut) and S. aureus [21]. Because they are considered safe for humans and animals, they are a promising candidate for decontaminating drinking water supplies at a low cost. Compared to traditional water treatment measures like membrane filtration or chemical precipitation, CaO-based treatment offers several benefits. They can be synthesized through “green” routes using biological waste like eggshells, molluscan shells, or plant extracts (e.g., mint or broccoli), reducing the need for expensive and hazardous chemicals. Unlike some chemical treatments that produce toxic byproducts, photocatalysis using CaO converts pollutants into benign substances. Photocatalytic techniques are relatively simple to implement and can reduce the half-life of toxic dyes from hours to just minutes. Barium Strontium Titanate (BST) — chemically Ba???Sr?TiO? is a perovskite-type oxide semiconductor formed by substituting some of the Ba²? in barium titanate (BaTiO?) with Sr²? (strontium) ions. It retains the ABO? perovskite crystal structure and exhibits tunable electrical and optical properties (e.g., band gap, polarization, charge separation) depending on the Ba/Sr ratio. This tuning makes BST a semiconductor photoactive material suitable for photocatalytic applications, especially in environmental remediation and pollutant degradation, where light-driven generation of electron-hole pairs initiates redox reactions. BST materials can act as photocatalysts under UV or visible light to degrade organic dye pollutants (e.g., methylene blue, tetracycline hydrochloride) in water [22, 23, 24]. The photocatalytic efficiency depends on composition, crystal structure, and charge

Experimental

1. Green Synthesis of CaO nanoparticles

Fresh mint leaves were collected, washed with distilled water and then transfer it to a beaker with 200 ml distilled water. The beaker is then boiled for 30 minutes in a water bath and the boiled suspension is then filtered through whatmann No: 1 filter paper. The filtrate was cooled at room temperature. 10 ml of this mint leaf extract was mixed with 10 ml Calcium nitrate solution and keep this mixture in a magnetic stirrer and stirred for 30 minutes. During stirring, add NaOH solution in drops resulting in the formation of a precipitate. The precipitate is then dried in oven for half an hour and washed with distilled water to remove impurities. The resulting CaO nanoparticles was then calcined for 3 hours at 500⁰C [16].

(b) Synthesis of Barium Strontium Titanate (BST) nanoparticles

0.005 moles of barium acetate and strontium acetate were dissolved in minimum quantity of glacial acetic acid .2 ml diethanol amine, 1.42 g titanium tetra isopropoxide were dissolved separately in 50 ml isopropyl alcohol stirred well and added to the above solution. It is then dried, powdered well and then sintered at 1000⁰C. Thus, we prepared Barium strontium titanate [25].

(c) Synthesis of CaO–BST Composites

Mixing CaO and BST powders and then anneal and mix well to form composites.

RESULTS AND DISCUSSION

Figure 1: XRD spectra of (A) mint leaf mediated CaO nanoparticles (B) undoped Barium Strontium Titanate.

In figure 1(A) the XRD pattern of the synthesized mint leaf mediated CaO nanoparticles exhibited prominent diffraction peaks at 2θ values of approximately 28⁰, 32.8°, 37.3°, 54°, 64.1⁰ and 67.3⁰ corresponding to the (111), (110), (200), (220), (311) and (222) planes of cubic CaO (JCPDS No. 37-1497).  For pure Barium Strontium Titanate (figure 1(B), peaks at 2θ values of 22.36, 31.92, 39.35, 45.87, 51.66, 57.09, 66.9, 76.17 corresponds to (100), (110), (111), (200), (201), (211), (220) and (310) phases respectively are obtained from the X-ray diffraction pattern.

Figure 2: FTIR soectrum of spectra of (A) mint leaf mediated CaO nanoparticles (B) undoped Barium Strontium Titanate.

The FTIR spectrum of the synthesized CaO nanoparticles and Barium Strontium titanate were recorded in the range of 4000-400 cm?¹ as in figure 2(A) and (B) respectively. For mint leaf mediated synthesised CaO nano particles, a broad band centered around 3437 cm?¹ corresponds to O–H stretching vibrations of surface hydroxyl groups and adsorbed moisture. A weak band near 1420 cm?¹ indicates the presence of carbonate species formed due to atmospheric CO? adsorption. The peak at 865cm^-1shows the presence of Ca-O-Ca bond.  The presence of minor organic-related phytoconstituents like carboxylic group and amides were confirmed by the bands at 2930 cm^-1 and 1420 cm^-1. Fore pure Barium Strontium Titanate, the band between 500-600 cm^-1 represents the Ti-O stretching modes in TiO6 octahedra. There is an OH stretching band at 1360 cm^-1 and a C=O stretching band at 1730 cm^-1. Also C-H stretching vibrations corresponds to 2800 and 2900 cm^-1 [26].

