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Process Safety and Environmental Protection 1 0 0 ( 2 0 1 6 ) 173–182

Contents lists available at ScienceDirect

Process Safety and Environmental Protection

journa l h om ep age: www.elsev ier .com/ locate /ps ep

otential of Melaleuca diosmifolia leaf as a low-costdsorbent for hexavalent chromium removal fromontaminated water bodies

aranya Kuppusamya,b,c,∗, Palanisami Thavamanic,d,allavarapu Megharajb,c,d, Kadiyala Venkateswarlue,

ong Bok Leea, Ravi Naidub,c,d

Institute of Agriculture and Life Science, Gyeongsang National University, Jinju 660-701, South KoreaCentre for Environmental Risk Assessment and Remediation (CERAR), University of South Australia,awson Lakes 5095, SA, AustraliaCooperative Research Centre for Contamination Assessment and Remediation of Environment (CRC CARE),O Box 486, Salisbury South 5106, SA, AustraliaGlobal Centre for Environmental Remediation (GCER), Faculty of Science and Information Technology,he University of Newcastle, Callaghan 2308, NSW, AustraliaFormerly Department of Microbiology, Sri Krishnadevaraya University, Anantapur 515055, India

r t i c l e i n f o

rticle history:

eceived 24 October 2015

eceived in revised form 10 January

016

ccepted 13 January 2016

vailable online 22 January 2016

eywords:

elaleuca diosmifolia

orest biomass

iosorption

hromium(VI) reduction

inetic model

astewater treatment

a b s t r a c t

The present study describes for the first time the utilization of dried twigs of Melaleuca dios-

mifolia, fallen off from the plant, to detoxify and remove hexavalent chromium or Cr(VI)

from aqueous systems. Initial characterization by gas chromatography revealed that the

selected biomaterial is one of the natural sources of eucalyptol. It constituted high concen-

trations of reducing compounds (iron, phenols and flavonoids). Batch studies revealed that

the biosorbent (5 g L−1) was able to remove 97–99.9% of 250 mg L−1 Cr(VI) at wide-ranging pH

(2–10) and temperature (24–48 ◦C). Adsorption kinetics was well described using the pseudo-

second-order kinetic model, while the equilibrium adsorption data were interpreted in terms

of the Langmuir isotherm model. The monolayer adsorption capacity was 62.5 mg g−1. Both

inductively coupled plasma optical emission spectrometry and liquid chromatography anal-

yses of the aqueous and solid phases revealed that the mechanism of Cr(VI) removal was

‘adsorption-coupled reduction’. Scanning electron microscope, infrared spectroscopy and

X-ray diffraction analyses of the biosorbent before and after adsorption also confirmed that

both adsorption and reduction of Cr(VI) to Cr(III) followed by complexation onto functional

groups of the active surface contributed to the removal of Cr(VI) from aqueous solution.

The selected biomaterial effectively (99.9%) removed Cr(VI) in lake and sea water samples,

highlighting its potential for remediating Cr(VI) in real environmental conditions.

© 2016 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.

∗ Corresponding author at: Institute of Agriculture and Life Science, GyeE-mail address: saran.miles2go@gmail.com (S. Kuppusamy).

ttp://dx.doi.org/10.1016/j.psep.2016.01.009957-5820/© 2016 The Institution of Chemical Engineers. Published by

ongsang National University, Jinju 660-701, South Korea.

Elsevier B.V. All rights reserved.

174 Process Safety and Environmental Protection 1 0 0 ( 2 0 1 6 ) 173–182

1. Introduction

Hexavalent chromium or Cr(VI) is the most dominant heavymetal in the natural environment, and is highly toxic evenwhen present at negligible concentrations (50 �g L−1) in waterbodies (Sarkar et al., 2010). Being an anion, Cr(VI) is highlymobile and enters the soil environment easily posing atremendous risk for groundwater contamination. Cr(VI) hascatastrophic health effects on the human body which includerespiratory diseases, skin ulceration, nasal irritation and lungcancer. Genotoxicity and carcinogenicity of Cr(VI) has becomea serious issue leading to a classification of Cr(VI) as a GroupA inhalation carcinogen by US EPA (United States Environ-mental Protection Agency), and Group I human carcinogen byIARC (International Agency for Research on Cancer) (Sreenivaset al., 2014). Due to its serious impact on the environment andpeople’s health, industrial effluents containing Cr(VI) must betreated as a rule. The maximum permissible limit for Cr(VI)in inland water is 0.1 mg L−1 and 0.05 mg L−1 for potable water(Saha et al., 2011; Sreenivas et al., 2014).

Several physical, chemical and biological technologies havebeen developed and implemented for Cr(VI) remediation.However, high chemical and energy requirement, generationof toxic by-products and incomplete removal limits the wideapplicability of these treatment techniques (Saha and Orvig,2010; Kuppusamy et al., 2016a). Thus, in recent years, sorptionhas been considered one of the most popular methods wherea natural or engineered sorbent material acts as a sink for thecontaminant and immobilizes it (Wang and Chen, 2009; Ataret al., 2012; C olak et al., 2013; Kuppusamy et al., 2016b).

