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Synthesis of TiO2/nZVI nanocomposite for nitrate removal from aqueous solution

  • Zahra HejriEmail author
  • Mehri Hejri
  • Maryam Omidvar
  • Sadjad Morshedi
Open Access
Research
  • 125 Downloads

Abstract

To develop a new adsorbent for removal of nitrate and to enhance the adsorbent separation from aqueous solution, surface modification of titanium dioxide nanoparticles with nano-zero-valent iron (nZVI) was performed through chemical coprecipitation of magnetic nanoparticles on TiO2 surface. Morphological, structural and magnetic properties of modified adsorbents (TiO2/nZVI) were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), Fourier transform infrared radiation (FTIR) and vibrating sample magnetometer (VSM). To determine the ionic strength effect and optimal removal conditions, the effect of contact time (60–210 min), pH (4–10) and adsorbent dosage (0.5–1.5 g/L) on adsorption efficiency were studied, using response surface method. Obtained results showed that the nitrate removal efficiency decreased with increasing ionic strength. The TiO2/nZVI nanocomposites exhibited a ferromagnetic behavior and its saturation magnetization was 795.28 memu/g. The maximum nitrate removal (98.226%) achieved by modified TiO2 was about 14.65% higher than the unmodified nanoparticles. The optimized adsorption parameters were: adsorbent dosage 0.982 g/L, pH 4.185 and the contact time 150.091 min.

Keywords

Titanium dioxide nZVI Nitrate Aqueous solution Adsorption 

Introduction

Nitrate is often found in drinking water because of human activities such as excessive utilization of chemical fertilizers, inappropriate disposal of industrial, human and animal wastes, etc. Nitrogen is converted to nitrate in the soil and since nitrate is dissolvable in water, it enters groundwater and eventually drinking water through the rain [1]. Increasing nitrate in drinking water has two adverse health effects: induction of blue-baby syndrome or methemoglobinemia, especially in infants, and the formation of carcinogenic nitrosamines [2].

Conventional nitrate removal technologies including ion exchange, reverse osmosis, electrodialysis, biological and chemical denitrification, are often costly and complex with low efficiency and sub-products [2, 3, 4, 5]. Adsorption has been proposed as an attractive technology for removal of different pollutants from water due to its process simplicity, selectivity and reusability of the adsorbent, low cost and environment-friendly nature [6, 7, 8, 9, 10, 11, 12, 13, 14]. Khezri et al. [15] investigated the adsorption of nitrate anions from aqueous solutions on ammonium-functionalized magnetic mesoporous silica. The removal efficiency of NO3 from solution was around 86.24% by the constructed adsorbent under the optimal experimental conditions. Nowadays, nano-adsorbents are widely used to efficiently eliminate the pollutants from water due to high surface-to-volume ratio, easy synthesis and rapid sorption [3, 16]. Bhatnagar et al. [2] have investigated the removal of nitrate from aqueous solution using alumina nanoparticles and achieved the maximum absorption capacity of 4 mg/g at 25 ± 2 °C and pH 4.4. Farasati et al. [17] eliminated nitrate from contaminated waters using anion exchanger Phragmites australis nanoparticles. The highest adsorption rate was obtained at pH 6 using 0.3 mg/L adsorbent. Zhao et al. [13] reviewed the recent works on the preparation of polymer composites and their application in the efficient removal of heavy metal ions from aqueous solutions under different conditions. Mohammadi et al. [18] synthesized carboxylated chitosan modified with ferromagnetic nanoparticles for adsorptive removal of nitrate anions from aqueous solutions. The maximum amounts of adsorption onto prepared nanoparticles were obtained in acidic conditions with 2 g/L of adsorbent. Yazdi et al. [19] removed nitrate from aqueous media by functionalized chitosan–clinoptilolite nanocomposites successfully.

