Imran Ali,Changsheng Peng ,3,*,Iffat Naz

1 College of Environmental Science and Engineering,Ocean University of China,Qingdao 266100,China

2 The Key Lab of Marine Environmental Science and Ecology,Ministry of Education,Ocean University of China,Qingdao 266100,China

3 School of Environmental and Chemical Engineering,Zhaoqing University,Zhaoqing 526061,China

4 Department of Biology,Qassim University,Buraidah 51452,Kingdom of Saudi Arabia(KSA)

Keywords:Green recipe Phytogenic magnetic nanoparticles Physical characterization of nanoparticles Lead and cadmium ions removal

ABSTRACT The present research study isfocused on green fabrication of superparamagnetic Phytogenic Magnetic Nanoparticles(PMNPs),and then itssurface functionalization w ith 3-Mercaptopropionic acid(3-MPA).The resulting material(i.e.3-MPA@PMNPs)characterized by FTIR,pow der XRD,SEM,TEM,EDX,VSM,BETand TGA techniques and then further employed for the investigation of the adsorptive removal of lead(Pb2+)and cad mium(Cd2+)ions from aqueous solutions in single and binary systems.The material show ed fastest adsorptive rate(98.23%)for Pb2+and(96.5%)Cd2+w ithin the contact time of 60 min at p H 6.5 in the single system.The experimental data w ere fi tted well to Langmuir isotherm,indicated monolayer adsorption of both metal ions onto 3-MPA@PMNPs and an estimated comparable adsorptive capacity of 68.41 mg·g-1(Pb2+)and 79.8 mg·g-1(Cd2+)at p H 6.5.However,kinetic data agreed well with pseudo-second-order model,and indicated that the removal mainly supported chemisorption and/or ion-exchange mechanism.Thermodynamic parameters such as ΔG o,ΔH o,andΔS o,were-3259.20,119.35 and 20.73 for Pb2+,and-1491.10,45.441 and 7.87 for Cd 2+at temperature 298.15 K,con fi rmed that adsorption was endothermic,spontaneous and favorable.The material demonstrated higher selectivity of Pb2+and its removal ef fi ciency was(98.20±0.3)%in binary system experiments.The material persisted performance up-to seven(07)consecutive treatment cycles without losing their stability and offered comparable fastest magnetic separation(35 s)from aqueous solutions.Therefore,it is recommended that the prepared material can be employed to remove toxic heavy metal ions from w ater/w astew aters and this“green”method can easily be implemented at large scale in low economy countries.

1.Introduction

Heavy metal(HM)contamination is creating an alarming environmental concern,due to their acute toxicity and non-biodegradability nature[1-7].HMs ion are inorganic pollutants,w hich mainly come from different kindsof industries i.e.pharmaceutical,glassmanufacturing,electroplating,batteries manufacturing,painting,pigments,mining,dyeing,coatings,plating,plastics,etc.and anthropogenic activities due to the ever-increasing urbanization and industrialization[8-20].These ions can bio-accumulate in the living cells because of their non-biodegradability nature,w hich may in fl uence both animals and humans,and leads to serious health issues e.g.lung infection,kidney infection,renal damage,liver infection,heart attacks,edema to the immune system,mental retardation,anemia,asthma,several types of cancer,hypertension,allergic reactions,renal abnormalities,reduction in fertility,and DNA damaging[21-27].HM ions such as mercury,nickel,cadmium,lead,chromium,and arsenic are the main sourcesof untreated and recalcitrant contaminants in w ater environment[28-30].In the current study,lead(Pb2+)and cadmium(Cd2+)are considered since they create acute poisoning and chronic disease in aquatic and human's life.For instance,Cd2+is w ell-known for causing cancer and kidney infection in humans,in addition to liver and lung infection[31-33].The disease caused by Cd2+poisoning is know n as “itai-itai”in Japan(literally “ouch ouch”disease)w hich cause severe pain in bodyache.Moreover,the presence of Pb2+in w ater can cause reduction in fertility,asthma and renal abnormalities to humans[11].That w ay,these HMs are included on the “red list”of priority pollutants of the USEnvironmental Protection Agency(US-EPA).Meanwhile,the allowable limit of Pb2+and Cd2+in drinking w ater is 0.010 and 3 mg·L-1for Pb2+and Cd2+respectively,asestablished by the World Health Organization(WHO)[34-36].Thus,the removal of these HM ions is an important area of research for water treatment experts to keep w ater quality good for drinking purposes.

There have been many treatment methods and processes(ionexchange resins,chemical precipitation,bio-adsorption,solvent extraction,membrane fi ltration,electrode deposition,reverse osmosis,ultra fi ltration,electrodialysis,chemical coagulation,fl occulation,photodegradation,activated sludge,colorimetric and electrochemicals)proposed and w ell documented in the literature for eradication of HMs from w astew ater[1,37-42].How ever,some speci fi c issues(generation of large quantity of toxic sludge,low kinetics,high installation and operational costs,etc.)are considered as hindrances for their implementation to commercial applications[2].However,adsorption technology is often considered comparatively much better than other w ater puri fi cation technologies,owing to its being inexpensive,easy to install and operate,w ith low design complexity,convenient operation process,high performance,and w ide applicability[25,43-46].Furthermore,many researchers have employed different kinds of adsorbents in adsorption technology to remove HM ions from w ater,including zeolites,clays,activated carbon,red mud,chitosan,lignocellulose,minerals,brewer'syeast,functionalized polymers,tea w aste,and plant w aste materials but their utilization in real applications is imperfect due to numerous hitches such as slow kinetics,low adsorptive capacity,involvement of high cost required for its preparation and activation,several steps required to regenerate/separate from fi nal ef fl uents and employment of toxic/hazardous chemicals for its fabrication[22-24].Therefore,these nuisances insist us to fi nd an alternative material which can deliver desired attributes.

Presently,nanoparticles(NPs)are being employed for the removal of HM ionsbecause of their largesurface area,permitting an ef fi cient removal of metal ions,higher number of active/vacant sites and diverse functions[28,34].However,the separation of NPs from fi nal ef fl uents and recovery of metal ions is the main drawback to its w idespread utilizations.Hence,to minimize this issue,magnetic nanoparticles(MNPs)and their magnetic nano-composites have been extensively employed in several applications i.e.magnetic resonance imaging(MRI),catalysis,targeted drug,gene delivery and lithium ion batteries including w astew ater treatment,HM ion removal and recovery because of their magnetic characteristics[1,2].Thus,MNPs as adsorbent are an ideal candidate for w ater treatment and to remove and recover metal ions from polluted w atersand soilsow ingto their easy separation,high reusability,high thermal stability and greater adsorptive performance[10].Due to their higher technological signi fi cance,different kinds of methods and processes have been utilized for the preparation of MNPs(including sol-gel,hydrothermal,chemical co-precipitation,electrochemical,microwave,milling,chemical etching etc.),but the use of toxic and highly reactive chemicals i.e.carbon monoxide,sodium borohydride,benzene and hydrazine is creating a harmful impact on the environment.In addition,these methods are expensive,time consuming,environmentally unfriendly and complex,and importantly thesefabrication methodsaregenerating secondary pollutantsdueto theuseof highly toxic chemicals[1,2].Therefore,making effective and ef fi cient MNPs by using environmental friendly route w ithout employing any toxic or reactive chemicals is of great research interest.To handle this situation,currently green nanotechnology is getting great attention due to their easy,simple,safe,clean and environmental friendly way to fabricate desired quality of MNPs w ithout employing any toxic and hazardous chemicals.Hence,the recent use of plant extracts to fabricate green MNPs through the reduction of metallic ions is found inexpensive,simple,easy to handle,eco-friendly,less time consuming,cheap and safe.How ever,only a few researchers have employed green method to make green MNPs for the adsorptive removal of HMs ions[1,37].

