1 Introduction

Freshwater scarcity represents one of the most critical issues for humanity and the global community. Moreover, increasing the contamination rate of water resources results from rapid growth of population and industrial revolution (Alizadeh et al. 2018; Asiabi et al. 2018; Tan et al. 2018). To complicated matter, presence of very traces of heavy metal or/and organic pollutants in water considers the main reason for several diseases such as vomiting, cholera, diarrhea gastroenteritis, and acute poisoning (Dixit et al. 2015; Fashola et al. 2016; Jacob et al. 2018). As reported in the WHO, copper is considered as a heavy metal with high toxicity level, increasing copper concentration in drinking water more than 2 mg/L, causing the anemia and damage of the kidney and liver tissues (Rojas 2014), more than 10.000 types of dyes enter in industrial products, where it is used in tanning textile, wool, silk, jute, leather, cotton paper, and acrylic coloring food agents. Malachite green is a cationic dye (Abdelkader et al. 2011). Malachite green causes environmental hazed, which is highly carcinogenic and toxic with dangerous and negative effect on aquatic organisms and human health (Chen et al. 2007; Santos et al. 2017; Tewari et al. 2018).

Among several water purification techniques such as chemical precipitation, reverse osmosis, coagulation, ion exchange, membrane (Jawad et al. 2017; Kheirandish et al. 2017; Wen et al. 2018),the adsorption process has been introduced as economic, green, and efficient strategy which achieves great benefits in heavy metal and organic decontamination (Efome et al. 2018; Yu et al. 2018). Unfortunately, adsorption process suffers from limitation of material with low adsorption capacity, low surface area, long time, and difficulties in recyclability (Pavlovic et al. 2009; Rouahna et al. 2018).

Layered double hydroxides (LDHs) are considered as inorganic brucite-like clay catalysts (Lin et al. 2011) with unique chemical structures and promising properties, such as environmental compatibility and high adsorptivity. LDH has been found in several fields with large scale, where it has been used as pharmaceuticals, catalysts, stabilizer, adsorbents, and ion exchangers. LDH exhibits an excellent performance in wastewater remediation of toxic heavy metals and dyes (Dash et al. 2018; Guo et al. 2018; Hatami et al. 2018; Omdeo et al. 2017). Due to high mechanical strength, good physiochemical properties and high surface area with multi-porous structures, electrospun nanofibers have been introduced as excellent materials for various applications especially in water treatment area (Chronakis 2005; Du et al. 2020; Kotp et al. 2019; Mohamed et al. 2019; Zhang et al. 2019). Polystyrene (PS) NF membranes are nontoxic, nondegradable, inexpensive, and recyclable and are obeyed in the remediation of polluted water for uptaking and adsorption of organic materials and heavy metals (Huang et al. 2014). The LDHs are very sensitive when used as adsorpent in acidic media unless they are composite form with a polymer, which preserves the LDH structure from degradation in acidic media; this is a new insights are still need and more research needs to be performed.. Consequently the main purpose of this introduced study is to prepare a novel nanofiber adsorbent (Zn/Al/GA LDH/PSNFs) based on PS with improved Cu2+ and dye sorption capacity in aqueous solutions. The influence of many different conditions on pollutant adsorption behavior as well as isotherm and kinetics models of adsorption was investigated. Moreover, we tested the multi-functionality of the prepared nanocomposite as antibacterial agent toward Gram-negative and Gram-positive bacteria (Graphical Abstract).

2 Experimental

2.1 Materials and Instruments

Polystyrene (Mw ~ 280,000 g/mol) was obtained from Sigma Aldrich, Aluminum chloride (AlCl3) , zinc chloride (ZnCl2) , . gallic acid (GA; Mw 170.12 g/mol) , cupric nitrate trihydrate (Cu(NO3)2.3H2O) , and malachite green (C23H25N2Cl) were purchased from LABAL Chemie (India). . The chemicals were obtained at high grade and used without purification.

