Introduction

The presence of reactive azo dyes in water stream can pose serious environmental problems. These dyes are released from the textile and dyeing industries [1]. Azo dyes have been predicted as potential genotoxic and carcinogenic chemical [2]. Approximately 15% of the dyes are lost during the dyeing process, and then, it released into environment [3]. Congo red (CR) is one of most widely used dye in the textile industries. This dye cannot be efficiently decolorized mainly because of presence of many benzene rings in their structure. Dyes removal from wastewater has been widely studied to reduce its environmental impact. There are different types of techniques available, which are adsorption [4], ozonation [5], Fenton-like degradation [6] and photocatalysis [7]. Fenton-like degradation is a rapid and cost-effective approach, and this is also simple to implant for the industrial scale to remove dyes from the wastewater before discharging into the environment. In the Fenton-like process, hydroxyl radicals are produced by the oxidation using hydrogen peroxide [8]. Currently, nanoparticles have been applied for azo dye removal; these particles are zero-valent iron (Fe0) [9] and bimetallic nanoparticle, such as Fe/Ni [10]. Adsorptive removal of CR has been studied by some researchers [11,12,13]. Recently, nano-metallic particles (NMPs) have been considered as a promising agent to treat the organic pollutants from the wastewater [8, 14]. However, application of NMPs for degradation of reactive azo dye (CR) is still limited.

Therefore, the present study was carried on the removal of CR by NMPs in the presence of hydrogen peroxide. NMPs were recovered from the fine fraction of automobile shredder residue (ASR). This has been considered that ASR is highly contaminated with heavy metals [15, 16]. In many countries, ASR waste is dumping into landfills, which may increase the risk of ground water pollution [15, 17]. The objective of the study was to treat the dye containing wastewater by applying NMPs (recovered from the leachate of ASR).

Materials and methods

The analytical grade chemicals were used in this work. A Congo red (C32H22N6Na2O6S2) sodium borohydride (NaBH4), hydrogen peroxide (H2O2) and hydrochloric acid (HCl) were supplied by Samchun Pure Chemicals Co. Ltd. (South Korea). Figure 1 shows the molecular structure of Congo red.

Fig. 1
figure 1

The molecular structure of hazardous dye Congo red (CR)

The details of the leaching of metals and recovery of NMPs from the ASR have been given in our previous study [14]. The recovery of NMPs was conducted at pH 7 of leachate solution through reduction method by using sodium borohydride (NaBH4). The experiment on CR degradation by NMPs was carried out in conical flasks of 250 mL at 25 °C in a water bath shaker (SWB-35, Hanyang Scientific equipment Co., Ltd., South Korea) at 60 rpm. A 100 mL of CR solution was taken in flasks, and then, suitable amount of NMPs (0.1 g/L, except dose study) and H2O2 (50 mM, except H2O2 concentration) was added to solution. Experiment was also carried out at different values of pH (pH 2, 3, 4 and 5) of the CR solution, and the pH of CR solution was maintained with 0.1 N HCl and 0.1 N NaOH. A solution of natural pH (6.5) also checked the CR degradation by NMPs. To measure the remaining CR concentration in solution, a 2 mL of sample was taken at various time intervals and then filtered through a 0.25-µm-size filter paper.

To determine the residual concentration of CR, UV–Visible spectrophotometer (UV 1601, Shimadzu) was used at a maximum wavelength (λ max) of 495 nm. The scanning electron microscope (SEM; S–4300CX; Hitachi, Japan) was applied to analyze morphology of NMPs. A Fourier transform infrared (FTIR) spectrophotometer (ISF 66/S, Bruker) was used to describe the functional groups on NMPs before and after reaction. X-ray diffraction (XRD) studies of the NMPs were analyzed using a D/Max–2500 diffractometer (Rigaku, Japan).

The rate of CR degradation by NMPs was determined by a pseudo-first-order reaction model [14],

$$ \ln ({{C_{t} } \mathord{\left/ {\vphantom {{C_{t} } {C_{0} }}} \right. \kern-0pt} {C_{0} }}) = - k_{1} t $$
(1)

where C 0 is the initial concentration of CR (mg/L), C t represents residual concentration of CR at time t (min), and k 1 is the pseudo-first-order rate constant (min−1).

