Toxic heavy metals and organic pollutants simultaneously exist in the wastewater of some industries. This study explores reduction of toxic divalent nickel ions, from either nitrate or sulfate salts, coupled with naphthalene (NA) degradation using titania photocatalyst in an efficient photo-sono reactor. A synergism appears when reduction and degradation treatments occur simultaneously in the media. With initial concentrations of [Ni(II)]0 = 5 mg/L and [NA]0 = 10 mg/L, under dominant mild conditions, removal efficiencies of 54.5% and 56.6% were obtained for Ni(II) and NA, respectively, when nickel nitrate was used. These efficiencies were enhanced to 59.2% and 57.5%, respectively, with nickel sulfate, all after 90 min operation. For evaluating the mechanism of reactions, reactive oxygen species analysis on solutions as well as Fourier transform infrared, scanning electron microscopy and Brunauer-Emmett-Teller analyses on the titania nanoparticles, before and after usage, was performed. The reaction kinetics was also followed for individual species in the mixed solution and, accordingly, the energy consumption was evaluated for one order of magnitude decrease in pollutant concentration. The high performance of the used method was revealed in comparison to the similar reported reduction/degradation processes.

Pollution of water and wastewaters by toxic heavy metals and organic compounds has been a major concern during recent decades. Among heavy metals, nickel is one of the potentially toxic metals, which has many applications in important industries such as stainless steel, nickel alloy, storage battery and electroplating (Joshi et al. 2011). Although Ni(II) has been identified as a component for participating in important metabolic reactions such as ureolysis, acidogenesis and methane biogenesis (Akhtar et al. 2004), but human contact with excessive amount of this ion can cause problems from skin irritation to lung damage and mal effects on the nervous system and mucous membranes (Caicedo 2008). In aquatic environments, nickel appears as Ni(II) and zero-valent Ni0; however, the potential toxicity for Ni(II) is reported as high whereas Ni0 is only slightly toxic (Chen & Ray 2001). The World Health Organization (WHO) recommends maximum concentration of nickel not to exceed 0.5 mg/L in waters (WHO 2008). Thus, reduction of Ni(II) to less harmful species would be very beneficial. Further, wastewaters from industries like electroplating usually contain organic compounds, which are used in different stages of manufacturing processes as balms, ductile material, polisher and so on. Naphthalene (NA), for instance, is widely used in different metal industries as brightener (Doan & Saidi 2008).

Several methods have been reported to treat wastewaters containing heavy metals and other pollutants. Ion exchange, membrane separation, adsorption, coagulation, electrolysis, and chemical precipitation are noteworthy. At the same time, it has been pointed out that these methods are coincident with some disadvantages including secondary pollution, large quantities of chemical reagents, high cost, poor treatment efficiency at low pollutant concentration and high energy consumption (Akhtar et al. 2004; Alqadami et al. 2017; Naushad et al. 2017).

A promising alternative is using advanced oxidation processes (AOPs) for pollutants removal due to high efficiency, wide adaptability and potential for simultaneous treatment of different pollutants (Saravanan et al. 2018). Among AOPs, heterogeneous photocatalysis by using semiconductors has been demonstrated with great success for the reduction of toxic metals as well as oxidation of organic compounds (Wang et al. 2004).

Upon irradiation of the semiconductor–solution interface with light energies greater than the semiconductor band gap, electron–hole pairs are formed in the photocatalyst conduction and valence bands, respectively (Barakat 2004). The electron–hole pairs are then separated and the adsorbed species on the sites of the catalyst undergo photo-induced redox reactions. For this aim, titania has been known as a very efficient photocatalyst since its band gap is around 3.2 eV with conduction band (CB) energy of −0.3 eV and valence band of +2.9 eV at pH 5.6 (Blake et al. 1991). Thus, any metal ion with a reduction potential less negative than −0.3 eV will be potentially reduced by photo-generated electrons of TiO2. Also, the photocatalytic process, under aerobic conditions, can generate several reactive oxygen species (ROS) including superoxide anion radical hydroxyl and hydrogen peroxide through reactions with dissolved oxygen (DO) (Nosaka & Nosaka 2017). The ROS contribute in the oxidation and reduction reactions.

