Introduction

Dyes are used in some industries such as textiles, paper, and plastics. Numerous industries discharge a substantial amount of wastewater into the aquatic environment. As a result, the life of plants, humans, and animals are under threat (Chhabra et al. 2020; Picos-Benítez et al. 2020; Shfieizadeh et al. 2022). The annual production amount of dyes is estimated to be approximately 700,000 tons, 15% of which is wasted during industrial processes (Azzam et al. 2019; Singh et al. 2019; Samsami et al. 2020). The discharge of synthetic dyes into the environment may lead them to be very persistent and contributes to serious health problems (Brindha et al. 2022). Also, the presence of dyes in waterbodies would reduce light penetration and could be a major obstacle to photosynthesis (Yoon et al. 2012). Anthraquinone dyes such as Acid Blue 25 (AB25) are used in various industries. This chemical compound has complex structures as well as high molecular weight resulting in a time-consuming breakdown of its bonds (Daneshvar et al. 2014).

Dye removal from wastewater can be accomplished by using several methods, such as adsorption, electrocoagulation, and advanced oxidation processes (Ahmadian et al. 2022). Advanced oxidation processes (AOPs) have proven to be one of the most effective ways to remove a variety of organic contaminants from wastewater. (Babaei et al. 2021; Guan et al. 2021). Among AOPs, photocatalysis is considered to be a sustainable and greener method for degrading organic pollutants (Wang et al. 2020; Chen et al. 2021b; Nippes et al. 2022). Several types of semiconductors, such as zinc oxide (ZnO), are widely used in photocatalytic reactions due to their high stability and low cost (Sakarkar et al. 2020; Sidik et al. 2020; Moawed et al. 2022). One of the best ways to improve photocatalytic efficiency is to increase the interaction between pollutants and photocatalysts. The mass transfer of contaminants onto catalyst surfaces can be enhanced by using materials such as activated carbon.

Pollutant degradation by photocatalysis occurs when a pollutant reacts with oxidizing species, some examples of which are hydroxyl radicals (·OH), valence band holes (hv+), and superoxide radicals (O2) (Palominos et al. 2008; Daimon et al. 2008; Li et al. 2011). The contributions of each oxidizing species in photocatalytic degradation depend heavily on the nature of the contaminant (Makama et al. 2020). Consequently, the identification of the oxidant, which is responsible for oxidizing the pollutant, is vital. In order to maximize efficiency, it is essential to pinpoint the precise role played by each oxidizing species in photocatalysis and understand how pollutant degradation occurs.

There are many inorganic ions in natural water and wastewater, including Cl−, HCO3–, NO3−, SO4−, and F− as anions and K+, Na+, Ca2+, and Mg2+ as cations (Wang et al. 2012; Zelmanov and Semiat 2015). A considerable amount of inorganic salts can be found in textile industries' wastewater (Guillard et al. 2003; Chen et al. 2021a). The presence of inorganic ions in wastewater can affect the removal of contaminants (Santiago et al. 2014; Tang et al. 2018). As an example, Cl− is an inorganic ion frequently used in textile dyeing as a promoter (Huang et al. 2018; Chen et al. 2021a).

This work aims to evaluate the effect of reactive species (hydroxyl radical (·OH), holes (h+), electron (e−), and superoxide radical (O2)) and the impact of inorganic anions and cations on the photocatalytic degradation of AB25 in order to provide better insight of the mechanism. To the best of our knowledge, no research has been conducted to examine the effect of reactive species and inorganic ions on the photocatalytic degradation of AB25. The results of this work may provide helpful guidance for other photocatalytic applications.