The removal of the CR by NMPs was also studied by a pseudo-second-order reaction kinetics [18],

$$ {1 \mathord{\left/ {\vphantom {1 {C_{t} }}} \right. \kern-0pt} {C_{t} }} = ({1 \mathord{\left/ {\vphantom {1 {C_{0} }}} \right. \kern-0pt} {C_{0} }}) + k_{2} t $$
(2)

The slope of the plot between 1/C t and time yields the pseudo-second-order rate constant k 2.

Results and discussion

Characterization of NMPs

Figure 2a represents a SEM image of the prepared NMPs. The results of the SEM analysis revealed that NMPs are not in spherical shape. Most of nanoparticles display a small clumping; this may be due to magnetic interaction by Fe present in NMPs [19]. The diameter of the NMPs was observed ≤100 nm. These NMPs tended to aggregate together to form chain structures, which might be due to the magnetic interaction between small particles [20]. As shown in Table 1, Fe was the most dominated element (54.65%) followed by carbon (26.47%), Zn (11.72%), and Al and Cu (3.02%). However, Ni, Pb and Mn were found to be in trace quantity.

Fig. 2
figure 2

SEM image of NMPs (a) and FTIR analysis of NMPs before and after reaction (b)

Table 1 Energy-dispersive X-ray spectroscopy (EDX) analysis of NMPs

FTIR spectra of NMPs before and after reaction with CR are shown in Fig. 2b. The peak was observed at 1396.7 cm−1; this peak specifies the presence of metals oxides (M–O) bands on the surface of NMPs. However, after reaction this peak was found to be slightly smaller and also shifted at 1413.6 cm−1. M–O band could indicate the M2O3 and M3O4 which designated partial oxidation of NMPs [14, 21]. The FTIR spectra of NMPs exhibited broaden bands at 3298 cm−1, which describe the O–H stretching [14]. After reaction, this peak was widen and shifted to 3206.5 cm−1. The peak at 1647.9 cm−1 represents the O–H stretching; however, intensity of this peak was improved and shifted to 1619.02 cm−1 [22]. However, the bands at 1036.3 cm−1 were observed after Fenton-like oxidation of CR, conforming the Fe–O stretching about Fe2O3 [21]. The XRD results exposed in Fig. 2c characterize the diffraction patterns of the NMPs. The diffraction peaks at 2θ = 30.8° indicate the formation of metals oxides (MxOy), where MxOy may suggest FeO, Fe2O3, Cu2O, etc. [23, 24]. The NMPs were observed in crystalline metal oxide phase. The broad metal oxide peak implies that the synthesized NMPs hold a chemically disordered crystal structure [25]. The diffraction peaks at 2θ = 61.2° indicate the formation of zero-valent Cu [26].

Degradation of CR by NMPs

Effect of NMPs dosages on CR degradation

The effect of various NMPs dosages (0.025, 0.05, 0.10 and 0.15 g/L) on the reaction rate was carried out at pH—3.0, CR initial concentration—10 mg/L, temperature—25 °C and concentration of H2O2—50 mM (Fig. 3a). The degradation efficiency of CR was increased with increasing dosages of NMPs up to 0.10 g/L, and this happened due to the increasing reaction sites [8]. However, with a further increase in dosages of NMPs, degradation efficiency of CR was reduced gradually. This reduction in the degradation efficiency of CR can be attributed to clustering of NMPs during the reaction [27]. Around 95.8% of CR degradation efficiency was attained in the 180 min of reaction time with dose of NMPs of 0.10 g/L. However, removal efficiency of CR was decreased to 91.9% with a dose of 0.15 g/L of NMPs. A high removal of CR by NMPs might be due to the small size of NMPs resulting high surface area. The CR degradation was increased with the increase in the dose of NMPs; this can be due to more availability of reactive site for electron transfer from the surface NMPs to H2O2 [28]. The optimum dosage of NMPs was selected as 0.10 g/L for subsequent CR degradation. Figure 4a shows the plot of ln C t /C o versus t for the degradation of CR with the different dosages of NMPs. Table 2 shows the values of the pseudo-first-order rate constant (k 1), pseudo-second-order rate constant (k 2) and R 2 for the degradation of CR with different dosages of NMPs. The value of k 1 was increased 21.4% with increasing dose up to 0.10 g/L. A value of k 1 was decreased with increasing dose of NMPs; a similar trend was also followed by k 2 value.

