Fabrication of novel bio-adsorbent and its application for the removal of Cu(II) from aqueous solution

As eco-friendly adsorption material, hydroxyapatite (Ca5(PO4)3OH, HA) has been extensively applied to the removal of heavy metal ions. However, separating and recovering of HA powder after the adsorption process limits their application. Alginate-based composite beads (HCA) encapsulated with HA and cellulose were designed to remove Cu(II) from aqueous solution. Fourier-transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), scanning electron microscopy (SEM), and X-ray photoelectron spectroscopy (XPS) were used for characteristic analysis. An extensive discussion in terms of HCA adsorption capacity, effect of various Cu(II) concentration, and analysis of the involved mechanisms of Cu(II) removal on the biosorption. HCA beads showed that the maximum adsorption capacity for Cu(II) of 64.14 mg/g at pH = 5 with 8 h contact time. The Langmuir adsorption isotherm and second-order kinetic model gave the closest fit. HCA beads display good regeneration ability after four cycles and offer potentiality for practical application.


Introduction
Attendant with the rapid growth of global industrialization and urbanization, water environment such as rivers, lakes, and reservoirs have been polluted by human-mediated activities. Emission of heavy metal ions has raised concern, due to their mobility and persistence in water which may eventually lead to their ingress into the food chain (Sun et al. 2018).
The ingress of toxic heavy metal ions through the food chain may result in their bio-accumulation in the human body, causing neurological disorders, kidney damage, cancer, etc. (Rastogi and Kandasubramanian 2020). To address the challenge of water pollution, many methods have been proposed for the separation and purification of contaminated water, such as chemical precipitation (Ebrahimi et al. 2017), membrane filtration (Foong et al. 2019), electrochemical treatment, and adsorption-based separation (Shalla et al. 2019). Among these various techniques, adsorption can be found to be widely used in water treatment owing to the cost-effectiveness, simplicity, and flexibility in design and operation (Joseph et al. 2019). Various adsorbents have been evaluated, including graphene oxide (GO), and metal organic frameworks (MOF) (Zhu et al. 2019;Yap et al. 2020). However, the added inconvenience in having to separate these powder materials after adsorption process limits their application for water treatment.
Sodium alginate (SA), a polyanionic polymer, can crosslink with divalent metal ions (e.g., Ca 2+ , Zn 2+ ) to form a stable framework with three-dimensional (3D) network structure, which could combined with nanomaterials, can overcome the barrier in separating and recovering of nanomaterials after adsorption process Zhao et al. 2021). Furthermore, their surface with hydroxyl and carboxyl groups provide large amounts of bonding sites, which are conducive to capture metal ions (Asthana et al. 2016). Baigorria et al. (2020) introduced bentonite-composite polyvinyl alcohol into the network structure of composite hydrogel beads, forming adsorbents that are stable and with enhanced arsenic (As) adsorption. The high specific surface area and sorption sites of these powder hydrogel composites provide enhanced contaminants removal ability (Lva et al. 2019;Zhang et al., 2019b). As reported by Xi et al. (2020), alginate immobilized Zr-bentonite hydrogel beads with an enhancement in the specific surface area and porosity, led to a high adsorption capacity for phosphate from aqueous solutions. HA is a form of the mineral calcium apatite, having a hexagonal crystal structure for calcium, phosphorus, and oxygen (Szcześ et al. 2017). Due to its nontoxicity, high adsorption efficiency, biocompatibility, and cost-effective production, HA may be developed as an alternative material for heavy metal removal.
Recent contributions report good adsorption capacity of heavy metals ions using carbohydrate biopolymer (Muya et al. 2016;Li et al. 2021). Natural biopolymers (e.g., cellulose, pectin and chitosan), as a biodegradable, abundant, and renewable resource, has attracted great attention for its use as sorbent materials from security concern (Xu et al. 2021). Cellulose, which is widely present in wood, cotton, and plants leaves, is considered to be renewable polymer material . Non-toxic, ecological, and abundantly available bio-polymers will be the preferred choice for treating water for human consumption ). In addition, cellulose can form a stable network structure through physical crosslinking, which is also helpful to provide more adsorption sites on account of its abundant hydroxyl groups (Luo et al. 2016a). Some researchers have employed cellulose composite beads for heavy metal removal from wastewater (Luo et al. , 2016b;Liu et al. 2020).
Inspired by the advantages of alginate-based material, the current research developed a nontoxic and eco-friendly composite bio-adsorbent for the removal of Cu(II) ion from aqueous solutions. The fabrication of the adsorbent was described in Scheme 1. The current research combines the adsorption performance of HA with the porous nature of cellulose and alginate material, to overcome the difficulty of separation of HA from water. In this way, the adsorption performance of hydrogel beads could be improved. The adsorption kinetics and isotherms of the composite beads were evaluated. A lab-scale fixed-bed glass column study was conducted to evaluate the potential of the HCA composite beads.