Recent studies have shown that zero-valent iron (Fe0), especially in nanoscale form, may potentially be used for water remediation, due to its affinity for a large number of contaminants, large specific surface area, and high surface reactivity [20, 21, 22, 23]. Muradova et al. [20] investigated the removal of nitrate from groundwater by Fe/Cu bimetallic nanoparticles. They found that the rate of nitrate reduction increased by adding the ratio of copper particles to ZVI in two-part metal particles. Zou et al. [24] reported the excellent removal capacity of nZVI-based materials for various heavy metal ions. Sepehri et al. [25], removed nitrate (up to 84%) from aqueous solution by zero-valent iron nanoparticles reinforced with natural zeolite. Furthermore, the adsorption of some contaminants including amido black dye (up to 88% removal), ibuprofen (92%), ametryn (88%), propranolol drug residue (90%), pantoprazole drug residue (89%), secbumeton herbicide (90%), β-estradiol (82%), atrazine herbicide(95%), cyanazine (80%) onto iron nanocomposite material as adsorbent has been investigated by Ali et al. [26, 27, 28, 29, 30, 31, 32, 33, 34].

Titanium dioxide is a nontoxic material that has been applied in environmental treatments such as water and air disinfection because of relatively low price, corrosion resistance and its unique properties such as strong photocatalytic activity and high physical and chemical stability [35, 36, 37, 38]. Titanium dioxide nanoparticles have been used to photocatalytic and adsorption removal of some pollutants [36, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48], but to the best of our knowledge, any research focused on removal of nitrate from water by adsorption on magnetized titanium dioxide nanoparticles and its comparison with unmodified TiO2 nanoparticles has not yet been reported. The main aim of this study was to investigate the nitrate removal efficiency of TiO2 nano-particles grafted with nZVI.

Experimental

Materials

Nano-TiO2 powder (anatase-phase crystal structure with average particle size of about 25 nm) was supplied by Nanolin, Germany. Potassium nitrate (99%), ferric trichloride (FeCl3·6H2O) with 99% purity, molecular mass 270.33 g/mol and density of 1.82 g/cm3, sodium borohydride (NaBH4) with a purity of 99%, molecular weight 37.83 g/mol and density of 0.0005 g/cm3 and ethanol (≥ 99% purity) from Merck, Germany were used.

Preparation of TiO2/nZVI nano-adsorbent

nZVI was synthesized in an anaerobic chamber via the reduction of Fe3+ ions with sodium borohydride as a reducing agent according to the method described by Huang [49]. Briefly, FeCl3·6H2O was dropped to NaBH4 solution in a 1:2 volume ratio. The black nZVI was separated and rinsed with pure ethanol and vacuum dried for 90 min at 160 mbar. The reduction process occurs according to the following equation [49]:
$$2{\text{Fe}}\left( {{\text{H}}_{2 } {\text{O}}} \right)_{6}^{3 + } + 6{\text{BH}}_{4}^{ - } + 6{\text{H}}_{2} {\text{O }} \to 2{\text{Fe}}^{0} + 6{\text{B}}\left( {\text{OH}} \right)_{3} + 21{\text{H}}_{2}$$
(1)

TiO2/nZVI nanocomposite was prepared according to Petala et al. [37] method, during two stages: wet impregnation and then reduction with sodium borohydride. For this purpose, 0.2 g ferric trichloride was dissolved in 7 mL of pure ethanol and 0.4 g TiO2 was added to this solution until saturation. Solvent evaporation was performed by heating in a hot water bath at 70 °C for 20 min. For reduction of Fe3 + to Fe0, a reducing solution (containing 0.2 g sodium borohydride in 7 mL of deionized water) was added to the solution as dropwise. The mixture was kept stationary for 30 min until deposition and then centrifuged at 15,000 rpm for 15 min. Finally, the solid phase was rinsed and vacuum dried. All of [TiO2]:[nZVI] ratios (0, 0.25, 0.5, 1, 2.5, 5 and 7), were produced in the same way. For this purpose, calculated mass amounts of TiO2 were added to ferric trichloride solution. [Fe3+]:[\({\text{BH}}_{4}^{ - } ]\) ratio was kept constant.