A variety of methods for simultaneous photocatalytic reduction of heavy metals and degradation of organic compounds have been proposed. For instance, the photo-reduction of Cu(II) and Se(IV) in the presence of formic acid and ethylene diamine tetra-acetic acid (EDTA) was investigated by Aman et al. (2011). Simultaneous Cr(VI) reduction and NA degradation in aqueous solutions by UV/TiO2 has been investigated in another work by Gutierrez et al. (2008). A synergic effect between Cr(VI) reduction and NA degradation by UV/TiO2 process was reported. In our recent study, simultaneous photocatalytic reduction of Cr(VI) and Ni(II) ions, coupled with degradation of sodium dodecyl benzene sulfonate, was reported (Saien & Azizi 2015). Enhanced photocatalytic treatment of pollutants is of interest due to a low dosage of titania nanoparticles and mild operating conditions.

Here, an attempt was made for the first time, for the photocatalytic reduction of nickel ions originating from nitrate or sulfate salts and in the presence of NA. The nitrate and sulfate salts are often present in the wastewaters of some industries like petroleum refining, electroplating, petrochemical, dyeing, textile and steel manufacturing (Ashour et al. 2008; Besharat 2010). NA, on the other hand, is extensively used as brightener and ductile agent in the nickel electroplating industries (Doan & Saidi 2008; Besharat 2010). Experiments were conducted in an efficient photo-sono reactor working with UV irradiation. For practical applications, the process performance was evaluated in terms of reaction kinetics, energy consumption and a specified process efficiency.

### Chemicals

All the used chemicals, including nickel nitrate hexa-aqua (98.5%), nickel sulfate hexa-aqua (99%), 1-(2-pyridylazo)-2-naphthol (PAN) (>99%), ethanol (99.9%) and Triton X-100, naphthalene (99%), sulfuric acid (98%), sodium nitrate (98.9%), sodium sulfate (99%), potassium iodine (99.8%), sodium acetate (99%), ammonium dimolybdate (>99%) and sodium hydroxide (>97%), were purchased from Merck. Titanium dioxide nanosize particles, P-25 (>99.5%), was an Evonik product (anatase to rutile weight ratio of about 80/20) with Brunauer-Emmett-Teller (BET) surface area of 50 ± 15 m2/g and average particle diameter of 21 nm. All aqueous solutions were prepared using deionized water with conductivity of less than 0.08 μS/cm.

### Photo reactor and experimental procedure

A 1.25 L cylindrical photo-sono reactor made of glossy stainless steel with 90 mm diameter and 200 mm height was used for the experiments. The light source was a 250 W mercury lamp (165 mm body length and 80 mm arc length) with wavelength range of 280–400 nm and the maximum emission of 365 nm. The lamp was installed centrally and immersed in the solutions. This configuration allows a homogeneous irradiation and perfect reflection for the beams contacting the reactor wall. At the beginning of each experiment, the ultrasound source (28 kHz, 60 W), located outside the bottom of the reactor, was working for 5 min, propagating waves in the reaction media in order to break up any particle cluster and providing homogeneous TiO2 nanoparticle suspensions. During experiments, temperature was maintained constant by means of a stainless steel water-flow jacket from a thermostat bath.

To run experiments, a solution (1 L) containing the specified concentration of a Ni(II) salt and NA was prepared and was transferred into the reactor after pH adjustment by using either sulfuric acid or sodium hydroxide dilute solutions. Temperature was then set to the desired value. An amount of the catalyst particles (100 mg/L) was added, and prior to light irradiation, the suspension was sonicated and then mixed for 30 min in the dark to ensure adsorption–desorption equilibria of the pollutants on TiO2 particles. All the experiments were performed while the content was continuously mixed with a simple agitator. Initial pollutant concentration in the mixed solutions was 5 mg/L of Ni(II) and 10 mg/L of NA (unless mentioned otherwise). The selection of nickel and NA initial concentration was with respect to the feasible final concentration at the end of treatments being close to the permissible limit for discharge into surface water: 2 and 0.06 mg/L for Ni(II) and NA, respectively (WHO 2008). According to a report, the NA concentration frequently found in electroplating, dyeing and textile wastewaters is within 0.1–300 mg/L (Nesterenko-Malkovskaya et al. 2012).

To follow the reaction progress, 2 mL samples (at least two samples) were withdrawn each time, and after vigorous centrifuging to separate nanoparticles, were divided into 1 mL samples for analyzing Ni(II) and naphthalene content.

