Introduction

Hygienic water is currently a widespread global issue. Safe drinking water is both a survival necessity and an ecological niche. Yet, negligence and mismanagement of water reservoirs have put the availability of fresh water at risk. Every year, millions of people die as a result of contaminated water and waterborne diseases (Kumar et al. 2022). Waterborne infections are undeniably caused by microorganisms. Coliform germs are a good indicator of current fecal excretion. Waterborne microbial infections are one of the leading causes of death (Luo et al. 2022).

Inorganic elements and heavy metals, although not very harmful in small amounts, can act as toxic waste in water over time. Organic contaminants, on the other hand, enter water bodies in similar ways: intentional dumping, leaching, industrial disasters, and human activities (Astuti et al. 2023).

This type of polluted water, especially in areas with high water intake, can cause serious health problems and even death in humans and other living things. In addition, organic pollutants and toxic substances from industrial wastes cause an acute stalemate between environmental and health problems. Aquatic fauna and flora are affected by trace amounts of dyes, as they are carcinogenic by nature. In particular, cationic dyes are more toxic than anionic dyes due to their aromatic ring arrangement with delocalized electrons and their synthetic nature (Karthigadevi et al. 2021). Moreover, the presence of these dyes reduces the productivity of agricultural land (Singh et al. 2021a).

Removing dye molecules from polluted water is now a top priority before it enters waterways. Considering the expected global demand for clean water, especially in Saudi Arabia, there is a strong push to develop low-cost, high-efficiency wastewater treatment systems and materials (Balabel and Alwetaishi 2021).

Various methods such as membrane separation, coagulation, photocatalysis, ion exchange, electrocoagulation, and adsorption are used to purify polluted water (Singh et al. 2021b; Sajjadifar 2017, Jabbar et al. 2022; Zhang et al. 2021; Al-Juboori et al. 2022; Shokri and Fard 2022; Rashid et al. 2021; Hiawi and Ali 2022). Among all available methods, adsorption is considered a cost-effective and easy-to-use approach for the purification of polluted water and can be applied from small residential units to large industrial facilities (Koh et al. 2022). A variety of conventional adsorbents, from activated carbon to inorganic wastes, have been investigated for wastewater treatment by a number of researchers worldwide (Lotfy and Roubik 2021; Lewoyehu 2021; Raza et al. 2022). However, most conventional adsorbents have non-uniform pore structure and low specific surface area, resulting in low adsorption capacity (Dou et al. 2022).

Recently, the use of nanomaterials such as nanoadsorbents, polymeric nanocomposites, and metal-doped nanomaterials has gained tremendous attention due to their unique properties that have a large number of active sites (Adeleye et al. 2016). A number of metal oxides such as zinc oxide, zirconium oxide, titanium oxide, binary metal oxides, and a combination of different metal oxides have been used to purify contaminated water and were found to be very effective compared to conventional adsorbents (Wang et al. 2020). Among various nanomaterials, alumina-based nanostructures are widely used due to their high pollution efficiency, cost efficiency, low energy intensity, and low environmental impact (Xie et al. 2015). Aluminum-based nanostructures are composed of metallic aluminum and alumina. Alumina nanoparticles are widely used for various applications, including biomedicine, drug delivery vehicles, ophthalmology, electronic devices, and as catalysts in various industrial processes (Roy 2021; Hassanpour et al. 2018; Wang et al. 2016; Yang et al. 2018; Atrak et al. 2018).

Metallic aluminum nanoparticles are very reactive and cannot be used in water treatment processes. However, the use of nano-Al2O3/hydrolyzed Al2O3 as adsorbents to regulate harmful ions in wastewater is very common (Ravindhranath and Ramamoorty 2017). The Al2O3 nanoparticles have shown versatile adsorption properties in decontamination of water/wastewater due to their large surface area, availability of surface groups to bind the pollutants and high surface energy (Sharma et al. 2021). The United Nations Environment Program (UNEP) has also recommended alumina-based adsorption as one of the best waste treatment technologies currently available (Prabhakar and Samadder 2018).

The literature survey described the synthesis of silver-doped alumina composites with antibacterial properties using sodium alginate as sacrificial template for the development of new biomedical applications. Alumina is a biocompatible and thermally stable ceramic, while silver was chosen for its antibacterial properties (Roque-Ruiz et al. 2018). Another study has reported two different routes for the synthesis of silver-doped anodic alumina nanocomposites with antimicrobial properties using electrochemical deposition and thermal reduction (Kiradzhiyska et al. 2022). Also, an addressed study showed the antimicrobial activity and sorption behavior of alumina/silver nanocomposites.

At the same time, Ag nanoparticles are being intensively researched (Maleki et al. 2015; Baghayeri et al. 2018) due to their unique optical, thermal, electrical, and biological properties (anticancer, antifungal, antiviral, and antibacterial activity). The excellent inhibitory activity of nano silver against many bacteria, including E. coli, M. luteus (Jagminas et al. 2015), K. pneumoniae, and B. pumilus (Mokhena and Luyt 2017), is well documented in the literature. Nanoscale species have higher bactericidal activity than microscopic particles, and when they are less than 10 nm in size, they can directly affect cell permeability. The significantly increased surface area of small particles improves the antibacterial activity and bioavailability of biomaterials (Calovi et al. 2021).

The chemical preparation of silver-doped alumina nanostructures with different phase compositions from metal salts has been studied (Zhou et al. 2012; Aparna et al. 2019; Kiseleva et al. 2019; Nasrollahzadeh et al. 2019). Various organic initiators and stabilizers were used to achieve high yield of reaction products and fixation of silver nanoparticles on the alumina surface. Nanostructures with high antibacterial activity were prepared by this method. One of the major drawbacks of chemical synthesis is the different phases and the formation of undesirable byproducts, the removal of which requires further procedures. In addition, the organic stabilizers used are often hazardous. Moreover, the conventional physiochemical processes for nanoparticle synthesis are time-consuming, expensive, require harmful substances, and are not environmentally friendly (Khatami and Iravani 2021). Therefore, the use of plant/plant waste extracts as reducing, capping, and stabilizing agents for the preparation of nanoparticles is more biocompatible and has lower sorption capacity compared to conventionally synthesized nanoparticles (Shameli et al. 2021; Rajoriya et al. 2021). Moreover, these processes are simple and rely on naturally occurring, nontoxic components in the abundant flora in our environment, making the complex syntheses tame (Angulo-Bejarano et al. 2021).