X-ray diffraction (XRD) was performed on a PANalytical (Empyrean) X-ray diffractometer using Cu Kα radiation (wavelength 0.154 nm) at an accelerating voltageof 40 KV, current of 35 mA, scan angle varying from 20 to 70°, and scan step of 0.02°. FTIR spectra were performed with a Bruker Vertex 70. The morphologies of the prepared materials were characterized by field emission scanning electron microscopy (FESEM), and energy-dispersive X-ray (EDX) spectroscopy was used for elemental analysis. The microstructure and surface morphology of Zn/Al LDH and Zn/Al/GA LDH were investigated using HRTEM (JOEL JEM-2100) with 200 KV as acceleration voltage. The specific pore volumes and pore sizes of the adsorbent materials were determined by N2 adsorption isotherm using an automatic surface analyzer (TriStar II 3020, Micromeritics, USA) with Brunauer-Emmett-Teller–specific surface areas. A pH meter (A∂wa–AD1030), atomic absorption spectrophotometer (AAS) (model ZEISS-AA55, Germany), and UV absorption spectrophotometer (Shimadzu UV-3600, Japan) were also used; the absorption of MG dye was determined at λmax 617 nm.

2.2 Preparation of Pristine PSNF-Based Support Membrane

Electrospun NFs of 20% PS were fabricated by dissolving 20 g of PS pellets (Mw~280,000) in 80 mL of dimethyl formamide (DMF), stirring at 80 °C for 6 h, and leaving the mixture at room temperature for 12 h. The PS solution was transferred into a syringe pump for electrospinning process at a fixed voltage of 18 KeV with feeding rate of 0.9 mL/hand the distance between the syringe jet and the collector was fixed at 14 cm. The produced PSNFs were dried in a vacuum oven overnight at 50 °C.

2.3 Preparation of Anchored Zn/Al/GA LDH/PSNFs

Zn/Al/GA LDH/PSNFs were synthesized using the co-precipitation method. Typically, different amounts of the as-prepared PSNFs were put in 0.1 M solution of GA with shaken at 150 rpm for 2 h. A mixture of 0.4 M ZnCl2 and 0.1 M AlCl3 was added to the GA-functionalized PSNF solution at pH ~ 9 using 0.1 M NaOH to fabricate the Zn/Al/GA LDH nanorods anchored to PSNFs. To ensure structural homogeneity, the nanocomposite was shaken for 12 h. The obtained product was washed twice with double distilled water.

2.4 Antimicrobial Activity of Zn/Al/GA LDH Nanocomposite, PSNFs, and ZnAl/GA LDH/PSNFs After Cu2+ Metal Ion or MG Dye Removal

The antimicrobial activity of Zn/Al/GA LDH/PSNFs and PSNFs after Cu2+ metal ion or MG dye was examined against different types of pathogenic bacteria, including K. pneumonia, S. pneumonia, S. aureus, E. coli, Salmonella sp., and P. aeruginosa. Whatman filter paper was punched into standard-sized (50 mm diameter) discs and placed in five screw-capped containers; to ensure sterilization, hot air oven at 150 °C for 30 min was used. Sterilized discs were soaked overnight with the two different tested concentrations of Zn/Al/GA LDH/PSNFs, PSNFs, or Zn/Al/GA LDH/PSNFs after Cu2+ metal ion or MG dye removal (1000 μg/mL). Normal saline was used for dilution at a concentration of 108 colony-forming units (CFU) mL−1 according to McFarland 0.5. Thereafter, 100 μL of the suspension was streaked on Mueller-Hinton agar plates. Sterile forceps are used for aseptically adding the discs on the Mueller-Hinton agar plates. All samples were incubated at 37 °C for 24 h, and the inhibition zone diameter was determined. All readings were taken in triplicate. This method was performed according to (Perez 1990; Wikler 2006). The minimum inhibitory concentration (MIC) of Zn/Al/GA LDH nanocomposite was estimated using the broth microdilution method. Several types of bacteria, including K. pneumonia, Streptococcus sp., P. aeruginosa, S. aureus, E. coli, and Salmonella sp., were cultured in saline solution with a fixed concentration of 108 CFU mL−1. The MIC of Zn/Al/GA LDH was determined according to (Wikler 2006). To perform the MIC test, freshly prepared broth cultures of the different examined bacterial strains were prepared at 108 CFU mL−1. Each 1 mL of broth culture was supplemented with 5000, 2500, 1250, 625, or 312 μg/L in Mueller-Hinton broth. Culture broth served as a negative control, while the nanomaterial served as a positive control. The control and treated samples were incubated at 37 °C for 24 h. All samples were tested in triplicate. A spread-plating technique was utilized to evaluate the MIC of the fabricated nanomaterials. Then, the number of colonies was measured against the concentration of bacterial cells (CFU mL−1 m) (Perez 1990). For MBC estimation, 10 μL was inoculated from all broth cultures without visible growth onto Mueller-Hinton agar, and the plates were incubated as previously mentioned in the MIC. The MBC indicated the lowest concentration of Zn/Al/GA LDH that causes complete bactericidal effects under aerobic conditions (Prakatthagomol et al. 2012).