Adsorption experiments

Adsorption experiments were conducted discontinuously to evaluate the nitrate adsorption efficiency. Certain concentrations of nitrate at specific contact times and pHs were subjected to different amounts of adsorbent, based on the relevant experiments design. Aqueous nitrate solution was stirred on a magnetic stirrer at 1000 rpm to allow the transfer of the pollutant onto the adsorbent. The main part of the magnetic adsorbent was separated from the solution using a magnet and the remainder was separated by centrifuge at 15,000 rpm for 15 min. The nitrate concentration was analyzed by spectrophotometry. The absorbance of the solutions was determined at λmax = 270 nm using a UV–Vis spectrophotometer model 8454/4000 (USA).

The nitrate adsorption efficiency onTiO2 and TiO2/nZVI nanocomposite was obtained by the following formula:
$${\text{Adsorption efficiency}} = \frac{{{\text{C}}_{0} - {\text{C}}_{\text{t}} }}{{{\text{C}}_{0} }},$$
(2)
where, C0 is the initial nitrate concentration in the aqueous solution; Ct is the concentration of nitrate in the aqueous solution at t min.

Characterization of adsorbent

The morphology of unmodified and modified nanoparticles was evaluated using Cambridge S360 scanning electron microscope (SEM) equipped with an Oxford EDX. All images were taken with an operating voltage 30 kV and 200, 500, 1000 and 2000 magnifications. All the specimens were sputter-coated with gold in a Quorum sputter coater model Q 150R ES. A closer look at the shape, size, and arrangement of the nanostructure adsorbent was carried out by Philips transmission electron microscope (TEM) model CM120. The X-ray diffraction analysis(XRD) was performed at an angular range of 5°–70° (2θ) with a step size of 2θ = 0.02° in Philips Analytical X-Ray diffractometer model X’ Pert PW 3040/60 using a Cu Kα radiation (λ = 1.5406 nm), 40 kV, and 30 mA. The diffractometer was equipped with 1° divergence slit and a 0.1 mm receiving slit. Fourier transform infrared spectroscopy (FTIR) was carried out by the Thermo Nicolet apparatus model Avatar 370, made in USA. All the peaks were obtained in the range of 4000–400 cm−1 for modified and unmodified nano-adsorbents. Magnetization of the prepared nanocomposite (TiO2/nZVI) was studied at room temperature using vibration sampling magnetometer (VSM) model 7400 (USA). This device is able to measure the magnetic properties of samples with an accuracy of 1 × 10−7 emu.

Regeneration and reusability of adsorbent

To evaluate the possibility of recycling the adsorbent, regeneration process of TiO2/nZVI nanocomposites was conducted. A 0.01 M NaOH solution was used as adsorbent recovery solution. TiO2/nZVI nanocomposites, previously saturated with nitrate, were agitated with NaOH solution for 3 h and followed by ultrasonication for 5 min to desorb the nitrate from them. Adsorbents were separated by centrifuge, and then washed by deionized water for 5 min. The denitrification performance of the regenerated adsorbent was measured in a new adsorption experiment. This adsorption–desorption cycle was repeated five times to test the reusability of the adsorbents [19, 50, 51].

