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Journal of Environmental Chemical Engineering

journal homepage: www.elsevier.com/locate/jece

Immobilization of citric acid and magnetite on sawdust for competitiveadsorption and extraction of metal ions from environmental waters

Amjad H. El-Sheikha,⁎, Ismail I. Fasfousa, Rawan M. Al-Salamina, Alan P. Newmanb

a Department of Chemistry, Faculty of Science, Hashemite University, P.O. Box 150459, Al-Zarqa 13115, Jordanb Faculty Research Centre for Built and Natural Environment, Faculty of Engineering and Computing, Coventry University, Coventry, UK

A R T I C L E I N F O

Keywords:Competitive biosorptionMagnetic solid phase extractionCitric acid-coated magnetiteWater treatmentEffluent treatment

A B S T R A C T

A promising new biosorbent with potential applications in both environmental monitoring and treatment is citricacid-coated magnetic sawdust. In this work, this product was prepared and compared with citric acid-coatedmagnetite in terms of surface characteristics, adsorption and suitability for magnetic solid phase extraction(MSPE) of Cu(II), Co(II) and Zn(II) in water. Five adsorbents were prepared: magnetite (Mag), citric acid-coatedmagnetite (cit-Mag), olive wood sawdust (OW), magnetic olive wood sawdust (MOW) and citric acid-coatedmagnetic olive wood sawdust (cit-MOW). The adsorption by cit-MOW (L-2 shape) was fast, physi-sorption,endothermic, spontaneous and favored. Cit-MOW had adsorption capacities (at pH5.6) of: 21.0, 14.4,17.6 mg g−1 for Cu(II), Co(II), Zn(II), respectively; while cit-Mag had adsorption capacities (at pH5.6) of: 5.0,1.6, 4.4 mg g−1 for Cu(II), Co(II), Zn(II), respectively. Competitive adsorption studies showed that Zn and Cowere significantly affected by the presence of Cu. A precise, accurate MSPE procedure was optimized using cit-MOW (at pH5.6) for extraction of metals in unspiked and spiked tap, well and rain water samples prior to theirdetermination by atomic absorption spectrometer.

1. Introduction

This work is concerned withextraction and adsorption of someheavy metals (Cobalt Co, Zinc Zn and Copper Cu) that present in en-vironmental waters. They may come to the environment through an-thropogenic sources, such as mining, smelting, electroplating, industrialdischarge, urban runoff and leachate from solid waste disposal sites,batteries, paints, plastics, alloy industries, ceramics, etc. [1–3]. Unlikeorganic substances, heavy metals cannot be destroyed and thereforethey accumulate in the environment and in living organisms. They maycause kidney failure, liver damage, cancer, nausea central nervoussystem, high blood pressure, pulmonary fibrosis, skin dermatitis, renaledema, diarrhea and vomiting [4]. Heavy metals have high affinity forthiol, carboxyl and amine groups, so that they inactivate enzymes andproteins from working properly [5]. They may also bind to cell mem-branes and inhibit transport processes through them [5]. Co is an es-sential trace metal in many body functions. It is part of vitamin B12, butits toxicity appears in concentrations more than 1mg kg−1 of bodyweight [6]. The maximum allowable limit of Cu in drinking water is1 mg L−1 [7]. The maximum allowable limit of Zn in drinking water is

5 mg L−1 [7]. So that there is a continuous need for development ofmetal monitoring and treatment methods.

Magnetic adsorption and magnetic solid phase extraction (MSPE)are among the most efficient water treatment and sample pre-concentration techniques. They are based on the use of magnetic ad-sorbents that can be separated by the use of an external magnetic field[8–10]. MSPE is used to separate analytes from complex matrices andraise their concentrations prior to analysis. Authors reported that nakedmagnetite (Mag) is hydrophobic in nature, making it unstable and witha tendency to form aggregates [11]. It has been suggested that mag-netite could be coated with appropriate materials to increase its stabi-lity in the working medium [11–13]. The most commonly used func-tional group is carboxylate because it forms stable covalent linkagewith the iron oxide surface [11]. The use of citric acid coated magneticnanoparticles was reported for removal of Cd from water where theadsorption capacity of Cd was 10.8mg g−1 ([14] [15],). Recently, areview summarized the applications of functionalized magnetic mate-rials for separation and preconcentration of pollutants in various ma-trices [16]. MSPE was used in extracting metal ions such as Cu [17], Co[4] Pb [4,18], Zn [18], Se [19], and Cr speciation [20]. Removal of Cd

https://doi.org/10.1016/j.jece.2018.08.020Received 28 March 2018; Received in revised form 6 August 2018; Accepted 7 August 2018

Abbreviations: Mag, magnetite; cit-Mag, citric acid-coated magnetite; OW, olive wood sawdust; MOW, magnetic olive wood sawdust; cit-MOW, citric acid-coatedmagnetic olive wood sawdust⁎ Corresponding author.E-mail address: elsheikh@hu.edu.jo (A.H. El-Sheikh).

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Available online 08 August 20182213-3437/ © 2018 Elsevier Ltd. All rights reserved.

T

by sawdust modified with magnetic nanoparticle was also reported [3].Many authors reported the use of olive waste materials as biosor-

bents for removal of metals from aqueous solutions (e.g. [21–24]).Chemical treatment of biosorbents is a popular method, which may beused to enhance biosorption capacity by modifying the availablefunctional groups. This can improve binding capacity; support its ri-gidity, durability and capability for regeneration. The use of citric acidmodified cellulose for adsorption of lead was reported by Kuo et al.[25].

The main objective of the present work is to prepare and char-acterize citric acid-functionalized magnetic olive wood sawdust thatcan be used for MSPE and adsorption of Cu(II), Co(II) and Zn(II) fromaqueous media. This work provides much basic data to facilitate up-scaling of the process of both manufacture and application of magneticcitric acid-coated sawdust. A comparison with citric acid-coated mag-netite is also conducted in terms of physio-chemical properties, ad-sorption properties and MSPE capability.