Preparation of the cellulose solution
α-cellulose (4 g) was rapidly dissolved into NaOH:urea:H 2 O( 7:12:81, weight ratio) solution at low temperature (− 12 ℃) following the method utilized by Qi (Qi et al. 2009). The resulting solution was stirred for 5 min, until a transparent cellulose solution (CS) was obtained.

Preparation of the HCA beads
The HA (0.1 g, 0.2 g, 0.4 g, 0.8 g, and 1.6 g) and cellulose solution (0.2 g, 0.4 g, 0.8 g, 1.6 g, and 3.2 g) were added into SA solution (1%, w/v). The mass ratios (HA, CS, SA) of mixed solution for (0.1, 0.2, 1.0), (0.2, 0.4, 1.0), (0.4, 0.8, 1.0), (0.8, 1.6, 1.0), and (1.6, 3.2, 1.0) were prepared. The mixture was stirred for 0.5 h to ensure it is thoroughly mixed. The mixed solution was dripped into CaCl 2 solution (2 wt%, 200 mL) using a syringe. After curing for 6 h, the obtained HCA beads were washed three times with deionized water (1000 mL). In addition, calcium alginate (CA) beads were prepared as the comparison sample, without the addition of the HA and CS. For the comparison sample, the SA solution (1%, w/v) was dripped into CaCl 2 solution (2 w %, 200 mL) using a syringe. Finally, the as-prepared beads were freeze-dried for 50 h in a refrigerant dryer for sample characterization. As-prepared beads were stored in deionized water prior to conducting the experiments.

Adsorption investigation
To systematically study Cu(II) adsorption behaviors of HCA beads, adsorption experiments were conducted using Cu(II) as model heavy metal ion. A measured quantity of wet adsorption beads was added to 50 mL of copper ion solution. The concentrations of the initial solutions ranged between 100 and 500 mg/L. The components are allowed to mix thoroughly. The concentration of the adsorbed solution was determined by spectrophotometry (Apha 2005). The experiments were conducted in triplicate. Adsorption capacity was calculated using the following equation: The removal efficiency of Cu(II) by HCA was calculated from each step and defined as: q e stands for the adsorption capacity of the bio-adsorbent for heavy metals (Cu(II), mg/g), m stands for the mass of (1) bio-adsorbent (g), V is volume of Cu(II) solution (L), and C 0 and Ce are the initial and equilibrium (mg/L) concentration of Cu(II), respectively.

Characterization methods
Fourier-transform infrared spectra (FTIR) of the product were performed on a spectrometer (Nicolet 6700). X-ray diffraction (XRD) test of sample was carried out by an XRD Rint-2000 diffractometer. The surface microstructure of the product was observed by a scanning electron microscopy (SEM) Nova Nano SEM 230 with an energy dispersive spectroscopy (EDS) for analyzing different elements. Elemental analysis of the materials was performed on X-ray photoelectron spectroscopy (XPS) ESCALAB 250Xi.

Regeneration experiment
To investigate the reusability of HCA beads for Cu(II) adsorption, the desorption experiments were carried out with 0.10 mol/L Ca(NO 3 ) 2 solutions, 0.01 mol/L HNO 3 , and deionized water. After elution, the regenerated HCA beads were subsequently evaluated in adsorption experiments to study their recyclability.

FT-IR analysis
To investigate functional groups and the interaction between the cellulose, HA, and CA, the sample was characterized by FTIR spectra. The results were presented in Fig. 2 (a-b). As shown in Fig. 1 a, FTIR analysis shows the characteristic absorbance of the HA (a), cellulose (b), CA (c), and HCA (d). The main functional groups of the as-prepared adsorbent in this experiment were shown in Table 1.The absorption peak at around 962 cm −1 is derived from stretching modes of P-O in HCA composite beads (Zhang et al., 2019c). After the HA and cellulose were introduced into CA, the asymmetric and symmetric stretching vibrations of the C = O bands shifted to 1630 cm −1 (HCA) from 1640 cm −1 (cellulose) and 1590 cm −1 (CA), which also provided evidence for the interaction between cellulose and sodium alginate matrix. These new peaks at 2900 cm −1 were corresponding to the stretching vibration of C-O, which confirms the binding of cellulose and alginate chains on the HCA surface by the electrostatic force (Luo et al., 2016b). A broad absorption peak at around 3600-3000 cm −1 was derived from the stretching vibrations of hydroxyl groups in the HCA, suggesting the stronger interaction among the alginate matrix.