Statistical analysis

Response surface method (RSM), central composite design(CCD) type was used for designing of experiments, analyzing resulted data and determining the influence of three factors “contact time”, “pH” and “adsorbent dosage” with three levels for each one,on “adsorption efficiency” response. The levels of factors have been selected according to the preliminary tests. Design expert software version 10 was used for analysis of variance (ANOVA). The 0.05 significance level was used. The optimal value for each of the three parameters was determined according to the obtained responses. Factors under study along with their levels are summarized in Table 1.
Table 1

Factors under study along with their levels

Factor

Level

Low

High

A: Contact time (min)

60

210

B: pH

4

10

C: Adsorbent dosage (g/L)

0.5

1.5

Results and discussion

Morphological properties

SEM micrographs of titanium dioxide nanoparticles and their modified form are shown in Fig. 1a–d. The micrographs, taken from the surface of samples, illustrate that the particles are largely spherical and the modified particles are slightly larger than unmodified particles.
Fig. 1

a, b SEM images of titanium dioxide nanoparticle; c, d SEM images of TiO2/nZVI nanocomposites; e, f TEM images of TiO2/nZVI nanocomposites

The larger size of TiO2/nZVI nanoparticles (up to 70 nm) indicates the proper integration of TiO2 nanoparticles with the modifier factor. As it is seen, the resulting nanocomposites are spherical with no agglomeration between particles.

TEM images taken from TiO2/nZVI magnetic nanoparticles are shown in Fig. 1e, f. In the prepared TiO2/nZVI nanocomposite structure, the coated core is visible that suggesting a good combination of TiO2 nanoparticles with nZVI. Based on TEM results, the size of prepared magnetic nanoparticles was below 100 nm.

Structural properties

The element composition of the synthesized materials was identified by an energy-dispersive X-ray spectroscopy system (EDX) coupled to the SEM. EDX analysis of nanocomposites (Fig. 11), confirmed the presence of titanium, iron, oxygen and chlorine elements in the composition of the compound. The presence of some oxygen (33.37%) in the composition is not surprising, since the occurrence of partial oxidation is inevitable.

X-ray diffraction patterns related to TiO2 nanoparticles and TiO2/nZVI nanocomposites at 2θ = 5°–70° are shown in Fig. 2. As it is seen, TiO2 exhibits a sharp peak at 2θ = 25.402° corresponding to the plane spacing (d-spacing) of 0.351 nm [35]. The X-ray diffraction pattern of the magnetic nanocomposite has the same pattern as for TiO2, with two excessive peaks in 2θ = 31.8° and 2θ = 44.9° which indicate the presence of Fe in the composition [38, 52]. The peak in 2θ = 25° can correspond to the presence of TiO2 and FeOOH in the nanocomposites [33]. Observed peak in 2θ = 31.8° is related to the presence of iron oxide (FeO) and the peak in 2θ = 44.9° indicating the presence of iron zero-valent crystalline phase [25, 53].
Fig. 2

X-ray diffraction patterns of TiO2 and TiO2/nZVI nanoparticles

Fourier transform infrared radiation (FTIR) technique is used to understand the adsorption mechanism [11]. A comparison between the FTIR spectrums of TiO2 and its modified form are shown in Fig. 3. In the nano-TiO2 spectrum, a strong and broad absorption band at 3430.52 cm−1 shows a large amount of –OH at the nano-TiO2 surface. Absorption band at 1629.16 cm−1 is related to Ti–OH bending vibration and the absorption band at 713.06 cm−1 indicates the Ti–O–Ti tensile vibration [54, 55].
Fig. 3

FTIR spectra of TiO2 and TiO2/nZVI nanoparticles

Comparison between FTIR spectra of TiO2/nZVI nanocomposite with the spectra of TiO2 nanoparticles shows that the surface modification has been occurred [56]. In the TiO2/nZVI spectrum, the broad band at 3415.20 cm−1 is related to tensile vibrations of –OH which should be in the range of 3200–3500 cm−1. The peaks at 1637.51 cm−1 and 3415.20 cm−1 are also indicative of the tensile vibrations of OH related to H2O and \(\alpha\)-FeOOH [53]. A shift in the absorption band of Ti–O tensile vibration from 713.06 cm−1 to 503.65 cm−1 indicates the Ti–O composition with other elements during the surface modification process [56].