Materials and methods

Materials

Mature bananas were purchased from a fruit store (Tehran, Iran). ZnO nanoparticles were bought from NAMAGO Company (Tehran, Iran). Ethylenediaminetetraacetic acid (EDTA, 99.995% purity), p-Benzoquinone (BQ, reagent grade ≥ 98%), sodium persulfate (Na2S2O8, ≥ 99%), potassium iodide (KI, ≥ 99%), potassium hydroxide (KOH, ≥ 85%), sodium bromate (NaBrO3, ≥ 99.5%), hydrogen peroxide (H2O2, 32%), sodium azide (NaN3, ≥ 99%), ethanol (≥ 99.9%), sodium sulfate (Na2SO4, ≥ 99%), potassium bicarbonate (KHCO3, 99.7%), sodium nitrate (NaNO3, ≥ 99%), sodium fluoride (NaF, ≥ 99.0%), sodium chloride (NaCl, ≥ 99.0%), hydrochloric acid (HCl, 37%), calcium sulfate (CaSO4)0.2H2O (≥ 99.0%), and aluminum sulfate (Al2SO4)3.18H2O (98.0%) were purchased from Merck.

Synthesis of BPAC, BPAC/ZnO nanocomposite

Tap and distilled water were used to wash the dirt and impurities of the banana peels. The banana peels were then chopped into small pieces and were dried in a laboratory oven at 100 °C for 48 h. Next, the banana peels were pulverized and then sieved through a laboratory sieve (200 µ). This powder was then carbonized at a temperature of 550 °C in a nitrogen gas muffle furnace for 1 h. KOH (45% aqueous solution) was used for activation, with a weight ratio of 1:4 of carbonized material to KOH. After 12 h of agitation on a magnetic stirrer, drying took place for 48 h in a laboratory oven at 120 °C. After drying, the sample was placed in a nitrogen gas muffle furnace at 750 °C for 1 h. Eventually, the BPAC was obtained after washing and neutralizing the pH (Mohammed and Chong 2014).

The physical mixing method was used to synthesize BPAC/ZnO nanocomposites. Different weight ratios (BPAC:ZnO) were used to synthesize this nanocomposite. A certain amount of BPAC was added to 100 ml of distilled water and stirred on a magnetic stirrer for 15 min. Furthermore, a certain amount of ZnO was added to 100 ml of distilled water and stirred for 15 min. Then, the ZnO solutions were added to the BPAC solution under continuous stirring and agitated for 2 h. Following this, the resulting solution was transferred to an ultrasonic bath for 2 h at 70 °C. Finally, the BPAC/ZnO nanocomposite was obtained after washing and drying with ethanol and distilled water (Alhan et al. 2019).

Characterization

The X-ray diffraction (XRD) was analyzed by PW1730, PHILLIPS, Netherlands. The microscopic morphology of the samples was analyzed by FESEM, MIRA III, TESCAN, Czech Republic. The specific surface area of the samples was calculated according to BET, BELSORP MINI II, BEL company, Japan. A Fourier transform infrared spectroscopy (FTIR) was obtained using the AVATAR model of Thermo company, USA.

Evaluation of the AB25 photocatalytic activity

Photocatalytic degradation of AB25 was carried out in a lab-scale reactor (10 cm diameter, 28 cm height, 5 mm thickness, two UVB lamps, and two aerator stones). The evaluation of activity was firstly studied by the optimization of effective parameters in different ranges of parameters. All the experiments were carried out for 150 min. Dark adsorption was conducted for 30 min to establish an adsorption–desorption equilibrium. To determine whether the pollutant concentration declines over time or not, 5 ml of the solution was taken and centrifuged at different time intervals. The concentration of AB25 was analyzed on a UV–Vis spectrophotometer at a wavelength of 602 nm. (Hach, DR4000). Experiments were carried out according to the OFAT method. Tests were performed three times with a maximum error rate of 5%. Also, to calculate the energy consumption of the system, Eq. 1 was used:

$$\frac{{{\text{Energy}}\;{\text{consumption}}}}{{{\text{Concentration}}}}\left( {\frac{{{\text{kWh}}}}{{{\text{kg}}}}} \right) = \frac{{\left( {{\text{W}}_{{{\text{aeration}}}} + {\text{W}}_{{{\text{lamps}}}} + {\text{W}}_{{{\text{fan}}}} } \right){\text{t}}}}{{{\text{V}}({\text{C}}_{0} - {\text{C}}_{{\text{t}}} )}}$$
(1)

Scavenging and inorganic ion investigations were accomplished under optimal conditions for effective parameters. The role of different reactive species and inorganic ions at different concentrations was investigated in the photocatalytic degradation of AB25.

