Detachment of Cu (II) and Co (II) ions from synthetic wastewater via adsorption on Lates niloticus fish bones using LIBS and XRF

Graphical abstract

Abstract

Natural fish bones, that are known to have unique adsorption capacity, have been used in the present work for removal of heavy metals, copper, and cobalt, from wastewater. It has been found that sorption process depends on the initial metal concentration and on the contact time. Laser-induced breakdown spectroscopy (LIBS) as a spectrochemical analytical technique was used for qualitative and quantitative analysis of the water samples. X-ray Fluorescence (XRF), as another spectrochemical analytical method, was exploited to characterize the remediation of wastewater. The optimum contact time values for the removal of Cu (II) and Co (II) were 270 and 300 min, respectively. Furthermore, the percentages of adsorbed Cu (II) and Co (II) were high for low initial concentrations and decreased with increasing the heavy metal initial concentrations. The Langmuir and Freundlich isotherm models were used to analyze the equilibrium adsorption data and Freundlich isotherm was found to represent the experimental results well with a correlation factor close to one. However, the pseudo-second-order kinetic model provided the best fit to the experimental data for the adsorption of heavy metals using fish bones compared to the pseudo-first-order model. The obtained results demonstrate the potential of using both LIBS and XRF in the analysis of contaminant wastewater effectively.

Introduction

Contamination of water by heavy metals poses serious ecological problems because of their pernicious effects on human, animals, and plants [1], [2]. Heavy metals such as copper and cobalt find their way to aquatic environment as a result of the rapid industrial development. Textile, mining, automotive metal-finishing, as well as metallurgical industries, release different heavy metals in nearby water drains [1], [2], [3], [4], [5]. Because of the high toxicity of these contaminants, extensive efforts have been exerted to remediate polluted wastewater. Conventional physical and chemical treatment techniques, including chemical precipitation, reverse osmosis, membrane filtration, ion exchange, and oxidation-reduction have been exploited to remove heavy metals from the wastewater [6]. Among these techniques, adsorption can be counted upon as an effective economic technique for the removal of heavy metals from wastewater due to its efficiency, versatility, and ease of operation [7]. More recently, biogenic hydroxyapatite (HAP) of different origins, such as fish and animal bones, bone chars, and food waste, has been used as sorbent materials for remediation of wastewater [8], [9], [10], [11]. Fishbones as a distinctive material of low cost and natural abundance have proven to be one of the most effective heavy metal sorbents used in industrial applications [12]. The adsorption efficiency of fish bones is due to the presence of hydroxyapatite (Ca₁₀(PO₄)₆(OH)₂) structure, that dependence on the exchange reaction with calcium ions with heavy metals [12], [13], [14], [15], [16].

Spectrochemical analytical techniques, such as Laser-Induced Breakdown Spectroscopy (LIBS) and X-ray Fluorescence (XRF) could be used effectively to follow up the adsorption procedure. LIBS uses laser-generated plasma as a source of material vaporization, atomization, and excitation. This technique has been successfully applied to analyze solid, liquid, and gaseous samples. LIBS also offers attractive features for real-time multi-elemental analysis at atmospheric pressure, including remote applications with no or minimal sample preparation in addition of being noninvasive and quasi-nondestructive. This laser spectroscopic technique has the capability for qualitative and semi-quantitative elemental analysis, with detection of most existing species such as major components and/or trace elements with low and high Z-number. It is also possible to use LIBS in situ with portable systems because of its simplicity and compactness of the required equipment contrary to other techniques such as Atomic Absorption Spectroscopy or Inductively Coupled Plasma Optical Emission Spectroscopy. LIBS has significant potential in the environmental applications, for tracing pollutants and for the detection of heavy metals contamination [17], [18], [19]. On the other hand, XRF as a well-established spectrochemical analytical technique offers some unique advantages as being fully non-destructive, requiring minimal sample preparation, simple and suitable for in situ use with portable equipment. Because of the simplicity of XRF analysis, it has been widely used for numerous environmental applications [20]. The limit of detection of both LIBS and XRF for metals is typically in the ppm range [19], [20], [21]. The main goal of this research work is to confirm the adsorption efficiency of fish bones for heavy metals. The variation of initial metals concentrations and contact times as adsorption parameters were examined. The LIBS results were validated by the XRF technique measurements. Adsorption isotherms and kinetics studies were reported to account for fish bones as an effective adsorbent of copper and cobalt from wastewater. Our resultsReference has been inserted in the text differ from the previously published research [19] mainly by focusing on the kinetics of the adsorption process and demonstrating the potential of LIBS technique as an environmental diagnostic technique by following up the concentration of the adsorbed copper and cobalt on fish bone.