Magnetic properties of TiO2/nZVI nano-adsorbent

The magnetic behavior of diamagnetic materials, paramagnetics, ferromagnetics, etc., can be measured in different shapes of powder, solid, thin film, single crystal, liquid, etc., using VSM with drawing of a residual curve. A representative hysteresis loop of TiO2/nZVI nanocomposites at ambient temperature and in the fields from − 20,000 to 20,000 Oersted is shown in Fig. 4. The obtained hysteresis loop, suggested a weak magnetic nature of prepared nanocomposites. The weak magnetism was most likely attributed to the existence of nonconductor TiO2 along with nZVI and the weak magnetic nature of nZVI itself. As it is seen, the response of prepared nanocomposite to the applied magnetic field indicates a ferromagnetic behavior. As expected, the residual is very low and also has a magnetic saturation. The saturation magnetization of the TiO2/nZVI nanocomposite was about 795.28 memu/g. Petala et al. [37] in their research on synthesis and characterization of nZVI/TiO2 for photocatalytic removal of chromium VI from water, achieved a ferromagnetic response with maximum magnetic properties of 16.1 Am2/kg at 5 K and 13.8 Am2/kg at 300 K without reaching saturation at any temperature. Lu et al. [38] reported a magnetic property of 1320 memu/g for Rectorite/TiO2/Fe3O4 composites.
Fig. 4

Magnetic property of TiO2/nZVI nanocomposite

Optimization of factors

To determine the optimal nitrate removal conditions, the effect of contact time (60–210 min), pH (4–10) and adsorbent dosage (0.5–1.5 g/L) were studied. For this reason, the specific concentration of the contaminant, under controlled pH conditions was contacted with different amounts of adsorbent during certain contact times based on the design of experiments.

Effect of contact time

The effect of contact time on nitrate adsorption efficiency by TiO2 and TiO2/nZVI nanoparticles are shown in Fig. 5a, b. As it can be seen, increasing the contact time has improved the efficiency of nitrate adsorption by the adsorbent. The main reason for increasing the amount of adsorption over time is increasing the collision chance of nitrate ions with active sites on the adsorbent. The major part of removal by unmodified TiO2 nanoparticles occurred in the first 150 min, and by approaching to equilibrium, the amount of adsorption had no considerable progress. The maximum adsorption on unmodified TiO2 nanoparticles was 82.9406% at 210 min. With increasing the contact time from 210 to 240 min, no further increase in adsorption efficiency occurred; this is due to adsorbent saturation over time. Bhatnagar et al. [2] had obtained the same result for removal of nitrate from water through adsorption on alumina nanoparticles. They observed that the adsorption process reached equilibrium within 60 min. Comparison of the obtained results showed that the maximum adsorption by modified TiO2 was about 14.65% higher than the unmodified nanoparticle; this is due to the increased adsorbent surface and the high ability of zero-valent iron nanoparticles integrated with TiO2 for removal of pollutants [20, 25, 57]. The obtained result matches the report by Muradova et al. [20] on the nitrate elimination from aqueous solution by nZVI/Cu nanoparticles. They also observed more reduction in concentration of nitrate at the beginning and in contact times more than 60 min, the concentration changes were little.
Fig. 5

Effect of contact time on nitrate adsorption by: a TiO2 nanoparticles, b TiO2/nZVI nanocomposites

