*2.2. Instruments*

Scanning electron microscopy (SEM) images were obtained on a Sirion200 microscope (FEI Company, Eindhoven, the Netherlands) at an accelerating voltage of 10.0 kV. The infrared spectra were obtained from a Nicolet 6700 Fourier transform infrared (FT-IR) spectrometer (Thermo Fisher Scientific, Waltham, MA, USA). X-ray diffraction (XRD) pattern was collected on a Bruker axs D8 advanced diffractometer (Bruker Corporation, Frankfurt, Germany) using Cu K α radiation. X-ray photoelectron spectroscopy (XPS) spectra were collected on a Shimadzu Axis-Ultra multifunctional X-ray photoelectron spectrometer (Shimadzu Corporation, Tokyo, Japan) using an Al K X-ray source. Metal concentrations were confirmed using a Perkin-Elmer Optima 2100 (Perkin-Elmer Company, Waltham, MA, USA) inductively coupled plasma-optical emission spectrometry (ICP-OES).

#### *2.3. Preparation of PS Colloidal Particles*

Monodisperse PS colloidal particles were synthesized by soapless emulsion polymerization [15]. Styrene is purified by vacuum distillation before using 150 mL of DI water, 0.175 g sodium chloride and 25 g styrene were added into a 250 mL three-port bottle, and stirred in the water bath at 70 ◦C. Then, 0.2 g of K2S2O8 was added to the bottle after ventilating nitrogen for nearly 20 min. With nitrogen protection, the polymerization reaction was conducted at 70 ◦C for 20 h. After the reaction, the particles were subjected to repeated centrifugal sedimentation at 9000 rpm and ultrasonic dispersion in water and ethanol to remove the styrene monomer and sodium chloride. Finally, the PS particles were mixed into a solution with 10% mass fraction.

#### *2.4. Preparation of mp-CA/GO*

For the fabrication of the mp-CA/GO, the GO (0.5 mg mL−1, 2 mL) and sodium alginate (1% *<sup>w</sup>*/*<sup>v</sup>*, 20 mL) solutions were mixed homogeneously. Then, PS colloidal particles solution (0.5 mL, 10 wt.%) was added to the sodium alginate and GO mixed solutions under magnetic stirring to form a homogeneous mixture. After that, this mixed solution was added dropwise to 100 mL of 0.2 M Ca2+ solution. A hydrogel sphere formed immediately when the mixed solution contacted with the Ca2+ solution. The hydrogel spheres were collected and washed with DI water, and placed into a low-temperature freezer. Then, the frozen hydrogel spheres were freeze-dried for 24 h under vacuum. The final mp-CA/GO composite aerogel were obtained by removing the PS colloidal particles with toluene and tetrahydrofuran exposure through ultrasonication [16].

#### *2.5. Adsorption and Desorption Tests*

In an adsorption test, ~ 50 mg of mp-CA/GO was added to 50 mL of 1.5 mM Pb2+, Cu2+, and Cd2+ solutions, respectively. The mixture was filtered after stirring for 40 min. The unextracted Pb2+, Cu2+, and Cd2+ in the filtrate were determined by ICP-OES.

In the desorption test, the Pb2+-, Cu2+-, or Cd2+-loaded mp-CA/GO was first immersed in 50 mL of 0.07 M HNO3 solution for 20 min. Then, the mp-CA/GO was separated and washed with DI water. The desorbed Pb2+, Cu2+, and Cd2+ in the eluent were also determined by ICP-OES. Each adsorption or desorption test was performed three times in parallel. The metal ion concentration was also determined three times in parallel and then averaged.

The adsorption capacity ( *Q*, mg/g) and Adsorption (%) were calculated as follows:

$$Q = \frac{\left(\mathbb{C}\_i - \mathbb{C}\_f\right) \cdot V \cdot M}{m} \tag{1}$$

$$\text{Adsorption} = \frac{\left(\text{C}\_i - \text{C}\_f\right)}{\text{C}\_i} \times 100\% \tag{2}$$

where *V* is the volume of the solution (L); *m* is the weight of the mp-CA/GO (g); *M* is the molar mass of metals (g mol−1); and *Ci* and *Cf* represent the initial and final concentration of metal ions in solution, respectively (mM).

