Introduction

Ethidium bromide (EtBr) is a potent mutagen and a toxic chemical [1]. It is also one intercalating common agent employed as a tag of fluorescent (nucleic acid stain) in laboratories of molecular biology for techniques, such as agarose gel electrophoresis [2]. Because the unique structure of EtBr resembles DNA, it can easily intercalate into DNA strand. Therefore, in the life science field, it is commonly used as nucleic acid fluorescent tag in various techniques.

Graphene is the thinnest known material, i.e., a sheet of carbon atoms could be arranged in hexagonal cells of only a single atom thick and yet be stronger than diamond [3]. Since it was experimentally isolated in 2004, it has been the object of intense theoretical and experimental research [4, 5]. Graphene oxide (GO) is similar to graphene, but presents oxygen-containing functional groups [5–8].

Recently, many studies have been done on the absorption process and have been compared with classical adsorbents, such as CNTs [9, 10], clay [11], activated carbon [12], graphene, and graphene derivatives, such as graphene oxide [13]. Graphene and graphene derivatives are more attractive recently because of their high selectivity, favorable physicochemical stability, and structural diversity. Extensive experiments have been conducted on the adsorption of organic or inorganic contaminants on graphene and graphene derivatives, such as Formaldehyde Molecule [14], Uranium(VI) [15], 1-naphthol [16], dyes [17], and adsorption of Pb(II) and Hg(II) [18]. Therefore, graphene and graphene derivatives might be good sorbents for the removal of contamination from water.

In this work, adsorption process was carried out for 17 min, 298 K, and pH 8 for removal ethidium bromide (EtBr) from aqueous matrices by graphene oxide (GO) surface as adsorbent from aqueous matrices. Adsorption of ethidium bromide on graphene oxide surface was interpreted well by type (I) of the pseudo-second-order kinetic model.

Materials and methods

Materials

Ethidium bromide (EtBr) (C21H20BrN3) used here was procured from Sigma-Aldrich Co. at 95% purity, and molecular weight was 394.31. NaOH and HCl were used to regulate the pH of the samples. The single-layer graphene oxide was prepared from Graphene Supermarket, (USA) with the following specifications: aqueous dispersion: concentration: 500 mg/L, 175 ml; composition: carbon (79%), oxygen (20%), flake size: 0.3–0.7 lm; thickness: 1 atomic layer–at least 80%; and brown color in all adsorption experiments as adsorbent. Specific surface area is determined to be 133 m2/g (see Fig. 1).

Fig. 1
figure 1

Structure of ethidium bromide (EtBr)

Full size image

Preparing GO surface

Graphene oxide was prepared from Graphene Supermarket, USA, and scanning electron microscopy of microstructures of GO surface used in this work is shown in Fig. 2.

Fig. 2
figure 2

SEM image of graphene oxide surface prepared from graphene Supermarket, (USA)

Full size image

Adsorption process study

Adsorption experiments were performed by adding 0.5 mg of graphene oxide (GO) surface as adsorbent into 20 mL of ethidium bromide (EtBr) solutions with known concentrations of 0.5 mg L−1. The samples were collected, and the concentration of ethidium bromide in the aqueous matrices was determined after a specified period of time at 2, 5, 8, 11, 14, 17, and 20 min by applying the spectrophotometer of UV–VIS (Thermo Electron Corporation, Aquamate) at 475 nm, respectively. Thereupon, to study adsorption kinetic parameter (q t ), in mg/g, for ethidium bromide at time t Eq. (1) was used [19].

$$q_{t} = \, \left( {\frac{{C_{0} - C_{t} }}{W}} \right) \, \times V,$$
(1)

where C 0 (mg L−1) was the initial ethidium bromide concentration, C t (mg L−1) was the ethidium bromide concentration at time t, q t (mg/g) was ethidium bromide adsorption capacity at time t, W (g) was adsorbent mass, and V (L) was the volume of ethidium bromide solution. Adsorption time curve for removal of ethidium bromide by GO as adsorbent is shown in Fig. 3.

Fig. 3
figure 3

Contact time effect of the adsorption ethidium bromide onto GO surface as adsorbent, initial concentration: 0.5 mg L−1; dosage of adsorbent: 0.5 mg; temperature: 298 K and pH: 8

Full size image

The graphene oxide adsorption experiments were performed using the batch technique to determine the ethidium bromide adsorption capacity. After 17 min, no noticeable change was observed in the amount of absorption capacity of EtBr on the GO surface as adsorbent. Therefore, 17 min was selected as optimum time for removal of ethidium bromide by graphene oxide surface as adsorbent in all adsorption experiments.

