Introduction

Oxidation of various organic pollutants on a solar-powered semiconductor photocatalyst provides a clean and environmentally friendly process [1]. Among the existing semiconductor materials, titanium dioxide (TiO2) is a very active, low cost, and non-toxic photocatalyst. TiO2 with a wide gap assigned by Eg 3.20 eV can only be activated under UV light [2,3,4,5,6,7,8,9,10,11,12,13,14,15,16], and consequently, it is less active under visible light exposure. This deficiency restricts the application of TiO2 photocatalyst under low cost sunlight, which is mostly composed of visible light [2, 8, 11]. The enhancement of TiO2 activity under visible light by mono-doping [2,3,4,5,6, 9, 10] and co-doping [7, 8, 11,12,13,14,15,16] has been proven to be the most successful strategy. By doping process, the gap can be narrowed and thus decreasing the Eg of TiO2 [2,3,4,5,6,7,8, 10, 11, 13,14,15], emerging in the visible radiation. Thereby, the visible response and the activity of organic pollutant degradation of the doped TiO2 under visible light exposure are considerably enhanced [2, 3, 5, 9,10,11, 13]. Besides, the improvement of the photodegradation of various organic pollutants underdoped TiO2 is also contributed by the ability of the dopant in suppressing the recombination of electrons (e-) and holes (h+) photogenerated by TiO2 during UV irradiation [5, 9, 15], as presented as Eqs. (1) and (2).

$$ {\mathrm{TiO}}_2+ hv\to {\mathrm{TiO}}_2\left({e}^{-}+{h}^{+}\right) $$
(1)
$$ {\mathrm{TiO}}_2\left({e}^{-}+{h}^{+}\right)\to {\mathrm{TiO}}_2+\mathrm{heat} $$
(2)

Compared to metal elemental dopants, the non-metals are more interesting due to their smaller size allowing them to be inserted into the TiO2 crystal lattice facilely [2, 4, 9,10,11]. Furthermore, among the non-metal dopants, nitrogen has attracted extensive interest as it can be easily introduced into the TiO2 structure, due to its atomic size, which is comparable with that of oxygen, its low ionization energy, and high stability [2,3,4, 15]. In accordance, N-doped TiO2 demonstrates significant photocatalytic activity under visible light irradiation [2,3,4]. It was also reported that increasing the dopant loaded has improved the photodegradation of organic pollutants, but the further increase was found to show the opposite effect [2, 5, 8,9,10,11, 15]. This unexpected trend is generated by the turning role of the dopant from preventing becomes servicing recombination [5, 6, 11, 15]. Consequently, the fast recombination may proceed that declines the effectiveness of the photocatalytic organic degradation.

For solving such drawback, co-doping TiO2 with two different non-metal elements has received intensive attention [6, 7, 11,12,13,14,15,16], as with low loading co-dopants can deliver a significant effect on increasing the photocatalytic degradation of various organic pollutants under visible light [2, 15]. Moreover, some studies have also proven that co-doped TiO2 showed higher activity than the corresponding mono-doped [8, 11, 15].

Considering the advantages of using N dopant, as presented above, the double elemental dopants of N combined with C [7, 12], with P [11], as well as with S dopants [13,14,15,16] have been intensively studied. Due to the pronoun effect shown by double dopants of N and S in the improving photodegradation under visible light exposure, co-doping with N and S is growing of interest [13,14,15,16]. Several studies have investigated the co-doping TiO2 with N-S atoms for accelerating the degradation of residual anti-inflammatory drug [13], antibiotic residual [14], p-chlorophenol [15], and phenol [16] in the presence of the visible light, and high results have been obtained.

To the best of our knowledge, the co-doped TiO2-N,S has not been examined for oxidation of hazardous heavy metals such as Pb (II). The heavy metal become an environmental concern due to its characters including wider disposal sources, rapid accumulative in biotic tissues, and high hazard for human health [17,18,19,20,21,22,23]. To prevent the dangerous effect of Pb(II) ion on the ecosystem and humans, an effective detoxification method is urgently required to treat the corresponding wastewater before reaching the environment. Removal of Pb(II) ions from water has been frequently performed by adsorption techniques [17,18,19]. Unfortunately, at the end of the process, the adsorbent saturated with Pb(II) can generate undesired hazardous solid waste, which creates a new problem in the environment. The most suitable method is believed to be oxidation of Pb(II) resulting from the less toxic and handleable PbO2 [20,21,22,23]. The effective Pb(II) oxidation can be obtained through the photo-Fenton process [20] and photocatalysis over TiO2 photocatalyst under UV illumination [20,21,22,23].