2. Materials and methods

2.1. Chemicals and reagents

Two olive branches (1-meter-long and 10 cm diameter) were col-lected from a tree in Al-Khalidiyah area at Al-Mafraq governorate inJune 2015. All chemicals were purchased from Sigma-Aldrich and wereof analytical grade. Distilled water was used throughout this work.Buffer solutions (0.05M) were used to maintain pH in adsorption andMSPE experiments. The following buffer solutions were used: glycine-HCl (pH2.2), acetate buffer (pH3.6 and 5.6), phosphate buffer (pH7.0),carbonate buffer (pH9.2 and 10.6).

2.2. Instruments and equipment

Absorbance measurement of ethanol extracts of olive wood wererecorded using a UV-1700 pharmaSpec UV–vis spectrophotometer,Shimadzu Corporation, Japan. pH measurements were recorded using aWeilheim (Germany) pH meter with a combined glass electrode(inoLab). An isothermal water bath shaker (GFL 1083, Germany) wasused in all MSPE and equilibrium adsorption experiments. A IKA mill(model MF 10B, Germany) was used for grinding the raw olive woodsorbents. A BWB-XP flame photometer was used for determination ofNa+ and K+ in real water samples. Sonication of solutions was carriedout using a soniclean, PTY, LTD sonicator (Australia). A BARNSTEAD/Thermolyne furnace with temperature range of (0–1200 °C (Dubuque,IOWA) was used for heating the adsorbents under inert atmosphere. AnAnalyst Thermo ICE 3000 atomic absorption spectrometer was used forthe quantitative determination of metals under the following opera-tional conditions: wavelength: Co: 240.7 nm, Cu: 324.8 nm, Zn:213.9 nm; lamp current: 75mA; slit width: Co: 0.2 nm, Cu: 0.5 nm, Zn:0.2 nm; burner height 7mm; acetylene flow: 1.1 L min−1; air flow: 10 Lmin−1. IR spectra for the adsorbents were measured by Bruker vertex70. Powder X-ray diffraction was performed using X-ray diffractometer,Shimadzu (Japan). Elemental analysis was carried out using Eurovectormodel E.A.3000 instrument (Italy) using copper sample-tubes. Nitrogenadsorption at 77 K was conducted with a NOVA-2200 VER. 6.11 for thedetermination of BET surface area.

2.3. Preparation of olive wood sawdust adsorbents

Three olive wood sawdust adsorbents were prepared. Initially barkwas removed and then branches were repeatedly passed through anelectric sawing machine to give wood flakes. They were then dried in anoven at 80 °C for 24 h to eliminate moisture. The olive flakes were thencrushed into small pieces and ground using a household blender andthen passed through 1mm sieve. 150 g of ground olive wood wasstirred for 45min with 1 L ethanol. The washing process was repeated

13 times until no absorption bands appeared in the absorption spectrumof ethanol wash. The ground-washed olive wood was labeled as OW.

Magnetic olive wood was prepared by using coprecipitation/ hy-drothermal method [20] as follows: 100ml of distilled water wasplaced in a 500ml filtration flask and N2 gas was purged for 10min.0.42 g of iron (II) chloride tetrahydrate and 1.16 g of Iron (III) chloridehexahydrate were then added to the flask and stirred under N2 gas for10min. After that 3.36 g of OW was added to the mixture (mass ratio1:7) and the mixture was stirred with N2 gas bubbling for 1 h. 5ml ofammonia (25%) was then added dropwise and the mixture was stirredin a water bath at 50 °C for 5 h. Finally, the magnetic adsorbent wasseparated using an external strong magnet and the supernatant wasdecanted. The product was washed with distilled water several timesand then dried in an oven overnight at 50 °C. The product was labeledMOW. Magnetite (Mag) was prepared in a similar procedure butwithout adding sawdust.

Citric acid modified magnetic olive wood was prepared based on amodified procedure reported by Thanh and Nhung [26]. 2.5 g of MOWwas stirred with 25ml of 0.15M NaOH overnight. The product wasthen separated using a strong magnet and then washed several timesusing distilled water to remove excess NaOH and then dried overnightat 90 °C. 2.0 g of the product was stirred at 70 °C with 14ml of 0.40Mcitric acid solution for 1 h and then oven-dried overnight at 50 °C. Theoven temperature was then raised to 130 °C for 90min, where thermos-chemical esterification is initiated. The sample was left to cool, washedwith distilled water several times, washed with 0.10M NaOH withstirring for 1 h, separated by using a strong magnet, washed with dis-tilled water several times, and finally dried in a drying oven for 24 h at50 °C. The produced sorbent was labeled as cit-MOW. Citric acid-coatedmagnetite (cit-Mag) was prepared in a similar procedure after repla-cing MOW with Mag.

2.4. Characterization of the adsorbents

The adsorbents were characterized by various physio-chemicaltechniques. This included: Fourier transform infrared (FT-IR) spectro-metry, powder x-ray diffraction (XRD), elemental analysis and nitrogenadsorption (BET) surface area. Boehm Titrations were also carried outto measure the total acidic groups and total basic groups and they wereconducted as described earlier in the literature ([21,23] [24],). Me-thylene blue adsorption was conducted as described elsewhere [22,24]to measure relative surface area. Point of zero charge (pHpzc) was es-timated by the mass titration method [24].

2.5. Physio-chemical characteristics of real water samples

Sodium and potassium ions determination was conducted usingflame photometer. Determination of calcium, magnesium and waterhardness was conducted using EDTA titration method. Water alkalinitywas estimated by titration against hydrochloric acid solution till pH4.5was reached. Total dissolved solids (TDS) was calculated using theformula (TDS=0.65 x electrical conductance).

