Eco friendly adsorbents for removal of phenol from aqueous solution employing nanoparticle zero-valent iron synthesized from modified green tea bio-waste and supported on silty clay

Shaimaa T.Kadhum,Ghayda Yassen Alkindi,Talib M.Albayati

1 Department of Civil Engineering,University of Technology,52 Alsinaa St.,Baghdad,PO Box 35010,Iraq

2 Department of Chemical Engineering,University of Technology,52 Alsinaa St.,Baghdad 35010,Iraq

Keywords:Wastewater treatment Environment Nano zero-valent iron Silty clay Phenol Adsorption

ABSTRACT The present research investigated a novel route for the synthesis of nanoparticle zero-valent iron(NZVI)utilizing an aqueous extract of green tea waste as a reductant with ferric chloride.Also,the supported nanoparticle zerovalent iron was synthesized using natural silty clay as a support material(SC-NZVI).The NZVI and SC-NZVI were characterized by infrared spectroscopy(FTIR),scanning electron microscope(SEM),X-ray diffraction(XRD),Brunauer–Emmett–Teller(BET),and zeta potential(ζ).The interpretation of the results demonstrated that the polyphenol and other antioxidants in green tea waste can be used as reduction and capping agents in NZVI synthesis,with silty clay an adequate support.Additionally,the experiments were carried out to explore phenol adsorption by NZVI and SC-NZVI.To determine the optimum conditions,the impact of diverse experimental factors(i.e.,initial pH,adsorbent dose,temperature,and concentration of phenol)was studied.Langmuir,Freundlich,and Tempkin isotherms were used as representatives of adsorption equilibrium.The obtained results indicated that the adsorption processes for both NZVI and SC-NZVI well fitted by the Freundlich isotherm model.The appropriateness of pseudo_first_order and pseudo_second_order kinetics was investigated.The experimental kinetics data were good explained by the second-order model.The thermodynamic parameters(ΔH0,ΔS0,and ΔG0)for NZVI and SC-NZVI were determined.The maximum removal rates of phenol at optimum conditions,when adsorbed onto NZVI and SC-NZVI,were found to be 94.8%and 90.1%,respectively.

1.Introduction

Until recently,80% of the total wastewater of the globe was discharged without treatment,including various contaminants from human waste to hazardous and toxic industrial wastewater[1].When water is polluted,it is costly,difficult,and usually infeasible to remove the contaminants.Water pollution not only causes the death of many organisms but also disrupts entire ecosystems.

Phenol is considered a ubiquitous and hazardous organic pollutant in water and wastewater and is very toxic even at low concentrations.Thus,phenol and its derivatives are a priority for removal,as they should not exceed the order of μg·L?1[2].Several techniques have been utilized to get rid of phenol from water and wastewater,such as chemical oxidation[3,4],microbial degradation[5,6],membrane separation[7],photocatalytic degradation[8],solvent extraction[9],ultrasonic degradation[10],enzymatic polymerization[11],and adsorption[12,13].Until now,adsorption has been the most agreeable technique as a result of its efficiency,low operational cost,and easy operation.

Nanoparticle zero-valent iron(NZVI)synthesized via chemical and physical methods has drawbacks including cost,requiring specific and expensive equipment,expending a great deal of power,producing inflammable hydrogen,and using toxic and costly chemicals,such as sodium borohydride,organic solvents,and dispersing and stabilizing agents[14].So,the synthesis of nanoparticle zero-valent iron has been advanced using an extraction from plants,such as green tea[15],eucalyptus[16],banana peel[17],Eriobotrya japonica[18],and mint[19].The green synthesis of nanoparticle zero-valent iron has emerged as a cost-effective,natural,and eco-friendly alternate process.Also,it does not use or produce hazardous materials [20].Nano zerovalent iron(NZVI)has high surface energy,high conductivity,and a large specific surface area,so it has considerable potential in the remediation of water and soil[21–23].Unfortunately,NZVI particles without any modification are inclined to aggregate and oxidize due to van der Waals and magnetic attraction forces[24],thus decreasing their performance in application.The agglomeration property has been confirmed to be highly reduced when a support is used for the reduction of an iron salt and also to produce,uniformly disperse,and accommodate the nano zerovalent iron[25].Hence,many immobilizing agents have evolved to overcome this problem,such as chitosan-stabilized nano zerovalent iron[26],montmorillonite-supported nano zerovalent iron[27],and carboxymethyl-cellulose-stabilized nano zerovalent iron[28].

