Cadmium biosorption potential of shell dust of the fresh water invasive snail Physa acuta

Cadmium biosorption potential of shell dust of the fresh water invasive snail Physa acuta

Journal of Environmental Chemical Engineering 1 (2013) 574–580 Contents lists available at SciVerse ScienceDirect Journal of Environmental Chemical ...

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Journal of Environmental Chemical Engineering 1 (2013) 574–580

Contents lists available at SciVerse ScienceDirect

Journal of Environmental Chemical Engineering journal homepage: www.elsevier.com/locate/jece

Cadmium biosorption potential of shell dust of the fresh water invasive snail Physa acuta Asif Hossain, Gautam Aditya * Ecology Laboratory, Department of Zoology, The University of Burdwan, Golapbag, Burdwan 713104, India

A R T I C L E I N F O

A B S T R A C T

Article history: Received 25 February 2013 Accepted 26 June 2013

The ability of shell dust (PSD) of an invasive freshwater snail (Physa acuta) to remove cadmium from contaminated water was evaluated. The results indicate that PSD, a waste biomaterial, bear potential of cadmium removal from contaminated water with biosorption capacity of 16.66 mg g1 at pH 6. The adsorption data at equilibrium fitted significantly more to Langmuir (R2 = 0.996) than Freundlich equations (R2 = 0.969). The kinetics of the adsorption process followed the pseudo-second order model (R2 = 0.996) better than the Lagergren model (R2 = 0.833). The FT-IR analyses support that the main 5O, mechanism of biosorption was cadmium chelating with different functional groups such as –OH, –C5 –C5 5C, and –C–C. The result obtained from the experiments show that the PSD can be used as an efficient, low cost, environmentally friendly biosorbent for cadmium from aqueous solution. ß 2013 Elsevier Ltd All rights reserved.

Keywords: Biosorption Cadmium Physa shell dust (PSD) Isotherm Kinetic

Introduction Aquatic pollution owing to heavy metals is a major concern for ecosystem functioning and biodiversity. The cascading effect of metal loads at various trophic levels causes decline in species richness of aquatic community, thereby interfere with ecosystem functions. Empirical evidences suggest that the metals like cadmium, chromium, copper, zinc, mercury, lead and alike, enters into the freshwater ecosystems through various modes as a byproduct of several industrial wastes. Considering cadmium as a heavy metal, the identified sources of its entry in freshwater ecosystems are metal refineries, battery industries, corrosion of galvanized pipes, paint industry, mining and natural deposits. Cadmium is a common heavy metal polluting the natural water bodies and ultimately causing severe damage to the human being and wild life. Entry of cadmium in the human body can damage liver and kidney, substitute calcium in bones, cause hypertension, initiate cancerous growth, while its accumulation in food chains may decline wildlife and species diversity [1,2]. Heavy metals as pollutants accumulate in the living system or stay in the environment. Removal of heavy metals from the environment is a sustainable alternative to reduce its ill-effects. Although heavy metal removal methods based on the principle of ion exchange, chemical precipitation and coagulation are well established,

* Corresponding author. Tel.: +91 342 2656566; fax: +91 342 253045. E-mail addresses: [email protected] (A. Hossain), [email protected], gautamaditya2001[email protected] (G. Aditya). 2213-3437/$ – see front matter ß 2013 Elsevier Ltd All rights reserved. http://dx.doi.org/10.1016/j.jece.2013.06.030

electrochemical basis and membrane technology alone or in combination, have evolved in the recent past [3,4]. The risks of generating secondary pollutants, associated with many of the metal removal methods are of great concern. As a consequence, use of biological materials for metal removal has been promoted to minimize the cost with increased efficacy [5]. Application of biological materials that are component of the natural system reduces the possibilities of yielding unwanted chemicals without inferring with the equilibrium state of the ecosystem functions. This is substantiated through the observations on the metal adsorption ability of different microorganisms [6], mushrooms [7] and hydrophytes [8] many of which are hyper accumulators of metals. Application of aquatic animals in metal adsorption [5] is also empirically tested as evident from the studies on living freshwater bivalve [9], bivalve shell [4,10] and the crab and acra shell biomass [11]. Uncontrolled application of living specimens like hydrophytes [8] and their derivatives for removal of heavy metals may facilitate biological invasion or secondary pollution thereby hindering the ecosystem functions. In many instances the use of the live specimens like mushroom [7] and the freshwater mussels Lamellidens marginalis [9] in metal removal may be constrained due to their food value and other economic values. While ability of metal removal may be high in biological species, their potential as aquaculture resources limits their application in metal bioremediation. Earlier studies demonstrate that unlike other approaches, calcium carbonate derivatives may be a potential cost-effective biosorbent for removal of heavy metals [3,10]. Shells of the freshwater snails can be considered as a cheap source of calcium