Natural biomaterials such as agricultural or plant wastesare cost-effective for immobilizing toxic environmentalcontaminants because they are highly accessible, cheap,ecofriendly, and have high adsorptive properties due to theirdiverse chemical and physical characteristics (Kuppusamyet al., 2015). Also, biomaterials can be modified using avariety of physico-chemical treatments to enhance their sorp-tive potential by changing surface properties. Recently, manyinvestigators have studied the feasibility of using low-cost andeasily available adsorbents such as banana peel (Memon et al.,2009), pomegranate husk carbon (Nemr, 2009), spent activatedclay (Weng et al., 2008), rice straw (Gao et al., 2008), sunflowerstem (Jain et al., 2009), sugarcane bagasse (Garg et al., 2009),dried water hyacinth roots (Mohanty et al., 2006), activated car-bon from tamarind wood (Acharya et al., 2009), ash gourd peel(Sreenivas et al., 2014), activated neem leaves (Babu and Gupta,2008), etc., for removing Cr(VI) from synthetic wastewater.There are four mechanisms for removing Cr(VI) by biomate-rials viz., anionic adsorption, adsorption-coupled reduction,anionic and cationic adsorption, and cationic adsorption (Sahaand Orvig, 2010). The search for alternative new, low-cost,efficient biosorbents to replace the commercially availableadsorbents is ongoing.

Melaleuca diosmifolia, commonly known as green honeymyrtle, is a plentiful, cheap and easily available shrub inAustralia, and is regarded as an environmental weed in Victo-ria. It is used as an excellent wind break in South Australianregions and is known to be non-toxic. Since the dried twigs ofthis plant are thrown away as garbage, this biomaterial can beput to good use as a sorbent for removing toxic metals fromindustrial effluents. To our knowledge, the use of M. diosmifoliaas low-cost sorbent for the removal of Cr(VI) from water has

not been investigated. With a ‘waste to resource’ approach, forthe first time we used biomaterial from M. diosmifolia without

any pretreatment through either physical or chemical means.Biosorption studies were carried out under various parame-ters such as pH, temperature, adsorbent dosage, contact timeand initial Cr(VI) concentration. Kinetic data and sorptionequilibrium isotherms were done in batch process, and wereanalyzed using different models. We also tested the appli-cability of the selected biomaterial for Cr(VI) removal in realenvironmental water samples and evaluated its reusability.

2. Materials and methods

2.1. Materials

Twigs that had fallen off from fully-grown M. diosmifoliaplants were collected from the road-sides in Mawson Lakes,South Australia during March 2014, washed with Milli-Q water(18 � cm−1, Milli-Q, ELGA labwater, UK) to remove the adher-ing soil particles, air-dried, ground and sieved to a particlesize of nearly 0.5 mm. The powdered material was stored in adesiccator with polythene sealing and used for experimentsas needed. Stock solution of the model Cr(VI) compound,potassium dichromate, was prepared in Milli-Q water. NaOH(0.1 mol L−1) and HCl (0.1 mol L−1) were employed to adjust thepH. All the chemicals used were of analytical grade purchasedfrom Sigma-Aldrich.

2.2. Characterization of biomaterial

2.2.1. Surface area and zeta potentialSpecific surface area of the material was measured on a Gem-ini V surface analyser (Micrometrics Instrument Corp., USA).Zeta potential was measured at different pH by a MalvernZetasizer Nano instrument (Malvern Instruments, USA).

2.2.2. ICP-MS and GC-MS analysesThe biomaterial was analyzed for total carbon (C) and nitro-gen (N) by dry combustion with a Trumac CN analyzer (Leco®

Corporation, US). Subsequently, portions (0.5 g) of the biomate-rial was digested in triplicates with 5 mL aqua regia in a Teflondigestion vessel using a microwave accelerated reaction sys-tem (CEM-MARS X®) as outlined in US EPA method SW 3051.The total elemental (mineral) contents in diluted suspensionsof the extracts were determined using a standard refer-ence material (Montana soil SRM 271, certified by NationalInstitute of Standards and Technology) and a blank in an Agi-lent 7500c (Agilent Technologies, Japan) inductively coupledplasma mass spectrometer (ICP-MS).

Predominant chemical constituents of the biomaterialwere estimated by gas chromatography (GC). Briefly, hotwater extract of the material was acidified with few dropsof conc. HCl, extracted by ultrasonication for 15 min (12 kHzsweep bandwidth, Soniclean pulse swept@ power ultrasonica-tor, Soniclean Pty. Ltd., Australia) with ethyl acetate, filtered(0.4 �m filter) and injected in a GC 5975 VL mass selectiondetector equipped with triple axis detector (Agilent Tech-nologies, USA). The GC injection port was configured for 1 �Lon-column injections, with an initial temperature of 50 ◦C,held for 5 min, and ramped up to 260 ◦C in 20 min. The flow ratewas 1.1 mL min−1 with a total run time of 77.5 min. An AgilentHP-5MS capillary column with 5% (v/w) phenyl-substitutedmethyl siloxane nonpolar stationary phase, cross-linked anddouble bonded to the capillary wall with excellent thermal

stability and low bleed levels was used. The dimensions ofthe column were 30 m × 250 �m × 0.25 �m. From the obtained

Process Safety and Environmental Protection 1 0 0 ( 2 0 1 6 ) 173–182 175

cmi

2StFmsmwsXJ

m6ruosS8pd

2Tosabs(

2

A0eA(crCceofusdtficigCf

R

tion study was also carried out with 10 mM phosphate buffer

Table 1 – Physico-chemical properties of biomaterialfrom M. diosmifolia.