2.6. Equilibrium, thermodynamic and competitive adsorption studies

Single-solute and multi-solute (competitive) equilibrium adsorptionexperiments were carried out by mixing 25mg of the desired adsorbentwith 25mL solution containing the desired concentration of metal ionand the pH was buffered to the desired value. The flasks were thencapped tightly and agitated in an isothermal water bath shaker at 30 °Cfor 24 h. The concentrations of metals in the supernatants were ana-lyzed by AAS. For determination of thermodynamic parameters, addi-tional adsorption isotherm experiments were conducted at 40 °C and50 °C. The amount of metal adsorbed on the adsorbent was calculatedby the equation:

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qe = (Co−Ce).V/m

where Co: initial concentration of the metal in the solution (mg L−1);Ce: equilibrium concentration of metal in the solution (mg L-1); qe:equilibrium concentration of metal on solid sorbent (mg g−1), V: vo-lume of the solution (L), m: mass of the adsorbent (g).

2.7. Kinetic adsorption experiments

Kinetic adsorption experiments were conducted as follows: 500mgof cit-MOW was stirred at 30 °C with 500mL solution of 40mg L−1 ofthe desired metal (Zn, Cu or Co) at pH 5.6. The decay in metals con-centration was followed by withdrawing 3mL samples from the mixtureusing a pipette at certain time intervals (t), and then the metal con-centration at that time (Ct) was analyzed by atomic absorption spec-trometer. The quantity of metal ion adsorbed at time t (min) on theadsorbent (qt (mg/g)) was calculated according to the following equa-tion:

qt = (Co – Ct).V/m

2.8. Magnetic solid-phase extraction (MSPE) procedure

The adsorbent cit-MOW was placed in an Erlenmeyer flask con-taining a water sample (spiked with the desired concentration of metalions and buffered to pH5.6). The mixture was then shaken for the de-sired time. The adsorbent was then separated from the water sampleusing an external magnet. Subsequently, metal ions adsorbed on thesorbent were eluted with nitric acid solution. The eluate was thendecanted from the flask and sent for analysis by AAS with the aid of anexternal magnet to keep the adsorbent in the bottom of the flask.

2.9. Application of optimum MSPE method on real water samples

Three environmental waters were used for evaluation of the pro-posed MSPE method; tap water, reservoir rain water and well water.Tap water samples were taken after allowing water to flow for 10minfrom water tap in our laboratory. Reserved rain water samples weretaken from Irbid (North Jordan) wells; which are known to store waterfor several months. Well water samples were collected from Al-Khalidyah; in which water comes out from artesian source. Unspikedand spiked water samples were analyzed (simultaneously) at0.1 mg L−1 level in three replicates.

3. Results and discussion

3.1. Characterization of the adsorbents

Characterization of the prepared adsorbents was conducted by FT-IR, XRD (Fig. 1), elemental analysis, BET surface area, methylene bluerelative surface area, pHpzc and Boehm titrations (Table 1). The ele-mental analysis showed that after magnetite deposition on OW, carboncontent decreased due to addition of a new constituent (magnetite).After citric acid treatment, the %C increased. Mag contained traces ofcarbon while hydrogen presence was due to FeeOH. Citric acid treat-ment of Mag increased the carbon and hydrogen contents in cit-Mag.Boehm titrations showed that OW contains total acidic groups of0.0676mmol g−1 and total basic groups of 0.0125mmol g−1. Afterdeposition of magnetite on olive wood, the total acidic groups de-creased to 0.0468mmol g−1 probably due to interaction between acidicgroups with magnetite (Table 1). After citric acid treatment of magneticolive wood, the total acidic groups increased up to 0.0603mmol g−1.The total basic groups were not affected by these treatments (Table 1).The total acidic groups in cit-Mag was very close to that in MOW. Aswould be expected, citric acid treatment of Mag increased the acidicgroups in cit-Mag.

The results of MB relative surface area (Table 1) showed that therewas no significant difference between OW and MOW while citric acidmodification of MOW tripled the MB relative surface area to reach avalue of 411m2 g−1. Similarly, cit-Mag has triple MB relative surfacearea compared to Mag, which indicated the role of carboxylic groups inthe MB adsorption mechanism (Table 1). The BET surface area of Magand cit-Mag were much higher than that of olive wood based sorbents(OW, MOW, cit-MOW) (see Table 1). The pHpzc of the adsorbentsshowed values within the range 5.8–6.6 (Table 1). Generally citric acidtreatment of the adsorbents increased total acidic groups and MB sur-face area; but slightly decreased the BET surface area and pHpzc

(Table 1). The XRD pattern of OW showed the presence of micro-crystalline cellulose (Fig. 1), which agreed with the XRD patterns re-ported in the literature ([14] [15],). Diffraction lines of crystallinecellulose appeared at 2θ=16, 22, 34°. Magnetite formation was con-firmed by XRD [27] where the main diffraction lines appeared at2θ=30, 35, 43, 53, 58 and 62° in MOW, cit-MOW, Mag, cit-Mag.

3.1.1. FT-IR of OWThe FT-IR spectrum of washed-ground olive wood (OW) is shown in

Fig. 1. The main absorption peak (strong, sharp) appeared at1028 cm−1 which is attributed to CeO stretching vibration in cellulose,hemi-cellulose and lignin. Detailed peak positions and assignments ofwood samples are discussed in the literature. For example, IR of Obechwood was reported by Fabiyi and Ogunleye [28]. Chen et al. [29] re-ported the IR spectrum of softwood and hardwood. Ofomaja and Nai-dooc [30,31], reported the IR assignments of pine cone. Rahman andIslam [32] reported IR spectrum of maple wood sawdust.

Chen et al. [29] reported that in softwood and hardwood, the po-sitions and relative intensity of peaks are different. For example, theband intensity of carbonyl stretching at 1739 cm−1 was greater inhardwoods than in softwoods [33]. Although carbonyl groups aremainly present in the hemicellulose branched chain component, theintensity of the bands depends on the ratio of holocellulose content tolignin content in all types of wood [34]. In the present work, this peakappeared as small-intensity band.