### Analytical method

The concentration of Ni(II) and NA was determined from their absorption in the spectra obtained by a UV-vis spectrophotometer (Jasco, V-630, Japan). For Ni(II) ion, colorimetry at λmax = 568 nm was performed using PAN color reagent as a standard method (Clesceri et al. 1998). Accordingly, the best conditions for the formation of Ni(II) complex were obtained by adding 2.5 mL of 1.0% Triton X-100 in water solution, 2 mL of buffer solution (pH 8.5), 1.0 mL of 0.01% PAN solution in ethanol, together with 1 mL of collected sample, all added into a 10 mL standard flask and making up to the marked level with deionized water. The NA analysis was performed from the change in the absorbance at its maximum wavelength of 275 nm (Mahmoodi & Sargolzaei 2014). Using these analysis procedures, the removal efficiency (RE) at any time was obtained from:
(1)
where and denote initial and an appropriate time concentrations of a pollutant species, Ni(II) or NA.

As a representative of DO, the amount of H2O2 present was measured by colorimetry method with iodide. Briefly, 0.2 mL of diluted solution was mixed with 1.6 mL deionized water, 0.1 mL 1 M potassium iodine, and 0.1 mL 1 M sodium acetate buffer containing a few drops of ammonium dimolybdate as catalyst for the oxidation of by H2O2 to . The absorbance of solutions was measured by the UV-vis spectrophotometer at λmax = 360 nm (Diesen & Jonsson 2014).

The separated TiO2 particles, after process utilization, were washed with deionized water and then were dried at room temperature in a dark chamber. These particles as well as pure TiO2 particles (before usage) were characterized in different ways.

### Reduction of Ni(II) ions

In preliminary experiments, the reduction of sole Ni(II) ions, originating from nitrate or sulfate salts, within concentration of 5–20 mg/L and with the conventional pH range of 7.5–9.5 (Saien et al. 2014; Saien & Azizi 2015), was examined. Titania photocatalyst concentration was fixed at 100 mg/L and temperature (T) was set within 20 to 40 °C. Under conditions of pH = 9.5 and T= 35 °C, removal efficiencies of 71.4% and 77.2% were achieved with 5 mg/L initial concentration of Ni(II), originating from nitrate and sulfate salts, respectively (Figure 1).

Figure 1

Effect of the salt source on the removal efficiency of Ni(II) after 90 min; [Ni(II)]0 = 5 mg/L, pH = 9.5 and T = 35 °C.

Figure 1

Effect of the salt source on the removal efficiency of Ni(II) after 90 min; [Ni(II)]0 = 5 mg/L, pH = 9.5 and T = 35 °C.

Close modal
Counter-anions can play a role of scavenging photo-generated hydroxyl radicals and the photo-generated holes and consequently enhance the photocatalytic reduction of metal ions, according to the following reactions (Burns et al. 1999):
(2)
(3)
(4)
(5)
or alternatively:
(6)
(7)

Thus, the capture of holes by counter-anions causes the photo-promoted electrons on the CB to become more readily available for nickel reduction, inhibiting electron–hole recombination. The interaction between counter anions and titania particles leads to the easier adsorption of nickel cations and therefore more ion reduction. It is noteworthy that the influence of ion strength on the electrostatic interaction between ions and the catalyst surface follows the charge-density order, presented in the Hofmeister series. Since anion has a higher ionic strength compared to , higher ion reduction will correspond to nickel sulfate. In a previous study on the photocatalytic treatment of metal ions and organic pollution by sulfuric acid-modified titania particles, it was reported that sulfate ions improve the photocatalytic activity (Samantaray et al. 2003). Meanwhile, ions from nitrate salt may convert to nitrite ions, , due to photolysis, and in turn generate H2O2 molecules, which is a disturbing factor in the reduction of Ni(II) ions (Tzou et al. 2008).

Figure 2(a) shows significant enhancement in the nickel RE with pH variation from 7.5 to 9.5. It is well known that TiO2 surface is negatively charged at pHs more than pHPZC (point of zero charge) which is within 6.7–7.5 for P-25 TiO2 (Wang et al. 2004). Increase in the solution pH causes the TiO2 surface to find negative charge, leading to higher Ni(II) ion adsorption. Thus, the nickel reduction increased from 23.6 to 69.8% for nickel sulfate and from 17.6 to 62.1% for nickel nitrate when pH was increased from 7.5 to 9.5, respectively. In addition, Ni(OH)2 formation and precipitation occurs under high alkaline pHs (Lin & Rajeshwar 1997; Shirzad-Siboni et al. 2012; Gutha et al. 2015). Accordingly, for treatments with just the photocatalytic process, the pH of solutions was limited to maximum 9.5.