Results and discussion

Characterization

Figure 1 displays the outcomes of the XRD analysis of the BPAC/ZnO nanocomposite. The aromatic zone in the graphitic structure of activated carbon is shown as a small peak in the XRD spectrum between 2θ = 20–30º. This peak is associated with (002) graphitic plane reflections of microcrystals that resemble graphite in activated carbon. This peak also shows that high porosity activated carbon has amorphous characteristics (Patel et al. 2021). The low BPAC peak was caused by the dominance of zinc oxide in the sample. Sharp diffraction peaks and no additional diffraction peaks were present, showing that the sample demonstrated high purity and crystallinity (Xu et al. 2022). Major ZnO peaks were detected at 2θ = 31.95º, 34.55º, 36.45º, 47.8º, 56.8º, 63.15º, 66.75º, 68.15º, 69.3º, 72.75º, 77.1º; which can be denoted as (100), (002), (101), (102), (110), (103), (200), (112), (201), (004), and (202) respectively.

Fig. 1
figure 1

XRD pattern of BPAC/ZnO

Full size image

FTIR analysis was utilized to identify and describe the chemical compounds and groups inside the samples’ structures. Figure 2 indicates an absorption peak relating to tensile vibrations of –OH at 3434.20 cm−1. Also, the 2924.05 cm−1 peak corresponds to C–H stretching. The wavenumbers of 1638.10 cm−1 and 1384.14 cm−1 were assigned to the C=C and C–H bonds. The wavenumber at 1071.70 cm−1 was ascribed to the C–O alcohol groups (Foroutan et al. 2022). Zn–O moiety’s stretching band was observed at 469.40 cm−1 in the BPAC/ZnO sample (Fatima et al. 2022).

Fig. 2
figure 2

Fourier transform infrared (FTIR) spectrum of the of BPAC and BPAC/ZnO

Full size image

The morphology of the prepared samples was characterized by Field Emission Scanning Electron Microscopy (FESEM), the results of which are presented in Fig. 3. Activated carbon contained a pore-like structure, which could provide a good place for the incorporation of the ZnO particles (Fig. 3a, b). Also, ZnO nanoparticles showed quite uniform particle size and shape, and the hexagonal structure of zinc oxide particles is quite clear (Fig. 3c, d). In Fig. 3e, f the successful formation of activated carbon and zinc oxide can be seen. The BPAC surface morphology was altered following the uniform loading of zinc oxide particles (Amir et al. 2022).

Fig. 3
figure 3

FESEM images of BPAC (a,b), ZnO (c,d), and BPAC/ZnO (e,f)

Full size image

Nitrogen adsorption/desorption isotherms of BPAC and BPAC/ZnO were measured to examine the BET surface area and pore size distribution (Table 1). The specific surface area and total pore volume of the BPAC (712.28 m2 g−1 and 0.3351 cm3 g−1, respectively) were higher than those of BPAC/ZnO nanocomposite (321.1 m2 g−1 and 0.1168 cm3 g−1, respectively). A high surface area of BPAC provides active sites that facilitate the diffusion of polluting products and reaction inside or outside the BPAC/ZnO photocatalyst, leading to an increase in the photocatalytic activity (Leichtweis et al. 2020). Moreover, the mean pore diameters of BPAC and BPAC/ZnO were obtained 2.089 nm and 3.946 nm, respectively. The loading of ZnO nanoparticles on the BPAC surface lowered the total pore volume of the BPAC/ZnO nanocomposite while increasing the mean pore diameter. This is because the BPAC pores were occupied by the ZnO nanoparticles (Table 2).

