Fenton and solar Fenton processes: inexpensive green technologies for the decontamination of wastewater from toxic Rhodamine B dye pollutant

The generation of extremely reactive ^•OH radicals by the advanced oxidation processes (AOPs) like photocatalysis, sonocatalysis, microwave (MW) catalysis, the Fenton process, and their combinations causes the mineralization of organic contaminants to CO₂, water, and salts (Hoffmann et al. 1995; Andreozzi et al. 1999; Suty et al. 2004; Lu 2013; Ameta & Ameta 2018; Elmobarak et al. 2021). Fenton-based AOPs are classified into homogeneous and heterogeneous types depending on the reactive phase. Classic Fenton (H₂O₂ + Fe²⁺), Fenton-like processes (Fe²⁺ + H₂O₂ + Metalⁿ⁺), sono-, photo-, electro-Fenton processes, and other Fenton processes based on O₃, H₂O₂, O₃-UV, H₂O₂-UV, and O₃-H₂O₂-UV, and so on are homogeneous AOPs. Processes involving suspended catalysts belong to the category of heterogeneous AOPs (Ndamitso et al. 2020; Onu et al. 2023). Reactant molecules get adsorbed at the active sites on the surface of the catalyst and the products get desorbed after the reaction. The use of FeS₂/SiO₂, (FeTiO₃)/TiO₂, titanomagnetite (Fe₃TiO₄), etc. as heterogeneous Fenton catalysts was also reported widely (Costa et al. 2008; Sivakumar et al. 2013; He et al. 2016; Diao et al. 2017; Luo et al. 2021). In the heterogeneous Fenton catalysts, ZnO has received high degree of attention since they are non-toxic, stable, inexpensive, and having high photosensitivity and oxidation capacities. ZnO has a flat band structure (3.35 eV approximately), has spectral overlap with solar emission (about 5%), and is easily applicable in ambient as well as harsh environments. It possesses a relatively large bandgap and offers several positively charged sites for scavenging photo-generated electrons. This prevents their recombination with the holes and promotes redox reactions.

Wastewater treatment using the Fenton process results in improved biodegradability, reduction of toxicity, and/or complete mineralization of the pollutant (Scaria et al. 2021; Vilela et al. 2021). In the classic Fenton process, when ferrous iron and H₂O₂ are combined, Fe²⁺ ions catalyze the decomposition of H₂O₂ to produce reactive ^•OH radicals under acidic conditions. The highly reactive species ^•OH radicals (oxidation potential 2.8 V) can attack organic molecules and lead to their mineralization through the formation of intermediates with rate constants in the range of 10⁶–10¹⁰ M⁻¹ s⁻¹ (Andreozzi et al. 1999; Khare et al. 2021). AOPs, with the possible exception of heterogeneous photocatalysis, have so far received only limited acceptance as viable and efficient techniques for the large-scale purification of wastewater due to the complexity of the process, high cost, and practical limitations. Accelerating the transformation of Fe³⁺/Fe²⁺ and managing the production of Fenton sludge are crucial for resolving the issues related to the Fenton process. The problem of Fenton sludge formation which is the main reason for its poor acceptance as an environment-friendly technology can be resolved by careful design of the component ratio and repeated recycling of Fe²⁺. The economic viability, input energy, recycling issues, stability of the process, etc. shall be taken into account for selecting the actual wastewater treatment process. The design of efficient photocatalytic reactors and modification of catalysts by new techniques, which are strongly relevant in catalyst recovery and controlling agglomeration, are also important research areas that require attention.

Recently, AOPs based on ionizing radiation, MWs, and pulsed plasma methods have also received a lot of attention, albeit their economic viability has not yet been established. The simple, low-cost conventional Fenton reaction has drawn renewed interest in this circumstance (Furia et al. 2021; Tang et al. 2021). The economic efficiency of the mineralization process by combining the Fenton reaction with other AOPs is being widely investigated. Photo-Fenton, sono-Fenton, MW-Fenton, photoelectro-Fenton, etc. are a few of them (Gou et al. 2021; Maroudas et al. 2021; Yu & Pei 2021; Li et al. 2022; Xia et al. 2022). In the classic Fenton process (without external activation), the H₂O₂ very slowly oxidizes ferrous to ferric ions leading to the creation of ^•OH radicals. In the presence of an external source of activation, this reaction is expected to be accelerated as H₂O₂ breaks down very quickly. This paper presents and discusses the results of the investigations on normal and modified Fenton processes as possible candidates for removing hazardous organics from wastewater.

