Green synthesis of granular activated carbon/zinc ferro nanocomposites-based bioleached laterite iron (BLaFe) for the removal of Rhodamine B in water using adsorption–Fenton's oxidation process

Contamination of environmental chemicals of dye from the textile industries is reflected in the usage of synthetic dyes for textiles (Dutta & Mukhopadhyay 2001). The complex structure of synthetic azo dyes is making them recalcitrant for degradation. Rhodamine B (RhB), N-[9-(2-carboxyphenyl)-6-(diethylamino)-3H-xanthen-3-ylidene]-N-ethylethanaminium, a unique fluorescent organic chloride salt dye that belongs to the group of xanthene dyes with its vast application in the textile industries (Table 1; Su et al. 2013). The fluorescence of RhB is temperature-dependent making it fluxional at room temperature. Direct discharge of processed water contaminated with dyes into natural streams harms biodiversity alarming potential danger to aquatic life (Islam & Mostafa 2018). RhB with poor biodegradability has extensive application in the paints and textile industries (Al-gheethi et al. 2022). The presence of RhD in natural streams indicates the direct discharge of industrial effluent to the natural stream posing threat to the aquatic environment (Hikmat et al. 2017).

Table 1

Details of Rhodamine B

Dye .	Rhodamine B .
Appearance	Red to violet
Molecular weight	479.0
Molecular formula	C28H31CIN2O3
Structural formula
Solubility in water (mg/L)	50

Dye .	Rhodamine B .
Appearance	Red to violet
Molecular weight	479.0
Molecular formula	C28H31CIN2O3
Structural formula
Solubility in water (mg/L)	50

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Treatment of wastewater contaminated with dye demands sophisticated treatment remedy techniques as traditional treatment fails to cope with the issues. Filtration, a biological treatment, is proven to be inefficient for dye removal. The advanced oxidation process uses a strong oxidizing agent to degrade a large number of organic pollutants persistent for other methods of treatment. However, several modifications have been made and experimentation was carried out has been by various research teams to make the process more efficient and cost-effective. Nano iron particles were proven to be efficient in the Fenton's oxidation with effective reduction in treatment time (Sangami & Manu 2017, 2018).

The use of modified nano catalysts like nanocomposites, carbon-based iron–graphene oxides and carbon gel doped with iron were found to be very effective in organic pollutant degradation (Ahmadi et al. 2021; Karim et al. 2022). Hassani et al. synthesized cobalt ferrite–graphene oxide nanocomposites using a surfactant-based thermal decomposition–reduction technique and achieved 90.5% of efficiency in organic dye removal. The team also limits the reusability of nanocomposites for five consecutive cycles with drop in 22% of removal efficiency (Hassani et al. 2017). Removal of the dye RhB by Fenton's oxidation has been reported by previous researchers (AlHamedi et al. 2009; Xue et al. 2009; Bhaskar et al. 2022). AlHamedi et al. (2009) studied the decoloration of RhB with UV/H₂O₂ process in the absence of catalyst and reported 73% of efficiency. Bhaskar et al. (2022) investigated EDTA-based biojarosite-driven Fenton's and UV–Fenton's oxidation of RhB claiming 80.0% of removal efficiency in near neutral pH. This marks the role of iron catalyst in Fenton's oxidation for RhB removal. Adsorption is known for dye removal in textile wastewater (Uddin et al. 2021; Velusamy et al. 2021). The combination of Fenton's oxidation and adsorption enhances the treatment efficiency (Nair & Kurian 2017; Rahimpour et al. 2020). Nair & Kurian (2017) investigated catalytic oxidation with nickel zinc ferrite nanocomposites claiming zinc doping increased the oxidizing power and surface acidity of nickel increased catalytic efficiency. Catalytic replacement of commercial iron from laterite iron in the Fenton's degradation has been studied and reported (Karale et al. 2014; Sangami & Manu 2018; Bhaskar et al. 2021). The usage potential of biogenic jarosite on EDTA-based Fenton's oxidation for the removal of RhB dye was studied and reported by Bhaskar et al. (2022). Zhou et al. (2016) reported the mechanism of dye degradation on Fenton's oxidation and claimed that organic functional groups are distorted under the action of hydroxyl radicals produced by the dissociation of hydrogen peroxide added.

Psidium gujava, a common tropical plant found in several parts of India, is known for its antimicrobial and anticarcinogenic properties. Rich phenolic compounds such as flavonoids, gallic acid, catechin, and rutin present in the leaves of P. gujava act as a capping and reducing agent in the synthesis of nanoparticles. The potential use of P. gujava for nanoparticles synthesis and characterization has been done by many researchers and synthesized nanoparticles showed better antimicrobial properties (Raghunandan et al. 2009; Khaleel et al. 2010; Parashar et al. 2011). Rashmishree et al. (2022) synthesized and characterized nano iron particles for their application in Fenton's oxidation of triclosan. The interaction of iron and zinc ions in the solution with polyphenols present in the plant extract results in the formation of zinc–iron composites (Devatha et al. 2016; Sangami & Manu 2017). Flavonoids, alkaloids and other chemicals present in the plant extract are essential as reducing and capping agents for the reduction of particle size.

Considering the treatment efficiency and cost-effectiveness of combined adsorption–Fenton's oxidation process, the present study deals with the synthesis of a novel nanocomposite catalyst using bioleached laterite iron (BLFe) and granular activated carbon (GAC) as a precursor and P. gujava plant extract. Catalyst synthesized was evaluated for its catalytic role in Fenton's oxidation process and adsorption studies have been conducted (Bhaskar et al. 2021).