Result and discussion

GO characterizations

Graphene oxide surface was prepared from (Graphene Supermarket, USA), and scanning electron microscopy of microstructures of GO surface used here is presented in Fig. 2.

The effect of pH

Removal of ethidium bromide from solution by graphene oxide adsorbent was carried out at 17 min as optimum time at temperature 298 K, and to also for find the optimum value of pH, experiments of ethidium bromide adsorption onto graphene oxide surface were performed at different pH values from 2 to 10. With increasing the initial pH value of solution from 2 to 8, removal of EtBr by GO surface as adsorbent increased, and maximum amount of adsorption of (EtBr) on (GO) surface was at pH 8. Then, as shown in Fig. 4, with increasing initial pH value of solution from 8 to 10, (EtBr) removal by (GO) surface decreased.

Fig. 4
figure 4

Effect of pH on the adsorption ethidium bromide onto GO surface as adsorbent, initial concentration: 0.5 mg L−1; dosage of adsorbent: 0.5 mg; temperature: 298 K and pH: 8

Full size image

Adsorption kinetics study

Adsorption kinetics experiments were conducted to obtain the resulting kinetic parameters and to investigate the effects of contact time. Figure 3 presents the variation in the adsorption of ethidium bromide by graphene oxide surface as adsorbent and as a function of contact time. It was noticed that after 17 min, no noticeable change was observed in the amount of absorption capacity of EtBr on the GO surface as adsorbent. Therefore, 17 min was selected as optimum time for removal of ethidium bromide by graphene oxide surface as adsorbent in all adsorption experiments. The adsorption kinetics of ethidium bromide was plotted using the Elovich, four types of the pseudo-second-order, the pseudo-first-order, and the intra-particle diffusion kinetic models.

The intra-particle diffusion kinetic model

For kinetic study, the constant rate for intra-particle diffusion is given by [20]:

$$Q_{t} = \, k_{i} (t)^{1/2} + \, C,$$
(2)

where ‘Q t ’ (mg/g) was the amount of ethidium bromide adsorbed on graphene oxide surface at different times t; C (mg/g) was a constant for the intra-particle diffusion model that gives an idea about the thickness of the boundary layer, and ‘k’ (mg/g min1/2) was the intra-particle diffusion rate constant [21], determined by plotting. Figure 5 shows q t versus plot t 1/2 q t .

Fig. 5
figure 5

Intra-particle diffusion adsorption kinetic of ethidium bromide on GO adsorbent. Conditions: initial concentration: 0.5 mg L−1; dosage of adsorbent: 0.5 mg; temperature: 298 K and pH: 8

Full size image

The Elovich kinetic model

At kinetic study the Elovich model is generally expressed as equation [22]:

$$\frac{{{\text{d}}q_{t} }}{{{\text{d}}t}} = \alpha \,{ \exp }( - \beta q^{2} ).$$
(3)

Elovich model liner form is expressed as Eq. (4) [22, 23]:

$$Q_{t} = \frac{1}{\beta } \ln \, (\alpha \beta ) \, + \frac{1}{\beta } \ln \, t,$$
(4)

where ‘β’(g/mg) was the extent of surface coverage, ‘Q t ’(mg/g) was the amount of ethidium bromide adsorbed on graphene oxide surface at different times t, and ‘α’ (mg/g min) was the initial adsorption rate. The intercept and slope at the linear relationship of the plot of q t versus ln t were used to determine α and β. Figure 6 shows the plot of q t versus ln t.

Fig. 6
figure 6

Elovich adsorption kinetic of ethidium bromide on GO adsorbent. Conditions: initial concentration: 0.5 mg L−1; dosage of adsorbent: 0.5 mg; temperature: 298 K and pH: 8

Full size image

The pseudo-first-order kinetic model

In general, at kinetic studies, the pseudo-first-order kinetic model equation is expressed as [24–26]

$$\frac{{{\text{d}}q_{t} }}{{{\text{d}}t}} = \, k_{1} (q_{\text{e}} \, {-} \, q_{t} ).$$
(5)

The integrated form of the pseudo-first-order kinetic model equation is as [27]

$$\log (q_{\text{e}} - q_{t} ) = \log (q_{\text{e}} ) - k_{1} t,$$
(6)

where q e and q t were the amounts of ethidium bromide removed by graphene oxide adsorbent at equilibrium and t time, respectively. k 1 was the constant rate. By plotting values of log (q e−q t ) versus t, q e and k 1 can be determined from the intercept and slope, respectively. Figure 7 shows the plot of log (q e−q t ) versus t.