Figure 2

Removal efficiency of Ni(II), after 90 min, versus temperature with [Ni(II)]0 = 5 mg/L (a) and versus nickel initial concentration under the same conditions and T = 35 °C (b).

Figure 2

Removal efficiency of Ni(II), after 90 min, versus temperature with [Ni(II)]0 = 5 mg/L (a) and versus nickel initial concentration under the same conditions and T = 35 °C (b).

Close modal

Figure 2(a) also demonstrates that temperature enhances the nickel RE; however, amounts more than 35 °C, do not provide much variation, which may be due to the low level activation energy in the photocatalytic reactions (Saien & Azizi 2015).

The influence of nickel initial concentrations is shown in Figure 2(b). Obviously, RE decreases with nickel concentration. This is because the catalyst surface becomes highly covered at high nickel concentrations and the light reaching the catalyst surface is diminished. Therefore, electron–hole formation becomes severe.

Moreover, kinetic studies revealed a pseudo-first-order kinetics for photocatalytic reduction of Ni(II) under conditions of [Ni(II)]0 = 5 mg/L, pH = 9.5 and T= 35 °C, for both used salts. The rate constants were obtained as 1.81 × 10−3 and 1.63 × 10−3 1/min for sulfate and nitrate salts, respectively.

### Simultaneous reduction of Ni(II) and degradation of NA

In this step, preliminary experiments on adsorption of Ni(II) and NA on TiO2 particles (100 mg/L) were performed in darkness. Adsorption of Ni(II) ions, originating from both the salts, on TiO2 nanoparticles was measured in the presence and absence of NA (10 mg/L). The percentage of adsorption after a typical 60 min time is shown in Figure 3. The surface charge of TiO2 particles in the used solution is slightly negative under natural solution pH of 7.5, which favors the electrostatic attraction between the opposite charged TiO2 particles and Ni(II) cations. This adsorption is decreased in the presence of NA due to competition between Ni(II) and NA species. NA is rather a neutral compound and its adsorption is favored around neutral pHs (Lair et al. 2008). On the other hand, NA adsorption is enhanced in the presence of Ni(II), so that 13.2% NA adsorption increases to 15.4% when accompanied by either of the nickel salts. The ‘salting-out’ effect can be the major reason for this observation in the mixture (Lair et al. 2008). An increase in the ionic strength due to the presence of nickel salts causes a decrease in the solubility of NA and the resulting adsorption of hydrophobic NA species on the surface of TiO2 particles.

Figure 3

Ni(II) and NA adsorption on TiO2 particles in darkness for different systems after 60 min; [Ni(II)]0 = 5 mg/L, [NA]0 = 10 mg/L, pH = 7.5 and T= 25 °C.

Figure 3

Ni(II) and NA adsorption on TiO2 particles in darkness for different systems after 60 min; [Ni(II)]0 = 5 mg/L, [NA]0 = 10 mg/L, pH = 7.5 and T= 25 °C.

Close modal

In the photocatalytic process with light emission, Figure 4 shows that, in contrast with darkness, the RE for Ni(II) reduction, from either of the salts, is enhanced upon coupling with NA degradation, and reaches 52.7% and 47.8% at pH 7.5. REs for Ni(II) reduction, in the absence of NA and under the same conditions were 25.8% and 22.3% for sulfate and nitrate salts respectively. This positive performance can be attributed to the consumption of photo-generated holes in NA degradation and inhibition of the electron–hole recombination. The photo-promoted CB electrons, generated by TiO2 particles under UV irradiation, become more readily available for Ni(II) ions and hence an easier reduction will be established. The NA degradation is also enhanced in the presence of nickel ions. The RE of NA was raised from 36.5% to 50.6% with nickel sulfate and to 48.3% with nickel nitrate. The stronger NA adsorption in the presence of nickel salts can be considered to be due to the salting-out effect. The enhancement of Ni(II) reduction and NA degradation implies a synergism for the process.

