Introduction

Degradation of toxic pollutants (harmful to environment and human) is difficult. UV light is potential source for degradation of such pollutants in the presence of semiconductor. TiO2 is a popular semiconductor due to non-toxicity, low cost, high chemical stability [1, 2]. Unfortunately, it has restricted practical applications due to its wide band gap (3.2 eV); so, it is triggered by UV radiation that represents only about 4–5% of natural solar radiation. Moreover, the mineralization processes through various redox reactions are encountered by the rapid recombination of the charge carriers. To overcome these problems, several approaches such as dye sensitization [3, 4] and composite [5, 6] are used. Recently, numerous efforts is paid to development of the visible light-driven (400–800 nm) photocatalysts (color catalysts) as visible light is abundant in solar spectrum [7, 8]. Nowadays, silver chromate (Ag2CrO4) is recognized as good visible light-sensitive photocatalyst due to its unique electronic and crystal structure. Ouyang et al. [9] discussed the correlation of crystal structures, electronic structures and photocatalytic properties in the three Ag-based oxides, AgAlO2, AgCrO2 and Ag2CrO4. They reported that Ag2CrO4 is a good photocatalyst which can remove organic contaminants under irradiation by visible light. Liu et al. [10] synthesized the Ag2CrO4 photocatalyst by microwave hydrothermal method. The catalyst exhibited high photocatalytic activity in degradation of pentachlorophenolate under visible light irradiation (98% after 5-h irradiation). Soofivand et al. [11] synthesized Ag2CrO4 and Ag2Cr2O7 by sonochemical method using silver salicylate and silver nitrate as precursors. They reported that increasing the sonication time changes the morphology of catalyst from nanorods to nanocapsules and nanoparticles. Photocatalytic activity of Ag2CrO4 was investigated by degradation of methyl orange. The percent of degradation was 87% after 280-min irradiation of visible light. Xu et al. [12] synthesized Ag2CrO4 by microemulsion, precipitation and hydrothermal methods to investigate the effect of preparation method on structure and photocatalytic activity of catalyst. The sample prepared by microemulsion method exhibited the highest photocatalytic efficiency on the degradation of methylene blue (MB) under visible light irradiation. Zhu et al. [13] synthesized Ag2CrO4 and AgBr/Ag2CrO4 by precipitation. Silver bromide enhanced the photocatalytic activity of Ag2CrO4 by increasing light absorption ability and enhancing the structure stability of Ag2CrO4. AgBr/Ag2CrO4 exhibited superior photocatalytic activity for photodegradation of rhodamine B about 93% while pure Ag2CrO4 degraded 73% under visible light irradiation. Luo et al. [14] synthesized Ag2CrO4/SnS2 composites with different content of Ag2CrO4 by chemical precipitation method. The as-prepared Ag2CrO4/SnS2 composites exhibited excellent photocatalytic efficiencies for the degradation of methyl orange under visible light irradiation. It was noticed that the rate constant of the Ag2CrO4(1.0 wt%)/SnS2 photocatalyst is 2.2 times as high as that of pure Ag2CrO4 and 1.5 times larger than that of pure SnS2, respectively. Habibi-Yangjeh and Akhundi [15] synthesized magnetic g-C3N4/Fe3O4/Ag2CrO4 nanocomposites, as visible light-driven photocatalyst, using refluxing method. As-prepared g-C3N4/Fe3O4/Ag2CrO4(20%) nanocomposite exhibited superior activity for degradation of rhodamine B under visible light irradiation. Photocatalytic activity of this nanocomposite was about 6.3- and fivefold higher than those of the g-C3N4 and g-C3N4/Fe3O4 samples, respectively. Furthermore, they investigated the influence of refluxing time, calcination temperature and scavengers of the reactive species on the degradation activity.