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carbonate, and thus the shells can be considered as biosorbent. To test this hypothesis the present study aimed at evaluation of metal biosorbent capacity of the shell dust of the invasive snail Physa acuta using cadmium as a model metal. The snail P. acuta is native to the North American continent and is considered as a globally invasive species with its spread in Asia including India [12]. A common inhabitant of pond, bogs and sewage drains, P. acuta is a prolific breeder and poses threat to the water distribution system, often clogging the drainage pipes rendering the biofilters ineffective [12,13]. Besides, P. acuta has the potential to alter the ecosystem functions interfering with the native snails, altering food chain interaction among the community members. In order to regulate huge production of P. acuta and stabilize native ecosystem functions, these snails are killed en masse, which offers a possible application of dead shell dust for the purpose of cadmium bioremediation. Thus the use of the snail shell dust (PSD) of the invasive snail, P. acuta can be promoted for removal of cadmium from aquatic system. In order to substantiate this proposition, the kinetics of adsorption and microscopic study of biosorption process were carried out and the results are highlighted in the following paragraphs. Materials and methods Preparation of the material The snails (P. acuta) were collected from local drains with hand nets. They were kept in aquaria until natural death. The tissue portions were discarded by boiling in water followed by wash in distilled water. The shells were sundried for 2 days and kept in oven at 50 8C for the next 2 days for complete dry. The shells were pulverized in mortar and pestles to fine granules. The fine granules were dried in an oven at 50 8C for 1 h and then sieved through 500 mm and consequently through 200 mm net. Granules of two different sizes were obtained – 500– 200 mm and 200 mm. Initial studies showed that the larger sized granules adsorbed less amount of cadmium and thus were not studied further. In order to determine the ash content, 1 g of fine granules (200 mm) was kept in a muffle furnace at 550 8C for 1 h. The ash content of the dust was 54.95% with water content capacity of 886.1 mg g1. The P. acuta shell is reported to contain 98.9  0.5% calcium carbonate by weight [14] interspersed on a fine matrix of organic matter made up of proteins and polysaccharides [15]. Preparation of the metal solution A stock solution of Cadmium of 1000 mg L1 was prepared in double distilled water and working solutions were prepared by appropriate dilution. The pH of the solution was adjusted by adding HNO3 (0.1 N) and NaOH (0.1 N). All the inorganic chemicals that have been used in these experiments were purchased from Merck India Ltd., India.

575

GBC Avanta 1.3, GBC Scientific Equipment (USA) LLC, USA). The influence of pH of the solution on biosorption equilibrium was studied after changing the pH of the solution in a range of 2–7. The effect of contact times between solution and the PSD were monitored by varying it from 10 to 80 min. For equilibrium studies the metal ion concentrations were used in seven concentrations between 25 and 1000 mg L1 while, for optimum biosorption study, the PSD biomass was varied between 200 and 1000 mg. The amount of Cadmium ion adsorbed on the PSD was estimated following the equation [16] qe ¼ ðco  ce Þv=m where qe = amount of metal adsorbed (mg g1), v = volume of solution (mL), m = mass of adsorbent in (g), co = initial concentration of the solution (mg L1) and ce = equilibrium concentration of the solution (mg L1). Analyses of FT-IR absorbance spectra of PSD IR spectra of protonated or Cd2+ loaded PSDs were recorded in a FT-IR spectroscopy (Perkin–Elmer FT-IR, Model RX1, Perkin–Elmer Inc., USA). Samples of 100 mg KBr discs contains 1% of the finally ground powder of each sample were prepared less than 24 h before recording. The scanning electron micrograph (SEM) study Raw and metal adsorbed PSDs were dried and prepared for scanning electron microscopic studies. The samples were attached with the stubs using cello tapes and gold plated in a sputter coater before use in the SEM. Electron acceleration potential of 20 kV was used for the microscopic observations. Photographs were taken in a Scanning Electron Microscope (HITACHI S530, Hitachi Ltd., Japan) at required magnification. Equilibrium modelling The adsorption equilibriums were studied for the estimation of maximum cadmium biosorption by the PSD. For the equilibrium study the experiments were performed at different initial cadmium ion concentration (25–1000 mg L1). Langmuir and Freundlich adsorption models were used in describing the equilibrium between adsorbed cadmium ions on the PSD (qe) and in solution (ce) at a particular temperature. The parameters of the Langmuir equation [17] were determined from a linear form of the following equation ce =qe ¼ ðce =aÞ þ 1=ab where a = maximum amount of metal ions/unit mass of adsorbent to form a monolayer (mg g1), b = equilibrium constant (L mg1). The linear plot of ce/qe against ce indicates the applicability of the Langmuir modelling in the experiment. Freundlich equation [18] is described as below

