The Fabrication of Calcium Alginate Beads as a Green Sorbent for Selective Recovery of Cu(II) from Metal Mixtures

: Calcium alginate (CA) beads as a green sorbent were easily fabricated in this study using sodium alginate crosslinking with CaCl 2 , and the crosslinking pathway was the exchange between the sodium ion of α -L-guluronic acid and Ca(II). The experimental study was conducted on Cu(II), Cd(II), Ni(II) and Zn(II) as the model heavy metals and the concentration was determined by inductively coupled plasma optical emission spectrometry (ICP-OES). The characterization and sorption behavior of the CA beads were analyzed in detail via using scanning electron microscopy (SEM), fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS). The adsorption experiments demonstrated that the CA beads exhibited a high removal e ﬃ ciency for the selective adsorption of Cu(II) from the tetra metallic mixture solution and an excellent adsorption capacity of the heavy metals separately. According to the isotherm studies, the maximum uptake of Cu(II) could reach 107.53 mg / g, which was signiﬁcantly higher than the other three heavy metal ions in the tetra metallic mixture solution. Additionally, after ﬁve cycles of adsorption and desorption, the uptake rate of Cu(II) on CA beads was maintained at 92%. According to the properties mentioned above, this material was assumed to be applied to reduce heavy metal pollution or recover valuable metals from waste water.


Introduction
In recent years, agriculture and the mining, chemical fertilizer, leather, battery and paper industries have developed vigorously, and the phenomenon of heavy metal wastewater directly or indirectly being discharged into the environment has become more and more serious, particularly in developing countries [1,2]. Heavy metal ions in wastewater are mainly comprised of zinc (Zn), nickel (Ni), cadmium (Cd), copper (Cu) and so on. Among them, copper is widely used in industrial production, such as in printed circuit boards (PCBs), and it is also one of the indispensable nutrients (trace elements) in the human body [3,4]. When copper-containing wastewater is discharged into the environment beyond its self-purification range, the high toxicity and non-biodegradability of copper ions poses a serious threat to animal and human health. Hence, investigating how to effectively remove copper ions from wastewater is very important to the ecological environment. In addition, the recovery of copper from wastewater also has certain economic benefits.

Preparation of Calcium Alginate Beads
Sodium alginate powder was added to distilled water to obtain a yellow viscous sodium alginate solution. The obtained solution remained stationary until there were no air bubbles. To synthesize the spherical bio-sorbent, sodium alginate solution was added dropwise into CaCl 2 (1%, w/v) solution under gentle stirring with a dropper and the bead was formed immediately. After 2 hours of curing, the sphere became compact, and a CA bead with a diameter of about 3 mm was obtained. The gel ball was rinsed with distilled water several times to remove free Ca(II) and stored in distilled water for further use.

Material Characterizations
The concentration of Cu(II), Zn(II), Ni(II) and Cd(II) used in all experiments was determined by inductively coupled plasma optical emission spectrometry (ICP-OES, Varian 700-ES, Walnut Creek, CA, USA), using 2% HNO 3 as the medium. The standard solution and the solution of heavy metal ions to be measured were acidic. Scanning electron microscopy (SEM, Hitachi SU8010, Ibaraki, Japan) was used to obtain the information of the physical structure and morphology of alginate beads. Fourier transform infrared spectroscopy (FTIR, PerkinElmer Spectrum Two, Waltham, MA, USA) was recorded in the 400-4000 cm −1 region on a FTIR spectrophotometer using a KBr disk method. Thermogravimetric analysis (TGA, TA Q600 SDT, New Castle, DE, USA) was carried out in a nitrogen gas flow from room temperature to 600 • C with a heating rate of 10 K/min. The point of zero charge (STARTER 2100, Parsippany, NJ, USA) was measured within the pH range from 3.0 to 10.0 by addition of 0.1 N HCl and NaOH. The samples were further analyzed by X-ray photoelectron spectroscopy (XPS, Thermo Fisher 250XI, Waltham, MA, USA) for the Cu 2p, Cd 3d, Zn 2p and Ni 2p regions. The charging shifts of the spectra were calibrated by placing the C1s peak at 284.8 eV from the adventitious carbon. Then, the results obtained from XPS were analyzed using the non-linear least squares curve fitting program (XPSPEAK4.1, software, Hong Kong, China).