2.5 Adsorption Experiment of Cu2+ Metal Ions and MG Dye

The adsorption experiment was conducted at room temperature using 50 mL Cu2+ metal ions or solution of MG dye in a 250-mL glass conical flask, which was shaken at 250 rpm using an orbital shaker. Parameters were varied as follows: pH 3–5.5 for Cu2+ metal ions and 3–8 for MG dye; dose of adsorbent 0.025–0.20 and 0.025–0.3 for MG dye; initial Cu2+ concentration 50 to 250 mg/L and for MG dye 20 to 200 mg/L; and effect of time from 10 to 300 min for Cu2+ metal ions and 5–480 min for MG dye. The pH was controlled with 0.1 M HCl and 0.1 M NaOH. The removal percentage of Cu2+ metal ions or MG dye adsorbed by the Zn/Al/GA LDH/PSNFs and the adsorption efficiency of the Zn/Al/GA LDH/PSNFs is estimated as shown in the following Eq. (1). The cycling stability of the Zn/Al/GA LDH/PSNFs was investigated at 25 °C for several cycles. Typically, 0.05 g of Zn/Al/GA LDH/PSNFs was immersed into 50 mL of 50 mg/L Cu2+ or MG dye solution for 24 h. After that the solution was removed and washed twice with bi-distilled water to use for another adsorption cycle. This process wasrepeated several times, and the dye and heavy metal removal efficiency were determinedafter each cycle by the following equations:

$$ q\%=\frac{C_0-{C}_e}{C_0}\times 100 $$
(1)
$$ {q}_e=\left({C}_0-{C}_e\right)\times \frac{V}{m} $$
(2)

where

C0:

is the initial concentration (mg/L)

Ce:

is the final concentration,

V:

is the volume of copper or dye solution,

m:

is the mass of adsorbate per gram and

qe:

is the amount of adsorbate adsorbed at equilibrium (mg/g).

The best fitting of kinetic models can be determined by using R2 and (χ2) chi-square statistic (Abdelhameed et al. 2019; Emam et al. 2018; Zheng et al. 2019)Eq. (3):

$$ \sum \limits_{i=1}^n\frac{{\left({q}_{exp}^i-{q}_{cal}^i\right)}^2}{q_{cal}^i} $$
(3)
$$ \frac{100}{n}\sum \limits_1^n\frac{q_{cal}-{q}_{exp}}{q_{exp}} $$
(4)

where

q exp :

is the experimental data

q Cal :

is the calculated values and

n :

is the number of observations in the experimental data.

3 Results and Discussion

3.1 Characterization

To elucidate the crystalline structure and configuration of the fabricated materials, XRD analysis was used, as shown in Fig. 1. The amorphous and broad peaks at 18° to 23° correspond to the PSNF structure of the support membrane (Fig. 1a). All characteristic peaks gave an indication of successful preparation of pristine Zn/Al LDH (Fig. 1b), and hydrotalcite-like structures very similar to LDHs were observed, with only one crystalline phase, which showed sharp and symmetric reflections of the basal (003), (006), (101), and (012) planes (Hu et al. 2018; Iftekhar et al. 2017; Zhang and Li 2014). The results confirmed the intercalation of GA into the interlayer of the nanocomposite and the formation of Zn/Al/GA LDH/PSNFs, according to the broad lower intensity asymmetric reflection for the nonbasal (018) plane. The XRD pattern of Zn/Al/GA LDH nanocomposite retained the characteristic structure of LDH at (003), (006), (009), (012), and (015) after GA intercalation, where the d-spacing at (003) [7.73 Å] changed to a d-spacing of 8.960 Å, which illustrates that GA was successfully intercalated into the interlayer galleries of Zn/Al/GA LDH /PSNFs. In the XRD pattern of (d), all peaks that appeared for sample (a) and (b) that confirmed the hierarchical structure of LDH were present but became broad, which may be due to the presence of PS, which is considered to be one of the polymers that causes exfoliation of LDH layers. The exfoliated layers were stacked face-to-face, so the hierarchical LDH construction was hidden due to the presence of PS (Applied Clay Science ACS Sustainable Chemistry and Engineering JAMA et al. 2018; Iftekhar et al. 2018).