Experimental

Preparation of sorbent

Lates niloticus fish bones (Egyptian Nile Perch) as a basic sorbent have been obtained from local markets in the vicinity of Cairo University. Fish bones were washed several times with hot distilled water after removal of fats and solid residues. Bones dried at 40 °C for 24 h were then ground and sieved down to about 300 µm size.

Preparation of synthetic wastewater

Standard synthetic wastewater stocks (1000 mg L⁻¹) of copper ion (Cu II) and cobalt ion (Co II) solutions were prepared by dissolving individually 3.93 g of crystalline copper sulfate “CuSO₄·5H₂O” and 4.036 g of crystalline cobalt chloride (CoCl₂) salt in 1-liter distilled water. The standard solutions of both Cu (II) and Co (II) were diluted to outfit different concentrations (50, 100, 150, 200, 250, and 300 mg L⁻¹) required in the experimental measurements.

Adsorption studies

The sorption experiments were carried out in 500 mL Erlenmeyer flasks by mixing 300 mL metal solution with 2.0 g dry weight of fish bones sorbent material. Mixtures have been stirred for predetermined time intervals, from 30 min until 6 h, at room temperature (25 ± 2 °C) on a magnetic stirrer at 200 rpm with an initial pH of (6.6 ± 0.1). Thereafter, the solutions have been filtered after 30 min in each case with 0.7 µm filter paper (Whatman, Cat No. 1001 125). The fish bones filtrates were then collected and dried, and the treated wastewater has been collected and stored in glass bottles.

LIBS setup

All experiments were carried out using a typical single pulse LIBS setup that employs a Q-switched, Nd: YAG laser (BRIO, Quantel, France) operating at a wavelength of 1064 nm. The laser pulse energy was 96 mJ, at 5 ns pulse duration and 10 Hz repetition rate. The measurements were performed in air at ambient atmospheric pressure. The laser was focused by a 10 cm focal length plano-convex quartz lens onto the target surface. A 2 m length optical fiber of 600 µm diameter has been mounted at a 45-degree angle with respect to the target surface to collect the emission from the plasma plum then fed it to the entrance slit of an echelle spectrometer (Mechelle 7500, multichannel, Sweden), covering the spectral wavelength range of 200–1000 nm (displayable in a single spectrum). An intensified CCD camera (DiCAM-PRO, PCO-computer optics, Germany) detects the spectra of the plasma emission. LIBS⁺⁺ software has been used for the analysis and identification of the obtained LIBS spectral lines. Each LIBS spectrum represents the average of 25 spectra taken as 5 spectra at 5 different positions on each fish bone sample target [16]. Detailed study of the experimental parameters of the present setup can be found in our previous work [19]. The optimum experimental conditions for LIBS analysis such as delay time (t_d) which is the time interval between firing the laser and triggering the detector (ICCD camera), and gate width (Δt) which is the time during which the detector is sensitive, were 1500 ns, and 2500 ns, respectively. These conditions provided very good spectral signal-to-noise ratio. For quantitative analysis using LIBS, the laser-induced plasma should satisfy the conditions of local thermodynamic equilibrium (LTE) [22]. Under the present experimental conditions, the plasma temperature T_e ranges from 6267 to 10676 K and the values of electron density N_e (cm⁻³) is greater than 10¹⁶ which fulfills McWhirter criterion (T_e > 5000 K, N_e > 10¹⁶).