3.1.2. FT-IR of mag and MOWAlthough the coprecipitaion-hydrothermal method is well-known, it

is necessary to optimize the process to deposit the minimum amount ofmagnetite on olive wood (to create suitable magnetic properties)without altering the adsorption properties of olive wood. Mikhaylovaet al. [35] reported that in the process of producing magnetite, ma-ghemite (γ-Fe2O3) may be formed as a byproduct. So that the pre-paration procedure should be optimized. The optimization of magnetitedeposition on olive wood included temperature, contact time, and massratio of Fe3O4-to-olive wood. The only criterion that was considered tochoose the best preparation condition was the magnetism test. A strongmagnet was used to do the magnetism test. Actually it was found thatmagnetite was formed at both 50 and 80 °C but it was not formed at25 °C. So that 50 °C was chosen. Regarding the contact time betweenolive wood and magnetite, it was found that five hours was sufficient toattach magnetite to the olive wood surface. And finally, the mass ratio1:7 (magnetite : wood) was found to be sufficient to provide magneticproperties to olive wood.

The FT-IR spectrum of Mag (Fig. 1) showed absorption bands at1632, 823, 694, 548 and 435 cm−1. The main band was the one at548 cm−1. In the literature, Casillas et al. [36] and Iyengar et al. [37]reported that IR absorption bands of magnetite appeared at 590 and450 cm−1 due to the Fe-O bond in tetrahedrical and octahedrical po-sitions. Furthermore, Li et al. [38] reported that magnetite showed aband at 572 cm−1 with a shoulder at 700 cm−1. In the present work,after deposition of magnetite on olive wood (adsorbent MOW, Fig. 1),the characteristic magnetite absorption bands appeared at 441 (shiftedfrom 435), 556 (shifted from 548), 834 (shifted from 823); while theabsorption bands 1632 and 694 cm−1 remained at similar positions.

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Fig. 1. A) FT-IR spectra, B) XRD patterns of the adsorbents.

Table 1Characteristics of various adsorbents.

BET b

(m2 g−1)SAMB

a,b

(m2 g−1)pHpzc %C %H Total acidic groups

(mmol g−1)bTotal basic groups(mmol g−1)b

Mag 138.6 54.2 6.6 0.65 ± 0.05 0.74 ± 0.05 0.0185 –Cit-Mag 124.5 173.8 6.3 3.42 ± 0.21 2.12 ± 0.17 0.0491 –OW 2.8 121.9 5.9 47.4 ± 1.5 5.9 ± 0.33 0.0676 0.0125MOW 8.5 130.9 6.0 43.4 ± 1.2 5.84 ± 0.21 0.0468 0.0125Cit-MOW 3.8 410.9 5.8 44.5 ± 0.9 5.96 ± 0.32 0.0603 0.0125

a SAMB: relative surface area by methylene blue adsorption.b Single experiments were conducted.

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3.1.3. FT-IR of cit-MOWGenerally, the IR spectrum of cit-MOW is very similar to that of OW.

However, the noticeable change in the IR spectrum of cit-MOW was theappearance of the absorption band at 1586 cm−1 and 1385 cm−1.

In a study reported by Kim et al. [39], citric acid was linked tomagnetic nanoparticles. The characteristic peaks of Fe3O4nanoparticlesappeared between 600 cm−1and 400 cm−1. These broad low-intensitybands can be associated with the stretching and torsional vibrationmodes of magnetite. In Kim et al. [39] work, the peaks corresponding tothe symmetric vibration of C]O at 1693 cm-1 and asymmetric CeOstretching at 1419 cm−1 from the carboxylic group of citric acid shiftedto 1579 cm−1and 1362 cm−1, respectively. These two new character-istic peaks indicated the presence of the COOeFe bond, which indicatedinteraction between Fe3O4nanoparticles and the carboxylic group ofcitric acid. Actually in the present work, we observed similar shifts (seeFig. 1). One of the two bands (1695 cm−1, which is assigned to C]Ostretching of citric acid) shifted to 1586 cm−1, while the one at1426 cm−1 (assigned to CeO asymmetric stretching) shifted to1385 cm−1. This indicated the formation of COOFe.

In another study reported by Gong et al. [5], citric acid was thermo-chemically esterified onto defatted cotton fiber to produce a carboxylcotton chelator (CCC). In comparison with the IR spectrum of defattedcotton, it could be seen that there was a new characteristic stretchingvibration absorption band of carbonyl group at 1753.4 cm−1 as shownin the IR spectrum of CCC [5]. They assigned this shift to citric acidesterification. In the present work, the peak at 1733 cm−1 in cit-MOWalready existed in MOW at 1724 cm−1. This peak was already assignedto C]O in ester. However, a slight shift in the peak (from 1724 to 1733cm−1) was observed but the sharpness of the peak remained un-changed. Thus our study could not confirm any esterification of thewood by citric acid.

3.1.4. FT-IR of cit-MagThe FT-IR spectrum of cit-Mag in the present work was very close to

those reported in the literature [13]. The band of citric acid that ap-peared at 1695 cm−1 (C]O symmetric stretching in the carboxylgroup) shifted to ∼1600 cm−1 in cit-Mag revealing that citric acidbonded to magnetite surface. The band of citric acid at 1426 cm−1

(asymmetric stretching of C]O in COOH group) shifted to 1401 cm−1.There was also a new band at 601 cm−1 that could be assigned tomagnetite after binding to citric acid.

