1 Introduction

Nanoparticle-sized materials have been the platform of intense investigations for their valuable applications in almost all fields of technology [1,2,3,4]. Among the key nanoparticle materials with successful practices is the metal oxide nanosized such as ZnO NPs, which has been proposed as a photocatalytic substance with a wide bandgap of 3.37 eV and an exciton-binding energy of 60 meV [5, 6]. ZnO NPs are effective adsorbates [7], coating elements for cellulose fibers [8], and magnetic materials used in information storage devices [9]. Besides, the small size and hence great surface area and surface energy of ZnO NPs allow different pharmacological and biomedical activities as antibacterial, fungicidal, and anticancer agents [10,11,12,13]. ZnO NPs showed high cytotoxic activity on different cell lines [14, 15], where the nanoparticles cause rounding of cells and reduction in the nuclear volume leading to nuclei fragmentation and release of apoptotic bodies [12]. It is well established that green synthesis of nanoparticles offers an economical preparation method and yields low toxic nano-products with versatile applications [16,17,18]. Seaweeds, microorganisms, and plant extracts are examples of biomaterials that cause metal reduction leading to nanoparticle formation [19, 20]. Another feature of ZnO NPs that attracted many research works is their superior effect as heavy metal removal in the water treatment process [7, 21,22,23]. Generally, the existence of heavy metal ions in water streams is a global problem. Some metal ions as Pb2+ and Cd2+ are lethal to the environment even in trace concentrations due to their high toxicity, relative bioavailability, and low degradability that results in a high tendency of accumulation. Many health problems arise from the intake of water contaminated by Pb2+ and Cd2+ [24, 25]. Due to the rapid and growing industrialization which increases the mass of annual discharge of metal ions to the water systems, there is a continuous need for developing methods for treating water from such contaminants. Various adsorption substances are efficient in capturing heavy metals from solutions such as humic acid, zeolite [26], and chitosan [27]. In consideration of the pertinent applications of ZnO NPs, the current work is our contribution to the green synthesis of ZnO NPs employing plant extract. The yielded nanoparticles were screened for their biological activities. Furthermore, the adsorption capacity of the synthesized ZnO NPs was assessed by removing Pb2+ and Cd2+ from environmental water samples collected from fresh, brackish, and seawater from different locations in Alexandria, Egypt.

2 Experimental

2.1 Chemicals

The chemicals utilized are of high analytical grades (Merck) and were used without further purification. Stock solutions of cadmium acetate dihydrate (purity ≥ 98%) and lead nitrate (purity ≥ 99%) were prepared in deionized double-distilled water and used as sources of the investigated ions Cd(II) and Pb(II) respectively. Zinc acetate dihydrate (Zn(CH3COO)2.2H2O, purity ≥ 98%) was used for the synthesis of ZnO nanoparticles. Also, analytical grade sodium acetate anhydrite (1 M) and hydrochloric acid (1 M) were used to prepare the buffering system of different pH values (pH = 2–7). The interfering ion solutions were prepared by dissolving 0.1 g of the salts; KCl, NaCl, MgSO4, KNO3, NaCO3, and CaCO3 in 25 ml of deionized water. The solution was then mixed with the investigated metal ion solutions under their specific optimum adsorption conditions which were determined in separate experiments.

2.2 Instruments

Different techniques were used in the characterization of ZnO NPs and their adsorption capacity. The pH value of water samples was measured by calibrated Electrochemistry Analyzer pH-meter (JENWAY 3505). The infrared spectrum was recorded in the range 400–4000 cm−1 with the BRUKER TENSOR 37 FT-IR spectrophotometer. The surface topography and particle size of the synthesized ZnO NPs were scanned by a scanning electron microscope (SEM, Model: JEOL-JSMIT 200). The nanoparticles sizes and shapes were further detected by transmission electron microscope (TEM, Model: JSM-1400 PLUS). Elemental analysis of the samples was performed by energy-dispersive X-ray emission spectroscopy, EDX. Also, the phase identification and the lattice spacing (d) in ZnO NPs sample were detected by the X-ray diffractometers using Cu Kα as a radiation source of wavelength (λ = 1.5406 Å) at a current of 30 mA with 40 kV. The data were scanned at 2θ in the range of 0 to 70°. The concentration of Pb2+ and Cd2+ after each experiment was recorded by atomic absorption spectrophotometer (Analytic Jena contra 300 Atomic Absorption, Germany). All instrumental measurements were achieved at the central laboratory of the Faculty of Science, Alexandria university.