Property Unit

Surface area (m2 g−1) 0.99Zeta potential (mV)

At pH 2 +0.81At pH 7 −6.77At pH 9 −8.37

Mineral compositionCa 502.1Na 9.8Pa 0.6Ka 6.4Caa 8.4Mga 1.7Naa 2.1Feb 148Alb 89.4Mnb 181Znb 12.3Cub 7.8Crb 0.5Sa 2.1Nib 2.5Cob 0.7

hromatogram the unknown compounds were identified byatching their specific retention time with those in the exist-

ng database.

.2.3. ESEM-EDX, XRD and DRIFTSurface structure and chemical composition of the bioma-erial before and after adsorption were investigated usingEI Quanta 450 FEG ESEM (environmental scanning electronicroscope) with an EDAX Apollo X SDD EDX (energy disper-

ive X-ray) detector, using an acceleration voltage of 15 kV andagnification ranging from 150 to 60,000-fold. M. diosmifoliaaste sample after Cr treatment was pressed in a stainless

teel sample holder and XRD pattern was obtained on a Lab X,RD-6000, Shimadzu diffractometer (Shimadzu Corporation,

apan).Infrared (IR) spectra of the original and Cr-sorbed bio-

aterial were recorded on an Agilent Cary FTIR (Series00, Australia) in the diffuse reflectance (DRIFTS—diffuseeflectance infrared Fourier transform spectroscopy) modesing the DRIFTS accessory. Spectra of the samples werebtained in the mid IR range (4000–400 cm−1) using KBr (potas-ium bromide) dilution and finely powdered KBr as reference.ample spectrum was obtained by collecting 64 scans at an

cm−1 spectral resolution. IR spectrum of the test compound,otassium dichromate, was also obtained using DRIFTS asetailed above.

.2.4. Total polyphenol, flavonoids and antioxidantsotal polyphenolic and flavonoid contents in water extractsf the plant material were determined following thetandard protocols (Singleton and Rossi, 1965; Khomdramnd Singh, 2011). Also, antioxidant potential was investigatedy determining DPPH (1,1-diphenyl-2-picrylhydrazyl) radicalcavenging activity (Blois, 1958), and ferric reducing powerOyaizu, 1986).

.3. Batch sorption experiments

ll batch kinetic and equilibrium studies were carried out with.1 g of the biosorbent material in 20 mL of Cr(VI) solution tovaluate the optimum values of the experimental parameters.fter each biosorption experiment, samples were centrifuged

3000 rpm for 10 min) and absorbance of the residual Cr(VI)oncentration was recorded (at 517 nm) using a microplateeader (SynergyTM HT, Bio-Tek). The effect of contact time onr(VI) removal was studied at neutral pH for initial Cr(VI) con-entration of 250 mg L−1 at 24 ◦C until the system reached anquilibrium. Similarly, the effect of initial Cr(VI) concentrationn the adsorption potential of the biomaterial was determinedor different concentrations of the metal (100 to 500 mg L−1)ntil equilibrium. The equilibrium adsorption isotherm wastudied using the Langmuir model. The effect of adsorbentose (1.25–5.0 g L−1), temperature (24, 37 and 48 ◦C) and solu-ion pH (2, 5, 7 and 10) on Cr(VI) removal was also examinedor an initial Cr(VI) concentration of 250 mg L−1. Kinetic exper-ments were carried out at the optimum pH and initial Cr(VI)oncentration, with 0.5 g L−1 biosorbent at 24 ◦C. All the exper-ments were executed in triplicate and the mean values areiven. The percentage of Cr(VI) removal (R) and amount ofr sorbed by the biomaterial (qe) can be calculated using the

ollowing equations:

(%) = Co − Ce

Co× 100 (1)

qe(mg g−1) = Co − Ce

M× V (2)

where, Co and Ce (mg L−1) are the initial and equilibrium Cr(VI)concentrations, V is the volume (L) of the metal solution andM is the mass (g) of the adsorbent used.

Since the selected biomaterial was rich in Fe with a con-siderable amount of total polyphenols and flavonoids withtheir reducing potential, all of which are the elements thatcontribute to the reducing potential of a material (Nadagoudaet al., 2010; Zhang et al., 2011), we queried whether the Cr(VI)removal would be an adsorption-coupled reduction process.Hence, in order to confirm Cr(VI) removal by the biomate-rial and to explore the mechanism of its removal (eitheras mere adsorption or as adsorption-coupled reduction), anadditional batch study was conducted at wide-ranging pHbetween 2 and 10 under optimum sorbent dose, temperatureand initial Cr(VI) concentration. After 2 h of incubation, theaqueous suspensions were centrifuged (3000 rpm for 10 min),filtered and analyzed for: (a) total Cr by inductively coupledplasma + optical emission spectrophotometer (ICP-OES, PerkinElmer, Optima 5300V, Japan); and (b) Cr(VI) species separatedby high-performance liquid chromatography (Agilent 1100,Japan) equipped with a guard column and separation column(Hamilton PRP-X100). They were quantified by ICP-MS (Agilent7500C, Japan). Concentration of total trivalent chromium orCr(III) was calculated by deducting the concentration of Cr(VI)from that of the total Cr.