Figure 3: UV-Visible absorption spectra of (A) mint leaf mediated CaO nanoparticles (B) Barium Strontium Titanate nanoparticles.

UV-Visible spectrum of CaO nanoparticles, figure 3(A), shows a prominent absorption band around 265 nm corresponding to intrinsic band gap transition, but no absorption was found in visible region. The visible region corresponds to energies of 1.8 to 3.1 eV but CaO has a wide band gap of 7 eV. So visible light does not have enough energy for exciting electrons from valence band to conduction band and hence no electronic transition occurs there [27, 28]. The UV-Vsible spectrum of BST, figure 3(B), exhibited a strong absorption edge in UV region around 290 nm corresponding to a band gap of about 3.0 eV. No significant absorption was observed in visible region [28].

Figure 4: Photocatalytic degradation curves of 10^-4M Methylene blue dye using mint leaf mediated CaO nano particles, BST (Barium Strontium Titanate) under visible light irradiation in absence and presence of sunlight.

[MB4 - Methylene blue of 10^-4M concentration stirred in dark, MBS4 - Methylene blue of 10^-4M concentration irradiated in sunlight, MB4+BST - Methylene blue of 10^-4M concentration stirred in dark with Barium strontium titanate, MBS4+BST - Methylene blue of 10^-4M concentration irradiated in sunlight with Barium strontium titanate, MB4+CaO - Methylene blue of 10^-4M concentration stirred in dark with CaO, MBS4+CaO-Methylene blue of 10^-4M concentration irradiated in sunlught with CaO, MBS4+BST+CaO -Methylene blue of 10^-4M concentration irradiated in sunlight with Barium strontium titanate and CaO.] The photocatalytic degration studies of CaO, BST and CaO-BST composites against 10^-4M methylene blue were given in Figure 4 A, B, C and D. Methylene blue usually shows a weak absorption band around 295 nm in the Ultra violet region as a result of higher energy electronic transitions [29]. In the visible region, it shows a shoulder band around 610 nm due to dimer formation in solution. A characteristic absorption maximum was observed around 665 nm, which is the principal electronic transition and the reason for intense blue colour [30]. When Methylene blue solution of 10^-4M is undergone photocatalytic treatment with CaO nanoparticles, the intensity of characteristic absorption peak of methylene blue at 665 nm decreases. Upon light irradiation, CaO nanoparticles can generate reactive oxygen species like hydroxy and superoxide radicals that attacks the conjugated chromophore structure of methylene blue thereby leading to mineralization of the dye [31]. But the photocatalytic efficiency of Barium strontium titanate nanoparticles towards methylene blue degradation is found to be lower than CaO nanoparticles which may be due to faster electron-hole recombinationrates, lower surface basicity and reduced dye adsorption capacity of BST. Also, the generation of reactive oxygen species is less effective leading to slower degradation rates. CaO-BST composites exhibits higher activity than BST but lesser than CaO alone. This is due to the strong surface basicity and increased dye adsorption provided by CaO for BST. But the overall activity of CaO-BST composite is lower than pure CaO due to dilution of more active phase.

Figure 5: Photocatalytic degradation curves of 10^-5M Methylene blue dye using mint leaf mediated CaO nano particles, BST (Barium Strontium Titanate) under visible light irradiation in absence and presence of sunlight.

The degradation is higher against dilute dye solution as in figure 5 by BST, CaO and CaO-BST. This is due to greater availability of catalyst surface sites relative to dye molecules, improved penetration of light through solution and more effective utilization of reactive oxygen species.

Table 1: Inhibition zone diameter (in mm) of CaO, BST and CaO-BST against Escherichia coli.

Figure 6: Images of agar plates showing results of antibacterial activity against Escherichia coli by (A) CaO nanoparticles (B) Barium strontium titanate (C) CaO-BST composites.