Fig. 1
figure 1

XRD patterns of PSNFs (a), Zn/Al LDH (b), Zn/Al/GA LDH (c), and Zn/Al/GA LDH/PSNFs (d)

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The FTIR pattern of the prepared materials was represented in Fig. 2. The GA pattern shows characteristic broad band at 3365 cm−1, which reflects OH phenolic vibrational stretching, and the characteristic peaks at 1706 cm−1 and 1242 cm−1 correspond to C=O and C–O of carboxyl groups. The peaks at 1619, 1538, 1450, and 1026 cm−1 are attributed to C=C vibration of aromatic rings (Hu et al. 2013) (Fig. 2a). The fabricated Zn/Al LDH (Fig. 2b) displays a broad band at 3457 cm−1, which may arise from the stretching of OH in LDH or physically adsorbed water molecules. The peaks at 1621 and 1365 cm−1 are related to the H–O–H bending vibration of interlayer LDH (Barnabas et al. 2016; Mahjoubi et al. 2017). The peaks at 685 and 599 cm−1 are related to M–O or O–M–O (where M = Zn or Al) (Islam and Patel 2010), and the peaks at 698 and 752 cm−1 are assigned to the in-plane bending vibration of substituted benzene, indicating the existence of C=C (Lu et al. 2018). Figure 2 c demonstrates the presence of characteristic peaks of both LDH and GA, which were used to create Zn/Al/GA LDH/PSNFs: a broad band at 3429 cm−1 (OH); peaks at 620 and 432 cm−1 corresponding to O–M (LDH); a peak at 2922 cm−1 (aliphatic CH2 and CH); a peak at 966 cm−1 corresponding to C=C in the aromatic benzene rings of PS; and peaks at 1546 and 1406 cm−1 (C=C of GA). The surface area and the pore size distribution of the sorbents were characterized by N2 adsorption-desorption isotherms; the results for Zn/Al LDH (Fig. 3a) show a significant increase in adsorption at a relative pressure of p/po > 0.02, meaning that nitrogen uptake below p/po 0.02 was negligible and micropores were blocked. Thus, our as-synthesized Zn/Al LDH sample corresponds to the type IV isotherm model, indicating that Zn/Al LDH is a mesoporous solid. The BET surface area of Zn/Al LDH was 40.64 m2/g. The low surface area of the LDH could be attributed to the preparation method, which allows agglomeration and partial stacking of platelet-like structures; this explanation was confirmed in the HRTEM images (Fig. 3b). In addition, the pore size distribution is represented in the inset of Fig. 3a, and the pore size was approximately 12 nm for Zn/Al LDH.

Fig. 2
figure 2

FTIR patterns of GA (a), Zn/Al LDH (b), Zn/Al/GA LDH (c), PSNFs (d), and Zn/Al/GA LDH/PSNFs (e)

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Fig. 3
figure 3

a N2 sorption isotherms for Zn/Al LDH; the inset shows the pore size distribution. b HRTEM image of Zn/Al LDH

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Figure 4Ia shows the good surface morphology of LDH, which has an agglomerated plate-like shape, as confirmed using BET (Fig. 3a). Some of the outer layers rolled up to create an exfoliated form, which can enhance the properties of Zn/Al LDH (Fig. 4Ia). The pure PSNFs possess a symmetric and smooth surface morphology with fiber-based nanostructures that lack beads (Fig. 4Ib). Moreover, the multiple pores on fiber surface represent the active sites to anchor the LDH onto the NFs and prevent loss from the membrane (Fig. 4Ic, d). Interesting images of the rod-like structure are shown in Fig. 4IIa–c; we will focus here on the reason for the formation of this LDH morphology and offer some perspectives for the future preparation of multi-functional LDH materials by using GA and PS. The reason for this layer-like structure in the rod-like morphology may be related to the possible intercalation process between GA and LDH. Hydrogen bonding between PS, GA, and the hydroxyl groups of LDH in crystallite orientation is proposed as the reason for the observed orientation (Kuang et al. 2010; Nagendran and Kamath 2017). The behavior is still unclear; however, as the reaction is thought to be nucleated at crystal defects on either the edge planes of the host crystallite or the basal plane surfaces of LDH. The morphological appearance here confirmed the shift in the characteristic peak at (003) and the change in d-spacing of pristine Zn/Al LDH.