2.4. Application to real samples

Applicability of the biomaterial to remove Cr(VI) in real envi-ronmental water samples was tested with lake and sea watercollected from Australia following the same procedure usedin the batch adsorption study. After adsorption, a desorp-

a mg g−1

b �g g−1

176 Process Safety and Environmental Protection 1 0 0 ( 2 0 1 6 ) 173–182

Table 2 – Identity of the chemical compounds present in M. diosmifolia.

GC peak no Retention time (min) Compound Molecular formula

1 9.8 Eucalyptol C10H18O2 11.1 Benzaldehyde, 4-methyl- C8H8O3 13.1 3-Cyclohexene-1-methanol, �, �,

4-trimethyl-C12H20O2

4 13.7 Benzene, 1,3-bis(1,1-dimethylethyl)-

C14H22

5 14.9 2-Furancarboxaldehyde C5H4O2

6 17.5 Phenol, 2,5-bis (1,1-dimethylethyl)- C14H22O

anion are also expected to increase the sorption capacity ofthe biomaterial with increasing initial Cr(VI) concentration. As

Fig. 1 – Effect of contact time and initial Cr(VI)

(20 mL, pH 6.4), and values were recorded after different timeintervals until equilibrium.

3. Results and discussion

3.1. Physico-chemical characteristics of biosorbent

The biosorbent material used had a BET surface area of0.99 m2 g−1. Surface charge of the material investigated byzeta potential showed +0.81, −6.77 and −8.37 mV at pH 2, 7and 10, respectively. This indicates that the biomaterial canbe tested for its sorption potential over both cationic andanionic pollutant if suitable pH of the solution is selectedin order to increase the ease of pollutant adsorption assurface charge of the sorbent varies with pH. Moreover,M. diosmifolia proved to be a rich source of macronutri-ents such as C (502.1 mg g−1) followed by N (9.8 mg g−1), Ca(8.4 mg g−1) and K (6.4 mg g−1) with small amounts of Naand P (Table 1). Among the micronutrients, Mn (181 �g g−1),Fe (148 �g g−1) and Al (89.4 �g g−1) constituted the maximumwith considerable amounts of Zn, Cu, Ni and S. How-ever, concentrations of Co and Cr were negligible (around0.5 �g g−1). GC-MS analysis revealed that the biomaterial isone source of eucalyptol followed by benzaldehyde, 4-methyl-;3-cyclohexene-1-methanol-�, �, 4-trimethyl-; benzene,1,3-bis(1,1-dimethylethyl)-; 2-furancarboxaldehyde and phenol, 2,5-bis (1,1-dimethylethyl)- (Table 2). Being the exclusive sourceof eucalyptol, M. diosmifolia is useful as a valuable naturalresource for a wide variety of industrial applications, forinstance as antiseptic, biopesticide, flavouring, etc. (Yani et al.,2014).

The ESEM EDX image revealed C, N, O, Na, Mg, Al, Si, P, S,Cl, K, Ca and Fe as the major chemical constituents on thebiomaterial’s surface, indicating that it is useful for extract-ing pollutants from water (Mallampati and Valiyaveettil, 2012).Both ICP-MS and ESEM-EDX analyses confirmed that the bio-sorbent (before adsorption) has negligible or no traces of Crspecies. The FTIR spectrum of M. diosmifolia powder in themid IR range revealed a broad band in the range 3262 cm−1

that corresponds to the hydrogen bonded alcohol. This is pri-marily a phenol group with OH bond stretch and the oneat 2925 cm−1 supports the presence of C H bonds (data notshown). The sharp peak at 1729 cm−1 was due to the C Ostretch of the aliphatic aldehydes, 1605 cm−1 corresponding toC C ring aromatic ring stretch, 1510 cm−1 to nitro compound,and 1447 cm−1 to C O bond of phenols. Presence of cyclic ether(C O), alkene (C C), C H bending and halo group (C Cl) is alsoconfirmed by the observation of peaks at 1243, 959, 822, 777,627 and 511 cm−1, respectively. It is evident from FTIR studies

that the selected biomaterial constitutes both positively andnegatively charged functional groups that would even more

favour the removal of a pollutant irrespective of its surfacecharge at wide-ranging pH.

3.2. Effect of initial pollutant concentration andcontact time

The effect of contact time on Cr(VI) removal at different initialconcentrations (0–500 mg L−1) is represented in Fig. 1. Thesedata were recorded at a constant pH of 7. The plot clearlydisplays that with initial Cr(VI) concentration of 0–250 mg L−1

there was an adsorption burst within a contact time of about2 h and then it levelled off, while with increasing concen-tration (300–500 mg L−1), the time taken to attain equilibriumwas prolonged (7–10 h). At the beginning the removal rate washigh because of the availability of abundant active groupsfor Cr(VI) biosorption. Following this the rate of removalbecame virtually insignificant owing to a quick exhaustionof the adsorption sites and a charge balance between theactive functional groups and Cr(VI) ions. Again, adsorptioncapacity of the biomaterial is dependent on initial Cr(VI) con-centration. This is because the essential driving force whichhelps to overcome all mass transfer resistance of the metalions between the aqueous and solid phases also increasesas the initial Cr(VI) concentration increases. The result isa higher probability of collision between sorbent and Cr(VI)ions, thereby causing higher Cr(VI) uptake (Malkoc, 2006). Also,higher availability of Cr(VI) in the solution and higher interac-tion between the cationic surface of the adsorbent and metal

concentration on Cr(VI) removal by M. diosmifolia. Sorbentdose, 5 g L−1; temp, 24 ◦C; pH, 7.0.

