Surface functionalization of Aspergillus versicolor mycelia: in situ fabricationof cadmium sulphide nanoparticles and removal of cadmium ions fromaqueous solution

Sujoy K. Das,*ab Ishita Shomea and Arun K. Guhaa

Received 9th December 2011, Accepted 17th January 2012

DOI: 10.1039/c2ra01273a

Xanthate functionalization of Aspergillus versicolor mycelia (AVM) was carried out to synthesize

cadmium sulphide (CdS) nanoparticles and for the removal of cadmium ions from aqueous solution.

The synthesized nanoparticles were characterized by spectroscopic and microscopic techniques.

Fourier transform infrared (FTIR) spectroscopy and elemental detection X-ray analysis (EDXA)

results confirmed the binding of cadmium with sulphur groups of the functionalized mycelia.

Scanning electron and atomic force microscopic studies revealed alteration of surface morphology

following binding of cadmium, while high resolution transmission electron microscopy (HRTEM)

and fluorescence micrographs demonstrated formation of CdS nanoparticles on AVM surface.

Formation of 3.0 ¡ 0.2 nm size CdS nanoparticles was confirmed from HRTEM images. The

maximum adsorption capacity of the functionalized mycelia for Cd+2 was enhanced to 141.5 mg g21

from the corresponding value of 70.5 mg g21 for pristine mycelia. An increase in adsorption capacity

was attributed to cadmium binding affinity of sulfur atoms due to soft acid–base reaction and

supported by a 2DG value. The experimental results thus suggest that xanthate functionalization of

AVM provides a feasible approach for CdS nanoparticle synthesis and also for efficient removal of

heavy metal ions.

Introduction

Surface and interfaces play an important role in many areas of

research ranging from nanoscience to environmental technology.

In recent years, template directed synthesis of nanoscale

materials has found potential applications in molecular electro-

nics, photocatalysis, solar energy conversion, and active electro-

nic devices.1,2 Moreover, a three dimensional dispersion of

nanomaterials fabricated on template molecules increases the

accessibility for catalytic reaction. Utilization of template

molecules in the fabrication of nanomaterials are currently being

explored in a number of systems like silica, metal oxide,

aluminum hydroxide-coated phospholipid tubules, polymers,

ceramics, cellulose, carbon nanotube, etc.3–7 Among these,

biological materials have gained much interest to modulate the

growth of a large variety of inorganic nanoparticles including

metal, semiconductor and magnetic particles. These biological

materials are useful because of their specific properties, such as

precise molecular recognition and the spatial organization that

they impart on the growth of nanoparticles through specific

binding affinities, nucleation and assembly.8–11 Besides, biologi-

cal fibers as scaffolds also allow the manipulation of size, shape

and even packing density of nanoparticles.12 Chemical modifica-

tion of a biomolecular scaffold with functional molecules

has therefore emerged as an attractive and practicable way to

rationally tailor the properties of the scaffolds in current

years.13,14 It creates preferential binding sites to nucleate and

organize nanoparticles on the surface. Meldrum and Seshadri

reported 15 nm porous gold nanostructures synthesis on skeletal

plates of echinoids (sea urchins) as templates.5 He et al.6

demonstrated the formation of porous and nonporous silver

nanostructures using cellulose fibers as the template. The

controlled interaction between surface functional groups and

nanoparticles yield a complex form of higher order hybrid

assemblies. Despite numerous reports on metal nanoparticles

assembling on biomaterials, very few reports are available on

the synthesis of semiconductor nanoparticles. Among various

nanoparticles, the template directed synthesis of cadmium

sulphide (CdS) nanoparticles has gained considerable attention

in current research due to its size-dependent tunable spectro-

scopic properties.15–17 We therefore, have attempted to functio-

nalize the surface of fibrilar fungal mycelia by covalently linking

xanthate groups for synthesis of CdS nanoparticles and removal

of these metal ions from water.