#### **3. Results and Discussion**

#### *3.1. Material Characterizations*

Figure 1A illustrates the GO and PS suspensions that were used to prepare the mp-CA/GO. PS colloidal particle suspension appears milky white, whereas GO suspension appear brownish black due to high concentration. GO and PS can be dispersed in DI water and form homogeneous suspensions after a mild ultrasonic treatment. The XRD pattern of GO is presented in Figure 1D. The strong diffraction peak of GO appears at 11.5◦, which can be indexed to the (002) reflections of stacked GO sheets with the interlayer distance of 0.771 nm, which allows alginate to exist between layers of GO [17–19]. The mp-CA/GO with excellent mechanical strength and structural stability was acquired by the help of GO. The morphology of PS and GO was characterized by SEM, as depicted in Figure 1B,C. The surface of the GO film has noticeable folds, which are attributed to the flexible, ultra-thin 2D lamination of GO (Figure 1C). As the most commonly used template material, PS colloidal particles have an excellent uniformity with a diameter of ca. 720 nm (Figure 1B), this uniformity is important for preparing materials with controlled porous structures.

**Figure 1.** (**A**) Digital photo of GO and PS solutions, SEM images of (**B**) PS colloidal particles and (**C**) GO film, and (**D**) XRD pattern of GO.

The FT-IR spectra of GO, calcium alginate (CA) and mp-CA/GO are presented in Figure 2. In the three spectra, the absorption peaks around 3360 and 1412 cm<sup>−</sup><sup>1</sup> are ascribed to the O–H and –COOH, respectively. The weak peak around 2930 cm<sup>−</sup><sup>1</sup> in the spectra of CA and mp-CA/GO belongs to the asymmetric stretching vibration of C–H, in which the "egg-box" structures formed by sodium alginate macromolecule and calcium ion limit the C–H stretching [20,21]. The peaks at 3179, 1715 and 1620 cm<sup>−</sup><sup>1</sup> in the spectrum of GO are attributed to the stretching vibrations of −OH, −COOH and C=C in the sp<sup>2</sup> carbon skeletal network, respectively [22]. Compared with the pure GO, the peaks of mp-CA/GO downshift from 3380 to 3346 cm<sup>−</sup><sup>1</sup> and 1612 to 1599 cm<sup>−</sup>1, respectively. It can be attributed to the intermolecular hydrogen bonds formed between GO sheets and alginate [23].

**Figure 2.** FT-IR spectra of (a) mp-CA/GO, (b) CA and (c) GO.

The SEM photos of the mp-CA/GO and CA are displayed in Figure 3. The PS beads with a diameter of ca. 720 nm are homogeneously dispersed in a composite aerogel sphere (Figure 3A), the spaces that were originally occupied by the PS beads remain as macropores after removing the particle template, and mp-CA/GO has a uniform diameter of approximately 720 nm macropore (Figure 3B). The surface appears more uneven in the mp-CA/GO with controlled pore structure than CA (Figure 3C), this phenomenon is beneficial to adsorb metal ions. In addition, the diameter of the pore structure can be regulated by changing the diameter of PS colloidal particle template, thus allowing us to design the required pore size of materials.

**Figure 3.** SEM photos of (**A**,**B**) mp-CA/GO/CA and (**C**) CA.

#### *3.2. Effect of pH*

In order to evaluate the effect of pH on adsorption performance, we conducted experiments at pH values that range from one to seven. As can be seen in Figure 4, adsorption capacity of the as-prepared mp-CA/GO increased significantly when the pH increased from one to three, and then remained stable with a further increase in pH. This result can be due to the change in the ionic state of the amino and carboxyl groups. When the mp-CA/GO was in strong acidic circumstance (pH < 3), functional groups (amino and carboxyl) were protonated. However, amino and carboxyl groups are deprotonated with an increase in the solution pH (3 ≤ pH ≤ 7) [10,24]. Therefore, mp-CA/GO has wide pH range to meet the needs of real application.

**Figure 4.** Effect of pH on metal ions adsorption. (~ 50 mg of mp-CA/GO was equilibrated with 50 mL of 1.5 mM Pb2+, Cu2+, or Cd2+ at 25 ◦C for 40 min).

#### *3.3. Effect of the Contact Time and Environmental Temperature*

The effect of contact time on the adsorption behavior of mp-CA/GO was also evaluated (Figure 5). The adsorption rate of Pb2+, Cu2+, and Cd2+ onto the mp-CA/GO was quite fast, thereby completing the adsorption process within 40 min. The adsorption rate was clearly high at the

initial adsorption period, possibly due to the controlled porous structures and abundant vacant sites of aerogel. The diffusion rate of metal ions was accelerated through the nanoporous structure of mp-CA/GO, thereby indicating the primary objective of preparing porous structure using PS colloidal particles as template. Furthermore, pseudo-first-order and pseudo-second-order kinetic models have been employed to investigate the adsorption kinetics during the adsorption process. The results summarized in Table 1 show that the pseudo-second-order kinetic model fits well with the kinetic data according to the values of R2, thus suggesting that chemical sorption is a rate determining step in the adsorption process.