Table 1 Physical properties of Banana peel activated carbon and BPAC/ZnO nanocomposite
Full size table
Table 2 Results of the optimization of the effective parameters on photocatalytic degradation of AB25
Full size table

Optimization of effective parameters

To achieve the best possible results, the mixing ratio of the synthesized nanocomposite (BPAC:ZnO) was changed, and the removal efficiency was evaluated. A 150 min experiment was conducted. Dark adsorption was conducted for 30 min to establish an adsorption–desorption equilibrium. According to the results, when a higher proportion of BPAC was included in the nanocomposite, the adsorption capacity was improved. The results can be correlated to the surface area of the nanocomposite. In order to determine the optimal mixing ratio, a study of energy consumption was also conducted. Results showed that the 3:7 mixing ratio demonstrated the highest removal efficiency and the lowest energy consumption; therefore, it was selected as the optimum mixing ratio.

The initial pH value strongly influences the photocatalytic degradation efficiency and surface charge of the nanocomposite. Based on the results, higher removal efficiency occurs at lower pHs on account of the anionic structure of AB25 dye and the positive surface charge of the nanocomposite at a pH lower than pHpzc of the nanocomposite, which was 6.8. The dye molecules adhered to the nanocomposite surface due to the electrostatic attractions of the nanocomposite surface and the AB25 molecules. Furthermore, hydroxyl radicals are more capable of oxidizing organic pollutants at low pH values compared to basic pH levels (Munagapati et al. 2018). Eventually, energy consumption calculations were carried out, and pH = 5 was determined as the optimal pH.

Generally, the dye removal efficiency decreases as the dye concentration rises. The nanocomposite active sites, consequently, were saturated with contaminants, reducing the efficiency of photocatalytic degradation. In the presence of high concentrations of AB25, fewer dye molecules made contact with the photocatalyst surface, resulting in lower photodegradation (Suresh and Sivasamy 2020). Furthermore, when dye concentration is high, the light is absorbed by the dye molecules before it reaches the nanocomposite surface. As a result, the generation of hydroxyl radicals is reduced. Finally, based on the calculations for energy consumption, 150 mg L−1 was selected as the optimum dye concentration.

The removal of dye is increased by increasing nanocomposite concentration. Based on the results of the study, the removal efficiency improved from 77 to 93% by increasing the nanocomposite from 0.2 g L−1 to 0.7 g L−1. This can be associated with the small amount of nanocomposite, which is inadequate to generate enough reactive species such as superoxide and hydroxyl radicals to reach a sufficient removal efficiency. The number of photocatalytic particles in the reactor increases when a high amount of the nanocomposite is added to the system (Suresh and Sivasamy 2020). Consequently, the active adsorption sites on the surface are multiplied, enhancing superoxide and hydroxyl radical generation and facilitating better photocatalytic degradation. Therefore, 0.6 g L−1 of nanocomposite was selected as the optimal dosage.

Dissolved oxygen plays a crucial role in the degradation of organic materials. The electron–hole recombination in photocatalytic processes is essential for energy waste. Oxygen molecules can function as electron acceptors and separate electrons from holes, reducing the likelihood of electron–hole recombination. Therefore, electron acceptors facilitate the separation of photogenerated electrons and holes in the photocatalytic process, leading to better photocatalytic performance (Thor et al. 2020). In fact, superoxide radicals are generated in the system and provide enough oxygen for photo-electron evacuation. Finally, the 5 L min−1 aeration rate showed the lowest energy consumption, and it was selected as the optimal value.

Photocatalytic degradation of contaminants is greatly influenced by UV light intensity. According to the results, the removal rates of AB25 were directly related to the intensity of UV light. As the light intensity increased, the number of photons increased, resulting in great light absorption by the photocatalyst surface (Mirmasoomi et al. 2017). Eventually, the 16W UV intensity was determined as the optimum value.

Scavenger experiments

The influence of different concentrations of several scavengers on the photocatalytic degradation of AB25 under the optimal condition of the effective parameters was accomplished. Potassium iodide (KI) and sodium azide (NaN3) as hydroxyl radical scavengers (·OH), ethanol and EDTA as photogenerated holes (h+), H2O2, Na2S2O8, and NaBrO3 as electron scavengers (e−), and p-BQ as a superoxide radical scavenger (O2) were used in this work. The concentrations of the scavengers used in this study were 3, 10, and 30 mM. The results of scavenger tests are presented in the figures below.