Coffee can be processed either dry or wet, producing a variety of byproducts. Dry processing is the most common process, with the main waste being the husk, which accounts for about 50% of the weight of coffee beans (Tizian et al. 2020). Coffee husks are a rich source of secondary metabolites such as caffeine and tannins, as well as phenolic acids such as chlorogenic acid, making them a promising resource for bioactive components for use in food. Chlorogenic acid is found throughout the plant kingdom, with coffee and its byproducts being the most prolific source (Janissen and Huynh 2018). These phenolic compounds are biologically active and have been associated with their antioxidant, antibacterial, and anticarcinogenic properties (Tungmunnithum et al. 2018).

The search for novel disinfection agents has become a critical topic in light of harmful pathogenic resistance in the water. The constant emergence of bacterial resistance has led researchers to develop new disinfectants. Among the most promising of these agents are metallic nanoparticles, which have shown excellent antibacterial activity in numerous studies. Even when new materials are introduced to the market, antibiotic-resistant bacteria appear in a relatively short time. However, nanoparticles with antibacterial activity are believed to have the potential to minimize or prevent the development of more resistant bacteria because nanoparticles target many biomolecules simultaneously, preventing the formation of resistant strains. Various mechanisms of antibacterial action of different nanoparticles have been reported, including interaction and regulation, ATP deprivation, and membrane interaction (Rahman et al. 2021; Aadil et al. 2021a, 2021b, 2022; Sumra et al. 2022).

Moreover, Dar et al. 2021 synthesized several transition metal oxides at low temperatures and examined their antibacterial efficacy against multidrug-resistant bacterial infections (Dar et al. 2021). Bioactive glass and poly (vinyl alcohol) composites produced by melting and sol–gel methods were recently used as antibacterial materials for disinfection in another study (Maximov et al. 2021; Asadpour et al. 2022).

Metal/oxide nanomaterials, carbonaceous nanomaterials, zeolites, and dendrimers are the most common nanoscale materials used as functional materials for water decontamination. These materials have a variety of physicochemical properties that make them attractive for use as separation and reaction media for water treatment (Chakraborty et al. 2021; Jimenez-Reyes et al. 2021).

The aim of this work is to propose a facile synthesis of silver-doped alumina nanocomposite using coffee husks, and the formed nanocomposite was characterized by various microscopic and spectroscopic techniques. The green synthesized silver-doped alumina nanocomposite using coffee husks was used for the first time as an antipathogenic agent for decontamination of wastewater. The antibacterial activity was investigated against two bacterial strains Escherichia coli and Salmonella typhimurium. The adsorption kinetics and isotherms were also studied.

Materials and methods

Chemicals and reagents

The chemicals aluminum nitrate nonahydrate (≥ 98.0%), ethanol (≥ 99.5%), silver nitrate (≥ 99.0%), and hydroxide sodium (99.9%) were purshaed from Sigma-Aldrich (Hambrug, Germany).

Botanical material

The coffee husks were collected from a local supermarket in Riyadh, Saudi Arabia, in December 2021. The botanical material was identified and authenticated by a taxonomist from the Department of Pharmacognosy at College of Pharmacy, King Saud University. A voucher specimen (CH-0421) was deposited in the same department.

Preparation of coffee husk extract

Coffee fruit husks were collected, washed thoroughly with deionized water, and air dried for 24 h. About 10 g of coffee husk powder was mixed with 200 mL of deionized water and magnetically stirred at 80 °C for 3 h. The resulting extract was set aside to cool at room temperature. It was centrifuged at 2500 rpm for 5 min and filtered using Whatman No. 41 filter paper. The obtained extract was then stored in a tight container in the refrigerator at 4 °C for further use (Scheme 1).

Scheme 1
scheme 1

General steps of coffee husk extract preparation

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Preparation of Ag NPs, Al2O3 NPs, and Ag-doped alumina nanocomposite

To prepare Ag NPs, 20 mL of coffee husk extract was placed in a volumetric flask and heated to 80 °C for 10 min under magnetic agitation (450 rpm). An appropriate amount (100 mL of 5.0 × 10–3 mol L−1, 0.0849 g) of silver nitrate solution was added to the heated extract. The pH was optimized to 10 with a few drops of sodium hydroxide (0.2 mol L−1, 0.799 g/100 mL). The brownish color of the mixture showed the conversion of Ag+ to Ag* nanoparticles (Zuorro et al. 2022).

Alumina nanoparticles (Al2O3 NPs) were synthesized green by mixing 10 mL of coffee husk extract with 50 mL of 2.0 mol L−1 aluminum nitrate nonahydrate (75.028 g/ 100 mL) under constant magnetic stirring for 30 min. A brownish Al2O3 precipitate was formed. The formed Al2O3 NPs were centrifuged at 2500 rpm for 5 min and filtered with Whatman filter paper No. 41. The resulting Al2O3 NPs were dried and stored in a tight container for further study (Manikandan et al. 2019).

To remove impurities, the precipitated particles were washed two or three times with ethanol. The powder particles were stored in a hot-air oven at 100 °C for one day and then in a muffle furnace at 400 °C for 3 h. Finally, the particles were ground into fine nanoparticles using a mortar and pestle. For the green synthesis of silver-doped alumina nanoparticles, 20 mL of coffee peel extract was mixed with 50 mL of deionized water containing silver nitrate (4.0246 g) and aluminum nitrate nonahydrate (37.514 g) to produce 0.5 and 2.0 mol L−1, respectively. The mixture was stirred magnatically for 2 h. The pH of the mixture was optimized to 12 by adding sodium hydroxide pellets. The formed silver-doped alumina nanoparticles were centrifuged and the same filtration and drying steps as above were performed for the synthesis of alumina nanoparticles (Nasrollahzadeh et al. 2019) (Scheme 2).