3.2. Adsorption of metals on various adsorbents: Single solute adsorptionstudies

3.2.1. Effect of pHThe pH of water sample affects the chemical speciation of metals

and the surface properties of the adsorbent. The values of pHpzc forvarious adsorbents are shown in Table 1. When pH < pHpzc this meansthat the surface has a net positive charge; while when pH > pHpzc thesurface has a net negative charge. Relative species distribution curves ofthe investigated metal ions were considered to explain our results. Itwas generally noted that free metal ions Co2+ and Zn2+ were pre-dominant until pH8 while Cu2+ was predominant until pH6. As pHbecame higher, M(OH)+ increased and then the neutral M(OH)2 ap-peared. At pH values> 12, M(OH)3− was predominant. Fig. 2 sum-marizes the effect of pH on metals uptake by the adsorbents.

It is generally noted that there was a general increase in metalsuptake with the increase in pH until the pHpzc, after which the uptakeremained almost constant or decreased depending on the type of ad-sorbent and the metal. The metals uptake by cit-MOW was generallymore pronounced compared to other adsorbents. Even cit-Mag ad-sorbent failed to compete with cit-MOW and MOW. For sorbent cit-MOW, it was clear that functionalization with citric acid increasedcarboxylic groups and this would participate in the adsorption process.It was clear that the adsorption of metals by cit-MOW was minimum at

low pH(2.2). The pKa value of carboxyl group in citric acid is 3. AtpH < 3, the pH was below the pHpzc and thus surface had a net po-sitive charge and the neutral form of carboxyl group (eCOOH) waspredominant. So that a possible repulsion might have occurred betweenthe positive metal ion and the net positively charged surface. AtpH > 3, the surface citrate anion was predominant and thus electro-static interaction between positively charged metals and citrate anionwill increase metals uptake. The highest metals uptake using cit-MOWwere obtained at pH 5.6 after which metals uptake started to decreasedue to formation of metal hydroxides where electrostatic interactiondecreased. Based on the above discussion, pH5.6 was selected as theoptimum pH for subsequent adsorption experiments.

3.2.2. Effect of concentration: Adsorption isotherms at pH 5.6Equilibrium adsorption study was conducted to determine the ad-

sorption capacity and favorability of adsorption at pH5.6 for all theadsorbents towards metal ions (Fig. 3). According to Giles and Smith[40] classification of adsorption isotherms, it is clear that all of theadsorbents involved in this study followed the “L-type” isotherm exceptcit-MOW followed “H-type” isotherm where the initial slope was ex-ceptionally high and the adsorbate exhibited a very high affinity to-wards the solid surface.

The most common adsorption isotherm models have been applied inthis work; the Langmuir isotherm model and the Freundlich isothermmodel. Langmuir equation can be written in the following linear form[41]:

Ce/qe= 1/(KL.Qmax) + (1/Qmax).Ce

where Ce (mg L−1) is the equilibrium concentration of remaining ad-sorbate in solution, qe (mg g−1) is the amount of adsorbate adsorbedper unit mass of sorbent at equilibrium, Qmax (mg g−1) is the amount ofadsorbate at complete monolayer coverage, and KL (L mg-1) is a con-stant that is related to the heat of adsorption. Higher values of KL in-dicate that the adsorbate favorably transfers from the solution to attachto the adsorbent surface.

The Freundlich adsorption isotherm model can be represented bythe following linear equation ([41]:

log qe = log Kf + n.log Ce

where Kf (mg1−n g-1 Ln) is a constant that indicates the adsorptioncapacity when equilibrium concentration equals one; Kf also representsthe average affinity of the adsorbate toward the solid surface adsorbent;n represents the degree of dependence of sorption on equilibriumconcentration; n is an indicator of adsorption intensity, surface het-erogeneity and adsorption favorability. When n<1: this means higherheterogeneity of the adsorbent surface and that sorption is favorableover the entire studied concentration range [41]. The adsorptionparameters of the Langmuir and Freundlich isotherms are presented inTable 2.

It was clear that adsorption data of all metal ions by all the ad-sorbents followed both Langmuir isotherm and Fruendlich isothermmodels (as indicated by R2 values). It was clear that cit-MOW gaveexceptionally higher adsorption capacities (higher Qmax and Kf values)and favority (higher KL) than other adsorbents (including cit-Mag). Theadsorbents followed the following order in terms of adsorption capacityand favority: cit-MOW > MOW > OW > cit-Mag > Mag. Generally,the wood-based adsorbents gave better adsorption results than Magbased sorbents.

3.2.3. Free energy of adsorption (E)The Dubinin-Radushkevich (D–R) model was applied to measure the

mean free energy of adsorption E [22]. The linear equation of thismodel is:

ln qe = ln qm – B[R.T.ln(1+ 1/Ce)]2

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Where the constant B (mol2 J−2) is related to the mean free energy ofadsorption E; R is the gas constant (8.314 J mol-1 K-1) and T is thetemperature (K). The free energy of adsorption E (J mol-1) is calculatedusing the formula:

E = (-2B)-1/2

E is defined as the change in free energy when one mole of ad-sorbate in solution is transferred from infinity to the adsorbent surface.The value of E gives information about the mechanism of adsorptionmechanism (chemical ion-exchange or physical adsorption). If8 < E<16 kJ mol−1 the adsorption is chemical (ion-exchange); whileif E < 8 kJ mol−1 then the adsorption is physical [22]. From Table 2, itwas noted that the values of E were generally less than 8 kJ mol−1

which indicated that the adsorption of metal ions on all adsorbents wasphysi-sorption in nature.

3.2.4. Thermodynamics of adsorptionThe thermodynamic parameters (free energy of sorption ΔG, en-

thalpy ΔH, and entropy changes ΔS) for adsorption of the undertakenmetals on various adsorbents were estimated by conducting the equi-librium adsorption experiments at various temperatures (30, 40 and50℃). The Langmuir constant KL was estimated at each temperatureand equilibrium adsorption constants K (L g−1) were calculated by theequation:

K=Qmax.KL

The values of ΔG for adsorption of Co, Cu and Zn on various ad-sorbents at 30 and 50 °C are presented in Table 2. These were estimatedusing the equations:

ΔG = −R.T.lnK, ΔG = ΔH – T. ΔS

where R (8.314 J mol−1 K−1) is the universal gas constant and T is theadsorption temperature in Kelvin. Table 2 also presents ΔH and ΔS foradsorption of Co, Cu and Zn on all the adsorbents.

