48 Using RSM, the dependency of effect on treatment can be directly represented as a
response curve or as response surface, and this curve or surface can be used to make decision not only about treatment structure but also about the relationship between
treatment and response. Knowledge of this relationship is important to find the treatment combination which gives the optimal highest or lowest response. The
exact relationship is never known but the approximation can be determined [101]. RSM has been widely used for the optimization of AOP’s for the degradation of
various contaminants. The reported literature on the application of RSM for the optimization of AOP experiments for the treatment of several pollutants are
summarized in Table 2.9.
2.5 Degradation Intermediate
Advanced oxidation process for the degradation of organic contaminants is ideally designed to completely mineralize the organic contaminant of concern to inorganic
products such as carbon dioxide CO
2
and water H
2
O, involving a highly reactive species i.e. hydroxyl radical. Since the reactivity of hydroxyl radical is very high, the
reaction between hydroxyl radical toward an organic contaminant occurs rapidly. Nevertheless, this reaction by itself does not directly results in mineralization but
produces organic oxidation by-products, which can further reacts with hydroxyl radical. Accumulation of by-product during advanced oxidation process might occur
when the reaction rate of hydroxyl radical toward the by-product is slow. Thus, this step can limit the rate of the complete mineralization of organic contaminant. Some
simple organic compounds such as acetic, maleic, and oxalic acid, as well as acetone, chloroform, and tetrachloroethane can not be readily oxidized using hydroxyl radical
[30]. However, they degrade slowly. The process may be enhanced considerably by selecting conducive process conditions.
49
Table 2. 9
Application of response surface methodology RSM in the advanced oxidation processes AOP’s area.
Contaminant References
AOP type Experimental
Factors Response
Experimental design Tool
Olive oil processing wastewater OMW
Ahmadi et al., 2005 [102]
Fenton peroxidation H
2
O
2
and Fe
2+
ratio, pH and OMW concentration
Total phenolics, color removal and aromatic
removal Central Composite
design 2
3
ful factorial Design Expert
version 5
Chemical laboratory wastewater
Benatti et al., 2006 [103] Fenton oxidation
[COD] and [H
2
O
2
] ratio, [H
2
O
2
] and [Fe
2+
] ratio and pH
COD removal 2
3
factorial with 6 center runs
Werkema and Aguiar 1996 online
Basic Red 2 BR2 dye
Körbahti and Rauf, 2008 [74]
UVH
2
O
2
BR2 concentration, H
2
O
2
concentration and pH BR2 degradation and
decolorization D-optimal design, with 3
replicates Design Expert 6.0
Terasil Red R dye Lim et al., 2009 [108]
Fenton-like H
2
O
2
pyridineCuII system
Screening process :
pH, H
2
O
2
concentration, Pyridine concentration
and Cu II concentration Optimization process
: concentration of H
2
O
2
, pyridine and CuII
COD reduction COD reduction
2
4
full factorial in triplicate, 3 blocks and 2
center point each 2
3
full factorial plus 4 center points, 3
replicates Minitab 14 PA,
USA Minitab 14 PA,
USA
Azo dye C.I. Basic Red 46 BR46
Khataee et al., 2010 [109]
Oxalate photoelectron- Fenton process using
carbon nanotube-PTFE cathode
initial concentration of dye, Fe
3+
, oxalate and electrolysis time
Decolorization efficiency
Central Composite Design CCD with total
31 experiments and 7 replication at the center
point Minitab 15 software
49
50
Leachate Li et al., 2010 [105]
Fenton treatment HRT hydraulic retention
time, Nitrogen concentration, CN ratio
COD and total nitrogen TN reduction
i 8 runs of 3 level factorial design, ii 6
runs at the so called star points and iii 1 center
point with 5 replicate each
Design Expert version 7.1.3, Stat-
Ease
Amoxicillin
Homem et al., 2010 [35] Fenton oxidation
Concentration of H
2
O
2
, concentration of Fe
2+
and temperature
CCo ratio C= concentration of
amoxicillin at t and Co= concentration of
amoxicillin at t=0 Central Composite
Design CCD with total 16 experiments: 8
factorial design, 6 expansions and 2 center
points JMP 5.01 software
Acid Red 27 AR 27 dye mix with Methyl
Red MR dye Naseri and Ayadi-
Anzabi, 2011 [106] Fenton treatment
Concentration of MT, AR 27, H
2
O
2
and Fe
2+
Decolorization efficiency
24 factorial points, 8 axial points star points
and 5 replications at center point
MINITAB® Minitab Inc. Realease 14.0
Phenol Hasan et al., 2011 [104]
Fenton’s peroxidation Phenol concentration,
H
2
O
2
and Phenol ratio, H
2
O
2
and Fe
2+
ratio, reaction time
TOC Removal Central Composite
Design CCD with 2 level factorial plus
additional experimental star point at 3 repetitions
OVAT
Oxitetracycline-HCl
OTC Rahmah et al., 2012 [107]
UVH
2
O
2
Ratio [OTC] to [H
2
O
2
], pH, and Temperature
TOC removal Box-Behnken
Statgraphics Centurion 15.2.11.0.
50
51 Oxidation of an organic compound containing nitrogen by hydroxyl radical may
proceed through the abstraction of hydrogen atoms and electrophilic addition leading
to the formation of carboxylic acids which is further degraded to smaller fragments
and eventually to CO
2
, NH
4 +
, NO
2 -
, NO
3 -
, N
2
, and H
2
O when enough hydroxyl radicals are generated in the reaction medium [89]. Organic acid and inorganic
compound containing nitrogen such as nitrite NO
2 -
, nitrate NO
3 -
and ammonia NH
3
ammonium NH
4 +
were found during the degradation of organic contaminant containing nitrogen using advanced oxidation processes [43, 63, 73, 80, 87
− 88, 110 – 112]. Glycine and ammonium was identified during the degradation of
monoethanolamine and diethanolamine by using Fenton’s reagent [43]. Alberici et al.
reported that ethylacetamide, acetaldehyde, pyrazine, acetic acid, carbon dioxide, ammonium NH
4 +
and nitrate NO
3 -
were found as the by-products during the degradation of diethylamine using
TiO
2
UV-VIS [88]. Identified by-products obtained from the mineralization process for different organic compounds containing nitrogen
using AOP’s are listed in Table 2.10.
2.6 Biodegradability of Pollutants