Fig. 3
figure 3

Effect of the different parameters: effect of dosages of NMPs (a), effects of pH values (b), effect of initial concentration of CR (c) and effect of concentration of H2O2 (d) on the CR degradation process

Fig. 4
figure 4

Kinetic study of CR degradation by NMPs with different parameters: NMPs dose (a), pH values (b), initial concentration of CR (c) and amount of H2O2 (d)

Table 2 Kinetic results for CR degradation by NMPs under Fenton-like process

Effect of pH values on CR degradation

A pH can affect the degradation of organic pollutants by NMPs. Acidic nature of dye solution generating more reactive sites on the surface of NMPs, triggering degradation of organic pollutants. However, an extremely acidic pH of solution (pH < 2) initiates fast corrosion of NMPs, which can reduce degradation efficiency of pollutants [29]. Many researchers have reported that most of advanced Fenton’s processes depend on pH and acidic conditions, which are favorable to the reaction [30, 31]. The effects of pH values (pH 2.0, 3.0, 4.0, 5.0 and 6.5) on the removal of CR were studied. The experimental conditions were: dose of NMPs—0.10 g/L, CR initial concentration—10 mg/L, temperature—25 °C and concentration of H2O2—50 mM (Fig. 3b). The degradation efficiency of CR was achieved approximately 97.9% in 180 min of reaction time at pH of 2.0. The degradation efficiencies of CR were decreased from 97.9 to 37.8% with increasing pH from 2 to 6.5 in a reaction time of 180 min; this can be due to the decrease in activity of the NMPs [32]. Figure 4b shows the plot of ln C t /C o versus t for the removal of CR with the various values of pH. The values of k 1 and k 2 were reduced around 86.6 and 98.7%, respectively, when the value of pH was increased from 2.0 to 6.5 (Table 2). Remarkably, rapid degradation of CR was detected at pH 2.0 of CR solution. The results also stated that reaction rate was decreased with increasing pH values, and the oxides of metals were available on the surfaces of NMPs, which were rapidly dissolved in acidic solution and the active sites are free and enhancing corrosion of metals including Fe [8]. At pH 2 of solution, more hydroxyl radicals were generated, which caused the very fast degradation of CR [27]. At pH 4, 5 and 6.8 limited the removal of CR was achieved due to the low concentration of H+ occurs in the solution, consequentially low generation of hydroxyl radicals. Furthermore, metals hydroxides are generated with the increase in the pH from 4 to 6.8 during the reaction. The accumulation of metal hydroxide precipitates on the surface of NMPs will hinder the generation of reactive oxygen species and decrease the MB removal [28].

Effect of initial concentrations of CR on the degradation of CR

Figure 3c shows the effect of initial CR concentrations (5, 10, 20 and 40 mg/L) on the degradation rate under the following experimental conditions: dose of NMPs—0.10 g/L, initial pH—3.0, temperature—25 °C and concentration of H2O2—50 mM. Degradation efficiency of CR was decreased from 97.9 to 91.0% with an increasing initial concentration from 5 to 40 mg/L. The active sites of NMPs were occupied by an increasing CR concentration. Figure 4c demonstrates the plot of ln C t /C o versus t for degradation of CR with the different concentrations of CR. The values of k 1 and k 2 were decreased around 43.2 and 96.6%, respectively, with increasing initial CR concentrations from 5 to 40 mg/L. Reduction in the rate of degradation of CR with high concentration of dye may be due to the adsorption of dye molecule on the surface of NMPs, and it could occupy the active sites of NMPs [27]. The values of k 1, k 2 and R 2 for the degradation rate of CR with various concentrations of CR are given in Table 2. The slow degradation of CR after 60 min of reaction time may be clarified by the difficulty in changing the N atoms of the CR into oxidized nitrogen compounds and the slow reaction of short chain aliphatics with hydroxyl radicals; furthermore, the higher number of azo-groups in dye molecule decreases the degradation process [33].