SEM and EDS analysis
The surface morphology of CA and HCA beads was observed by SEM. As shown in Fig. 2 (a-b), we can see that the morphological structure of CA with smooth fracture surface, which may be caused by dehydration of the hydrogel beads. For the HCA composite beads in Fig. 2 (c-d), as expected, the surface showed an irregular wrinkled structure, which would be conducive to the diffusion and distribution of heavy metal ions onto surface or within the adsorbent.
The new structure of composite beads appeared to contain large pores, providing physical space for adsorption. The adsorption of heavy metal ions Cu(II) through these pores may occur within the beads as there is sufficient space for the metal ions to travel (Tao et al. 2021). As shown in the Fig. 2 e, EDS analysis of HCA shows that the surface of composite hydrogel beads was mainly composed of the elements C (7.29%), O (13.57%), P (8.73%), Cl (7.62%), and Ca (62.79%). The element of P was observed from the EDS spectrum of HCA, indicates that HA was successfully encapsulated within the alginate-based matrix. In addition, The Ca/P ratio for the formed HCA beads was 7.19, which was significantly higher than Ca/P stoichiometric ratio (1.67) of the HA. This is further indication that the Ca had crossinglinked with alginate to form novel types of HCA beads.

XPS analysis
In order to demonstrate the changes of related elements before and after the adsorption of novel engineered and designed adsorbent, the XPS spectra were performed as presented in Fig. 3 (a-c). The high resolution energy spectrum shows the elemental composition of the HCA hydrogel beads, and the binding energy signals at 284.8 eV, 347 eV, and 532 eV correspond to the C1 s peak, Ca2 p peak, and O1 s peak in HCA, respectively (Zhang et al. 2018). When Cu(II) was adsorbed, the characteristic peak of Cu2 p appeared, indicating that the heavy metal Cu(II) ion was successfully adsorbed to the surface and/or interior of HCA. For the high-resolution Cu2 p spectra (Fig. 3 c), the XPS peak binding energies of Cu2 p1/2 and Cu2 p3/2 are 951.6-954.7 eV and 931.6-934.4 eV, respectively (Godiya et al. 2019). In addition, a small satellite peak is located at the binding energy ∼944.3 eV , which also reveal that Cu(II) exists on the surface of HCA hydrogel beads. The outward observation of the beads for Cu(II) before and after the adsorption is provided in Fig. 3 d. Note that the color change in the beads is indicative of copper ion removal.

Adsorption kinetics of HCA beads
In order to further understand the adsorption process, the adsorption experiment was evaluated by the following kinetic model (Bo et al. 2020).
The pseudo-first-order kinetic Eq. (3):  The pseudo second-order kinetic Eq. (4): where q e represents adsorption capacity of the HCA (mg/g), and q t represents the adsorption capacity at time t (min). k 1 (L/min) and k 2 (g/mg.min) represent adsorption rate constants.
t q e Table 2 shows the different mass ratios of the HCA samples with HA, CS, and SA in the preparation process. The influence of different mass ratios of HA, CS, and SA on the adsorption capacity was also investigated (Fig. 4 a). Among the different mass ratios of samples, the maximum adsorption capacity (48.25 ± 2.92 mg/g) of Cu(II) occurred at HCA-4 (HA: ∼0.8%, CS: ∼1.6%, SA: ∼1%). Such improvements may be owing to the reason that cellulose and HA participated in the adsorption behavior and in the formation of the network structure of HCA beads, in order to investigate their adsorption properties. HCA beads were placed in conical flask containing 50 mg/L, 150 mg/L, and 300 mg/L Cu(II) solution (50 mL, pH = 5), respectively. With the increase of initial Cu(II) concentration, the adsorption capacities of HCA beads obviously increased, which was consistent with the Fig. 4 b. The Cu(II) adsorption capacities of the HCA beads reach up to 55.20 mg/g, better than those of other hydrogels materials (Zhang et al., 2019b;Wu et al. 2019). In addition, the experimental results show that the pseudo-second-order kinetic model (300 mg/L: R 2 = 0.998, 150 mg/L: R 2 = 0.998, 50 mg/L: R 2 = 0.997) can better fit the experimental data than the first-order kinetic model (300 mg/L: R 2 = 0.996, 150 mg/L: R 2 = 0.957, 50 mg/L: R 2 = 0.991) for Cu(II) adsorption. The calculated theoretical maximum adsorption capacity is closer to the real experimental value. Therefore, the above results show that the adsorption process of HCA for Cu(II) is more consistent with chemical adsorption (Pu et al. 2018).