Fresh leaves of P. gujava were collected from the campus of NITK, Surathkal. Leaves were washed thoroughly with distilled water, dried and cut into small pieces. Cut pieces of leaves were boiled in distilled water at 60–70 °C for 1 h. The extract were filtered and stored for further use at 4 °C.

BLFe was biologically leached out from the laterite soil using the novel acidophilic bacteria Acidithiobacillus ferrooxidans (Bhaskar et al. 2021). BLFe solution of 1 mM concentration and 15 mM zinc solution was added to plant extract at a 1:1 ratio followed by 0.2 g of GAC and mixed thoroughly in the shaker at 200 rpm for 1 h. Solutions were filtered using Whatman's filter paper No. 1 and oven-dried at 105 °C overnight. The extracted particles were stored in moisture-free containers for further use.

Oxidative degradation of RhB using BLFe-based GAC/zinc ferro nanocomposites as catalyst was carried out with 10 mg/L of initial dye concentration (Su et al. 2013; Li et al. 2016; Zhou et al. 2016). BLFe-based GAC/zinc ferro nanocomposites were added at an incremental dose into RhB solution in a conical flask. The solution was adjusted to pH 3 using 0.5 M H₂SO₄ and allowed for 60 min to ensure proper mixing and uniform distribution of BLFe-based GAC/zinc ferro nanocomposites powder in the solution before the addition of H₂O₂. The investigation was conducted with appropriate experimental conditions by considering different dosages of nanocomposite particles (1.0–10 g/L) and H₂O₂ quantities (100–1,000 mg/L). Samples were extracted at regular intervals for analysis. During sampling, each time 1 ml of sodium thiosulfate was added to halt the reaction (Khan et al. 2009). The concentration of RhB was measured using a double-beam UV–Vis spectrophotometer at the wavelength of 554 nm (Systronics Make, Model No. AU-2701). Chemical oxygen demand (COD) was measured by the colorimetric method as per 5220D of Standard Methods for Examination of Water and Wastewater (APHA Method 4500-F 1992). pH was measured using a digital pH meter (Model – edge, HANNA Make). Ferrous iron was measured by the 1,10-phenanthroline method using a UV–Vis spectrophotometer at a wavelength of 510 nm (Systronics Make, Model No. AU-2701) (APHA Method 4500-F 1992). Ferric iron was measured by the potassium thiocyanate method using a UV–Vis spectrophotometer at a wavelength of 510 nm (Systronics Make, Model No. AU-2701) (Woods & Mellon 1941). H₂O₂ was measured using a double-beam UV–Vis spectrophotometer at a wavelength of 470 nm (Systronics Make, Model No. AU-2701) (Eisenberg 1943). A recoverability and reuse test for the spent catalyst was conducted. BLFe-based GAC/zinc ferro nanocomposites powder was filtered, collected, dried and reused as a Fenton's catalyst for the degradation of RhB.

BET surface areas were found to be 389 m²/g with a pore volume of 0.51 cm³/g, respectively, confirming the mesoporous structure of the nanoparticles obtained.

The dosage and form of iron is the prime factor in the Fenton oxidation process (Barbusi & Filipek 2001; Chen et al. 2016). The effect of catalyst dosage was studied by increasing BLFe-based GAC/zinc ferro nanocomposites dosage in the range of 1.0–10.0 g/L. During the study, the degradation of RhB increased with an increase in the load of catalyst. The efficiency of dye removal was observed to increase by 6.7% on increasing catalyst dosage from 1 to 5 mg/L and 7.9% on an increase to 10 mg/L. This observation indicates that BLFe-based GAC/zinc ferro nanocomposites play a catalytic role in the process by decomposing hydrogen peroxide into hydroxyl radicals, thereby accelerating the formation of active sites on the catalyst. Iron leached out of nanocomposites accounts for 0.14 g/L at this stage. A continuous increase in ferric iron concentration was observed during the study.

It is the high redox active metal ions that contribute to more redox reactions by increasing the oxidation property in zinc-doped ferro nanocomposites leading to a higher removal rate compared with other nanocomposites (Nair & Kurian 2017). The adsorption capacity of different materials for the removal of RhB is presented in Table 6. It is observed from the previous studies that sodium montmorillonite acts as a good adsorbent with an adsorption capacity of 35.45 m²/g (Selvam et al. 2008).

Phytochemical synthesis of green synthesis of BLFe-based GAC/zinc ferro nanocomposites led to the formation of honeycomb-like nanocomposites. The presence of iron and zinc was confirmed with EDS and XRD studies. Synthesized nanocomposites exhibited a catalytic role in the Fenton's oxidation along with surface adsorption contributing maximum dye removal of 96.2% within 200 min of treatment. COD removal of about 64.2% observed within 40 min of treatment indicates better oxidation during the treatment. Adsorption studies suggest the Freundlich Adsorption Isotherm model with maximum adsorption. Recoverability and reusability studies suggest that nanocomposites can be reused effectively for up to five consecutive cycles for the removal of RhB dye.

The authors have consent to publish this article.

All authors contributed to the study's conception and analysis. Material preparation and design were performed by S.B., R.K.N., B.M. and M.Y.S. Analysis was carried out by S.B. and R.K.N. The first data of the manuscript was written by S.B. and all authors commented on previous versions of the manuscript. All authors read and approved the final manuscript.

No funding was received for conducting this study.

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

The authors declare there is no conflict.