Adsorption Experiments
For kinetic studies, an adsorption experiment was carried out using 200 mg/L of Cu(II), Zn(II), Ni(II) and Cd(II) in distilled water, respectively, of which the initial pH was 5.5, 6.3, 6.6 and 6.6, respectively. The saturated adsorption time was determined, and the samples were taken at a predetermined time interval. The adsorption isotherms of CA beads in a monomer solution and a tetra-metallic mixture solution were studied to evaluate their saturated adsorption capacity. The beads (1 g) were fully dispersed in 50 mL of monometallic solution and tetra-metallic mixture solution with different concentrations ranging from 50-800 mg/L at 120 r/min for 12 h at a temperature of 25 • C in a thermostabilized warm bath. In the pH effect experiment, five groups of tetra-metallic mixture solutions with a pH of 2.0, 3.0, 4.0, 5.0 and 6.0, respectively, were prepared and adjusted by adding NaOH (0.1 mol/L) or HNO 3 (0.1 mol/L).
The suspended impurities in the solution were removed by membrane filtration. The concentration of the metal ions was determined by ICP-OES and the amount of adsorption could be calculated using the following equation: where q is the adsorption of the metal ions (mg/g), C 0 and C are the initial and final metal ion concentrations, respectively (mg/L), V is the total volume of suspension (L), and M is the dry mass of adsorbent (g).

Desorption and Reuse Experiments
Desorption experiments were also conducted using 1% CaCl 2 solutions and 0.1 M HNO 3 as the desorption reagents. In this experiment, 1 g of CA beads were added to 50 mL tetra-metallic mixture solutions with pH 5.7. After sorption, the heavy metal loaded CA beads were subsequently suspended in a 0.1 M HNO 3 eluting agent to evaluate the desorption performance. Then, the CA beads were washed with 1% CaCl 2 solutions to make them neutral and distilled water several times to remove the free Ca(II) ions from the beads. Four sorption-desorption cycles were conducted to assess the reusability of CA beads.

Characterization of CA Beads
The CA beads were synthesized through crosslinking with a diameter of 3 mm. Figure 1a shows the overall appearance of CA beads after drying; the diameter was reduced to about 1 mm. Figure 2b,c shows that there were a lot of ravines on the surface of CA beads, which increased the surface area and provided more adsorption sites. The surface of the CA bead was composed of wire-like Ca-alginate and a honeycomb network could be observed in the internal structure of CA ( Figure 1d). suspended in a 0.1 M HNO3 eluting agent to evaluate the desorption performance. Then, the CA beads were washed with 1% CaCl2 solutions to make them neutral and distilled water several times to remove the free Ca(II) ions from the beads. Four sorption-desorption cycles were conducted to assess the reusability of CA beads.

Characterization of CA Beads
The CA beads were synthesized through crosslinking with a diameter of 3 mm. Figure 1(a) shows the overall appearance of CA beads after drying; the diameter was reduced to about 1 mm. Figure 2(b-c) shows that there were a lot of ravines on the surface of CA beads, which increased the surface area and provided more adsorption sites. The surface of the CA bead was composed of wirelike Ca-alginate and a honeycomb network could be observed in the internal structure of CA ( Figure  1(d)). To evaluate the hydration and thermal decomposition of the CA beads, thermogravimetric analysis (TGA) was applied. As seen in Figure 2, the first mass loss was attributed to the dehydration process and a 12.6 wt% loss up to 210 °C. The second part was a thermal degradation stage, which resulted in pyrolysis from 210 °C to 510 °C with a weight loss of 45.8 wt%. The third part represented the conversion of the remaining materials to carbon residues and CaCO3 and the residual weights were 40.2 wt% [23].