Green tea waste (GTW) is one of the abundant biowastes due to heavy tea consumption by consumers.Polyphenol and other antioxidants in GTW could serve as a reductant for adsorbent synthesis.In this work,we have reported for the first time the synthesis of green NZVI using the extract of GTW as a reduction agent with iron salt.Also,we have focused on a novel preparation of supported nanoparticle zero-valent iron by using silty clay.In addition,a detailed characterization for both synthesized adsorbents(i.e.,NZVI and SC-NZVI)was reported,followed by investigation of the performance of synthesized NZVI and SC-NZVI as adsorbents to adsorb phenol in a batch experiment system.

2.Materials and Methods

2.1.Materials

The commercial leaves of green tea were the source of polyphenol utilized in this work.Ferric chloride anhydrous(FeCl3),phenol crystal(C6H5OH),0.01 mol·L?1sodium hydroxide(NaOH),0.01 mol·L?1sulfuric acid(),and deionized water were used.All chemicals were purchased from Thomas Baker Chemicals(India).The natural silty clay used in the present study was obtained from Mosul,Iraq.

2.2.Preparation of green tea waste extract

The tea extract was formed according to the procedure that followed by Wang et al.[29] (with some modification) by adding 20 g of tea waste to 200 ml of deionized water.The solution was heated at 90°C for 30 min and filtered through Whatman No.1 filter paper.The resulting filtrate was collected and stored in a clean,dried beaker for further use.

2.3.Synthesis of NZVI

During the synthesis of NZVI,both ferric chloride(FeCl3)and green tea waste extract were mixed in a clean,sterilized beaker in a 1:1 proportion.Two hundred ml of green tea waste extract was added to 200 ml of freshly prepared 0.09 mol·L?1aqueous FeCl3.After the addition of green tea waste extract to the ferric chloride solution,there was an immediate color change from golden to black,as shown in Fig.1.The pH of the resulting solution was 2.16.The immediate color change of the mixture and the reduction in the pH indicate nano iron formation.The pH was increased to 6 by using 0.1 mol·L?1NaOH.A magnetic stirrer mixed the solution for 1 h,and then,it was filtered by Whatman No.1 filter paper,which was dried at 65°C for 4 h to recover granular NZVI on the filter paper.

2.4.Synthesis of silty clay-supported nanoparticle zero-valent iron(SCNZVI)

A liquid-phase reduction method with silty clay as a support material was used to synthesis SC-NZVI.First,2 g of silty clay was dissolved in 150 ml of deionized water and mixed with 150 ml of freshly prepared 0.1 mol·L?1aqueous FeCl3.Second,100 ml of green tea waste extract was added to the iron/silty clay mixture.As in the synthesis of NZVI,there was an immediate color change and reduction in the pH value.After that,the pH was increased to 6 using 0.1 mol·L?1NaOH.A magnetic stirrer mixed the solution for one h,after which it was filtered by Whatman No.1 filter paper and dried at 65°C for 4 h to recover granular SC-NZVI on the filter paper.

2.5.Characterization

The external morphology texture,crystalline structure,and orientation of materials making up the NZVI and SC-NZVI samples were analyzed using a TESCAN VEGA III (Czech Republic) scanning electron microscope(SEM).The θ-2θ X-ray diffraction(XRD)patterns were collected over the 2θ range of 0–80°on an XRD-6000(Shimadzu,Japan)diffractometer equipped with Cu Kαradiation (λ=0.154060 nm).The FTIR spectrum of the NZVI and SC-NZVI were also scanned within the spectral range of 4000–400 cm?1using an IRAffinity-1S(Shimadzu,Japan) spectrophotometer.The zeta potentials of NZVI and SC-NZVI were measured by a ZetaPlus(Brookhaven Instruments)at 25°C to analyze the surface charge.The specific surface area,pore volume,and pore size of the NZVI and SC-NZVI were measured via the BET–“N2”adsorption–desorption method using the micromeritics ASAP 2020.