Generally, the values of the ΔG reflect the adsorption spontaneity.Also, it indicates in the range of 0 to −20 kJ mol−1 that the process ismainly physi-sorption while in the range -80 to −400 kJ mol−1 it in-dicates that the process is chemisorption [23]. The value of ΔH reflectsthe enthalpy of the reaction (exothermic or endothermic reaction). Thevalue of ΔS reflects the change in randomness and disorder of the ad-sorption process.

The adsorption of Co(II), Cu(II) and Zn(II) on cit-MOW was spon-taneous, physical and thermodynamically favorable. The values of ΔGbecame more negative with increasing temperature. The opposite wasobserved with magnetite based adsorbents, where positive ΔG valueswere generally recorded that indicated a non-spontaneous unfavoredadsorption process. The values of ΔG became more positive with in-creasing temperature. The ΔH values indicated that the adsorptionprocess was sometimes exothermic and sometimes endothermic. Thepositive ΔS values for adsorption of metals on cit-MOW indicated anincrease in disorder at the solid-liquid interface due to some structuralchanges in the adsorbate and adsorbent during the adsorption process

Fig. 2. Effect of pH on adsorption (% uptake) of Cu(II), Co(II) and Zn(II) using various adsorbents. %RSD range: a) 1.6–8.4%; b) 2.1–9.1%; c).1.2–8.1%.

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Fig. 3. Equilibrium adsorption (single solute) isotherms of Cu(II), Co(II) and Zn(II) at pH5.6 on various adsorbents at 30 °C.

Table 2Adsorption and thermodynamic parameters for adsorption of Cu(II), Co(II) and Zn(II) at pH5.6 on various sorbents at 30 °C.

Langmuir Model Freundlich Model D-R Model Thermodynamic parameters

Qmax (mg g−1) KL (L mg−1) R2 KF n R2 E(kJ mol−1)

R2 ln K ΔG (kJ mol−1) ΔH (kJ mol−1) ΔS(J K−1 mol-1)

CuMag 1.4 0.3 0.9932 2.4 0.4 0.9229 0.5 0.9678 −0.85 2.14 31.1 0.10Cit-Mag 5.0 0.6 0.9870 2.0 0.4 0.9808 2.2 0.8730 1.13 −2.84 −76.2 −0.24OW 3.5 0.9 0.9889 1.7 0.3 0.9383 1.0 0.9369 1.13 −2.90 −61.4 −0.19MOW 4.3 1.2 0.9966 3.0 0.2 0.8775 2.2 0.9667 2.77 −7.00 −8.5 −0.010Cit-MOW 21.0 1.5 0.9991 10.4 0.3 0.8556 2.4 0.9546 3.4 −8.4 3.7 0.04

CoMag 8.7 0.2 0.9231 1.2 0.7 0.9425 0.8 0.9747 0.29 −0.73 −23.3 −0.07Cit-Mag 1.6 0.1 0.8616 0.2 0.7 0.7846 0.5 0.8983 −1.60 4.00 18.7 −0.08OW 3.2 0.3 0.9838 0.9 0.5 0.8840 0.8 0.9472 0.02 −0.06 −131.3 −0.43MOW 4.3 0.6 0.9958 1.7 0.4 0.9417 1.1 0.8763 0.88 −2.20 −31.7 −0.10

Cit-MOW 14.4 1.3 0.9991 7.2 0.3 0.795 2.2 0.9775 2.9 −7.2 −4.2 0.01ZnMag 1.9 0.4 0.9892 0.7 0.4 0.9903 0.8 0.8690 −0.34 0.86 −11.9 −0.040Cit-Mag 4.4 0.2 0.9817 0.7 0.6 0.9850 0.7 0.9461 −0.30 0.76 −19.6 −0.70OW 5.6 0.2 0.9618 1.2 0.5 0.9897 0.7 0.9384 0.16 −0.40 −23.1 −0.075MOW 5.1 0.8 0.9987 2.2 0.4 0.9411 1.3 0.9585 1.39 −3.50 −11.1 −0.025Cit-MOW 17.6 1.5 0.9995 8.8 0.3 0.8580 2.2 0.9942 3.2 −8.1 8.6 0.06

Fig. 4. Competitive adsorption (multi-solute) isotherms of Cu(II), Co(II) and Zn(II) at pH5.6 on cit-MOW at 30 °C.

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[18].

3.3. Competitive adsorption study of the undertaken metals on cit-MOW

Competitive (multi-solute) adsorption isotherms of metals on cit-MOW at pH5.6 in bi-solute solutions ((Cu+Co), (Cu+Zn), (Co+ Zn))and tri-solute solution (Cu+Co+Zn) was studied in separate sets ofexperiments at 30 °C. The purpose of these experiments was to in-vestigate the influence of each metal ion on adsorption of other metals.The competitive adsorption isotherms are shown in Fig. 4. The Lang-muir adsorption isotherm was obeyed by all bi-solute and tri-solutecompetitive adsorption experiments where Qmax and KL values wereestimated for each metal ion (Table 3). A competition factor (CF) wascalculated as follows:

CF=Qmax single solute / Qmax multi solute

It was generally noted that competitive adsorption of metals in biand tri-solute solutions resulted in a decrease in Qmax values of eachmetal ion relative to Qmax in single-solute adsorption experiments. Thiswas reflected on the values of CF which were all smaller than 1, whichindicated a negative influence of metals on each other. It was also thatthe sums of competitive Qmax values (in bi and tri-solute experiments)were larger than each single-solute Qmax value. This indicated that thepresence of two or three metals together in the same solution increasedthe total Qmax of the surface. From the CF values in bi-solute adsorptionsystems, it was clear that the presence of Cu significantly decreased theadsorption capacity of both Co and Zn. Contrary, the presence of Co andZn slightly decreased the adsorption capacity of Cu. Regarding thecompetition between Co and Zn, it was clear from CF values that bothmetals had equivalent negative influence on each other. In the tri-soluteadsorption system, the CF values for all the three metals decreased tohalf the values relative to bi-solute adsorption systems. However, Cuwas the least metal affected by other metals while Co and Zn weredoubly affected relative to Cu and relative to the bi-solute adsorptionsystems. If this sorbent was to be used for practical water or wastewatertreatment these factors would need to be taken into account whenformulating a treatment process for a stream containing several metals.