Figure 4

Removal efficiencies for different investigated processes; [Ni(II)]0 = 5 mg/L, [NA]0 = 10 mg/L, pH = 7.5 and T = 25 °C.

Figure 4

Removal efficiencies for different investigated processes; [Ni(II)]0 = 5 mg/L, [NA]0 = 10 mg/L, pH = 7.5 and T = 25 °C.

Close modal

It is worth noting that similar to a previous report on photocatalytic degradation of aniline with TiO2 particles (Wang et al. 2009), a surface charge transfer complex can form between opposite charges of Ti4+ and O2− from the TiO2 structure and C and H+ from NA, respectively (Rei et al. 2002). Based on these assumptions, for which a simple scheme is illustrated in Figure 5, the formed NA-TiO2 surface species can be photo-excited upon light absorption and initiate a direct electron transfer into the CB of TiO2 particles. The photo-induced electrons, injected into CB, can easily be trapped by Ni(II) ions which act as electron acceptor and accelerate NA conversion to a radical cation (+). Subsequently, intermediates such as 1-naphthol, 1,2-benzenedicarboxaldehyde, 1,4-naphthoquinone, alkyl phthalates and some others are formed by a series of electron transfer mineralization mechanisms (Wang et al. 2009). The in situ formation of Ni/TiO2 and NiO/TiO2 nanocomposites is one advantage of this process. This product can be used as an efficient photocatalyst in degradation of pollutants under UV or solar irradiations (Sharma & Lee 2015).

Figure 5

Schematic presentation of the simultaneous photocatalytic reduction of Ni(II) and oxidation of NA with TiO2 particles.

Figure 5

Schematic presentation of the simultaneous photocatalytic reduction of Ni(II) and oxidation of NA with TiO2 particles.

Close modal

The Fourier transform infrared (FTIR, Perkin-Elmer, Spectrum 65, USA) spectra of TiO2 powders, before and after usage, are shown in Figure 6(a). The broad peaks centered between 3,600 and 3,400 cm−1 and at 1,630 cm1 correspond to the stretching vibrations of O-H groups and the bending vibrations of the adsorbed water molecules, respectively. The peak appearing below 1,200 cm1 corresponds to Ti-O-Ti vibrations. Further, in Ni/TiO2 composite, a weak band is observed at 2,925 cm1, which can be due to the presence of Ni over the TiO2 surface (Begum et al. 2008). Other bands at 1,713.8 cm1 and 1,387.5 cm1 are assigned to N-O in nitrate ions (Hankare et al. 2011) and the band between 1,450 and 1,350 cm1 is assigned to S=O in sulfate ions (Samantaray et al. 2003). The other appeared peaks correspond to the adsorption of trace amounts of sulfate or nitrate anions on the surface of titania nanoparticles. Further, the peak at about 1,000 cm1 can be assigned to Ti-O-C stretching bond (Pavia et al. 2009). It is significant that there is no strong peak, relevant to aromatic C-H and C-C stretching vibrations, within 2,800–3,000 cm1 and 1,6721,696 cm1 regions, respectively. These confirm that adsorbed NA species on the TiO2 surface are completely oxidized.

Figure 6

FTIR spectra of NA and also TiO2 particles before and after usage in simultaneous photocatalytic reduction of Ni(II) and degradation of NA (a), and SEM image of TiO2 particles in the simultaneous photocatalytic treatment of NiSO4 and NA, before and after usage (b); [Ni(II)]0 = 5 mg/L, [NA]0 = 10 mg/L, pH = 7.5 and T= 25 °C.

Figure 6

FTIR spectra of NA and also TiO2 particles before and after usage in simultaneous photocatalytic reduction of Ni(II) and degradation of NA (a), and SEM image of TiO2 particles in the simultaneous photocatalytic treatment of NiSO4 and NA, before and after usage (b); [Ni(II)]0 = 5 mg/L, [NA]0 = 10 mg/L, pH = 7.5 and T= 25 °C.

Close modal

The scanning electron microscopy (SEM, Tescan, Mira 3, Czech Republic) was used to determine the morphology of the TiO2 nanoparticles before usage and of collected and dried particles after usage. The SEM image (Figure 6(b)) reveals that the formed nanocomposite particles have regular and spherical shape after TiO2 doping with nickel species (Ni0 and NiO). The bigger found particle size (25.6 nm compared to 20.2 nm) is similar to a report for synthesizing mesoporous (Ni2+/Ni3+)-TiO2 particles with combined sol-gel and thermal decomposition methods (Rajendran et al. 2018). In a previous study using X-ray photoelectron spectroscopy to analyze used photocatalyst particles (Saien et al. 2014), the presence of nickel species on the titania surface (Ni0 and NiO) was confirmed, because of immediate oxidation of metallic nickel.