Process Safety and Environmental Protection 1 0 0 ( 2 0 1 6 ) 173–182 177

Fig. 2 – Effect of (a) adsorbent dosage and pH, and (b)adsorbent dosage and temperature on the removal of Cr(VI)by M. diosmifolia. Initial Cr(VI) concentration, 250 mg L−1,and contact time, 2 h.

MCc

3

Tidi5aatiaawGpwtpft

Fig. 3 – Kinetic and isotherm models for removal of Cr(VI)by M. diosmifolia. (a) Pseudo-second-order kinetics (sorbentdose, 5 g L−1; temp, 24 ◦C; pH, 7.0; and initial Cr(VI)concentration, 250 mg L−1); (b) Langmuir isotherm (sorbentdose, 5 g L−1; temp, 24 ◦C; pH, 7.0; and initial Cr(VI)concentration, 100–500 mg L−1).

. diosmifolia offered a finite number of active adsorption sites,r(VI) adsorption showed saturation at higher initial Cr(VI)oncentration.

.3. Effect of adsorbent dosage, pH and temperature

he data in Fig. 2 indicate the effect of adsorbent dose, whereinncrease in Cr(VI) removal was observed with increase inosage rate from 0.5 to 5 g L−1. This can be attributed to the

ncrease in surface area. Among various adsorbent dosages, g L−1 could remove 97–99.9% of Cr(VI). The removal of Cr(VI)s a function of pH and temperature was also examined overn initial pH range of 2–10 (Fig. 2a) and at varying tempera-ures, i.e. 24, 37 and 48 ◦C (Fig. 2b). Although a slight increasen Cr(VI) removal with decreasing pH and increasing temper-ture was observed, the differences were not that significantfter 2 h. Previous researchers reported using ash gourd peelaste (Sreenivas et al., 2014), olive pomace (Malkoc, 2006) andular fruits (Rao and Rehman, 2010), finding that the optimumH was 1–3 for maximum Cr(VI) removal and the reactionsere either endo or exothermic in character. In this study,

he adsorbent had a relatively higher and broader workingH and temperature range. This may be associated with theunctional groups on the adsorbent surface which can be pro-

onated even at pH and temperature extremes, thus allowing

Cr(VI) adsorption to occur through electrostatic attraction (Luoet al., 2011).

We observed the complete removal of Cr(VI) within 3–5 minwhen the initial pH of the solution was acidic (pH 2) (datanot shown); however, as the pH increased (5 to 10), the timetaken to achieve maximum Cr(VI) removal lasted up to 2 h.This is because at lower pH, Cr(VI) species is mainly in univa-lent form (HCrO4

−), thus requiring one exchangeable site forone molecule of Cr(VI) species and making biosorption morerapid and easier. However, with increasing pH, the divalentforms of Cr(VI) species (Cr2O7

2− and CrO42+) are abundantly

present, necessitating two exchangeable sites for adsorptionto occur, thereby delaying the process of metal removal (Dengand Ting, 2005). The high resistance for wide-ranging pH andtemperature with maximum adsorption capacity thus gives M.diosmifolia the potential to be used in many industrial applica-tions.

3.4. Sorption kinetics

Fitting the kinetic data into various mechanistic models, itappears that a pseudo-second-order kinetic equation best fitsthe adsorption data of M. diosmifolia (R2 = 1) (fitting parame-

ters for other models not presented). The following equation

178 Process Safety and Environmental Protection 1 0 0 ( 2 0 1 6 ) 173–182

Table 3 – Langmuir and pseudo-second-order kineticconstants for the removal of Cr(VI) by M. diosmifolia.

Constant Unit

Pseudo-second-order kinetic modelqe exp (mg g−1) 49.38K2 (g mg−1 min−1) 0.15qe pred (mg g−1) 50.0h (mg g−1 min−1) 378.8R2 1.0p <0.0001

Langmuir constantsqm (mg g−1) 62.50KL (L g−1) 0.13R2 0.98p <0.0001RL 100a = 0.07

200a = 0.04300a = 0.02400a = 0.02500a = 0.01

a Initial Cr(VI) concentration in mg L−1.

of total Cr by the biomaterial was not related with the removal

describes the linear form of pseudo-second-order kinetics (Xieet al., 2014):

t

qt= 1

kq2e

+ t

qe(3)

where, qe and qt (mg g−1) are the amount of Cr(VI) adsorbed atequilibrium and at time t, respectively, and k (g mg−1 min−1)is the pseudo-second-order kinetic rate constant. A plot of tversus t/qt results in a linear graphical relationship indicatingthe applicability of second-order kinetics for Cr(VI) as shownin Fig. 3a. Pseudo-second-order kinetic parameters elucidatedin Table 3 were obtained from the slope and intercept of theplot. The initial adsorption rate (h) (mg g−1 min) was calculatedif t = 0 from the following equation:

h = kq2e (4)

