Coupling of Coagulation and Fenton-Like Oxidation for Decolorization of Congo Red Dye in Water

. Azo dyes are widely utilized in a variety of industries, including food, cosmetics, and textiles. Removal of azo dyes from wastewater by the conventional biological process is challenging d ue to its toxicity. Alternative treatment technology is needed to remove the dye eﬀectively and in a relatively short duration. The processing technology is expected to be able to reduce pollutant materials before they enter water bodies which are a source of agricultural irrigation. In this work, dye removal was studied in some physical and chemical processes, including coagulation (Coag.), Fenton-like oxidation (FLO), and its combination. Synthetic Congo Red (CR) solution was used as a contaminant model of dye wastewater. The assays were performed in a laboratory Jar-Test apparatus with varying FeCl3 coagulant doses (20-30 mg/L), H2O2 doses (42-1,680 mg/L), and coupling mode (Coag.-FLO; FLO-Coag.; FLO/Coag.). The congo red decolorization up to 87% was observed in the coupling of Coag.-FLO process at 24 mg/L FeCl3, 280 mg/L H2O2, pH 8 (Coag.) and pH 3 (FLO). Compared to the removal eﬃciency of the Coag. (color removal 45%) and FLO (color removal 62%) under selected circumstances. In addition to the higher CR removal, the coupling Coag.-FLO process showed potential cost saving due to less H2O2 dose and partially shift to coagulant.


Introduction
Azo compounds are part of organic substances that have an azo bond and an alkyl (aromatic) bond.With their ability to provide intense color through aromatic groups, azo dyes are often used in the textile, cosmetic, and food industries [1].However, with the hazardous (carcinogenic) properties produced and the high level of solubility, wastewater containing dyes needs to be treated first so as not to affect humans and the environment adversely [2], [3].Specific to the environmental quality, water bodies (rivers, lakes, seas) which become the final estuaries of wastewater also function as a source of water in agricultural activities, especially in the field of irrigation.Meanwhile, the quality of irrigation affects the quality of plant growth which can be proven through the presence of azo dyes in plants which have an impact on root damage and inhibit plant growth and photosynthesis because sunlight is blocked by dyes [4].In terms of human health, azo dye also brings consequences to customer's health due to regular intake of azo-toxic dye-treated foods or by direct contact with it [5].Therefore, various treatment technology options are needed to set it aside.
Current wastewater treatment is generally carried out through wastewater treatment plants (WWTPs) that use conventional processes, such as biological and physical-chemical treatment [6].Conventional biological treatment typically is an activated sludge process that well known to remove high levels of organic matter [7].However, this process has many shortcomings from various problems that majority arise, such as sludge production, bulking and foaming, and less organic removal efficiency due to the lack of incoming biomass [8].On the other hand, activated sludge treatment uses a large amount of land due to the long hydraulic retention time (HRT), mostly 10-60 days, so it is unsuitable to be applied in densely populated areas [9].
The wastewater treatment method called physical-chemical treatment involves coagulation, flocculation, and sedimentation processes.The advantage of the coagulation-flocculation (CF) process is the faster processing time compared to biological treatment.In addition, CF is considered to be more capable of removing dyes than biological processes since manufactured dye compounds frequently have highly intricate structures and are intently engineered to be refractory with limited biodegradability [10].The CF process works by forming contaminants, in this context dyes, into small particles, becoming larger flocs or particles, which are then removed through the sedimentation process [11].Through the CF process, dissolved color in wastewater can be removed up to 96% [12].Despite the high removal, this process must be coupled with other treatments coagulation itself is mainly used as the primary treatment for the removal of organic and inorganic solids, so that other additional treatments are needed to obtain other processing functions (secondary/tertiary) in order to achieve better water quality [13].Additionally, the use of more and more chemical compounds is equilibrium to the formation of sludge, so other treatments are needed to reduce the workload of coagulation [14].
To assist coagulation, the Fenton method is one of the technologies that can be applied to remove organic, inorganic, and microbial contaminants from water.Fenton oxidation is part of advanced oxidation processes (AOPs) that work by producing various reactive oxygen species to destroy pollutants into water and carbon dioxide [15].With its simple operation and powerful performance, Fenton is a technology that attracts much attention compared to other types of AOPs [16].This method can remove dyes in water, which can be seen from the removal of dyes reaching >98% in just a short time [17].Even more, a study that utilizes construction waste as a catalyst proving that Fenton is competent to remove dyes (methyl orange) up to 97% [18].
Several studies have been conducted combining the CF and Fenton methods to remove organic substances.The combination of the CF and Fenton process resulted in an increase in the removal of organic substance waste from coffee processing to 66-77% [19], organic substance removal of 85.2% in sugarcane vinasse waste [20], and 45-70% in printing waste [21].In essence, this combination treatment has the concept of reducing the load on the Fenton process as a second process if coagulation is carried out first.Meanwhile, if the Fenton process is the first process, then the aim is to attack the component compounds of the pollutant first before being adsorbed and removed through coagulants.Therefore, the present work aims to investigate the decolorization of synthetic wastewater containing Congo Red by coagulation, Fenton reaction, and its possible combination.

