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REMOVAL OF TETRACYCLINE FRO M AQUEOUS SOLUTIONS USING NANOSCALE ZERO VALENT IRON AND FUNCTIONAL PUMICE MODIFIED NANOSCALE ZERO VALENT IRON.

Introduction

Tetracylines (TCs) have been widely used in human therapy and the livestock industry (Zhao et al. 2011). TCs are poorly adsorbed in metabolizm. 30-50% of the initial TCs amount could be absorbed by digestive tract of humans and animals (Song et al. 2014). Residues of TCs were excreted to the environment via different pathways such as direct discharge of agriculture products and the excretion of substances in livestock urine and feces (Zhao et al. 2011; Boxall 2003). Although TC concentrations and their byproducts were low in the environment, they may lead to development of antibiotic resistant microbial populations as well as environmental and human health problems (Zhao et al. 2011; Daughton, Ternes 1999).

The TC molecule has three multiple ionisable functional groups: a tricarbonylamide group (C-1-C-3), a dimethylammonium group (C-4) and a phenolic diketone group (C-10-C-12) and is amphoteric (Fig. 1) (Zhao et al. 2012; Li et al. 2010; Chen et al. 2011). They can undergo protonation-deprotonation reactions and present different ionic species depending on the solution pH (Zhao et al. 2011). TC species are a cation ([H.sub.3][TC.sup.+], pH< 3.3), neutral ([H.sub.2][TC.sup.0], 3.3 < pH < 7.70), an anion ([HTC.sup.-], 7.70 < pH < 9.70) or two anion ([TC.sup.2-], pH > 9.70) (Zhao et al. 2012).

Previous studies show that the principal removal mechanism of TC was adsorption process. Natural and nano materials such as goethite (Zhao et al. 2012), activated sludge (Song et al. 2014), kaolinite (Li et al. 2010), clay minerals (Chang et al. 2009a), illite (Chang et al. 2012), and magnetite nanoparticles (Zhang et al. 2011a) have been used in previous studies. Recently, nZVI in powder or granular form has been used to remove pollutants such as heavy metals, chlorinated organic, dyes, pharmaceutical compounds (Fang et al. 2011; Dickinson, Scott 2010; Kim et al. 2013; Li et al. 2012; Chen et al. 2013). This is because of its extremely small particle size, large specific surface area, high density, effective and high reactivity (Uzum et al. 2009; Cao, Zhang 2006; Kanel et al. 2006). Based on these characteristics, nZVI technology could become a promising approach for treating antibiotic wastewater. However, nZVI has usually been agglomerated in conventional systems along with a decrease in its reactivity and mechanical strength, which have limited its practical applications (Liu et al. 2014; Cumbal et al. 2003). Recently, porous materials such as zeolite (Kim et al. 2013), kaolinite (Uzum et al. 2009), pillared bentonite (Li et al. 2012) and pillared clay (Zhang et al. 2011b) have been widely used as mechanical supports to enhance the dispersibility of nZVI particles. All of these studies suggest that the use of supported nZVI is promising for the remediation of contaminated sites (Li et al. 2012). However, some immobilization methods are quite complex, so the development of more efficient immobilization systems is required for practical applications (Li et al. 2012).

Pumice is a light, porous volcanic rock and has a large surface area and skeleton structure (Asgari et al. 2012). Pumice is generally pale in color, ranging from white, cream, blue or grey to greenish-brown or black (Guler, Sarioglu 2014). Pumice has been used in water treatment as a low-cost adsorbent, filter and support media (Fang et al. 2011; Liu et al. 2014; Kitis et al. 2007). Only a few heavy metal removal studies have used pumice with nZVI (Liu et al. 2014; Moraci, Calabro 2010). For these reasons, this study focused on removal of TC onto functional pumice modified nanoscale zero valent iron (P-nZVI). The aim of this study was to understand the removal of TC antibiotic compounds from wastewaters using nZVI and PnZVI. The synthesis of the stable nZVI and P-nZVI composites, characterization, effect of various parameters on the removal process, kinetic studies of the adsorption and reduction process; adsorption isotherms and thermodynamic parameters, and possible removal mechanisms are reported in this study.