The results show that the calculated qe value is close tothe experimental data. Attainment of faster adsorption equi-librium is supported by the calculated pseudo-second-orderrate constant (k) which was 0.152 g mg−1 min−1. In addition,the initial adsorption rate (h) was 378.8 mg g−1 min−1. High ‘h’value is expected due to the presence of more cationic activeadsorption sites that can readily hold Cr(VI) anion. After theexhaustion of the active sites, the adsorbate anions need tomigrate to the inner pores of the biomaterial, thus resultingin slower adsorption at later stages (Sarkar et al., 2010). Thus,the best fitness of the pseudo-second-order model for Cr(VI)adsorption onto the biomaterial confirms that the presentadsorption process is controlled by both physical and chemicalprocesses over the whole range of adsorbate concentrationsstudied (Ho and McKay, 1998; Ncibi et al., 2007).

3.5. Sorption isotherm

Adsorption isotherm defines the relationship between themetal ions in the fluid phase and the concentration ofadsorbed metal at a constant pH, temperature or adsor-bent dose. Langmuir and Freundlich isotherm models were

tested for the present data, and the Langmuir isotherm modelwas found to best fit the data. The basic assumption of the

Langmuir model is that adsorption occurs at specific homoge-nous sites within the adsorbent and there is no interactionbetween the adsorbed species. The Langmuir isotherm modelis given by the following equation (Sarkar et al., 2010):

Ce

qe= 1

qmK+ Ce

qm(5)

where, Ce (mg L−1) is the equilibrium Cr(VI) concentration, qe

(mg g−1) is the amount of Cr(VI) adsorbed at equilibrium, qm

(mg g−1) is the maximum Cr(VI) adsorption capacity by the bio-material and K (L−1 mg) is the Langmuir adsorption constantrelated to the free energy of adsorption. A plot of Ce versus Ce/qe

results in a linear graphical relationship indicating the appli-cability of the Langmuir model for Cr(VI) as shown in Fig. 3b.The Langmuir constants were obtained from the slope andintercept of the straight line for Cr(VI), and are elucidated inTable 3. Since M. diosmifolia adsorption data fit well to thismodel, it is clear that the active sites of the biosorbent sur-face are homogenous for Cr(VI) binding. Also, high value ofcorrelation coefficient (R2 = 0.98) indicates a good agreementbetween the parameters and confirms the monolayer adsorp-tion of Cr(VI) onto the adsorbent surface. The constant qm, ameasure of the adsorption capacity to form monolayer, was ashigh as 62.50 mg g−1 at initial pH range of 7. The constant KL

which denotes adsorption energy was equal to 0.13 L g−1. Theqm value for biomaterial from M. diosmifolia at pH 7 is not com-parable with the existing literature as in all the cases optimumpH for Cr(VI) removal was acidic (1–5.7 pH), wherein the valueswere: 69.3 mg g−1 for activated carbon at pH 3.2 (Hu et al., 2003);13.57 mg g−1 for modified organoclays at pH 5 (Sarkar et al.,2010); 18.69 mg g−1 for olive pomace at pH 2 (Malkoc, 2006);and 18.7 mg g−1 for ash gourd at pH 1.0 (Sreenivas et al., 2014).Our results clearly indicate the superiority of the unmodifiedmaterial of M. diosmifolia over other adsorbents reported so farin the literature. Thus, this non-conventional biosorbent hasa great potential in the removal of Cr(VI).

A further analysis of the Langmuir equation can be madeon the basis of a dimensionless equilibrium parameter, RL

(Babu and Gupta, 2008), also known as separation factor givenby

RL = 11 + KCo

(6)

where K (L−1 mg) is the Langmuir adsorption constant and Co

(mg L−1) is the initial Cr(VI) concentration. RL value indicatesany one of the four possible sorption characteristics: RL > 1 forunfavourable sorption; RL = 0 for irreversible sorption; RL = 1for linear sorption, and 0 < RL < 1 for favourable sorption. Inthe present study, the dimensionless parameter, RL remainedbetween 0.01 and 0.07 (0 < RL < 1) (Table 3), which is consistentwith the requirement for a favourable adsorption process overthe Cr(VI) concentration range tested (100–500 mg L−1).

3.6. Evidence for the occurrence of ‘adsorption-coupledreduction’

To clearly understand the mechanism for removing Cr(VI) bythe selected biosorbent, we analysed the fractions of differentCr species in the aqueous and solid phases. Removal efficiency

rate of Cr(VI). Though LC-ICPMS confirmed the removal ofnearly 99% Cr(VI) (remaining Cr(VI) was 0.1–1.65 mg L−1) from

Process Safety and Environmental Protection 1 0 0 ( 2 0 1 6 ) 173–182 179

Fig. 4 – ICP-MS and ICP-OES data on adsorption-coupledreduction of Cr(VI) by M. diosmifolia. (a) Total Cr, Cr(VI) andCr(III) remained in the solution at different pH (sorbentdose, 5 g L−1; temp, 24 ◦C; incubation time, 2 h; initial Cr(VI)concentration, 250 L−1); (b) Total Cr (mg) sorbed by 100 mgpowder after 2 h at different pH.

