Iron nanocatalyst for its potential application as Fenton's catalyst for the degradation of methylene blue dye was synthesized with the fruit extract of Citrus maxima using bioleached laterite iron as a precursor. Synthesized iron particles were characterized suitably and their catalytic role in the degradation of methylene blue and rhodamine B by Fenton's oxidation was evaluated. The synthesized nanocatalyst exhibits heterogeneous catalytic properties in the degradation of methylene blue and rhodamine B with a degradation efficiency of 93.6 and 91.3%, respectively. Observed rate constants are consistent with the increase in catalyst dosage as it speeds up the reaction. The degradation of methylene blue and rhodamine B follows a pseudo-first-order reaction with a linear fit. Reusability studies confirm the reduction in the catalytic efficiency of the synthesized iron nanoparticles after five consecutive cycles.

  • Synthesis and characterization of bioleached laterite nanoparticles.

  • Fenton's oxidation.

  • Degradation of selective dyes.

  • Sustainable replacement of natural laterite iron for commercial iron.

  • Reusability studies on the catalyst.

Graphical Abstract

Graphical Abstract
Graphical Abstract

Diverse chemical reagents used in the textile industries and large volume of water leads to the generation of wastewater contaminated with environmental persistent chemicals. Dyes by the virtue of their nature are non-amenable to degradation of exposure to water and other chemicals (Dutta & Mukhopadhyay 2001). Dyes usually have benzene and naphthalene rings but may also contain aromatic or aliphatic groups. It is the side group attached to the dye that imparts the color. It is the complex nature of dyes that makes them more interesting in the field of chemical remediation. Synthetic azo dyes are carcinogenic to humans in nature with potential toxic properties. Discharge of wastewater containing dye to natural streams and rivers may harm the biodiversity, posing toxicity to aquatic life (Islam & Mostafa 2018). Methylene blue belongs to the phenothiazine group of organic dyes. Discovered in the year 1876, methylene blue has its application as a tracer in the field of medicine for the radioactive detection of cancer (Nour n.d.; Simmons et al. 2003).

Many treatment methods like ozonation, electrochemical method, and the photochemical methods have been suggested for dye degradation out of which the photochemical method is proven to be effective because of its application and simplicity (Huang et al. 1993; Kim et al. 2004; Huang et al. 2008). Hydroxyl peroxide produced by the dissociation of hydrogen peroxides acts on the complex structure of organic dyes, breaking it, and causing degradation. The dissociation of hydrogen peroxide is a slow process for which a divalent cation, usually ferrous iron, is used to accelerate the reaction.

Various studies have been carried out on Fenton's oxidation of dyes in water and wastewater (Kim & Kan 2014). The generation of a large quantity of sludge and the cost of iron catalyst for the treatment are the limitations for the application of Fenton's oxidation on a large scale. Studies on the replacement of commercial iron from lateritic iron have been conducted and reported. However, the cost-effectiveness of chemical leaching remains in question. In this article, cost-effective bioleaching of lateritic iron for its application as Fenton's catalyst in the degradation of organic compounds is reported (Bhaskar et al. 2021).

Nanoparticles, being an effective tool in water and wastewater treatment, have an application in Fenton's oxidation. Heterogenous Fenton's oxidation using nano iron catalyst is much more effective in the degradation of organic compounds specific to organic dyes (Bishnoi et al. 2018). However, the efficiency of such treatment depends on the type of catalyst and method used in the synthesis of iron nanocatalyst.

The present study encompasses the synthesis and characterization of iron nanoparticle catalysts synthesized using bioleached laterite iron as a precursor and Citrus maxima fruit extract and its application in the Fenton's degradation of methylene blue and rhodamine B dyes.

Fruit pulp extract-based synthesis of bioleached laterite nano iron catalyst

Green synthesis of bioleached laterite nano iron catalyst (BLaNFeCs) was carried out as described in Bhaskar et al. 2020 (Bhaskar et al. 2020). Pulp of C. maxima was collected, washed in distilled water, and crushed to extract the juice. The juice extracted was filtered with Whatman's filter paper (No. 42) and stored at 4 °C for further use. Phytochemical extracts of C. maxima were added to bioleached laterite iron solution dropwise on heating at 80 °C until the color turned black. Solutions were filtered using Whatman's filter paper No. 1 and oven-dried. The extracted particles were stored in moisture-free containers. Synthesized nanoparticles were characterized using scanning electron microscopy, X-ray diffraction, electron dispersive spectrophotometry, and BET analyzer.