Experimental procedure

qe ¼ k f ce 1=n

The batch sorption experiments were performed in 250 mL Erlenmeyer’s flask that contained 100 mL solution of a particular cadmium ion concentration at required pH and relevant amount of P. acuta shell dust (PSD). The flasks were sealed with wax paper and shaken in a shaking incubator (Lab Companion, SI-300R, India) at 150 rpm with appropriate time and temperature. After shaking for a particular time period, the solution of the flasks was centrifuged at 2000 rpm. The supernatant was filtered using Whatman 42 filter paper (Sigma–Aldrich, UK) for estimation of metal concentration by atomic absorption spectroscopy (Model

where kf (mg g1) and 1/n are the Freundlich constants indicating the adsorption capacity and intensity respectively. The linear plot of log qe against log ce indicates the applicability of the Langmuir modelling in the experiment. Kinetic model The pseudo-first order model in adsorption was proposed by Lagergren. It describes the rate of adsorption is proportional to the number of unoccupied binding sites of the biosorbent. This model

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works well in the region where the biosorption process occurs quickly [19]. Lagergren equation [20] or pseudo-first order reaction is mathematically expressed as

(g mg1 min1). A linear plot of t/q vs. t indicates whether this model of biosorption is applicable for this case or not. Results

dq=dt ¼ kl ðqe  qt Þ where qe = amount of adsorbed metal ion on biosorbent at equilibrium (mg g1), qt = amount of adsorbed metal ion on biosorbent at time ‘t’ (mg g1), kl = Lagergren constant (min1). Integrating the above equation and transforming to log scale logðqe  qt Þ ¼ log qe  kl ðt=2:303Þ linear plot of log(qe  qt) against time indicates whether this kinetic model is applicable or not for biosorption process. The pseudo-first order model for prediction of biosorption is not suitable for a long period of adsorption process [21] and the pseudo-second order equation in this case as described by Ho is dqt =dt ¼ k2 ðqe  qt Þ2 or qt ¼ qe ½ðqe k2 tÞ=ð1 þ qe k2 tÞ

The metal adsorption increased as a positive function of metal ion concentration of the solution. A range of 25–1000 mg L1 ion

b Cd 2+ Uptake by PSD (mg g-1)

10

Cd 2+ uptake by PSD (mg g-1)

The biosorption procedure was maintained over a range of pH 2–7. The sorption procedure was affected by the pH of the medium in two ways – metal solubility and total charge of the functional groups of the biosorbent. The optimum pH, at which the procedure shows highest adsorption for the biosorbent was estimated. At high pH precipitation of the metal was observed. At low pH, possibly due to high protonation, metal sorption capacity decreased. The experiment was carried out using 100 mL solution of 100 mg L1 Cd2+ and 100 mg of the PSD at 30 8C in reference to varying pH of the solution. The pH dependent adsorption of the metal ion by PSD (Fig. 1a) indicated that the metal sorption was negligible at pH 2 which increased with increase in pH. At pH 6, highest adsorption was observed that declined with further increase in pH. Effect of initial metal ion concentration

where qe = amount of adsorbed metal ion on biosorbent at equilibrium (mg g1), qt = amount of adsorbed metal ion (mg g1) 1) on biosorbent at time ‘t’, k2 = second order rate constant

a

Effect of pH of the solution

8 6 4 2

20 18 16 14 12 10

0

8 6 4 2 0

1

2

3

4

5

6

7

0

pH

d

9 8.8

Cd 2+ uptake by PSD (mg g-1)

Cd 2+ uptake by PSD (mg g-1)

c

8.6 8.4 8.2 8 7.8 7.6 7.4 7.2 7

200

400

600

800

Cd 2+ conc.

in solution (mg

0

400

1000

L-1)

10 9 8 7 6 5

0

20

40

60

Adsorption time (min)

80

200

600

800 1000

Amount of PSD (mg/100ml)