Fig. 4
figure 4

FESEM images (I) of Zn/Al/GA LDH (a), pure PSNFs (b) and Zn/Al/GA LDH/PSNFs (c) and high magnification of Zn/Al/GA LDH nanocomposite (d); FESEM images (II) of Zn/Al/GA LDH/PSNFs during the formation of LDH nanorod-like structures, from (a) to (c)

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3.2 Adsorption Experiment

3.2.1 Influence of pH on the MG Dye and Cu2+ Metal Ion Removal Process

The impact of pH on uptake of copper metal ions or MG dye is shown in Fig. 5. The stability of MG dye was tested at various pH values from 3 to 8.00 (Fig. S1). The main peak of MG dye was appeared at 617 nm showing no change in color or disappearance of dye color (Liu et al. 2020; Wu et al. 2020). At lower pH levels, the large number of H+ ions caused the protonation of the catalyst surface, which decreased the interaction between the catalyst surface and MG dye which have the same positive charges. As the pH increased, the surface of the adsorbent became negatively charged due to the functional groups present on GA (pKa values of 4.12 and 8.32 (Fazary et al. 2008)), which facilitated its interaction with the positively charged MG cations through electrostatic interactions and hydrogen bonds (George and Saravanakumar 2018; Guo et al. 2013) (95% at pH 8).

Fig. 5
figure 5

Impact of pH on the adsorption capacity for MG dye (a) and Cu2+ metal ions (b) using Zn/Al /GA LDH/PSNFs

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In the case of Cu2+ metal ion adsorption, when the pH of the solution increased to pH 4 (Fig. 5), the removal efficiency was approximately 67% because the GA functional groups at the catalyst surface became negatively charged, so there are electrostatic attraction between the negatively charged surface and Cu2+ metal ions and complex formation between GA and Cu2+ metal ions with a high stability constant 9.75 (Fig. S2) (Gul et al. 2018). Similar investigations are reported for the adsorption of Cu2+ metal ions on LDHs as shown in Table 1 (Moran et al. 1997; Morck et al. 1983). Further increase in pH beyond 6 gradually caused precipitation of copper metal ions; for this reason, we avoid the adsorption of copper metal ions beyond pH 6. To confirm this behavior (hydrolysis), the uptake amount of Cu2+ in the absence of the adsorbent as a reference experiment was investigated at higher pH (Fig. S1) (Fu and Wang 2011; Schiewer and Wong 2000).

Table 1 Reported isotherm parameters of Cu2+ metal ion adsorption by Zn/Al/GA LDH/PSNFs
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3.2.2 Influence of Zn/Al/GA LDH/PSNFs Dosage on the Adsorption Efficiency for Copper and MG Dye

The effect of catalyst dosage on the Cu2+ metal ion and MG dye removal efficiency is tested in 50 mL of Cu2+ solution or MG dye with an initial concentration of 50 mg/L at 25 °C for 24 h, as shown in Fig. 6a and b. The removal efficiency increased with increasing Zn/Al/GA LDH/PSNFs dosage, which may be related to the increase in available adsorption sites on Zn/Al/GA LDH/PSNFs. The maximum removal efficiency of Zn/Al/GA LDH/PSNFs for both Cu2+ and MG dye reached 94% at 0.05 g and 95% at 0.05 g, respectively. With increasing Zn/Al/GA LDH/PSNFs dosage, no increase in the removal efficiency due to the saturation of adsorbent sites .Furthermore, the removal efficiency of the MG dye decrease with increasing the dose of adsorpent, it can be attributed to the aggregation of LDH/GA particles through the membrane takes place (Padmavathy et al. 2016).

Fig. 6
figure 6

Impact of dosage on Zn/Al/GA LDH/PSNFs removal of Cu2+ metal ions and MG dye

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3.2.3 Influence of Time and Kinetics of Adsorption

The influence of time on the adsorption of Cu2+ metal ions and MG dye onto Zn/Al/GA LDH/PSNFs is shown in Fig. 7a and b. Cu2+ adsorption gradually increased in the first 60 min, which may be due to the presence of many vacant sites and the fast diffusion rate of Cu2+ metal ions from the solution onto the outer surface of the catalyst. The slow increase in adsorption from 60 to 300 min may be due to the limited vacant sites on the catalyst and the long diffusion time of Cu2+ metal ions from the solution into the inner sphere of the catalyst. No further uptake of Cu2+ metal ions took place after equilibrium was reached at 120 min (Fig. 7a) (Bakr et al. 2016). The adsorption efficiency of MG dye gradually increased in the first 3 h, and then, equilibrium was reached after 240 min, at which point the adsorptivity reached 95% (Fig. 7b).