XRF setup

As mentioned above, the samples have been also analyzed via the XRF technique. An XRF spectrometer (Portable XRF, Thermo Scientific, NITON/XLt 8138, 592 GKV, USA) having a 40 kV X-ray tube with a gold anode excitation source. The detection range of this spectrometer expands from sulfur to uranium with a low limit of detection for high-Z elements. The advanced NITON software has been used for the analysis of the obtained XRF spectra [23].

Scanning electron microscopy (SEM)

Scanning electron microscopy (SEM, FEI Quanta FEG 250 series, Japan) [24] investigations were performed at magnifications of 10× to 10,000×. It has been used for the morphological characterization of the samples to elucidate the porous properties of the biosorbents. For cross-sectional inspection, the fragmented samples were embedded in carbon tab.

Results and discussion

Scanning electron microscope (SEM) analysis

To show clearly the adsorption effect on the bones surface morphology, the physical morphology of fish bones surface is shown in Fig. 1. The SEM micrographs depict the surface morphology before and after adsorption processes at the highest concentration of 300 mg L⁻¹ for 270 min at initial pH (6.6 ± 0.1).

Fig. 1.

SEM images of fish bones (a) before adsorption (b) after adsorption of Cu (II) (c) after adsorption of Co (II) [in 300 mg L⁻¹ concentration after 270 min with initial pH (6.6 ± 0.1)].

For comparison studies, all images have had the same 2000× magnification. Analysis of SEM image presented in Fig. 1(a) revealed that the dried pure fish bones have numerous small pores on the surface which are responsible for increasing the surface area and consequently the increase of adsorption capacity and efficiency [25]. Fig. 1(b) and (c) displays a comparison between the adsorption processes of both Cu (II) and Co (II) on fish bones surface. Fig. 1(b) indicates that all pores appearing on the adsorbent surface are almost completely covered by Co (II) ions. On the other hand, Fig. 1(c) apparently shows that fish bones surface is partially covered by Co (II) ions.

Influence of contact time and metal ions initial concentration on removal process

Fig. 2(a), (c) depicts the effect of contact time on adsorption uptake of Cu (II) and Co (II) onto fish bones from synthetic wastewater at different concentrations using LIBS analysis, respectively. The results indicate that both LIBS intensity and adsorption uptake increase with increasing contact time until reaching the equilibrium point of 270 min for Cu (II) and 300 min for Co (II). The effect of contact time on adsorption uptake of Cu (II) and Co (II) onto fish bones is accentuated by making use of XRF analysis at the same experimental conditions as shown in Fig. 2(b), (d). The trend of the XRF curves indicates a significant consistency; that lends confidence to the LIBS results.

Fig. 2.

Effect of contact time on adsorption of Cu (II) and Co (II) on fish bones for different concentrations using LIBS (a, c) and XRF (b, d).

Fig. 3(a), (b) shows the effect of contact time on removing Cu (II) and Co (II) respectively from the synthetic wastewater by means of XRF analysis with same initial concentrations. In Fig. 3(a) it is clear that the reduction in the -intensities arises as a consequence of increasing the contact time. By repeating the experiment for Co (II), Fig. 3(b) demonstrates similar trend of decreasing intensities; which shows the increase in the removed amount of Co (II) from synthetic wastewater with longer contact time for the same initial concentrations measured for Cu (II).

Fig. 3.

Effect of contact time on removal of (a) Cu (II) and (b) Co (II) from synthetic wastewater for different concentrations using XRF.

It should be noted that the metal cations adsorption on the fish bones is higher in the beginning due to the availability of a large surface area with specific sites of the adsorbent. Reaching saturation means that all active sites in the adsorbent are occupied [26], [27].

Adsorption isotherm

The adsorption percentage efficiency of metal ion removal E has been calculated by the following equation:

$$E=(c_i-c_e)/c_i\times 100$$ (1)

where $c_i$ is the initial metal ion concentration (mg L⁻¹), and $c_e$ is the equilibrium metal ion concentration (mg L⁻¹). The calculations are performed at a fixed contact time of 30 min with initial pH (6.6 ± 0.1) and temperature (25 ± 2 °C).