Preparation of Materials
Congo Red (CR), FeCl 3 .6H 2 O, 30% H 2 O 2 , and H 2 SO 4 were purchased from Merck.CR was used to model pollution of organic compounds (dye) in water.FeCl 3 was used as a coagulant in the CF process and a catalyst in the FLO process.Solution of 1 M H 2 SO 4 and 1 M NaOH were prepared for pH adjustment.

Experimental Protocols
Coagulation (Coag.)processes were carried out in a typical Jar Test equipment.The dose variation of the FeCl 3 coagulant used was 20-30 mg/L.This experiment contained 500 mL of Congo Red (CR) with a concentration of 50 mg/L, a rapid mixing speed of 120 rpm (1 minute), a slow mixing speed of 20 rpm (20 minutes), a settling time of 15 minutes, and a solution pH of 8.The experiment was carried out by adding the coagulant dose to the sample solution, then fast stirring, slow stirring, and sedimentation within the specified time.The supernatant from the process was then taken and tested for CR absorbance.The optimum dose obtained from this experiment will be used as the iron catalyst dose in the FLO experiment.
Fenton-like oxidation (FLO) assays were also conducted in a standard Jar Test apparatus.The dose of H 2 O 2 varied was 42 -1,680 mg/L (H 2 O 2 /COD ratio: 0.1-4x).In addition, the dose of iron catalyst was taken from the optimum dose from the Coag.process.CR with a concentration of 50 mg/L as much as 500 mL was processed under conditions of 120 rpm agitation speed (21 min) and pH of 3. Experiments were carried out by preparing CR solution and pH adjustment through the addition of 1 M H 2 SO 4 .Then, the addition of iron catalyst and H 2 O 2 solution was given in sequence to the solution, followed by stirring for 21 minutes. 1 M NaOH was added at the 21st minute until the pH reached 7, then the stirrer was turned off, and the precipitation process was carried out for 15 minutes.The supernatant from the process was then taken and tested for CR absorbance.The optimum H 2 O 2 /COD ratio obtained from this experiment will be used as a reference for calculating the concentration of H 2 O 2 in the combination process.
Several possible combinations of Coag.and FLO were investigated, such as Coag.-FLO;FLO-Coag.;FLO/Coag.The difference in experimental models is based on the order in which the experimental process is carried out.The optimum coagulant/catalyst dosage and H 2 O 2 /COD ratio were based on the Coag.and FLO processes.Generally, the conditions used were the same with each process.The first treatment process is carried out at a volume of 500 mL, and then the supernatant will be submitted as much as 400 ml to the second process.Especially for the FLO/Coag., no FeCl 3 coagulant was added to the process because they wanted to utilize the iron precipitated at the end of the FLO process (pH 8).Moreover, FLO/Coag.does not have settling time after FLO and immediately proceeds to the Coag.process.The determination of the best combination model is based on the highest effectiveness of dye removal.