In addition, the TiO2 particles, before and after usage, were characterized by N2 adsorption–desorption porosimetry using a BET analyzer (BELSORP, Mini II, Japan). The results, presented in Table 1, indicate that the average surface area and the average diameter of TiO2 particles before usage agree with the manufacturer's reported values (‘Materials and methods' section). Meanwhile, the collected TiO2 particles after usage consistently have less surface area and large pores, compared with initial particles, respectively. These kinds of changes have also been reported for nanocomposites such as Ag/TiO2 and NiO/TiO2, synthesized by methods other than photocatalytic reduction (Rajendran et al. 2018; Saravanan et al. 2018).

Table 1

BET surface analysis of TiO2 nanoparticles in the photocatalytic treatment of NiSO4 and NA

SampleSurface area (m2/g)Average diameterBET (nm)Pore volume (cm3/g)Pore size (nm)
TiO2 before usage 50 ± 15 21 0.17 16.4
TiO2 after usage 43 ± 10 26 0.24 20.2
SampleSurface area (m2/g)Average diameterBET (nm)Pore volume (cm3/g)Pore size (nm)
TiO2 before usage 50 ± 15 21 0.17 16.4
TiO2 after usage 43 ± 10 26 0.24 20.2
(8)
(9)
Figure 7

Removal efficiencies after 90 min treatments, [Ni(II)]0 = 5 mg/L, [NA]0 = 10 mg/L: effect of pH at T = 25 °C (a), effect of temperature at pH = 7.5 (b), effect of purging nitrogen gas on REs (c), and comparison of H2O2 generation (d) under given conditions.

Figure 7

Removal efficiencies after 90 min treatments, [Ni(II)]0 = 5 mg/L, [NA]0 = 10 mg/L: effect of pH at T = 25 °C (a), effect of temperature at pH = 7.5 (b), effect of purging nitrogen gas on REs (c), and comparison of H2O2 generation (d) under given conditions.

Close modal

The respective average Ni(II) and NA REs show an ascending and then a flat variation with pH. The highest average RE was found at pH around 7.5 for both the sulfate and nitrate nickel salts.

Investigation on temperature effect (Figure 7(b)) revealed that nickel reduction and NA degradation increase with temperature increase from 20 to about 35 °C and then remain almost constant perhaps due to a temperature disfavor effect on their adsorption. An average enhancement of about 12.4% was achieved for treatments below pH 7.5 after 90 min. Temperature helps the photocatalytic reactions to compete with the non-favorable electron–hole pair recombination; however, the low level activation energy of photocatalytic reactions gives a small temperature effect (Saien & Azizi 2015). Temperature of 35 °C can suit operations, giving 58.4% average RE (59.2% for Ni(II) and 57.5% for NA) for sulfate and 55.5% average RE (54.5% for Ni(II) and 56.6% for NA) for nitrate salts. In addition to the reasons given in the section on just Ni(II) reduction, more electron–hole recombination is expected under a low level of metal reduction, which in turn gives lower NA degradation.

The role of ROS in the process was investigated by continuous nitrogen gas purging into solutions. Figure 7(c) shows that 61.5% average RE (74.3% for Ni(II) and 48.8% for NA) and 54.2% average RE (65.9% for Ni(II) and 42.5% for NA) were obtained for sulfate and nitrate salts, respectively, and a higher reduction of Ni(II) ion and a lower oxidation of NA occurred. It has been reported that reduction of metals, with low reduction potential, is difficult with DO due to competition between metal ions and DO to attract electrons in the CB of TiO2 (Yang et al. 2012). Meanwhile the efficiency of NA oxidation is diminished under these conditions. DO acts as an effective electron acceptor to extend the hole's lifetime and also may be involved in the formation of ROS, such as H2O2 molecule, which contribute in photocatalyst reactions of NA degradation (Nosaka & Nosaka 2017).