Therefore,to the best of our know ledge,the use of the plant leaf extract of Fraxinus chinensis Roxb for the fabrication of green MNPs has not yet been reported.F.chinensis Roxb,know n as“Chineseash”isa species of fl ow ering trees.Its leaves have been used in traditional Chinese medicine(TCM)for dysentery disorders.The genus is widespread across Europe,Asia and North America.In order to investigate the adsorptive performance of green MNPs to remove HM ions(Pb2+and Cd2+)from aqueous solutions,the follow ing core objectives w ere designed:(i)to prepare phytogenic magnetic nanoparticles(PMNPs)using theplant leaf extract of F.chinensis Roxb asa reducing and capping agent;(ii)to functionalize prepared PMNPs by 3-Mercaptopropionic acid(3-MPA)ligand i.e.3-MPA@PMNPs for the expected adsorptive removal of HMs ions because bare PMNPs tend to be unstable due to the oxidation of the particles.In addition,according to the Hard-Soft Acid-Base(HSAB)theory,it w as also expected that Pb2+and Cd2+w ould expose good removal ef fi ciency due to strong interaction betw een metal ions and mercapto/thiol(--SH)and hydroxyl(--OH)groups because of thesoftnessof theacid(Pb2+and Cd2+);(iii)to confi rm thefabrication and functionalization of 3-MPA@PMNPs,by application of different techniques i.e.FTIR,XRD,SEM,TEM,EDX,TGA,BETand VSM;(iv)to investigate the adsorptive performance of 3-MPA@PMNPs in terms of sorption capacity and sorption kinetics;(v)to evaluate the in fl uence of different operational parameters including solution p H,temperature,initial dye concentration,contact time,adsorbent dosages on the adsorptive performance of 3-MPA@PMNPs in single system(vi)to employ various isotherms,kinetics and thermodynamic equations for scrutinizing removal mechanisms;and(vii)to investigate the in fl uences on removal ef fi ciency in binary system by 3-MPA@PMNPs;(viii)fi nally,to determine the potential of reusability of 3-MPA@PMNPs for consecutive treatment cycles.

2.Materials and Methods

2.1.Chemicalsand instruments

The chemicals i.e.Pb(No3)2,Cd SO4·4H2O,copper sulfate,(Merck,99%),3-mercaptopropanoic acid(HSCH2CH2CO2H),ferric trichloride,ferric chloride hexahydrate(FeCl3.6H2O),sodium hydroxide(NaOH),potassium ferricyanide,ferrous sulfate(FeSO4.7H2O),MgSO4,acetate buffer,Na2CO3,HCl(Aldrich,99%),ethanol(C2H5OH)(Merck,99.9%),tripyridyltriazine TPTZ,HPLC grade(Anaqua,99.9%),andgallic acid(Merck,99.9%)w ere utilized.Distilled w ater w as obtained from Qingdao w ater puri fi cation department for making all aqueous solutions.Further,for the preparation of 3-MPA@PMNPs and for the execution of adsorption studies,different kinds of instruments including temperature controlled rotary shaker(THZ-92A,China),magnetic stirrer(MS-M-S10,Biobase Biodustry,Shandong Co.,Ltd.),Atomic Absorption Spectrometry(Solar M6,Thermo Elemental,USA),thermostatic w ater bath shaker(Max Q 7000,Thermo Scienti fi c),w eighing balance(Sartorius Entris 64-1S,German),centrifuge machine(model C-30BL),p H meter(m V/ORPMETTLERTOLEDO),volumetric fl asks and Erlenmeyer fl asks(JOAN Lab Pyrex Glass Erlenmeyer Flask,Ningbo Yinzhou Joan Lab Equipmennts,china)w ere utilized.

2.2.Bio-synthesisof 3-MPA@PMNPs

For the bio-synthesis of 3-MPA@PMNPs,fi rst,PMNPs w ere synthesized using leaf extract of‘Fraxinus chinensis Roxb’as areducing and capping agent and its potential to reduce and the existence of phenolic compounds w ere estimated using ferric reducing antioxidant pow er(FRAP)assay[47]and Folin-Ciocalteu method[47,48],elaborated procedure is available in supplementary information(Text-Sa).Brie fl y herein,80 ml of plant extract(PE)solution was prepared by adding 10 g plant leaves pow der into 80 ml distilled w ater and then heated for at-least 90 min at 80 °C.Separately,7.321 g(FeSO4·7H2O)and 7.321 g(FeCl3·6H2O)w ere added into 50 ml of distilled w ater to make1/1 metal salt(MS)solution(Fe+2:Fe+3)ratio.After this,50 ml of PEand 50 mlof MSsolution were merged and boiled at 80°Conto magnetic stirrer heater for at-least 60 min.The solution p H w as maintained at 12 by using 0.1 mol·L-1NaOH solution drop-w ise.Thereafter,the formation of PMNPs was ensured as the mixture color changed from pale green to dark black and then it w as allow ed to stand for at-least 60 min.Theblack color particleswerecollected by centrifuging themixture at 8000 r·min-1for at least 20 min and then vacuum-fi ltered through 0.22μm membrane fi lter paper.The synthesized back pow der was washed/cleaned thoroughly twice using 50 ml of ethanol solution and then again vacuum-fi ltered.Then,it w as dried in oven at 80°Cfor at-least 20 min.Thereafter,for the fabrication of 3-MPA@PMNPs,5 g pow dered PMNPs and 2.35 g 3-MPA w ere mixed together by ultrasonication into 50 ml distilled w ater for at least 12 h at room temperature and pressure and the solution p H w as maintained at 8 by adding drop-w ise 0.1 mol·L-1NaOH solution.After 12 h of mixing time,the black particleswere collected via vacuum-fi lter and cleaned thoroughly tw ice by 50 ml of ethanol solution and then again vacuum-fi ltered through 0.22μm membrane fi lter paper.Finally,the black particles w ere oven dried at 80°Cfor at least 20 min and then employed in the heavy metal removal experiments.

2.3.Characterization of 3-MPA@PMNPs

The size,shape,surface properties,magnetic nature,elemental contents,morphology and thermal stability of thebio-synthesized 3-MPA@PMNPs were investigated by using different analyses techniques i.e.Fourier transforms infrared spectroscopy(FTIR,Bruker Vertex 70),Scanning Electron Microscope w ith integrated energy dispersive X-ray system(SEM-EDX,JSM-6610 LV,Japan Electronics),transmission electron microscopy(TEM,JEM-2100HR,Japan),X-ray diffractometer(XRD,Philips Electronic Instruments),Brunauer-Emmet-Teller(BET)analyzer(Micromeritics'ASAP2020),X-ray photoelectron spectroscopy(XPS,ESCALAB 250,German),Vibrating sample magnetometer(VSM,VSM600)and Thermo-gravimetric analyzer(TGA,NETZSCH TG 209F3).