Unfortunately, Ag2CrO4 is slightly soluble in aqueous solution which decreases its structural stability and photocatalytic ability [13]. Therefore, this study focused on utilizing the properties of PbCrO4 such as absorption in visible region and insolubility to use as visible light photocatalyst. The characterization of PbCrO4/P-25 was investigated using developed techniques such as XRD, SEM, N2 adsorption–desorption isotherm and UV–Vis spectrophotometer. Rh B was chosen as the model of pollutant and tungsten lamp 200 W as visible light source. The effects of PbCrO4/P-25 dosage, initial concentration of Rh B and initial solution pH on degradation of the Rh B were investigated. The photocatalytic degradation mechanism was proposed, and the regeneration and reusability PbCrO4/P-25 were also examined.

Experimental

Materials

Rh B and PbCrO4 were purchased from Sigma-Aldrich. Degussa P-25 was obtained from Evonik Degussa India Pvt. Ltd.; it consists of 25 and 75% rutile and anatase phases of TiO2, respectively, with a specific BET (Brunauer–Emmett–Teller) surface area of 50 m2/g and primary particle size of 20 nm. All these chemicals were used without further purification. Properties of investigated dye are listed in Table 1. PbCrO4/P-25 composite was prepared by grinding.

Table 1 Properties of the investigated dye
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Stock solution is prepared by dissolving accurately weighed sample of dye in bidistilled water to give a concentration of 10−3 mol L−1. Desirable concentrations of dye are obtained by serial dilution.

Methods

X-ray diffraction patterns were carried out by X’PERT-PRO-PANalytical (the Netherlands) X-ray diffractometer, with CuKα (λ = 1.5406 Å) radiation in the 2θ range from 4 to 80o. The scanning mode is continuous with step size of 0.02° and scan time of 0.5 s. The average crystallite sizes are calculated from the diffraction of peak broadening using the Debye–Scherrer equation, Eq. (1):

$$ D = \frac{k}{\beta \cos \theta } $$
(1)

where D is the average crystallite size, β is the full width half maximum (FWHM) of the highest intensity peak (110 peak), k is a shape factor of the particles (it equals to 0.89), θ and λ are the diffraction angle and the wavelength of the X-rays, respectively.

Adsorption–desorption isotherms of purified N2 at 77 K were determined using Nova 3200 system (USA). BET method was utilized to calculate the specific surface area from adsorption data. The Barrett–Joyner–Halenda model was used to estimate the average pore size from the desorption data.

The nanostructure of a prepared sample was investigated by scanning electron microscope, SEM using JEOL, model Jed 2300 (Japan) microscope.

The band gap energy of PbCrO4/TiO2 sample was determined using DRS. The band energy is calculated by Eq. (2):

$$ E_{\text{g}} = {\text{h}} \frac{C}{\lambda } = \frac{1240}{\lambda } $$
(2)

where E g is the band gap energy (eV), h is the Planck’s constant, c is the light velocity (nm s−1) and λ is the wavelength (nm).

pH of solution was conducted with a Griffin pH meter fitted with glass calomel electrode.

Determination of point zero charge

pH at the point of zero charge (pHpzc) for PbCrO4/P-25 catalyst was determined by the batch equilibration technique [16]. Sodium chloride solution (0.1 mol/L NaCl) was used as an inert electrolyte. Initial pH values (pHinitial) of the NaCl solutions were adjusted from 2 to 12 by addition of 0.1 mol/L HCl or NaOH. 0.1 g of PbCrO4/TiO2 was added into 25 mL of 0.1 mol/L NaCl solution. The suspension mixture was allowed to equilibrate for 3 h in a shaker maintained at room temperature. Then, the suspension solution was filtered and the pH values (pHfinal) were measured.

Photocatalytic experiments

Photocatalytic activity was evaluated by monitoring the degradation of Rh B under visible light irradiation (200 W Tungsten lamp). 100 mL aqueous solution of dye with certain amount of photocatalyst was taken in Pyrex vessel which was surrounded by a circulating water jacket to cool the sample. Prior to light irradiation, the suspension was stirred in the dark for 30 min to reach the equilibrium between the dye and catalyst. During the course of light irradiation, 5 mL aliquot was withdrawn at a various time intervals. Then the samples were centrifuged for 10 min at 1800 rpm. Supernatant concentration was determined spectrophotometrically at λ max = 554 nm of Rh B using thermostat Evolution 300 UV–Vis spectrophotometer. The removal % is calculated by Eq. (3):

$$ {\text{Removal \% }} = \, \left[ {\left( {A_{\text{o}} {-} \, A_{\text{t}} } \right)/A_{\text{o}} } \right] \, \times \, 100 $$
(3)

where A o and A t are the absorbance of dye after equilibrium in dark and at time ‘t’ of irradiation, respectively, at λ max = 554 nm.

