Nanotechnologies have prominent applications in the field of science and technology owing to their size-tunable properties providing a promising approach for degradation of various pollutants. In this scenario, the present work aims to study the effect of nickel ferrite nanoparticles on the degradation of Irgalite violet dye by Fenton's reaction using oxalic acid as an oxidizing agent in the presence of sunlight. The effect of pH and adsorbent dosage on the rate of dye degradation was monitored. Based on these studies it was observed that 99% dye degradation was achieved for catalyst dosage of 0.2 g, 400 ppm dye concentration and 2.0 mM oxalic acid at pH 3.0 within 60 min. The studies reveal that the degradation follows pseudo-first-order kinetics and the catalyst reusability remained constant almost for five cycles. Further, nickel ferrite nanoparticles are proven to be an efficient alternative for the removal of dyes from coloured solutions.
Water is a fundamental and valuable source for all forms of life on Earth. This vital source is being contaminated due to urbanization and industrialization where effluents are let into water bodies without proper treatment. Large quantities of wastewaters with high colour intensity, high chemical oxygen demand and fluctuating pH are being discharged into water bodies causing serious environmental issues affecting flora and fauna. Dyes have a complex aromatic structure and are resistant towards chemicals, heat and light, while their biodegradation is a significantly slow process. From a toxicological as well as from an ecological point of view it is essential to treat effluents containing these dyes. These diverse pollutants when allowed into the soil by migration and transformation modes cause huge global environmental issues.
Most of the conventional methods are ineffective due to the large degree of aromatics/phenols present in dye molecules. Further, they call for high operational and maintenance costs, and therefore concerns lie with the treatment of effluents by economically viable methodologies (Ye et al. 2017a, 2017b, 2019a, 2019b). Various physico-chemical methods like coagulation, flocculation, ion exchange, membrane separation, oxidation, nanofiltration, adsorption, catalytic ozonation (El et al. 2019) etc. are available for the treatment of dyes. However, some of the major limitations, such as the removal of adsorbents or the concentrated sludge produced post-remediation, are the major drawbacks of these methods. One of the alternatives to the traditional methods are the advanced oxidation processes (AOPs) which have been used to remove contaminants from drinking water and effluents. AOPs include photolysis, heterogeneous photocatalysis, electro-Fenton and photo-Fenton. Fenton is considered as an AOP that combines ferrous ions and hydrogen peroxide and other hydroxyl generating acids to promote the generation of the hydroxyl radicals in aqueous solution (Khosravi & Eftekhar 2013; Lassoued et al. 2018). The photo-Fenton process is a typically enhanced Fenton reaction with a higher rate and faster mineralization of recalcitrant organics than the dark reaction process and can take advantage of utilizing UV irradiation from solar light. According to the light sensitivity of the photo-Fenton reaction with respect to the sunlight spectrum, the utilization of a solar light is reported to be very economic and a simple alternative compared to UV lamps, especially for treatment of wastewater at full scale. In the reaction of the photo-Fenton process, Fe2+ ions are oxidized by oxalic acid to Fe3+ to generate hydroxyl radicals. The application of Fenton's reagent as an oxidant for wastewater treatment is attractive. Among the advantages of Fenton's process relative to other oxidation techniques are the simplicity of equipment and the mild operation conditions (atmospheric pressure and room temperature); mainly for these reasons, the Fenton process has been regarded as the most economical alternative (Alalm et al. 2015; Deng & Zhao 2015; Santos-Juanes et al. 2017). In order to enhance the degradation of organic pollutants, external sources of energy have been added to the Fenton process, such as light source, and this process is called photo-Fenton process (Suna et al. 2007; Maaz et al. 2009; Shihong et al. 2009; Youssef et al. 2016). The reaction of ferrous ion (Fe2+) and oxalic acid (C2H2O4) generate hydroxyl radicals where ferric ion (Fe3+) regenerates Fe2+ by reacting with an excess of C2H2O4 and hydroperoxides (•OOH). The photo-Fenton process enhances the production of hydroxyl radicals by photo-reduction of Fe3+ to Fe2+ and photolysis.
Recently, various nanomaterials have been used as nanocatalysts for various applications such as sensing, detection, degradation and electrical applications (Qin et al. 2017; Yi et al. 2018a, 2018b, 2018c, 2019; Ahmadi et al. 2019; Baghban et al. 2019; Ramezanizadeh et al. 2019; Wang et al. 2019; Yang et al. 2019). ZnO, gold and TiO2 are the other nanoparticles which are often reported, however, the use of magnetic NiFe2O4 nanoparticles for the photo-Fenton degradation of organic model pollutants in the presence of oxalic acid is studied. It is well known that the magnetic and electrical properties of MFe2O4 nanoparticles can be varied by changing the identity of the divalent M2+ cation or by partial substitution, while maintaining the basic crystal structure. For magnetic nanodevices and biomedical applications, MFe2O4 ferrites of size-tuned ferrite particles of superparamagnetic threshold were used at room temperature (Li et al. 2015; Weng & Huang 2015; Yahya et al. 2016; Behin et al. 2017; Shi et al. 2017; Sohrabi et al. 2017).