Rhodamine B (RhB) (>99.6%) and ZnO (>99.5%) were purchased from Sigma Aldrich India and are used as such. H₂O₂ (HP) (30.0% w/v) and FeSO₄.7H₂O (FS) were purchased from Qualigen (India) and used as such without further purification. Unless otherwise stated, other chemicals used were also of AnalaR grade or equivalent.

The dye solution is combined with the required concentration of FS and HP in a 250-mL closed Pyrex glass reactor in a conventional experiment setup without external activation. The reactor is wrapped in black paper for classic Fenton experiments to prevent any potential photochemical reactions. The solar experiments were carried out on sunny days on the rooftop of our laboratory building in Kochi, Kerala, India. The samples were placed in a 500-mL pyrex glass reactor and at periodic intervals, samples were taken, filtered, and analyzed. An ultrasonic bath (Equitron make) of a frequency of 53-KHz power of 100 W was used for US experiments. By the circulation of water from a thermostat, the required temperature (29 ± 1 °C) is maintained in the sonicator (10 L). A specially designed jacketed Pyrex glass reactor was used for specific experiments at different temperatures. Experiments using MW radiation were performed in a commercial MW oven (2,450 MHz and 800 W power) modified by making an opening at the top for introducing the reactor. Using a magnetic stirrer, the sample solutions are continuously mixed. The pH of the solution was adjusted to the desired level as needed using 1 N (normal) H₂SO₄ or NaOH.

An appropriate quantity of the respective aqueous solution (except in the case of the catalyst, which was introduced as solid itself) was added into the RhB solution for experiments with added components, such that the net concentration of the dye, as well as of the additive, would be as desired. In all cases, UV-Vis spectrophotometry at 554 nm was used to monitor the degradation efficiency by analyzing RhB remaining in the solution system. Using standard iodometry methods, H₂O₂ was measured. A fixed amount (0.1 g) of the catalyst in 50 mL of RhB solution in a 250-mL flask was used for adsorption studies (Jain et al. 2009; Tijani et al. 2022). The pH was adjusted as required for the study. To reach equilibrium, the suspension was constantly stirred for 2 h at a temperature of 29 ± 1 °C. Keeping the suspension overnight before centrifuging had no discernible effect indicating that the 2 h were sufficient to complete the adsorption. Then, it was centrifuged at 3,000 rpm. The colorimetric method was used to measure the concentration of RhB in the centrifugate.

Thorough studies were conducted to determine the best ratio of two crucial components FS and HP for RhB degradation under various conditions, i.e., normal Fenton with no external activation (NF), and with activation using SL, US, and MW, by keeping the concentration of RhB constant. Thus, by altering one variable and keeping the other constant, the quantity of FS and HP was optimized. The effect of the FS/HP ratio on the efficiency of the Fenton process is investigated and the ratio is optimized. The RhB concentration of 15 mg/L was kept constant. The concentration of HP was increased from 2.5 to 20 mg/L, keeping the concentration of FS constant at 2.5 mg/L. In order to study the effect of FR-pretreated ZnO, the optimized ratio of FR (1:3) solution is vigorously mixed with 200 mg/L ZnO for 60 min. The ZnO is then filtered, and dried at room temperature and the solar degradation experiment is then carried out. FR-pretreated ZnO exhibits higher RhB degradation after 60 min of solar exposure.

The influence of persulfate on the solar degradation of RhB is investigated with and without ZnO. The persulfate doses ranged from 2.5 to 12.5 mg/L. The net concentration of H₂O₂ in the RhB/FR/ZnO (15/2.5:7.5/200 (mg/L)) system was measured following the decolorization of the suspension both in the presence and absence of FR at pH 6.5 and 3.5. Experiments were carried out to determine if the drop in H₂O₂ concentration in the system containing Fe²⁺ is caused by the latter's rapid interaction with the former during solar irradiation. After decolorization under SL, the quantity of H₂O₂ in sample solutions of RhB + ZnO and RhB + ZnO + FR is measured. Then, each sample solution is separated into two portions. One half (C) receives 2.5 mg/L of Fe²⁺ addition and is exposed to SL for 60 min. The other half (B) is exposed to radiation for 60 min without the addition of Fe²⁺ and the H₂O₂ is calculated.