1. Materials and methods

1.1. Materials and chemicals

Pumice was provided by Kayseri-Basakpinar in Turkey and the chemical composition was 69.27% Si[O.sub.2], 22.1% [Al.sub.2][O.sub.3], 3.89% [K.sub.2]O, 3.61% [Na.sub.2]O, 2.90% [Fe.sub.2][O.sub.3], 1.82% CaO and small amounts of Mg, Ti, S, P, Mn, Ba, Zr, Cr and Zn. After washing with distilled water, it was oven dried at 50 [degrees]C. Later, the particle size of dried pumice stone was ground particles smaller than 0.125 mm. Iron (II) chloride tetrahydrate (Fe[Cl.sub.2]*4[H.sub.2]O), sodium borohydride (NaB[H.sub.4]) and ethanol were supplied by Sigma Aldrich. TC was provided by a pharmaceutical factory in Turkey. All chemicals were of analytical grade purity.

1.2. Preparation of nZVI and P-nZVI

nZVI and P-nZVI were synthesized using borohydride reduction method (Uzum et al. 2009). Fe[Cl.sub.2]*4[H.sub.2]O and sodiumborohydride (NaB[H.sub.4]) were used and pumice was used as a support material [16]. Briefly, the preparation of P-nZVI composites had pumice to nZVI mass ratio of 1:1, 1:2 and 2:1. For the 1:1; Fe[Cl.sub.2]*4[H.sub.2]O (5.34 g) was dissolved in ethanol and water mixture (30 mL; 4:1 v/v), then 1.5 g of pumice was added to this solution and stirred for 30 min. Meanwhile, a prepared 1.0 M NaB[H.sub.4] solution (3.05 g NaB[H.sub.4] in 100 mL of deionized water) was added dropwise into the pumice-[Fe.sup.2+] mixture on the magnetic stirrer. Black solid particles of nZVI appeared immediately after the first drop of NaB[H.sub.4] solution was added. After adding all of the borohydride solution, the mixture was left stirring for an additional 10 min. The reduction of iron ions by borohydride ions can be represented by the following reaction (Wang et al. 2006):

>[Fe.sup.2+] + 2[B.sup.H-.sub.4(aq)] + 6[H.sub.2]O [right arrow] [Fe.sup.0] + 2B[(OH).sub.3] + 7[H.sub.2][up arrow]. (1)

The synthesized materials were separated from the liquid phase via the vacuum filtration. Solid particles were washed with absolute ethanol. The synthesized material was oven dried at 50 [degrees]C. The pumice:nZVI mass ratios (1:2 and 2:1) were synthesized following the same method. nZVI and P-nZVI composites were stored in brown bottles for later use.

1.3. Experiments with different pumice to nZVI mass ratios

0.05 g of P-nZVI prepared by different mass ratios of pumice to nZVI (1:1, 1:2 and 2:1) was added into 100 mL of 50 mg/L TC solutions. Solution pH values were adjusted to 4.00 by ADWA pH meter. The Erlenmeyer were shaken in a shaker at 200 rpm for 60 min. After centrifugation, the supernatant solutions were analyzed with a spectrophotometer (CHEBIOS) at 357 nm wavelength.

1.4. Batch experiments

Batch experiments were performed in Erlenmeyer with 100 mL of aqueous solution. Effect of nZVI and P-nZVI amounts (2:1) (2-10 g/L), solution pH (2-10), initial TC concentration (25-300 mg/L) and temperature (25-45 [degrees]C) on TC removal using nZVI and P-nZVI was investigated. The solutions were shaken in a temperature controlled shaker (GERHARD) at 150 rpm. Sampling was made at a certain time-interval (5-180 min) and then the supernatant solutions were transferred to falcon tubes. The samples were centrifuged by centrifugation (Hettich EBA 21). All experiments were performed in duplicate. The details of the experimental conditions are presented in Table 1.