In this scenario, the ferrite materials based on nickel and cobalt are the subject of extensive research due to important magnetic properties like good remnant magnetization, high coercivity, magnetic anisotropy and moderate saturation magnetization. They also exhibit high electrical resistivity, and good thermal and chemical stability (Sivakumar et al. 2011; Sagadevan et al. 2018). Nickel ferrite (NiFe2O4) has an inverse spinel structure with crystal structure as AB2O4, in which tetrahedral sites (A) are occupied by Fe3+, octahedral sites (B) by Fe3+ and Ni2+ ions formed by cubic close packing of oxygen. However, due to random distribution of divalent ions in tetrahedral and octahedral sites reverse cationic distribution in nanocrystalline spinel takes place. For instance, utilizing mesoporous ZnFe2O4-H2O2 under visible light for photocatalytic degradation of orange II, enhanced degradation was observed within 2 h. NiFe2O4 nanoparticles were used for the degradation of rhodamine B under visible light/oxalic system (Liu et al. 2012) and it was observed that they were highly stable even after seven cycles. Similarly, Fenton-like performance was also observed by which the nZVI/H2O2 system yielded 91.95% removal efficiency of magenta flexographic dye (Kecić et al. 2018). Sodium hypochlorite, hydrogen peroxide (H2O2) and ozone (O3) are the other oxidizing agents used. Fenton treatment with the addition of oxalic acid has high reaction rates with ·OH and can also act as a scavenging agent in the reaction process. Due to the above-mentioned advantages, this study aims to determine the effect of nickel ferrite nanoparticles and oxalic acid using the photo-Fenton process in the degradation of Irgalite violet dye.
Irgalite violet belongs to the category of azo pigments with strong colour intensity, good transparency and rheology for aqueous, solvent-based and energy-curable inks. These characteristics of the dye make it difficult for the dye industry to decolourise or degrade the dye wash water and its effluent. Considering the unsustainable water use and water pollution due to dye effluents, in this work, the combined photo-Fenton and ferrite process was used to treat persistent dyes. Nickel ferrites due to their relatively dry and more compact physical structure than ferric hydroxide considerably reduce sludge volume in ferrite formation. Hence, nickel ferrite is considered as a heterogeneous photo-Fenton catalyst.
MATERIALS AND METHODS
Photocatalytic experimental studies
NiFe2O4 nanoparticle synthesis was carried out by co-precipitation method as mentioned in previous work (Kollarahithlu & Balakrishnan 2018). Photocatalytic degradation of Irgalite violet dye was carried out utilizing NiFe2O4 nanoparticles in the presence of sunlight between 11a.m. and 3p.m. (IST) with an average light intensity of 6.2 kWh/m2/day, at a temperature of 28–32 °C. Prior to irradiation, the solution was magnetically stirred in the dark to attain adsorption-desorption equilibrium for 30 min between the dye and photocatalyst. The photo-Fenton degradation of the dye was performed by varying the amount of NiFe2O4 (0.05–0.20 g), oxalic acid (0.5–2 mmol), dye concentrations (200–1,000 ppm) and pH (2.0–4.0) in a 250 mL conical flask.
Degradation efficiency estimation
RESULTS AND DISCUSSION
Effect of pH
The degradation of Irgalite violet dye by NiFe2O4 nanoparticles was studied between pH 2.0 and 4.0. At initial pH the degradation rate was minimal. The degradation efficiency was found to increase with increase in pH and attained a maximum of 99.9% degradation at pH 3.0 (Figure 1). This could be due to the increase in ferric/oxalate species and similar findings were reported by Liu et al. (2012), where the degradation of rhodamine B in the presence of oxalic acid was more effective at pH 3.0 confirming that the conducted Fenton oxidation process is optimum at pH 3.0 (Suna et al. 2007). Above pH 4.0 the degradation rate slows down to 78%; the probable reason might be due to the lower concentration of H2O2 production by the oxalic acid (Monteagudo et al. 2008). The acid environment is beneficial for the Fenton-like process, the working pH for the reaction is confined to 1.0–5.0 and the highest efficiency of the Fenton reaction occurs at pH 3.0. At pH higher than 4.0, the Fenton reaction appears to terminate due to the precipitation of Fe3+. The amount of iron ions decreases because Fe3+ precipitates after use due to its extremely low solubility product constant even at the pH 4.0. The degradation rate increases as the pH decreases and reaches an optimum at initial pH 3.0. At the initial pH of 2.0, a colourless solution is observed at 40 min; at pH 3.0, 99.9% of the dye is converted at 60 min reaction time and at pH 4.0 it is 78%. The oxidation potential of •OH is lower at pH 4.0 than that at pH 3.0. At pH > 3, the formation of hydroxyl radicals slows down because of hydrolysis of Fe3+ and Fe3+ precipitation as Fe (OH)3 from the solution. Thus, the efficiency of Fenton oxidation decreases at pH higher than 3. Also, ferric hydroxide formation could decompose oxalic acid into O2 or H2O and, consequently, the oxidation rate is decreased as a result of the low concentration of hydroxyl radicals. Similar performance in the oxidation of dye solutions using iron-alumina catalyst in the heterogeneous Fenton process was reported where a detrimental effect on decolouration at pH > 3 was found (Barrera-Salgado et al. 2016; Zhou et al. 2016; Yi et al. 2018a, 2018b, 2018c).