2.4.Experiments for the adsorptive removal of lead(Pb2+)and cadmium(Cd2+)by singe and binary system

2.4.1.Single system experiments

All the Pb2+and Cd2+adsorption experiments w ere conducted in batch mode by shaking Erlenmeyer fl asks in temperature controlled rotary shaker(THZ-92A,China)under atmospheric pressure and room temperature(25 ± 2)°Cby adjusting shaking speed at 100 r·min-1.Initially,various operational parameters w ere optimized i.e.solution p H(2-8),temperature(298.15-313 K),initial metal ions concentration(10-1000 mg·L-1),dosages(0.1-1.5 g·L-1)and contact time(0-360 min).For making stock solution of Pb2+and Cd2+(1000 mg·L-1),the required quantity of Pb(NO3)2(1.59 g)for Pb2+and Cd SO4·4H2O(2.49 g)for Cd2+w ere dissolved into 1000 ml of pure distilled water.Thereafter,the solution was further diluted by using dilution method(C1V1=C2V2)to obtain desired metal ion concentration by keeping constant metal ion solution volume(50 ml).The initial metals ionsconcentration w as200 mg·L-1during the optimization of adsorbent dosages,p H,contact time and temperature.How ever,the solution p H w as kept constant at 6.5 throughout all other experiments(excluding the solution p H studies).The p H adjustments w ere made by using 0.1 mol·L-1NaOH and HCl solution through all the experiments,wherever needed.After optimizing in fl uencing factors,the sorption isotherm and kinetic studies w ere performed.A 0.5 g·L-1amount of 3-MPA@PMNPs w as added in a 50 ml of metal ion solution(50-1000 mg·L-1)at p H 6.5 and the mixture p H w as calculated using a p H meter(mV/ORP METTLER TOLEDO).The mixture was kept in temperature controlled rotary shaker under atmospheric pressure and room temperature(25 ± 2)°Cby adjusting shaking speed at 100 r·min-1.Themixture w aspermitted to react w ith the3-MPA@PMNPsfor aperiod of 0-360 min and the fi nal metal ion concentration w as estimated at the end of 5,10,15,30,60,90,120,180,240,300 and 360 min using Atomic Absorption Spectrometry(Solaar M6,Thermo Elemental,USA)atλmax=283.3 nm for Pb2+andλmax=254 nm for Cd2+.The experimental data w ere further fi tted by using different isothermsand kineticsmodelsto investigate the probable removal mechanism(equations are given in Text-Sb).The values of regression coef fi cient(R2)were calculated and implied to observe the best fi tting of the experimental data and consequently fi nal debate w as made.The thermodynamic studies w ere also conducted and thermodynamic parameters(equations are given in Text-Sb)i.e.change in free energy ΔGo(kJ·mol-1),enthalpy ΔHo(k J·mol-1)and entropy ΔSo(kJ·mol-1)w ere evaluated at 298.15,303.15 and 313 Kin a temperature controlled rotary shaker by employing 0.5 g·L-1adsorbent dose at an initial metals ions concentration of 200 mg·L-1under atmospheric pressure and room temperature(25 ± 2)°Cby adjusting at the shaking speed of 100 r·min-1.At the end of each experiment,the superparamagnetic material(3-MPA@PMNPs)was separated by using simple hand-held magnet and the fi nal metals ions concentrations were determined using Atomic Absorption Spectrometry.Lastly,the removal ef fi ciency and adsorptive capacity at different time interval and equilibrium was estimated using follow ing equations:

w here Co(mg·L-1)istheinitial metalsionsconcentration in solution,Ct(mg·L-1)is the fi nal metals ions concentration in solution at different time interval,Ce(mg·L-1)is the concentration of metals ions in solutions at time equilibrium,M(g)is the amount of adsorbent and V(L)is the volume of metals ions solution.

For the determination of material stability and reusability,acidic medium w as chosen to regenerate material and then consecutive sorption and desorption cycles w ere examined up to ten times.An amount of 0.5 g·L-13-MPA@PMNPs w as fi rst added in 50 ml metals ion solution(200 mg·L-1)and then after measuring fi nal metal ion concentration at equilibrium(360 min),20 ml of 25%HCl acidic solution was mixed and sonicated for 20 min.Then,the material w as separated by using magnet and w ashed to neutrality using distilled w ater and it w asre-employed in theremoval of metalsions.Theadsorption-desorption ef fi ciencies were estimated by using follow ing equations:

w here,Mdesorbedis the quantity of metals ions sorbed onto 3-MPA@PMNPs,Msorbedis the amount of metal ions desorbed from 3-MPA@PMNPs,Co(mg·L-1)is the initial metal ions in feed solution,Ce(mg·L-1)is the metal ion concentration at equilibrium,V(L)is the volume of feed solution,Cr(mg·L-1)is the metal ion concentration in solution after regeneration and Vris the volume of regeneration solution.

2.4.2.Binary system experiments

For investigating adsorption competition,binary system(Pb2+and Cd2+)experiments w ere also tested in batch mode at different ratios of metal ion concentration.The presence of co-existing ions can create selectivity for the removal of targeted pollutants during adsorption process.A mixed Pb2+and Cd2+aqueous concentrated solution was prepared by keeping Pb2+concentration(200 mg·L-1)constant and the Cd2+concentration w as varied as 10,50 and 100 mg·L-1respectively.A fi xed amount of 0.5 g·L-1adsorbent dose w as added in a 50 ml of mixed solution containing metal ions(Pb2+/Cd2+)w ith the ratio of 1:20,1:04,and 1:02 respectively.The mixture p H was maintained at 6.5.Thereafter,the mixture w askept in temperature controlled rotary shaker under atmospheric pressure and room temperature(25±2)°Cby adjusting at the shaking speed of 100 rpm.Then,it waspermitted to react w ith the3-MPA@PMNPsfor aperiod of 120 min and the fi nal metal ion concentration w ere estimated by using Atomic Absorption Spectrometry and the removal ef fi ciencies w ere estimated by using Eq.(1).

3.Results and Discussion

3.1.Characterization of 3-MPA@PMNPs

3.1.1.FTIR,XRD,EDX,SEM and TEM analyses

Fig.1.The Fourier transform infrared spectroscopic(FTIR)spectrum of(a)Plant extract;(b)fabricated phytogenic magnetic nanoparticles(PMNPs);(c)phytogenic magnetic nanoparticles functionalized w ith 3-Mercaptopropionic acid(3-MPA@PMNPs);and(d)after adsorption of lead(Pb2+)ions onto 3-MPA@PMNPs.

Fourier Transform Infrared Spectroscopic(FT-IR)study w as conducted to understand the capping/coating of 3-MPA on the surface of PMNPs and its alteration after the adsorption of metal ions(Fig.1).The FTIRresults clearly illustrated the formation of Fe3O4and coating of 3-MPA on the surface of fabricated PMNPs.The presence of polyphenols,alcohols,carboxyl,aliphatic,primary amine,glucose,alkene and ether in the plant leavesextract isclearly demonstrated in Fig.1a.In addition,the peak at 630 cm-1indicates or can be attributed to the characteristic band of Fe--O,w hich suggests the successful formation of Fe3O4NPs(Fig.1b)or PMNPs.The FTIRspectrum of 3-MPA@PMNPs has fi ve important functional groups(i.e.a broad--OH stretching from 3357 to 2962 cm-1,C--H stretching from 2891 to 2812 cm-1,thiol(--SH)at 2659&2511 cm-1,carboxylic(COOH)at 1633 cm-1and free stretching of CSH at 898 cm-1(Fig.1c).The occurrence of(--SH)group and the binding nature of carboxylate group(COO-)on the surface of 3-MPA@PMNPs,con fi rmed the coating/capping of 3-MPA(Fig.1c),as earlier reported by Venkatesw arlu et al.[49].In contrast,no peak appeared in the FTIRspectrum of plant extract(Fig.1a)and fabricated PMNPs(Fig.1b)in the range of 2511-2659 cm-1.The tw o peaks at 2659-2511 cm-1show the vibration of(--SH)group on thesurfaceof PMNPsand then therewasashift to 2632-2501 cm-1,indicating the sorption of metal ions.Further,after the sorption of Pb2+ions,a new peak appeared at 435 cm-1,indicating the bonding betw een S--H and Pb2+(Pb--S).Overall,the FTIRresults con fi rmed the fabrication of PMNPs,smooth functionalization of PMNPs by 3-MPA and adsorption of metal ionsonto 3-MPA@PMNPs via bonding betw een--SH and--OH functional groups by forming chelate/chelation process(Fig.1).