Fig. 7
figure 7

Pseudo-first-order kinetic of ethidium bromide on GO adsorbent. Conditions: initial concentration: 0.5 mg L−1; dosage of adsorbent: 0.5 mg; temperature: 298 K and pH: 8

Full size image

The pseudo-second-order kinetic model

In general, at kinetic studies, the pseudo-second-order kinetic model equation is expressed as Eq. 7 presented by Ho in 1995 demonstrated how the rate depended on the adsorption equilibrium capacity [28, 29]:

$$\frac{{{\text{d}}q_{t} }}{{{\text{d}}t}} = \, k\,(q_{\text{e}} {-} \, q_{t} )^{2} .$$
(7)

An integrated pseudo-second-order rate can be obtained from Eq. (7) for the boundary conditions q t  = 0 to q t  = q t and t = 0 to t = t, which is given by [30]

$$\frac{1}{{(q_{\text{e}} - q_{t} )}} = \frac{1}{{q_{\text{e}} }} + \, kt.$$
(8)

By rearranging, Eq. (8) can take a linear form as

$$\frac{t}{{q_{t} }} = \frac{1}{{k_{2} q_{{{\text{e}}^{2} }} }} + \frac{t}{{q_{\text{e}} }},$$
(9)

where k 2 (g mg−1 min−1) was the equilibrium rate constant of pseudo-second-order model; t (min) was the reaction time; q e (mg g−1) was the amount of adsorbate at equilibrium; and q t (mg g−1) was the amount of adsorbate at time t.

In this study, four types of linear forms of the pseudo-second-order kinetic model [31, 32] were used. Figures 8, 9, 10, 11 show the plots of four types of linear forms of the pseudo-second-order kinetic model.

Fig. 8
figure 8

Type 1 of pseudo-second-order kinetic of ethidium bromide on GO adsorbent. Conditions: initial concentration: 0.5 mg L−1; dosage of adsorbent: 0.5 mg; temperature: 298 K and pH: 8

Full size image
Fig. 9
figure 9

Type 2 of pseudo-second-order kinetic of ethidium bromide on GO adsorbent. Conditions: initial concentration: 0.5 mg L−1; dosage of adsorbent: 0.5 mg; temperature: 298 K and pH: 8

Full size image
Fig. 10
figure 10

Type 3 of pseudo-second-order kinetic of ethidium bromide on GO adsorbent. Conditions: initial concentration: 0.5 mg L−1; dosage of adsorbent: 0.5 mg; temperature: 298 K and pH: 8

Full size image
Fig. 11
figure 11

Type 4 of pseudo-second-order kinetic of ethidium bromide on GO adsorbent. Conditions: initial concentration: 0.5 mg L−1; dosage of adsorbent: 0.5 mg; temperature: 298 K and pH: 8

Full size image

The Chi-square statistic (χ 2) was used to evaluate the fitness of kinetic equations to the experimental data [13]. Chi-square statistic can be defined as

$$\chi^{2} = \sum\nolimits_{i}^{N} {\frac{{\left( {q_{\text{e,exp}} - q_{\text{e,cal}} } \right)^{2} }}{{q_{\text{e,cal}} }}} .$$
(10)

χ 2 will be a small number if the data from the experimental data are similar to the model, and χ 2 will be a large number if they are different. N is the number of observations in the experimental data, and the subscripts “calc” and “exp” show the calculated and experimental values, respectively.

According to the results of kinetic study listed in Tables 1 and 2, removal of ethidium bromide (EtBr) by graphene oxide (GO) surface as adsorbent from solution was well interpreted by pseudo-second-order kinetic model the type (I) because of low value of the Chi-square statistic (χ 2) and high value of the correlation coefficients (r 2).

Table 1 Pseudo-first-order, Elovich and intra-particle diffusion kinetic parameters for adsorption ethidium bromide on GO adsorbent
Full size table
Table 2 Four types of the pseudo-second-order kinetic of ethidium bromide on GO adsorbent
Full size table

Conclusions

In summary, adsorption capacity of ethidium bromide (EtBr) on graphene oxide (GO) surface as solution adsorbent was investigated. To remove ethidium bromide by graphene oxide adsorbent, 17 min was selected as optimum time as after 17 min, no noticeable change was observed in the amount of absorption capacity of EtBr on the GO surface. The results of pH effect showed that high amount of adsorption capacity of (EtBr) on (GO) surface was at pH 8; therefore, solution pH was fixed at pH 8. All adsorption experiments were performed at temperature 298 K. For kinetic study and to test adsorption experimental data the Elovich, the pseudo-first-order, the intra-particle diffusion, and the pseudo-second-order kinetic models were used. The results of kinetic study showed that adsorption of ethidium bromide on graphene oxide surface was well interpreted by type (I) of the pseudo-second-order kinetic model because of the low value of the Chi-square statistic (χ 2) and high value of the correlation coefficients (r 2).