Effect of oxalic acid concentration
The effect of initial concentration of the oxidizing agent oxalic acid was studied to determine the rate of degradation from 1.0 to 3.0 mM for the experimental condition of 0.15 g of catalyst loading at dye concentration 400 ppm and pH 3.0 for 60 min. It can be observed that the degradation rate increased with the increase in oxalic acid concentration up to 2 mM (99.5%), reached a plateau, and decreased at 2 mM by 1% for various dye concentrations (Figure 2). As the dye concentration increased, the degradation efficiency decreased; this is because at higher concentrations of dye the number of active sites decreases, which is due to absorption of more dye molecules on the catalyst surface. This could be attributed to the hindering effect of the dye molecules on the production of •OH radicals and H2O2 leading to reduction in the rate of degradation. A concentration of 1.0 mM of magnetic NiFe2O4 and oxalic acid increased to 2 mM concentration leads to a high degradation rate under irradiation, indicating that NiFe2O4 is a heterogeneous Fenton catalyst. The degradation rate increases with increased oxalic concentration from 0.1 mM to 2.5 mM and then reaches a plateau at concentrations greater than 1.0 mM. This phenomenon can be attributed to forming of the predominant species [FeIII(C2O4)n]3−2n on the solid catalyst surface; its concentration reaches a maximum when the concentration of oxalic acid is 2.0 mM in solution. The reason for the optimum degradation at 2 mM of oxalic acid could be due to the generation of more OH radicals and Fe2+ ions (Liu et al. 2012) With the addition of oxalic acid, the degradation ratio of the dye is only 3.9% without irradiation. A 5% increase in decolourisation ratio is observed in the solution of oxalic acid and Irgalite violet without NiFe2O4 under irradiation. However, 99.99% degradation was achieved in the presence of the magnetic catalyst NiFe2O4 with the addition of 2 mM of oxalic acid crystals to the solution. The major role of oxalic acid is to enhance the oxidation process, and the reason for decreases at 2 mM may be because the excess production of OH radicals might compete among themselves and inhibit the efficiency of the degradation process.
Effect of catalyst loading
Figure 3 shows the dependence of Irgalite violet dye 400 ppm degradation on the loading of the NiFe2O4 catalyst in the presence of oxalic acid with an initial concentration of 2.0 mM. The degradation ratio at 20 min is 68.4%, 75.0%, 78.2%, 79.3% and 81.5% when the dosage of the NiFe2O4 catalyst is 0.05 g, 0.10 g, 0.15 g, 0.20 g and 0.25 g, respectively. When the concentration of the catalyst is further increased a decrease in degradation efficiency is observed. At the initial stage, the increase of the dosage of the catalyst leads to an obvious rise of the degradation ratio. However, the dosage of nickel ferrite from 0.20 g to 0.25 g only results in a small change of 2% and this difference almost disappears at 30 min. The observed trend is explained on the basis that on increasing the Fe2+ ions in the reaction mixture, there is an enhanced generation of •OH radicals, consequently increasing the rate of photodegradation (Kakodia et al. 2013). After the addition of optimum Fe2+ ions, the higher dose of Fe2+ results in a brown turbidity that causes the recombination of •OH radicals and Fe2+ reacts with •OH as a scavenger. Therefore, on further increase, the rate decreases. Hence, 0.15 g NiFe2O4 was used in our experiments.
Effect of dye concentration
The concentrations ranging from 200 ppm to 1,000 ppm were taken individually and degradation studies carried out. The maximum degradation percentage observed for Irgalite violet is 99.995% for 400 ppm within 60 min at pH 3.0, as shown in Figure 4. There is a gradual decrease in the percentage of degradation as concentration of the dye concentration increases due to the occupancy of active sites on the catalyst by the dye molecules (Ameta et al. 2012). Since the amount of hydroxyl radicals generated by the given concentration of photocatalyst has been scavenged by the increase of dye molecules, a decrease in degradation efficiency is observed.