The pow der X-ray diffractometer(XRD)results indicated that the fabricated PMNPs w ere highly crystalline,majority of them show ed the sign of magnetite/hematite NPs and they could be clearly assigned to thecubeshapeof metallic iron(Fig.2).Thehigh characteristic diffraction peaks appeared at 2θ=32.5°,35.2°,45.4°,57.3°and 62.8°respectively,associated w ith(220),(311),(400),(511)and(440)planes of Fe3O4as reported in JCPD reference pattern 019-0629 and JCPDSCard No.82-1533.The peaksat 2θ=35.2°and 62.8°w ere mainly illustrating the presence of iron oxide(Fig.2).The fabricated PMNPs resulted in the mean crystallite size of～39 nm,as calculated by using Scherres equation.Moreover,the absence of additional diffraction peaks for other iron oxide phases i.e.α-Fe2O3,α-FeOOH,and γ-Fe2O3in the XRDpattern,hinting the phase purity of the fabricated Fe3O4/PMNPSand 3-MPA@PMNPs(Fig.2).The weak diffraction intensity and peak broadening may indicate the formation of nano-sized magnetic particles.Moreover,an additional characteristic peak grow n at 2θ =56°after the coating of 3-MPAon the surface of PMNPs,thismay indicate the capping of(--SH)group w ith PMNPs or due to the dispersion by the 3-MPA(Fig.2).In addition,the intensity of the characteristic peaks at(+)was also found slightly reduced.This w as probably due to the coating 3-MPAon the surfaceof PMNPs.It might also bedue to the presence of carboxylate group(COO-)on the surface of PMNPs(as previously confi rmed by FTIR analysis).Overall,the XRD results con fi rmed the formation of PMNPs,smooth capping of 3-MPAon the surface of PMNPs.

Fig.2.The X-ray diffractometer(XRD)patterns of phytogenic magnetic nanoparticles(PMNPs)and phytogenic magnetic nanoparticles functionalized with 3-Mercaptopropionic acid(3-MPA@PMNPs).

Fig.3.Energy dispersive X-ray system(EDX)image of phytogenic magnetic nanoparticles functionalized w ith 3-Mercaptopropionic acid(3-MPA@PMNPs)(inset table is the elemental composition of the fabricated 3-MPA@PMNPs).

The size,shape,elemental contents and morphologies characteristics of the fabricated 3-MPA@PMNPs w ere investigated by SEM,EDX and TEM analyses.The atomic percentages as obtained by(energy dispersive X-ray system)EDX quanti fi cation w ere Fe(24.49%),C(24.75%),O(49.02%),C(24.75%)and S(1.74%).The presence of sulfur homogenously distributed throughout the sample indicated capping of sulfur containing ligand(3-MPA)on the surface of 3-MPA@PMNPs,though in a small amount.How ever,the higher percentages of C(24.75%)indicates the sign of the involvement of plant bio-molecules in the reduction of metal ions and stabilization of PMNPs(Fig.3).

Scanning Electron Microscope(SEM)results indicated that 3-MPA@PMNPs show ed granular homogenous spherical shaped structure of Fe3O4(magnetite)with diameter in the range of 30-50 nm(Fig.4).Moreover,the transmission electron microscope(TEM)results show ed that 3-MPA@PMNPs w ere fi ne,monodisperse,compact and irregular in shape.Majority of them show ed cube shape morphology and some of them w ere found spherical in shape.The averagediameter of the majority of particles(e.g.＞85%of 3-MPA@PMNPs)was in the range of 35-55 nm,w hich is w ell in agreement w ith the results obtained from pow der XRD and SEM analyses(Fig.4).The particles w ere agglomerated because 3-MPA ligand became interlinked on the surface of PMNPsdueto theinvolvement of(--OH)/hydroxyl groupsand COO-groups.The coating of plant organic molecules and 3-MPA ligands played an important function in controlling their aggregation and improving their scattering and colloidal stability.In addition,the existence of high speci fi c surface area and mesopore structure hints that large number of vacant/active sites could subsist on the surface of 3-MPA@PMNPs to adsorb heavy metals ions and toxic dyes from aqueous environment(Fig.4).The proposed mechanism for the formation of PMNPs,3-MPA@PMNPs and or the reactions that might be occurred in the preparation processis shown in Fig.4.

3.1.2.XPSand BETanalyses

Fig.4.The proposed mechanism for the formationof phytogenic magnetic nanoparticles functionalized w ith 3-Mercaptopropionic acid(3-MPA@PMNPs);TEM Image of 3-MPA@PMNPs;scanning electron microscope(SEM)image of 3-MPA@PMNPs;and expected chemical reactions that might be happened during the formation of 3-MPA@PMNPs;

Fig.5.High resolution X-ray photoelectron spectra(XPS)of phytogenic magnetic nanoparticlesfunctionalized with 3-Mercaptopropionic acid(3-MPA@PMNPs)(a)Fe2p(before and after theadsorption of Pb2+and Cd2+;(b)O1s(before and after the adsorption of Pb2+and Cd2+;(c)C1s(beforeand after theadsorption of Pb2+and Cd2+;and(d)S2p(beforeand after the adsorption of Pb2+and Cd2+).

The X-ray photoelectron spectrum(XPS)analysis w as carried out to understand the surface composition and structure of fabricated 3-MPA@PMNPs(Fig.5).The results demonstrated three major peaks at 710.58/724.6,529.95,and 284.79 eV,matching to Fe 2p,O 1s and C 1s,respectively.In addition,a small peak at 163.5 eV for S 2p also appeared,suggestingthepresence/coatingof 3-MPAon thesurfaceof 3-MPA@PMNPs(Fig.5d).The high resolution XPSspectra of Fe 2p,O 1s,C 1s,and S2p were studied by carefully investigating the structure of fabricated PMNPs,as show n in Fig.5a-d.In Fig.5a,Fe 2p spectrum is shown,with two major peaksat 710.58 and 724.6 ev in the binding energy range of 700-740 eV,w hich can be correlated to Fe 2p3/2and Fe 2p1/2respectively,proving the formation/phase purity of Fe3O4.However,there was no satellite peak at/or around 719 eV,w hich w as a typical characteristic feature for the magahemite phase i.e.γ-Fe2O3,indicating the phase purity of Fe3O4[22-24].In Fig.5b,the O 1s spectra showed a major peak at 529.95(～530)eVin addition to a small peak at 532.8 eV.The peak at 529.95 eV can be attributed to the lattice oxygen atoms bonding w ith Fe(Fe--O),w hile the peak at 532.8 eV can be assigned to O in the--OH groups of the PMNPs or 3-MPA(Fig.5b).The C 1s pro fi le depicted,main peak at 285.7 eV in C 1s,w hich can be assigned to polyphenolic(O--H)or alcoholic(C--O)groups that might be associated w ith the capping membrane of organic functional groups onto PMNPs(Fig.5c).Overall,the XPSresults in addition to results obtained from EDX,FTIRand XRD clearly con fi rmed the formation of 3-MPA@PMNPs by surface functionalization of PMNPs by 3-MPA.

The N2adsorption-desorption analysis at 77 Kwas employed to determine the surface properties of 3-MPA@PMNPs(Fig.6).The obtained isotherms show hysteresis loop of different intensities correlated with type IV isotherm model with an H4-type hysteresis loop,as classi fi ed by IUPC(Fig.6).The hysteresis loops took place at relative pressures between 0.46 and 0.99,and evoked the presence of mesoporous feature of the sample(Fig.6).The speci fi c surface area of the material w as 115.42 m2·g-1,as determined by the Brunauer-Emmett-Teller(BET)method.Surprisingly,the speci fi c surface area of 3-MPA@PMNPs w as much better than most of the previously reported green magnetic nanoparticles[3,22-24].The results indicated that the average pore radius of the material w as 17.052 nm,as determined by Barrett-Joyner-Halenda(BJH)model(the pictorial representation of the size distribution is show n in theinset of Fig.6.Thetotal volume of theporesw asdetermined by the single-point adsorption value at P/Po=0.9989 that w as 0.238 cm3·g-1,indicating the presence of loose mesoporous structure of the sample(Fig.6).Overall,the prepared material indicates the presence of mesoporosity w hich can assist the diffusion of pollutants through the porous material.In addition,it can also facilitate in the adsorptive removal of toxic dyes and separation of metal ions from the environment.