Scheme 2
scheme 2

General steps of Al2O3 NPs and Ag-doped alumina nanocomposite preparation

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Characterization of Al2O3 NPs and Ag-doped alumina nanocomposite

Spectrophotometric (UV–Vis) characterizations of the synthesized Al2O3 NPs and Ag-doped alumina nanocomposite were measured using the Ultrospec 2100 biochrom spectrophotometer (Biochrom Ltd., Cambium, Cambridge, UK). Fourier transform infrared spectroscopy (FTIR) (PerkinElmer, Yokohama, Japan) was performed to monitor the surface functional groups of the synthesized Al2O3 NPs and Ag-doped alumina nanocomposite. The elemental composition of Al2O3 NPs and Ag-doped alumina nanocomposite was investigated at 20 kV using an energy dispersive X-ray device (EDX, JSM-7610F; JEOL, Tokyo, Japan). Scanning electron microscope (SEM, JEOL Ltd., Tokyo, Japan) and transmission electron microscope (TEM, JEM-1400, JEOL Ltd., Tokyo, Japan) were used to study the surface morphology of the synthesized nanoparticles. The surface morphology of Al2O3NPs and Ag-doped alumina nanocomposite was measured using JEM-2100F scanning electron microscope and JEM-1400 transmission electron microscope (JEOL Ltd., Tokyo, Japan).

Antibacterial activity

E. coli (ATCC 25922) and Salmonella typhi (ATCC 14028) were taken from the American Type Culture Collection (ATCC) and used as model pathogens in the current study to measure the antibacterial potential of Ag-dopped alumina nanocomposite. The bacterial strains were grown on agar plates and the freshly obtained colonies were inoculated into 100 mL of tryptone-soya broth. Using a UV–vis spectrophotometer, growth was monitored every 4 h until the optical density reached 0.1 at 600 nm (OD of 0.1 corresponded to a concentration of 108 CFU/mL medium). The minimum inhibitory concentration (MIC) of the pre-synthesized Ag-dopped alumina nanocomposite was tested by inoculating 1.0 mL of a fully grown culture with 100 mL of a freshly prepared culture broth containing different concentrations (6, 12, 24, and 30 µg mL−1) of the Ag-dopped alumina nanocomposite. Pathogenic cultures were grown under dark conditions and treated with Ag-dopped alumina nanocomposite samples as described above. All culture flasks were incubated in a rotary shaker at 37 °C and 150 rpm. Concertation of bacterial strains was determined by optical density (OD) at 600 nm absorbance at 4 h intervals over 16 h using a UV spectrophotometer. E. coli and S. typhi without the addition of Ag-dopped alumina nanocomposite were treated as control samples.

Determination of bacterial toxicity

The fluorescence staining method was applied to measure the number of dead and live bacteria at specific time intervals using a dead/live BacLight bacterial viability kit. The E. coli and S. typhi bacterial strains were stained with fluorescent dyes obeying the manufacturer’s instructions and were monitored under confocal laser scanning microscopy (CLSM) as previously described (Alovisi et al. 2022).

Adsorption of Indigo carmine dye on Ag-doped alumina nanocomposite

The batch experiment study was used to evaluate the activity of Ag-doped alumina nanocomposite in removing IGC from water. The effect of contact time and kinetics study were performed by treating 200 mL of IGC dye solution (160 mg L−1) with 85 mg of Ag-doped alumina as sorbent under continuous stirring. Then, 10 mL of the reaction mixture was withdrawn and filtered, and the absorbance of IGC was recorded by UV–vis spectroscopy. The adsorption capacity in each time interval was calculated using Eq. (1), and the best sorbent was selected for further study.

$${q}_{t}=\frac{({C}_{0}-{C}_{t})\times \mathrm{V}}{\mathrm{M}}$$
(1)

where C0 and Ct represent the IGC dye concentration (mg L−1) at zero and t time, and V and M represent the volume (L) IGC dye solution and mass (g) of sorbent Ag-doped alumina nanocomposite, respectively.

The results obtained were applied to evaluate the adsorption kinetics of IGC. Pseudo 1st order model (FOM) and 2nd order model (SOM) kinetics were used to estimate the sorption rate by applying Eqs. (2) and (3).

$$\mathrm{ln}\left({\mathrm{q}}_{\mathrm{e}}-{\mathrm{q}}_{\mathrm{t}}\right)=\mathrm{ln}\left({\mathrm{q}}_{\mathrm{e}}\right)-{k}_{1}\times \mathrm{t}$$
(2)
$$\frac{1}{{q}_{t}}=\frac{1}{{k}_{2}{q}_{2}2t}+ \frac{1}{{q}_{e}}$$
(3)

However, the step involved in controlling adsorption, intra-particles model (IM), and liquid film model (LM) diffusion was analyzed by applying Eqs. (4) and (5).

$${q}_{t}={K}_{IM}\times {t}^{1/2}+{C}_{i}$$
(4)
$$\mathrm{ln}\left(1-\mathrm{F}\right)=-{K}_{\mathrm{LM}}\times \mathrm{t}$$
(5)

where K1 and K2 represent the rate constant of FOM (min−1) and SOM (g mg−1 min−1); KIM (mg g −1 min−0.5 and KLM (min−1): the IM and LM are constants, the qe and Ci represent the equilibrium adsorption capacity (mg g−1) and boundary layer thickness (mg g−1) parameters.

In addition, IGC adsorption on the Ag-doped alumina nanocomposite was investigated at different pH values. The IGC solution (160 mg L−1) was adjusted to a pH between 2 and 10. Two hundred milliliters of the optimized IGC solution was treated with 85 mg of the Ag-doped alumina nanocomposite under constant stirring until equilibrium, and the remaining reaction mixture was used as a reference standard.