1.5. Characterization and analytical methods

The adsorption capacity ([q.sub.e], mg/g) is given below (Guler, Sarioglu 2014):

[q.sub.e] = ([[TC].sub.o] - [[TC].sub.e])V/m, (2)

where [[TC].sub.o] and [[TC].sub.e] are the initial and the equilibrium concentration (mg/L), V is the volume of solution (L) and m is the amount of nZVI and P-nZVI (g).

The removal efficiency of TC (%) is described as follows (Guler, Sarioglu 2014):

Sorption(%) = [[[TC].sub.o] - [[TC].sub.e]/[[TC].sub.o]] x 100. (3)

The nZVI and P-nZVI samples were collected after reacting with TC solution for 5, 10, 15, 30, 60, 120 and 180 min. Morphological analyses were performed using a scanning electron microscope (SEM) (LEO, 440) and transmission electron microscope (TEM) (JEOL JEM 1220). All TEM samples were prepared from depositing a drop of dilute ethanol solution of the nanoparticles onto a carbon film. The chemical bonds between the atoms and the functional groups of material were identified by FTIR method. The FTIR samples in KBr pellets were analyzed by a Perkin Elmer Spectrum 400 spectrometer. The crystalline phases of nZVI and P-nZVI were determined by used XRD diffractometer. A Cu K[alpha] incident beam ([lambda] = 1.54 nm) was used (Bruker AXS D8 Advance). The specific surface area (BET) was measured following the Brunauer-Emmett-Teller (BET) [N.sub.2] method.

2. Results and discussion

2.1. Characterization of materials

The FTIR spectrum of the nZVI and P-nZVI composites before and after TC removal are shown in Figure 2. Broads bands at 3200-3600 [cm.sup.-1] may have resulted from O-H stretching. O-H bending was observed at the 1645 [cm.sup.-1] band. Bands at <900 [cm.sup.-1] in the nZVI may be related with iron oxides. These bands in the P-nZVI are weaker due to reduced oxidation of pumice modified [Fe.sup.0]. According to Kim et al. (2013), the pumice support may reduced Fe hydroxide formation. Similar results were obtained from other studies (Kim et al. 2013; Yuan et al. 2009). Bands at 1336-1128 [cm.sup.-1] resulted from ethanol used in preparing the composites. Composites may include bands associated with sulfate green rust and lepidocrocite formation on some [Fe.sup.0] surfaces (Kim et al. 2013; Andrade et al. 2009). The FT-IR spectra of TC-nZVI and TC-P-nZVI are different from those of nZVI and P-nZVI.

XRD spectrums of nZVI, TC-nZVI, P-nZVI, and TC-P-nZVI are presented in Figure 3. There were peaks at 22[degrees] and 28[degrees] in P-nZVI, which are associated with mineral dachiardite (Ca, Na, K, Al, Si and [H.sub.2]O) (Guler, Sarioglu 2014; Ersoy et al. 2010). The apparent peak at 44.9[degrees] in nZVI and P-nZVI indicated that [Fe.sup.0] nanoparticles were modified onto the pumice surface (Chen et al. 2011; Fang et al. 2011; Zhang et al. 2011c). The peaks at 30[degrees] and 33[degrees] are related to [Fe.sub.3][O.sub.4]/[gamma]-[Fe.sub.2][O.sub.3] and FeO, respectively. These iron oxides also appeared in P-nZVI (Zhang et al. 2011c; Kanel et al. 2005).

The SEM images of nZVI (a), P-nZVI (b), TC-nZVI (c), and TC-P-nZVI (d) are shown in Figure 4. The nZVI and P-nZVI composites were aggregated from the van der Waals and magnetic forces (Kim et al. 2013; Wang et al. 2013). The SEM images after reaction showed that the external surfaces of nZVI and P-nZVI were covered by TC.

In Figure 5 is presented TEM images of the nZVI and P-nZVI. The laboratory prepared nZVI and P-nZVI were spherical. TEM images also show that most particles formed chain-like aggregates (Sun et al. 2006). Aggregation of the nanoparticles is caused by the large surface area and magnetic dipole-dipole interactions of the particles. TEM analysis of a aged sample of nZVI showed that the chainlike structure was stil dominant but indicated also the presence of some larger floc aggregates, stemming possibly from the enhancement in oxide formation (Uzum et al. 2008). The BET surface area, total pore volume and mean pore size of nZVI and P-nZVI were 21.67 [m.sup.2][g.sup.-1], 0.10 [cm.sup.3][g.sup.-1], 40 nm and 12,41 [m.sup.2][g.sup.-1], 0.05 [cm.sup.3][g.sup.-1], 27 nm, respectively.