Fig. 4 shows the percentage removal efficiency calculated by Eq. (1) versus different initial metal concentrations, under the previously specified conditions. These results assure that the removal efficiency for copper is higher than that of cobalt at all concentrations. The difference in ion exchange capacity on the adsorbent surface for the two elements could justify this difference in removal efficiency [28]. However, the dependence on charge density of each element, extent of hydrolysis, and solubility of hydrolyzed metal ions in the solution can also be taken into consideration in this issue [29].

Fig. 4.

Effect of initial metal concentration on the percentage removal efficiency of Cu (II) and Co (II). The error bars represent the standard deviation of the experimental data.

Adsorption isotherm models

At a fixed temperature, the adsorbate quantity adsorbed to that remaining in the solution is called adsorption isotherm and it describes the equilibrium relation between the concentrations in both adsorbent and solution phases [29]. Langmuir and Freundlich isotherm models are the most widely adsorption isotherm models that are used to quantify the sorption capacity of adsorbate.

Langmuir isotherm

This model assumes that adsorbent has sites with uniform energy for adsorption of adsorbate providing a monolayer homogeneous adsorption [30], [31]. The linear form of Langmuir isotherm is presented as follows [32]:

$$\frac{1}{q_e}=\frac{1}{{\mathit{bc}}_e{q}_{\mathit{max}}}+\frac{1}{q_{\mathit{max}}}$$ (2)

$q_e$ (mg g⁻¹) is the amount of metal ion adsorbed at specified equilibrium, $q_{\mathit{max}}$ (mg g⁻¹) is the maximum amount of the metal ion per unit weight of sorbent and $b$ (L mg⁻¹) is Langmuir adsorption equilibrium constant related to the energy of adsorption.

Fig. 5(a), (b) shows the Langmuir adsorption isotherm plot of $1/c_e$ versus $1/q_e$ for Cu (II) and Co (II), respectively. The values of Langmuir constants, $q_{\mathit{max}}$ and $b$, are calculated from the intercept is equal to $1/q_{\mathit{max}}$ and the slope is equal to $1/{\mathit{bq}}_{\mathit{max}}$ through linear regression. Usually, high correlation coefficient, 0.8888 and 0.8623 respectively, indicates that the application of the Langmuir equation supports monolayer formation on the surface of the adsorbent.

Fig. 5.

Langmuir adsorption isotherm for the adsorption of (a) Cu (II) and (b) Co (II) by fish bones.

The Langmuir isotherm constants for the adsorption of copper and cobalt ions are given on the corresponding figures. The obtained values of $b$ and $q_{\mathit{max}}$ are 0.25 and 35.12 for Cu (II) and 0.06 and 23.46 for Co (II), which prove that the adsorption process depends on both the concentration and contact time.

Langmuir isotherm can be described by a dimensionless constant known as the separation factor or the equilibrium factor $R_L$, given by [28], [33], [34]:

$$R_L=\frac{1}{1+{\mathit{bc}}_o}$$ (3)

$b$ is Langmuir constant (L mg⁻¹) and $c_o$ is the initial concentration (mg L⁻¹). $R_L$ is used to predict the shape of the isotherms, which gives information about the favorability of the adsorption of metal ions on the adsorbent. According to equation (3), the values of the separation factor ($R_L$) for all selected concentrations (50–300 mg L⁻¹) of metal ions are found to be less than 1, which indicates the favorable biosorption conditions. Fig. 6 provides the relation t between $c_o$ (mg L⁻¹) and $R_L$ values which shows that by increasing the concentration the $R_L$ values decrease exponentially in the range of 0 < $R_L$ < 1. This consequently assures that the adsorption of Cu (II) and Co (II) is still favorable even at higher concentrations.

Fig. 6.

The calculated separation factor R_L versus the initial concentrations of Cu (II) and Co (II).