Effect of pH value

Figure 6a, b shows the effect of pH on nitrate removal efficiency by nano-TiO2 and TiO2/nZVI nanocomposites. The role of pH is crucial, as it may influence both the reactivity with the pollutants and the actual composition of adsorbent. In acidic conditions, the adsorbent surface is protonated and has a higher positive charge. This causes electrostatic interactions between adsorbents and negative charged ions in the water. In alkaline conditions, the surface of adsorbent has a negative charge and the interaction between adsorbent and positive ions will increase [58]. We found that the nitrate removal efficiencies decreased with increasing pH value. The adsorption mechanism was strong electrostatic interaction between H+ ions that had been increased positive charges on adsorbent surface and negative charged nitrate ions. In high pHs because of the competition between \({\text{OH}}^{ - }\) ions present in alkaline environment and anionic nitrate for sit on adsorbent active sites, nitrate removal was decreased. The highest adsorption efficiencies on nano-TiO2 and TiO2/nZVI nanocomposite were 82.9406% and 95.092% at pH 4, respectively. The maximum adsorption efficiency by TiO2/nZVI has indicated an improvement equal to 14.45% in the presence of nZVI. Bhatnagar et al. [2] also observed that the adsorption efficiency of nitrate by nano-alumina was increased in pH range from 3 to 4.4; it then decreased along with an increase in pH. Xie and Gao [39] also had used TiO2 nanoparticles to remove heavy metals from water. They had observed less absorption efficiency for metal cations in low pHs because of the competition between H+ and positively metal ions; Conversely, at high pHs, the adsorbent was deprotonated and was more likely to absorb cationic metals. Ai et al. [59] have reported that the sorption of U(VI) towards graphene oxide was primarily influenced by the pH values of the aqueous solution, and its adsorption performance was strongest at high pH levels. The adsorption mechanism was strong electrostatic interaction between the uranyl ion and the negative charged O atoms of the oxygen-containing functional groups, which were the main adsorption sites also. Based on their opinion, in addition to the pH influence, an increase in functional groups and negative charges on the GO surface can improve the adsorption ability towards uranyl ions.
Fig. 6

Effect of pH on nitrate adsorption by: a TiO2 nanoparticles, b TiO2/nZVI nanocomposites

Effect of adsorbent dosage

Effect of adsorbent dosage on the adsorption efficiency is shown in Fig. 7a, b. As it is clear from this figure, the efficiency of nitrate adsorption initially has improved by increasing the amount of adsorbent and then remained constant. This result is similar to what proposed by Ali et al. in their studies on molecular uptake of Congo red dye from water on iron composite nano-particles [60] and fast removal of fluoride from water by iron nano-impregnated adsorbent [61].
Fig. 7

Effect of adsorbent dosage on nitrate adsorption by: a TiO2 nanoparticles, b TiO2/nZVI nanocomposites

When using unmodified TiO2, with an increase in the amount of adsorbent from 0.3 to 1 g/L, the adsorption efficiency increased from 14.285 to 82.637% (approximately 478.5% improvement) while with increasing the adsorbent from 1 to 1.5 g/L, only 0.36% increase in removal efficiency was observed. Therefore, increasing the amount of adsorbent to more than 1 g/L will not increase the adsorption efficiency significantly. The maximum nitrate removal efficiency with TiO2/nZVI nanocomposites was 95.092% using 1.5 g/L adsorbent, which is 14.65% higher than that of the unmodified TiO2 with the same adsorbent dosage. We attribute these results to good surface characteristics of prepared composite nano-adsorbent and its suitable interaction with nitrate molecules. Consequently, grafting the TiO2 nanoparticles with nZVI has improved the adsorption capability of the adsorbent. Statistical analysis, also showed that the influence of adsorbent dosage on removal efficiency was significant (P < 0.05). The high performance of nano-zero-valent iron to remove various pollutants had been reported previously [24, 25, 37, 62].

Regeneration and reusability of modified TiO2

The regeneration of an adsorbent is one of the most important aspects for an economical adsorption process. Therefore, to reduce costs and waste production, regeneration tests for recycling TiO2/nZVI nanocomposites were conducted and the results are shown in the Fig. 8. Results indicated that the adsorption efficiency decreased after each cycle of adsorption–regeneration slightly and there was no significant difference during five adsorption–regeneration cycles. The first-time regenerated TiO2/nZVI nanocomposites could remove 95.9% of nitrate (only 0.992% less than the fresh adsorbent) and 85.001% after five time regeneration. Therefore, the obtained results confirm that the majority of adsorbent can recycle. Qin et al. [63] in their study on mesoporous TiO2–SiO2 adsorbent for ultra-deep desulfurization of organic-S had observed that the first-time regenerated adsorbent could recover 99% of the breakthrough capacity as compared to a fresh adsorbent and after the fifth regeneration, it could recover 94.5%.
Fig. 8