Adsorption equilibria

The effect of the initial concentration of IGC on its removal by the good sorbent was studied using different IGC concentrations (40, 80, 120, and 160 mg L−1), and the effect of temperature on IGC removal was studied at different temperatures (30, 40, and 50 °C). The obtained results were used to analyze the sorption isotherms and thermodynamics.

Application to natural water samples

The seawater sample was collected from the coast of Jeddah, Saudi Arabia. The water sample was mixed with an appropriate amount of IGC solution (160 mg L−1) to obtain a concentration of 10 mg L−1. The contaminated seawater was treated with 85 mg Ag-doped alumina nanocomposite under constant stirring at room temperature, and the removal efficiency of the nanocomposite was evaluated by applying Eq. (6).

$$\% E= \frac{({C}_{0}-{C}_{t})\times 100}{{C}_{0}}$$
(6)

Statistical analysis

The statistical analysis of the current study was carried out by ANOVA using SPSS software version 19.0. with numerous comparisons and the least significant difference.

Results and discussion

Recently, most natural water reservoirs have high levels of pathogenic bacterial contamination, and consumption of this contaminated water is the main cause of various diseases that are widespread in developing countries (Kristanti et al. 2022). Nanomaterials with interesting properties have revealed new ways to improve waste treatment techniques. These techniques often involve coating pollutants to reduce the toxicity of constituents and kill pathogens, as well as cleaning contaminants from water due to the reactive surfaces of smart materials. Therefore, the development of such antibacterial nanomaterials is of great importance for the development of water purification systems.

In the current study, well-developed Al2O3 NPs and Ag-doped alumina nanocomposites were synthesized by a green approach using coffee husk waste. The Al2O3 NPs were prepared by mixing coffee husk extract with aluminum nitrate under continuous magnetic stirring. The Ag-doped alumina nanocomposite was prepared by combining equimolar amounts of coffee husk extract with aluminum nitrate and silver nitrate under constant magnetic stirring. The experimental conditions such as duration, temperature, and appropriate pH suitable for obtaining consistent Ag-doped alumina nanocomposite-like morphology were set as 2 h, 25 °C, and pH 12. The morphological characteristics showed the pre-synthesized metal oxide and nanocomposite. Identification and characterization of the formed Al2O3 NPs and Ag-doped alumina nanocomposite were carried out using various spectroscopic and microscopic techniques.

Characterization

The optical behavior of the Ag NPs, Al2O3 NPs, and Ag-doped alumina nanocomposite prepared with coffee husk extract was investigated by UV–vis spectroscopy. The coffee husk extract showed an absorption peak at 314 nm in the UV region. The synthesized Ag NPs exhibited remarkable optical properties, which were directly attributed to the surface plasmon resonance (SPR) absorption peak at 420 nm, resulting from the reduction of silver ions to silver nanoparticles. The observed broad peak of Ag NPs can be explained by the formation of polydisperse in the prepared sample (Yang et al. 2010). However, for Al2O3 NPs and Ag-doped alumina nanocomposite, two distinct absorption peaks were recorded at 274 and 420 nm (Fig. 1). Moreover, the nanoparticles exhibited comparatively high stability even after 2 weeks, as evidenced by the negligible changes in their absorption peak. This stability is likely due to the presence of antioxidants and phenolic compounds in the coffee husk extract, which protect the nanoparticles from deformation and decomposition (Ribeiro et al. 2019).

Fig. 1
figure 1

Optical behavior (UV–Vis) absorbance spectra of Ag-doped alumina nanocomposite, Ag NPs, extract, Al2O3 NPs, and blank

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The band gaps of the synthesized Ag NPs, Al2O3 NPs, and Ag-doped alumina nanocomposite were calculated using the following relationship:

$$\mathrm{Energy\;quantum\;mechanics\;}{E}_{\mathrm{g}}=\mathrm{hC}/\uplambda$$
(7)

where the values of h, C, and λ represent the expressed bank constant (6.626 × 10–34 J.s), the speed of light (2.99 × 108 m/sec), and the absorption peak, respectively. The estimated values of the band gap were given as 2.95 eV, 4.52 eV, and 3.87 eV, respectively, for the above nanomaterials. The reduced shape of the surface is due to surface plasmon resonance (SPR), which also increases the scattering probability and penetration of radiation. These mechanisms, which include the creation of holes on the surface and the separation of electrons, accelerate the oxidation process. In addition, it has been demonstrated that changes to the dielectric matrix can affect the absorption peak that appears on an SPR. It is known that the refractive index of the matrix is directly related to its effective dielectric behavior.

The FT-IR technique provides the probable functional groups present in biomolecules that may be responsible for the reduction process to nanoparticles, capping, and stabilization of the synthesized nanostructures. The spectrum of the coffee shell FT-IR (Fig. 2 a) shows two bands at 1058 and 777 cm−1 due to the presence of polysaccharides, as previously reported in the literature (Cangussu et al. 2021), where 1250–700 cm−1 represents the fingerprint region of polysaccharides. The broad band at 4325 cm−1 expresses the intermolecular O–H bond of the alcohol or the average N–H stretching vibration of the primary amine. The two prominent low absorption bands at 2926 and 2847 cm−1 express the C–H stretching vibration of the methyl group of the caffeine molecule (Cruz et al. 2013). The spectrum of the coffee husk sample also showed increased wavenumber values between 1700 and 1500 cm−1. The bands in this region (1621 and 1437 cm−1) can be attributed to the axial deformation of the C = C and C = N bonds in the aromatic ring of trigonelline (Reis et al. 2013). The FT-IR spectrum of Ag NPs (Fig. 2 a) shows two bands, one broad around 3405 cm−1 and others at 2923 and 2370 cm−1, attributed to the O–H (aromatic or aliphatic alcohol), the average C–H stretching vibration of the alkane, and strong N–H stretching of amine salt, respectively. The observed bands (Fig. 2 b) at 1706 and 1605, 1446, 756, 695, and 536 cm−1 are attributed to the strong C = O stretching of the carboxylic acid, the mean C = C stretching of the conjugated alkene, the mean O–H bending of the carboxylic acid, the trisubstituted C = C of the alkene, the strong halogen compound, and the Ag NPs, respectively (Aadil et al. 2020).