BLaNFeCs-based Fenton's oxidation of selective dyes

BLaNFeCs-based Fenton's degradation was carried out with an initial dye concentration of 10 mg/L for both methylene blue and rhodamine B (Dutta & Mukhopadhyay 2001). BLaNFeCs was added at an incremental dose to the dye solution taken separately in a conical flask. The solution was adjusted to a pH of 3 using 1 N H2SO4 and allowed to settle for 10 min to ensure proper mixing and uniform distribution of BLaNFeCs in the solution before the addition of H2O2. The experiment was conducted under a set of conditions for dye degradation with different dosages of BLFeNCs (0.1–1 g/L) and H2O2 (100–1,000 mg/L). Samples were drawn at regular intervals for analysis. During sampling, each time 1 ml of n-butanol was added to arrest the reaction (Khan et al. 2009). The concentration of methylene blue was measured using a UV–vis spectrophotometer. Chemical oxygen demand (COD) was measured using the calorimetric method (APHA Method 4500-F: 1992). pH was measured using a digital pH meter. Ferric iron was measured by the potassium thiocyanate method using a UV–vis spectrophotometer (Mellon & Woods 1941). H2O2 was measured using a UV spectrophotometer (Eisenberg 1943).

BLaNFeCs formation and its characterization

Iron nanoparticles were formed during the reaction of fruit extract and leached laterite solution and were confirmed with SEM, XRD, and EDS analysis (Devatha et al. 2016; Sangami & Manu 2017). The iron particles formed were seen as spherical particles on SEM images and their size was observed to be 100–200 nm. EDS data show us three peaks corresponding to iron (Figure 1(b)) and reveal that the nanoparticles formed contain iron at 12.28% by weight composition as shown in Table 1.
Table 1

Chemical composition of BLaFeNCs

Element Fe Cu 
Weight % 7.10 18.92 2.02 12.28 0.08 0.08 
Element Fe Cu 
Weight % 7.10 18.92 2.02 12.28 0.08 0.08 
Table 2

Overview of Fenton's oxidation of selective dyes in water

Target compoundOperating parameter
Degradation (%)COD removal (%)Rate constant k (min−1)Regression R2
BLaFeNCs (g/L)H2O2 Dosage (mg/L)
Methylene blue 0.5 500 93.6 87.6 0.0236 0.9177 
Rhodamine B 0.5 1000 91.3 81.5 0.0200 0.9224 
Target compoundOperating parameter
Degradation (%)COD removal (%)Rate constant k (min−1)Regression R2
BLaFeNCs (g/L)H2O2 Dosage (mg/L)
Methylene blue 0.5 500 93.6 87.6 0.0236 0.9177 
Rhodamine B 0.5 1000 91.3 81.5 0.0200 0.9224 
Figure 1

Scanning electron microscopic and EDS images of BLaNFeCs showing the morphological appearance and elemental composition of synthesized nanoparticles.

Figure 1

Scanning electron microscopic and EDS images of BLaNFeCs showing the morphological appearance and elemental composition of synthesized nanoparticles.

Close modal

Six peaks observed at 2θ 30.48, 35.89, 43.52, 53.99, 57.49, and 63.10 correspond to iron oxide (PDF: 01-075-0449) and 90.56 corresponds to magnesium iron gallium oxide (PDF: 00-033-0896). Broad peaks observed are due to organic material coatings stabilizing the nanoparticles and the formed nanoparticles are amorphous in nature. The low signal-to-background ratio observed shows poor crystallinity confirming the amorphous nature of nanoparticles (Chen et al. 2017). Chemical composition by percentage weight for formed nanoparticles is tabulated in Table 1.

BET surface areas for BLFeNPs were found to be 87.75 m2/g with pore diameters of 7.225 nm confirming the mesoporous structure of the nanoparticles obtained (Soon & Hameed 2011) (Figure 2).