Analytical Methods
The pH of the solution was measured using a pH meter.The absorbance of CR was measured with a spectrophotometer at 499 nm (DR2000, HACH).The absorbance readings were converted to CR concentration (mg/L).The percentage removal of CR was calculated in the formula: CR Removal = (CR 0 -CR 1 )/CR 0 × 100% Meanwhile, chemical oxygen demand (COD) was also measured using a spectrophotometer (Hach Method 8000) as a benchmark for the doses of H 2 O 2 .Based on Figure 1, it can be noticed that the addition of the FeCl 3 doses from 20 to 30 mg/L (equivalent to 6.9 -10.4 mg/L Fe 3+ ) results in a significant increase in CR removal from 20% to 91%.Trend lines that show an increase in the percentage of removal when coagulants are added can generally be found in studies that use the coagulation method.Adding a coagulant dose increases the amount of CR that is destabilized to form flocs that can be removed during sedimentation [14].From all these six dose variations, the coagulant dosage of 30 mg/L resulted in the greatest CR elimination.At the same time, the dose of 28 mg/L brings a tiny CR removal gap, which is a difference of only 3%, consequently this coagulant dose is indicated to be the optimum dose.The optimum dose obtained is not much different from similar studies, such as the elimination of Direct Red 23 (50 mg/L) reaching 97.7% at a dose of 40 mg/L FeCl 3 [22].The difference in dye removal results could occur because the number of coagulant doses (40 mg/L) used was higher than in this study (28 mg/L).Another study also found Direct yellow 11 removal of 87.4% at 24.4 mg/L FeCl 3 which was lower due to the use of less coagulant [23].

Fenton-Like Oxidation
This experiment was carried out with variations in the concentration derived from variations in the H 2 O 2 /COD ratio (0.1 x -4 x), including 42; 210; 420; 840; 1,260; and 1,680 mg/L.Furthermore, the FeCl 3 catalyst dose of 24 mg/L was selected for FLO.
OOH • + Organic Pollutants → CO 2 +H 2 O (4) Within 21 minutes of processing time, CR dye removal increased from 35% to 67% due to the impact of the extra dose of H 2 O 2 .The results obtained that the H 2 O 2 dose of 1.68 g/L showed the most outstanding CR dye removal.However, at 840 mg/L, dye removal reached up to 62.2%, and there was no significant difference in the percentage of removal when H 2 O 2 concentrations were doubled, particularly at dosages of 1,260 mg/L and 1,680 mg/L.According to Haber and Weiss (Eq.5), this could be attributed to the auto-decay of H 2 O 2 to oxygen and water (Eq.4) and the recombination of OH • radicals (Eq.6) [25].
H 2 O 2 works as an OH • radical scavenger because it reacts with the OH radical.In order to obtain the best degradation result, the H 2 O 2 should be added appropriately.Therefore, the H 2 O 2 /COD ratio= 2x, expressed at an H 2 O 2 concentration of 840 mg/L, is determined as the optimum concentration ratio of H 2 O 2 in the FLO experiment.
When discussing the removal of dyes which only reached 62.2% in the FLO process, this can also be found in similar studies using ferric chloride as a Fenton catalyst, only able to remove organic matter (COD) of 63.2% [20].In addition, there is also another Fenton study that has found an organic matter removal of 49.4% [19].This modest removal of contaminants could be due to using FeCl 3 as a catalyst which produces more OOH • than OH • when reacting with H 2 O 2 (Eq.1).Meanwhile, OH • , generally produced in the Fenton oxidation reaction (Eq.2), not Fenton-like oxidation, is known to have a higher oxidation capacity than OOH • , resulting in lower contaminant removal in the FLO process than in the normal Fenton oxidation process [26].
Another possibility that can cause low dye removal is the overuse of iron.As seen in Eq.1, the addition of iron is directly proportional to the formation of OOH • .However, if it is used excessively, the excess OOH • production can have a scavenging effect on other reactive oxygen species.On the other side, based on Eq.1, the FLO process can make Fe 2+ ions which can also produce reactive oxygen species (OH • ) on Eq.2, which will be affected by the scavenging effect of excess OOH • .This fact has been proven in a study that showed that consumption of H 2 O 2 increased as the catalyst dose was added but did not provide better removal [20].Therefore, it is advisable to re-adjust the dose and not use the optimum dose of the coagulation process due to FLO and Coag.are two distinct processes.