In the context of environmental science the surface and

interface also plays a crucial role. The most commonly used

aDepartment of Biological Chemistry, Indian Association for theCultivation of Science, Kolkata, 700 032, IndiabEnvironmental Technology Division, Council of Scientific and IndustrialResearch (CSIR)-Central Leather Research Institute (CLRI), Chennai,600 020, India. E-mail: sujoy@clri.res.in; Fax: +914424916351;Tel: +914424437132

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techniques for the removal of heavy metals like cadmium from

water bodies are lime precipitation, ion exchange, ultrafiltration

and reverse osmosis. But these techniques suffer from limitations

like high operating cost, incomplete precipitation, and genera-

tion of a huge amount of metal-bearing toxic sludge. Adsorption

is a recently developed technique for metal removal, but lack

of affinity and inadequate uptake capacity of the adsorbent

materials requires a long time to reach equilibrium.18,19 Surface

functionalization of adsorbents with suitable functional group is

believed to increase the uptake capacity and also increase the

affinity of the adsorbents for the desired metal ions and hence

improve their performance. Therefore, surface functionali-

zation of adsorbent has practical significance in efficient removal

of metal ions. Among different functional groups, xanthate

functionalization is usually preferred due to their easy preparation

procedures, low solubility products and high stability constant

values of the metal complexes formed.20 In this manuscript we

explored in situ synthesis of CdS nanoparticles on fibrilar

Aspergillus versicolor mycelia (AVM) through surface functiona-

lization by xanthate modification. Moreover, the functionalized

mycelia exhibited high cadmium binding capacity compared to

the pristine mycelia. We therefore, strongly believe that in situ

synthesis of CdS nanoparticles and removal of cadmium ions from

aqueous solution by surface functionalization has significant

practical implication in terms of nanoparticles synthesis and

bioremediation of environmental pollutants.

Experimental section

Chemicals

Cd(NO3)2,4H2O, was purchased from Merck, Germany.

Microbiological media were procured from Himedia, India. All

other reagents were of analytical reagent grade and purchased

from E-Merck, India.

Metal solution and analysis

Aqueous solutions (1000 mg L21) of cadmium were prepared by

dissolving the required amount of Cd(NO3)2,4H2O in double distilled

water and diluted to get the desired concentration. Concentration of

the cadmium was measured by atomic adsorption spectrometer

(Varian Spectra AA 55) using the respective standard solution.

Preparation and functionalization of A. versicolor mycelia

(AVM). A. versicolor used in this study was maintained and

cultivated in potato dextrose (20% potato extract and 2% dextrose)

medium.21 The organism was grown in an 250 mL Erlenmeyer

flask containing 75 mL media by inoculating with spore suspension

(4 6 107/mL) and incubated at 30 uC for 5 days under shaking

(130 rpm) condition. At the end of incubation, mycelia was

harvested by filtration, washed with deionized water and dried by

lyophilization. Xanthate functionalization of A. versicolor was

carried out as described before.22 In brief, 5 g of dried A. versicolor

was treated with a mixture of carbon disulphide (20 mL) and

NaOH solution (25 mL of 14% aq.) and incubated for 5 h at 10 uCunder shaking conditions. The resulting yellow product was

filtered and washed repeatedly with deionized water until neutral

and finally dried by washing with acetone. The dried functionalized

A. versicolor was stored at 4 uC for use.

Synthesis of CdS nanoparticles

Functionalized 0.2 g of A. versicolor was treated with 25 mL

Cd+2 solution (500 mg L21) under shaking condition for 24 h at

30 uC. After 24 h, the mycelia were collected by centrifugation at

15 000 rpm for 15 min and dispersed in ultrapure water by

sonication followed by filtration to remove large mycelium. UV-

vis spectroscopic measurement of the dispersed solution was then

recorded on a Varian Carry 50 Bio spectrophotometer. The

control experiment was performed under identical condition

excepting without addition of AVM.

Characterization of as synthesized CdS

The synthesis of CdS on AVM was characterized by JEOL JSM

6700F field emission scanning electron microscope equipped

with an energy dispersive X-ray spectrometer (FESEM-EDAX).