It is evident that with 1, 3, and 10 mM of EDTA, the photocatalytic removal efficiency dwindled to 80%, 65%, and 59%, respectively. Also, it decreased to 84%, 73%, and 68% when ethanol was added to the solution. The results are presented in Fig. 4. In the presence of EDTA and ethanol, the removal efficiency declined, indicating that the valence band holes which produce hydroxyl radicals have an important role in the degradation of AB25. The presence of EDTA and ethanol in the system, as a result, decreased the holes and the ·OH radicals, which eventually translated into a diminution in removal efficiency.

Fig. 4
figure 4

Changes in AB25 removal efficiency in the existence of ethanol (a) and EDTA (b)

Full size image

Then, the dye removal efficiency was decreased by adding 3, 10, and 30 mM NaN3 and KI as hydroxyl radical scavengers. It was decreased to 92%, 90%, and 85% when different concentrations of NaN3 were introduced to the solution. The addition of KI, furthermore, led to depletion in removal efficiency, which was 91%, 89%, and 82% for 3, 10, and 30 mM, respectively. The results are presented in Fig. 5. KI contains an (I−) ion that reacts with hydroxyl radicals and holes (Eq. 2–6), which causes a decrease in the removal efficiency (Li et al. 2011). In light of this, ·OH radicals played a supplementary role in the photocatalytic degradation process. Also, the decrease in AB25 removal efficiency due to the presence of NaN3 could be linked to the reaction of NaN3 with singlet oxygen and hydroxyl radical.

$$I^{ - } + {}_{{}}^{ \bullet } OH_{ads} \to I^{ \bullet } + OH_{ads}^{ - }$$
(2)
$$I^{ - } \to I^{ \bullet } + e^{ - }$$
(3)
$${e}^{-}+ {h}_{VB}^{+}\to {Heat}$$
(4)
$$I^{ \bullet } + I^{ - } \to I_{2}^{ - \bullet }$$
(5)
$$I_{2}^{ - \bullet } \rightleftharpoons I_{2} + e^{ - }$$
(6)
Fig. 5
figure 5

Effect of different concentrations of KI (a) and NaN3 (b) on the photocatalytic degradation of AB25

Full size image

According to the results, after adding 2 mM of Na2S2O8, H2O2, and NaBrO3 to the photocatalytic reactor, the removal efficiency improved to 95%, 96%, and 96%, respectively. While by adding 10 mM of Na2S2O8 and NaBrO3, the removal efficiency increased to 96% and 99%, the removal efficiency dwindled to 86% when 10 mM of H2O2 was added. Likewise, by adding 30 mM of Na2S2O8 and NaBrO3, the removal efficiency improved to 98% and 99%, whereas the removal efficiency reduced to 78% in the presence of 30 mM of H2O2. The results are presented in Fig. 6. Due to Na2S2O8 ability to create sulfate radicals (SO4·−), using Na2S2O8 increased the efficiency of the process (Eq. 7 and 8.) (Govindan et al. 2017).

$$S_{2} O_{8}^{2 - } + e^{ - }_{CB} \to SO_{4}^{ \bullet - } + SO_{4}^{2 - }$$
(7)
$$SO_{4}^{{ \bullet - }} + Dye \to Degraded\;products + CO_{2} + H_{2} O$$
(8)
Fig. 6
figure 6

Changes in removal efficiency of AB25 in the presence of H2O2 (a), Na2S2O8 (b), and NaBrO3 (c)

Full size image

Also, the electron and the bromate ion reaction reduced the electron–hole recombination, improving the removal efficiency (Eq. 9.) (Makama et al. 2020).

$$Br{{O}^{-}}_{3}+{{e}^{-}}_{CB}+6{H}^{+}\to B{r}^{-}+3{H}_{2}O$$
(9)

This increase in removal efficiency indicates the positive effect of bromate ions on dye removal by trapping electrons. The presence of H2O2 in the process could increase the efficiency, but in this study, it reduced the removal efficiency of AB25 due to its possible reaction with the hydroxyl radicals (Eq. 10 and 11) (Govindan et al. 2017).