Combination of Coagulation and Fenton-Like Oxidation
Figure 3 is the data presented to compare the performance of the three process integration concepts under the same conditions in each Coagulation and FLO process.Based on the Figure 3, the three combined treatment schemes show different CR dye removal results by using the selected conditions in both processes.
The highest CR dye removal, was found in the Coag.-FLOintegration model (FeCl 3 =24 mg/L; H 2 O 2 =280 mg/L), which was 87%.In addition, the CR removal percentage was followed by the combination model of FLO-Coag.(FeCl 3 =24 mg/L; H 2 O 2 =840 mg/L) at 66% and the FLO/Coag.model.(FeCl 3 =24 mg/L; H 2 O 2 =840 mg/L) by 57%.When these conditions are compared with each coagulation and FLO treatment using the same conditions, the combination treatment concept in Coag.-FLO and FLO-Coag.produces higher CR removal (FLO = 62%; Coag.= 45%).Furthermore, since dose of H 2 O 2 in FLO process is based on the COD concentration, coupling of Coag.-FLO potentially reduces H 2 O 2 dose (from 840 mg/L to 280 mg/L) thanks to partially COD removal to during the Coag.step (Table 1).This reduction in oxidant consumption further strengthens the choice to choose the Coag.-FLOmode over FLO alone because there is a much greater cost reduction (Table 2) and removal percentage.Meanwhile, with an insignificant removal percentage difference, the use of FLO alone in removing CR is better than combined, such as the FLO-Coag.process which costs more.
On the other hand, the concept of process integration in FLO/Coag.also brings higher removal than the coagulation process (FeCl 3 = 24 mg/L) under the same conditions but lower when compared to the same conditions in the FLO process (FeCl 3 = 24 mg/L; H 2 O 2 = 840 mg/L) alone, which has achieved CR removal of up to 62%.This is expected to occur due to the elimination mechanism on the FLO/Coag.process is only via FLO, because the FLO reaction is faster than Coag.and during FLO the iron is converted into complex iron compounds, which may not be able to function as a coagulant.Related to reagent cost (Table 2), FLO/Coag.and FLO itself uses the same amount of money, so it is better to use FLO mode, which has a higher percentage of removal, or use Coag.alone resulting in less removal but reduced cost.
Based on Table 1, Coag.-FLO is able to set aside COD values of up to 87%.However, in the absence of a standard that regulates one particular type of dye, the maximum limit for dye contamination could be seen from the COD value which is included in one of the standard parameters for wastewater quality.While the current challenge is the difficulty of conventional technology to achieve the targeted effluent quality [8].Therefore, with a high percentage of removal, the selection of this water treatment technology can be an option to maintain environmental water quality through pollutant concentrations that do not exceed environmental quality standards.If related to the agricultural context, wastewater treatment technology has an important role in reducing pollutant materials before they enter water bodies which are a source of water for irrigating plants (agricultural activities).

Conclusion
Coagulation (Coag.) and Fenton-like oxidation (FLO) was tested individually and coupled to remove color from Congo Red (CR) synthetic dye wastewater.The experimental result showed CR removal from Coag.And FLO was 45% and 62%, respectively.Regarding the coupling process, Coag.-FLO showed the highest CR removal up to 87% compared to FLO/Coag.And FLO-Coag.integration mode.In addition to the higher CR removal, the coupling Coag.-FLO process showed potential cost saving due to less H 2 O 2 dose and partially shift to coagulant.

(pH= 3 ;Fig. 2 .
Fig.2.Effect of H 2 O 2 Concentration According to Figure 2, the percentage of CR dye removal rises as the concentration of H 2 O 2 increases.This is in accordance with the theoretical basis, which shows that the addition of peroxide compounds will boost the chances of the formation of hydroxyl radicals that function to oxidize pollutants into CO 2 and H 2 O [24], shown in (Eq.1-3).Fe 3+ + H 2 O 2 → Fe 2+ + OH -+ OOH • (2)

Table 1 .
COD Removal in Individual and Coupling Process

Table 2 .
Reagent Cost in Individual and Coupling Process * : Calculated based on laboratory-scale experiment