Fig. 1. (a) Effect of pH of the solution on biosorption of cadmium on PSD at 30 8C, initial cadmium ion concentration of 100 mg L1, contact time of 60 min and biosorbent dose of 1 g/100 mL solution. (b) Effect of initial cadmium ion concentration of solution on biosorption on PSD at 30 8C, pH 6, contact time of 60 min and biosorbent dose of 1 g/ 100 mL solution. (c) Effect of time on biosorption of cadmium on PSD at 30 8C, pH 6, initial cadmium ion concentration of 100 mg L1, and biosorbent dose of 1 g/100 mL solution. (d) Effect of biosorbent dose on biosorption of cadmium on PSD at 30 8C, pH 6, initial cadmium ion concentration of 100 mg L1, contact time of 60 min.

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a.

577

15.9 14 2346.05

12

466.36

2364.50

10 2522.89 2923.89

8 %T

712.54 699.73

1786.87

1083.11

6 4 2

1031.26

860.55

1638.70 3447.86 1475.31

0 -2.0 4400.0 4000

3000

2000

cm-1

1500

1000

400.0

b. 15.9 15 14

468.07

13 12

2522.95

11 2363.53

10

2345.81

2927.46

9

712.41 699.56

1786.66

%T 8 1082.84

7 6 5 4

860.04 3450.03

1634.73

3 2

1474.63

1.0 4400.0 4000

3000

2000

cm-1

1500

1000

400.0

Fig. 2. FT-IR absorbance spectra of snail shell dust (PSD) before (a) and after (b) biosorption of cadmium.

concentrations were used for the study taking 7 different doses in series. The 25 mg L1 initial metal ion concentration showed the lowest adsorption while the 800 mg L1 concentration showed the highest adsorption and the adsorption remained same with further increase in Cd2+ ion in the solution (Fig. 1b). Influence of the biosorbent dose The cadmium biosorption potential of the PSD augmented over its increase in amount in the treatment solution. The more the amount of biosorbent was present, the more the free binding sites or exchanging groups were available to adsorb the metal ion from the solution. For the 100 mg L1 metal ion concentration the increase in biosorbent amount, resulted in increased metal ion adsorption and above a certain dose it remained same or slightly higher due to comparatively higher number of free sites and lesser number of metal ions (Fig. 1c).

Effect of contact time The sorption potentials of the PSD over time were monitored from 10 min to 20, 40, 60, 80 min by using 100 mL of 100 mg L1 Cd2+ at pH 6 (Fig. 1d). At the beginning, metal adsorption was less due to more binding sites remained free when treated for the short period of time and increased rapidly as the treatment time increases. It showed lowest adsorption when treated for 10 min and increased over time to saturate at 60 min and after that the uptake remained almost same. The variation in uptake of the cadmium ions with time were used in fitting the kinetic models. FT-IR study In order to study the mechanism of cadmium removal and the main functional groups responsible for Cd2+ binding the FT-IR spectrum of the PSD were performed. The IR spectra of protonated and cadmium loaded PSD are shown in Fig. 2a and b, respectively. A

A. Hossain, G. Aditya / Journal of Environmental Chemical Engineering 1 (2013) 574–580

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Fig. 3. Scanning electron micrograph (SEM) of snail shell dust (PSD) before (a) and after (b) the biosorption of cadmium (magnification at 4000).

peak at 3447 cm1 is indicating the presence of hydroxyl (–OH) groups. Strong band at 2923 cm1 is due to C–H stretching frequency and peak at 1638 cm1 is due to –C5 5O stretching mode of the primary and secondary amides [22]. Weak band at 1475 cm1 is attributed to aromatic C5 5C and the strong band at 1031 cm1 is due to C–O stretching of alcoholic groups [23]. The FT-IR of metal loaded PSD shows that distinct shift of some band as well as change in intensity informs some ion exchange behaviour of the PSD. The scanning electron micrograph (SEM) study The surface structure of the free and cadmium loaded PSD were analyzed under scanning electron microscope. The scanning electron micrographs of the dried PSD before and after the Cd2+ treatment at 4000 magnification are shown in Fig. 3. It indicates the irregular morphological structure of the particles and lamellar stratified surface of the PSD. The SEM image of the biosorbent after exposure to the Cd2+ shows a spongy layer indicating surface precipitation occurred during the sorption [3]. In case of cadmium sorption by CaCO3 compound generally follows surface precipitation due to similar ionic radii [24] of divalent calcium and cadmium. Adsorption isotherm The biosorption isotherm is important in waste water treatment as it implies estimation of biosorption capacity of the adsorbent. The linear representations of Langmuir and Freundlich isotherm of cadmium adsorption at 30 8C are given in Fig. 4. The correlation coefficient and constants obtained from the equations are presented in Table 1. The correlation of determination is high in Langmuir equation (R2 = 0.99) contrast to Freundlich equation Table 1 Coefficients of the Langmuir and Freundlich isotherm models for cadmium biosorption by PSD. Metal