However, detoxification of Pb(II) by using N,S-coped TiO2 in the presence of visible light so far is untraceable in the literature. The application of the N,S co-doped TiO2 for photo-oxidation of Pb(II) in the solution contributes plausibly a novelty in Pb(II) remediation as well as the application of TiO2-N,S photocatalyst. Under the circumstance, in the present research, photooxidation of Pb(II) over TiO2-N,S photocatalyst under visible light irradiation is addressed.

Concerning the co-doping N,S into TiO2, urea, and thiourea are the most employed as the sources of the N and S dopants [2, 3, 9, 10, 13, 14, 16] and other organic compounds [2, 11, 12]. However, using organic amine for N and S dopant sources can inevitably lead to organic residues on the photocatalyst surface [4, 5] that can decrease the activity of the photocatalyst. To avoid such weaknesses, in this paper, nitric and sulfuric acids are proposed as the simple inorganic N and S dopant sources respectively, into the TiO2 structure. The co-doping is performed in one step suggesting a fast and practice process. Furthermore, to reach the maximum Pb(II) photo-oxidation result, the influences of some important parameters controlling the effectiveness of Pb(II) oxidation such as dopant amount in the photocatalyst, photocatalyst mass, irradiation time, and solution pH are also evaluated.

Methods

Materials

The main materials used were TiO2 (Degussa P25), HNO3 (65%), H2SO4 (95–97%), and Pb(NO2)3, which were purchased from E. Merck company and used without any purification.

Co-doping of TiO2 by N and S atoms

The co-doping TiO2 with N and S elements was employed by a pressured hydrothermal technique in the autoclave [24]. Powder of TiO2 about 0.8 g was suspended in 20 mL of a mixture of distilled water (10 mL) and ethanol (10 mL), followed by stirring for 30 min to make a good suspension. Into the suspension, 20 mL of HNO3 5 M and 20 mL of H2SO4 5 M were added, accompanied by stirring for 30 min to get a homogenous mixture. The mixture was then put in the autoclave and heated at a constant temperature of 150°C for 6 h. After that, the co-doped TiO2 photocatalyst was collected and washed with deionized water three times to remove the weakly adsorbed species. Then, the photocatalyst was dried at 110°C and continued with calcination at 400°C for 2 h. The quantities of TiO2, HNO3, and H2SO4 introduced would give a mole ratio of Ti:N:S = 1:1:1. This as-prepared photocatalyst was coded as TiO2-N,S(1:1:1).

The same procedure was copied for processes by addition of HNO3 and H2SO4 with the same concentration (5M) but different volume, giving mole ratio of Ti:N:S = 1:1:1.5 and 1:1:0.5, as well as by using single HNO3 and H2SO4 with the same volume (20 mL) and concentration (5 M). The photocatalyst samples prepared were given codes as TiO2-N,S(1:1:1.5), TiO2-N,S(1:1:0.5), TiO2-N(1:1), and TiO2-S(1:1), respectively.

Characterization

The effect of co-doping on the crystallinity of TiO2 was followed by using an X-ray diffractometer of 6000X Shimadzu with Cu K radiation. The XRD patterns were recorded from 4–40o of the 2 theta angles. The absorption edge and band gap energy of the samples were measured using a UV/visible spectrophotometer equipped with a Specular reflectance accessory of the UV-1800 series. The SRUV spectra were taken from 300–800 nm of the wavelength. Infrared spectra (IR) with the wavenumber of 4000–400 cm−1 were recorded on a Shimadzu Prestige 21 Infrared spectrophotometer that was used to evaluate the success of the N,S doping. The determination of Pb(II) concentration in the solution from the photooxidation process was carried out by using 3110 Perkin-Elmer flame Atomic Absorption Spectrophotometry elemental analysis.