Adsorption Kinetics
In industrial applications, adsorption kinetics are important for process design and operation, which required us to achieve adsorption equilibrium under certain system conditions. The kinetic behavior of the metal ions removed by CA beads is presented in Figure 3. The experimental data were fitted with pseudo-first-order and pseudo-second-order kinetics [24,25].
Pseudo-first-order model: q = q 1 − exp(−k t) (2) To evaluate the hydration and thermal decomposition of the CA beads, thermogravimetric analysis (TGA) was applied. As seen in Figure 2, the first mass loss was attributed to the dehydration process and a 12.6 wt% loss up to 210 • C. The second part was a thermal degradation stage, which resulted in pyrolysis from 210 • C to 510 • C with a weight loss of 45.8 wt%. The third part represented the conversion of the remaining materials to carbon residues and CaCO 3 and the residual weights were 40.2 wt% [23].

Adsorption Kinetics
In industrial applications, adsorption kinetics are important for process design and operation, which required us to achieve adsorption equilibrium under certain system conditions. The kinetic behavior of the metal ions removed by CA beads is presented in Figure 3. The experimental data were fitted with pseudo-first-order and pseudo-second-order kinetics [24,25].
where both q 1 and q 2 are the amount of metal ions adsorbed at equilibrium (mg/g), q t is the amount of metal ions adsorbed at any time t (mg/g), k 1 and k 2 are equilibrium rate constants of the pseudo-first-order and pseudo-second-order, respectively (g/mg min).   Table 1. Parameters of pseudo-first-order and pseudo-second-order kinetic models. q1 and q2 = the amount of metal ions adsorbed at equilibrium; k1 and k2 = equilibrium rate constants of the pseudofirst-order and pseudo-second-order, R 2 = coefficient of determination, respectively.  The results for the kinetic model parameters are summarized in Table 1. The higher coefficient of determination (R 2 ) value of the pseudo-second-order model demonstrated that the adsorption rate was dominated by the rate of the chemical reaction [26]. In this experiment, the ion exchange occurred between the metal ions and Ca(II) and the adsorption rate of the CA was dependent on the exchange rate. The obtained k 2 values for Cu(II), Zn(II), Ni(II) and Cd(II) adsorption onto CA beads indicated that the adsorption rate was Ni(II) > Zn(II) > Cd(II) ≈ Cu(II). Table 1. Parameters of pseudo-first-order and pseudo-second-order kinetic models. q 1 and q 2 = the amount of metal ions adsorbed at equilibrium; k 1 and k 2 = equilibrium rate constants of the pseudo-first-order and pseudo-second-order, R 2 = coefficient of determination, respectively.

Adsorption Isotherm
In order to evaluate the maximum adsorption capacity of CA beads, the adsorption isotherms were measured in the initial metal concentrations ranging from 0-800 mg/L. The adsorption equilibrium data were fitted with Langmuir and Freundlich isotherm models [27,28].
Freundlich model : q e = K F C 1/n e (5) where q e is the equilibrium adsorption capacity of heavy metal ions (mg/g), q m indicates the maximum adsorption capacity for heavy metal ions (mg/g), C e is the equilibrium concentration after adsorption (mg/L), K L and K F denote equilibrium constants of Langmuir (L/mg) and Freundlich (mg/g) (L/g), respectively, and n is the Freundlich exponent. Figure 4 shows the adsorption isotherm of Cu(II), Zn(II), Ni(II) and Cd(II) separately on the CA beads and Table 2 lists the parameter values along with their correlation coefficients. The CA beads showed good adsorption towards Cu(II), Zn(II), Ni(II) and Cd(II), with a maximum adsorption of 140.55 mg/g, 174.60 mg/g, 114.69 mg/g and 216.82 mg/g, respectively. As shown in Table 3, calcium alginate beads exhibited significant advantages over other low-cost biosorption materials in terms of their maximum adsorption capacity for Cu(II). Figure 5 shows the selective removal of Cu(II) on CA beads. With the increase of equilibrium concentration, the adsorption effect of CA beads on Cu(II) first increased rapidly and then tended to slow until equilibrium was reached, while the adsorption capacity of Zn(II), Ni(II) and Cd(II) decreased slowly. The fitting parameters of the Langmuir and Freundlich models are shown in Table 2 and the adsorption data were more consistent with the Langmuir model (R 2 = 0.9920) than the Freundlich model (R 2 = 0.8126) according to the Cu(II)* adsorption coefficient, which suggested that the adsorption of Cu(II) was the mono-layer sorption during the sorption process. Due to the competitive sorption between Cu(II) and other metals, the isotherm data of Zn(II), Ni(II) and Cd(II) were unable to be fit by the Langmuir or Freundlich models. Therefore, the CA beads showed better sorption capacity toward Cu(II) selectively than the other three heavy metals, indicating that the CA beads could be applied to the selective recovery of Cu(II) from polymetallic solutions. Table 2. Parameters of the Langmuir and Freundlich models. q m = the maximum adsorption capacity for heavy metal ions; K L and K F = equilibrium constants of Langmuir and Freundlich, respectively; n = Freundlich exponent; R 2 = coefficient of determination.

