3.2.3. Component Analysis by TGA

In order to clarify the developed nanocomposites construction of the prepared product, TGA was used to demonstrate the existence of SF protein in SF@Cu-NFs based on the gravity measurement, shown as spectrum (i), (ii), and (iii) in Figure 6 and Table 1 for Cu3(PO4)2, SF@Cu-NFs and SF protein, respectively. As can be see, although both the weight loss of Cu3(PO4)2 and SF@Cu-NFs could be

divided into three stages, but there were some differences between them. For Cu3(PO4)2 as shown in spectrum (i), its weight loss included (1) due to the physical combination water; (2) due to part of chemical crystalline water; (3) due to rest of chemical crystalline water. However, the three weight loss stages of SF@Cu-NFs as shown in spectrum (ii) included (1) due to the physical combination water; (2) due to the chemical crystalline water; (3) due to the decomposition of amino acid residues and major peptide chains of SF protein. The weight loss of SF protein showed three stages, including (1) due to the physical combination water; (2) due to chemical crystalline water; (3) due to decomposition of amino acid residues and major peptide chains. The weight loss of chemical crystalline water showed the difference between Cu3(PO4)2 and SF@Cu-NFs, which was mainly resulted from the different water content. This was mainly because Cu3(PO4)2 was well-known to contain more crystalline waters like CuSO4·5H2O crystal but protein@Cu-NFs were generally considered to contain three crystalline waters in the form of Cu3(PO4)2·3H2O [31,39].

**Figure 6.** TGA spectrum of (i) Copper phosphate; (ii) SF@Cu-NFs nanoflower; (iii) SF protein.

**Table 1.** Thermogravimetric data of TGA spectrum (i) copper phosphate; (ii) SF@Cu-NFs nanoflower; (iii) SF protein.


### *3.3. SF@Cu-NFs Adsorption Performance for Pb(II)*

### 3.3.1. Investigation of pH Influence on Pb(II) Adsorption

The pH value is a very key factor for the investigation of HMIs adsorption, and the pH effect on the adsorption of Pb(II) by the prepared SF@Cu-NFs was investigated from the following two aspects in this work.

On one hand, pH can affect the existing form of the adsorbate metal ion. Generally speaking, HMI would produce hydrolysis with the change of pH and Pb(II) would exit with different forms at different pH (free ionic Pb2+ or solid Pb(OH)2). In order to precisely study the adsorption process, Pb(II) should be excellently prevented from precipitating to be Pb(OH)2). Consequently, the initial pH must be controlled for the accurate monitoring of Pb(II) adsorption. The pH can be calculated for Pb(II) by the following precipitation–dissolution equilibrium

$$\text{Pb(OH)}\_{2} \Leftrightarrow \text{Pb}^{2+} + 2\text{OH}^{-} \tag{3}$$

Its equilibrium constant was

$$K\_{sp(\text{Pb(OH)}\_2)} = \left[\text{Pb}^{2+}\right][\text{OH}^-]^2 \tag{4}$$

Then, *p*OH and *p*H were calculated to be

$$p\text{OH} = -l\lg\sqrt{\frac{K\_{\text{sp}}(\text{Pb}(\text{OH})\_2)}{\left[\text{Pb}^{2+}\right]}}\tag{5}$$

and

$$p\text{H} = 14 + \lg\sqrt{\frac{K\_{\text{sp}}(\text{Pb}(\text{OH})\_2)}{\left[\text{Pb}^{2+}\right]}}\tag{6}$$

with *Ksp*(Pb(OH)2) = 1.2 × 10−<sup>15</sup> and trace amount -Pb2+ < 10−<sup>7</sup> mol·L−1, the *p*H for Pb(II) adsorption should be *p*H < 10 by Equation (6).

On the other hand, pH can affect the surface charge of the adsorbent material, which is a main factor influencing the adsorption capacity [62]. Then the zeta potentials of SF@Cu-NFs were analyzed at different pH conditions by calculating the average of 10 measurements. The test shown in Figure S3a indicates that the surface of the prepared SF@Cu-NFs adsorbent was positively charged at pH < 5 but negatively charged at pH > 5. The isoelectric point of SF@Cu-NFs was calculated to be pH = 4.2, at which the dispersion system of SF@Cu-NFs showed the lowest stability. Meanwhile, SF@Cu-NFs showed the highest stability with the highest absolute zeta potential of 12.83 mV at pH = 5.0, which was expected to have the best adsorption result.

Combinding the results of above two aspects, the influences of pH value on the adsorption were correspondingly investigated in the range of pH = 4.0–9.0 for 200 mg L−<sup>1</sup> Pb(II) and the responses of adsorption capacity were shown in Figure S3b. It was noted that the adsorption capacities increased sharply with increasing pH value from 4.0 to 5.0, and then decreased slowly from 5.0 to to 9.0. As a result, the prepared SF@Cu-NFs showed the maximum adsorption capacity at pH = 5.0, which was consistent with above result of zeta potential investigation. Subsequently, pH = 5.0 was selected as the optimum condition to obtain the best Pb(II) adsorption.