Photocatalytic removal of Pb(II) from water under visible irradiation

In this typical process, 25 mg of the co-doped photocatalyst mixed with 50 mL of a solution containing Pb(II) 10 mg/L was stirred to get a homogenous mixture. The mixture was placed in a container glass, and the container was put in the photo-process apparatus equipped with 4 visible lamps (TL-D Intensity @20 W, 2000 lm/m2). Then, the visible lamps were turned on, and the mixture was magnetically stirred for 30 min. After the desired time, the solution in the container was filtered under 41 Whatman filter paper to collect the solution containing the Pb(II) residue. The concentration of Pb(II) ions left in the solution was analyzed by using AAS based on the respective standard curve. The removed Pb(II) (in %) from the solution was calculated by the following equation:

$$ \frac{Co-C}{Co}\ x\ 100\% $$
(3)

where Co is the initial amount of Pb(II) (mg) and C represents the amount of Pb(II) left in the solution (mg).

The same procedure proceeded with various mole ratios of Ti:N:S, photocatalyst masses, irradiation time, and solution pH, as well as by using mono-S-doped and N-doped photocatalyst as a comparison.

Result and discussion

Characterization of The N,S Co-doped TiO2

SRUV spectra

The SRUV spectra of the co-doped TiO2 along with un-doped photocatalysts are exhibited in Fig. 1. By taking the intersection of the absorbance at the respective wavelength, the maximal absorbance can be obtained. From the maximal absorbance, the band gap energies of the photocatalyst can be calculated, and the calculation results are presented in Table 1.

Fig. 1
figure 1

The SRUV Spectra of a TiO2, b TiO2-N,S(1:1:0.5), c TiO2-N,S(1:1:1), and d TiO2-N,S(1:1:1.5)

Full size image
Table 1 The effect of co-doping on the decreasing Eg
Full size table

Data in the Table 1 indicates that by co-doping, the absorption of the photocatalyst shifts significantly into a longer wavelength, entering the visible region. The shifts are due to the considerable decrease of the band gap energy, resulting from narrowing the gap, which is synergically created by the two dopant (N and S) atoms. Further, the decreasing band gap energy is observable in more effective when the amount of dopant S is enlarged giving larger narrowing the gap. This data provides evidence of the success of co-doping. The decrease of Eg and absorption shift allow TiO2 to be highly responsive under visible irradiation. Similar findings have also been frequently reported [2, 5, 8,9,10, 15].

XRD data

Figure 2 displays the XRD patterns of un-doped TiO2 along with the co-doped TiO2-N,S samples. The XRD pattern of TiO2 is characterized by peaks appearing at 25.091, 37.651, 48.021, 53.891, 55.071, 62.381, 68.701, 70.041, and 75.001 of the 2θ under Cu-Kα that well fit with those of the standard anatase phase of TiO2 recorded by JCPDS card number of 01-071-1167 [2].

Fig. 2
figure 2

XRD patterns of a TiO2, b TiO2-N,S(1:1:0.5), c TiO2-N,S(1:1:1), and d TiO2-N,S(1:1:1.5)

Full size image

Similar patterns of the co-doped photocatalyst samples to the pattern of the undoped TiO2 are also clearly observed. The difference that appeared in the intensities of the doped photocatalyst is lower than the intensities of the undoped one. With the increase of the amount of S in the co-doped photocatalyst, the intensities are seen to be considerably decreased. The lowering intensities imply the crystallinity deforms partially, due to the insertion of the atom dopants in the crystal lattice [2, 10]. The more amount of S dopant produces, more S inserted into the lattice leading to higher deformation of the crystal. This alteration of the intensities is good proof of the success of the N,S co-doping in TiO2. Other studies have also obtained the same trend [2, 5, 8, 11].

