3.3.1. Effect of Common Salt Ions
As the best compatible combination assessed in batch tests, the xanthan–KMnO4 mixture was used in this part of the test.
(1) Effect of salt ions on solution viscosity
Although the solution viscosity varies with the shear rate, its fluctuation trend at each shear rate showed consistency. The detailed analysis of the viscosity change here assumes a shear rate of 100 s−1
(medium level within the test range). For MgCl2
, and Na2
, the solution viscosity showed a steadily decreasing trend with increasing addition amount. Relative to the initial viscosity of the xanthan–KMnO4
mixture (5.54 cP), when a salt concentration of 100 × 10−3
mol/L was added, the viscosity retention rates for MgCl2
, and Na2
were approximately 70%, 75.8%, and 80.7%, respectively (Table 4
). In contrast, an obvious rise in the solution viscosity was observed during the addition of KCl, NaCl, NaHCO3
, and CaCl2
. A low concentration (0.01 × 10−3
mol/L) of KCl displayed a slight viscosity thickening effect; however, as the KCl concentration increased, the solution viscosity began to decrease. A similar phenomenon occurred with NaCl and NaHCO3
: when the amount added reached 0.1 × 10−3
mol/L, the viscosity rebound was approximately 11.7% and 8.0%, respectively. The addition of 100 × 10−3
also resulted in a significant increase in the solution viscosity, which was approximately 15.2% higher than the initial viscosity. Except for CaCl2
, the maximum viscosity loss occurred at the highest concentration (100 × 10−3
mol/L) of the salts, but all losses were below 30%.
The relative effects of the cations Na+, K+, Mg2+, and Ca2+ on the solution viscosity were greatly affected by the concentration. When a small amount was added, the effect of Na+ was the largest. With increasing concentration, K+ and Mg2+ showed a more pronounced effect, while Ca2+ maintained a moderate effect on the viscosity under all concentrations. In addition, the effect of SO42− on the solution viscosity was apparently higher than that of the other anions at lower concentrations (10−5, 10−4, 10−3 mol/L); as the concentration increased, its impact gradually decreased. In contrast, for HCO3− and NO3−, the intensity of the ionic effect was positively correlated with the concentration.
(2) Effect of salt ions on the rheological behavior of the xanthan–KMnO4 mixture
Compared with the other cations, Ca2+
could significantly affect the solution rheological behavior. A low concentration of CaCl2
caused a decline in the STP value, and when its concentration reached 10−1
mol/L, the rheological behavior of the solution was greatly enhanced with an STP value of 5.44% (Table 5
). Similar trends were also observed when MgCl2
was added, and a low concentration could slightly weaken the rheological behavior of the solution. When the concentration increased to 10 × 10−3
mol/L, a small upward trend occurred (STP = 2.67%, which was larger than 2.10% for 1 × 10−3
mol/L). The effects of Na+
on the rheological behavior of the mixture solution were relatively stable; in general, an increase in the concentration caused the rheological behavior to be weaker, and among these cations, Na+
had the least effect on the STP, which was reflected in the fact that the STP value did not change much at each concentration. Moreover, lower concentrations of NaNO3
, and Na2
could slightly improve the solution STP (when the concentration was between 0.01 and 0.1 × 10−3
mol/L, the STP values of NaNO3
were all above the initial value of 3.2%), and as the concentration increased, this effect was stably diminished; for example, when the addition amount increased to 1 × 10−3
mol/L, the rheological behavior of the mixed solutions was close to that of the initial state (3.2%) and showed a gradual weakening trend with increasing concentration. In contrast, the addition of NaCl resulted in a continued weakening of the solution’s rheological behavior. Among the species added, HCO3−
exhibited the most obvious effect on that solution STP, while the effects of NO3−
were similar. For Cl−
, at higher concentrations, its effect on solution was more pronounced than that of other ions.
Although the salt ions showed a certain degree of influence on the viscosity and rheological behavior of the xanthan–KMnO4 mixture in the range of test concentrations, considering that the concentration range set in the test was relatively large, combined with the actual groundwater conditions, this impact could be acceptable.
3.3.2. Effect of pH
The initial pH of the xanthan–KMnO4
mixture was measured to be 6.43, or weakly acidic. The solution pH was adjusted to increase/decrease at a rate of 1 each step, and six pH values were obtained, which were 4.51, 5.46, 6.43, 7.53, 8.49, and 9.46. These values fell within the pH range of all five categories of groundwater quality standards. Figure 6
illustrates the rheological behavior of the mixture solution and its apparent viscosity changes with changing pH values. Under all pH conditions, the mixture solution exhibited good shear-thinning characteristics, and the degree of shear thinning varied slightly with the pH value (from pH 4.51 to 9.46, the STP values were 10.38, 11.09, 11.35, 11.36, 11.09, and 10.79) (Figure 6
a). The shear-thinning performance was better when the solution approached neutral conditions, which was consistent with the results for the solution viscosity under a variety of shear rate conditions (Figure 6
b). As the degree of acidity or alkalinity increased, the viscosity presented a downward trend, especially under alkaline conditions. The trend was steeper, and the lowest solution viscosity occurred when the pH was 9.46, with a viscosity retention range of 91–95% (compared with the viscosity under initial conditions, where pH = 6.43) that varied with the shear rate. Even so, the viscosity loss was still within the range of 10%. The results implied that the impacts of the groundwater pH on the rheological behavior of the xanthan–KMnO4
mixture and its viscosity were minor.