2.2. Parameters affecting TC removal

2.2.1. Pumice:nZVI mass ratio

Figure 6a showed that removal efficiency of TC increased the mass ratio as increased from 1:2 to 2:1. Using pumice as support material for nZVI created a synergetic effect for the TC removal process (Kim et al. 2013). Similar trends were reported for nitrate (Zhang et al. 2011b), organic contaminants (Zhang et al. 2011d) and Cr(VI) removal (Li et al. 2012).

2.2.2. Effect of nZVI and P-nZVI amounts

The removal efficiency of TC varied with contact time, from 5 min to 180 min under with different amounts of nZVI and P-nZVI (Fig. 6b). Removal efficiency of TC increased with increasing nZVI and P-nZVI amount and contact time. The removal efficiency of TC increased from 67% to 77% and 37% to 52% by increasing amounts from 1 to 5 g/L of nZVI and P-nZVI amount, respectively. Figure 4b shows that 5 g/L nZVI and P-nZVI exhibited better removal efficiency than 1 g/L nZVI and P-nZVI. The removal efficiency of TC by nZVI was much higher than P-nZVI. These results may be explained with the more adsorptive and active surface sites (Chen et al. 2013). The optimum dosage of nZVI and P-nZVI was selected as 5 g/L which was used in further experiments.

2.2.3. Effect of pH value

The pH is an important factor affecting removal efficiency of TC by iron (Chen et al. 2011). The effect of pH on the TC removal rates and the zero point of charge ([pH.sub.pzc]) are presented in Figure 6c. The effects of different initial solution pH (2-4-6-8-10) were studied using a 100 mL solution containing 50 mgTC/L with contact times of 5-10-15-30-60-120-180 min. [pH.sub.pzc] values of nZVI and P-nZVI were found to be approximately 9.30.

After 180 min at pH 2, 4 and 6, the average removal efficiency of TC was approximately 92% and 88% for nZVI and P-nZVI; at pH 8 and pH 10, it was approximately 88% and 72%, respectively. At pH < 3.3; 3.3 < pH < 7.7 and 7.70 < pH < 9.70, [H.sub.3][TC.sup.+] (cationic), [H.sub.2][TC.sup.0] (zwitterionic) and HTC- (anionic) species of TC were dominant (Zhao et al. 2012; Chen et al. 2011). When pH was lower than the [pH.sub.PZC] of nZVI and P-nZVI, the composites surface was positively charged. The removal mechanism might be via surface complexation and/or cation exchange on the surface sites (Fang et al. 2011). When pH was greater the [pH.sub.PZC] of nZVI and P-nZVI, surfaces areas were negatively charged. In this situation, the decrease in removal efficiency may be explained by electrostatic repulsion between surface areas and TC molecules (Ersoy et al. 2010). At lower pH, corrosion of nZVI composite was accelerated and [Fe.sup.3+]-TC hydroxides on the iron surface contributed to the increase of TC removal (Li et al. 2012). Similarly, TC removal by P-nZVI decreased when initial pH increased. The reason for this condition may have an additional effect of functional groups on pumice. These results indicated that the TC removal using nZVI and P-nZVI can be studied over a wide pH range.