Effect of the H2O2 addition on CR degradation

As shown in Fig. 3d, the effect of H2O2 concentrations (0, 25, 50, 100 and 150 mM) on the degradation rate was studied under the following experimental condition: dose of NMPs—0.10 g/L, pH value—3.0, temperature—25 °C and initial concentration of CR—10 mg/L. The CR degradation rate was found to be low with NMPs and without addition of H2O2; this is because of insufficient or no creation of hydroxyl radicals during the reaction. The hydroxyl radical is one of the most strongest oxidizing agents, which attacks organic compounds present at the surface of the NMPs [27]. The CR removal efficiency was increased from 83.2 to 95.8% by increasing the concentration of H2O2 from 0 to 50 mM; this is due to the generation of huge amount of hydroxyl radicals [27]. However, with a further increase in the concentration of H2O2, degradation efficiency was decreased slightly; this can be attributed to recombination of hydroxyl radicals and scavenging effect of H2O2, causing decrease in the oxidation of NMPs by H2O2 [14]. Furthermore, hydroperoxyl radicals were generated from the excess hydroxyl radicals, which were not participated in the oxidative degradation of the CR due to very less reactivity of hydroperoxyl radicals. The degradation of the CR was found by mainly reaction with hydroxyl radicals [1]. Figure 4d displays the plot of ln C t /C o versus t for the degradation of CR with various concentrations of H2O2. The value of k 1 was increased around 80% with an increasing concentration of H2O2 up to 50 mM. However, with further increase in the concentration of H2O2, a value of k 1 was slightly reduced. However, the value of k 2 was increased with increasing concentration of H2O2 from 0 to 150 mM. The removal efficiency of CR was enhanced suggestively in the combination of NMPs and hydrogen peroxide in acidic pH of solution as compared to the NMPs alone. Table 2 shows the values of k 1, k 2 and R 2 for the degradation rate of CR with various concentrations of H2O2.

Probable mechanism for the removal of CR by NMPs

The oxidation of CR by hydroxyl radicals was mainly responsible for the degradation of CR in an aqueous solution. Hydroxyl radicals are generated when NMPs can react with the aqueous H2O2 [28]. The highest rate of CR degradation may be due to the high corrosion of NMPs at acid pH of solution and availability of numerous active sites for reaction. An oxidation of CR by H2O2 in the acidic pH of solution was mainly due to the generation of hydroxyl radicals in the reaction [34]. The removal mechanism of CR using NMPs can be expressed as follows [14]:

$$ {\text{NMPs }} + {\text{ H}}_{2} {\text{O}}_{2} + {\text{ H}}^{ + } \to {\text{M}}^{{{\text{n}} + }} + {\text{ OH}}^{ \cdot } + {\text{ OH}}^{ - } $$
(3)
$$ {\text{M}}^{n + } + {\text{ H}}_{2} {\text{O}}_{2} \to {\text{M}}^{{({\text{n}} + 1) + }} \to {\text{OH}}^{ \cdot } + {\text{ OH}}^{ - } $$
(4)
$$ {\text{OH}}^{ \cdot } + {\text{ CR }} \to {\text{Intermediate }} + {\text{ CO}}_{2} + {\text{ H}}_{2} {\text{O}} $$
(5)
$$ {\text{M}}_{\text{x}} {\text{O}}_{\text{y}} + {\text{ nH}}^{ + } \to {\text{nM}}^{{{\text{n}} + }} + {\text{ nH}}_{2} {\text{O}} $$
(6)

where “M” represents metal, “n” is number of valences or moles, and “O” represents oxides.

The studies on degradation of Congo red have been represented in Table 3. When NMPs compared with all other catalyst showed almost highest % degradation than other materials such as Carissa edulis extract capped ZnO, ZnO and TiO2 nanoparticles.

Table 3 Comparative study for the degradation of CR by different nanomaterials

Conclusion

The degradation of CR by NMPs in the presence of H2O2 was found to be highly effective. The CR degradation efficiencies were increased from 90.1 to 95.8% with increasing the dosages of NMPs from 0.025 to 0.10 g/L and further increase in dosages, efficiency was not increased. However, degradation efficiency was decreased from 97.9 to 37.8% with increasing the value of pH from 2.0 to 6.5. The degradation efficiency of CR was reduced with an increasing concentration of CR. However, degradation efficiency was increased with an increasing concentration of H2O2 up to certain limit. The degradation efficiency of CR by NMPs was most suitable under acidic pH of solution. The degradation of CR by NMPs under Fenton-like process followed the pseudo-second-order kinetics as compared to the pseudo-second-order kinetics. A very fast degradation rate was attained at pH 2.0 of CR solution. The present study confirmed that the CR can be removed successfully by even very small amount of NMPs under the Fenton-like process.