FTIR data

Several characteristic peaks belonging to TiO2 are notable in the FTIR spectra as seen in Fig. 3. The characteristic peaks of TiO2 appearance at 700–500 cm-1 of the wavenumbers belonged to the O–Ti–O bond. The other peaks also appear at about 3400 and 1630 cm-1 associated with O–H from water adsorbed on the TiO2 surface [9, 10, 15]. The spectra of all TiO2-N,S samples are similar to that of TiO2, with several new peaks at about 1390–1405, 1219, 1134–1140, and 1049 cm-1 of the wavenumbers. Further, the intensities of the new peaks are seen sharper as the enlargement of the S content. The peak appearing at 1396–1404 cm-1 was related to NO vibration. The additional peaks observed at 1049 cm-1 is attributable to S–O–bond that of at 1134–1140 and 1219 cm-1 correspond to S=O from sulfate [9, 10, 15]. The absorbance of S-O and S=O vibrations are notable only in the co-doped TiO2 with higher S content, and the absorbance is insignificant in TiO2-N,S with the lowest S content. Further, the absorbance of S–O and S=O vibrations are sharper with an increase in the amount of the S doped. It is assigned that with the small sulfuric acid concentration, the S doping is undetectable, and by introducing higher concentration doping N and S have been successfully proceeded [9, 10, 15].

Fig. 3
figure 3

FTIR spectra of a TiO2, b TiO2-N,S (1:1:0.5), c TiO2-N,S (1:1:1), and d TiO2-N,S (1:1:1.5)

Full size image

Photocatalytic activity of the co-doped TiO2 in the temoval of Pb(II) under visible light

Influence of co-doping

It is observable in Fig. 4 that co-doping promotes higher removal of Pb(II) under visible light compared to the undoped. Moreover, increasing the amount of S doped in co-doped TiO2 has improved the removal, but the opposite effect is seen when the amount of S dopant is further enlarged. The improvement of the photo-oxidation in the presence of the co-doped under visible light illumination is promoted by narrowing the gap or reducing the Eg, allowing TiO2 to be responsive by visible light. In addition, the dopant atoms can also service separation of the photogenerated electrons and hole by capturing the electrons [5, 11, 12, 14], which detains the recombination. In contrast, when the S dopant is in excessive amount, the role of the dopant switches into the recombination center [5, 6, 11, 15], which accelerates the recombination, and thus diminishes the Pb(II) removal.

Fig. 4
figure 4

The results of Pb(II) photo-oxidation under visible exposure over: (1) TiO2, (2) TiO2-N,S(1:1:0.5), (3) TiO2-N,S(1:1:1), and (4) TiO2-N,S(1:1:1.5)

Full size image

The Pb(II) ion removal from the aqueous media under visible light and in the presence of TiO2-N.S can be stimulated by 3 possible reactions. The first reaction is precipitation of Pb2+ with -OH anionic into Pb(OH)2, written as reaction (4), that can only occur at pH higher than 7. The photocatalytic reaction was conducted at pH 5, suggesting that no precipitation of Pb(OH)2 occurs.

$$ {\mathrm{Pb}}^{2+}+2{\mathrm{OH}}^{-}\to \mathrm{Pb}{\left(\mathrm{OH}\right)}_2 $$
(4)

The second possibility is the reduction of Pb2+ induced by electrons released by TiO2 during light irradiation into Pb0 following reaction (5) [22]. Since the standard reduction potential is a negative value suggesting that the reduction is impossible thermodynamically [20, 21].

$$ {\mathrm{Pb}}^{2+}+2\ {\mathrm{e}}^{-}\kern0.5em \to {\mathrm{Pb}}^0\kern2em {\mathrm{E}}^{\mathrm{o}}=-0.13\ \mathrm{V} $$
(5)

The last possible reaction is oxidation by hole (h+) with oxidation potential as much as 3.5 V, exhibiting a strong oxidizing agent. In addition to the hole, OH radical also acts as a strong oxidizing agent with 2.8 V of the oxidation potential. The oxidation of Pb2+ with OH radicals to be Pb(IV)O2 is represented by reactions (6) and (7).

$$ {h}^{+}{+}^{\hbox{--}}\mathrm{OH}\to \mathrm{OH} $$
(6)
$$ {\mathrm{Pb}}^{2+}+2\ \mathrm{O}\mathrm{H}\to {\mathrm{Pb}\mathrm{O}}_2+2{\mathrm{H}}^{+} $$
(7)

It is known that the standard reduction potential of Pb(IV)/Pb(II) is −0.67 V [21], implying that the reduction is unlikely to proceed. Hence, it is obvious that photooxidation of Pb(II) is more favorable. The more possible oxidation of Pb(II) is in agreement with the study reported [23] that Pb(II) could be oxidized by chlorine and was accelerated by Mn(VII).