2.6.Batch adsorption experiments

Batch adsorption experiments were conducted to assess the performance of the synthesized NZVI and SC-NZVI to adsorb phenol and remove it from the aqueous solution.A calibration curve of the phenol was made using a UV-9200 (Biotech Engineering Management Co.Ltd.),with a λ max of 268 nm.All individual experiments were conducted in a 500-ml beaker.A specific weigh of NZVI or SC-NZVI was added to the prepared solution of a specific concentration of phenol(C6H5OH)and deionized water.All pH regulations were done by 0.1 mol·L?1NaOH or 0.1 mol·L?1H2SO4.The produced solution was agitated at 250 r·min?1using a magnetic stirrer.Samples were filtered using a 0.45-μm syringe filter.The absorbance of phenol was acquired by UV spectroscopy.Depending on the absorbance and calibration curve,the concentration and removal rate of phenol were calculated.

Fig.1.Color change due to the addition of tea extract to ferric chloride solution.

where C0and Ctare the concentrations of phenol at the times zero and t,respectively.

2.7.Adsorption isotherm

The adsorption isotherm parameters can be applied to demonstrate the adsorbent–adsorbate reaction,which provides significant information about the surface characteristics,activity of the adsorbent,and mechanism of adsorption[30].

The Langmuir adsorption isotherm described the adsorbateadsorbent system equilibrium,where the adsorption of the adsorbate is confined to one molecular layer(monolayer)either at or before a relative pressure of unity is reached.The straight shape of the Langmuir formula is reported as follows[31]:

where qm(mg·g?1)is the Langmuir maximum adsorption capacity;KL(L·mg?1)is the constant relating to the adsorption capacity;Ceqis the adsorbate concentration at the equilibrium in the solution(mg·L?1);and qeis the adsorbate concentration at equilibrium onto the adsorbent(mg·g?1).

The form of this isotherm can also be written in terms of the dimensionless equilibrium parameter RL(separation factor),which is express as follows[32]:

where C0is the initial adsorbate concentration(mg·L?1).The isotherm is considered favorable if RL<1,unfavorable if RL>1,irreversible if RL=0,and linear if RL=1[33,34].

The Freundlich isotherm for heterogeneous surface energy systems is given as follows:

where n and Kfare the Freundlich constants.These constants can be obtained from the linear plot of lnqeversus lnCeq,which represent the slope and intercept,respectively.

The Temkin isotherm was used in the following form[32,35]:

where β=RT/b;R is the universal gas constant;T is the absolute temperature(Kelvin);β is the Tempkin constant(J·mg?1);and α and β are computed from the slop and intercept,respectively,of qevs.lnCe.

2.8.Kinetics

Adsorption kinetics is a linear relation that explains the rate of retention or release of adsorbate from an aqueous environment to a solidphase interface at different conditions.

The pseudo-first-order(Lagergren model)explains the adsorption of adsorbate onto the adsorbent following the first order mechanism.

where qtis the adsorbate adsorbed at time t(mg·g?1);and K1is a constant(min?1),which is determined by plot ln(qe–qt)versus t.

Pseudo-second-order assumes that the rate of adsorption of the adsorbate is proportional to the available sites on the adsorbent.The linearized form is[36].

2.9.Adsorption thermodynamics

The temperature impact and thermodynamic parameters(i.e.,ΔH0,ΔS0,and ΔG0)for phenol adsorption onto NZVI and SC-NZVI were investigated.These parameters were computed according to the following formulas[37]:

Fig.2.SEM and EDS images:(a)silty clay;(b)NZVI and(c)SC-NZVI;(d)dispersion of iron on the surface of bentonite.

where ΔG0is the free energy change,ΔH0is the enthalpy change,ΔS0is the entropy change,and R is the universal gas constant (8.314 J·mol?1·K?1).The slope and interception of the plot of lnKDversus 1/T determines the values of ΔH0and ΔS0.

3.Results and Discussion

3.1.Characterization of NZVI and SC-NZVI

Fig.3.XRD patterns of samples.

