Table 1 Face centered central composite design, representing Cr⁶⁺ removal by P. alcaliphila strain NEWG-2 as influenced by yeast extract, glucose, pH and incubation time.

Run			Variables				Growth (OD)	Final pH	Cr⁶⁺ removal percent		Residuals
Standard	No	Type	X₁	X₂	X₃	X₄	Growth (OD)	Final pH	Observed	Predicted	Residuals
28	1	Center	0	0	0	0	0.397	5.26	96.03	96.29	− 0.27
20	2	Axial	0	1	0	0	0.263	5.51	95.84	95.88	− 0.04
2	3	Factorial	1	− 1	− 1	− 1	0.413	6.14	94.44	94.49	− 0.05
1	4	Factorial	− 1	− 1	− 1	− 1	0.238	6.23	93.27	93.32	− 0.05
8	5	Factorial	1	1	1	− 1	0.341	6.03	93.67	93.72	− 0.05
3	6	Factorial	− 1	1	− 1	− 1	0.233	6.08	93.45	93.43	0.02
23	7	Axial	0	0	0	− 1	0.317	6.03	95.43	95.29	0.14
30	8	Center	0	0	0	0	0.418	5.32	96.59	96.29	0.30
11	9	Factorial	− 1	1	− 1	1	0.463	4.55	93.54	93.66	− 0.12
24	10	Axial	0	0	0	1	0.296	5.84	95.42	95.59	− 0.18
7	11	Factorial	− 1	1	1	− 1	0.342	6.02	93.63	93.64	− 0.01
13	12	Factorial	− 1	− 1	1	1	0.913	4.33	94.74	94.70	0.04
29	13	Center	0	0	0	0	0.421	5.36	96.26	96.29	− 0.04
5	14	Factorial	− 1	− 1	1	− 1	0.412	5.91	93.83	93.96	− 0.13
17	15	Axial	− 1	0	0	0	0.292	5.5	95.89	95.71	0.18
14	16	Factorial	1	− 1	1	1	0.333	4.74	94.54	94.57	− 0.03
27	17	Center	0	0	0	0	0.393	5.08	96.44	96.29	0.15
26	18	Center	0	0	0	0	0.445	5.34	96.45	96.29	0.15
6	19	Factorial	1	− 1	1	− 1	0.388	6.36	94.33	94.19	0.14
22	20	Axial	0	0	1	0	0.251	5.48	95.76	95.83	− 0.07
25	21	Center	0	0	0	0	0.415	5.22	96.11	96.29	− 0.18
10	22	Factorial	1	− 1	− 1	1	0.316	5.41	94.11	94.08	0.03
4	23	Factorial	1	1	− 1	− 1	0.334	6.12	94.44	94.46	− 0.02
19	24	Axial	0	− 1	0	0	0.281	5.59	95.92	95.91	0.01
18	25	Axial	1	0	0	0	0.189	6.36	95.94	96.16	− 0.22
15	26	Factorial	− 1	1	1	1	0.382	4.55	94.70	94.66	0.04
21	27	Axial	0	0	− 1	0	0.275	5.46	95.51	95.48	0.03
12	28	Factorial	1	1	− 1	1	0.383	6.09	94.44	94.33	0.11
16	29	Factorial	1	1	1	1	0.252	5.19	94.45	94.38	0.07
9	30	Factorial	− 1	− 1	− 1	1	0.358	5.04	93.30	93.26	0.04

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