Influence of photocatalyst mass

The photocatalytic removal of Pb(II) is significantly controlled by photocatalyst mass, as displayed in Fig. 5. Notably, the effectiveness of the Pb(II) removal enhances sharply with the enlargement of the photocatalyst mass, but the oxidation is detrimental when the mass is in excess. With the higher photocatalyst mass, more OH radicals can be provided. Also, enhancing the photocatalyst mass can enlarge the active surface of the photocatalyst that improve the Pb(II) adsorption to be oxidized by OH radicals. The more number of OH radicals and Pb(II) adsorbed is beneficial to accelerate the photo-oxidation that reaches the maximum result.

Fig. 5
figure 5

Influence of the photocatalyst mass on the photo-oxidation over: a TiO2-N,S (1:1:1), b TiO2-N (1:1), and c TiO2- S(1:1)

Full size image

The photocatalyst mass exceeding the optimum level (35 mg) can escalate significantly the turbidity, which hinders the penetration of the light. Consequently, the number of OH radicals formed may be declined and thus diminish photooxidation. A study also obtained similar results [10, 15].

Moreover, the figure also demonstrates that co-doping results in higher oxidation compared to the mono-doped. It is evidence that co-doping can noticeably increase the visible responsive of TiO2 and effectively prevent the recombination, due to the synergic effect of the double dopants [8, 11,12,13,14]. The higher photooxidation of Pb(II) shown by TiO2-N over that of TiO2-S implies that the effect of doping N is more effective than S dopant. This trend was also reported as well [6].

Influence of the irradiation time

Figure 6 demonstrates a sharp incremental of the Pb(II) photooxidation as the extending time up to 30 min, but there is a negative effect with the longer time than 30 min. Prolonging time of the irradiation facilitates TiO2 to provide more OH radicals and also allows OH radicals to contact with Pb2+ effectively. With the longer than 30 min, the solid of PbO2 resulting from the oxidation may be formed in a larger amount [9, 10, 15, 20], which can cover the surface of the photocatalyst. In accordance, the formation of OH radicals is fewer, and thus, the photooxidation of Pb(II) declines. Some studies have also found a similar trend [9, 10, 15, 20].

Fig. 6
figure 6

The influence of the irradiation time on the photo-oxidation results at a pH 3, b pH 7, and c pH 11

Full size image

Influence of pH solution

The photooxidation effectiveness with the alteration pH can also be seen in Fig. 6. Increasing pH up to 7 exhibits an elevation of the effectiveness, but the contrary effect is seen at pH higher than 7. At lower pH, the surface of TiO2 is protonated to be positive charge [8, 10, 11, 21], presented as reaction (7), that prevents providing OH radicals, and Pb(II) exists prominently as Pb2+ [19,20,21]. The same charges of the photocatalyst surface and Pb(II) ions restrict the adsorption of Pb2+ on the surface of TiO2. The fewer OH radicals and low Pb2+ adsorption explain the low photooxidation of Pb2+ by OH radicals on the surface of the photocatalyst. At higher pH, the protonation should decrease or quiet, allowing TiO2 to form OH radicals maximally and to adsorb Pb2+effectively, which is beneficial in the high photooxidation. When the pH is climbed up to basic condition (pH 9), the most surface of TiO2 is in the negative forms, as seen in reaction 6 [11, 21], that is to form OH radicals. In such pH Pb(II) precipitates as Pb(OH)2 [19, 21]. These conditions are certainly adverse for Pb2+ photooxidation by OH radicals.

Conclusions

It is evidence that N and S atoms from HNO3 and H2SO4, respectively, have been successfully co-doped into TiO2. The co-doping N,S into TiO2 has significantly decreased the Eg from 3.2 eV into 2.53–2.95 eV depending on the dopant loaded and thereby noticeably improves the photocatalytic removal of Pb(II) under visible light. Furthermore, the effectiveness of Pb(II) photooxidation is found to be considerably directed by the fraction of dopants, and the optimum mole fraction of the dopants are Ti:N:S = 1:1:1. Additionally, by applying 0.5 g/L of the TiO2-N,S (1:1:1) photocatalyst dose in 45 min of time and pH 7, the highest removal of 10 mg/L Pb(II) could be achieved that is approximately 98%. The removal of Pb(II) from the aqueous is due to the photocatalytic oxidation induced by OH radicals to form handleable PbO2.