## 3 Results and Discussion
### Effect of a Single Factor on the AS and Suspension of EFLPs
#### 3.2.2 Hydrochemical Component Concentration  
The critical coagulation concentration (CCC), defined as the minimum concentration of counterions required to induce coagulation, is one of the most important basic properties of a colloidal dispersion and is often applied to assess its status. Once its concentration reaches or exceeds this threshold, the particles will be aggregated and deposited and then the stability of the colloidal system will be destroyed [45].  
In this study, the influence of copper (Cu\({}^{2+}\)) ions was obvious and increased with increasing concentration (Figure 2I). The increase was great when the ion concentration was low (less than 0.1 mmol/L) and small when the ion concentration was high, hardly changing until its concentration was more than 0.1 mmol/L, indicating that the CCC value for Cu\({}^{2+}\) was approximately 0.1 mmol/L in the studied systems. The same characteristic was found for the influence of sodium (Na\({}^{+}\)) ions, and their CCC value was about 50 mmol/L (Figure 2 II). Obviously, the CCC value for Cu\({}^{2+}\) was smaller than that for Na\({}^{+}\), which was mainly because Cu\({}^{2+}\) could provide two countercharges, while sodium ions provided only one counter charge at the same concentration [46; 47]. Regarding EFLP dispersion, the CCC values for Cu\({}^{2+}\) and Na\({}^{+}\) were greater than their respective concentrations in the groundwater over Guanzhong Plain [29; 34], implying that the EFLPs released from the loess are likely to be suspended in groundwater, further proving that EFLPs can migrate in the groundwater.  
The CCC value for Cu\({}^{2+}\) from Cu(NO\({}_{3}\))\({}_{2}\) was 0.65 mmol/L, as reported by [PERSON] et al. (2011 and 2019) [48; 49], which is much higher than the 0.1 mmol/L found in the present study, implying that the effect of the same hydrochemical component varies with different systems. CCC values for Na\({}^{+}\) have also been reported in other references and are generally unequal and even differ greatly. For example, for Na\({}^{+}\) from sodium chloride (NaCl), its CCC value was 91.6 mmol/L according to [PERSON] et al. (2017) [50], and 62.47 (soil nanoparticles) and 45.69 mmol/L (colloidal particle) according to [PERSON] et al. (2020) [36], implying that these differences are due to the interaction between Na\({}^{+}\) and different particles. However, even in the same conditions, the CCC values reported for Na\({}^{+}\) (from NaCl) and K\({}^{+}\) (from KCl) were 91.6 mmol/L and 47.8 mmol/L in one report [50], respectively, and 11.38 mmol/L (KCI) and 46.84 mmol/L (KNO\({}_{3}\)) in another reports [48; 49], respectively. [PERSON] et al. (2011, 2023) [46; 47] reported that the CCC values for K\({}_{2}\)SO\({}_{4}\), KCl and KH\({}_{2}\)PO\({}_{4}\) were 9.915, 11.38 and 180.7 mmol/L, respectively. [PERSON] et al. (2013) reported [51] a CCC value of 3 mmol/L for CaCl\({}_{2}\) and 4 mmol/L for MgCl\({}_{2}\). All these values are different, although they had equivalent countercharges in all studied systems. These differences are ascribed to the effects of anions on particle AS and suspension [46]. Anions and cations coexist and both inevitably have impacts on the fine particles in a solution; in other words, there may be a combined effect between their impacts [48; 52; 53] that needs to be further proved.  
Figure 2: Effects of Cu\({}^{2+}\) concentration (I) and Na\({}^{+}\) concentration (II) on the AS rate of EFLPs.