Fig. 2
figure 2

FT-IR findings for a coffee husk extract, b Ag NPs, c Al2O3 NPs, and d Ag-doped alumina nanocomposite

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Figure 2 c shows the functional groups formed during the synthesis of Al2O3 NPs from coffee husk extract. The recorded bands at 3465, 2444, 1636, 1359, 1052, 834, and 618 cm−1 express the presence of strong intermolecular bonds of alcoholic O–H stretching vibrations, weak S–H stretching vibrations of thiol groups, medium strength C = C stretching vibrations of disubstituted alkenes, medium strength C–H bending vibrations of alkanes, strong C-O stretching vibrations of alkyl aryl ethers, and strong halogen compounds and Al-O nanoparticles, respectively. The functional groups observed in Fig. 2 d showed combined functional groups of Ag and Al–O spectra, indicating the formation of Ag-doped alumina nanocomposite.

The XRD spectrum was utilized to study the crystalline nature of the pre-synthesized nanomaterials. As demonstrated in Fig. 3 a, the XRD spectrum showed the crystallite nature of Ag NPs which displayed five remarkable peaks at 38.13° (1 1 1), 46.12° (2 0 0), 54.55° (1 4 2), 67.74° (2 2 2), and 76.84° (3 1 1) crystalline planes, respectively. These values were in agreement with the indexed values of JCPDS. No. 04–0783 of Ag NPs and revealed the crystalline structure of the synthesized Ag NPs (Feret et al. 2000). The XRD spectrum of Al2O3 NPs was also studied to explain their crystalline nature (Fig. 3 b).

Fig. 3
figure 3

XRD patterns of pre-synthesized a Ag NPs, b Al2O3 NPs, and c Ag-doped alumina nanocomposite using coffee husk extract

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Different observed peaks were recognized at 29.49° (0 1 2), 35.48° (1 0 4), 38.99° (1 1 0), 42.62° (1 1 3), 55.69° (0 2 5), 59.94° (1 1 6), 66.76° (2 1 4), and 68.73° (3 0 0) crystalline planes. The recorded values well matched the standard JCPDS card No. 46–1212 of Al2O3 NPs (Holzwarth and Gibson 2011). Furthermore, the XRD spectrum of Ag-doped alumina nanocomposite showed the presence of three significant peaks of Ag corresponding to (2 0 0), (2 2 0), and (3 1 1) crystalline planes revealing the formation of Ag-doped alumina nanocomposite (Fig. 3 c). The average crystallite size of the pre-formed nanomaterials was estimated from the Debye‐Scherrer equation (Song et al. 2013):

$$\mathrm{Average\;crystallite\;size\;}D=0.94\uplambda /\mathrm{\beta Cos\theta }$$
(8)

where these values are D (the average crystallite size), λ (the absorption wavelength, 1.54056 Å for Cu Kα radiation), β (the width of the peak at ½ maximum intensity), and θ (the peak position angle), respectively. The average estimated crystallite size of Ag NPs, Al2O3 NPs, and Ag-doped alumina nanocomposite was 17.3 ± 4.2, 19.2 ± 2.4, and 23.9 ± 1.1 nm, respectively. The absence of any additional peaks related to other elements confirmed the high purity of the synthesized nanomaterials.

Cell length (a & c), volume (V), x-ray density (ρx-ray), total density (ρm), and porosity (%) were calculated using the Cell Software. All these parameters were estimated using Eqs. 9–12 (Bibi et al. 2021; Touqeer et al. 2020; Aamir et al. 2021; Kousar et al. 2022) and the results are shown in Table 1.

Table 1 Cell parameters of Ag NPs, Al2O3 NPs, and Ag-doped alumina nanocomposite
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$$\mathrm{Cell\;volume}={a}^{2}\times c\times 0.866$$
(9)
$${\rho }_{\mathrm{x}-\mathrm{ray}}=\frac{\mathrm{Z }\times \mathrm{M}}{{\mathrm{V}}_{\mathrm{cell}}\times {\mathrm{N}}_{\mathrm{A}}}$$
(10)
$${\rho }_{\mathrm{m}}=\frac{\mathrm{m}}{\uppi {\mathrm{r}}^{2\times \mathrm{h}}}$$
(11)
$$\mathrm{Porosity\;}\left(\mathrm{\%}\right)=1-\frac{\mathrm{Bulk\;density}}{\mathrm{x}-\mathrm{ray\;density}}$$
(12)

After co-doping, the unit cell volume decreased from 68.31 × 10–24 cm3 to 52.14 × 10–24 cm3, while the X-ray density increased from 27.12 to 58.43 g/cm3. This could be due to the replacement of smaller host cations by larger cations caused by the expansion of the unit cell. The porosity of the prepared materials ranged from 61.32 to 78.65%, which is consistent with previous results.

One of the most popular tools for characterizing nanomaterials and nanostructures is the scanning electron microscope (SEM). The signals resulting from the interactions between electrons and the sample provide details about the sample, such as its chemical composition and surface appearance. SEM was used to study the morphology of fine coffee husk powder (Fig. 4 a). The acquired image show that the coffee husk has an uneven surface with particle size in the range of 0.5–100 µm (Lanje et al. 2010). SEM was used to assess the shape of Al2O3 NPs. The results of the SEM examination used to evaluate the morphology are shown in Fig. 4 b. The nanoparticles were hemispherical in shape and had an average size of 80 nm. The SEM image of the Ag NPs synthesized from coffee husk extract showed relatively spherical nanoparticles. The particle size of the synthesized Ag NPs was about 50 nm (Fig. 4 c). The surface morphology of the Ag-doped alumina showed spherical, non-uniformly distributed particles. The large particle size of about 100 nm was due to the agglomeration of the crystallites (Fig. 4 d). The gray-white mass indicated the successful diffusion of the silver metal on the surface of the Al2O3 NPs.