ts1td2dcpapeToapdtttsoo3p

bmFr

was extractable, clearly suggesting that the adsorption pro-

he aqueous solution at a pH range of 2–10, ICP-OES analy-is of the aqueous fractions revealed the presence of nearly4.8–25.22 mg L−1 of the total Cr (Fig. 4a). The occurrence ofotal Cr in the aqueous phase implies Cr(VI) reduction to Cr(III)ue to its coming into contact with the biomaterial (Park et al.,007). Maximum Cr(VI) reduction was observed at acidic con-ition (pH 2 followed by 5). Reduction reaction implies theonsumption of protons and is therefore favoured at in a lowH state. Not only protons but a source of electrons is needed,nd in the case of a biomaterial, the organic compoundsresent in the surface of the material supply the necessarylectrons for a reduction to occur (Lopez-Garcia et al., 2013).he first evidence of anionic adsorption and partial reductionf Cr(VI) into Cr(III) below pH 3 by biomaterials such as sug-rcane bagasse, sawdust, maize cob and sugar beet pulp wasrovided by Sharma and Forster (1994). Park et al. (2007) alsoemonstrated reliable evidence of adsorption-coupled reduc-ion by natural biomaterials at an initial pH of 2. It is notablehat the maximum Cr removal (total) was at near neutralo alkaline pH. This could be attributed to the presence ofome positively charged functional groups over the surfacef the biomaterial enhancing the sorption of Cr(VI). More-ver, the digested solid phase (sorbed material) constituted–3.8 mg g−1 of total Cr, highlighting the higher adsorptionotential of the selected biomaterial (Fig. 4b).

The mechanism of adsorption-coupled reduction by theiosorbent was further confirmed by examining the Cr-sorbedaterial under SEM-EDX, FTIR and XRD. A comparison of

TIR spectrum of the biomaterial before and after Cr removal

evealed the disappearance of C C (822 cm−1) and C O H

(711 cm−1) bonds with the appearance of few C H bonds(2856.3, 1659, 1370 and 772 cm−1) in the Cr-sorbed material(Fig. 5a). The appearance of new C H bond reveals the occur-rence of a reduction process. In general, if either of the C Oor C C bond is broken to form C H bond, then the reac-tion is meant to be ‘reduction’ (Mason, 1949). On the otherhand the appearance of a new bond at 882 cm−1 correspondsto the hexavalent Cr (Fig. 5b), confirming the adsorption ofCr(VI) over the biomaterial surface. Thus, FTIR observationsupports that the reaction involves both adsorption and reduc-tion. Additionally, XRD analysis (Fig. 5c) showed that bothtrivalent (as chromium oxide) and hexavalent (as dipotassiumchromate) forms of Cr exist on the surface of the biomaterialafter treatment, clearly indicating that both Cr(VI) adsorp-tion and Cr(VI) reduction contributed to the overall removal ofCr(VI) from aqueous solution. Both the ESEM and EDX analysesrevealed the presence of Cr in the biomaterial after sorption,wherein the untreated sample did not show any incidence ofCr (Fig. 5d). It is clear from the data that the pores and surfacesof adsorbent are covered, and hence became smooth due tothe adsorbate (Fig. 5e). Consequently, the SEM analysis alsoholds well as a second confirmation of Cr adsorption by thebiomaterial.

The reducing property of the biomaterial is expected pri-marily due to its chemical characteristics. Notably, some ofthe constituents like Fe (Zhang et al., 2011) and polyphe-nols (Scalbert and Williamson, 2000) are well-known reducingagents. In fact, the material from M. diosmifolia was found toconstitute as high as 148 �g g−1 Fe (Table 1), 42.7 mg g−1 totalphenols and 39 mg g−1 flavonoids (Table 4). The iron reduc-ing power of the plant extract was found to be 27.7 mg g−1

which confirms its high reducing potential. Furthermore anantioxidant potential of about 90.6% (based on DPPH radicalscavenging activity) of the biomaterial indicates its potentialfor use as a natural antioxidant (Moure et al., 2001) whichwarrants further investigation. Since the present biomaterialis rich in polyphenols that have recently been implicated inthe green synthesis of nanoparticles (because of their reduc-ing or stabilizing activity) (Moulton et al., 2010), we hope to usethe extract from M. diosmifolia for green synthesis of nanopar-ticles and test its potential to bioremediate Cr-contaminatedwastewater in the near future.

In general, the mechanism for removing Cr(VI) by thebiomaterial is supposed to be a complex process involv-ing adsorption followed by direct and/or indirect reductionmechanisms. Any one or a combination of the following mech-anisms possibly result in better Cr(VI) removal by the selectedplant material: (1) sorption of Cr(VI) directly onto the adsor-bent surface, (2) reduction of sorbed Cr(VI) to Cr(III) by reactionwith adjacent electron-donor groups, (3) release of the reducedCr(III) into aqueous phase due to electronic repulsion betweenthe positively-charged groups and the Cr(III) ion, (4) complex-ation of Cr(III) with adjacent functional groups (adsorption oradsorption-coupled reduction), (5) reduction of Cr(VI) to Cr(III)in the aqueous phase by direct contact with the electron-donorgroups of the biomaterial, and (6) either complexation of thereduced form with the adsorbent surface or remaining in theaqueous phase (reduction or reduction-coupled adsorption).

