Bioleached laterite nano iron catalyst (BLaNFeCs)-based Fenton's degradation of selective dyes in water

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.

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 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).

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).

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).

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.