Scanning electron microscope (SEM) analysis was carried out to study the morphology and structure of the surface of silty clay,NZVI,and SC-NZVI.The results indicate that the silty clay particles exhibited a dendritic structure,as shown in the SEM images in Fig.2(a).This structure increased the available surface area for the reaction.Fig.2b indicates that there were many nodular protrusions on the surface of the synthesized NZVI.These protrusions represent the prepared zerovalent iron(Fe)particles.They were cubical in shape,with many having a diameter of around 100 nm.The aggregation of NZVI,which is clearly demonstrated in Fig.2(b),caused a reduction in the reaction activity of the NZVI,which explains the reasons that NZVI is commonly immobilized on support materials.In contrast,the SC-NZVI seemed in a fine,dispersive state,as illustrated in Fig.2(c).These graphs confirmed that the agglomeration of the supported NZVI was restricted and provided a homogeneous dispersion on the surface of the silty clay.The distribution of iron on silty clay surface was characterized by EDS[Fig.2(d)],result found iron particles distributed uniformly on the silty clay surface.This further proved silty clay was able to disperse Fe0particles and could prevent the aggregation of NZVI effectively.

The X-ray diffraction(XRD)technique has been employed to determine the materials and crystalline structure of silty clay as well as synthesized NZVI and SC-NZVI(Fig.3).The XRD pattern indicated that the silty clay contained quartz as a non-clay mineral,represented at 2θ=26.7°[38].The sharp peak at 2θ=29.5°represents calcite[39].As exhibited in Fig.3b,less obvious characteristic peaks of zerovalent Fe(α-Fe)were observed at about 2θ of 44°and 65°,which corresponds to standard data(JCPDS File No.00-006-0696).This result indicates to the low degree of crystallinity of NZVI[40].The distinguished reflections at 2θ=33.3°and 62.2°were identified as iron oxides(i.e.,Fe2O3and Fe3O4) [41,42].The peak at 2θ=24.6° represents the organics that adsorbed from the green tea waste as capping/stabilizing agents[26,43].The XRD pattern of SC-NZVI is shown in Fig.3(c).The sharp peak at 2θ=26.7° represents quartz.The organic materials Fe2O3,Fe3O4,and Fe0were observable in the XRD spectra of the SC-NZVI.

Fig.4.FTIR analysis of samples.

Fourier transform infrared spectroscopy(FTIR)is an analytical technique used to identify organic,polymeric,and,in some cases,inorganic materials.FTIR spectra provide information about the surface functional groups on the NZVI and SC-NZVI.The spectra for NZVI and SC-NZVI are shown in Fig.4.For the FTIR spectra of NZVI,the highest peak was in the range between the wave numbers 3410 cm?1and 3354 cm?1,which correspond to polyphenols[24,43,44].This indicates that many specific phenolic functional groups can reduce Fe3+to Fe0[45–47].The same result appeared in the range between the wave numbers 3431 cm?1and 3305 cm?1of the FTIR spectra of SC-NZVI.The presence of bands attributed to polyphenolic compounds at 1624 cm?1in NZVI and 1629 cm?1in SC-NZVI is due to a C=C aromatic ring stretching vibration,whereas 1018 cm?1and 1080 cm?1relate to a C—O—C and O—H absorption peak.These peaks reveal the prominent phenolic functional groups in the FTIR analysis.The abundance of phenolic groups provides an appropriate molecular structure for the effective delocalization of the unpaired electron,which is attributed to the free radical scavenging potential of the green tea waste extract.Moreover,there is a positive correlation between the antioxidant activity and the number of the phenolic hydroxyl groups[48].

A strong adsorption peak observed in the FTIR spectra of SC-NZVI at 1028 cm?1indicates the functional group of silicate ions[49].The FTIR spectra of NZVI and SC-NZVI revealed several peaks around 540 cm?1and 420 cm?1,which refer to Fe-O stretches of Fe3O4and Fe2O3,affirming the generation NZVI,which were then oxidized to iron oxide after being exposed to air and water[50].

Zeta potential analysis is a technique for monitoring the electrokinetic charge of the surface of the nanoparticles in solution.Zeta potential is the most important index of the colloidal dispersions'stability.In this work,Zeta potential was ?16 mV for NZVI,while for SC-NZVI,it was+62 mV(Fig.5).Zeta potential values foresee colloidal stability.The high positive charge(greater than+25)and negative charge(less than ?25)indicate the repulsion among the particles,and thereby,the higher stability of the formation[51].The results of Zeta potential analysis affirm that silty clay is a good support that increases dispersion stability and limits agglomeration due to van der Waals interparticle attractions.