H2O2 is a rather stable molecule of the ROS, and the amount of this species was measured easily with iodide method (Nosaka & Nosaka 2017) either with or without nitrogen purging. The results in Figure 7(d) show that H2O2 formation highly increases at initial times and then tends to decrease due to its consumption in NA oxidation. As expected, H2O2 formation is very low in continuous nitrogen purging due to the absence of DO.

### Kinetic study

Owing to practical applications, the kinetics of photocatalytic reactions was investigated under mild mentioned operating conditions and during 90 min treatment. Results show that Ni(II) photocatalytic reduction and NA degradation in the mixed solution are well described by pseudo-first-order kinetic models as where , and k are the initial concentration and at any time, t, of Ni(II) or NA, and the kinetic rate constant, respectively. The rate constants as well as the coefficient of determination (R2) for different investigated cases are listed in Table 2.

Table 2

The rate constants of Ni(II) reduction and NA oxidation; [Ni(II)]0 = 5 mg/L, [NA]0 = 10 mg/L, pH = 7.5, T= 35 °C and during 90 min treatments

Processk (1/min)R2
Ni(NO3)2 0.0027 0.970
NiSO4 0.0037 0.952
NA 0.0054 0.983
Ni(NO3)2 in (Ni(NO3)2 + NA) 0.0080 0.983
NiSO4 in (NiSO4 + NA) 0.0086 0.992
NA in (Ni(NO3)2 + NA) 0.0075 0.997
NA in (NiSO4 + NA) 0.0082 0.993
Processk (1/min)R2
Ni(NO3)2 0.0027 0.970
NiSO4 0.0037 0.952
NA 0.0054 0.983
Ni(NO3)2 in (Ni(NO3)2 + NA) 0.0080 0.983
NiSO4 in (NiSO4 + NA) 0.0086 0.992
NA in (Ni(NO3)2 + NA) 0.0075 0.997
NA in (NiSO4 + NA) 0.0082 0.993

### Energy consumption analysis

Among several factors in selecting a method for pollutants treatment, economics is vital. In this regard, electrical energy consumption, , has the main contribution in the photochemical processes. Here, can be calculated according to the proposal of the Photochemistry Commission of the International Union of Pure and Applied Chemistry (IUPAC) for the first-order reactions as (Bolton et al. 2001):
(10)
where P is the electric power (kW) of the photochemical system, V is the solution volume (L) in the reactor and t is the reaction duration time (min). In order to take into account the impact of reaction temperature variation on the calculations, Equation (10) can be given as the following equation, noting that the constant represents the rate constant k (1/min):
(11)

Based on obtained rate constants, the electrical energy related to the photocatalytic removal of each pollutant under dominant conditions was obtained as 1,116.3 and 1,199.8 kWh/m3 for Ni(II) and 1,170.7 and 1,280.1 kWh/m3 for NA, when sulfate and nitrate nickel salts were used in the mixtures, respectively. The calculated values for simultaneous treatments are less than the electrical quantities required for treatment of each pollutant individually (Table 3). Considering the current electrical energy (industrial sector) price in the US market as 0.0702 US$/kWh in 2018 (US EIA 2018), the electrical energy cost relating to the above four electrical energy amounts is estimated as 78.4, 84.2 and 82.2, 89.9 US$/m3, respectively.

Table 3

The performance of the used photoatalytic process for simultaneous Ni(II) reduction and NA oxidation in comparison with other reported processes