Fenton's degradation of selective dyes by BLaNFeCs

The catalytic performance of synthesized nanoparticles was confirmed by Fenton's oxidation with dye degradation efficiency of 93.6 and 91.3% for methylene blue and rhodamine B, respectively, on addition of BLaNFeCs dosage of 0.5 g/L and H2O2 dosage of 500 and 1,000 mg/L, respectively (Figure 3). Maximum degradation of 88.2 and 84% was observed at 40 min of treatment with a rate constant of 0.0236 and 0.020 min–1 for methylene blue and rhodamine B, respectively. It is observed that for methylene blue the degradation was increased by 5% with an increase in catalyst loading from 0.1 to 0.5 g/L (Figure 2). However, further increase in catalyst dosage to 1 g/L has led to a degradation drop by 8.2%. For rhodamine B, the degradation is shown to be increased by 1.3% with an increase in catalyst loading from 0.1 to 0.5 g/L and there is a decrease in efficiency by 5.3% on a further increase of catalyst loading. In all cases, treatment time is reduced by 40 min leading to maximum degradation. An increase in H2O2 dosage is observed to have increased the degradation efficiency up to 500 mg/L and further increase to 1,000 mg/L led to a drop in the degradation efficiency by 8.8%, indicating the scavenging effect (Khan Wirojanagud & Sermsai 2009), whereas maximum degradation of rhodamine B was observed at 1,000 mg/L of H2O2 dosage. It is to be noted that both for 0.5 and 1 g/L of catalyst load, the treatment time is reduced to half. An increase in catalyst load is more beneficial than increasing hydrogen peroxide since iron increases the speed of the reaction (Andrades et al. 2021). The result obtained is consistent with previous works on the degradation of methylene blue and rhodamine B (Dutta & Mukhopadhyay 2001; Zhao & Zhu 2006; Hou et al. 2011; Tak et al. 2015; Zhou et al. 2016; Bishnoi et al. 2018; Karim et al. 2022).

The degradation mechanism of methylene blue and rhodamine B was demonstrated by previous researchers. Degradation of methylene blue involves the breaking of the C–N bond connected to the benzene ring generating degradation products, namely H2SO4 and HCl (Amini et al. 2015). Degradation of Rhodamine B follows N-de-ethylation, chromophore cleavage, ring opening, and mineralization stages. Zhou & co-workers (2016) identified three intermediate products due to the de-ethylating steps of N,N′-diethylammonium groups as N,N,N′-triethyl rhodamine, N,N′-diethyl rhodamine, and rhodamine (Zhou et al. 2016). Hou et al. (2011) claim oxalic acid, formic acid, and acetic acid as chromophore cleavage and ring opening intermediates. Tertiary amines formed during the degradation can be oxidized to amine oxides which further oxidize to aldehyde intermediates and then to carboxylic acids (Hou et al. 2011) (Figure 4).
Figure 2

XRD images of BLaNFeCs representing the corresponding peaks of synthesized nanoparticles.

Figure 2

XRD images of BLaNFeCs representing the corresponding peaks of synthesized nanoparticles.

Close modal
Figure 3

Oxidative degradation of methylene blue with the following BLaNFeCs dosages: (a) 0.1 g/L; (b) 0.2 g/L; (c) 0.5 g/L; and (d) 1.0 g/L.

Figure 3

Oxidative degradation of methylene blue with the following BLaNFeCs dosages: (a) 0.1 g/L; (b) 0.2 g/L; (c) 0.5 g/L; and (d) 1.0 g/L.

Close modal
Figure 4

Oxidative degradation of rhodamine B with the following BLaNFeCs dosages: (a) 0.1 g/L; (b) 0.2 g/L; (c) 0.5 g/L; and (d) 1.0 g/L.

Figure 4

Oxidative degradation of rhodamine B with the following BLaNFeCs dosages: (a) 0.1 g/L; (b) 0.2 g/L; (c) 0.5 g/L; and (d) 1.0 g/L.