2.3 Synthesis of ZnO NPs by plant extract

The leaves of Ocimum tenuiflorum plant (commonly known as Holy basil) were freshly collected, cleaned with running tap water to remove dirt, and then dried in sunlight. The dried powder was preserved in an airtight container. Ten grams of powdered leaves were added to 100 ml of deionized water then boiled for 10 min until the mixture turned red, then cooled. The leaf extract was centrifuged for 5 min at 5000 rpm, filtered, and refrigerated. ZnO NPs were produced by adding 2 ml of the O. tenuiflorum leaves aqueous extract dropwise to a 0.02 M solution of Zn(CH3COO)2.2H2O under conditions of continuous stirring in 70 °C water bath for 2 h. Drops of 0.5 M sodium hydroxide were added until pH ~ 12. The light-yellow precipitate of ZnO was formed. After repeated re-dispersions in deionized water, the precipitate was centrifuged and filtered. Calcination of the obtained precipitate was done in a ceramic crucible at 550 °C for 3 h [15, 26]. The yielded yellow powder was dried overnight at 80 °C and stored in airtight bottles for characterization. The preparation scheme is shown in Fig. S1.

2.4 Methodology

2.4.1 Batch equilibrium method

The adsorption capacity of the synthesized ZnO NPs for the metal ions under investigation (Pb2+ and Cd2+) was tested by the batch equilibrium method [17, 28]. A stock of adsorbate solution (1000 ppm) of each ion was prepared in deionized water. As an initial experimental condition, a certain amount of the green adsorbent was added and left in contact with the adsorbate solution under continuous stirring for 30 min in a rotary shaker at a rate of 150 rpm. Different experimental conditions including pH, adsorbent dosage, contact time, adsorbate concentration, and interfering ions were changed and tested separately to discover the optimum conditions of metal ions adsorption. After each experiment, the concentration of the residual metal ions in the filtrate was detected by AAS to evaluate the metal ions uptake.

2.4.2 Calculation of mass adsorption capacity

The mass adsorption capacity (qe) and the percentage removal (% R) of the investigated metal ions (Pb2+ and Cd2+) by ZnO NPs were quantified by the following formulae [21]:

$${q}_{e}=\frac{\left({C}_{o}- {C}_{e}\right)V}{W}$$
$$\%R= \frac{\left({C}_{o}-{C}_{e}\right)\times 100}{{C}_{o}}$$

where Co is the initial concentration of the metal ions in solution in ppm, Ce is the concentration of the residual metal ions at equilibrium after applying the batch experiment, V is the volume of solution in liter, and W is the mass of ZnO NPs in grams.

2.4.3 Multistage microcolumn technique

The removal of heavy metals from real water samples by adsorption on the synthesized ZnO NPs was evaluated by the multistage microcolumn technique [26, 29]. The column was packed with 0.1 g of the sorbent. One liter of the water sample was pre-tested for the content of metal ions of interest and other physicochemical properties including percentage salinity (S%), pH, and oxidizable organic matter (OOM). Samples were also pre-tested for dissolved nutrient salts (nitrate (NO3-N), nitrite (NO2-N), dissolved inorganic phosphate (DIP), and dissolved inorganic silicate (DSi). Additionally, the major constituents Ca2+, Mg2+, Na+, Li+, K+ were assessed, Table S1. Water samples were allowed to pass through the packed column at a slow flow rate of 0.5 ml min−1. After the completion of three successive extractions, the filtrate was analyzed for Cd2+ and Pb2+ ions by AAS.