Samples were coated with platinum before FESEM-EDAX

analysis. The atomic force microscopy (AFM) images were

recorded on a multimode AFM (Veeco Metrology, Autoprobe

CP-II, Model No AP0100). The sample was prepared as described

by Das et al.23 In brief, the functionalized AVM, before and after

treatment with cadmium ions, were incubated the with an

ultrasonically cleaned glass cover slip for 60 min, followed by

repeated washing with ultrapure Millipore water (18.2 MV) to

remove loosely attached AVM. The cover slip was then mounted

for AFM study. Imaging in air at ambient conditions (20 ¡ 2 uC)

was carried out using silicon probes (RTESPA-M, Veeco, Santa

Barbara, CA) and in tapping mode for minimizing sample damage

by the scanning tip. The cantilever used had long tips (aspect ratio

4 : 1) with spring constants ranging from 20 to 80 N m21 and

resonance frequencies of 245–285 kHz. The mycelium was

scanned in both front and back directions several times before

capturing an image to ensure minimal effects of non-linearity,

such as hysteresis.

For HRTEM images, samples were prepared by drop-casting

methodology. The dispersed solution of functionalized AVM

after treatment with cadmium was drop casted on a carbon

coated copper grid and then micrographs were recorded on a

JEOL JEM 2010 high resolution transmission electron micro-

scope operated at 200 kV. FTIR spectra of the samples

were taken with Shimadzu FTIR Spectrometer under ambient

condition. Pressed pellets were prepared by grinding the powder

specimens with spectroscopic grade KBr with a sample/KBr ratio

y1/100 in an agate mortar. The FTIR spectra were recorded

with 500 scans at a resolution of 2 cm21. The fluorescence

microscopy images of cadmium treated functionalized AVM

were recorded on a fluorescence microscope (Olympus BX-61)

using an excitation filter of BP460–495 nm and a band

absorbance filter covering wavelengths below 505 nm. The

samples were excited with a 50 W mercury lamp. Fluorescent

microscopy images of several randomly selected sites were

captured with a digital camera connected to the microscope.

Adsorption experiment

Adsorption experiments were conducted in a batch process in

100 mL Erlenmeyer flasks to study the uptake capacity of

functionalized mycelia. Effects of pH, kinetics and concentration

were studied. The optimum pH for adsorption was determined

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by suspending 4 g L21 pristine or functionalized A. versicolor

mycelia in 100 mL Erlenmeyer flasks containing 50 mg L21

cadmium, at different pH values (2.0–7.0). 50 mM citrate-

phosphate buffer was used to prepare different pH solution

containing 50 mg L21 cadmium ions. The flasks were then

incubated with shaking (120 rpm) at 30 uC (ambient temperature)

for 24 h. At the end of incubation, adsorbent was separated by

centrifugation (10 000 rpm for 15 min) and the concentration of

cadmium in the supernatant was measured by atomic absorption

spectrometry (AAS) as described above. The amount of cadmium

adsorbed by the mycelia was calculated using the mass balance

equation as described elsewhere.21 The equilibrium adsorption

isotherm was carried out similarly in a batch process at pH 6.0 but

with different cadmium concentrations (5–1000 mg L21). Other

experimental parameters were the same as described above. The

kinetics of adsorption process was followed at regular intervals up

to 6 h using 50 mg L21 cadmium at pH value 6.0. As samples were

collected from individual flasks, no correction was necessary

regarding the withdrawal of the sampling volume. In all cases, the

control experiments were conducted under identical conditions

excepting without addition of any types of AVM.

Elution of cadmium from loaded AVM. The functionalized

AVM adsorbed with 50 mg L21 cadmium solution was

incubated with low pH (, 2.0) solution under shaking at

130 rpm for 60 min. On completion of the incubation period, the

concentration of metal ions eluted from the loaded mycelia was

measured by AAS.

