Table 2 Analysis of variance for the response surface of Cu²⁺ removal % by Azotobacter nigricans NEWG-1 obtained by the Box-Behnken design.

9

98.7

6163.84

684.87

43.49

0.0003^*

58.89

Linear effect

X₁

1

0.7

45.41

45.41

2.88

0.1502

2.38

X₂

1

64.6

4029.78

4029.78

255.92

<0.0001^*

22.44

X₃

1

12.9

807.02

807.02

51.25

0.0008^*

10.04

Total linear

3

78.2

4882.2

1627.4

103.4

0.000^*

Square effect

X₁²

1

5.1

340.93

340.93

21.65

0.0056^*

−9.61

X₂²

1

8.7

494.41

494.41

31.40

0.0025^*

−11.57

X₃²

1

2.6

159.30

159.30

10.12

0.0245^*

6.57

Total square

3

16.4

1020.6

340.2

21.6

0.003^*

Interaction effect

X₁X₂

1

0.3

17.35

17.35

1.10

0.3420

2.08

X₁X₃

1

1.4

87.70

87.70

5.57

0.0647

−4.68

X₂X₃

1

2.5

156.00

156.00

9.91

0.0254^*

−6.25

Total interaction

3

4.2

261.1

87.0

5.5

0.048^*

Error effect

Pure Error

2

0.1

6.85

3.42

Lack-of-Fit

3

1.2

71.88

23.96

7.0

0.1276

Total error

5

1.3

78.7

15.8

14

100.0

R²

98.74%

Std. Dev.

3.97

Adj R²

96.47%

Mean

51.10

Pred R²

81.33%

C.V.%

7.77

Adeq Precision

20.1125

PRESS

1165.54