3.4. Adsorption Kinetics and mechanism of adsorption of metals on cit-MOW

The metals uptake profiles of each metal on cit-MOW at pH5.6 areshown in Fig. 5. These curves confirmed that monolayer coverage wasachieved within one hour. Pseudo-first-order and pseudo-second-orderkinetic models were applied on the kinetic adsorption data [22]:

pseudo-second-order kinetic equation: log (qe - qt) = log qe – (k1/2.303).t

pseudo-second-order kinetic equation: t/qt= 1/(k2.qe2) + (1/qe).t

where qe and qt (both in mg g−1) are the amounts of heavy metals

adsorbed on the adsorbent at equilibrium and at time t (minutes), re-spectively; k1 (min−1) and k2 (g mg−1 min−1) are the first-order andthe second-order rate constants of adsorption, rspectively. The experi-mental values of qe were estimated by extrapolating the adsorption datato t = ∞. It was found that pseudo-second order model gaveR2>0.9978 and more realistic qe values; while pseudo-first ordermodel gave R2<0.4857.

3.4.1. Intra-particle diffusionEarlier studies showed that kinetic data could be used to investigate

the mechanism of adsorption [[22]]. The process of biosorption may becontrolled either by external mass transfer (film diffusion) or intra-particle diffusion (pore diffusion) or both. Indeed, the slower step willbe the rate-limiting step. The participation of intra-particle diffusioncan be confirmed by applying the Weber-Morris equation:

qt = kid t0.5 + C

Table 3Competitive adsorption parameters (estimated by Langmuir adsorption model) of Cu(II), Co(II) and Zn(II) at pH5.6 on cit-MOW adsorbent at 30 °C.

Cu(II) Co(II) Zn(II) Sum of Qmax

(mg g−1)Qmax Cu

(mg g−1)KL CF R2 Qmax Co

(mg g−1)KL CF R2 Qmax Zn

(mg g−1)KL CF R2

Single solute: 21.0 1.5 – 0.999 14.4 1.3 – 0.999 17.6 1.5 – 0.998 –Bi-solute:Cu+Co 16.8 0.9 0.8 0.974 7.1 1.8 0.49 0.993 – – – – Qmax Cu + Qmax Co= 23.9Cu+Zn 18.6 1.1 0.89 0.999 – – – – 8.1 2.5 0.46 0.999 Qmax Cu + Qmax Zn=26.7Zn+Co – – – – 9.4 2.0 0.65 0.998 10.6 1.6 0.60 0.997 Qmax Co + Qmax Zn=20.0

Tri-solute: Cu+Co+Zn 14.0 1.0 0.67 0.990 5.1 2.5 0.35 0.999 5.0 4.0 0.28 0.993 Qmax Cu + Qmax Co + Qmax Zn=24.1

Fig. 5. Kinetics for adsorption of Cu(II), Co(II) and Zn(II) at pH 5.6 on cit-MOWat 30℃.A) adsorption (uptake) profile, B) intraparticle diffusion plots.

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where kid is the rate constant of intra-particle transport (mg g−1/

2 min−1/2) of the adsorbate and C is a constant that is related to thethickness of the boundary layer. Plots of qt against t0.5 are shown inFig. 5. In the literature, it was reported that if a straight line passingthrough the origin is observed, then the intra-particle diffusion is theonly rate-limiting step. However, if a linear relationship, with an in-tercept, is observed (over a period of time), then this indicates thatintra-particle diffusion is involved but it is not the only rate-limitingstep because it did not pass through the origin. If a biphasic (two linearparts) plot is observed, then the first linear part of the plot (usually theone of steeper slope) indicates that film diffusion (surface biosorption)occurs first and the second linear part of the plot (usually the one oflower slope) indicates that pore diffusion (intra-particle diffusion) oc-curs later. Indeed, this is the observed case in the present work.Therefore, the biosorption process proceeds first by surface biosorption(film diffusion) followed by intra-particle diffusion (pore diffusion). Theintra-particle diffusion seems to be slower (rate-limiting step).

3.5. Application of cit-MOW for MSPE of metal ions from real waters

The purpose of this part was to investigate the ability of this sorbentto be used for MSPE of the investigated metal ions at pH5.6 prior totheir determination by flame atomic absorption spectrometer (FAAS)with specific aim of facilitating trace metal analysis of environmentalsamples. The choice of cit-MOW and pH5.6 was based on the resultspresented in section 3.2. The effect of various parameters that mayaffect the recovery in MSPE process were optimized and the followingMSPE method was proposed: 150mg cit-MOW is added to 100ml watersample containing the metal ions under study at pH 5.6. The mixture isshaken for 15min and then the magnetic adsorbent is separated fromthe solution using an external magnet. The adsorbed metals are elutedusing 10mL of 0.1 M nitric acid and the supernatant is decanted whilean external magnet is used to hold the adsorbent within the flask. Fi-nally, the eluate is then taken for FAAS analysis. The reusability of cit-MOW was confirmed by re-using the adsorbent five times where therecovery of Zn(II) and Cu(II) decreased by ∼5%, while for Co(II) thedecrease was ∼14%.