2.5 Cytotoxic screening against Panc-1, PC-3, CACO-2, and MRC-5

The cytotoxic activity of the green synthesized ZnO NPs was tested against three human cancer cell lines; pancreatic cancer cells (Panc-1), intestinal carcinoma cells (CACO-2), prostate cancer cells (PC-3), and normal human lung fibroblast cells (MRC-5) utilizing the colorimetric (MTT) assay, 3-(4,5-dimethylthiazole-2-yl)-2,5-diphenyltetrazolium bromide in a microtiter plate [30]. The results are indexed by IC50 which represents the inhibition of the growth of the cells by 50% relative to cisplatin, the standard anticancer drug. The measurements were performed at the Regional Center for Mycology and Biotechnology, Al-Azhar University, Egypt. Of the seeded cells, 96-well plates were incubated at 37 °C for 24 h. Each sample concentration in DMSO was managed in triplicates and the results were averaged. All mammalian cell lines were obtained from the American Type Culture Collection (ATCC, Rockville, MD).

3 Results and discussion

3.1 Characterization of the synthesized ZnO NPs

3.1.1 FT-IR spectroscopy

FT-IR of the synthesized ZnO NPs exhibits six characteristic peaks (Fig. 1). The broad band at 3308 cm−1 is assigned to the νOH of the phenolic group of the plant extract which is capping the particles. The small peak at 2344 cm−1 is attributed to νC-H. Also, the bands that appeared at 1493 and 1394 cm−1 are due to the stretching vibrations of the C–N and –COO groups, respectively, which are the common functional groups of the Ocimum tenuiflorum plant that are accountable for the reduction of Zn2+ ion and the formation of the metal nanoparticles [31]. Moreover, the band at 901 cm−1 could be assigned to the bending mode of the O–H group. The sharp band that appeared at 474 cm−1 is attributed to the metal–oxygen stretching which demonstrates the formation of ZnO NPs [32]. FT-IR spectra of the adsorbed Pb2+ and Cd2+ on ZnO NPs are shown in Fig. 1. Clear band shifts and intensity changes are observed in the FT-IR spectra after metal ion adsorption. For example, the disappearance of the bands at 3308 cm−1 (νO–H), 2344 cm−1 (νC–H) 1493 cm−1 (νC–N), and 1394 cm−1 (νCOO) upon metal ions adsorption indicates the role of these functional groups in the adsorption process. Besides, the emergence of new IR bands in the range 650–790 cm−1 is assigned to the stretching vibration of Pb–O and Cd–O bonds [21, 33]. Also, the band at 901 cm−1 (δO–H) suffered a shift upon adsorption to 1014 and 1016 cm−1 for Cd2+ and Pb2+, respectively, which is further evidence of the involvement of the O–H group in the adsorption process.

Fig. 1
figure 1

FT-IR of the synthesized free ZnO NPs (top) and the modified ZnO NPs after adsorption of Cd2+ (middle) and Pb.2+ (bottom)

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3.1.2 X-ray diffractometry (XRD)

Eight Sharp XRD peaks of the synthesized ZnO NPs were recorded (Fig. 2). The values of lattice spacing (d) corresponding to each diffraction angle with their intensity counts are displayed in Table S2. The peaks located at Bragg’s diffraction angles (2θ) of 31.73°, 34.38°, 36.23°, 47.50°, 56.59°, 62.77°, 67.96°, and 69.12° were assigned to the planes (100), (002), (101), (102), (110), (103), (112), and (201) according to the Joint Committee of Powder Diffraction Standards of ZnO file (JCPDS 36–1451) [34]. The diffraction results confirmed the crystallization of the synthesized ZnO NPs in the hexagonal wurtzite structure which is the most stable and common form at normal conditions of temperature and pressure [35]. Also, the experimental interplanar spacing 2.817 Å, 2.60 Å, and 2.477 Å were in excellent agreement with the standard values of JCPDS 2.814 Å (100), 2.603 Å (002), and 2.476 Å (101) for hexagonal ZnO.

