## Materials and Methods
### Experimental Study on Dynamic Adsorption Characteristics of Solute
With the measured mass concentration as the horizontal coordinate and the OD\({}_{600}\) value as the vertical coordinate, the relationship curve between the mass concentration and the OD\({}_{600}\) value was obtained as shown in Figure 4. The relationship between the mass concentration of _S. pasteurii_ and the OD\({}_{600}\) value was established, so that the mass concentration of bacteria could be estimated directly by the OD\({}_{600}\) value in the later test. It can be seen from Figure 4 that there was a linear relationship between the mass concentration of _S. pasteurii_ and the OD\({}_{600}\) value. The curve of the relationship between the mass concentration of _S. pasteurii_ and the OD\({}_{600}\) value was fitted, and the fitting equation was as follows:  
\[y =6.28\times\mathrm{OD}_{600}-1.16, \tag{3}\]  
where \(y\) is the mass concentration of _S. pasteurii_, mg/mL. Through the relationship curve and the fitted Formula (3), the corresponding mass concentration of the bacterial solution with different OD\({}_{600}\) values could be calculated, and the corresponding mass concentration of _S. pasteurii_ could be accurately and quickly obtained by measuring the OD\({}_{600}\) value during the test. Compared with other methods, this method is convenient and feasible, reduces the test time, and improves the test accuracy. (2) Adsorption kinetics of CaCl\({}_{2}\) solution  
The experimental method was the same as that of the adsorption kinetics of _S. pasteurii_. We changed the bacterial solution of _S. pasteurii_ to 111 g/L of CaCl\({}_{2}\) solution (1.0 mol/L of CaCl\({}_{2}\) solution). The test was intended to measure the change in Ca\({}^{2+}\) concentration. EDTA complexometric titration has the advantages of sensitivity, rapidity, and simplicity and is currently the most widely used method for measuring calcium content. The EDTA method was used to measure the concentration of Ca\({}^{2+}\). An indicator is needed to determine the end point of titration. The calcium red indicator is a common end point indicator for the EDTA determination of calcium. The structural formula is shown in Figure 5.  
Figure 4: Curve of the relationship between the mass concentration of _S. pasteurii_ and the OD\({}_{600}\) value. The relationship between the color of the calcium indicator and the pH is shown in Figure 6.  
The calcium red indicator itself is blue. When the pH exceeds 13, it will form a wine-red complex with Ca\({}^{2+}\), but the stability of the complex formed by the calcium red indicator and Ca\({}^{2+}\) is slightly worse than that of the colorless complex formed by EDTA and Ca\({}^{2+}\). Therefore, when the solution is wine red, EDTA is used for determination. After the complexation of free calcium ions, the Ca\({}^{2+}\) in the calcium red indicator complex is captured to release Ca\({}^{2+}\) in the complex formed by the calcium red indicator and Ca\({}^{2+}\), and the solution changes from wine red to blue. This is the end point of the EDTA titration of Ca\({}^{2+}\), as shown in Figure 7.  
Figure 5: The structural formula of the calcium red indicator. Figure 6: The relationship between the color of the calcium indicator and the pH. \[Y=8.59\times 10^{7}Z^{1.3627}, \tag{2}\]0 The solution before and after the end point of titration: (**a**) wine-red solution before titration end; (**b**) blue solution at titration end point. The EDTA complexometric titration steps were as follows. Firstly, an appropriate amount of water sample containing Ca2+ was added to a 250 mL conical flask and diluted to 100 mL with distilled water. Then, an appropriate amount of 2 mol/L sodium hydroxide solution was added to the whole solution, which caused the pH value of the whole solution to exceed 13. Then, the solution was titrated immediately to prevent the solution from settling for too long and causing turbidity. A small amount (approximately 0.05 g) of calcium red indicator was dropped into the solution and then the conical flask was shaken to homogenize the solution.