In the Fenton process, the excess reagent successfully employed to degrade the fresh substrate is tested by progressively adding more FR/HP/FS (1:3/7.5/2.5 mg/L) to a reaction system containing optimized concentrations of pollutant and reagents where the pollutant degradation leveled off due to insufficient reagent. The recycling of Fe²⁺ is done when pollutant degradation in the SF case reached a peak. An extra 7.5 mg/L of H₂O₂ was introduced to the system to efficiently use the remaining FS and fresh RhB (15 mg/L) solution is supplied to the system once the decomposition of the RhB is completed. More H₂O₂ is injected once the degradation has stabilized and more RhB is added when degradation is completed, which is done continually. After initial use, the system's ability to recycle Fe²⁺ is examined by periodically replacing the H₂O₂ that was used up in the FR.

The degradation efficiency (47% for SF , 43% for NF, 42% for US Fenton, and 30% for MW Fenton within 30 min of time) at the respective optimum ratio of Fenton components is in the order: SF > NF > US-Fenton > MW-Fenton.

The COD determines the effectiveness of any AOP and in the study COD decreases after decolorization, indicating that at least some of the intermediates are unstable and are getting mineralized. However, the COD does not really alter much after the initial reduction, most likely because some of the intermediates generated might be more resilient and persistent. Even after prolonged hours, the Fenton process does not degrade them. Thus, it can be concluded that the Fenton process is an effective inexpensive way to decolorize RhB contaminants from water. Complete mineralization, on the other hand, occurs more slowly, suggesting the presence of resistant, slowly degrading intermediates. To accomplish total mineralization in such circumstances, a combination of the Fenton process with external sources of activation like photo, sono, and MW may be needed.

Combination of Fenton reaction with light as the source of activation, i.e., photo-Fenton is known to enhance the efficiency of the process (Babuponnusami & Muthukumar 2011; Sannino et al. 2012; Pouran et al. 2015; Çalık & Çifçi 2022). The current study, as reported earlier in this paper, shows that the Fenton process in combination with MW/US activation is not effective for the complete mineralization of RhB pollutants in water while the SL-Fenton combination (SF) gave encouraging results (Figure 3). Hence, the SF process is examined in depth for the degradation/mineralization of RhB pollutants. The efficiency of the Fenton process depends on the relative concentrations of FS and HP (Gulkaya et al. 2006; Ghosh et al. 2010; Asgari et al. 2020). Hence, the ratio of these critical components is optimized initially.

The interaction between the RhB molecule and the reactive free radicals will be more effective as the concentration of the dye is increased. This leads to an increase in the rate of reaction. Beyond the optimum, the rate of degradation slows down because the relative concentration of ^•OH radicals to interact with all the RhB molecules will be less. This leads to a steady state or even a decrease in the degradation rate. At higher concentrations, the dye molecules are excessively large in comparison to the amount of FR needed for efficient degradation. Since H₂O₂ is primarily responsible for degradation, any hindrance to the ability of H₂O₂ to interact with Fe²⁺ to produce ^•OH radicals will result in a reduction in degradation.

The trend indicates that in the presence of ZnO, the solar degradation of RhB (photocatalysis) will continue and finally approach the same degradation as in the presence of FR/SL. RhB degraded more rapidly in FR-pretreated ZnO. This suggests that inhibition (by ZnO) of classic Fenton degradation of RhB may be at least partially due to the adsorption of FR over ZnO.

Due to the generation of extremely reactive radical anions, persulfates have been proven to be powerful oxidants in AOPs (Norzaee et al. 2017; Venâncio et al. 2022). The influence of persulfate on the solar degradation of RhB indicated that persulfate has a positive influence on degradation, most likely as a result of efficient production.