Fig.6.N2 adsorption-desorption isotherm at(77 K)p ore size distribution(inset)of phytogenic magnetic nanop articles functionalized w ith 3-Mercap topropionic acid(3-MPA@PMNPs).

3.2.Adsorptive removal of Pb2+and Cd2+by 3-MPA@PMNPs by single system

3.2.1.In fl uence of various operating parameters

Fig.7.In fl uence of various operating parameter on the adsorptive performance of phytogenic magnetic nanoparticles functionalized w ith 3-Mercaptopropionic acid(3-MPA@PMNPs)in the removal of heavy metal ions,(a)doses of 3-MPA@PMNPs;(b)solution p H;(c)initial metal ion concentration(mg·L-1)and(d)temperature(K).

3.2.1.1.In fl uenceof adsorbent dosage.In order to investigate the in fl uence of adsorbent dosage on the adsorptive removal of Pb2+and Cd2+by 3-MPA@PMNPs,the adsorbent dosage w as varied from 0.1 to 1.5 g·L-1at p H 6.5 using metal solution concentration of 200 mg·L-1for 2 h(Fig.7a).The results indicated that the adsorptive removal was optimized by increasing adsorbent dosage.The removal of Pb2+rapidly increased from 74.8%to 98.23%as the amount of adsorbent increased from 0.1 to 0.5 g·L-1then it w as almost stable w hen there w as further increase in the dosage from 0.5 to 1.5 g·L-1(Fig.7a).On the other hand,the removal of Cd2+sharply increased from 27.6%to 96.5%w ith the increase in the dosage from 0.1 to 0.5 g·L-1and then it further increased up-to 98%as the adsorbent dosage w as increased from 0.5 to 1.5 g·L-1(Fig.7a).This increase in removal ef ficiency might be due to the availability of greater vacant/active sites on the surface of adsorbent or the presence of higher surface area for metal ion sorption.How ever,it w as observed that beyond the adsorbent dosage of 0.5 g·L-1,the removal for both metal ions w as negligibly improved,indicating the saturation/equilibrium between metal ions and adsorbent.Thus,0.5 g·L-1of 3-MPA@PMNPS,as an optimum adsorbent dosage,w as chosen and it w as used in subsequent studies.

3.2.1.2.In fl uence of solution p H.The in fl uence of solution p H on the adsorptive removal of Pb2+and Cd2+by 3-MPA@PMNPs w as observed in the range of p H 2-8 at an initial metal ion concentration of 200 mg·L-1for 2 h(Fig.7b)because the p H of the solution can in fl uence on its chemistry,binding sites and chemical interaction by altering the ionization state of the functional groups present on the surface of the adsorbent/3-MPA@PMNPs.The fi ndings depicted that initially the removal of Pb2+and Cd2+increased in the p H range of 2-6.5/7,and then decreased beyond p H 7(Fig.7b).It is well-known that Pb2+and Cd2+exist as different fractions i.e.Pb2+/Cd2+,Pb(OH)+/Cd(OH)+,Pb(OH)2/Cd(OH)2,and Pb(OH)3/Cd(OH)3at different p H values w hich can in fl uence on the adsorptive removal.In the current study,the removal of Pb2+increased from 45.61%to 98.23%as p H increased from 2 to 6.5/7 and then reduced slightly to 95.6%in p H range of 7-8.On the other hand,the removal of Cd2+w as increased from 9.6%to 96.5%as p H w as increased from 2 to 6.5/7 and then decreased in the p H range of 7-8(Fig.7b).At low p H/acidic conditions,the removal ef fi ciency of both metal ions w as low,because the higher generation of(H+)ion concentration in solution due to the protonation of surface functional groups competed w ith Pb2+and Cd2+for the adsorption sites,and meanwhile created Coulombic repulsion/electrostatic repulsion betw een metals ions and positively charged adsorbent surface that signi fi cantly reduce the adsorption process.Thus,by increasing the solution p H,the amount of(H+)ionswas reduced and the amount of adsorbed metal ions increased reaching maximum at p H 6.5/7.This behavior can also be explained in this w ay that,the p HPZCof the adsorbent/3-MPA@PMNPs w as 5.19,as estimated by p H drift method reported by El Haddad et al.[50](Text-Sc;Fig.Sa).Thus,at p H＜.p HPZC,the surface of the adsorbent/3-MPA@PMNPs was positively charged thereby increasing electrostatic repulsion w ith positively charged metal ions,and meantime the presence of excess H+ion concentration in the solution competed the adsorption sites w hich inhibited the adsorption of metal ions.In contrast,at p H＞6.5/7,the removal of Pb2+and Cd2+decreased again(Fig.7b).It might be due to the aggregation effect of metals ion on OH-,asreported earlier by other researchers[22-24].Overall,a maximum of 98.23%of Pb2+and 96.5%of Cd2+removal ef fi ciency w as achieved at p H 6.5/7 and 200 mg·L-1of initial metal ion concentration.

Fig.8.In fl uence of contact time on the adsorptive performance of heavy metal ionsby phytogenic magnetic nanoparticlesfunctionalized with 3-Mercaptopropionic acid(3-MPA@PMNPs)in single system(C o=200 mg·L-1;Dosage=0.5 g·L-1).

3.2.1.3.In fl uence of initial metal ion concentration,temperature and contact time.In order to investigatethein fl uenceof initialmetal ion concentration on the removal performance by 3-MPA@PMNPs,the initial metal ion concentration varied from 10 to 1000 mg·L-1at p H 6.5 using adsorbent dosage of 0.5 g·L-1(Fig.7c).The results show ed that as metal ion concentration increased,the removal performance w as decreased,w hile sorption capacity w as improved(Fig.7c).This might be due to thedecrease in the availableactive/vacant siteson the surplus metal ions.The effect of temperature on the removal of HMs ions w as also investigated in the temperature range of 298.15-313 Kat an initial metal ion concentration of 200 mg·L-1.The results show ed that the removal ef fi ciency decreased for both metals as the temperature increased from 298.15 to 313 K(Fig.7d).How ever,the adsorptive capacity w as almost stable,w hich indicated that the removal of both metals w as spontaneous and endothermic in nature.In addition,it may suggest that the degree of freedom/randomness at the solid/liquid interface w as increased during the adsorption process in aqueous solution.Similarly,the effect of contact time was investigated at p H 6.5 and initial metal concentration of 200 mg·L-1by using adsorbent dosage of 0.5 g·L-1.The results indicated that initially the removal effi ciency of both metal ions rapidly increased in the time range of 0-30 min and then approached to equilibrium after the contact time of 60 min.It is a popular phenomenon that initially adsorbate adsorbed on the surface of adsorbent ow ing to availability of greater number of active/vacant sites for sorption process.Then,after a certain period of time,it becomes dif fi cult to fi ll the remaining active sites because of the creation of repulsive forces betw een solute molecules in the adsorbent surface and thosein thesolution,that caused to inhibit the adsorption process.According to the results obtained,as show n in Fig.8,the optimum equilibrium time w as 60 min for the maximum sorption of both HM ion at p H 6.5 and initial metal ion concentration of 200 mg·L-1using 0.5 g·L-1adsorbent dosage.

3.2.2.Adsorption kinetics of Pb2+and Cd2+onto 3-MPA@PMNPs

In order to investigate the kinetics of HMs ions sorption onto 3-MPA@PMNPs,the in fl uence of contact time on metals ions sorption w as observed by varying initial metals ions concentration from 50 to 1000 mg·L-1at p H 6.5 using adsorbent amount of 0.5 g·L-1.Initially,sorption capacity rapidly increased w ithin the contact time of 15 to 30 min and then approached equilibrium after 60 min(Fig.Sb,c).The equilibrium adsorption time w as different in the different metals ions concentration.At low metal ions concentrations,equilibrium adsorption time w as attained in the contact time of less than 60 min.On average,the fast adsorption rate w as achieved w ithin 30 min of contact time w hile at 60 min adsorptive equilibrium w as attained along w ith 98.23%of Pb2+and 96.5%of Cd2+removal ef fi ciency.Meanwhile,at equilibrium,a maximum adsorptive capacity of 79.8 and 68.41 mg·g-1of Pb2+and Cd2+respectively,was achieved by 3-MPA@PMNPS(Fig.Sd).This fast adsorption of metal ions onto 3-MPA@PMNPs can also be assigned to the synergistic complexation betw een metal ions and the thiol/(--SH)functional groups attached on the surface of 3-MPA@PMNPs,w hich w as formerly checked by FTIR and XPS analyses.