Metal
Langmuir Model Freundlich Model q m (mg/g) K L (L/mg) R 2 K F (mg/g) (L/g) 1  140.55 mg/g, 174.60 mg/g, 114.69 mg/g and 216.82 mg/g, respectively. As shown in Table 3, calcium alginate beads exhibited significant advantages over other low-cost biosorption materials in terms of their maximum adsorption capacity for Cu(II).   where is the initial concentration of Cu(II) (mg/L) and b is the dimensionless constant of Langmuir. The values of indicated that the adsorption process may be an unfavorable trend ( > 1), linear ( = 1), favorable (0 < < 1) or irreversible ( = 0) [34]. Here, the adsorption process of Cu(II) by CA beads showed that the b value was 0.0639, the calculated RL value was within the range of 0.0175-0.2434 and just fell within the range of 0-1, indicating that CA beads were favorable to the Cu(II) adsorption process.

Effect of pH
The effect of pH on Cu(II) adsorption by CA beads in the mixed metal solutions was investigated with the pH value in the range of 2-6. Figure 6 shows the uptake capacities of heavy metals using the CA beads at various pH values. When the pH value was lower than 2.0, few heavy metal ions were A dimensionless R L constant was introduced to reveal the essential characteristics of the Langmuir model, and the adsorption process was further evaluated. The correlation values of R L could be calculated from the following equation.
where C i is the initial concentration of Cu(II) (mg/L) and b is the dimensionless constant of Langmuir. The values of R L indicated that the adsorption process may be an unfavorable trend (R L > 1), linear (R L = 1), favorable (0 < R L < 1) or irreversible (R L = 0) [34]. Here, the adsorption process of Cu(II) by CA beads showed that the b value was 0.0639, the calculated R L value was within the range of 0.0175-0.2434 and just fell within the range of 0-1, indicating that CA beads were favorable to the Cu(II) adsorption process.

Effect of pH
The effect of pH on Cu(II) adsorption by CA beads in the mixed metal solutions was investigated with the pH value in the range of 2-6. Figure 6 shows the uptake capacities of heavy metals using the CA beads at various pH values. When the pH value was lower than 2.0, few heavy metal ions were adsorbed due to the competition of the many H + ions. With the increase of initial pH value from 2 to 6, the adsorption capacity of CA beads to Cu(II) increased significantly, and when the pH was 6, the maximum adsorption was observed. However, there was no obvious change in the adsorption amount of the other three kinds of metal ions, which were only slightly increased.
The point of zero charge (PZC) was employed to analyze the surface charges of the CA beads, and the measured value was 8.2. When the pH was lower than 8.2, the surface of the material was positively charged because of the introduction of calcium ions. At a pH above 8.2, there was a negative charge due to the reaction between the -OH on the surface and the OH· in the solution, leading to the formation of a negatively charged functional group, O − . Therefore, the adsorption process of heavy metal ions by CA beads could be affected by pH, and there is a competition mechanism between H + and metal ions with carboxyl groups in acidic environments. Additionally, the carboxylic (-COOH) and hydroxyl (-OH) groups of the CA beads were changed to the protonated forms and the Ca(II) was released into the solution under acidic conditions [35]. adsorbed due to the competition of the many H + ions. With the increase of initial pH value from 2 to 6, the adsorption capacity of CA beads to Cu(II) increased significantly, and when the pH was 6, the maximum adsorption was observed. However, there was no obvious change in the adsorption amount of the other three kinds of metal ions, which were only slightly increased. The point of zero charge (PZC) was employed to analyze the surface charges of the CA beads, and the measured value was 8.2. When the pH was lower than 8.2, the surface of the material was positively charged because of the introduction of calcium ions. At a pH above 8.2, there was a negative charge due to the reaction between the -OH on the surface and the OH· in the solution, leading to the formation of a negatively charged functional group, O − . Therefore, the adsorption process of heavy metal ions by CA beads could be affected by pH, and there is a competition mechanism between H + and metal ions with carboxyl groups in acidic environments. Additionally, the carboxylic (-COOH) and hydroxyl (-OH) groups of the CA beads were changed to the protonated forms and the Ca(II) was released into the solution under acidic conditions [35].

