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With the help of the Design Expert (6.0.8), a 2³ complete factorial central composite design was obtained. A total of 20 tests in duplicate were used to the central composite design matrix according to this approach, as shown in Table 3.

Table 3

CCD for independent operational variables and the recorded response for As (III)

Std	Run	Factor 1	Factor 2	Factor 3	Response breakthrough time
		A: Influent As(III) concentration	B: Influent flow rate	C: Bed depth	Experimental values	RSM predicted
		μg/L	mL/min	cm	min
10	1	1500.00	6.00	10.00	412	417
7	2	300.00	9.00	15.00	858	861
9	3	109.08	6.00	10.00	496	501
4	4	1500.00	9.00	5.00	232	238
5	5	300.00	3.00	15.00	934	941
16	6	900.00	6.00	10.00	468	463
14	7	900.00	6.00	15.00	778	782
17	8	900.00	6.00	10.00	468	463
18	9	900.00	6.00	10.00	468	463
3	10	300.00	9.00	5.00	281	277
2	11	1500.00	3.00	5.00	298	291
13	12	900.00	6.00	5.00	245	240
8	13	1500.00	9.00	15.00	741	735
6	14	1500.00	3.00	15.00	792	789
11	15	900.00	6.00	10.00	468	463
20	16	900.00	6.00	10.00	468	463
19	17	900.00	6.00	10.00	468	463
12	18	900.00	11.05	10.00	425	419
1	19	300.00	3.00	5.00	312	318
15	20	900.00	6.00	10.00	468	463

Std	Run	Factor 1	Factor 2	Factor 3	Response breakthrough time
		A: Influent As(III) concentration	B: Influent flow rate	C: Bed depth	Experimental values	RSM predicted
		μg/L	mL/min	cm	min
10	1	1500.00	6.00	10.00	412	417
7	2	300.00	9.00	15.00	858	861
9	3	109.08	6.00	10.00	496	501
4	4	1500.00	9.00	5.00	232	238
5	5	300.00	3.00	15.00	934	941
16	6	900.00	6.00	10.00	468	463
14	7	900.00	6.00	15.00	778	782
17	8	900.00	6.00	10.00	468	463
18	9	900.00	6.00	10.00	468	463
3	10	300.00	9.00	5.00	281	277
2	11	1500.00	3.00	5.00	298	291
13	12	900.00	6.00	5.00	245	240
8	13	1500.00	9.00	15.00	741	735
6	14	1500.00	3.00	15.00	792	789
11	15	900.00	6.00	10.00	468	463
20	16	900.00	6.00	10.00	468	463
19	17	900.00	6.00	10.00	468	463
12	18	900.00	11.05	10.00	425	419
1	19	300.00	3.00	5.00	312	318
15	20	900.00	6.00	10.00	468	463

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Table 4

Sum of squares sequential model

Source	Sum of squares	DF	Mean square	F value	Prob > F
Mean	5.091E + 006	1	5.091E + 006
Linear	7.755E + 005	3	2.585E + 005	95.91	< 0.0001
2FI	4,948.30	3	1,649.43	0.56	0.6497
Quadratic	34,824.08	3	11,608.03	34.63	< 0.0001	Suggested
Cubic	3,352.06	4	838.01	6.366E + 007	< 0.0001	Aliased
Residual	0.000	6	0.000
Total	5.910E + 006	20	2.955E + 005

Source	Sum of squares	DF	Mean square	F value	Prob > F
Mean	5.091E + 006	1	5.091E + 006
Linear	7.755E + 005	3	2.585E + 005	95.91	< 0.0001
2FI	4,948.30	3	1,649.43	0.56	0.6497
Quadratic	34,824.08	3	11,608.03	34.63	< 0.0001	Suggested
Cubic	3,352.06	4	838.01	6.366E + 007	< 0.0001	Aliased
Residual	0.000	6	0.000
Total	5.910E + 006	20	2.955E + 005

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