Nitrate removal efficiency of TiO2/nZVI adsorbent (at optimal conditions) over five regeneration cycles

Statistical results

Analysis of the results by Design Expert software showed that the quadratic model is statistically well matched to the obtained data for nitrate adsorption onto the TiO2 nanoparticles before and after grafting with nZVI. The obtained equations for nitrate adsorption efficiency by TiO2 nanoparticles based on the coding and real factors, respectively, were:
$${\text{Adsorption efficiency }}\left( \% \right) \, = \, + 7 4. 3 1+ 2 5. 7 5*{\text{ A }} - 7. 7 4*{\text{ B }} + 1 4. 2 5*{\text{ C }} - 1 3. 7 7*{\text{ A}}^{ 2} - 10. 9 2*{\text{ C}}^{ 2}$$
(3)
$$\begin{aligned} {\text{Adsorption efficiency }}\left( \% \right) \, = \, - 70. 7 7 10 2 { } + 1.00 4 3 5*{\text{ Time }}\left( { \hbox{min} } \right) \, - 2. 5 7 9 1 1*{\text{ pH }} + 1 1 5. 8 5 30 3* \hfill \\ {\text{ adsorbent }}\left( {{\text{g}}/{\text{l}}} \right) \, - { 2}. 4 4 8 3 4 {\text{E}} - 00 3*{\text{ Time }}\left( { \hbox{min} } \right)^{ 2} - 4 3. 6 80 2 2*{\text{ adsorbent }}\left( {{\text{g}}/{\text{l}}} \right)^{ 2} \hfill \\ \end{aligned}$$
(4)
The obtained equations for nitrate adsorption efficiency by TiO2/nZVI nanocomposites based on the coding and real factors, respectively, were:
$${\text{Adsorption efficiency }}\left( \% \right) \, = \, + 8 5. 7 4 { } + { 24}. 7 8 { }*{\text{ A }} - 9. 1 6 { }*{\text{ B }} + { 13}. 3 6 { }*{\text{ C }} - 1 4. 9 3*{\text{ A}}^{ 2} - 8. 6 2*{\text{ C}}^{ 2}$$
(5)
$$\begin{aligned} {\text{Adsorption efficiency }}\left( \% \right) \, = \, - 4 7.0 8 4 4 8 { } + 1.0 4 7 1 7 { }*{\text{ Time }}\left( { \hbox{min} } \right) \, - 3.0 5 3 7 1 { }*{\text{ pH}} \hfill \\ \, + 9 5. 6 9 3 1 3*{\text{ adsorbent }}\left( {{\text{g}}/{\text{l}}} \right) \, - 2. 6 5 4 6 4 {\text{E}} - 00 3*{\text{ Time }}\left( { \hbox{min} } \right)^{ 2} - 3 4. 4 8 1 7 9*{\text{ adsorbent }}\left( {{\text{g}}/{\text{l}}} \right)^{ 2} \hfill \\ \end{aligned}$$
(6)
Design Expert software has been used to determine the optimal removal conditions within the tested range. Predicted and experimental optimum conditions for nitrate removal by TiO2/nZVI along with the maximum adsorption value are presented in Table 2.
Table 2

Predicted and experimental optimal conditions for nitrate removal by TiO2/nZVI

Contact time (min)

pH

Adsorbent (g/L)

Adsorption efficiency (%): predicted

Adsorption efficiency (%): from experiment

Error (%)