**Figure 5.** Adsorption kinetics studies of the (**A**) Pb2+, (**B**) Cd2+, and (**C**) Cu2+. (~ 50 mg of mp-CA/GO was equilibrated with 50 mL of 1.5 mM Pb2+, Cu2+, or Cd2+ solution at 25 ◦C).


**Table 1.** Kinetic parameters for Pb2+, Cd2+, and Cu2+ ions adsorption on the mp-CA/GO.

Furthermore, the effect of environmental temperature on the adsorption behavior for heavy metal ions was also investigated. As seen in Figure 6, the adsorption capacity of the mp-CA/GO increased slowly with the increase in temperature. However, its improvement in terms of adsorption performance was vague, possibly because the large specific surface area and controlled porous structure of mp-CA/GO allowed the metal ions to acquire additional binding sites and rapid ion diffusion rate on the material. The adsorbent's weak sensitivity to temperature is crucial to practical applications, thereby enabling the mp-CA/GO to be potentially applied to the practical treatment of heavy metal ions.

**Figure 6.** Effect of the environmental temperature on metal ions adsorption. (~ 50 mg of mp-CA/GO was equilibrated with 50 mL of 1.5 mM Pb2+, Cu2+, or Cd2+ solution at different temperatures for 40 min).

#### *3.4. Maximum Adsorption Capacity of the mp-CA/GO*

The maximum adsorption capacity is one of the most important features for the adsorbent. As seen in Figure 7, the adsorption capacity increases dramatically with the initial concentration from 0.1 mM to four mM for the mp-CA/GO. After exceeding four mM, the increment in mp-CA/GO has leveled off, possibly because of the lack of adequate functional groups to accommodate additional metal ions. The maximum adsorption capacities of mp-CA/GO for Pb2+, Cu2+, and Cd2+ are 368.2, 98.1 and 183.6 mg/g, respectively. These values are higher than most of the reported heavy metal ion adsorbents [25–43] (Table 2).

**Figure 7.** Effect of the initial metal ion concentration on adsorption. (~ 50 mg of mp-CA/GO was equilibrated with 50 mL of Pb2+, Cu2+, or Cd2+ solution at 25 ◦C for 40 min).



To study the adsorption behavior and predict which adsorption system is favorable. Langmuir adsorption isotherm and Freundlich adsorption isotherm models were applied in this study (Figure 8). The isotherm constants were calculated from the experimental data and are presented in Table 3. The results show that the Langmuir isotherm appears to be a favorable model to supervise the adsorption process, which indicates the adsorption process was a monolayer adsorption.

**Figure 8.** (**A**) Langmuir and (**B**) Freundlich adsorption isotherms studies of the Pb2+, Cd2+, and Cu2+. (~ 50 mg of mp-CA/GO was equilibrated with 50 mL of Pb2+, Cu2+, or Cd2+ solution at 25 ◦C for 40 min).

**Table 3.** Isotherm parameters for Pb2+, Cd2+, and Cu2+ ions adsorption on the mp-CA/GO.


## *3.5. Adsorption Mechanism*

In order to clarify the adsorption mechanism of the mp-CA/GO, we monitored the amount of each metal ion in the adsorption process. It can be seen from Figure 9 that the mole amount of Pb2+, Cu2+ and Cd2+ in the solution decreased while Ca2+ increased as the reaction proceeded. This illustrates that, in the first adsorption process, the adsorption of heavy metal ions is mainly completed by ion exchange. However, the mole amount of the adsorbed Pb2+, Cu2+, or Cd2+ is higher than that of the

desorbed Ca2+ during the different adsorption periods, indicating that the ion exchange is only one of the mechanisms. In order to fully understand the essence of adsorption, we further analyzed the O one s spectra before and after Pb2+, Cu2+, and Cd2+ adsorption (Figure 10). The two peaks at 530.88 and 529.767 eV in the O one s spectrum are attributed to the oxygen-containing functional groups. New peaks appeared at 530.461, 530.728, and 530.866 eV, respectively, after the adsorption of Pb2+, Cu2+, and Cd2+, indicating that the oxygen groups of on mp-CA/GO were involved in chemisorption of Pb2+, Cu2+, and Cd2+. Therefore, the excellent adsorption performance of mp-CA/GO is attributed to the combined action of chemical coordination and ion exchange.

**Figure 9.** The mole amount of (**A**) Pb2+, Ca2+, (**B**) Cu2+, Ca2+ and (**C**) Cd2+, Ca2+ in different adsorption times.

**Figure 10.** O1s XPS spectra of (**A**) mp-CA/GO, (**B**) mp-CA/GO with Pb2+ adsorption, (**C**) mp-CA/GO with Cu2+ adsorption, and (**D**) mp-CA/GO with Cd2+ adsorption.