Fig. 4
figure 4

SEM images of a coffee husk powder, b Ag NPs, c Al2O3 NPs, and Ag-doped alumina nanocomposite

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The elemental composition and mapping of the synthesized Ag NPs, Al2O3 NPs, and Ag-doped alumina nanocomposites were determined using SEM in conjunction with an EDX spectrometer. The results showed the presence of two peaks between 3 and 4 kV associated with the characteristic Ag lines K and L with a weight fraction of 71.36% and an atomic fraction of 37.99%, respectively. The maxima observed at 0.2 kV and 3.6 kV correspond to C and Cl with a weight fraction of 16.42% and 12.22%, respectively, and an atomic content of 35.13% and 28.88%, respectively. The presence of carbon could be due to the presence of stabilizers with alkyl chains (Puchalski et al. 2007). The presence of a weak chlorine peak is attributed to phytochemicals that may be present in the coffee husk and are undoubtedly functionalized on the synthesized AgNPs (Fig. 5 a).

Fig. 5
figure 5

EDX spectrum of a Ag NPs, b Al2O3 NPs, and c Ag-doped alumina nanocompoiste prepared from coffee husk extract

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The EDX spectrum of Al2O3 NPs displayed the presence of four recognized peaks related to Al with wight% 17.46 and atomic % 10.46%. Furthermore, two strong peaks were observed at 0.2 and 0.4 kV related to C and O, respectively. The estimated weight% was 18.49% and 64.06%, and atomic% 24.87% and 24.87% for C and O, respectively (Fig. 5 b). The EDX of Ag-doped alumina nanocomposite showed the presence of two peaks between 3 and 4 kV corresponding to Ag with weight% 13.95% and atomic % 9.25%. Additionally, three peaks were noticed at 1.5, 1.9, and 5.5 kV attributed to Al with a weight% of 31.85% and atomic% of 9.93%. These peaks confirmed the successful formation of Ag-doped alumina nanocomposite. Moreover, two remarkable peaks were recorded at 0.2 and 0.4 kV related to C and O with weight% of 28.65%, 36.38%, and atomic% of 44.81%, and 35.74, respectively (Fig. 5 c). Furthermore, the synthesized Ag NPs, Al2O3 NPs, and Ag-doped alumina nanocomposite using coffee husk extract were also investigated by elemental mapping. The results revealed the good distribution of the formed nanomaterials (Figs. 6 a, c).

Fig. 6
figure 6

Elemental mapping of a AgNPs, b Al2O3NPs, and c Ag-doped alumina nanocomposite

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Antibacterial activity

The WHO defines safe drinking water as water with a theoretical microbial load of zero. Water supply can be improved through good sanitation, resource management, and hygiene. In addition, it is believed that improving water quality can help prevent 10% of all infections worldwide. In this context, nanotechnology has recently increased the potential for safe water purification techniques. The antibacterial potential of Ag-doped alumina nanocomposite was investigated against E. coli and S. typhi, two Gram-negative bacterial strains. The growth rate was measured at regular intervals based on OD. In the current study, optimal growth was achieved during the log phase, which lasted 4–12 h. Thereafter, the rate of doubling phase decreased and a stationary phase was observed in both E. coli and S. typhi control cells. Five different concentrations of the Ag-doped alumina nanocomposite were used to test the antibacterial potential. The minimum inhibitory concentration of the Ag-doped alumina nanocomposite was found to be 12 µg mL−1 for antibacterial activity (Fig. 7 a–b), and as the concentration of the nanocomposite increased, the inhibition also increased. Significant inhibition was observed at a concentration of 30 µg mL−1 for both bacterial strains. However, inhibition was higher for E. coli (Fig. 7 a) than for S. typhi (Fig. 7 b).

Fig. 7
figure 7

Kinetic growth of E. coli cells and S. typhi a exposed to different concentrations of Ag-doped alumina nanocomposite NCS. The experiments were performed three times and a difference was estimated to be significant statistically at p < 0.05 value. SPSS-19.0 statistical software was used for statistical computations. At all the selected concentrations (6, 12, 24, and 30 µg mL−1 (light), 30 µg mL−1 (dark), the OD values were significant at p < 0.05 value. The OD value for all the abovementioned concentrations was significant at a value p < 0.05 d and f at the culture time of 12 and 16 h. Representative CLSM images of c E. coli and e S. typhi control cells treated with to Ag-doped alumina nanocomposite (30 µg m L−1)

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It is interesting to note that the Ag-doped alumina nanocomposite showed antibacterial effect on E. coli and S. typhi cells even under dark conditions. However, the antibacterial effect was low under dark conditions with similar concentrations for both pathogenic strains. Moreover, the highest concentration (30 µg mL−1) had the most deleterious effect on E. coli cells. Our results suggest that contact time and concentration influence the inhibitory effect. The exposed bacterial cells were stained with fluorescent dyes to gain insight into cell death, as these dyes are specific for nucleic acids. The kit included SYTO9 and propidium iodide dyes (PI). The representative CLSM images of E. coli and S. typhi before and after staining are shown in Figs. 7 c–e and d–f.

The green fluorescence of SYTO9 is able to penetrate all cells and was used to assess total cell number (Figs. 7 c and e), whereas the red fluorescence of PI penetrates only cells with disrupted cytoplasmic membranes (Figs. 7 d and f). Living cells are impermeable to dyes such as PI because their membranes are intact. It is clear from the CLSM images that only dead E. coli and S. typhi cells were observed at a higher concentration of the Ag-doped alumina nanocomposite (30 µg mL−1). Once again, it was proved that toxicity depends on contact time and concentration. The harmful effects of nanomaterials on biological systems need to be understood by biomedical sciences. The main causes of bacterial toxicity of nanomaterials can be explained on the basis of membrane and oxidative stress, as described in the literature already available (Ezealigo et al. 2021).