Fig. 2
figure 2

XRD pattern of the synthesized ZnO NPs using Cu anode at λ = 1.5406 Å

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3.1.3 Scanning electron microscopy (SEM), transmission electron microscope (TEM), and energy-dispersive X-ray (EDX)

The surface morphology of the synthesized ZnO NPs was examined by SEM technique. The images (Fig. 3) show that the particles have spherical shapes of particle sizes ~ 19 nm. No evidence of aggregation was observed in the scan which is due to the stabilization of nanoparticles by the plant extract. SEM images of the surface of ZnO nanosphere after the adsorption of Cd2+ and Pb2+ (Fig. 4) demonstrate clear changes owing to the occupation of the active sites by these metal ions yielding a non-uniform covering around the ZnO NPs surface. The observed changes on the surface verified the physical adsorption mechanism [36]. The particle size of the synthesized ZnO NPs was further approved by the TEM technique (Fig. 5). Also, the connection of EDX to the SEM technique provides a tool to identify the elemental composition of the scanned sample. EDX spectrum (Fig. 6) revealed that the abundant elements in the synthesized ZnO NPs are zinc and oxygen at binding energies of 1 and 0.5 keV with measured mass percent of 56.17 and 39.08%, respectively [37]. No discernible peak was noted for impurities. The data point to the high purity of the synthesized ZnO nanoparticles [34].

Fig. 3
figure 3

SEM of ZnO NPs showing the spherical nature of the particles with a size of 18.68 nm

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Fig. 4
figure 4

Morphology changes due to the adsorption of (a) Cd2+ and (b) Pb2+ on ZnO NPs

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Fig. 5
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Images of the synthesized ZnO NPs by TEM technique

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Fig. 6
figure 6

EDX profile of the synthesized ZnO NPs

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3.2 Effect of pH, adsorbent dosage, contact time, initial metal ion, and interfering ions concentrations on the adsorption capacity

The adsorption of Pb2+ and Cd2+ by the synthesized ZnO NPs was measured at pH range (2–7) at room temperature by the batch equilibrium approach where 0.02 g of ZnO was added to 25 ml of samples loaded with equimolar of the tested ions. The maximum adsorption of Pb2+ and Cd2+ (96.73% and 21.35%, respectively) was achieved at pH = 7 (Table 1). Generally, the small size and the high mobility of hydrogen ions in solutions result in a higher affinity for adsorption on surfaces than the affinity for Pb2+ and Cd2+ [38, 39]. Therefore, the high concentration of hydrogen ions at low pH values leads to great competition between protons and metal ions for the active sorption sites, and hence low adsorption capacity was observed for the investigated metal ions in the strong acidic solutions. However, at pH > 5, the deprotonation of the surface enhances the uptake of the metal ions from the surrounding medium. Also, the adsorption capacity (qe) values detected at pH 7 for Cd2+ and Pb2+ are 24.18 and 5.34 mgg−1, respectively. This implies high selectivity of ZnO NPs material for Pb2+ compared to Cd2+.

Table 1 The impact of pH on the % removal and metal sorption capacity (mgg−1) of ZnO NPs
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Additionally, the impact of sorbent dosage on the Pb2+ and Cd2+ removing capacity was determined at various weights in the range of 10–200 mg of ZnO NPs at the optimum pH value (pH = 7). The results showed that the maximum adsorption capacity for Pb2+ (qe = 25 mgg−1) was achieved at a sorbent dosage of 40 mg of ZnO NPs. However, the adsorption of Cd2+ exhibited high value (qe = 25 mgg−1) at 200 mg of ZnO NPs (Fig. 7). Noteworthy, the small sorbent dosage required for the maximum removal of ions is good evidence for the efficiency of ZnO NPs as an adsorbing surface.