Fig. 7
figure 7

Effect of contact time on the adsorption of Cu2+ metal ions (a) and MG dye (b): C0 = 50 mg/L; pH 5 and 8 for Cu2+ and MG dye, respectively; dose = 0.05 g/L; and temperature 25 ± 2 °C

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As the adsorption process depends on the physical and chemical characteristics of the adsorbents, we studied the kinetics, which provided important information about the adsorption mechanism and the rate of pollutant uptake from solution and provided evidence for the adsorbent design (Moaty et al. 2017) (Fig. S3 and Table S1). Several models were applied to the examined kinetic experimental data, including nonlinear fit for the following pseudo-first-order, pseudo-second-order, and mixed 1,2 order models (Farghali et al. 2020; Younes et al. 2019; Zaher et al. 2020) (Table 2 and Fig. 8).

Table 2 Statistical parameters of kinetic models for Cu2+ metal ions and MG dye adsorption onto Zn/Al/GA LDH/PSNFs
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Fig. 8
figure 8

Regressions fitting of kinetic models for the removal of Cu2+ metal ions (a) and MG Dye (b)

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The kinetic study of the adsorption plays an important role in the identification of adsorbent design. Also, the kinetics study provides the key of the mechanisms and the rate of pollutant removal. Adsorption kinetics includes four steps: bulk transport, film transport, intra-particle transport, and adsorption on the adsorbent (Abdelhameed et al. 2018a). The pseudo-first-order kinetic considers a physisorption process where it performed with a diffusion mechanism and not dependent on the concentrations of both reactants (Abdelhameed et al. 2017; Abdelhameed et al. 2018b); pseudo-second-order kinetic which the chemical reaction seems significant in the rate-controlling step so it could be called chemisorption process and the mixed 1,2 order. The nonlinear equations of these three kinetic models were plotted (Fig. 8), and Table 2 provides the model coefficients. Comparing the correlation coefficients for each expression using R2, χ2, and ARE% results showed that the pseudo-first-order rate model is not completely valid for the Cu2+ system and pseudo-second-order for MG dye system. Moreover, the other remaining models are showing fit of goodness to the experimental data as illustrated in Fig. 8. The mixed 1,2 order kinetic model gives good correlation coefficient (0.95) for Cu2+ removal system, however, 0.97 for MG dye one using pseudo-first-order model. This shows that the sorption of pollutants under study in solutions by Zn/Al/GA LDH/PSNFs adsorbent is controlled kinetically by mixed 1,2 order kinetic reaction for Cu2+ metal ions system and pseudo-first-order for MG dye. The calculated value for the adsorption capicity (qe,calc) value from the mixed 1,2 order kinetic for Cu2+ was 34.5 mg/g and value from the pseudo-first-order for MG dye 48.8 mg/g, which is very close to the value of the equilibrium experimental quantity adsorbed, q(e,exp) = 32.9 and 49 mg/g, respectively. So, the interaction proceed by physical adsorption by electrostatic interactions or van der Waals force in the state of MG dye (Qu et al. 2018); however, in the Cu2+ system, the interaction proceeds by chemical and physical adsorption (Amin et al. 2019; Zaher et al. 2020).

3.2.4 Adsorption Isotherms

Adsorption isotherms were established by adding 0.05 g catalyst to a constant volume (50 mL) of copper solution and MG dye with different initial concentrations (50–250 mg/L and 20–200 mg/L, respectively), shaking at 250 rpm and incubating at room temperature for 24 h. The adsorption isotherm is very important because it can explain how the ions or molecules of an adsorbate interact with adsorbent surface sites and gives a hypothesis about the surface of the adsorbent and adsorption process (Jiang et al. 2016). The experimental adsorption isotherm data were fitted using three isotherm models, including two-parameter isotherm models ( Langmuir (1918) and Freundlich (1907)) and a three-parameter isotherm model (Sips (1948)). The experimental adsorption isotherms together with the predicted data obtained from the adsorption isotherm models are shown in Fig. 9, and their expressions and corresponding parameters are reported in Table 3. The adsorption process was best fitted with the two-parameter and three-parameter isotherm models with respect to R2 and the monolayer maximum adsorption capacity (qmax = 190 mg/g and 61.7 mg/g) for Cu2+ metal ions and MG dye, respectively. The calculated monolayer maximum adsorption capacity (190 mg/g) obtained from the two-parameter and three-parameter isotherm models was greater than that of many adsorbents found in the literature (Table 1), as listed in Table 3.