Membrane stress was caused by direct contact between the Ag-doped alumina nanocomposite and the selected bacteria, ultimately leading to destruction of the bacterial cell wall and cell death (Godoy-Gallardo et al. 2021). The antibacterial effect of silver nanomaterials has been reported previously, and the authors theorized a possible mechanism. According to their study, silver nanoparticles were dispersed in a liquid medium, and Ag+ ions interacted with and accumulated in the walls of bacterial cells (Ekrikaya et al. 2021). In the current study, there is a possibility of a photocatalytic effect in which reactive radicals are generated after being exposed to visible light. The Ag-doped alumina nanocomposite continuously interacted with bacterial cells, exerting remarkable toxicity against the selected E. coli and S. typhi. The OD was recorded as a function of time, as shown in Figs. 7 a and b. The deleterious effect of the nanocomposite on selected bacteria was further illustrated by CLSM images. Figures 7 c, e, d, and f show that E. coli, S. typhi, nanocomposite treated E. coli, and nanocomposite treated S. typhi have higher concentration.

This is how the plausible plot technique is described. It was hypothesized that the Ag-doped alumina nanocomposite adhered to the outer cell membrane of the bacterium, which eventually led to cell lysis after the cell was further broken. However, the Ag-doped alumina nanocomposite first adhered to the outer cell wall and then penetrated the inner cell membrane, damaging the cell membrane and allowing the disorganized intracellular contents to leak out and completely destroy the bacterial cell. In Gram-negative bacteria such as E. coli and S. typhi, there is only a thin layer of peptidoglycan between the cytoplasmic membrane and the outer membrane. It is believed that the Ag+ atoms in the Ag-doped alumina nanocomposite interact with the bacterial cells and attach to the bacterial cell walls when the Ag-doped alumina nanocomposite is dispersed in the medium. This is due to the excess of carboxyl and other groups that dissociate at biological pH and negatively charge the surface of the bacterial cells (Zhao et al. 2022). The opposite charges of the bacteria and Ag+ atoms in the Ag-doped alumina nanocomposite and the electrostatic forces could be the cause of their adhesion and bioactivity. Thus, the free radicals and electrostatic forces are most likely the reason for the harmful effect of the Ag-doped alumina nanocomposite in the current study. Thus, the result of the study is that the formed Ag-doped alumina nanocomposite is a potential candidate for use as an antibacterial agent for wastewater treatment.

Effect of surface morphology

The surface and porosity properties of the Ag-doped alumina nanocomposite were determined by analyzing the N2 adsorption–desorption isotherm (Fig. 8 a). The Barrett-Joyner-Halenda (BJH) method was used to measure the pore diameter and volume, while the surface area was measured by the Brunauer–Emmett–Teller method (BET). The Ag-doped alumina nanocomposite showed H3-type loop hysteresis with cylindrical pores associated with slit-like non-rigid aggregate particles. The Ag-doped alumina nanocomposite showed the highest surface area in the BET technique, which was confirmed by the SEM results showing that the formed nanocomposite has the best adsorption capabilities (Table 2).

Fig. 8
figure 8

a N2 adsorption isotherm and pore size distribution, b contact time of IGC removal, and c effect of pH on the IGC removal by Ag-doped alumina nanocomposite

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Table 2 The surface features of the formed Ag-doped alumina nanocomposite
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IGC adsorption estimation

The effect of contact time on IGC adsorption of Ag-doped alumina nanocomposite is shown in Fig. 8 b. Although adsorption equilibrium did not develop in the first 3 h, a total of 92% of the adsorbed substances were adsorbed in the first 50 min. The qt value obtained for the Ag-doped alumina nanocomposite was 197.6 mg g-1. These results are consistent with the high surface area obtained by the BET estimation of the Ag-doped alumina nanocomposite.

Effect of pH

The effect of pH on the removal of IGC by Ag-doped alumina nanocomposites is shown in Fig. 8 c. At low pH, IGC sorption was decreased, and it was even more decreased in the high pH solution. The performance of the Ag-doped alumina nanocomposite remained constant in the pH range from 5 to 8, with pH 6 being preferred. The reason may be due to the molecular structure of IGC, which contains a high concentration of H+ ions that can protonate the electron-rich sites of the ionic dye and lead to the repulsion of some sites of the metal oxide nanoparticles. In this way, the electron-rich sites are occupied, which reduces the availability of the dye and participates in the sorption process. As a result, the -OH groups can occupy the active sorption sites of the sorbent at higher pH (Elamin et al. 2023).

Adsorption kinetics

The adsorption order and rate control mechanism for IGC dye adsorption were evaluated on the surface of Ag-doped alumina nanocomposite. The R2 values for IGC sorption were 0.9923 at a K1 value of 0.043 min−1 and 0.9336 at a K2 value of 0.008 g mg−1 min−1 for FOM and SOM, respectively. However, evaluation of the rate control mechanism yielded R2 values of 0.9962 with an equilibrium constant of 0.034 mg g−1 min−0.5 and 0.9852 with an equilibrium constant of 16.23 min−1 for LDM and IM, respectively (Figs. 9 a, c). The obtained results indicate that the sorption of IGC dye on the surface of Ag-doped alumina nanocomposite is by FOM and the sorption is controlled by LDM (An 2020). Following the FOM and LDM mechanism indicates that the particle size of Ag-doped alumina nanocomposite is small and has high affinity for IGC dye removal (Hameed and El-Khaiary 2008).

Fig. 9
figure 9

Kinetics studies for IGC dye removal including a FOM, b SOM, c LDM, and d IM on the surface of Ag-doped alumina nanocomposite

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Adsorption isotherms

The adsorption isotherms were studied using the results from the effect of concentrations. The models of Langmuir (LIM) and Freundlich (FIM) were applied to study the adsorption probabilities of monolayers and multilayers corresponding to Eqs. (13) and (14).

$$\frac{1}{{q}_{e}}=\frac{1}{{K}_{L}{q}_{m} } . \frac{1}{{C}_{e}}+\frac{1}{{K}_{L}}$$
(13)
$$\mathrm{ln}{q}_{e}=ln{K}_{F}+\frac{1}{n} ln{C}_{e}$$
(14)

KL, Ce, qm, qt, KF, and 1/n represent the constant LIM (Lmg−1), the concentration of the equilibrium solution (mg L−1), the estimated maximum (mg g−1), the equilibrium constant (L mg−1), and the heterogeneity factor of FIM, respectively. The plots of LIM and FIM are shown in Figs. 10 a and b, and the calculated results are given in Table 3. The adsorption of IGC on the surface of Ag-doped alumina nanocomposite matched well with LIM (R2 = 0.9783), while the R2 value of FIM was 0.9557. Moreover, a Freundlich heterogeneity factor of 0.687 was obtained, indicating excellent sorption of IGC on the surface of the Ag-doped alumina nanocomposite (Hamdaoui and Naffrechoux 2007).

