The Dichromate Method versus the Photoelectrochemical Method: the Synergistic Influence of Turbidity and Chlorides on Chemical Oxygen Demand Analysis

Quintana, Gabriel O.; Fagnani, Enelton; Candello, Fernando P.; Guimarães, José R.

doi:10.21577/0103-5053.20170161

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Articles • J. Braz. Chem. Soc. 29 (3) • Mar 2018 • https://doi.org/10.21577/0103-5053.20170161 copy

The Dichromate Method versus the Photoelectrochemical Method: the Synergistic Influence of Turbidity and Chlorides on Chemical Oxygen Demand Analysis

Authorship SCIMAGO INSTITUTIONS RANKINGS

This paper compares the dichromate method with the photoelectrochemical method (PeCOD), with regards to the influence of chloride and turbidity with chemical oxygen demand (COD) determination. Whereas the upper limits of chloride concentration are well established for both techniques, the influence of turbidity and the combined interference of underdosing chlorides and turbidity together have never been reported. When only potassium hydrogen phthalate or sorbitol were analyzed, the results show no significant difference in either method when 607 mg Cl^- L^-1 or 230 NTU of turbidity were added to the samples within the range of 100-900 mg L^-1 COD. However, a combined effect of these two interferents leads to significantly different results, mainly for low COD range, as evidenced by the Student's t-test for paired samples (p = 0.05).

Keywords:
advanced oxidation process; interference study; organic matter determination; wastewater characterization

Criteria	Dichromate COD method	PeCOD method
Waste generation	potentially toxic metals in strongly acidic media	aqueous solutions with low toxic potential
Operation cost, maintenance and consumables	low-cost reagents, simple instrumentation	patented instrumentation, high-cost replacement parts and consumables
Type of process	batch	continuous flow
Average time analysis for each sample / h	3	0.35
Efficiency to degrade nitrogenous analytes	unable to completely degrade NOCs	able to completely degrade NOCs
Labor qualification	less	more
Risk to operator	medium	low
Portability	difficult	easy
Interference	chloride and other reduced inorganic species, suspended solids	chloride, pH < 4.0, suspended solids

Code (color)	COD range / (mg L^-1)	Ratio sample:electrolyte	Max Cl^- allowed^b b after dilution with electrolytes the chloride concentration remains below 200 mg L-1. / (mg L^-1)
Blue	up to 25	4:3	350
Green	up to 150	1:1	400
Yellow	up to 1,500	1:9	2,000
Red	up to 15,000	1:49	10,000

Curve	Added NaCl/Cl^- / (mg L^-1)	Final turbidity / NTU
1	0/0	0
2		35
3		230
4		2,300
5	100/60.7	0
6		35
7		230
8		2,300
9	1,000/607	0
10		35
11		230
12		2,300

Substance	Aliquot in the final solution / mL
Substance	Blank	Calibrant	Standard (1:9)
Deionized water	50	-	-
Commercial calibrant	-	25	-
KHP/sorbitol	-	-	2
Commercial electrolyte^a a Commercial formulation containing water, natural sugars and lithium nitrate (up to 20%). KHP: potassium hydrogen phthalate; COD: chemical oxygen demand.	450	225	18

Parameter	KHP	Sorbitol
Initial pH	4.3 ± 0.1	5.4 ± 0.3
Initial turbidity / NTU	0.07 ± 0.01	0.75 ± 0.50
Initial apparent color / (mg Pt-Co L^-1)	2 ± 0	6 ± 0
EC for 607 mg Cl^- L^-1 adjusted standards / (mS cm^-1)	2.392 ± 0.072	2.070 ± 0.027

Kaolin / (g L^-1)	Expected turbidity / NTU	Final turbidity ± standard deviation / NTU (RSD / %)
Kaolin / (g L^-1)	Expected turbidity / NTU	KHP	Sorbitol
0.060	30	35 ± 12 (35)	34 ± 15 (44)
0.390	200	230 ± 40 (16)	230 ± 50 (22)
2.180	2,000	2,300 ± 600 (25)	1,900 ± 400 (22)

Analyte	Intercept	Slope	R²	Standard deviation (S_y/x)	Limit of detection / (mg L^-1)	Limit of quantification / (mg L^-1)
KHP	0.0045	3.89375 × 10^-4	0.99978	0.00221	17	57
Sorbitol	-0.00189	3.84777 × 10^-4	0.99902	0.00465	36	121

Theoretical COD / (mg O₂ L^-1)	Control COD / (mg O₂ L^-1)	Recovery / %
		COD tested for Cl^-		COD tested for turbidity
		60.7 mg L^-1	607 mg L^-1	35 NTU	230 NTU	2,300 NTU
100	96	94	101	109	109	140
200	104	101	100	101	104	119
500	99	103	101	102	103	109
700	100	100	100	104	103	104
900	100	100	97	101	102	105

Control COD / (mg O₂ L^-1)	Recovery variation between control and test COD values / %
	COD tested for Cl^-		COD tested for turbidity
	60.7 mg L^-1	607 mg L^-1	35 NTU	230 NTU	2,300 NTU
96	2	-5	-13	-13	-44
104	3	4	3	0	-15
99	-4	-2	-3	-4	-10
100	0	0	-4	-3	-4
100	0	3	-1	-2	-5
Paired t-test parameters (p = 0.05)
d	0.2	0	-3.6	-4.4	-15.6
s_d	2.7	3.7	5.9	5	16.5
n	5	5	5	5	5
n - 1	4	4	4	4	4
\|t_calc\|	0.17	0	1.36	1.96	2.12
t _crit	2.78	2.78	2.78	2.78	2.78
H₀	retained	retained	retained	retained	retained

Theoretical COD / (mg O₂ L^-1)	Control COD / (mg O₂ L^-1)	Recovery / %
		COD tested for Cl^-		COD tested for turbidity
		60.7 mg L^-1	607 mg L^-1	35 NTU	230 NTU	2,300 NTU
100	100	93	88	95	102	117
200	84	84	84	95	100	112
500	93	93	100	98	100	105
700	94	87	96	87	99	101
900	92	96	92	96	96	99

Theoretical COD / (mg O₂ L^-1)	Control COD / (mg O₂ L^-1)	Recovery / %
		COD tested for Cl^-		COD tested for turbidity
		60.7 mg L^-1	607 mg L^-1	35 NTU	230 NTU	2,300 NTU
100	114	76	137	106	170	128
200	159	142	153	120	130	149
500	142	139	142	126	141	122
700	134	125	139	127	120	106
900	135	127	135	127	129	115

Theoretical COD / (mg O₂ L^-1)	Control COD / (mg O₂ L^-1)	Recovery / %
		COD tested for Cl^-		COD tested for turbidity
		60.7 mg L^-1	607 mg L^-1	35 NTU	230 NTU	2,300 NTU
100	133	60	108	84	76	4
200	135	71	110	90	92	54
500	148	88	117	94	106	83
700	142	93	116	106	106	87
900	111	91	108	106	108	91

Theoretical COD / (mg O₂ L^-1)	Recovery / %
	Dichromate method		PeCOD method
	KHP	Sorbitol	KHP	Sorbitol
100	112	124	0	0
200	122	106	41	27
500	110	104	86	83
700	111	101	65	81
900	105	100	102	89

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