2.3. Adsorption isotherms

The equations of the Langmuir, Freundlich and D-R models are given below (Guler, Sarioglu 2014; Gao et al. 2012):

[q.sub.e] = [Q.sub.m]b[[TC].sub.e]/1 + b[[TC].sub.e]; (4)

[q.sub.e] = [k.sub.F][[TC].sub.e.sup.1/n]; (5)

[mathematical expression not reproducible]; (6)

[q.sub.e] = [RT/[b.sub.T]]ln([K.sub.T][[TC].sub.e]), (7)

where, [[TC].sub.e] (mg/L) indicates the equilibrium concentrations in aqueous phase after the adsorption of TC on nZVI and P-nZVI. [q.sub.e] (mg/g) is the equilibrium adsorption capacity of nZVI and P-nZVI x [Q.sub.m] (mg/g) and [q.sub.D-R] (mol/g) are the maximum adsorption capacities, b (L/mg), [k.sub.F] (L/g), and [beta] ([mol.sup.2]/[J.sup.2]) are the constant of Langmuir, Freundlich and D-R models, respectively. n and ? (J/mol) are the Freundlich linearity index and the Polanyi potential, respectively. The mean free energy E (kJ/mol) is expressed as below:

E = 1/[square root of (2[beta])]. (8)

Isotherm graphics are given in Figure 7 a, b and the isotherm parameters and correlation coefficients for TC are listed in Table 2.

As shown in Table 2, experimental data excellent fit both the Langmuir and Freundlich isotherm models (all exceed 0.960). Thus, the adsorption process occurred on a homogeneous adsorbent surfaces and on a reversible heterogeneous surfaces in the adsorption sites (Chen et al. 2013). The maximum adsorption capacity ([Q.sub.m]) of TC for nZVI and P-nZVI was 105.46 mg/g and 115.13 mg/g, respectively. These values were much higher than adsorptions capacities of other adsorbents (Table 3) (Li et al. 2010; Chen et al. 2011; Xu, Li 2010; Chang et al. 2009b; Figueroa et al. 2004). P-nZVI had a much higher b (L/mg) (Langmuir constant) than nZVI, which verifies the advantageous TC adsorption of P-nZVI (Xu, Li 2010). The E value of adsorption was higher than 8 kJ/mol for all results, suggesting that the adsorption mechanism may be a chemisorption.

2.4. Kinetic studies

The TC removal on nZVI and P-nZVI was contained adsorption and reduction process.

2.4.1. Adsorption kinetics

The non-linear form of the pseudo-first order, pseudo-second order and intra particle models are applied to adsorption studies. These models depended on the physical and/ or chemical properties of the adsorbent (Chen et al. 2013). The pseudo-first-order model is given below:

[mathematical expression not reproducible], (9)

where [q.sub.e] and [q.sub.t] (mg/g) are the amounts of TC molecules adsorbed on the nZVI and P-nZVI at equilibrium and different time and [k.sub.1] ([min.sup.-1]) is the rate constant of the pseudo-first-order model for the adsorption process.

The pseudo-second-order model can be shown as follows:

[q.sub.t] = [k.sub.2][q.sub.e.sup.2]t/1 + [k.sub.2][q.sub.e]t, (10)

where [q.sub.e] and [q.sub.t] (mg/g) are the amounts of TC molecules adsorbed on the nZVI and P-nZVI at equilibrium and different time and [k.sub.2] (g.[mg.sup.-1][min.sup.-1]) is the rate constant of the pseudo-first-order model for the adsorption process.

Intraparticle model is given below:

[q.sub.t] = [k.sub.i][t.sub.0.5] + C, (11)

where: [k.sub.i] (mg/g x [min.sup.0.5]) and C are the rate constant of intraparticle diffusion model and the intercept, respectively. The larger the intercept is the greater is the contribution of the surface sorption in the rate controlling step. The plots of qt vs. t for pseudo-first order and pseudo-second order kinetic model and the plot of qt vs. [t.sup.0.5] for intra particle model are presented in Figure 8a, b. The calculated coefficients and correlation coefficients are listed in Table 4.

According to the results, adsorption kinetics for all studies fit well with the pseudo-second-order model; the correlation coefficient (R2) ranged from 0.997 to 0.999. This showed that the rate controlling step in adsorption process may be chemisorptions and TC adsorption occurs probably via van der Waals forces or ion exchange between the adsorbent and TC (Zhao et al. 2011; Wang et al. 2013). In addition, intraparticle diffusion model is not the one rate controlling step due to it was not linear (Guler, Sarioglu 2014; Ersoy et al. 2010). Therefore; TC adsorption was affected both intra particle diffusion and boundary diffusion.