Fig. 1 UV-vis spectra of dispersed solution of functionalized AVM

before and after treatment with cadmium solution.

Fig. 2 SEM images of functionalized AVM before (A, low magnification; C, high magnification) and after (B, low magnification; D, high

magnification) binding with cadmium; AFM images of functionalized AVM before (E) and after (F) binding with cadmium.

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Results and discussion

Synthesis and characterization of CdS

Incubation of Cd(NO3)2 solution with functionalized AVM for

10 h caused a color change of the mycelia from pale yellow to

orange yellow, indicating the formation of CdS nanoparticles on

the AVM surface. The orange yellow colored mycelia (CdS

fabricated mycelia) were collected by centrifugation (10 000 rpm

for 10 min), dried by lyophilization and dispersed in ultrapure

water. The UV-vis spectra of the dispersed solution exhibited an

absorption maximum at about 380 nm (Fig. 1) due to the surface

plasmon resonance (SPR) band of the CdS nanoparticles.24,25

However, the control functionalized AVM showed no such

absorption band. This indicated formation of CdS nanoparticles

on the surface of functionalized AVM. Fig. 2 showed the

FESEM and AFM images of functionalized AVM before and

after interaction with Cd(NO3)2. FESEM images (Fig. 2A–D)

showed that the surface morphology of control functionalized

AVM changed conspicuously following binding with cadmium.

Compared to the control AVM (Fig. 2C), the surface of

cadmium treated AVM became more rough and appearance of

globular structures of CdS was observed in a high magnification

image (Fig. 2D). AFM images demonstrated that functionalized

AVM has domain like layer structures (Fig. 2E) on the surface.

Following interaction with cadmium, disappearance of layer

structures and subsequent appearance of globular structures of

CdS nanoparticles (Fig. 2F) on AVM surface were witnessed. A

similar structure was also reported by Liu et al.26 on the formation

of CdS on a reduced graphene oxide surface. The TEM

micrograph clearly shows the formation of CdS nanoparticles

on the surface of functionalized mycelia (Fig. 3A). CdS

nanoparticles were uniformly distributed throughout the surface.

The HRTEM image as shown in Fig. 3B suggested formation of

spherical particles with an average size (n = 100) of 3.0 ¡ 0.2 nm.

The measured d-spacing of the lattice fringes in the HRTEM

image was 3.3 A, which corresponds to the (111) plane of cubic

face CdS.25 The SAED pattern (Fig. 3B, inset) obtained from CdS

nanoparticles showed Scherrer ring patterns characteristic of

(111), (220), and (311) atomic planes of cubic CdS structure.

Energy dispersive X-ray analysis (EDXA) of the functiona-

lized AVM (Fig. 3C) showed the presence of C, N, O, Na, Ca

and S peaks. The C, N, O and Ca peaks appeared from

carbohydrate and protein molecules present on AVM surface,

whereas S and Na peaks demonstrated functionalization of

AVM. The pristine AVM showed peaks of only C, N, O and Ca

(data not shown). Following interaction with Cd+2 solution,

functionalized AVM showed the presence of C, N, O, S and Cd

peaks. It is interesting to note that in the post treated mycelia,

the peaks of alkaline earth metal ions (Na and Ca) disappeared

and concomitantly Cd peaks appeared (Fig. 3D) on the surface.

This indicated that the formation of CdS nanoparticles on the

surface occurred through ion exchange mechanism.

The FTIR spectra of the functionalized AVM showed

perceptible changes after binding of cadmium ions. The xanthate

functionalized AVM exhibited peaks at 655, 1040, 1052, 1081,

1103 and 1225 cm21 (Fig. 4A) corresponding to cc-s, cc=s, ccss (a),

Fig. 3 TEM image (A) of functionalized AVM after binding with cadmium; HRTEM image (B) of CdS nanocrystals formed on the functionalized

AVM. SAED pattern (B, inset) of CdS nanocrystal; EDXA spectra of functionalized AVM before (C) and after (D) binding of cadmium.