Determination of chemical oxygen

COD is the measurement of amount of oxygen in aqueous solution consumed in oxidation of pollutant in wastewater and is used to calculate the mineralization percent by Eq. (4):

$$ {\text{mineralization }}\;{\% } = \frac{{{\text{COD}}_{\text{b}} - {\text{COD}}_{\text{a}} }}{{{\text{COD}}_{\text{b}} }} \times 100 $$
(4)

where CODb and CODa are the COD values of pure dye before and after irradiation in the presence of catalyst, respectively.

Results and discussion

Characterization of catalyst

XRD

XRD is widely used technique in the investigation of the crystalline parameters and size of the nanoparticles. Figure 1 displays the diffraction patterns of pure TiO2 (P-25), pure PbCrO4 and PbCrO4/P-25 nanoparticles. On careful examination of the diffraction patterns, one can notice the interference of peak at 2θ = 25.3 of P-25 with that one for chromate and disappearance of 38.5 assigned to P-25 in diffractogram of PbCrO4/P-25. It is interesting to notice that the crystalline pattern for the PbCrO4/P-25 sample resembles the XRD pattern of the pure PbCrO4. There is no remarkable change noticed for PbCrO4 sample; however, a reduction in peak intensity of PbCrO4 is observed. This is due to mixing of P-25 with chromate nanoparticles which reduced the concentration of chromate in PbCrO4/P-25; consequently, peak intensity decreased. The analysis of our experimental results pointed out that the crystallite size of pure TiO2(P-25), PbCrO4 and PbCrO4/P-25 is about 23, 77 and 72 nm, respectively, which is consistent with reported early [17].

Fig. 1
figure 1

XRD patterns of pure PbCrO4, TiO2 (p-25) and PbCrO4/TiO2 (p-25) composite

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SEM

The SEM images of pure PbCrO4, TiO2(P-25) and PbCrO4/P-25 composite are shown in Fig. 2. As shown in Fig. 2, morphology of pure PbCrO4 is composed of regular rods and the surface of PbCrO4 is very smooth. As indicated in Fig. 2, TiO2 (P-25) nanoparticles are distributed on the surface of PbCrO4. The morphology of the PbCrO4/P-25 composite is approximately similar to that of PbCrO4 except for the relatively rough surface in the composite due to deposition of TiO2(P-25) nanoparticles on PbCrO4 surface.

Fig. 2
figure 2

SEM Images of TiO2 (p-25), PbCrO4 and PbCrO4/TiO2 (p-25) composite

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DRS

The UV–Vis diffuse reflectance spectra of pure PbCrO4, P-25 and PbCrO4/P-25 samples are presented in Fig. 3. Pure PbCrO4 shows a sharp absorption edge near 565 nm, corresponding to the band gap of 2.2 eV. Mixing P-25 with PbCrO4 has no effect on absorption edge of PbCrO4.

Fig. 3
figure 3

DRS spectra of PbCrO4, TiO2 (p-25) and PbCrO4/TiO2(p-25) composite

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Textural characterization PbCrO4/P-25 of nanoparticles

Adsorption–desorption isotherms of N2 adsorption at 77 K on PbCrO4/P-25 nanoparticles are presented in Fig. 4a. The adsorption isotherm is classified as type II according to IUPAC, exhibiting H3 hysteresis loop which refers to plate-like pores. The specific surface area (A BET) of the prepared sample is about 29.43 m2/g which was estimated by using the BET equation in its normal range of applicability and adopting a value of 16.2 Å for the cross-sectional area of N2. However, the total pore volume (Vp) was taken at a saturation pressure and expressed as liquid volume = 0.066 cc/g. According to pore size distribution (Fig. 4b), some of pores radii are micropores with peak maximum located at 13.52 Å and other with pore radii greater than 50 Å (mesopores). t-plot (Fig. 4c) showed the presence of mixture of micropores (downward deviation) and mesopores (upward deviation) that confirmed the suggested data from pore size distribution.