Cadmium

Langmuir constant

(R2 = 0.96). It indicates Langmuir model is more suitable for describing the biosorption equilibrium of Cd2+ on the snail shell dust (PSD). The high qmax value (16.66 mg g1) obtained following the model (Langmuir equation) is more close to experimental data obtained from atomic absorption spectrophotometer (8.86 mg g1). The value of b (0.066) from Table 1 indicates that the adsorption isotherm follows Langmuir isotherm model. The equilibrium models of cadmium biosorption support that under optimum conditions (pH 6, biosorbent dose of 1 g, Cd2+ concentration of 100 mg L1 and 60 min time period) 16.66 mg g1 is the maximum biosorption capacity of PSD. The maximum cadmium biosorption capacities of low cost biosorbents comparable to PSD are shown in Table 3. Biosorption kinetics The kinetic model is necessary for determination of optimal condition of the biosorption process. For the evaluation of differences in sorption process the kinetics of metal uptake were described by pseudo first order and pseudo second order model [21]. The linear plots obtained from pseudo first order and pseudo second order model at 100 mg L1 initial Cd2+ concentration, pH 6 and at studied temperature are shown in Fig. 5. The rate constants, expected metal uptake and correlation coefficients have been described in Table 2. For the pseudo first order model the correlation coefficient (R2 = 0.833) is high and the calculated metal uptake with this model is very lower (1.25 mg g1) than the expected metal uptake (8.86 mg g1) (Fig. 5a and Table 2), indicating that the biosorption process is not a first order reaction. The pseudo second order reaction in biosorption is based on the sorption capacity on the solid phase. The correlation coefficient in the pseudo second order reaction (R2 = 0.999) is high and the calculated metal adsorption (8.84 mg g1) is much nearer to the expected value (8.86 mg g1) (Fig. 5b and Table 2), suggesting that the biosorption process comply pseudo second order model. Discussion

Freundlich constant

qmax (mg g1)

b (L mg1)

R2

n

kF

R2

16.66

0.066

0.996

3.41

0.448

0.969

It is apparent from the results that the shell dust (PSD) of the invasive aquatic snail P. acuta adsorb considerable amount of cadmium from the medium, which vary the concentration of the

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a 60

579

a 0.2

y = 0.060x + 0.914 R² = 0.996

50

0.1

y = -0.010x + 0.097 R² = 0.833

0 log (qe-qt)

-0.1

ce/qe

40 30

-0.2 -0.3 -0.4 -0.5 -0.6

20

-0.7 -0.8

10

0

20

40

60

80

100

80

100

Time (min)

0

b 10

0

500

1000

9

ce t/q

b 0.6

y = 0.293x - 0.348 R² = 0.969

0.5

7 6 5 4 3

0.4

2

0.3

log qe

y = 0.113x + 0.179 R² = 0.999

8

1

0.2

0 0

0.1

20

40

60 Time (min)

0 Fig. 5. (a) Pseudo-first order plot for biosorption of cadmium on PSD (pH 6, temperature 30 8C, biosorbent = 1 g, cadmium ion concentration = 100 mg g1). (b) Pseudo-second order plot for biosorption of cadmium on PSD (pH 6, temperature 30 8C, biosorbent = 1 g, cadmium ion concentration = 100 mg g1).

-0.1 -0.2 -0.3 0

1

2 log ce

3

4

Fig. 4. (a) Langmuir isotherm plot for biosorption of cadmium on PSD (pH 6, temperature 30 8C, biosorbent = 1 g, cadmium ion concentration = 100 mg g1). (b) Freundlich isotherm plot for biosorption of cadmium on PSD (pH 6, temperature 30 8C, biosorbent = 1 g, cadmium ion concentration = 100 mg g1).