150.091

4.185

0.982

98.226

96.892

1.358

Effect of ionic strength

To study the effect of ionic strength on nitrate removal efficiency, the salt content of the feed solution was adjusted for the optimized conditions (adsorbent dosage 0.982 g/L, pH 4.185 and the contact time 150.091 min). Obtained results illustrated that the nitrate removal decreased as the ionic strength increased (Fig. 9). This removal decrease may be because of the competitive adsorption among NO3 and interfering Cl ions toward the adsorbent. Similar findings were reported by Kim and Zazouli in their works on nitrate adsorption by nZVI [22, 64]. Ali et al. [33] also reported the same result for atrazine herbicide adsorption onto the iron nanocomposite.
Fig. 9

Effect of ionic strength on nitrate adsorption efficiency

Effect of different TiO2:nZVI ratios

The efficiencies of nitrate removal by TiO2, nZVI and TiO2/nZVI were studied at determined optimized conditions. A removal efficiency of 78.9% and 65.73% were able to be achieved using TiO2 and nZVI nanoparticles alone, while the nitrate removal was increased up to 96.892% using TiO2/nZVI nanocomposites. Figure 10 shows the nitrate removal efficiencies with different ratios of TiO2/nZVI adsorbents at constant nitrate concentration of 200 mg/L and optimum conditions. On the basis of obtained results, the effect of grafting the TiO2 nanoparticles with nZVI on nitrate removal efficiency is apparent. It can be seen that TiO2:nZVI ratio has strongly influenced the nitrate removal performance of the adsorbent. By increasing the amount of TiO2 in the adsorbent composition, its performance was significantly improved. However, increasing the TiO2:nZVI ratio to more than 7 times, reduced the efficiency of the adsorbent.
Fig. 10

Effect of different TiO2/nZVI ratios on nitrate adsorption efficiency

The major problem for Fe0 application as an adsorbent is its high agglomeration and the formation of a surface oxide layer and that for TiO2 is the limit of the efficiency due to the recombination phenomenon [49]. Our results indicated that grafting TiO2 with ZVI nanoparticles has overcome these problems as well as to improve their performance. Huang et al. [49] in a similar study, reported that the efficiency of TiO2/Fe0 composite on azo dye reduction is much more than microscale and nanoscale Fe0 and TiO2 particles. Ulucan-Altuntas et al. [65] in their study on the effect of activated carbon/nZVI ratio on removal of nickel ion from water, indicated that the adsorption capacity was increased from 125 and 820 mg/g for activated carbon and nZVI, respectively, to 1190 mg/g for 50% AC/nZVI nanocomposite.

Conclusions

The developed functionalized TiO2/nZVI nanocomposites were effective for nitrate elimination from water and had a good reusability. Denitrification of nitrate by nanoscale ZVI particles includes the direct reduction by metallic iron and indirect reduction by the iron corrosion product, hydrogen; but with respect to the using of TiO2 in the composite structure, it seems that adsorption has been the main mechanism for nitrate removal. On the basis of SEM and TEM morphological analyses, the prepared nanocomposites were spherical with no agglomeration between particles and the coated core was visible. In VSM study, the response of prepared nanocomposite to the applied magnetic field indicated a ferromagnetic behavior. This magnetic property of the particles makes it easier to separate them from the solution. Comparison of the obtained results showed that the maximum adsorption by modified TiO2 was about 14.65% higher than the unmodified nanoparticles. Therefore, the combination of nano-TiO2 with nZVI would greatly improve its nitrate adsorption efficiency. The optimal conditions predicted for a maximum nitrate separation of 98.226% by TiO2/nZVI nanocomposites were: adsorbent dosage 0.982 g/L, pH 4.185 and the contact time 150.091 min. Regeneration process was carried out with NaOH and proven it was an effective agent in the discharge of nitrate.

Notes

Acknowledgements

The authors sincerely thank the officials at Islamic Azad University, Quchan Branch, for their financial support and the provision of laboratory equipment.

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Open AccessThis article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made.

Authors and Affiliations

  1. 1.Department of Chemical Engineering, Quchan BranchIslamic Azad UniversityQuchanIran

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