As a result, the rate of degradation decreases as reaction time improves. The number of reactive free radicals available for the degradation of RhB decreases and the degradation process slows down as the substrate, intermediates, and persulfate compete for the ⁠. The effect of ZnO on the FR/PS/SL degradation of RhB revealed that ZnO had no influence on the FR/PS-induced degradation at different PS concentrations. ZnO results in the formation of additional ^•OH radicals. They might, however, be interacting more with the ⁠, which would cause both to be destroyed by chain termination. As a result, the concentration of ^•OH and varies over time, causing the degradation to either decrease or stabilize.

It is observed that persulfate emerges as a potential candidate to be used as an alternative oxidant in the photo-Fenton process as it reacts with Fe²⁺ to form sulfate radical (⁠⁠) which has a longer lifespan when compared to ^•OH. This could be due to the fact that persulfate has the ability to form reactive radicals in the presence of SL in addition to the presence of ^•OH radicals from FR. Compared to rates reported for ^•OH, reaction rates between ⁠, natural organic matter, and ions are slower because only reacts by electron transfer while ^•OH can react through three separate processes (OH addition, OH reduction, and OH oxidation). As a result, is more stable when treating real matrices with a variety of components. Additionally, by reacting these radicals with hydroxide ions (OH) in FR reagent, ^•OH may be simultaneously produced in the presence of ⁠.

The optimal FS/HP ratio in this case contains an excess of H₂O₂. The addition of Fe²⁺, when the RhB degradation has stabilized at ∼50% only slightly, increases the degradation demonstrating that even under photo-irradiation FS does not become inactive and that the leveling off is not caused by a lack of sufficient FS. The experiments FR and H₂O₂ addition in between further confirm this. In both situations, the degradation is greatly accelerated by the addition of additional reagents at the level-off stages (60 min: 50% degradation; 105 min: 80% degradation). The enhancing effect of the gradual addition of FR and H₂O₂ is diminished with each addition. As a result, adding FR/H₂O₂ after 150 min, when the deterioration is stable at 95%, has no noticeable effect. Since most of the dye has already been degraded at this point, the effect is negligible as there is not enough dye available to fully utilize the FR/H₂O₂ that is already present. This also indicates that in the case of FR, a moderate excess of reagent is always advisable to produce good substrate degradation due to the simultaneous self-decomposition of H₂O₂. Another possibility is that the reactants or intermediaries block the Fe²⁺ or interact ineffectively to produce transient complexes. This aspect requires additional research, which is beyond the focus of this study.

As previously mentioned, RhB degrades faster as the concentration of Fe²⁺ increases. Extra ferrous iron does not, however, impact the rate of degradation after a critical optimum. Other researchers also reported comparable observations (Kavitha & Palanivelu 2005; Liu et al. 2019; Mahtab et al. 2021; Gao et al. 2022). The unfavorable loading of dissolved Fe²⁺ in the effluent water with prolonged use of this procedure for water treatment is one of the critiques leveled against the use of the Fenton process for water purification. The rate of degradation accelerated immediately after the addition of extra H₂O₂ (7.5 mg/L) as indicated in Figure 14.

Repeating the cycle (addition of H₂O₂ followed by fresh RhB) several times would confirm that Fe²⁺ could be recycled thereby lowering its excessive loading in the discharged water. As a result, a key objection to the use of FR for water treatment is addressed here. Similar studies have been reported in the context of classic Fenton (Gayathri et al. 2016).

The intermediates identified at 50% degradation of RhB by LC-MS analysis is displayed in Table 1. The precise identification of several of the low molecular weight fragments is not given here. The mass spectral measurements may result in the formation of numerous transitory products, although these are not particularly relevant to the present situation.