Freundlich isotherm

Freundlich model [35] is an empirical expression used to describe both the heterogeneous surfaces and multilayer sorption. The mathematical form of Freundlich adsorption isotherm is represented by the following equation:

$$\ln q_e=\ln k_f+\frac{1}{n}\ln c_e$$ (4)

where $k_f$ and $n$ are constants, being indicative of the extent of adsorption and the degree of non-linearity between solution and adsorbent, respectively. The coefficient $k_f$ (mg^1−1/n g⁻¹ L^1/n) is a measure of the adsorption capacity; the greater is the surface accessible for adsorbate particles, the greater is the value of $k_f$ [5]. Plotting $\ln c_e$ versus $\ln q_e$ will yield an intercept of $\ln k_f$ and a slope of $1/n$.

Fig. 7(a), (b) shows the fitting plot of Freundlich isotherm for Cu (II) and Co (II), respectively. The constant values obtained from Freundlich adsorption isotherm and its correlation coefficient $R^2$ are summarized in the figures. Regression values of 0.992 for Cu (II) and 0.981 for Co (II) are acceptable to describe the adsorption of both heavy metals on fish bones. The constants obtained from $(1/n)$ are 0.689 for Cu (II) and 0.561 for Co (II) indicate favorable and high-affinity adsorption of fish bones for metallic ions.

Fig. 7.

Freundlich adsorption isotherm of (a) Cu (II) and (b) Co (II) by fish bones.

Finally, from all parameters of both isotherms, it has been found that the equilibrium data are well-fitted to Freundlich isotherm. This assumes that it is applicable for non-ideal adsorption on heterogeneous adsorbent surfaces.

Adsorption kinetic model

To evaluate the kinetics of the adsorption of the Cu (II) and Co (II) from wastewater, the pseudo-first-order, and pseudo-second-order kinetic models were tested to interpret the experimental data.

Pseudo-first order kinetic model

The pseudo-first-order equation of Lagergren is generally expressed as [36]:

$$\frac{\mathit{dq}}{\mathit{dt}}=q_e(1-e^{-k_{1}t})$$ (5)

where $\frac{\mathit{dq}}{\mathit{dt}}$ is the adsorption capacity at time $t$, $q_e$ (mg g⁻¹) is the equilibrium adsorption capacity, and $k_1$ (min⁻¹) is the rate constant of pseudo-first order adsorption. Integrating and applying boundary conditions, $q_t=0$ at initial time $(t=0)$ and $q=q_t$at a given time $(t=t)$, equation (5) takes the form:

$$\ln (q_e-q_t)=\ln q_e-k_{1}t$$ (6)

Fig. 8(a), (b) shows the plot of $\ln (q_e-q_t)$ versus $t$ for different metal concentrations to obtain the rate constant $k_1$ from the slope and $q_e$ from the intercept.

Fig. 8.

The linear pseudo first-order kinetic sorption data for (a) Cu (II) and (b) Co (II) at different concentrations.

Table 1 shows the parameters from the pseudo-first-order model for both Cu (II) and Co (II). By comparing the presented results, it is clear that the rate of cobalt adsorption on fish bones is less than that of copper for all concentrations. Therefore, the adsorption of Cu (II) onto fish bones is much higher than that of Co (II). On the other hand, there is an observable difference between calculated adsorption capacities and the experimental values for both metallic ions.

Table 1.

Pseudo-first order kinetic model parameters for different initial concentrations of Cu (II) and Co (II).

Initial metal concentration (mg L−1)	K1 (min−1) × 10−3		R2		qe (cal.)		qe (exp.)
	Cu (II)	Co (II)	Cu (II)	Co (II)	Cu (II)	Co (II)	Cu (II)	Co (II)
50	15.53	7.36	0.867	0.973	4.542	4.074	7.3	6.6
100	19.58	16.61	0.933	0.979	22.471	17.898	14.4	12.2
150	19.2	16.06	0.815	0.948	38.477	18.667	20.9	14.4
200	22.28	8.85	0.917	0.962	83.386	14.366	27.6	18.3
250	12.25	7.7	0.983	0.985	37.156	13.762	32.6	22.2
300	10.58	10.99	0.939	0.907	39.636	26.731	36.4	24.8

Pseudo-second order kinetic model

The pseudo-second-order equation is also based on the sorption capacity of the solid phase and is expressed as [37]:

$$\frac{{\mathit{dq}}_t}{\mathit{dt}}=k_2{(q_{e-}{q}_t)}^2$$ (7)

where q_t is the adsorption capacity at time t and k₂ (g mg⁻¹ min⁻¹) represents the pseudo-second-order rate constant.