Fig. 9
figure 9

Isothermal adsorption of Cu2+ (a) adsorption isotherm of MG dye (b) on Zn/Al/GA LDH/PSNFs using three different models

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Table 3 Results of nonlinear isotherm models for Cu2+ metal ions and MG dye
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The FTIR results for the catalyst before and after MG dye adsorption are performed as shown in Fig. 10. For Zn/Al/GA LDH/PSNFs, all characteristic bands were found; the bands at 1035, 1404, and 1546 cm−1 are attributed to the C=C vibration of aromatic rings; the broad band at 3417 cm−1 is attributed to OH; and the peak at 2922 cm−1 corresponds to aliphatic CH2 and CH. The Zn/Al/GA LDH/PSNFs after MG adsorption possess all bands related to, slight change in the intensity of the band for OH stretching group at 3417 cm−1, and a small-intensity peak related to –C–N groups in MG dye appears at 1171 cm−1 with a slight change in position. Notably, the –C–N group stretching intensity is significantly decreased due to the binding behavior between the Zn/Al/GA LDH/PSNFs and MG dye. All of the changes can be attributed to the electrostatic interaction between the cationic MG dye and the different functional groups present in the Zn/Al/GA LDH/PSNFs and the consequent formation of H-bonds (Ramezani et al. 2013; Yang et al. 2018).

Fig. 10
figure 10

FTIR spectra of MG dye and Zn/Al/GA LDH/PSNFs before and after adsorption of MG dye

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The XRD of the Zn/Al/GA LDH/PSNFs after Cu2+ adsorption was investigated using XRD and EDX, as presented in Fig. 11a. In addition to the characteristic main peaks for LDH at (003), (006), (009), (018), and (015), there are two peaks at 27.6° and 40.39°, which are related to the adsorbed Cu2+ metal ions. Additionally, the change in the peak intensity and shift confirmed the adsorption process. Furthermore, EDX elemental analysis was performed to confirm the adsorption of Cu2+ metal ions onto the Zn/Al/GA LDH/GA/PSNFs after adsorption, as shown in Fig. 11b; Cu2+ metal ions were one of the principal elements of the sample.

Fig. 11
figure 11

XRD patterns of Zn/Al/GA LDH/PSNFs before and after Cu2+ metal ion adsorption (a) and EDAX spectrum of Zn/Al/GA LDH/PSNFs after adsorption (b)

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To demonstrate that Zn/Al/GA LDH/PSNFs can be reused more than one time, adsorption/desorption cycles were repeated five times. The desorption of Cu2+ ions and MG dye cations from Zn/Al/G ALDH/PSNFs was performed using 0.1 M HCl and 0.1 M NaOH solution, respectively, for 2 h. The results of cycle tests are shown in Fig. 12. At the end of the fourth cycle, the Zn/Al/GA LDH/PSNFs retained more than 80% adsorption efficiency for Cu2+ metal ions and an adsorption efficiency of more than 90% for MG dye. Then, the adsorption efficiency gradually decreased. This decrease should be due to the loss of some useful functional groups of the Zn/Al/GA LDH/PSNFs due to the acidic and basic media.

Fig. 12
figure 12

Cycling stability of Zn/Al/GA LDH/PSNFs for Cu2+ metal ions and MG dye adsorption

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3.3 Antimicrobial Activity Studies

The greatest zones of inhibition were achieved by Zn/Al/GA LDH/PSNFs, followed by Zn/sAl/GA LDH/PSNFs after the adsorption of Cu2+ metal ions and by Zn/Al/GA LDH/PSNFs after adsorption of MG dye, as shown in Fig. 13. The main mechanism of the antimicrobial activity of the Zn/Al/GA LDH/PSNFs is local rupturing or pore formation in the cell membranes of Gram-negative and Gram-positive bacteria, with consequent leakage of essential intracellular constituents of the bacterial cell(Borges et al. 2013) (Fig. 14a). Furthermore, our findings were in accordance with results showing that Zn/Al LDH/GA nanocomposite inhibited biofilm formation in broth culture for 48 h at 25 °C (37.72%) and 24 h culture at 37 °C (34.23%). Furthermore, the MIC of Zn/Al/GA LDH was estimated at a concentration of 8 mg/mL (Fig. 15); this material showed higher antibacterial activity to Gram-negative bacteria, such as E. coli, than Gram-positive bacteria such as Staphylococcus aureus (Yang et al. 2018). The affinity of GA to the bacterial cell membrane was very high, and the increased lipophilicity depended upon the intercalation of GA between the Zn/Al LDH layers (Shalaby and Shanab 2013). The intercalation of GA in Zn/Al LDH has selective inhibitory activity against Gram-positive bacteria rather than Gram-negative bacteria, as demonstrated in our study (Lee et al. 2017).