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cc-o-c and ccss (s) of xantahte groups.22,27,28 Downfield shifts of the

wavenumber as well as reduction of intensity in the region 800–

1200 cm21, particularly 655, 1040 1052, 1081, 1103 and 1225 cm21

were noted after binding of cadmium (Fig. 4B). The FTIR

spectrum thus confirmed that the xanthate groups on the

functionalized mycelia were the main binding sites for cadmium

ions.20 Cadmium reacts with the sulphur atom of the xanthate

group and forms CdS nanoparticles on the functionalized AVM.

However, the absorption band for Cd–S was not detected on the

current scale of the spectrum as it appeared at y250 cm21.29

XRD patterns also confirmed the formation of CdS on

functionalized mycelia. The XRD pattern (Fig. 4C) exhibited

diffraction peaks at 26.4u, 43.8u and 51.5u corresponding to

(111), (220) and (311) planes of cubic phase CdS (JCPDS 10-

454), respectively. The XRD data were in good agreement with

TEM results and supported the successful synthesis of CdS

nanoparticles on the surface of AVM. This result further showed

that the main diffraction peaks of CdS-AVM composites are

similar to pure CdS and demonstrated that in situ fabrication of

CdS on AVM does not result in the development of new crystal

orientations of CdS.

It is well known that CdS nanoparticles have luminescence

property under UV light. Therefore, the luminescence properties

of the synthesized CdS were recorded by fluorescence microscopy.

The as synthesized CdS has absorption maximum at about

380 nm, however cadmium treated functionalized AVM was

excited at 460 nm to overcome the strong background fluores-

cence. The bright field and fluorescence images of cadmium

treated functionalized AVM are illustrated in Fig. 4D and E,

respectively. The bright green color under fluorescence micro-

scopy further confirmed the formation of CdS nanoparticles on

xanthate functionalized AVM following the binding of cadmium

ions. Similar fluorescence properties of CdS were noted by Peretz,

et al.30 by embedding CdS on polyvinyl pyrrolidone matrices.

Fig. 4 FTIR spectra of functionalized AVM before (A) and after (B) treatment with cadmium solution; XRD pattern (C) of the as-synthesized CdS

nanoparticles; bright field (D) and fluorescence microscopy (E) image shows luminescence property of CdS nanocrystals formed on the functionalized

AVM. Micrographs were recorded on a fluorescence microscope (Olympus BX-61) using an excitation filter of BP460-495 nm and a band absorbance

filter covering wavelengths below 505 nm.

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These results therefore clearly demonstrated CdS nanoparticle

synthesis in a single step process employing a simple functiona-

lization technique.

Batch adsorption experiment

Functionalized AVM was further tested for adsorptive removal

of cadmium ions from water bodies. The surface functionaliza-

tion of AVM through the xanthate group is believed to increase

the adsorption capacity compared to the pristine mycelia as the

sulphur atom of the xanthate group has a strong affinity for

cadmium. The pH is an important factor and plays a crucial role

in the adsorption of metal ions by changing the surface charge

density on both the adsorbent and adsorbate. Moreover the

metal speciation, sequestration, and/or mobility are strongly

influenced by solution pH. Adsorption of cadmium by functio-

nalized AVM was found to increase with increase in pH of the

solution, with an optimum pH range 5.0–6.0 (Fig. 5A). This

high adsorption at pH value 5.0–6.0 was associated with the

formation of positively charged metal species having strong

affinity for the surface functional groups. The experiment was

restricted beyond the pH value of 6.0 due to precipitation of

metal hydroxides such as [Cd(OH)3]2 or [Cd(OH)4]22, which

have a lower binding affinity due to repulsive interaction with

the negatively charged binding sites of the adsorbent.31,32

Speciation studies of cadmium salt demonstrated the forma-

tion of Cd(OH)(aq) and Cd(OH)2(aq), species at pH 6.0 and

Cd(OH)32(aq), and Cd(OH)4

22(aq) species beyond pH value

6.0.31,32 However, cadmium exists as Cd+2(aq) at low pH values.