Specific surface area measurements of NZVI and SC-NZVI were made by the Brunauer,Emmett and Teller(BET)method of the adsorption of nitrogen gas.The analysis results are exhibited in Table 1 and indicate that the specific surface area of SC-NZVI(98.5 m2·g?1)is significantly higher than that of NZVI(42 m2·g?1).The decreasing value of the specific surface area of NZVI might result from the surface of NZVI being covered by the organic materials of tea waste extract.In spite of this decreasing value of the specific surface area of NZVI,it still larger than that of commercial Fe powder(<10 μ),which has a specific surface area of only 0.9 m2·g?1[52].The high specific surface area of SC-NZVI demonstrates its high adsorption capacity.Therefore,the supported NZVI has many more applications in remediation activities than NZVI[53].

Fig.5.Zeta Potential analysis of samples.

Table1 BET analysis of samples.NZVI;SC-NZVI

3.2.Batch adsorption experiments

3.2.1.Effect of pH

The pH of a phenol solution is a key affecting factor for the adsorption onto NZVI and SC-NZVI.The chemical properties of the adsorbent-adsorbate system are influenced by the pH value by transforming the equilibrium dissociation process of solutes and surface functional groups of the sorbent toward ionized and non-ionized forms.The adsorption experiments were conducted in the pH range around the optimum value for the maximum reaction rate of phenol[54,55].The experiments were performed in the pH range of 2–5,with the following initial conditions:1 g·L?1NZVI or SC-NZVI;0.5 g·L?1phenol;and 25°C.Fig.6 shows the effect of the initial pH of the phenol solution on the removal rate of phenol.As shown in this figure,the maximum removal rate of phenol was at pH=2.5 and 3 for NZVI and SC-NZVI,respectively.After these values,the removal rate of phenol reduced with raise in the pH.

Oxonium ions,which exist at lower pH values,will prohibit the dissociation of surface acidic groups.In these circumstances,forming H bonds and π-π reactions between adsorbate(phenol)and adsorbent(NZVI or SC-NZVI) is privileged.Thus,adsorption capacity is at its highest[56].

3.2.2.Effect of contact time

As illustrated in Fig.7,the effect of the contact time on the removal efficiency of phenol was studied over 90 min and at these initial conditions:phenol concentration 0.5 g·L?1,NZVI or SC-NZVI dose=1 g·L?1,temperature=25°C,and pH=2.5 and 3 for NZVI and SC-NZVI,respectively.This figure clarifies the rapid increase in the removal rate of phenol in the first 5 min of the contact time.Also,the adsorption of phenol onto the adsorbents(NZVI and SC-NZVI)had a positive correlation with the contact time until it reached stability.Adsorption equilibrium was reached within 25 min for both NZVI and SC-NZVI and with phenol removal rate of 80% and 66.6%,respectively.After this time,no more phenol uptake was observed,perhaps because most of the surface binding sites were filled[57].

Fig.6.Effect of pH on the removal rate of phenol (adsorbent dose=1 g·L?1,phenol concentration=0.5 g·L?1,temperature=25°C).

Fig.7.Effect of contact time on the removal rate of phenol(phenol concentration=0.5 g·L?1,NZVI or SC-NZVI dose=1 g·L?1,temperature=25°C,pH=2.5 and 3 for NZVI and SC-NZVI,respectively).