After integration and application of the boundary conditions q_t = 0 at t = 0 and q_t = q_t at t = t, the integrated form becomes:

$$\frac{t}{q_t}=\frac{1}{k_2{q}_e^2}+\frac{t}{q_e}$$ (8)

Fig. 9(a), (b) depicts a linear relationship between $\frac{t}{q_t}$ and $t$ according to Eq. (8), regarding both metallic ions measurements for their different concentrations.

Fig. 9.

The linear pseudo second-order kinetic sorption data for (a) Cu (II) and (b) Co (II) at different concentrations.

The kinetic parameters given in Table 2 for the pseudo-second-order model were determined from the slopes and intercepts of the lines in Fig. 9(a), (b) for different concentrations Cu (II) and Co (II), respectively.

Table 2.

Kinetic parameters for the adsorption of Cu (II) ion and Co (II) ion onto fish bones based on the pseudo-second-order kinetic model.

Initial metal concentration (mg L−1)	K2 (min−1) × 10−3		R2		qe (cal.)		qe (exp.)
	Cu (II)	Co (II)	Cu (II)	Co (II)	Cu (II)	Co (II)	Cu (II)	Co (II)
50	0.688	1.32	0.985	0.984	9.634	6.590	7.3	6.6
100	0.490	3.95	0.998	0.936	19.360	15.045	14.4	12.2
150	0.355	1.02	0.997	0.987	27.787	16.392	20.9	14.4
200	0.366	1.42	0.986	0.974	36.405	17.293	27.6	18.3
250	0.493	2.06	0.968	0.996	39.186	23.186	32.6	22.2
300	1.62	1.03	0.946	0.948	39.038	23.946	36.4	24.8

The best fitting of adsorption data was obtained for the pseudo-second-order kinetic model as shown in Fig. 9(a), (b). This model assumes that a chemisorption mechanism is involved in the adsorption process and the rate of the site is proportional to the square of the number of unoccupied sites. The adsorption kinetics of Cu (II) and Co (II) ions onto fish bones, suggests that the rate-limiting step could be chemical sorption or ion exchange [37], [38].

Conclusions

In the present work, Lates niloticus fish bones (Egyptian Nile Perch) that dried and ground to 300 µm size have been used as a sorbent for the removal of the toxic heavy metals (Cu (II) and Co (II)) from wastewater. LIBS and XRF as well-established spectrochemical analytical techniques were applied for the qualitative and quantitative monitoring of the heavy metals removal. The efficiency of fish bones in adsorption of heavy metals, is mainly due to its content of the natural hydroxyapatite (HAP) that depend on the ion exchange reaction with calcium ions on the bone surface. The obtained optimum contact time values for the heavy metal ion removal of Cu (II) and Co (II) were 270 and 300 min, respectively. Furthermore, the highest percentage values of adsorbed Cu (II) and Co (II) were found at the low initial ion concentrations. Based on correlation coefficients, the best fit model is the Freundlich isotherm that was found to provide the best correlation of Cu (II) and Co (II) adsorption onto fish bones. The kinetic studies revealed that the adsorption process of both ions followed well the pseudo-second-order kinetic model. These experimental studies accentuate the potential of using LIBS and XRF as powerful spectrochemical analytical techniques for environmental analysis, which develop an appropriate technology regarding the removal of heavy metals from contaminated industrial effluents. However, the results obtained are preliminary and further studies are planned in future work on real wastewater samples and highly optimized experimental conditions.

Conflict of interest

The authors have declared no conflict of interest.

Compliance with Ethics Requirements

This article does not contain any studies with human or animal subjects.