The reduced adsorption observed at low pH value (, 3.0) may

be attributed to (i) higher hydrated [Cd+2(aq)] species having low

mobility and (ii) protonation of the surface functional groups.

Competition between Cd+2(aq) species and H+ or H3O+ ions

present in the solution also hindered the approach of metal

species due to coloumbic repulsion. Moreover, the xanthate

group was found to be unstable at low pH values and dissociated

from the mycelia with the elimination of carbon disul-

phide.22,33,34 At higher pH values (5.0–6.0), more functional

groups are available for metal ion binding due to deprotonation,

resulting in high adsorption. Therefore, maximum adsorption of

cadmium within the pH values of 5.0–6.0 might be due to partial

hydrolysis of species like Cd(OH)(aq), and Cd(OH)2(aq), having

strong affinity for the negatively charged functional groups of

the mycelia. EDXA data show (data not shown) that cadmium

ions replace the Na and Ca peaks after cadmium adsorption on

functionalized AVM, demonstrating that the cadmium adsorp-

tion process occurred through an ion-exchange mechanism.

The kinetic results (Fig. 5B) indicated that the cadmium

adsorption process was very fast and reached equilibrium within

20 min in the functionalized AVM against 6 h for pristine

mycelia (inset, Fig. 5B). The reasonably fast kinetics reflected

good accessibility of the binding sites of the functionalized AVM

to cadmium ions. The enhanced adsorption rates therefore have

significant practical advantage in terms of time and space over

the conventional techniques.

The functionalized AVM was further used to study the

enhanced adsorption capacity of this mycelia compared to

pristine mycelia. The maximum adsorption (Fig. 5C) capacity of

the xanthate-functionalized AVM for cadmium was found to be

145.5 mg g21 compared to 70.5 mg g21 for pristine AVM. The

isotherm profile in functionalized mycelia was much steeper than

that of pristine mycelia and approached to an ideal type-1

isotherm according to IUPAC classification35 and best fitted

with the Langmuir isotherm36 model with regression coefficient

(r) 0.995. On the other hand, a regression coefficient of 0.875 for

Fig. 5 Effect of pH (A) on cadmium adsorption on functionalized

AVM. Adsorption kinetics (B) of cadmium on the functionalized AVM;

inset figure depicts adsorption kinetics on pristine AVM. Adsorption

isotherm (C) of cadmium on the functionalized and pristine AVM. Data

represent an average of four independent experiments ¡ S.D. shown by

the error bar.

Table 1 Parameters associated with adsorption of cadmium on both functionalized and pristine AVM

Type of mycelia Qmax (mg g21) Kd (L g21) KL (L g21) DG (kJ mol21)

Functionalized AVM 145.45 0.17 6.65 24.77Pristine AVM 70.05 0.073 0.46 21.95

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pristine mycelia plausibly indicated a lack of energy uniformity37

of the binding sites for metal ions compared to xanthate

functionalized mycelia. The cadmium removal capacity of the

adsorbent could also be expressed in terms of distribution

coefficient (Kd).37

Kd~Q

Ceq

where Q is amount of metal species adsorbed (mg g21), and Ceq

is equilibrium concentration (mg L21).