3.2.3.Effect of adsorbent dose

The influence of the adsorbent dosage was studied at the following initial conditions:phenol concentration 0.5 g·L?1,contact time=25 min,temperature=25°C,and pH=2.5 and 3 for NZVI and SC-NZVI,respectively.The doses of NZVI and SC-NZVI were adjusted from 0.75 g·L?1to 2 g·L?1.As demonstrated in Fig.8,the trend of the phenol removal rate increasing occurred when the doses of NZVI and SC-NZVI were less than 1 g·L?1;however,a downward trend occurred when the dose was larger than 1 g·L?1.The maximum removal rates of phenol using NZVI and SC-NZVI were 81%and 69.4%,respectively.This phenomenon could result from several reasons.First,the aggregation could be due to the increasing dose of the adsorbent leading to a decrease in the specific surface area and an elongation of the diffusion path of the organic pollutants.Hence,one can observe a sharp decrease in the NZVI line compared with the SC-NZVI line.Second,because the concentration of phenol was low,the high dose of NZVI and SC-NZVI caused unsaturated adsorption sites and an increase in NZVI and SC-NZVI,which oxidized to Fe2O3and Fe3O4.As a result,the equilibrium adsorption and reduction capacity of NZVI and SC-NZVI decreased when the dose arrived a specific value[58].Thus,the optimal dose of both NZVI and SC-NZVI is 1 g·L?1.

3.2.4.Effect of adsorbate concentration

Fig.8.Effect of the adsorbent dose on the removal rate of phenol(phenol concentration=0.5 g·L?1,contact time=25 min,temperature=25°C,pH=2.5 and 3 for NZVI and SCNZVI,respectively).

Fig.9.Effect of the adsorbate concentration on the removal rate of phenol (contact time=25 min;NZVI or SC-NZVI dose=1 g·L?1;pH=2.5 and 3 for NZVI and SC-NZVI,respectively;temperature=25°C).

The effect of the initial phenol concentration was evaluated using a 0.5–1.5 g·L?1phenol concentration(NZVI or SC-NZVI dose=1 g·L?1;contact time=25 min;pH=2.5 and 3 for NZVI and SC-NZVI,respectively;temperature=25°C),as clarified in Fig.9.The removal rate decreased with increasing concentrations of phenol due to the increased competition among the phenol ions toward the adsorption sites[59,60].

3.2.5.Effect of temperature

The influence of temperature on the adsorption of phenol onto NZVI and SC-NZVI was studied at 15,25,35,and 45°C under the following conditions:NZVI and SC-NZVI dose=1 g·L?1,phenol concentration=0.5,contact time=25 min,pH=2.5 and 3 for NZVI and SC-NZVI,respectively.The obtained results are demonstrated in Fig.10.As observed in this figure,the adsorption of phenol onto the surface of NZVI and SCNZVI increased with increasing solution temperature,hence indicating that the process is endothermic.This increase could be due to increases in the phenol mobility[61].In addition,the oxidation during the phenol sorption triggered both diffusion and incremental increases in the specific surface area of NZVI and SC-NZVI[62].The results are similar to those reported in the literature[63].

3.2.6.Optimum operating conditions

The optimal operating circumstances for various examined parameters were obtained for the maximum removal rate of phenol for the adsorbents NZVI and SC-NZVI.The optimum operating conditions are(1)a pH of 2.5 and 3 for NZVI and SC-NZVI,respectively,(2)an adsorbent dose of 1 g·L?1),(3)a contact time of 25 min,(4)a phenol concentration of 0.5 g·L?1,and(5)a temperature of 45°C.The maximum removal rates of phenol at optimum conditions were 94.8%and 90.1%for NZVI and SC-NZVI,respectively.

Fig.10.Effect of temperature on the removal rate of phenol (contact time=25 min,phenol concentration=0.5 g·L?1,NZVI or SC-NZVI dose=1 g·L?1,pH=2.5 and 3 for NZVI and SC-NZVI,respectively).

3.3.Adsorption isotherm

The adsorption isotherms of phenol on NZVI and SC-NZVI are illustrated in Figs.11 and 12,respectively.Isotherm models (Langmuir,Freundlich,and Tempkin)were applied to fit the experimental data.The values of the isotherm parameters and the correlation coefficients(R2)are summarized in Table 2.The correlation coefficient(R2)values for the Langmuir isotherm model of NZVI and SC-NZVI are 0.9143 and 0.937,respectively.The values for the Freundlich isotherm model of NZVI and SC-NZVI are 0.9226 and 0.9388,respectively,while for the Temkin isotherm of NZVI and SC-NZVI,they are 0.9138 and 0.8983.These results indicate that the Freundlich isotherm has a higher R2for NZVI and SC-NZVI when compared with the Langmuir and Tempkin isotherm models.According to these results,it can be concluded that NZVI and SC-NZVI show a better fitting model with the experimental data[64].