2.4.2. Reduction kinetic

The pseudo-first-order kinetic model was used for TC removal by nZVI and P-nZVI. Pseudo-first-order kinetics is showed as below (Chen et al. 2013; Zhang 2003):

ln[[TC].sub.e]/[[TC].sub.e] = -[k.sub.obs]t, (12)

where [k.sub.obs]([min.sup.-1]) is the rate constant of a pseudo-first-order kinetic model (Table 5). The removal of TC by nZVI and P-nZVI composites fitted well to the pseudo-first order kinetic model. The [k.sub.obs] decreased for nZVI and P-nZVI as the initial TC concentration increased from 25 to 300 mg/L, respectively. This indicated that the reduction of TC occurs at the interface of nZVI and P-nZVI (Chen et al. 2013; Shi et al. 2011). The rate of reduction was related to the active sites of adsorbent and the initial TC concentration (Chen et al. 2013). Moreover, the rate constants [k.sub.obs] of P-nZVI were higher than nZVI. Pumice provided a mechanical supports and reactivity (Chen et al. 2013; Choi et al. 2009).

2.5. Thermodynamic parameters

The standard free energy change ([DELTA][G.sup.o]; kJ/mol), standart enthalpy change ([DELTA][H.sup.o]; kJ/mol) and standart entropy change ([DELTA][S.sup.o]; kJ/mol.K) were calculated using the following equations (Chen et al. 2013):

[DELTA][G.sup.o] = -RTln[K.sub.c]; (13)

ln[K.sub.c] = [[DELTA][S.sup.o]/R] - [[DELTA][H.sup.o]/RT], (14)

where T is the reaction temperature (Kelvin) and [K.sub.c] is the [q.sub.e]/[C.sub.e]. [DELTA][H.sup.o] and [DELTA][S.sup.o] were determined from a plot of ln [K.sub.L] versus 1/T. The thermodynamic parameters are listed in Table 6.

Negative values of [DELTA][G.sup.o] and [DELTA][H.sup.o] indicate that TC adsorption was spontaneous, feasible and exothermic. The negative value of [DELTA]S showed that the adsorption process is due to an associative mechanism and decreased randomness. The negative [DELTA]S value (-0.51 kJ/mol for P-nZVI) mean that the decreased randomness at the solid/liquid interface during adsorption.

2.6. Proposed mechanism for TC removal by nZVI and P-nZVI

The core and shell structure of nZVI composites provide two uptake mechanisms. The core is an electron source and can reduce ions, so it has a higher reduction potential than iron. The shell has hydroxyl groups and it provides uptake on the nZVI surface of adsorbate via surface complexation (Uzum et al. 2009).

Based on the results presented here, possible mechanisms for TC removal from aqueous solutions involves (i) TC adsorption by nZVI and P-nZVI and (ii) TC reduction via oxidation of [Fe.sup.0] to [Fe.sup.3+] (Li et al. 2012; Chen et al. 2013; Zhang et al. 2012). The yellow flocculent precipitates were observed, indicating that [TC.sub.x] [Fe.sub.(1-x)] [(OH).sub.3] complexes were formed. Similar results were reported by other researches (Chen et al. 2011). Possible reactions and mechanism are as follows (Eqs (15)-(19) and Fig. 9):

TC + nZVI or P-nZVI [right arrow] TC-nZVI or TC-P-nZVI (adsorption); (15)

TC + [Fe.sup.0] + 2[H.sup.+] [right arrow] [Fe.sup.2+] + [H.sub.2] + TC + 2[e.sup.-] (oxidation in acidic solution); (16)

TC + [Fe.sup.0] + 2[H.sub.2]O [right arrow] [Fe.sup.2+] + [H.sub.2] + 2O[H.sup.- ] + TC + 2[e.sup.-] (oxidation in basic solution); (17)