Fig. 13
figure 13

Optical images of the diameters of the zones of inhibition against different bacterial strains and at different concentrations for (a) PSNFs, (b) Zn/Al/GA LDH/PSNFs, and (c) Zn/Al/GA LDH/PSNFs after adsorption of Cu2+ metal ions and (d) Zn/Al/GA LDH/PSNFs after adsorption of MG dye

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Fig. 14
figure 14

Zone of inhibition in mm of (a) Zn/Al/GA LDH/PSNFs, (b) PSNFs, and (c) Zn/Al/GA LDH/PSNFs after adsorption of Cu2+ metal ions and (d) Zn/Al/GA LDH/PSNFs after adsorption of MG dye with different volumes (1, 0.5, 0.25, and 0.125 mL)

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Fig. 15
figure 15

Antibacterial activity (MIC and MBC) of Zn/Al-GA LDH against Gram-positive (S. aureus and S. Pneumoniae), Gram-negative bacteria (E. coli and P. aeruginosa), and two different fungal species (A. flocculosus and A. nigricans). Lower drug concentration needed for either bacteriostatic or bactericidal activity against Gram-positive bacteria than other species (higher antimicrobial activity)

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Additionally Zn/Al/GA LDH/PSNFs changed the bacterial hydrophobicity, the polar, and electron-acceptor components of the bacterial cells. After exposure to Zn/Al/GA LDH/PSNFs, the electron-acceptor ability increased for Gram-positive bacteria and decreased for Gram-negative bacteria. This result demonstrates that Zn/Al/GA LDH/PSNFs are electrophilic materials and seem to significantly interact with bacterial surface components; this conclusion is in accordance with our results (Borges et al. 2013).

After Cu2+ metal ion removal, good antimicrobial activity observed in this material showed higher antibacterial activity to Gram-negative bacteria, such as E. coli, than Gram-positive bacteria such as Staphylococcus aureus(Yang et al. 2018). The affinity of GA to the bacterial cell membrane was very high, and the increased lipophilicity depended upon the intercalation of GA between the Zn/Al LDH layers(Shalaby and Shanab 2013). The intercalation of GA in Zn/Al LDH has selective inhibitory activity against Gram-positive bacteria rather than Gram-negative bacteria, as demonstrated in our study (Lee et al. 2017).

Notably, some metal ions, including Cu2+ and Zn2+, have the ability to create strong bonds with imidazole, amino, or thiol groups on the bacterial cell membrane due to structural changes and increased cell permeability, causing disorders in regular transport through the plasma membrane in microorganism cells and hence cell death (Stanic et al. 2010).

This study demonstrated that after being used to remove Cu2+ metal ions and MG dye, the prepared nanomaterials Zn/Al LDH, PSNFs, and Zn/Al/GA LDH/PSNFs showed effective bactericidal activity against Gram-positive and Gram-negative microorganisms.

4 Conclusion

A novel hierarchical nanocomposite, Zn/Al/GA LDH nanocomposites anchored on PSNFs, was prepared via an effective, simple, easy, and inexpensive method to synthesize Zn/Al/GA LDH/PSNFs using the co-precipitation method. The prepared material was used in the uptake of Cu2+ metal ions and MG dye from wastewater. The maximum adsorptivity percentage was obtained at pH 5 and 8 for Cu2+ metal ions and MG dye, respectively. The Zn/Al/GA LDH/PSNFs yielded an optimum dosage of 0.05 g/50 mL at room temperature, and the maximum adsorption capacity was 190 mg/g and 61.70 g/g for Cu2+ metal ions and MG dye, respectively. The adsorption kinetic data for Cu2+ metal ions were fitted to the pseudo-second-order model, with R2 = 0.97, and the MG dye data were fit to the pseudo-first-order model, with R2 = 0.99. The Zn/Al/GA LDH/PSNFs retained more than 80% of the adsorption efficiency for Cu2+ metal ions and more than 90% efficiency for MG dye. This catalyst is a good candidate adsorbent for Cu2+ metal ions and organic MG dye. The antimicrobial activity was investigated, and the results showed that the synthesized Zn/Al/GA LDH, PSNFs, Zn/Al/GA LDH/PSNFs, and Zn/Al/GA LDH/PSNFs after the removal of Cu2+ metal ions or MG dye showed effective bactericidal activity against Gram-positive and Gram-negative microorganisms. Consequently, this material can be considered as a promising material in the development of new adsorbents.