Fig. 7
figure 7

The adsorption capacity of Pb2+ and Cd2+ at different dosages of ZnO NPs

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Also, the optimal contact time for the adsorption of Pb2+ and Cd2+ on the surface of ZnO NPs was assessed at time intervals between 10 and 60 min and under the optimum experimental conditions for each investigated ion. Clearly, the mechanism started with rapid adsorption of the positive ions on the surface of ZnO NPs (Fig. 8) which was then slowed after the saturation of most of the available active sites. The maximum adsorption of Cd2+ was obtained at 50 min with removal percentage and capacity of metal sorption are 40.7% and 10.18 mgg−1, respectively. On the other hand, the adsorption of Pb2+ reached its maximum at a contact time of 20 min with removal percentage and capacity of metal sorption values of 99.99% and 25 mgg−1, respectively. In the current work, the detected short equilibrium contact time for the adsorption of Pb2+ provides an economic benefit for large-scale wastewater treatment applications.

Fig. 8
figure 8

The time-dependent adsorption of Pb2+ and Cd2+ by ZnO NPs

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Moreover, the effect of the initial metal ion concentration (C°) on the adsorption capacity was tested at different Pb2+ and Cd2+ concentration values in the range of 5–100 ppm at the optimum pH, sorbent dose, and contact time. The profile of percentage metal ions removal as a function of initial metal ions concentration (Fig. 9) revealed a primarily increase in the metal uptake with the increase of Pb2+ concentration. However, at higher concentrations of the tested ion, the removal decreases probably due to the surface saturation. Maximum removal of 99.24% of Pb2+ from the solution was attained by using an initial concentration of (C° = 20 ppm). However, the peak adsorption for Cd2+ corresponds to a removal of 56.98% was reached early at C° = 5 ppm (Fig. 9). This was followed by a rapid decline in adsorption due to fewer available sites for adsorption. The results imply that ZnO NPs have a high adsorption affinity for Pb2+, and hence is an efficient adsorbent for the elimination of this ion from aqueous solutions [40]. The data of the adsorption experiments were fitted to different isotherm and kinetic models.

Fig. 9
figure 9

The effect of initial concentration of Pb2+ and Cd2+ on their % removal by ZnO NPs

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Based on the batch experiments, the optimum adsorption conditions for the two investigated ions were established. The initial concentration of 5 ppm Cd2+, contact time = 50 min., and 200 mg ZnO NPs were the best conditions in the case of Cd2+. However, 20 ppm Pb2+, contact time = 20 min, and 40 mg ZnO NPs are the condition for maximum adsorption of Pb2+. Furthermore, the influence of interfering ions, on the adsorption capacity of ZnO NPs was examined by adding 25 ml of 0.1 g of some selected ions, NaCl, KCl, MgSO4, KNO3, NaCO3, and CaCO3 in deionized water to the standard solution containing Cd2+ and Pb2+ under the optimum experimental adsorption conditions at pH = 7. The results are summarized in Table 2. Generally, ions that coexist in solutions greatly hinder the adsorption of the ions of interest by competing for the sorbents’ active sites. The mechanism is mainly governed by the hydrated radius which is inversely related to the ionic radius and to the affinity of adsorption [41]. The small hydration shell of Pb2+ facilitates its mobility across the boundary layer of the adsorbent surface [42]. In the present work (Table 2), the absence of interfering ions yields the highest adsorption capacity towards Cd2+ and Pb2+ with a percentage removal of 98.32 and 99.61%, respectively. The results showed that NaCl ions exhibited the highest interfering effect in the case of the Cd2+ adsorption which lowered the removal percentage of Cd2+ by about 25%. However, Na2CO3 and CaCO3 cause insignificant changes in the adsorption capacity of Cd2+. Besides, most of the selected ions did not interfere significantly in the removal of Pb2+ by ZnO NPs except for the Na2CO3 which causes a reduction in the efficiency of Pb2+ removal by about 10%.