TC + 2[Fe.sup.2+] + 2[H.sup.+] + 1/2[O.sub.2] [right arrow] [Fe.sup.3+] + TC + [H.sub.2]O (oxidation in acidic solution); (18)

xTC + (1-x)[Fe.sup.3+] + 3[H.sub.2]O [right arrow] [TC.sub.x][Fe.sub.(1- x)][(OH).sub.3] + 3[H.sup.+] (reduction). (19)

Conclusions

This study demonstrated that nZVI and P-nZVI composites can be used successfully used for TC removal in aqueous solutions. Pumice was an effective support material as a dispersant and stabilizer that reduced nZVI aggregation. Mean pore diameters of nZVI and P-nZVI were 40 nm and 27 nm, respectively. pH significantly affects removal efficiency. FTIR, XRD, SEM and TEM analyses indicated that nZVI particles were dispersed on the surface of pumice and aggregation of nZVI was decreased. The removal of TC by nZVI and P-nZVI may comprise two processes: (i) reduction (ii) adsorption.

Finally, the high removal efficiency (91% and 92% for nZVI and P-nZVI, respectively) demonstrated that nZVI and P-nZVI are promising materials for treating pharmaceutical compounds in wastewaters.

https://doi.org/10.3846/16486897.20161210156

Acknowledgements

This work was supported by the Scientific Research Project Fund of Cumhuriyet University under Grant [number M-479].

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Ulker Asli GULER. Doctor of Environmental Science (2010). Ass. Prof. Dr of Environmental Engineering Department of Cumhuriyet University. Research interest: wastewater treatment, adsorption/biosorption, environmental technologies.

Ulker Asli GULER

Department of Environmental Engineering, Engineering Faculty, Cumhuriyet

University, Sivas, Turkey

Submitted 16 Feb. 2016; accepted 04 Jul. 2016

Corresponding author: Ulker Asli Guler

E-mail: ulkerasli@gmail.com

Caption: Fig. 1. Structure of TC

Caption: Fig. 2. FTIR patterns of nZVI (a), TC-nZVI (b), P-nZVI (c), and TC-P- nZVI (d)

Caption: Fig. 3. XRD spectrums of nZVI (a), TC-nZVI (b), P-nZVI (c), and TC-P- nZVI (d)

Caption: Fig. 4. SEM images of of nZVI (a), P-nZVI (b), TC-nZVI (c), and TC-P- nZVI (d)

Caption: Fig. 5. TEM images of of nZVI (a), and P-nZVI (b)

Caption: Fig. 6. Effect of different parameters on TC removal (a) mass ratios of pumice to nZVI (Conditions: 298 K, 200 rpm, 50 mgTC/L, [pH.sub.(initial)]: 4.0, 5 g/L, 100 mL), (b) adsorbent amount and [pH.sub.PZC] (Conditions: 298 K, 200 rpm, 50 mgTC/L, [pH.sub.(initial)]: 4.0, pumice:nZVI = 2:1, 100 mL), (c) solution pH (Conditions: 5 g/L, 298 K, 200 rpm, 50 mgTC/L, pumice:nZVI = 2:1, 100 mL)

Caption: Fig. 7. Langmuir, Freundlich (a) and D-R (b) isotherm plots for adsorption of TC onto nZVI and P-nZVI

Caption: Fig. 8. Pseudo-first, pseudo-second order kinetic (a) and intra particle model (b) of TC onto nZVI and P-nZVI

Caption: Fig. 9. Proposed mechanism for TC removal by nZVI and P-nZVI
Table 1. Experimental conditions for adsorption studies

                                     Experimental conditions

Set   Aim of experiment        Pumice:nZVI      pH      TC conc.
                               mass ratio                (mg/L)

1     Pumice:nZVI             1.1, 1:2, 2:1    4.0         50
      mass ratio

2     Effect of nZVI and           2:1         4.0         50
      P-nZVI amounts

3     Effect of pH value           2:1        2-4-6-       50
                                               8-10

4     Adsorption isotherms         2:1         8.0     25-50-100-
                                                        200-300

5     Adsorption kinetics          2:1         4.0         50

6     Effect of temperature        2:1         8.0         50

                           Experimental conditions

Set    Adsorbent      Contact time (min)     Temperature
      amount (g/L)