3.7. Regeneration of biomaterial

Only about 21 mg L−1 out of the total 220 mg L−1 of sorbed Cr

cess of Cr(VI) onto the biomaterial is partially irreversible,

180 Process Safety and Environmental Protection 1 0 0 ( 2 0 1 6 ) 173–182

Fig. 5 – (a) FTIR spectra of M. diosmifolia before and after Cr(VI) removal; (b) FTIR spectrum of potassium dichromate; (c) XRDiomaterial (d) before and (e) after treating with Cr(VI) solution.

Table 4 – Total polyphenol (TP), total flavonoid (TFd)contents and antioxidant activity in M. diosmifolia.

Chemicals/activity Value

TP 42.7 ± 6.7 mg GAE g−1 on dry wtbasis

TFd 39.0 ± 3.5 mg Quercetin g−1 on drywt basis

Antioxidant activityDPPH radical scavenging 90.6 ± 3.9%

spectrum of Cr sorbed biomaterial; SEM EDX images of the b

and this in turn indicates the strong complexation of Cr tothe adsorbent surface. Out of 20 mg L−1 Cr (total Cr desorbed),Cr(VI) species constituted only about 3 mg L−1. It is evident thatmost of the Cr bound on the surface exists as Cr(III), and thisreduced form may be more strongly bound to some functionalgroups like C O, thus making it non-extractable (Suksabyeet al., 2007).

3.8. Implications for remediation

Water samples collected from a lake (pH 7.8) and sea (pH 8.4)

were used in order to evaluate the applicability of the presentremoval process based on adsorption-coupled reduction of

activityReducing power 27.7 ± 5.1 mg ascorbic acid g−1

Process Safety and Environmental Protection 1 0 0 ( 2 0 1 6 ) 173–182 181

Table 5 – Potential of biomaterial from M. diosmifolia in remediating Cr-contaminated wastewater.

In aqueous solution (mg L−1) Total Cr (mg) sorbed by 100 mg leaf powder

Cr(VI) Total Cr Cr(III)

W1 0.14 9.94 9.8 3.02W2 0.47 4.61 4.14 3.95

Sorbent dose, 5 g L−1; temp, 24 ◦C; pH, 7.0; initial Cr(VI) concentration, 250 mg L−1.

Ccbo4teicTtCimior

MpfiwotrTitoifcm

4

TaacbcowIoac

A

SAt

r(VI) onto M. diosmifolia biomaterial. Appreciable removal effi-iencies were obtained for Cr(VI) (Table 5). In 2 h, the amendediomaterial (sorbent dose = 5 g L−1) was able to remove 99%f added Cr(VI). Also, the aqueous solution contained only.61–9.94 mg L−1 of the total Cr, and the total Cr sorbed byhe material (100 mg) was about 3–3.9 mg. The Cr(VI) removalfficiency witnessed in the present study is far superior ast achieved almost complete removal of Cr(VI) (initial Cr(VI)oncentration of 250 mg L−1) in real environmental samples.his is the first evidence for the remarkable potential of dried

wigs of M. diosmifolia in the detoxification and removal ofr(VI) in real water samples, suggesting its suitability for

ndustrial wastewater treatments. As this material has higheretal removal potential and works well in wide-ranging pH,

ts applicability can be investigated for the removal of severalther organic pollutants, and metals like As, Pb, Zn, Cu, etc. ineal samples.

Being a weed with no human utility or commercial value,. diosmifolia is easily accessible and inexpensive. Use of thislant material as a biosorbent for Cr(VI) removal not only puri-es contaminated waters but also achieves the purpose ofaste minimization and its re-utilization as a resource. More-

ver, Cr(VI) is detoxified to Cr(III) and is independent of pH oremperature, which implies the application of M. diosmifolia ineal problematic environments with wider physical settings.he properties of the biosorbent increase the probability of

ts additional practical use as a biological source for eucalyp-ol, natural antioxidants and polyphenols. Certainly, disposalf the exhausted adsorbent laden with Cr ions is crucial for

ts practical application. Curing could be a useful technologyor efficient disposal of the exhausted adsorbent, and this willertainly prevent the secondary pollution of Cr in the environ-ent.

. Conclusions

he present study highlights the superiority of M. diosmifolias a potential biosorbent with reducing potential which willssist in the efficient detoxification and removal of Cr(VI) fromontaminated wastewaters. This natural adsorbent proved toe a useful and valuable means for controlling water pollutionaused by toxic metal ions. Utilization of the biomaterial with-ut pretreatment is one of the significant features of this studyith respect to decreasing the cost of the adsorption process.

t can be concluded that the powdered form of the dried twigsf M. diosmifolia is simple, more economical and excellentdsorbent than those commercially available for remediatingontaminated water bodies.

cknowledgments

K thanks the Australian Government, University of South

ustralia (UniSA) and Cooperative Research Centre for Con-

amination Assessment and Remediation of the Environment

(CRC CARE) for the International Postgraduate Research Schol-arship (IPRS), and CRC CARE for top-up fellowship duringPhD.

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