Fig. 4
figure 4

N2 adsorption–desorption isotherms (a), pore size distribution curve (b) and t-plot (c) for PbCrO4/TiO2 (p-25) composite

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Point zero charge

Figure 5 shows the plot of pHinitial against pHfinal of PbCrO4/P-25 catalyst suspension in 0.1 M NaCl. The presence of the plateau indicates that the catalyst has amphoteric properties and acts as a buffer in this range of pH (4–8). In this range, the pHfinal is almost the same for all values of pHinitial and corresponds to pHpzc. The pHpzc of PbCrO4/P-25 catalyst is observed to be pH 3.

Fig. 5
figure 5

Plot pHfinal against pH initial for PbCrO4/TiO2 (p-25) composite in 0.1 N NaCl

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Photocatalytic activity of PbCrO4/P-25

Many studies focused on the evaluation of photocatalytic activity of Ag2CrO4 [9,10,11,12], others on the modification and improvement of their properties [13,14,15]. In the literature, few studies concerned with the preparation of PbCrO4. Moreover, no studies concerned with photocatalytic activity of PbCrO4 as visible light catalyst. Therefore, the present study concerned with photocatalytic activity of PbCrO4 which evaluated by monitoring the degradation of Rh B under visible light irradiation (200 W tungsten lamp). Photocatalytic activity of PbCrO4/P-25 is compared with photocatalytic activity of Ag2CrO4 toward different pollutants (Table 2). Table 2 shows that PbCrO4/P-25 is good visible light photocatalyst in comparison with Ag2CrO4 which is slightly soluble in aqueous.

Table 2 Photocatalytic activity of PbCrO4 against Ag2CrO4 under visible light irradiation
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The photodegradation rate was affected by several parameters such as pH, catalyst dose and initial dye concentration. Moreover, a linear plot between log At (absorbance of Rh B) against time was obtained, which indicates that the photocatalytic degradation of Rh B follows first-order kinetics. The rate constant for this reaction was measured by Eq. (5)

$$ k = \, 2.303 \, \times {\text{ slope}} $$
(5)

pH value

pH is one of the important factors controlling the adsorption of dye on adsorbent and consequently the photodegradation rate. The effect of pH studied in the range of 2–10. Increasing pH decreases the rate constant of photodegradation of Rh B (Table 3). Since PZC is about 3, the surface of catalyst is positively charged at pH values less than 3. Moreover (pKa of COOH = 3.7 [18]), consequently expected rate at pH = 2 is the slowest one due to the repulsion between cationic Rh B and the surface of catalyst. This is contradicted with experimental results where complete photodegradation of Rh B was observed at pH = 2 (Fig. 6a); this will be discussed later.

Table 3 Observed first-order rate constants of the photodegradation of Rh B on PbCrO4/P-25 composite using 200 W tungsten lamp
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Fig. 6
figure 6

UV–Vis spectra for photodegradation of Rh B on PbCrO4/TiO2 (p-25) using 200 W tungsten lamp, pH = 2 (a) and pH = 10 (b), inset color change of Rh B with time [Rh B] = 10−5 M 0.2 g/100 ml PbCrO4/TiO2 (p-25) (color figure online)

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Furthermore, at 3 < pH < 10, drastic decrease in photodegradation rate constant was observed (Table 3). This is due to (1) repulsion between carboxylic group and negatively charged surface of catalyst and (2) aggregation of dimer resulted from electrostatic attraction between COO− and N+ of xanthane group of zwitterion [19, 20] which obscures reaching light to the catalyst surface. The constancy of rate constant in the range of pH 4–8, (Table 3) is in coincidence with the behavior of catalyst in this pH range (Fig. 5).