To explore the sorption mechanism and to calculate the kinetic parameters of metal ion adsorption onto 3-MPA@PMNPs,various adsorption kinetic models were employed.It is assumed that different independent processes may be involved in controlling sorption kinetics during adsorptive removal of pollutantsthat can perform in seriesor parallel,for instance,(a)bulk transport,(b)fi lm diffusion/external mass transfer,(c)intraparticle diffusion,and(d)chemisorption/chemical reaction.For this purpose,the kinetic models including Pseudo-fi rst-order,Pseudo-second-order,Elovich,Intraparticle diffusion/Webber and Morris and Liquid fi lm diffusion w ere employed to fi t the experimental data(see Text-Sb).The obtained resultsof the fi tsare given in Tables1&2.According to Fig.9 and the regression coef fi cient values(R2=0.99)in Tables 1&2,it can be suggested that metal ion adsorption data onto 3-MPA@PMNPs were successfully described by pseudo-second order,indicating the presence of chemisorption and/or ion exchange mechanism.Similarly,the values of k2w ere low er compared to h(Tables 1&2),indicating initially a fast sorption of metal ions and then followed by slow adsorption.

Adsorption and desorption w ere also discovered by using Elovich's kinetic model.The values ofαw ere calculated by the ratio of qe/t,w hereas the value ofβwas measured from the slope of graph plotting betw een ln t vs qt.The values ofα w ere higher than that ofβ (Tables 1&2),w hich indicated that 3-MPA@PMNPs had a higher adsorption rate than the desorption rate.The regression coef fi cient values w ere R2＜0.90 show ing the non-applicability of this model.To check the diffusion mechanism between metal ions and 3-MPA@PMNPs,Weber and Morriskinetic equation w asused.Theapplicability of thismodel w asestimated by plotting graph betw een t0.5and qt.Generally,this model is used to check the follow ing steps that might happen in the adsorption process:(a)bulk diffusion;(b)fi lm diffusion;(c)intraparticle diffusion;and(d)chemical reaction via ion-exchange or thesorption of pollutants at an active/vacant site on the adsorbent surface.The values of the regression coef fi cient(R2)poorly correlated w ith the experimental data(Tables 1&2).It can be seen in Fig.Se that the graph line could not pass through the origin,indicating non-applicability of intraparticle diffusion model.

Finally to con fi rm the fi lm diffusion mechanism,liquid fi lm diffusion kinetic model w as employed to the experimental data.The values of kfdw ere calculated from the slope of the straight line of graph plotted betw een t vs ln(1-F).The values of intercept and kfdare listed in Tables 1&2.The straight line of the plotted graph did not pass through the origin,that indicated nearnessto the origin.Thisdeviation might be due to the presence of rapid mixing during the batch sorption experiments which can create discrepancy between rates of mass transfer in the starting and last stages.In addition,theregression coef fi cient valuesw ere(R2＜0.90)showing the non-applicability of thismodel.Hence,according to the sorption kinetic studies and fi tting of experimental data,thepseudo-second-order model indicates that the sorption of metal ion onto 3-MPA@PMNPs w as mainly due to chemisorption and/or ionexchange mechanism.

Table 1 Kinetic parameters for the adsorption of lead ions(Pb2+)onto phytogenic magnetic nanoparticles functionalized w ith 3-Mercaptopropionic acid(3-MPA@PMNPs)at different initial concentrations

Table 2 Kinetic parametersfor theadsorption of cadmium ions(Cd2+)onto phytogenic magnetic nanoparticlesfunctionalized w ith 3-Mercaptopropionic acid(3-MPA@PMNPs)at different initial concentrations

3.2.3.Sorption isotherm study

For investigating sorption isotherm,four different equilibrium models(Langmuir isotherm,Freundlich Isotherm,Temkin isotherm and Dubinin-Radushkevich isotherm)were employed to explain themetal ion interaction with 3-MPA@PMNPs(see Text-Sb).Thecalculated valuesof isotherm constants for the adsorption of metal ions onto 3-MPA@PMNPs are enlisted in Table 3.The value of R2(linear regression correlation coef fi cient)w asemployed asan indication of best fi tting of different models.According to the fi ndings,Langmuir modelgavethebest fi t to the experimental data of metal ions since it show ed higher R2=0.98(for Pb2+)and R2=0.97(for Cd2+)values than the other isotherm models(Fig.10),w hile the range of KLvalue for Pb2+w as 0.99-0.08 and for Cd2+w as 0.99-0.11,indicating that sorption is favorable.The favorability of the sorption isotherm depends upon the KLvalue.If KL＞1 sorption isotherm isunfavorable,if KL=1 sorption isotherm isliner,if KL=0 sorption is irreversible isotherm and if 0＜KL＜1 then sorption is favorable.The fi ndings suggested the monolayer metal ion sorption onto homogenoussitesof 3-MPA@PMNPs.The estimated maximum sorption capacities for both metals ions are enlisted in Table 3.

Altogether,the evidences obtained from the kinetic and isotherm studies along with the verity that metal ions and functional groups on 3-MPA@PMNPs are oppositely charged,indicating that the ion exchange might be the governing adsorptive removal mechanism of metal ions onto 3-MPA@PMNPs.Furthermore,the surface function using the 3-MPA ligand is an important segment for uniform distribution of active sites for metal ion sorption w ith monolayer coverage that improved the adsorptive capacity and removal ef fi ciency by PMNPs.In addition,the formation of chelation happened due to the bonding of positively charged metal ions w ith mercapto/--SH groups attached on the surface of 3-MPA@PMNPs.

Fig.9.The linear plot of pseudo-second-order model for(a)lead(Pb2+)and(b)cad mium(Cd 2+)sorp tion onto 3-phytogenic magnetic nanoparticles functionalized w ith 3-Mercaptopropionic acid(MPA@PMNPs)(dosage=0.5 g·L-1).

Table 3 Isotherm constants for adsorption of lead and cadmium ions(Pb2+and Cd2+)onto phytogenic magnetic nanoparticles functionalized with 3-Mercaptopropionic acid(3-MPA@PMNPs)

Fig.10.The linear plot of Langmuir isotherm model for(a)lead ions(Pb2+)and(b)cadmium ions(Cd 2+)sorption onto phytogenic magnetic nanoparticles functionalized w ith 3-Mercaptopropionic acid(3-MPA@PMNPs)(dosage=0.5 g·L-1).

3.2.4.Thermodynamic studies

The negative values ofΔGoindicated thermodynamically feasible spontaneous nature of the metal ions sorption onto 3-MPA@PMNPs.However,the valuesofΔGoincreased with theincrease in temperature;this suggested a higher feasibility of sorption at high temperature(Table 4).The value of enthalpy of sorption(ΔHo)w as positive for both metal ions and this indicates the endothermic nature of sorption in the temperature range of 298.15-313 K.Generally,enthalpy or heat of sorption ranging betw een 2.1 and 20.9 k J·mol-1associates to physical sorption w hile ranging from 20 to 418 kJ·mol-1is considered as chemical sorption.Hence,the value of(ΔHo)suggests that the sorption of both metal ions onto 3-MPA@PMNPs occurred mainly due to chemisorption and/or ion exchange mechanism,as demonstrated earlier in kinetic and isotherm studies.The value of entropy(ΔSo)w as estimated to be 20.739 k J·mol-1for Pb2+and 7.8733 kJ·mol-1for Cd2+,and this positive value show s some dissociative process and randomness/adsorbed species degree of freedom at the solid/solution interface during thew holesorption process.It might bedueto thestructural changes of the adsorbents/material and adsorbates/metal ions that occurred during the adsorption process.A slight decrease in the removal ef ficiency w as also noticed by increasing temperature(from 298.15 to 313 K),w hile adsorptive capacity w as almost stable(Fig.Sf).