Table 2 The effect of interfering ions on the percentage removal and metal sorption capacity (mgg−1) of Cd2+ and Pb2+ on ZnO NPs
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3.3 Adsorption modeling

3.3.1 Isotherm models

Four different isotherm models, namely Langmuir, Freundlich, Tempkin, and Dubinin–Radushkevich, were fitted to the adsorption data of Cd2+ and Pb2+ ions on ZnO NPs to correlate the experimental results with the model. The best-fitting model displayed a correlation coefficient (R2) equal to unity [43]. The model equations with the definition of each parameter are stated in the supplementary data, Table S3. Also, the calculated parameters of the isotherm models are collected in Table 3. Langmuir isotherm assumes that adsorption is a monolayer formation process where one ion is adsorbed per active site with all spots being energetically equivalent, and no interaction occurs between the adsorbed ions [44]. The results revealed that the experimental data for the adsorption of Cd2+ and Pb2+ on ZnO NPs fitted well to the Langmuir isotherm (Fig. 10), with a correlation coefficient of R2 = 0.9659 and 0.8557, respectively. Also, the maximum adsorption capacity (qmax) of the adsorption of Pb2+ is almost twice the adsorption of Cd2+. Moreover, the separation factor RL (Table S3) indexes the feasible nature of adsorption in the Langmuir isotherm. Under the current experimental conditions, the calculated low value of RL (0.01–0.69) indicates that the adsorption is favorable [45].

Table 3 Calculated isotherm model parameters for the adsorption of Pb2+ and Cd2+ ions on ZnO NPs
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Fig. 10
figure 10

Langmuir adsorption isotherm for the adsorption of Cd+2 and Pb+2 by ZnO NPs

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Likewise, the experimental data demonstrated a good fit to Freundlich isotherm (Fig. 11 and Tables S3 & 3), with R2 = 0.9600 and 0.9295 for Pb2+ and Cd2+, respectively. This isotherm states that the concentration of ions adsorbed on the sorbent surface increases with the concentration of the adsorbate with the possibility of multi-layers formation. The value of the adsorption intensity (n) being greater than one in the case of the Cd2+ adsorption indicates the ease of ions separation from solution and hence the viability of adsorption [46].

Fig. 11
figure 11

Freundlich adsorption isotherm for the adsorption of Cd+2 and Pb+2 by ZnO NPs

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Furthermore, the physical or chemical nature of the adsorption process can be justified by Dubinin–Radushkevich’s isotherm (D-R) isotherm (Table S3 and 3). Physical adsorption is assigned for systems with apparent energy also called the mean free energy, ED, in the range (1–8) kJmol−1 [47]. Based on the calculated D-R parameters, the experimental data fitted well with the (D-R) isotherm model, R2 = 0.931 and 0.963 for Pb2+ and Cd2+, respectively (Fig. 12). The calculated ED values for the current work, 1.58 and 0.707 kJmol−1 confirm that the adsorption is a physisorption process [48]. Tempkin isotherm (Table S3 & 3) is a useful model to predict the heat of adsorption. The graphs of qe versus ln Ce (Fig. 13) showed a linear relation with a correlation coefficient of 0.972 and 0.918 for Pb2+ and Cd2+, respectively, indicating the viability of Tempkin’s isotherm. The Tempkin parameters, variation of the heat of adsorption (BT) and the equilibrium binding constant (AT), can be extracted from the slope and the intercept of the linear plot, respectively. The positive value of BT point to an exothermic adsorption process. Also, the affinity of Pb2+ to the active sites at ZnO NPs is greater (AT = 4.55 Lmg−1) than the affinity toward Cd2+ (AT = 0.68 Lmg−1) which suggests an extent of selectivity of the surface to Pb2+ [49]. Furthermore, the heat of adsorption (BT) are found to be 0.027 and 0.474 KJmol−1 for Pb2+ and Cd2+ at T = 298 K.