At pH10 part of PbCrO4 formed soluble Na2CrO4 and no photodegradation of Rh B was observed (Fig. 6b). This could attribute to Na2CrO4 works as screen which prevents reaching of light to catalyst, consequently decreasing the production of reactive oxygen species responsible of degradation of dye. To prove this, known concentration of Na2CrO4 added to photocatalytic batch led to reduction of the removal percent to 20% on irradiation of 6 h.

Photocatalyst dose

Photocatalyst dose may affect the photodegradation of Rh B, so different amounts of photocatalyst are used. The removal percent increased from 29 to 87% by increasing the amount of photocatalyst from 0.05 to 0.5 g/100 mL. Also rate of photodegradation increased with increases in the amount of the photocatalyst (Table 3). This is attributed to the fact that as the photocatalyst amount increased, exposure surface to light increased and consequently the rate of photodegradation increased.

Rhodamine B concentration

The effect of dye concentration was studied by changing the concentration of dye in range 0.5 × 10−5–2 × 10−5 mol dm−3 while other variables were kept constant. Increasing dye concentration from 0.5 × 10−5 to 2 × 10−5 mol dm−3 decreases the rate of degradation, from 6.17 × 10−3 min−1 to 1.64 × 10−3 min−1 (Table 3). This is attributed to dye which works as an inner filter prevents the passage of light to the semiconductor and consequently decreases the number of photoelectrons and number of photoholes, so rate of photodegradation decreases.

Radical scavenger

In order to investigate the role of reactive oxygen species (O •−2 and •OH) involved in the photodegradation of Rh B, experiments were carried out under optimum reaction conditions (100 mL of 10−5 mol dm−3 of Rh B, 0.2 g of catalyst and irradiation time 5 h) in the presence of scavengers (10−5 mol dm−3) such as ethanol (isopropyl alcohol) for •OH [21], MV2+ for the electron [22], ascorbic used as positive hole scavenger [23] and benzoquinone for O •−2 [24]. The obtained results showed that the photodegradation percentage of Rh B was reduced from 70 to 62, 50, 50 and 48%, respectively, after the addition of MV2+, isopropyl, benzoquinone and ascorbic. This is due to the decrease in the concentration of reactive oxygen species in the presence of scavengers, and therefore, both O •−2 and •OH were actively involved in the photodegradation process. The percentage of removal of Rh B in the presence of isopropyl and benzoquinone is almost the same. This indicates that O •−2 and •OH were involved in photodegradation by the same order. Furthermore, increasing the concentration of benzoquinone from 10−5M to 10−4M decreases the removal percent of Rh B from 50 to 30% which refers to involvement of O •−2 in photodegradation; i.e., O •−2 has pronounced role in photodegradation of Rh B.

The product of photocatalytic decomposition of Rh B

Figure 7 shows the temporal UV–Vis change of Rh B in the presence of PbCrO4/P-25 under irradiation by visible light at natural pH. The decomposition of Rh B is accompanied by hypsochromic shift from 554 to 500 nm with color changed from rose to fluorescent yellowish green. According to earlier reports [18, 25,26,27,28,29], deethylation product (546, 532, 504, 500 nm) is corresponding to N,N-diethyl-N′-ethyl rhodamine 110, N-ethyl-N′-ethyl rhodamine 110, N-diethyl rhodamine 110 and rhodamine 110, respectively. This is attributed to dye anchor on the surface of catalyst via N+ near to adsorbed O2. Moreover, at pH = 2 complete photodegradation of rhodamine 110 was obtained as presented in Fig. 6a. This is attributed to formation of HO2., which is more reactive than O •−2 in acidic medium [30] and it has high redox (+1.44 V vs. NHE) in comparison with redox potential of O •−2 (+ 0.89 V vs. NHE) [18]. It can say that under visible light irradiation, Rh B underwent two competitive processes which occurred simultaneously during the photoreaction: (1) N-deethylation processes are preceded by formation of a nitrogen-centered radical and (2) destruction of dye chromophore structure is preceded by generation of a carbon-centered radical [18, 25, 26].