Effect of contact time
It was observed that the degradation efficiency of nickel ferrite magnetic nanoparticles over Irgalite violet dye effluent was greater in the presence of solar radiation compared to the absence of solar light, as shown in Figure 5. In the presence of light source, the photons excite the surface electrons of the catalyst, whereby these electrons move from the valence band to the conduction band leaving positive holes in the valence band, which further reacts with water to release hydroxyl ions that degrade the dyes. Oxalic acid also contributes to the production of hydroxyl radicals which enhances the rate of degradation. As the irradiation time proceeds the degradation efficiency was found to increase. It is evident that in the presence of solar irradiation the photo catalyst (nickel ferrite) had degraded about 99.995% of Irgalite violet whereas in the absence of solar irradiation, the degradation efficiency had dropped to 29% within 60 min. In addition, the degradation rate was observed to remain almost constant after 60 min due to the competitiveness of the parent dye molecules with intermediate/short chain aliphatics during the photocatalytic degradation process (Muruganandham & Swaminathan 2006). Since the batch studies were carried out in aqueous solutions, these radicals contribute to the degradation of dye under solar irradiation in the absence of nickel ferrite nanoparticles and showed 17% degradation efficiency. This may be due to the presence of minute concentrations of hydroxyl radicals generated in water. The photons of solar light generate hydroxyl (OH) radicals due to the water molecules present in the system.
The major challenge encountered during this work is optimizing the pH, since the optimum pH is around 3.0 in the Fenton treatment which is acidic. The modified Fenton oxidation process is improved by using oxidizing agents which, in turn, increase the •OH radicals' formation to improve the efficiency of the system, and the use of NiFe2O4 nanoparticles, due to the presence of Fe2+, showed strong reactivity to catalyse dyes at various conditions. The band gap of the synthesized NiFe2O4 nanoparticles for this study is 2.14 eV. Moreover, its catalytic stability can be maintained for multiple cycles and it can be a promising treatment. Even though economic feasibility was not the focus of this investigation, since this method uses solar light for the Fenton process costs, the only costs involved in the study are NiFe2O4 nanoparticles and the oxidizing agent oxalic acid. Furthermore, the use of solar energy has brought advantages since it widened the range of reactant concentrations that could be used and still achieved very satisfactory treatment performances, making it reasonable to state that the lower the number of reactants, the more economically feasible the treatment becomes (Pignatello et al. 2006; Hansson et al. 2012; Usman et al. 2016).
Kinetic study of photodegradation
The reaction rate constant was calculated from the slopes C/Co against time as shown in Figure 6. The correlation constant for the fitted line was calculated to be R2 = 0.99 and rate constant (k) was calculated to be 0.0223 min−1. The half-life, t1/2 of the pseudo-first-order reaction given by t1/2 = 0.693/k was deduced to be 31.07 min.
The possible mechanism for photodegradation of Irgalite violet dye over MFe2O4 (M = Ni) and the active species for photodegradation of dye could be produced by three pathways. When ferrite was irradiated in the presence of solar light, the electron/hole (e−/h+) pair was generated on the surface:
Since OH· was produced by three pathways, enhancement in the rate of degradation was observed. The hydroxyl radical produced by the above three pathways was the main active species for degradation of Irgalite violet dye.
Recyclability of the nanoparticles
The same catalyst was washed with ethanol and distilled water and dried at 60 °C for seven times. The catalyst exhibited activity with almost the same efficiency up to six runs, as shown in Figure 7. With the reuse of catalyst the active sites of the catalyst become occupied, therefore a decrease in the efficiency is observed after six runs. The decrease in activity could be due to the loss in the activity of the catalyst during washing and drying. Figure 8(a) and 8(b) depict the morphology of nanoparticles before and after six runs.
The nickel ferrite nanoparticles synthesized by co-precipitation method were found to be stable under acidic conditions for the degradation of Irgalite dye effluent. Further, the dye was degraded completely within 60 min in the presence of 0.15 g of the nickel ferrite catalyst (k= 0.0223 min−1) and adding oxalic acid (2 mM) enhanced the degradation efficiency of the nanoparticles (99.95%). The UV absorbance peaks of the dye solution after the photodegradation process showed no peaks of the dye which confirms the degradation of the dye. Therefore, it can be concluded that this study can effectively be used for the degradation of dye effluents from industries. Further, this approach to dye degradation offers a simple and efficient catalyst and does not require any special conditions for the degradation process. The major limitation of this work is the pH as it is operated at acidic pH 3.0. The work can be improved by changing various heterogenous catalysts and oxidizing agents to widen the spectrum of the working pH, thereby improving the efficiency. However, the present work is a cost-effective and feasible method. Further, due to nickel ferrite's relatively dry and more compact physical structure than ferric hydroxide, ferrite formation considerably reduces sludge volume.