Fig. 12
figure 12

D-R model for the adsorption of Cd2+ and Pb2+ by ZnO NPs at room temperature

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Fig. 13
figure 13

Tempkin model for the adsorption of Cd2+ and Pb2+ by ZnO NPs at room temperature

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3.3.2 Kinetics of adsorption

Four kinetic models were employed to assess the mechanism of adsorption process of metal ions on the synthesized ZnO NPs. The results of the batch adsorption experiments were fitted to pseudo-first-order, second-order, liquid film, and intraparticle diffusion kinetic models (Table S4). The fitting results revealed a good agreement with the second-order model where the calculated correlation coefficient values are of about unity (Table 4). Also, applying the second-order model yielded a calculated adsorption capacity qe, which is in accord with the experimental values (Table 4), with a perfect linear relation along the whole range of the metal ion concentration (Fig. 14). The excellent fitting of the experimental data to the second-order model implies kinetic mechanism that involves both the metal ion and the surface where the rate-determining step is electron exchange between these two entities [50]. Moreover, to evaluate the nature of the diffusion process that took place in the bulk system prior to the adsorption, intraparticle diffusion and liquid film diffusion models were applied (Table S4). Noticeably, there is a clear divergence from linearity between the experimental results and the intraparticle diffusion model with a low calculated correlation coefficient R2 being in the range 0.196–0.845 (Table S5). This divergence from the model ruled out the dominance of intraparticle diffusion in controlling the adsorption mechanism. However, fitting the data to the liquid film diffusion model (Table S4) showed better correlation values (R2 is in the range 0.6783–0.9897, Table S5) which suggests that the mechanism is mainly governed by the liquid film surrounding the adsorbent surface. Nevertheless, the deviation of the straight lines of the film model from passing through the origin (Fig. S2) indicates the existence of a combination of different diffusion mechanisms leading to a heterogeneous diffusion process [51].

Table 4 Fitting parameters of pseudo-first-order and second-order kinetic models for the Cd2+ and Pb2+ adsorption by ZnO NPs
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Fig. 14
figure 14

Pseudo-second-order kinetic plots of metal ions adsorption by ZnO NPs

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3.4 Adsorption of Cd 2+ and Pb 2+ from environmental water samples

The applicability of the green synthesized ZnO NPs for the elimination of Cd2+ and Pb2+ from environmental water samples was investigated by using a multistage column packed with the sorbent. Samples were gathered from distinct locations to represent different types of water. For example, El-Mex Bay sample represents the polluted brackish water and the sample collected from the West Dessert Operating Petroleum Company discharge exemplifies the wastewater. Samples were also taken from the Eastern Harbor to represent normal seawater and El-Mahmoudia Canal as freshwater. The characteristic physicochemical parameters of each sample are measured and listed in Table S1. The removal percentages in each run of the adsorption of Cd2+ and Pb2+ from the samples are shown in Table 5. Undoubtedly, the best metal ions removal was extracted after the third run of the experiment, where the percentage removal reached 99.95% in the case of the Petroleum Company sample. After the three consecutive extractions, the % removal of Cd2+ and Pb2+ were in the range 48.82–97.36% and 94.93–99.95%, respectively, which points to the feasibility of adsorption of Pb2+ compared to Cd2+. This detected selectivity of ZnO NPs towards Pb2+ has been previously reported [26] and it could be explained by the difference in ionic radius, hydration diameters, and solubility between these ions. Also, the high percentage elimination of Cd2+ from the Freshwater (El-Mahmoudia Canal, 97.36%) implies that interfering ions in these samples have a big influence on adsorption. Moreover, the less removal percentage of the heavy metals of interest from El-Mex Bay station may be due to different impurities and high concentrations of nutrient salts (Table S1). The outlined results in this contribution provide evidence that ZnO NPs synthesized by plant extract are efficient low-cost surface for the removal of Cd2+ and Pb2+ from water samples.