$$H_{2} O_{2} + h_{VB}^{ + } \to HO_{2}^{ \bullet } + H^{ + }$$
(10)
$$HO_{2}^{ \bullet } + HO_{{}}^{ \bullet } \to H_{2} O + O_{2}$$
(11)

In this reaction, a hydroperoxyl radical was produced, which consumed the hydroxyl radical (Monteagudo et al. 2020). Therefore, it can reduce removal efficiency. If the concentration of H2O2 exceeds a certain level, it could lead to a diminution in removal efficiency. In this study, the concentration of 10 and 30 mM H2O2 caused a reduction in efficiency from 94 to 86% and 78%. The results are presented in Fig. 6a.

Eventually, the effect of p-BQ was investigated. In the presence of photogenerated electrons, oxygen molecules are reduced to a superoxide anion (O2·). p-BQ was capable of capturing O2 radicals (Silvestri et al. 2019). It can consequently decrease degradation rates, showing the existence of O2 radicals in the reactor (Eq. 12.) (Palominos et al. 2008). Based on the results, dye removal efficiency reduced from 94 to 68% and 49% in the presence of 2 and 10 mM of p-BQ in the solution. The results are presented in Fig. 7.

$$BQ + O_{2}^{ \bullet - } \to BQ^{ \bullet - } + O_{2}$$
(12)
Fig. 7
figure 7

Impact of various concentrations of BQ on the removal efficiency of AB25

Full size image

According to the above results, it is worth mentioning that superoxide radicals and photo-generated holes have a decisive role in the photocatalytic degradation of AB25. When p-BQ was used as superoxide radical scavengers, the photocatalytic degradation of AB25 was significantly inhibited by O2 radicals. Moreover, photo-generated holes can produce ·OH radicals, and the production of ·OH radicals was reduced in the presence of EDTA and ethanol, which led to a diminution in AB25's removal efficiency. In addition, electron acceptors, two examples of which are Na2S2O8 and NaBrO3, accelerated the degradation of AB25. As a result, SO4− and BrO3− oxidants are capable of decomposing AB25. Also, the presence of H2O2 in the solution caused a reduction in the photocatalytic performance. Furthermore, the results of the study revealed that KI and NaN3 had a low impact on AB25 degradation. Therefore, it is worth noting that in the photocatalytic degradation of AB25, hydroxyl radicals played a supplementary role.

Effect of inorganic ions

Effect of anions

Salts are usually found in aqueous media, and a number of researchers claim that the inorganic salts in water play an important role in the photocatalytic degradation process of organic compounds. In order to investigate the anion effect, the impact of several anions (Cl−, HCO3–, NO3−, and F−) on the photocatalytic degradation of AB25 was investigated.

Cl− inhibited the photocatalytic degradation of AB25, and the inhibitory effect improved by increasing Cl− concentration. Regarding the pHpzc of BPAC/ZnO, the catalyst's surface was positively charged when the pH of the solution was lower than 6.8. Consequently, anions such as Cl− could be adsorbed on the surface of the nanocomposite and block the active sites, which results in the reduction of the removal efficiency. Moreover, I− in the chloride is an excellent scavenger for valence band holes. Therefore, Cl− could also react with the valence band hole and compete with AB25 for the holes, decreasing the photocatalytic degradation rate. The effect of chloride ions is given by (Eq. 13–16) (Santiago et al. 2014). During the degradation of organic compounds, chloride anions act as effective inhibitors. In this study, the removal efficiency dwindled from 94 to 93%, 90%, and 85% by adding 2, 10, and 30 mM NaCl to the solution, respectively (Fig. 8a). In order to confirm the adsorption of Cl− onto the nanocomposite surface, once the process completed, FT-IR analysis was conducted, and the results are presented in Fig. 10b.

$$Cl^{ - } + h^{ + } \to Cl^{ \bullet }$$
(13)
$$Cl^{ - } + {}^{ \bullet }OH \to ClOH^{ \bullet - }$$
(14)
$$ClOH^{ \bullet - } + H^{ + } \to Cl^{ \bullet } + H_{2} O$$
(15)
$$Cl^{ \bullet } + Cl^{ - } \to Cl_{2}^{ \bullet - }$$
(16)
Fig. 8
figure 8