Adsorption Mechanism Analysis
FTIR was used to analyze the molecular structure of chemical compounds and the FTIR spectra of CA beads is shown before and after metal ion adsorption in Figure 7. For the CA beads before adsorption, the dominant peak at 3425.28 cm −1 was due to the vibration stretching of the O-H bond, indicating that hydroxyl (-OH) groups existed in the beads. Bands at 2926.01 cm −1 referred to the vibration stretching of -CH. The absorption peaks around 1610.24 cm −1 , 1418.32 cm −1 and 1034.37 cm −1 could be attributed to the asymmetric and symmetric stretching vibrations of -COO (carboxylate) and the stretching vibration of C-O, respectively [36]. After the sorption of metal ions, the FTIR spectra displacement slightly changed. This may be because the increased ionic volume weakened the stretching and torsional vibration of the functional groups, thus causing the displacement of adsorption peaks. The decrease of peak vibration intensity indicated that metal ions bind to -OH and -COO formed in the CA beads. In addition, no new peaks were produced, indicating that the functional groups of the adsorbents did not change, and the ion exchange process between the metal ions and the CA beads was possible.

Adsorption Mechanism Analysis
FTIR was used to analyze the molecular structure of chemical compounds and the FTIR spectra of CA beads is shown before and after metal ion adsorption in Figure 7. For the CA beads before adsorption, the dominant peak at 3425.28 cm −1 was due to the vibration stretching of the O-H bond, indicating that hydroxyl (-OH) groups existed in the beads. Bands at 2926.01 cm −1 referred to the vibration stretching of -CH. The absorption peaks around 1610.24 cm −1 , 1418.32 cm −1 and 1034.37 cm −1 could be attributed to the asymmetric and symmetric stretching vibrations of -COO (carboxylate) and the stretching vibration of C-O, respectively [36]. After the sorption of metal ions, the FTIR spectra displacement slightly changed. This may be because the increased ionic volume weakened the stretching and torsional vibration of the functional groups, thus causing the displacement of adsorption peaks. The decrease of peak vibration intensity indicated that metal ions bind to -OH and -COO formed in the CA beads. In addition, no new peaks were produced, indicating that the functional groups of the adsorbents did not change, and the ion exchange process between the metal ions and the CA beads was possible.    .96 eV, respectively, after deconvolution, indicating that the adsorbed metal ions were in chemical states without further oxidation or reduction and the valence states were not changed during the ion exchange process [37][38][39][40]. Figure 7. FTIR images of CA beads before and after the adsorption. Figure 8 shows the XPS diagram of Cu 2p, Cd 3d, Zn 2p and Ni 2p (after sorption), further exploring the adsorption mechanism. The binding energy of Cu 2p, Cd 3d, Zn 2p and Ni 2p were 933.55/953.44 eV, 405.59/412.34 eV, 1021.97/1044.94 eV and 856.96 eV, respectively, after deconvolution, indicating that the adsorbed metal ions were in chemical states without further oxidation or reduction and the valence states were not changed during the ion exchange process [37][38][39][40]. Cu(II) preferentially bound to the active sites of CA beads when the four ions were coexistent at the same concentration ( Figure 9). The selective adsorption mechanism of copper ions on CA beads was via divalent metal ions exchanging ions with Ca(II) in the "egg box" structure [41]. The adsorption sites were mainly negative groups (COO-) that could adsorb the cation in the solution, and the stability of the bond with Cu(II) was better [42]. Therefore, compared with Zn(II), Ni(II) and Cd(II), the affinity for Cu(II) was the strongest in the ion exchange process between Ca(II) and metal ions in CA beads. This competitive relationship made the CA bead a potential material that could be applied to the recovery the copper ions from heavy metal mixtures. Cu(II) preferentially bound to the active sites of CA beads when the four ions were coexistent at the same concentration ( Figure 9). The selective adsorption mechanism of copper ions on CA beads was via divalent metal ions exchanging ions with Ca(II) in the "egg box" structure [41]. The adsorption sites were mainly negative groups (COO-) that could adsorb the cation in the solution, and the stability of the bond with Cu(II) was better [42]. Therefore, compared with Zn(II), Ni(II) and Cd(II), the affinity for Cu(II) was the strongest in the ion exchange process between Ca(II) and metal ions in CA beads. This competitive relationship made the CA bead a potential material that could be applied to the recovery the copper ions from heavy metal mixtures.