metal, pH of the medium and time of exposure. Equilibrium models and biosorption kinetics of cadmium ions support that under optimum conditions (pH 6, biosorbent dose of 1 g, Cd2+ concentration of 100 mg L1 and 60 min time period) maximum biosorption capacity by the PSD is 16.66 mg g1. Maximum cadmium biosorption capacities of similar low cost biosorbents [25–36] are shown in Table 3. While the materials of plant origin like rice straw [29], wheat bran [30] and alike (Table 3) show the ability to adsorb materials, potential risk of leaching of lignin,

tannin and other metabolites exist that may interfere with the aquatic food chain and disrupt ecosystem functions. In contrast, PSD is mainly composed of calcium carbonate, degradation of which, if any, will not yield unwanted compound to the ecosystem. Presence of calcium will instead facilitate adsorption of cadmium because of the similarities in ionic radii that enhance ion exchange [3]. In view of metal removal from lentic and lotic ecosystems, use of materials of biological origin is preferred to reduce the chances of addition of contaminants and recalcitrant compounds that may alter ecosystem functions. Although materials like Brewer’s yeast [27] and mushroom [7,35] have shown ability to adsorb cadmium to a greater extent, the cost associated is significantly high compared to waste shells of animal origin. Besides, mushrooms and yeast have high food value. In a similar way, use of microorganisms in heavy metal removal from aquatic system does not yield benefits due to limitations in desorption and continuous flow sorption [5]. Empirical evidences suggest that

Table 2 Comparison of rate constants and equilibrium metal uptake for cadmium binding by PSD at pH 6 and initial metal ion concentration of 100 mg L1. Metal

Cadmium

qe

exp

8.86

(mg g1)

Pseudo-first order model k1 (min1)

qe

0.023

1.25

cal.

Pseudo-second order model (mg g1)

R2

k2 (g mg1 min1)

qe

0.833

0.071

8.849

cal.

(mg g1)

R2 0.999

580

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Table 3 Comparative data of biosorption capacities (qmax – maximum metal uptake capacity) for cadmium by different biosorbents. Adsorbent

qmax (mg g1)

Reference

Raw corn stalk Olive waste Brewer’s yeast Corncob Rice straw Wheat bran Castor seed hull Walnut tree sawdust Bamboo charcoal Chitosan/bentonite Mushrooms Coconut copra meal Snail shell dust (PSD)

3.39 6.55 10.17 4.73 13.9 15.71 6.98 5.76 12.08 12.05 34.96 4.99 16.66

[25] [26] [27] [28] [29] [30] [31] [32] [33] [34] [35] [36] Present study

animal wastes like crab and acra shells and mussel shells are comparatively more efficient for metal removal in terms of availability in nature and cost benefit ratio [3,4,10,11]. Consistent with the observation on egg shells and crab shells, the present study demonstrates that PSD can be a suitable material for adsorption of cadmium from aquatic ecosystem. Considering the ill effects of P. acuta as invasive snails on the native ecosystems, mass killing is obvious. As a consequence easy availability of the shell dust with little processing can make the shell dust as efficient biosorbent. Use of the PSD can be promoted for removal of cadmium from aquatic system while restoring ecosystem functions in situations where P. acuta is a common invasive snail. Thus dual benefit of cadmium removal and restriction of P. acuta in local ecosystems can be achieved Although metal desorption and extraction from PSD needs to be evaluated through further studies, the present study is a pioneer effort to establish that shell dust of a common invasive snail P. acuta can be used in cadmium removal from aquatic ecosystem. Conclusions The present work indicates that at pH 6, the maximum biosorption capacity of PSD is 16.66 mg g1. The main functional group responsible for chelation are OH, C5 5O, C5 5C and C–C, as supported by FT-IR analysis. The isotherm model follows Langmuir model (R2 = 0.996) better than Freundlich model (R2 = 0.969). The biosorption process followed the pseudo second order (R2 = 0.999) kinetics better than pseudo first order (R2 = 0.833) and. PSD may prove as efficient, low cost and environment friendly biosorbent for cadmium bioremediation. Acknowledgements The authors acknowledge the comments of three anonymous reviewers in upgrading the manuscript to its present version. The authors are thankful to the respective Heads, Department of Zoology, Department of Environmental Science and Department of Chemistry, The University of Burdwan, Burdwan, India for the facilities provided including DST-FIST. AH thankfully acknowledges the financial assistance provided by Council of Scientific and Industrial Research (CSIR), New Delhi, India [Sanction No. 20-12/ 2009(ii)EU-IV dated 31-5-2010]. References [1] N. Johri, G. Jacquillet, R. Unwin, Heavy metal poisoning: the effects of cadmium on the kidney, Biometals 23 (2010) 783–792. [2] A. Bernard, Cadmium and its adverse effects on human health, Indian J. Med. Res. 128 (2008) 557–564.

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