The uptake capacity (Qmax), distribution coefficient (Kd), and the

Langmuir adsorption constant (KL),36 related to the adsorption

energy for Cd(aq) species is summarized in Table 1. The Qmax, Kd,

and KL values for cadmium on the functionalized mycelia were

higher than those on the pristine AVM, indicting high affinity of the

functionalized mycelia for cadmium. In addition the adsorption

capacity of AVM through functionalization was increased signifi-

cantly compared to other reported adsorbents.38–42 For examples,

kaolinite clay after pre-treatment with tripolyphosphate adsorbed

113.64 mg g21 of cadmium, whereas untreated kaolinite clay

adsorbed only 13.23 mg g21.38 Sodium tetraborate treated kaolinite

clay adsorbed 44.05 mg g21 of cadmium.39 Granular activated

carbon and activated clay adsorbed 11.75 and 8.718 mg g21

cadmium, respectively;40 whereas, other types of carbon adsorbed

40–97 mg g21 of cadmium.41 Epichlorohydrin treated, NaOH

treated and sodium bicarbonate treated rice husk adsorbed 11.12,

20.24 and 16.18 mg g21 cadmium, respectively.42 The increased

adsorption of cadmium in xanthate functionalized AVM is thus

attributed to the metal-binding ability of sulphur groups with

cadmium.43,44 Cadmium and sulphur groups are soft acid and soft

base, respectively, hence high adsorption of cadmium by functio-

nalized mycelia can be explained by soft acid–base interaction

according to the Pearson rule.45

A. versicolor is easily grown in a cheap and simple growth

medium. The handling of this fungus is very easy and growth

rate is also high compared to other fungi. It secretes large

amount of proteins and is widely used in the production of

important enzymes including amylase, cellulase, xylanase, and

pectinolytic enzymes, and also in biodiesel production.46,47 Large

amount of waste mycelia are generated from these industries.

Therefore, dissimilatory properties of this fungi could be

exploited for low-cost and environmental friendly removal of

metal ions. Xanthate functionalization of AVM not only

increases the adsorption capacity, but also increases the affinity

towards cadmium.42 Xanthate functionalized AVM took only

20 min to reach equilibrium, whereas other reported adsorbent

took 5–10 h to attain the equilibrium.40,42,48,49 Most importantly,

more than 85% of the adsorbed cadmium ions were eluated from

the loaded AVM by low pH (, 2.0) solution. Functionalization

of AVM thereby offers a low-cost green chemical approach

toward reclamation of cadmium ions from water bodies.

The metal ions’ binding affinity of xanthate functionalized

AVM can be explained by a model reaction X2S + M+2 A MS +

2X+ employing thermodynamics data as described by Brown

et al.50 We therefore measured the DG (Gibbs free energy)35

values for the present adsorption process, considering above

reaction with respect to cadmium and the result came out to be

24.77 kJ mol21. The negative DG value indicates the high degree

of spontaneity and energetically favorable adsorption process. In

addition, formation of CdS nanoparticles following binding with

the sulfur atom of the xanthate group is also responsible for

higher adsorption of cadmium in the functionalized AVM. Thus

increased cadmium binding efficiency of the functionalized

mycelia demonstrated that the xanthate group has a strong

binding affinity for cadmium and this has practical significance

in process scale up for removal of heavy metal ions.

Conclusions

We developed a novel method for the fabrication of metal sulfide

nanoparticles on the surface of fibrilar fungal mycelia and heavy

metal removal through a xanthate functionalization process. The

method includes in situ synthesis of CdS nanoparticles on the

mycelia surface and removal of cadmium ions from water bodies.

SEM and AFM images supported the appearance of globular

structures of CdS nanoparticles on the surface of AVM

following the binding with cadmium ions. TEM image showed

that synthesized CdS nanoparticles have an average size of 3.0 ¡

0.2 nm, while the EDXA result confirmed the involvement of an

ion-exchange mechanism in the binding process. Fluorescence

microscopy images showed the luminescence properties of the

synthesized CdS nanocrystals. Functionalized mycelia also

adsorbed 141.5 mg g21 cadmium at pH value 6.0, while under

identical conditions the pristine mycelia adsorbed 70.5 mg g21

only. Kinetic results demonstrated very fast removal of cadmium

by functionalized AVM and was completed within 20 min. This

increased adsorption of cadmium by functionalized mycelia has

significant practical application in process development.

Acknowledgements

We thank Mr. R. N. Banik and Mr. S. Majhi of our Institute for

their cooperation during AFM and FESEM experiments,

respectively. Gratitudes are also due to Ms. Mousumi Basu

(Institute of Environmental Studies and Wetland Management,

Kolkata) for Atomic Absorption Spectroscopic analysis.

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