Desorption and Reusability Experiment
Regeneration of loaded sorbent is a key factor in water treatment processes and the desorption of the adsorbed Cu(II), Zn(II), Ni(II) and Cd(II) ions with 0.1 M HNO3 was investigated. Figure 10 showed the reusability of CA beads with four heavy metal ions. The results revealed that after five cycles, the CA beads still had selective adsorption for Cu(II) and the adsorption capacity was more than 92%. Therefore, it can be explained that CA beads have good reusability during reuse

Desorption and Reusability Experiment
Regeneration of loaded sorbent is a key factor in water treatment processes and the desorption of the adsorbed Cu(II), Zn(II), Ni(II) and Cd(II) ions with 0.1 M HNO 3 was investigated. Figure 10 showed the reusability of CA beads with four heavy metal ions. The results revealed that after five cycles, the CA beads still had selective adsorption for Cu(II) and the adsorption capacity was more than 92%. Therefore, it can be explained that CA beads have good reusability during reuse experiments and have economic potential in wastewater treatment.

Desorption and Reusability Experiment
Regeneration of loaded sorbent is a key factor in water treatment processes and the desorption of the adsorbed Cu(II), Zn(II), Ni(II) and Cd(II) ions with 0.1 M HNO3 was investigated. Figure 10 showed the reusability of CA beads with four heavy metal ions. The results revealed that after five cycles, the CA beads still had selective adsorption for Cu(II) and the adsorption capacity was more than 92%. Therefore, it can be explained that CA beads have good reusability during reuse experiments and have economic potential in wastewater treatment.

Conclusions
A spherical CA bead with a diameter of 3 mm was prepared by crosslinking the hydroxyl and carboxyl groups of sodium alginate with Ca(II) to form an insoluble hydrogel; there were a large number of active sites in the porous honeycomb structure for metal ions to attach. According to the pseudo-second-order and Langmuir isotherm model, the adsorption mechanism was explained well and the chemical reaction dominated the rate of the adsorption process. The maximum uptake of Cu(II) could reach 107.53 mg/g in the mixed heavy metal solution and the valence state of the four metal ions was not changed according to XPS analysis during the adsorption process. Cu(II) exchanged ions with Ca(II), binding with α-L-guluronic acid in the "egg box" structure. The selective

Conclusions
A spherical CA bead with a diameter of 3 mm was prepared by crosslinking the hydroxyl and carboxyl groups of sodium alginate with Ca(II) to form an insoluble hydrogel; there were a large number of active sites in the porous honeycomb structure for metal ions to attach. According to the pseudo-second-order and Langmuir isotherm model, the adsorption mechanism was explained well and the chemical reaction dominated the rate of the adsorption process. The maximum uptake of Cu(II) could reach 107.53 mg/g in the mixed heavy metal solution and the valence state of the four metal ions was not changed according to XPS analysis during the adsorption process. Cu(II) exchanged ions with Ca(II), binding with α-L-guluronic acid in the "egg box" structure. The selective adsorption was indicated through the isotherm experiments, giving this material a high potential for the continuous treatment of the selective recovery of copper from multi-metal solutions.