3.3.Proposed removal mechanism of Pb2+and Cd2+by 3-MPA@PMNPs

The clue obtained from isotherm,kinetics and thermodynamic studies w as further ensured by using FTIRand XPSanalysis techniques to explore the probable removal mechanism of Pb2+and Cd2+by 3-MPA@PMNPs.How ever,it w as noticed that 3-MPA ligand had an important role in the removal of Pb2+and Cd2+,but the exact mechanism is not yet clear.In the FTIRspectrum,the tw o peaks at 2659 cm-1and 2511 cm-1show s the vibration of mercapto/--SH groups attached on the surface of PMNPs before adsorption of Pb2+,and then it shifted to 2632 and 2501 cm-1after adsorption of Pb2+(Fig.1d).The peaks shifted to low er w avelength indicating the Pb2+coordination w ith mercapto/--SH groups[49].Furthermore,after the sorption of Pb2+metal ions,a new peak appeared at 435 cm-1(Fig.1d),indicating the bonding betw een S--H and Pb2+(e.g.Pb--S).Overall,the FTIR resultsdemonstrated that metal ionswere mainly removed via thebonding/interaction/coordination betw een mercapto/--SH and--OH functional groups by forming chelate or via chelation process(Fig.1d).

Moreover,the XPSspectra of 3-MPA@PMNPs w ere also studied before and after the adsorption of Pb2+and Cd2+(Fig.11a-d).The high resolution spectra of Fe 2p,O 1s,C1s and S2p w ere also studied before and after the sorption of metal ions by 3-MPA@PMNPs(Fig.5a-d).After the adsorption of metal ions onto 3-MPA@PMNPs,the peak at 163.5 eV for S2p shifted to low er intensity and the bending energy slightly decreased from 163.5 to 163.2 eV(Fig.5d).This may suggest that mercapto/--SH groups attached onto 3-MPA@PMNPs w ere affected due to the interaction w ith metal ions.On the other hand,the peak at 532.8 eVfor--OHfunctional groups attached onto 3-MPA@PMNPs was reduced after the interactions w ith metal ions(Fig.5b).This may show the contribution of--OH functional groups in the removal of metal ions via ion-exchange or chemisorption.Similarly,the peaks that appeared in the high resolution spectra of Fe 2p,and C 1s w ere also affected after the sorption of metal ions(Fig.5a,c).

To better understand the removal mechanism of Pb2+and Cd2+by 3-MPA@PMNPs,the high resolution spectra of Cd 3d and Pb 4f w ere also obtained after the sorption of metal ions(Fig.11b,d).The XPSspectra of 3-MPA@PMNPs after the sorption of metal ionsclearly show ed the changes in the peaks of Fe 2p,O 1s,C 1s and S2p,in addition to the new peaks that appeared for Cd 3d and Pb 4f(as highlighted in Fig.11a,c).The high resolution XPSpro fi le of Pb 4f depicted tw o independent peaks at 138.4 and 143.38 eV,w hich w ere associated w ith the Pb2+metal ion binding energies for the 4f7/2and 4f5/2orbitals.How ever,there w as no peak that appeared at about 136.6 eV,w hich is speci fi ed for the binding energy of neutral Pb.On the other hand,the high resolution XPS pro fi le of Cd 3d depicted tw o independent peaks at 405.46 and 412.36 eV,w hich w ere associated w ith the Cd2+metal ion binding energies for the 3d5/2and 3d3/2orbitals.The presence of the XPSpeaks for Pb2+and Cd2+indicated the adsorption of these metal ions on the surface of 3-MPA@PMNPs.Altogether,the results obtained from XPSanalysissuggestthat Pb2+and Cd2+mainly sorbed on the surface of 3-MPA@PMNPs by forming chalet/chelation,ow ing to the presence of mercapto/--SH and--OH functional groups via ion-exchange/chemisorption mechanism instead of reduction,as earlier elucidated in FTIR,isotherm,kinetic and thermodynamic studies.The proposed removal mechanism of Pb2+and Cd2+by 3-MPA@PMNPs is shown in Fig.12 and the possible interaction of 3-MPA@PMNPs with metal ions to form chelation and neutral complexes in the p H range of 2-8 is also illustrated in Fig.12.

Table 4 Thermodynamic parameters values for the sorption of lead and cadmium ions(Pb2+and Cd2+)onto phytogenic magnetic nanoparticles functionalized with 3-Mercaptopropionic acid(3-MPA@PMNPs)at different temperature(K)

Fig.11.XPSspectrum of(a)phytogenic magnetic nanoparticles functionalized w ith 3-Mercaptopropionic acid(3-MPA@PMNPs)before and after the adsorption of cadmium ions(Cd2+);(b)high resolution XPSspectra of Cd 3d(after the adsorption of Cd2+);(c)3-MPA@PMNPs(beforeand after the adsorption of Pb2+)metal ions;and(d)high resolution XPSspectra of Pb 4f(after the adsorption of Pb2+).

Fig.12.Proposed removal mechanism for lead and cadmium ions(Pb2+and Cd2+)by phytogenic magnetic nanoparticlesfunctionalized with 3-Mercaptopropionic acid(3-MPA@PMNPs)and possible interaction of 3-MPA@PMNPs w ith metal ions to form chelation and neutral complexes in the p H range of 2-8.

3.4.Adsorptive removal of Pb2+and Cd2+by 3-MPA@PMNPs by binary system

In order to observe the adsorption competition and selectivity,a binary(Pb2+and Cd2+)system w as prepared by keeping concentration of metal constant at 200 mg·L-1at p H 6.5.It can be seen in Fig.13 that the prepared material show ed much higher removal ef ficiency of Pb2+and Cd2+,and the material persisted Pb2+removal ef fi ciency(98.20±0.3)%even by increasing the initial metal ion(Pb2+/Cd2+)concentration ratio(from 1/20 to 1/02)in the solution(50 ml).In contrast,the removal ef fi ciency of Cd2+w as signi fi cantly reduced from 82.3%to 67.6%in the presence of Pb2in the solution.The higher selectivity for Pb2+in the presence of Cd2+might be discussed in this w ay that Pb2+is a borderline acid and preferentially offers to the borderline base i.e.mercapto/--SH functional groups on the surface of 3-MPA@PMNPs and this group played a critical role in the high selectivity of Pb2+compared to Cd2+due to the softness of the base,according to the Pearson acid-base classi fi cation or Hard-Soft Acid-Base(HSAB)theory.Though,Cd2+are also a soft acid but Pb2+competed to connect w ith borderline base ow ing to the borderline acidic nature of Pb2+.Our fi ndings w ell matched w ith results reported by other researchers[22-25].Overall,the removal of Pb2+persisted in the presence of Cd2+and the removal of Cd2+w ere inhibited.It happened due to the generation of electrostatic repulsion among excess positively charged Cd2+present in the solution and/or the repulsion betw een solute molecules and the attached metal ions on the surface of adsorbents.

3.5.Stability,magnetic measurement and reusability of 3-MPA@PMNPs

The thermal stability and presence of 3-MPA ligands w ere investigated by using thermo gravimetric analysis(TGA)technique.The TGA curve show ed mainly tw o mass loss steps(Fig.14).A 2.53%mass loss appeared at temperature below 203.3°C,suggesting the elimination of water/H2Oor residual solvent,physisorbed,and chemisorbed H2Omolecules in the sample[3,22-24].The second major mass loss(17.31%)happened in the temperature range of 203.3-597.6°C,suggesting the elimination or decomposition/loss of capping biomolecules and 3-MPA[49].Further,there w as no mass loss noticed above 600°C and 3-MPA@PMNPs exhibits 80.16%mass residue.The TGA results suggested that about 17%of plant biomolecules+3-MPA coating/capping w as observed on the surface of 3-MPA@PMNPs and it possessed high thermal stability.

