where A refers to the value of pH, B is ultrasonic reaction time, and C is the dosage of zero-valent iron powder.
The statistical significance of the model equation was evaluated by the F-test for the ANOVA. The ANOVA evaluations of this model, shown in Table 2, imply that this model can describe the experiments. In Table 2, the prob > F-values for relative density and bending strength were lower than 0.05, which indicated that the quadratic models were significant [18]. The coefficient of determination (R2) was 0.7629, which was close to 1, and indicated a correlation between the observed and the predicted values. The “lack-of-fit tests” compare the residual error to the “pure error” from replicated experimental design points. Values of p > 0.05 for both responses indicated that the lack of fit for the model was insignificant. Adequate precision measures the signal-to-noise ratio, and a ratio greater than 4 is desirable. The adequate precision for Y was 7.747. This high value of adequate precision demonstrated that the model was significant for the process.