Figures of merit of the proposed MSPE method were estimated afterapplication of the proposed MSPE method (n=3) on distilled watersamples spiked (simultaneously) with the investigated metal ions.Calibration sensitivity (m) equals the slope of the calibration curve. Thedetection limit Cm was calculated by the equation: Cm=3 × Sbl / m,where: Sbl is the standard deviation of the blank signal (n= 5), whilethe limit of quantification LOQ=10 × Sbl / m. Figures of merit are

presented in Table 4. It is clear that the proposed method gave rea-sonable results in terms of detection limit and sensitivity. The methodselectivity was investigated by application of the optimum method onwater samples containing the following possible interfering ions: 100mgNa L−1; 50mg K L−1; 50 mgCa L−1; 20 mgMg L−1. The reduction inthe recovery was less than 5%.

The optimum MSPE method was then tested on environmentalwaters (tap water, well water and rain water samples). As shown inTable 4, only zinc could be detected by the proposed MSPE method,while copper and cobalt were lower than the limits of detection of theproposed MSPE method. To allow further evaluations, environmentalwater samples were spiked simultaneously with 0.1mg L−1 of eachmetal. The results are shown in Table 4. It was found that the recoveryranged between 57.0–92.0 % (%RSD range: 2.3–10.5 %). The resultsindicated that the MSPE-FAAS method had an appropriate validity forsimultaneous determination of Co, Cu and Zn in complex water sam-ples. The physiochemical characteristics of the investigated waters wereas follows: Tap water: Na+: 72mg L−1, K+: 4mg L−1, Ca+2:18mg L−1, Mg: 9, pH: 7.6, electrical conductivity: 1250 μS cm−1, totaldissolved solids: 813mg L−1, alkalinity: 175mgCaCO3 L−1, hardness:82mgCaCO3 L−1; Reservoir rain water: Na+: 7mg L−1, K+: 2mg L−1,Ca+2: 8mg L−1, Mg: 4mg L−1, pH: 7.7, electrical conductivity: 869 μScm−1, total dissolved solids: 565mg L−1, alkalinity: 135mgCaCO3 L−1,hardness: 37mgCaCO3 L−1; well water: Na+: 91mg L−1, K+: 5mg L−1,Ca+2: 22 mg L−1, Mg: 12, pH: 7.8, electrical conductivity: 1327 μScm−1, total dissolved solids: 863mg L−1, alkalinity: 254mgCaCO3 L−1,hardness: 104mg L−1;

3.6. Potential applications of magnetic olive wood-based sorbents in real lifewater and wastewater treatment

Whilst this study has provided significant fundamental data on theuse of citric acid treated-magnetized wood as a potential inexpensiveand effective sorbent and has been shown to offer a useful environ-mental monitoring approach (particularly where ICP-MS or methodswith similar sensitivity is not available) there would be much to con-sider before a proposal to use the sorbent in a full scale wastewater orwater treatment application whether this be as a primary treatmentsystem or as part of a final polish following treatment by more con-ventional means. This might also open up an opportunity to use thistechnology for small scale water treatment in communities which relyon metal contaminated groundwater. One might imagine a water col-lection vessel with an external cavity to hold the magnet to allow in-dividual families to locally treat their own well water. The social

Table 4Figures of merit and application on real water samples of the MSPE-AAS analytical method.

Co(II) Cu(II) Zn(II)

Detection limit (μg L−1) 2 13 9Working range (mg L−1) 0.006 - 0.300 0.043 - 0.500 0.030 - 0.500Bias range, % (-15.0) - 12.0 (-30) - (-6.0) (-22.2) - 18.0RSD (n=3), % 0.46 - 5.59 1.69 - 12.4 0.97 - 13.6Sensitivity (L mg−1) 0.62 0.7 2.34Enrichment factor range 8.5 - 11.2 7.0 - 9.4 7.8 - 11.8Correlation coefficient (R2) 0.996 0.997 0.996

Application on real waters

Tap waterun-spiked (n=3, mg L−1± sd) ND ND 0.581±0.009% Recovery (0.1 mg L−1 added) 92.0±3.3 73.0± 4.1 76.0±2.6Rain waterun-spiked (n=3, mg L−1± sd) ND ND 0.072±0.026% Recovery (0.1 mg L−1 added) 75.0±5.3 65.0± 4.6 57.0±10.5Well waterun-spiked (n=3, mg L−1± sd) ND ND 0.085±0.020% Recovery (0.1 mg L−1 added) 88.0±9.1 85.0± 2.3 83.0±7.2

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benefits of this could be great.One of the greatest challenges may be how the system would deal

with the small volume of final effluent produced after regenerating thesorbent. Conversely, if a suitable regeneration scheme cannot be de-veloped then both the economic and environmental costs of the use of a“once through” sorbent will need to be studied closely, the energy andother resource inputs of producing the sorbent (including the requiredthermal treatment). And since wood is used we might also take intoaccount the lost opportunity of using the wood as a renewable energysource.

4. Conclusions

Magnetic sawdust coated with citric acid is a novel, and potentiallyinexpensive biosorbent that has an exceptional ability for adsorptionand MSPE of a range of metal ions from water compared to other sor-bents (cit-Mag, MOW and OW). It seems that bonding citric acid tomagnetite is more favored than esterification of wood which increasedthe acidic groups content. The adsorption on cit-MOW is fast, physi-sorption, endothermic, spontaneous, favored, and high adsorption ca-pacities were recorded. It can be applied for the determination of metalions in environmental water samples and the sorbent may make a goodcandidate as an effective biosorbent for use in water and wastewatertreatment. Competitive adsorption studies showed that Zn and Co aresignificantly affected by the presence of Cu.

Acknowledgments

Authors would like to thank the Deanship of Scientific Research atthe Hashemite University for financial support. Thanks also to BasemNasrallah, Fatma Soubani and Ala’a El-Khateeb for technical assistance.

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