Fig.13.Removalof lead and cadmium ions(Pb2+and Cd 2+)in binary system by phytogenic magnetic nanoparticles functionalized with 3-Mercaptopropionic acid(3-MPA@PMNPs)(Pb2+=200 mg·L-1;Cd2+=10,50 and 100 mg·L-1;dosage=0.5 g·L-1).

Fig.14.Thermo gravimetric analysis(TGA)plot/curve of phytogenic magnetic nanoparticles functionalized with 3-Mercaptopropionic acid(3-MPA@PMNPs).

Vibrating sample magnetometer(VSM)study w as employed to obtain the hysteresis loop of 3-MPA@PMNPs at temperature 300 K by applying magnetic fi eld from-15 to+15 k Oe(Fig.15a).The fi ndings demonstrated that our fabricated material(3-MPA@PMNPs)had superparamagnetic behavior by keeping saturation magnetization(Ms)value of 50.95 emu·g-1.Meanw hile,the value of remanent magnetization(M r)and coercivity(H c)w as zero.How ever,the low er value of Msmight be due to the increase in surface area or reactions among coating agent and PMNPs as reported earlier by other researchers for green magnetic nanoparticles[3,22-24].

Fig.16 depicts the results of sorption-desorption of Pb2+and Cd2+by 3-MPA@PMNPs for ten(10)consecutive treatment cycles.The prepared material maintained its performance up-to seven(07)consecutive treatment cycles and then gradually decreased.Since the desorption ef fi ciency w as not 100%,it can be assumed that a segment of sorbed metal ions remained inside the 3-MPA@PMNPs.This deposition of metal ions onto 3-MPA@PMNPs might be responsible for maintaining desorption ef fi ciency in the fi rst few treatment cycles.Moreover,during the fi rst seven treatment cycles,the leakage of iron w as not found.This suggests that there w as no striping of 3-MPA capped PMNPs formed.How ever,later the reusability decreased w ith increased treatment cycles.This might be due the destruction of 3-MPA@PMNPs surface caused by the dissolution of iron particles.The sorption-desorption ef fi ciency w as greater than 80%and 78%for Pb2+and Cd2+respectively,up to seven cycles w ithout loss in stability,w hereas,it decreased to 38%and 31%for Pb2+and Cd2+respectively,up to ten cycles.This decline in sorptiondesorption ef fi ciency might be due to the leakage of iron,change in morphology or de fi ciency of the availability of active/vacant sites on the surface of 3-MPA@PMNPs(because portion of the sorbed dye remained inside).Overall,thecapping of 3-MPAonto PMNPsw asstable up-to seven consecutive treatment cycles and it can successfully remove metal ions from the w astew aters in a continuous process.

Overall,the prepared material show ed comparatively much better magnetic separation time than most of the other reported materials(Table 5).The material can be easily separated from aqueous environment w ithin 35 s w ith the help of permanent hand-held magnet(Fig.15b).Altogether,this kind of material is perceived to be incredibly prerequisite for w ater/w astew ater treatment process for enhancing operational ef fi ciency and retaining treatment cost,thus is economically feasible.In addition,the fabricated material can be reused for consecutive treatment cycles and recovery of metal ions can also be obtained.

Fig.15.(a)M-H hysteresis loop/VSM measurement of phytogenic magnetic nanoparticles functionalized w ith 3-Mercaptopropionic acid(3-MPA@PMNPs)at temperature 300 K;and(b)Magnetic separation study of 3-MPA@PMNPs using simple hand-held magnet(distance betw een the magnet and sample was 5 cm).

Fig.16.Study of the stability and reusability of phytogenic magnetic nanoparticles functionalized w ith 3-Mercaptopropionic acid(3-MPA@PMNPs)for consecutive sorption-desorption cycles of Pb2+and Cd2+(20 ml of 25%HCl acidic regeneration solution;adsorbent dosage=0.5 g·L-1,initial concentration of metal ions(C o)=200 mg·L-1).

Table 5 Comparison of 3-MPA@PMNPs with other magnetic sorbent in term of magnetic separation time and saturation magnetization(M s)

3.6.Comparison of 3-MPA@PMNPs with other sorbents

Finally,in comparison to other sorbentsemployed for theremovalof Pb2+and Cd2+from w astew ater,it w as observed that 3-MPA@PMNPs had superior performance in terms of high adsorptive capacity along w ith high reusability up-to fi ve(07)consecutive treatment cycles(Table 6).In addition,green fabrication,fast and easy separation from fi nal ef fl uents just by using simple magnet makes 3-MPA@PMNPs a desirable candidate for the adsorptive removal of Pb2+and Cd2+from domestic and industrial w astew aters.

Table 6 Comparison of PMNPs@3-MPAwith other sorbents for the removal of lead and cadmium ions(Pb2+and Cd2+)from wastew ater

4.Conclusions

A “green”recipe was successfully employed to devise cost-effective,non-toxic and environmental friendly PMNPs using the leaf extract of F.chinensis Roxb as reducing and capping agents w ithout using toxic or hazardous chemicals.Furthermore,the prepared PMNPs w ere successfully coated/capped by 3-MPA ligand via a facile method as confi rmed via FTIR,pow der XRD,EDX,TEM,SEM,VSM,TGA and BET analysis techniques.The prepared material w as effectively employed to eradicate toxic HMs ion i.e.Pb2+and Cd2+by single and binary systems.The follow ing are the key fi ndings of this research w ork:

·Amaximum removal ef fi ciency of 98.23%and 96.5%of Pb2+and Cd2+respectively,w as noticed w ithin the contact time of 60 min at p H 6.5 and 200 mg·L-1of metal ion concentration during single system experiments.

·The kinetic experimental data fi tted w ell w ith pseudo-second-order kinetic model,suggesting that removal mainly supported chemisorption and/or ion-exchange mechanism.

·The adsorption date agreed w ell w ith Langmuir isotherm,demon strating monolayer sorption of metal ionsonto 3-MPA@PMNPs.

·The prepared material achieved comparable adsorptive capacity of 68.41 and 79.8 mg·g-1for Pb2+and Cd2+respectively,at p H 6.5.·Thermodynamic parameters such asΔGo,ΔHo,and ΔSo,were-3259.20,119.35 and 20.73 for Pb2+,and-1491.10,45.441 and 7.87 for Cd2+at temperature 298.15 K,con fi rming that adsorption wasendothermic,spontaneous and favorable.

·The higher removal ef fi ciency(98.20±0.3)%of Pb2+ions w as also observed in binary system even by increasing concentration of Cd2+from 10 to 100 mg·L-1.

·The FTIRand XPSresultssuggested that mainly metal ionssorbed due to the presence of electrostatic interaction and the formation of chelate or chelation with mercapto/--SH ligand and--OH functional groups attached to the surface of 3-MPA@PMNPs.

·The material show ed removal ef fi ciency greater than 80%up-to seven(07)consecutive treatment cycles and show ed comparable fastest separation time of 35 s just by simply applying a hand-held magnet due to their superparamagnetic nature.

·The domestic and industrial w astew aters can be effectively treated

from toxic heavy metals by cost effective green technologies.

Altogether,the prepared material(3-MPA@PMNPs)demonstrated comparably high adsorptive removal performance and kinetics.Thus,this“green technology”illustrates a bright future to remove toxic HM ions and it can easily be implemented for w ater/w astew ater treatment particularly in low economy developing countries.

Supplementary Material

Supplementary datato thisarticle can be found online at https://doi.org/10.1016/j.cjche.2018.03.018.