Table 5 Percentage removal of Cd2+ and Pb2+ by the green sorbent ZnO NPs from environmental samples
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3.5 Cytotoxic activity of the green synthesized ZnO NPs

The antiproliferative activity of the green synthesized ZnO NPs has been examined against three mammalian cancer cell lines: Panc-1 (human pancreatic cancer cell line), PC-3 (human prostate carcinoma), and CACO-2 (intestinal carcinoma) as presented in Table 6. The cytotoxicity of ZnO NPs was also investigated versus normal human lung cell MRC-5. The concentrations of ZnO NPs required to inhibit 50% of the examined cells, IC50, are extracted from the graphical relation between the applied ZnO NPs concentrations and the surviving cells (Fig. 15(a–c)), where the viable cells were identified by a colorimetric technique using the MTT method. Many reports have proposed mechanisms of ZnO NPs cytotoxicity against cancers. ZnO NPs is approved to induce oxidative stress inside the cancer cell by increasing the level of reactive oxygen which leads to cell death after the formation of lipid peroxides and the damage of cell protein [52, 53]. Also, the nanoparticles cause a series of observed cell alterations including cell rounding and shrinkage, chromatin aggregation, and the formation of apoptotic bodies that leads to cell apoptosis [12, 54, 55]. In the present study, a superior activity for ZnO NPs against the tested cancer cells has been observed. The high inhibitory activities of the synthesized ZnO NPs (1.70, 3.67, and 5.70 µgml−1) compared to that of the standard therapeutic agent, cisplatin, (3.57, 5.09, 7.75 µgml−1) under the same experimental conditions, points to a promising anticancer candidate. Moreover, the low inhibition efficacy of ZnO NPs against normal cells, MRC-5, implies its potency as a selective low side effect anticancer drug. Furthermore, comparison of the cytotoxic activity of the synthesized ZnO NPs with cisplatin revealed that they exerted nearly equipotency against the normal cell MRC-5 with IC50 of 22.40 ± 1.28 and 22.50 ± 0.73 respectively (Fig. 16).

Table 6 Cytotoxicity activity (IC50) in µgml−1 of the green synthesized ZnO NPs
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Fig. 15
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Antiproliferative activity of ZnO NPs against (a) Panc-1, (b) PC-3, and (c) CACO-2

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Fig. 16
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Cytotoxic activity of ZnO NPs and cisplatin against normal human lung cells

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4 Conclusion

Nano-ZnO was synthesized by an eco-friendly method and then carefully characterized by different spectroscopic techniques. The synthesized ZnO NPs showed significant adsorption capacity for Pb2+ and Cd2+ at pH = 7. The short contact time required for the maximum adsorption of Pb2+ (20 min) pointed to a high degree of selectivity of ZnO to this ion. Perfect removal of Cd2+ and Pb2+ from environmental water samples was accomplished where the highest removal percentage was observed at El -Mahmoudia Canal with a percentage removal of 97.36% and 97.97% for Cd2+ and Pb2+ respectively. The synthesized green sorbent showed substantial efficiency in the metal ions elimination, and hence can be considered a promising eco-friendly and low-cost agent in the remediation of polluted water. Cytotoxic activity of ZnO NPs is another important application that was explored in the current work. The three investigated cancer cell lines (Panc-1, PC-3, and CACO-2) were extremely sensitive to ZnO NPs with IC50 of 1.70, 3.67 and 5.70 µgml−1 respectively. The detected activity was more potent than that of cisplatin, the standard anticancer drug. Interestingly, the nanoparticles exhibited less inhibition effect on the normal cell (MRC-5) compared to their effect on cancer cells. The cytotoxic results shed light on the ZnO NPs synthesized using Ocimum tenuiflorum leaves as promising highly effective, low-price, and low-side effects antiproliferative agent. The combination of reaction conditions, precursors, as well as the method of preparation used in the present contribution, is considered a novel practice that yielded ZnO nanoparticles of very small size (18.68 nm) which led to high removal capacity of Pb2+ and Cd2+ and superior cytotoxicity against the investigated cancer cell lines.