Changes in removal efficiency of AB25 in the existence of different concentrations of inorganic anions

Full size image

Fluoride also would inhibit the production of ·OH radicals by covering the nanocomposite surface. As a result, when F− was present in the solution, the degradation was predominantly caused by ·OH radicals. Participation of holes and surface hydroxyl radicals in photocatalytic degradation could occur in the absence of F− (Dugandžić et al. 2017). As can be seen in Fig. 8b, based on the differences between the reaction rates in the absence and presence of F− anions, it should be noted that holes and surface hydroxyl radicals are involved in the photocatalytic degradation of AB25. In this study, the removal efficiency decreased from 94 to 93%, 84%, and 80% by adding 2, 10, and 30 mM NaF to the AB25 solution, respectively (Fig. 8b).

The inhibition effect of NO3− was much greater than that of Cl−. In the presence of low concentrations of nitrate anions (below 0.1 mM), the degradation of pollutants can be improved (Dugandžić et al. 2017). NO3− ions could be adsorbed on the nanocomposite surface and compete for the active sites when excessive amounts of this ion were available in the solution. It was observed that the removal efficiency was inhibited at higher concentrations of NO3− in this study. The inhibition was much more significant than chloride anions on account of the larger size of NO3− anions, which led to a more effective blocking of the nanocomposite surface. The photocatalytic degradation efficiency was reduced from 94 to 89%, 82%, and 75% by adding 2, 10, and 30 mM NaNO3− to the solution, respectively (Fig. 8c). Also, nitrate anion could act as radical scavengers by reacting with holes and hydroxyl radicals (Eq. 17 and 18) (Dugandžić et al. 2017). Also, to confirm the adsorption of NO3− onto the BPAC/ZnO surface, FT-IR analysis was carried out after the completion of the process, the result of which is presented in Fig. 10d. Reduction at bands 3400 cm−1 and 1600 cm−1 showed that NO3− was adsorbed onto the surface of BPAC/ZnO nanocomposite.

$$NO_{3}^{ - } + h^{ + } \to NO_{3}^{ \bullet }$$
(17)
$$NO_{3}^{ - } + {}_{{}}^{ \bullet } OH \to NO_{3}^{ \bullet } + OH^{ - }$$
(18)

When HCO3− reacted with −OH, carbonate radicals, which are weak oxidizing reagents and rarely react with other organic molecules, were formed (Santiago et al. 2014). The photocatalytic degradation of AB25 was mainly due to O2·−, not due to OH. Thus, HCO3– had an inconsiderable inhibitory effect on degradation. The removal efficiency decreased from 94 to 92%, 88%, and 82% by adding 2, 10, and 30 mM KHCO3− to the solution, respectively (Fig. 8d). It has been reported that bicarbonate ions can scavenge h + and other radicals based on the reactions given below (Santiago et al. 2014).

$$HCO_{3}^{ - } + h^{ + } \to HCO_{3}^{ \bullet - }$$
(19)
$$HCO_{3}^{ - } + {}_{{}}^{ \bullet } OH \to CO_{3}^{ \bullet - } + OH^{ - }$$
(20)

Finally, inorganic additives inhibit the photocatalytic removal of AB25 in the following order:

$$NO_{3} ^{ - } \ge F^{ - } \ge HCO_{3} ^{ - } \ge Cl^{ - }$$

Effect of cations

Wastewaters contain numerous cations of different metals. These ions, including K+, Na+, Ca2+, and Mg2+, can affect photocatalytic degradation (Dugandžić et al. 2017). In this study, the photocatalytic degradation of AB25 was studied in the presence of different concentrations of Ca+ and Al3+ cations. These two salts exhibited an inhibition effect on the AB25 photocatalytic degradation. In this study, the removal efficiency was reduced from 94 to 87%, 82%, and 68% by adding 2, 10, and 30 mM (Al2SO4)3 to the AB25 solution, respectively (Fig. 9). Also, the removal efficiency dwindled from 94 to 90%, 85%, and 83% by adding 2, 10, and 30 mM (CaSO4) to the AB25 solution, respectively. AB25 was strongly inhibited by aluminum ions in photocatalytic degradation, resulting from Al3+ adsorption. Calcium ions had a similar impact on the photocatalytic degradation rate to Al3+. Although the nanocomposite surface and Al3+ were charged positively when the pH value was below the pHpzc of the nanocomposite, Al3+ adsorption occurred. Also, after the completion of the process, FT-IR analysis was carried out to confirm the adsorption of Al3+ onto the BPAC/ZnO surface. The results are presented in Fig. 10b. The FTIR spectra ensured a reduction at bands 3400 cm−1 and 1600 cm−1 when Al3+ was adsorbed on the nanocomposite.

Fig. 9
figure 9

Effect of different concentrations of (CaSO4) and (Al2SO4)3 on the photocatalytic degradation of AB25

Full size image
Fig. 10
figure 10

FT-IR spectrum of BPAC/ZnO nanocomposite (a), changes in FT-IR spectrum after adsorption of Cl− (b), Al3+ (c), and NO3− (d)

Full size image

Photocatalytic mechanism

The mechanism of AB25 photocatalytic degradation by BPAC/ZnO nanocomposite is illustrated in Fig. 11. BPAC’s pore structure caused the adsorption of AB25 molecules. Because of this, there was a greater opportunity for photocatalysts to interact with pollutants. The generated e− and h+ experienced a transition to the photocatalyst surface during the photocatalytic process. When an electron traveled to the photocatalyst surface, O2 was converted to O2·−. The hole also oxidized H2O into OH· when moved to the catalyst surface. It is important to note that electron–hole recombination on the photocatalyst surface reduced the removal efficiency. The photocatalytic process reached a higher efficiency when electron–hole recombination was prevented. Considering that BPAC acts as an electron acceptor in the photocatalytic system, it inhibits electron–hole recombination. The electrons could have a reaction with O2, leading to the production of many oxidizing radicals like O2·− and OH· (Lu et al. 2019). Eventually, final products such as CO2 and H2O were produced because of the disintegration of AB25 molecules into smaller products.

Fig. 11
figure 11

Photocatalytic mechanism of AB 25 by BPAC/ZnO nanocomposite

Full size image

UV–visible spectra changes

Figure 12 shows the absorption spectral changes when the synthesized nanocomposite degrades the AB25 aqueous solution. As shown in Fig. 12, wavelengths at 305 nm, 394 nm, and 602 nm were allotted to AB25 maximum wavelengths. Peaks at 390 nm and 305 nm were attributed to carbonyl and benzene, respectively. The main peak at 602 nm was associated with the anthraquinone chromophoric group. Figure 12 indicates the diminution of AB25 wavelengths, informing that the benzene rings were broken up on account of photocatalytic degradation. The AB25 anthraquinone structure, moreover, was broken down because of the O2·−, h+, and OH· produced in the system.

Fig. 12
figure 12

Absorption spectral changes of AB25 aqueous solution during the photocatalytic process

Full size image

Conclusion

The photocatalytic decomposition of Acid Blue 25 by BPAC/ZnO nanocomposite was investigated. The findings indicated an elimination efficacy of 94% after 150 min under optimally effective conditions. Under the optimal conditions of effective parameters, the impact of several reactive species on photocatalytic degradation of AB25 was examined. The results showed that h+ and O2·− played a major role, while hydroxyl radicals played an auxiliary role in the photocatalytic degradation of AB25. The presence of electron scavengers such as Na2S2O8 and NaBrO3 increased the degradation rate. Also, the impact of inorganic anions and cations on the photocatalytic degradation of AB25 was examined. Among the anions, NO3− exhibited the greatest inhibition ability. Inorganic anions inhibited the removal efficiency of AB25 in the following order\(NO_{3}^{\_} \ge F^{\_} \ge HCO_{3}^{-} \ge Cl^{\_}\). Similarly, Al3+ showed the highest inhibition capacity among other cations.