The isotherm study of HCA
In order to further evaluate the adsorption performance of HCA for Cu(II), Langmuir and Freundlich isothermal models are used to fit the adsorption experimental data. The formula of Langmuir and Freundlich adsorption isothermal model (Kim et al. 2017) is: K f (L/mg) represents the Langmuir equilibrium adsorption constant, q m represents the maximum uptake adsorption capacity (mg/g), b (L/mg) was the adsorption coefficient of Langmuir equilibrium, C e represents the concentration (mg/L), and q e (mg/g) represents the adsorption capacity of the two models. n represents a heterogeneous factor.
The relevant parameters of Langmuir and Freundlich models are shown in Table 3. As you can be seen, the correlation coefficient R 2 of the Langmuir sorption (R 2 = 0.990) Fig. 3 The XPS survey spectra of HCA before and after Cu(II) adsorption is higher than that of the Freundlich sorption (R 2 = 0.825), indicating that the adsorption type of Cu(II) on HCA beads may be better represented by the Langmuir adsorption isotherm model. The adsorption of Cu(II) on HCA suggested monomolecular layer adsorption plays the dominant role and occurred on a heterogeneous surface, which was also consistent with the previous study (Luo et al. 2016a, ;Hu et al. 2018).

Regeneration experiment
The regeneration of HCA beads is highly important for further evaluation of its practical application. The material was regenerated in 0.10 mol/L Ca(NO 3 ) 2 and 0.01 mol/L HNO 3 solution (Wang et al. 2016;Oulguidoum et al. 2021). After cleaning with deionized water, the regenerated samples were rinsed with deionized water several times to remove trace salt, and then, the adsorption test was carried out to study their regenerated performance. Figure 5 d shows the relationship between the removal efficiency and times regeneration. After 4 times regeneration of HCA, the removal efficiency decreased slightly, but still maintained a good removal efficiency. The HCA beads not only show good reusability but also can be separated by gravity, which has potential industrial applications.

Fixed-bed glass column adsorption tests
A lab scale column was set up using the HCA beads as adsorbents for Cu(II) removal as shown in Fig. 6 a. A low concentration of Cu(II) (3 mg/L) was applied to the column adsorption experiment. It can be seen (Fig. 6 b) that the Cu(II) ions were efficiently removed by the HCA beads. For a rather long period of time (102 h), the removal efficiency of Cu(II) was found to be higher than 80%. After 125 h, Cu(II) breakthrough was observed, with

Mechanisms of Cu(II) removal
The removal of Cu(II) is a complicated process, in which physical effects (e.g., pore filling mechanism) and chemical interactions (e.g., cation-exchange, electrostatic attraction) might participate and vary in involvement (Liu et al. . The proposed removal mechanism of Cu(II) by HCA beads is shown in Scheme 2. The HCA beads formed a relatively irregular wrinkled skeleton structure after the filling/ crosslinking. In terms of the physical structure, the surface of HCA beads appeared visible huge porous, providing inside physical space for capture metal ions. Cation-exchange might participate in the adsorption process, where calcium ions in hydrogel matrix (− COO…Ca…OOC-) may be replaced with the free Cu(II) ions . The abundance of COOand O-containing groups on the surface of beads can also easily coordinate with Cu(II) to form complexes . Besides, HA and cellulose possess abundant hydroxyl functional groups, which are beneficial for Cu(II) adsorption from wastewater. The current research shows that HCA beads may have potential for water decontamination applications.

Conclusion
In this study, eco-friendly, micro-and nanostructured and reusable bio-adsorbent was fabricated via a crosslinked technology for the removal of Cu(II). The stable surface structure of the adsorbent was confirmed by further FTIR, XRD, SEM, and XPS characterization analysis. The adsorption performance was evaluated by using batch adsorption experiment, and the maximum adsorption capacity of HCA for Cu(II) was calculated to be 64.14 mg/g by Langmuir model. The adsorption kinetics studies showed that the adsorption process for Cu(II) was mainly controlled by chemical adsorption, and Langmuir model fitted the adsorption parameter better. The adsorption-desorption experiment was regenerated 4 times and HCA still maintained a high removal efficiency. These experiments demonstrate that the HCA beads have potential to be used as a bio-adsorbent for the removal of Cu(II) from wastewater. Further studies may be conducted on actual wastewater applications, pilot-scale and largescale treatment, and disposal of the exhausted biosorbent.