Table 1

List of intermediates detected by LC-MS analysis during SL/FR/RhB degradation

Sl. No. .	m/z .	Intermediates identified during the degradation of RhB .
1	443	Rhodamine B (parent compound)
2	431	Hydroxy N-ethyl-N′ ethyl rhodamine
3	415	N,N-diethyl-N′-ethylrhodamine
4	403	Hydroxy amino N′ ethyl rhodamine
5	387	N-ethyl-N′-ethylrhodamine
6	359	N-ethyl rhodamine
7	359	N-ethylrhodamine
8	138	3-Hydroxybenzoic acid
9	117.06	Succinic acid
10	91.05	Oxalic acid

Sl. No. .	m/z .	Intermediates identified during the degradation of RhB .
1	443	Rhodamine B (parent compound)
2	431	Hydroxy N-ethyl-N′ ethyl rhodamine
3	415	N,N-diethyl-N′-ethylrhodamine
4	403	Hydroxy amino N′ ethyl rhodamine
5	387	N-ethyl-N′-ethylrhodamine
6	359	N-ethyl rhodamine
7	359	N-ethylrhodamine
8	138	3-Hydroxybenzoic acid
9	117.06	Succinic acid
10	91.05	Oxalic acid

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The highly reactive free radicals may interact with other radicals, and/or H₂O₂ or may get deactivated by self-scavenging (Equations (12)–(15)).

Under photolysis/photocatalysis, the various other possible reactions are given as follows (Equations (21)–(29)).

Many of the above mentioned reactive free radicals are produced by the Fenton process even in ambient conditions, i.e., conventional Fenton. They can generate intermediates and finally mineralize the pollutant by interacting with RhB in a number of ways. The decomposition and mineralization processes can be accelerated when solar energy is used as an external energy source. When ZnO, which functions as an effective photocatalyst, under UV/visible light (SL) is present, the degradation is further accelerated. Based on the results of the current study, it is seen that the SF process and solar photocatalytic-Fenton process significantly enhance the efficiency of the conventional Fenton process for the degradation and mineralization of RhB.

The work demonstrates that the traditional Fenton reaction, which uses H₂O₂ and a simple ferrous salt at ambient conditions, may be revisited and used as a powerful advanced oxidation technology to remove hazardous contaminants from water. Fenton reaction is a convenient option for cost-effective and efficient wastewater treatment due to the relative simplicity of the procedure. The RhB dye is used as a model pollutant to illustrate the efficiency of the Fenton process for complete degradation and mineralization under SL. Solar activation improves the efficiency of the classic Fenton process for purifying water from dye contamination. Mineralization is successfully completed using ZnO as the catalyst and SL as the activation source. It also reports the difference in the behavior of ZnO in classic Fenton and solar photocatalytic-Fenton. By recycling the Fenton sludge and replenishing the used-up H₂O₂ regularly, the drawbacks of accumulation of Fe²⁺/Fe³⁺ can be eliminated. During the process itself, the Fe²⁺ that is converted into Fe³⁺ is converted back to Fe²⁺. FS/HP ratio and pH are crucial for maximizing efficiency. By using LC-MS analysis, several intermediates formed during the degradation are identified. The effectiveness of the solar photocatalytic Fenton process for treating wastewater economically is demonstrated here. Solar energy is a natural, inexhaustible, and renewable source of energy. Thus, as a green and efficient degradation technology, SL/FR/ZnO is favored for its advantages of high removal rate, short time use, wide concentration range, low cost, good stability, and no secondary pollution.

Financial support from the University Grants Commission (UGC), India, is gratefully acknowledged by way of the Senior Research Fellowship to P.V.G. The authors would like to thank Kerala University of Fisheries and Ocean Studies (KUFOS), St. Albert's College, and Cochin University of Science and Technology (CUSAT) for providing the necessary laboratory facilities, equipment, materials, etc. to conduct the study. The authors also thank two anonymous reviewers for their constructive evaluation of our manuscript, which significantly enhanced the overall quality of the manuscript.

P.V.G. conceived the idea, conducted the research work, and wrote the manuscript. S.J. reviewed and edited the manuscript and conducted overall supervision of the work. E.P.Y. designed the study, and reviewed and edited the manuscript. S.Y. supported the experimental setup and reviewed the manuscript. All authors read and approved the final manuscript.

The first author was supported by the University Grants Commission (UGC), India by way of a Senior Research Fellowship, and Kerala University of Fisheries and Ocean Studies (KUFOS) by way of a Post-Doctoral Fellowship.

All ethics were followed during the preparation of the manuscript. Unethical actions were not made.

All authors gave consent to participate in this publication.

All authors gave consent for publication in the journal Water Practice and Technology.

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.