Close modal
Variation of iron and decolorization of methylene blue and rhodamine B during Fenton's oxidation are presented in Figures 5 and 6. It is observed from the graph that the ferrous form of iron leached out of nanocatalyst undergoes an oxidation process forming the ferric iron species which is confirmed with iron determination (Figures 5(a) and 6(a)) and later there is a drop in the ferric form of iron in the solution. Observed results are consistent with previous works (Khan et al. 2009; Bhaskar et al. 2019, 2021; Bhaskar, Basavaraju Manu 2020). Figures 5(b) and 6(b) present the decolorization efficiency during the oxidation process which is indicated by COD reduction (Dutta & Mukhopadhyay 2001). The maximum reduction in COD observed was 87.6 and 81.5% for the optimum degradation of methylene blue and rhodamine B, respectively. The reduction in COD is also attributed to the adsorption process other than Fenton's oxidation (Masomboon 2009). Complete degradation was observed within 40 min of treatment. Fenton's treatment is shown to be active in acidic pH. During the Fenton's process, hydrated ferrous ions get transformed into the colloidal ferric species forming the ferric hydroxyl complexes, thereby reducing the efficiency at basic pH. During the study, pH varied slightly from 3.0 to 4.4 which favored the oxidation process forming hydroxyl radicals (Kang & Hwang 2000; Burbano et al. 2005). It is to be noted that the rate of oxidation in Fenton's process depends on the dissolution rate of ferrous iron that has leached out of the nanocatalyst. The amount of catalyst increases the degradation efficiency which might be due to adsorption on the surface of the nanocatalyst particles, making the process heterogenous in reaction (Barbusiński 2005). Figure 7 indicates the linear fit for the Fenton's oxidation of methylene blue and rhodamine B follows a pseudo-first-order kinetics with a rate constant of 0.0236 and 0.020 min–1, respectively. The reusability of spent catalyst was studied for the Fenton's oxidation of methylene blue. Degradation of methylene blue is amenable to spent catalyst reuse. Figure 8 shows us the degradation of methylene blue in catalyst reuse. Degradation on the first cycle of reuse was 92.0% with a rate constant of 0.0188 min–1, whereas on the second cycle of reuse the efficiency was 88.4% with a rate constant of 0.0159 min–1. For the third cycle of reuse, degradation efficiency was 73.4% with a rate constant of 0.0102 min–1. For the fourth and fifth cycles of reuse, degradation efficiencies were 64.5 and 68.8% with rate constants of 0.0088 and 0.0086 min–1, respectively. It is observed that the degradation efficiency is decreasing on catalyst reusage with a decrease in the rate constant. The drop in degradation might be due to the loss of iron from nanoparticles by leaching in each cycle. However, it is observed that the catalyst can be reused for three consecutive cycles (Table 2).
Figure 5

(a) Variation of iron and (b) decolorization efficiency of methylene blue during Fenton's oxidation.

Figure 5

(a) Variation of iron and (b) decolorization efficiency of methylene blue during Fenton's oxidation.

Close modal
Figure 6

(a) Variation of iron and (b) decolorization efficiency of rhodamine B during Fenton's oxidation.

Figure 6

(a) Variation of iron and (b) decolorization efficiency of rhodamine B during Fenton's oxidation.

Close modal
Figure 7

Linear fit for Fenton's oxidation of (a) methylene blue and (b) rhodamine B using BLaNFeCs.

Figure 7

Linear fit for Fenton's oxidation of (a) methylene blue and (b) rhodamine B using BLaNFeCs.

Close modal
Figure 8

Catalyst reusability on Fenton's oxidation of methylene blue using BLaNFeCs.

Figure 8

Catalyst reusability on Fenton's oxidation of methylene blue using BLaNFeCs.

Close modal

Iron nanocatalyst was sustainably synthesized by a phytochemical method using bioleached laterite iron as a percussor and C. maxima fruit extract. Fenton's oxidation of methylene blue and rhodamine B dyes has been carried out using a synthesized nano iron catalyst for the evaluation of its potential as a Fenton's catalyst in the degradation of dyes. It is observed that at acidic pH synthesized nanocatalyst is proven for its catalytic role in dye degradation with degradation efficiency of 93.6 and 91.3% with 1:1 and 1:2 BLaNFeCs:H2O2 ratio for methylene blue and rhodamine B, respectively. Degradation lasts for 80 min with a maximum degradation in 40 min following a pseudo-first-order kinetics with rate constants of 0.0236 and 0.020 min–1. An increase in catalyst dose led to a shortening of treatment time with an increase in degradation efficiency. Reusability tests confirm that spent catalyst can be reused for three consecutive cycles with better efficiency in dye removal. The study confirms the sustainable use of natural laterite iron-based nanoparticles as a catalyst in Fenton's degradation of dyes.

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

The authors declare there is no conflict.

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