Fig. 7
figure 7

UV–Vis spectra for photodegradation of Rh B on PbCrO4/TiO2 (P-25) using 200 W tungsten lamp [Rh B] = 10−5 M 0.2 g/100 ml PbCrO4/TiO2 (p-25) pH = 6

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Mineralization

The photocatalytic experimental results indicate a pronounced reduction in the intensities of the bands at 554 and 300 nm, suggesting that both chromophore and aromatic parts of rhodamine B were breaking down (Fig. 6a). The chemical oxygen demand (COD) is the amount of oxygen equivalent to the amount of organic and inorganic matter in the sample. A remarkable reduction in COD is evidence for the oxidation and/or decrease in carbon content in the sample, hence indicative of the extent of mineralization. It was found that COD decreased from 16 to 5.4 mg/L after exposing the sample to visible light irradiation for 10 h at pH = 2, indicating that about 70% of dye is completely removed from the solution. This result is in agreement with the photodegradation (Fig. 6a).

Mechanism

From obtained results, the proposed mechanism is that dye and catalyst absorb visible light and dye in excited state injects electron in conduction level of catalyst which in turn transfers it to O2 adsorbed on surface forming superoxide, O •−2 . Then superoxide reacts with cationic form of Rh B to form the final product as follows:

$$ {\text{Rh B}}/{\text{PbCrO}}_{ 4} + {\text{ h}}\upupsilon \, \to {\text{ Rh B}}^{*}/{\text{PbCrO}}_{ 4} $$
(6)
$$ {\text{Rh B}}^{*}/{\text{PbCrO}}_{4} \to {\text{ Rh B}}^{ + \cdot } /{\text{PbCrO}}_{4}^{ - } $$
(7)
$$ {\text{Rh B}}^{ + \cdot } /{\text{PbCrO}}_{4}^{ - } + {\text{ O}}_{2} \to {\text{ Rh B}}^{ + \cdot } /{\text{PbCrO}}_{4} + {\text{ O}}_{2}^{ - \cdot } $$
(8)
$$ {\text{HO}}_{2}^{ \cdot } \leftrightarrow {\text{ O}}_{2}^{ - \cdot } + {\text{ H}}^{ + } \quad {\text{K}}_{\text{a}} = \, 4.69 \, \left[ {18} \right] $$
(9)
$$ {\text{Rh B}}^{ + \cdot } /{\text{PbCrO}}_{4} + {\text{ O}}_{2}^{ - \cdot } \to N - {\text{deethylate product}} $$
(10)
$$ {\text{Rh B}}^{ + \cdot } /{\text{PbCrO}}_{4} + {\text{ HO}}_{2}^{ \cdot } \to \, N - {\text{deethylate product }} \to {\text{ CO}}_{2} + {\text{ H}}_{ 2} {\text{O}} . $$
(11)

Stability of catalyst

Successive experiments under optimum condition were done. After each experiment, the catalyst was washed by bidistilled water several times and dried in oven at 50 °C and reused in new experiment. It was found that the photocatalytic activity was diminished, and after third cycle, the catalyst lost its activity although no change in XRD (Fig. 8).

Fig. 8
figure 8

XRD of photocatalyst before and after the 3rd cycle

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Conclusion

This research work reflects the potentiality of PbCrO4/P-25 nanoparticles as an effective and preferential photocatalyst for removal of Rh B under irradiation by visible light. The structure, crystalline and morphology feature of PbCrO4/P-25 nanoparticles were investigated using XRD, BET and SEM techniques. The effects of pH, photocatalyst dose and dye concentrations were investigated. Many photocatalytic experiments were performed in the presence of various scavengers to investigate the active species which is responsible for dye degradation. The experimental results have pointed out that the rate is much suppressed in the presence of isopropyl and benzoquinone solution, revealing that •OH and O •−2 radicals are the main active species in the photodegradation of Rh B dye. The product of photocatalytic degradation of Rh B depended on the pH of medium. In natural pH, the product was rhodamine 110, while at pH = 2, CO2 and H2O were the final products.