Fig. 10
figure 10

The adsorption efficiency of IGC dye on the surface of Ag-doped alumina nanocomposite using a LIM and b FIM using different concentration (40, 80, 120, 160 mg L−1) at 30 °C

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Table 3 Isotherms and thermodynamic parameters for IGC dye adsorption on the surface of Ag-doped alumina nanocomposite
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Thermodynamic

The effects of temperature and concentration on the removal of IGC by Ag-doped alumina nanocomposite are shown in Fig. 11. The Ag-doped alumina nanocomposite showed an experimental qt value of 462.7 mg g−1 from 160 mg L−1 at 50 °C. The qt value is directly proportional to the temperature in an endothermic process (Darryle et al. 2021). To better understand the sorption of IGC on the surface of the Ag-doped alumina nanocomposite, the thermodynamics were evaluated. The enthalpy (∆H°) and entropy (∆S°) were calculated using Eq. (15) and their values were used to determine the Gibbs free energy (∆G°) using Eq. (16). The results are shown in Table 3.

Fig. 11
figure 11

a The influence of initial fed of IGC concentration on the sorption efficiency of Ag-doped alumina nanocomposite at various temperatures (30, 40, and 50 °C), b thermodynamic study of IGC sorption on the surface of Ag-doped alumina nanocomposite using different concentrations at 40, 80, 120, and 160 mg L−1 at the same temperature degrees, and c

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$${\mathrm{lnK}}_{\mathrm{c}}=\frac{{\Delta H}^{\mathrm{o}}}{\mathrm{Rt}}+\frac{{\Delta S}^{\mathrm{o}}}{R}$$
(15)
$${\Delta G}^{^\circ }={\Delta H}^{^\circ }-{T\Delta S}^{^\circ }$$
(16)

As in the above calculation, the value 0.0081345 kJ mol L−1 was used as the deal gas constant (R). The positive (∆H°) obtained are in agreement with the previous endothermic results. However, the negative values of (∆G°) indicate the spontaneity of IGC sorption on the nanocomposite surface (Elamin et al. 2019). Moreover, the results showed that the G° values were less than 20 kJ mol L−1, indicating a physio-sorption process (Elamin et al. 2021). The results showed that the spontaneity of sorption increased with decreasing concentration, indicating the effectiveness of the Ag-doped alumina nanocomposite for the treatment of water, for which a lower concentration was required.

Application of water sample and Ag-doped alumina nanocomposite regeneration

The results showed that the use of nanocomposites to remove IGC from contaminated water samples (Fig. 11 c). The study showed that the nanocomposite effectively treated and completely removed 98.45% of IGC concentration of 10 mg L−1 in six water samples. In addition, the regeneration of the proposed Ag-doped alumina nanocomposite was tested. The nanocomposite used was filtered and sonicated with 10 mL ethanol for 10 min, followed by 20 mL distilled water, filtered, rinsed with distilled water, and dried at 100 °C for 2 h. The initial removal efficiency was assumed to be 100%, and the subsequent power was approximated. The average efficiency of the Ag-doped alumina nanocomposite was 95.72%, with an RSD value of 3.6%, while the lowest efficiency was 89.32%. This result was consistent with the sorption matching of the LFDM, indicating that the penetration of the contaminant into the internal sorption sites was easy and hindered the recovery.

Conclusion

A novel Ag-doped alumina nanocomposite with an average size of 100 nm was successfully prepared using coffee husk extract by environmentally friendly biogenic synthesis. The morphological features showed the spherical structure of the Ag-doped alumina nanocomposite with non-uniform distribution of particles. The formed nanocomposite showed the highest surface area (126.58 m2g−1), average pore diameter (145.95 Å), and average pore volume (0.45 cm3g−1) in the BET technique. The evaluation of antibacterial capabilities revealed that the Ag-doped alumina nanocomposite possessed significant antibacterial activity with minimum inhibitory concentration of the nanocomposite 12 µg mL−1 against E. coli and S. typhi, two very common water contaminants. The as-synthesized Ag-doped alumina nanocomposite also showed antibacterial activity against both bacterial strains under dark conditions. However, the antibacterial effect of the Ag-doped alumina nanocomposite was comparatively less significant on pathogenic bacteria in the dark. Thus, our results clearly indicate that the pre-synthesized nanocomposite has potential antibacterial properties and could be used to combat waterborne pathogens. Clean water is a vital requirement for life, and our research introduces a novel method for purifying wastewater using Ag-doped alumina nanocomposite. In addition, the nanocomposite has multiple effects as a sorbent to remove IGC from contaminated water. The adsorption order and rate control mechanism for IGC dye adsorption were evaluated on the surface of Ag-doped alumina nanocomposite. The R2 values for IGC sorption were 0.9923 at a K1 value of 0.043 min−1 and 0.9336 at a K2 value of 0.008 g mg−1 min−1 for FOM and SOM, respectively. However, evaluation of the rate control mechanism yielded R2 values of 0.9962 with an equilibrium constant of 0.034 mg g−1 min−0.5 and 0.9852 with an equilibrium constant of 16.23 min−1 for LDM and IM, respectively. The LFM affected the IGC removal on the surface of the Ag-doped alumina nanocomposite and the IGC adsorption was in accordance with the equilibrium model of LI. However, further research on the other physicochemical properties of the Ag-doped alumina nanocomposite is needed before this material can be used to purify contaminated water, especially for drinking water purposes.