1          5                  60             25[degrees]C

2        2-5-10      5-10-15-30-60-120-180   25[degrees]C

3          5         5-10-15-30-60-120-180   25[degrees]C

4          5         5-10-15-30-60-120-180   25[degrees]C

5          5         5-10-15-30-60-120-180   25[degrees]C

6          5         5-10-15-30-60-120-180      25-35-
                                             45[degrees]C

Table 2. Isotherm parameters and correlation coefficients of TC
adsorption on nZVI and P-nZVI

                     nZVI

                Langmuir model

[Q.sub.m] (mg/g)   b (L/mg)   [R.sup.2]

105.46              0.0063      0.962

                    P-nZVI

                Langmuir model

[Q.sub.m] (mg/g)   b (L/mg)   [R.sup.2]

115.13              0.0070      0.970
                      nZVI

                  Freundlich model

[k.sub.F](L/g)    1/n    [R.sup.2]   [q.sub.D-R] (mol/g)

2.267             0.62     0.979           0.0009

                      P-nZVI

                  Freundlich model

[k.sub.F](L/g)    1/n    [R.sup.2]   [q.sub.D-R] (mol/g)

2.356             0.65     0.982           0.0012

                          nZVI

                        D-R model

E (kJ/mol)   [beta] ([mol.sup.2]/[J.sup.2])    [R.sup.2]

9.63               5.3978.[10.sup.-9]            0.970

                         P-nZVI

                        D-R model

E (kJ/mol)   [beta] ([mol.sup.2]/[J.sup.2])    [R.sup.2]

9.38               5.6889.[10.sup.-9]            0.975

Table 3. TC adsorption capacities using different adsorbents

Adsorbent          [Q.sub.m] (mg/g)    Reference

nZVI                    105.46         This study
P-nZVI                  115.13         This study
Kaolinite                4.32          (Li et al. 2010)
Marine sediments       16.7-33.3       (Xu, Li 2010)
Palygorskite             61.8          (Chang et al. 2009b)
Montmorillonit            54           (Figueroa et al. 2004)

Table 4. Adsorption kinetic parameters of the TC removal by the
nZVI and P-nZVI

                             Pseudo-first order

[q.sub.e,exp]      [k.sub.1]      [q.sub.1]
(mg/g)           ([min.sup.-]1)    (mg/g)     [R.sup.2]

nZVI

9.10                 0.198          7.79        0.728

P-nZVI

9.16                 0.221          8.23        0.768

                               Pseudo-second order

                    [k.sub.2]
[q.sub.e,exp]    (g x [mg.sup.-1]   [q.sub.2]
(mg/g)            [min.sup.-1)       (mg/g)     [R.sup.2]

nZVI

9.10                  0.038           8.35        0.900

P-nZVI

9.16                  0.043           8.77        0.937

                      Intra particle diffusion

                   [k.sub.i]
[q.sub.e,exp]       (mg/g x
(mg/g)           [min.sup.0.5])    C     [R.sup.2]

nZVI

9.10                 0.277        5.33     0.984

P-nZVI

9.16                 0.253        5.99     0.975

Table 5. [k.sub.obs] for removal of TC

TC conc (mg/L)  [k.sub.obs] ([min.sup.-1])    [R.sup.2]

nZVI     25                0.012                0.902
         50                0.007                0.959
         100               0.002                0.952
         200               0.001                0.861
         300               0.002                0.943

P-nZV    25                0.018                0.972
         50                0.008                0.964
         100               0.004                0.963
         200               0.002                0.974
         300               0.004                0.982

Table 6. Thermodynamic parameters obtained for TC removal
onto nZVI and P-nZVI composites

         [DELTA][H.sup.o]    [DELTA][S.sup.o]
             (kJ/mol)          (kJ/mol x K)

nZVI           -2.87               52.84
P-nZVI        -17.95               -0.51

          [DELTA][G.sup.o] (kJ/mol)

         298 K    308 K    318 K

nZVI     -18.85   -18.82   -19.93
P-nZVI   -19.13   -17.44   -17.93


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Author:Guler, Ulker Asli
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Date:Sep 1, 2017
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