Photocatalytic process[Ni(II)] or [NA] (mg/L)[TiO2] (mg/L)pHLamp power (W)t (min)Accompanied reagentRE (%)EEC (kWh/m3)PE 1/[(kWh/m3) (mg/L)]Reference
Ni(NO3)2 + TiO2 15 1,000 7.0 125 120 – 55.0 720.9 7.63 × 10−5 Karimi et al. (2013)
Ni(NO3)2 + ZnO 1,000 7.0 125 120 – 41.0 1,091.1 3.76 × 10−5 Kabra et al. (2007)
NiSO4 + TiO2 100 7.5 250 90 – 32.1 2,594.6 1.24 × 10−4 This work
Ni(NO3)2 + TiO2 100 7.5 250 90 – 27.9 3,555.5 7.85 × 10−5 This work
NiSO4 + TiO2 100 7.5 250 90 NA, 10 mg/L 59.2 1,116.3 5.30 × 10−4 This work
Ni(NO3)2 + TiO2 100 7.5 250 90 NA, 10 mg/L 54.5 1,199.8 4.54 × 10−4 This work
NA + TiO2 10 100 7.5 250 90 Ni(NO3)2, 5 mg/L 56.6 1,280.1 4.42 × 10−4 This work
NA + TiO2 10 100 7.5 250 90 NiSO4, 5 mg/L 57.5 1,170.7 4.91 × 10−4 This work
NA + TiO2 10 100 7.5 250 90 – 36.5 1,777.8 2.50 × 10−4 This work
Photocatalytic process[Ni(II)] or [NA] (mg/L)[TiO2] (mg/L)pHLamp power (W)t (min)Accompanied reagentRE (%)EEC (kWh/m3)PE 1/[(kWh/m3) (mg/L)]Reference
Ni(NO3)2 + TiO2 15 1,000 7.0 125 120 – 55.0 720.9 7.63 × 10−5 Karimi et al. (2013)
Ni(NO3)2 + ZnO 1,000 7.0 125 120 – 41.0 1,091.1 3.76 × 10−5 Kabra et al. (2007)
NiSO4 + TiO2 100 7.5 250 90 – 32.1 2,594.6 1.24 × 10−4 This work
Ni(NO3)2 + TiO2 100 7.5 250 90 – 27.9 3,555.5 7.85 × 10−5 This work
NiSO4 + TiO2 100 7.5 250 90 NA, 10 mg/L 59.2 1,116.3 5.30 × 10−4 This work
Ni(NO3)2 + TiO2 100 7.5 250 90 NA, 10 mg/L 54.5 1,199.8 4.54 × 10−4 This work
NA + TiO2 10 100 7.5 250 90 Ni(NO3)2, 5 mg/L 56.6 1,280.1 4.42 × 10−4 This work
NA + TiO2 10 100 7.5 250 90 NiSO4, 5 mg/L 57.5 1,170.7 4.91 × 10−4 This work
NA + TiO2 10 100 7.5 250 90 – 36.5 1,777.8 2.50 × 10−4 This work
Finally, a valid criterion of process efficiency (PE), corresponding to RE when assigned to unit electrical energy consumption and the employed photocatalyst dosage, can be introduced as:
(12)

This criterion reflects the efficiency, achieved with respect to the level of energy consumption as well as the catalyst dosage. Accordingly, the effect of temperature and the reaction progress are involved in PE via the EEC parameter.

The removal efficiencies as well as other described criteria are listed in Table 2 in comparison with other previously reported studies on photocatalytic reduction of Ni(II). The pseudo-first-order reactions are relevant under corresponding operating conditions. It is seen that despite nearly the same order of , the PE values for nickel ion, in this work, are about one order of magnitude higher than those of referenced works. Effective light in the used photoreactor and the synergism action of the reduction/degradation process are the main reasons for this very significant preference.

In this study the enhanced photocatalytic reduction of Ni(II) was demonstrated when simultaneously performed with naphthalene degradation and using a low dosage of titania nanoparticles. The photocatalyst hole consumption by naphthalene causes lowering of the electron–hole recombination and acceleration of the electron trapping by Ni(II) ions. Further, the formed naphthalene-TiO2 species can be photo-excited upon light absorption and provide a direct electron transfer into Ni(II) ions. Confirming the results, the TiO2 particles before and after usage were characterized by means of several techniques and it was found that average particles size, after usage, was increased due to nickel doping while the pore size was increased. The other important findings were: (i) the optimum mild operating conditions were pH 7.5 and 35 °C; (ii) under pertinent conditions, the removal efficiency of Ni(II) ions, for both the used salts, was about twice that in the absence of naphthalene; (iii) there appears to be more opportunity for the reduction of Ni(II) ions when the sulfate salt is used, compared with nitrate salt, individually or simultaneously; (iv) nitrogen purging in the samples confirmed the favorable DO role in naphthalene degradation and negative role in nickel reduction; (v) the Ni(II) ion reduction and naphthalene degradation obey pseudo-first-order kinetic reactions; (vi) electrical energy consumption analysis showed a relatively low required energy for the simultaneous treatment systems in comparison with individual treatments under the same conditions; and (vii) based on different criteria, including economic parameters, the significant preference of the process was revealed.

The authors wish to thank the university authorities for providing the financial support to carry out this project. The authors also acknowledge Evonik Industries for supplying TiO2 (P-25) as a gift to our research group.

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