Fig.11.NZVI isotherm models.

Fig.12.SC-NZVI isotherm models.

Table2 Isotherm parameters and correlation coefficients for adsorption of phenol onto NZVI and SC-NZVI

The RLvalues are between 0 and 1,showing that the adsorption of phenol onto NZVI and SC-NZVI is favorable.The values of n for NZVI and SC-NZVI were 0.81 and 0.42,respectively.These numbers indicate that phenol adsorbed onto NZVI with more power in comparison with SC-NZVI.

3.4.Adsorption kinetics

The kinetics of phenol adsorption on NZVI and SC-NZVI were studied using pseudo-first and pseudo-second-order kinetics models,as demonstrated in Figs.13 and 14,respectively.Kinetic model parameters and correlation coefficients R2are recorded in Table 3.For NZVI and SC-NZVI,the correlation coefficient of the pseudo-second-order model is higher than for the first-order model.These results emphasize that the rate-limiting step is chemisorption,including valence forces through the sharing or exchange of electrons[65].

Table3 Kinetic parameters for the sorption of phenol onto NZVI and SC-NZVI

3.5.Thermodynamics

Fig.13.Adsorption kinetics for the adsorption of phenol onto NZVI.

Fig.14.Adsorption kinetics for the adsorption of phenol onto SC-NZVI.

The removal rate of phenol increases with a rise in temperature(Fig.10).The increase in the adsorption capacities of NZVI and SCNZVI was attributed to the increase of activity of the adsorbent surface with temperature and an increase in the pore size.The values (ΔH0,ΔS0,and ΔG0) are listed in Table 4.The calculated positive values of the ΔH0of NZVI and SC-NZVI(42.65 and 39.74 kJ·mol?1,respectively)indicate that the adsorption of phenol onto both adsorbents is an endothermic reaction.The negative values of ΔG0(in the ?20 to 0 kJ·mol?1range)indicates the spontaneous nature of the adsorption process[66].The positive values of ΔS0for NZVI and SC-NZVI (0.15 and 0.14 kJ·mol?1·K?1,respectively)reflect the random nature of the process at the solid/solution interface and the affinity of NZVI and SC-NZVI for phenol adsorption[67,68].

Table4 Thermodynamic parameters for the sorption of phenol onto NZVI and SC-NZVI

4.Conclusions

In this study,green tea waste was successfully utilized as a reductant in NZVI synthesizing process.The synthesis of NZVI using green tea waste is easy,eco-friendly,and economical.Also,it has been shown that silty clay has the ability to act as a dispersant and stabilizer during the synthesis of silty clay-supported nanoparticle zerovalent iron (SC-NZVI).The SEM images show that the prepared zero-valent iron(Fe0)particles appeared as cubical,nodular protrusions,and many of them were about 100 nm in diameter.This work also clearly demonstrates that supporting NZVI with silty clay achieves a high degree of dispersion.The organic materials Fe2O3,Fe3O4,and Fe0,which were adsorbed from green tea waste extract as capping/stabilizing agents,were observable in the XRD spectra of NZVI and SC-NZVI.The presence of functional polyphenols in both NZVI and SC-NZVI was detected by FTIR.Zeta potential results detected that SC-NZVI was much more stable than NZVI.Additionally,SC-NZVI proved to be of a high specific surface area,and hence,had a high reactivity and adsorption capacity.

From the experimental results,it was found that the optimum conditions for removing aqueous phenol are contact time=25 min;phenol concentration=0.5 g·L?1;NZVI or SC-NZVI dose=1 g·L?1;temperature=45°C;and pH=2.5 and 3 for NZVI and SC-NZVI,respectively.The maximum removal rate of phenol at optimum conditions,when it adsorbs onto NZVI and SC-NZVI,was 94.8% and 90.1%,respectively.

Linear form of Freundlich model was found best fitted with higher value of determination coefficient(R2).The kinetic studies suggested pseudo-second order of reaction,while thermodynamic studies demonstrated endothermic nature of reaction.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

The authors gratefully acknowledge the scientific support and help from the Civil Engineering Department and Chemical Engineering Department,University of Technology,Baghdad,Iraq.