Adsorption of cadmium Ions from aqueous solution using different adsorbents

Indian Journal of Scientific &Industrial Research Vol. 63,May 2004, pp. 4LO-416

me

Adsorption of cadmium(II) Ions from aqueous solution using different adsorbers

utili mu con ads Ajay Kumar Meena, G K Mishra, Satish Kumar, Chitra Rajagopal* and P N Nagar**

Centre for Fire, Explosive &Environment Safety, Brig S K Mazumdar Road, Delhi 110054 Received 30September 2003; accepted 15March 2004

A comparative study on the adsorption of cadmium from aqueous solutions on a few low cost and locally available untreated and chemically treated adsorbents iscarried out. Mustard husk, carbon aerogel and treated GAC are found tobe most effective adsorbents in addition to treated GAC for the removal of cadmium from the aqueous solution solution at varying process parameters such as, pH (2-12) adsorbent dose (0.5-1.2 g/lOO ml.), contact time (24-72 h) and initial cadmium concentration (1-5 rng/L). Treated GAC, carbon aerogel, and mustard husk show 100, 87 and 72 per cent adsorptive removal of cadmium, respectively, under optimized conditions of pH 4 and dosage 1g/lOO mL for 2 mg/L cadmium aqueous solutions in 48 h. The adsorption parameters are determined using both Langmuir and Freundlich isotherm models. Surface complexation and ion exchange are the major removal mechanisms involved. The adsorption isotherm studies indicate that the adsorption process isa monolayer coverage of cadmium onsurface of treated GAC and fits into the Langmuir model. The adsorptive behaviour of cadmium on untreated mustard husk and carbon aerogel satisfiesnot only the Langmuir assumptions but also the Freundlich assumptions, i.e., multilayer formation on the surface of the adsorbent with an exponential distribution of site energy. The results of the experimental studies as well as the model parameters arepresented.

Keywords: Adsor?tion, Cadmium(II) ions, Aqueous solution IPC Code: Int.C!. C 23 C22/05

Introduction

Most of the heavy metals above trace quantities are harmful to humans, animals and plants. Federal and local agencies have therefore stipulated discharge limits on the levels of these heavy metals in the effluents being discharged into the environment.

Cadmium, used in this study, is one of these metals. It is an irritant to the respiratory tract, and prolonged exposure to this pollutant can cause anemia and a yellow stain that gradually appears on the necks of the teeth. Acute toxicity is almost always caused by inhalation of cadmium fumes or dust, which are produced by heating this metal^I. The industrial uses of cadmium are wide spread and are progressively increasing in electroplating, paint pigents, plastics, silver cadmium batteries", smelting.', cadmium nickel batteries, stabilizer, phosphate fertilizer, mining and alloy industries". One of the major sources of cadmium discharge into natural waters is from the electroplating industries, which accounts for about 50 per cent of the annual cadmium consumption.

*Author for correspondence

**Department of Chemistry, University of Rajasthan, Jaipur 302 004

(E-mail: chit_rajagopal@hotmail.com.

ajaysheera@ yahoo.corn.)

(I)

S by

con

(II) (a)

Removal of cadmium from effluents beforethey are discharged into the environment can be accomplished by processes such as, chemiul precipitation, cementation, solvent extraction, reverse osmosis, and ion exchanges. These processes are, sometimes, neither effective nor selective andsomeof them are very expensive", Adsorption is a processfor the removal of heavy metals, which is quite selective and effective, and is able to remove very lowlevelsof heavy metals from the aqueous solutions/wastewater.

Ion exchange resin and activated carbon^7,8 are well known materials that are used for this purpose.

Recently, some agricultural and forestry products/wastes have been recognized as new adsorbents. In general the cost of these biomaterialsis negligible, compared to the cost of ion exchangeand chemically prepared adsorbents. Activated carbon produced from almond shell", sawdust based GAelo•

tree bark treated with formaldehyde and sulphuric acid^II, bone char, tea leaves, wood charcoal ^12,coconut shell carbon^13, suI phurized acti vated carbor"

ozonized activated carbon ^IS, rice hulls and ricebran"

pine bark", treated sawdust" and agricultural wase"

have been used with and without treatment forthe removal of heavy metals. Adsorption of heavymet~s by these materials might be attributed totheir protein.

carbohydrates and phenolic cornpounds'" whichhave

effi apl1 Th

bents

MEENA et al.: ADSORPTION OF CADMIUM(II) IONS 411

metalbinding functional groups such as, carbonyl, hydroxyl,sulphate, phosphate, and amino groups".

Moreoverthe removal of metal ions from their solutionsinthe presence of agricultural materials may bedue to the adsorption on surface and pores, and

~soto complexation by these materials.

The work aims to study the possibility of the utilization of carbon aerogel, treated GAC, and mustardhusk for the adsorption of cadmium from contaminated aqueous solution/wastewater. The adsorption process parameters such as, initial concentration, pH, contact time and adsorbent dose forthe maximum removal of cadmium from its aqueoussolutions/wastewater were optimized.

rle be at ial .nt IL 8h

Materialsand Methods

)n

its ot le el

(I)Test Solution

Synthetic stock solution of cadmium was prepared by dissolving required quantity of cadmium metal in 5 mL concentrated HCI and 20 mL distilled demineralised water. The stock solution was further dilutedwith distilled demineralised water to desired concentrationfor obtaining the test solutions.

e they

III be

emical everse

.s are,

irne of ess for ective /els of water.

~well rpose.

uestry new ials is e and arbon AC^IO , ihuric conut oon", ran ", aste'"

Ir the ietals )tein, have

(II)Preparation ofAdsorbent

(a)Preparation of Treated GAC

Untreated GAC has poor adsorptive removal efficiencyfor cadmium ion because it lacks the appropriatechemical functional group on its surface.

Therefore,chemical treatment is required to introduce suitablefunctional group (- SNa, -SH) on its surface forimproving the adsorption capacity".

99.5 g of GAC (coconut based) is immersed for 24 h inminimum 750 mL of distilled water containing 0.5 g of Na2S. The mixture was heated almost to drynessand then dried in an oven for 4 h at 110°C.

Thedried sample was washed with distilled water severaltimes till it gave negative test for sulphide.

Thewashed sample was again dried at 110°C for 4 h andstored in a dessicator for use.

(b)Carbon Aerogel

Carbon aerogel (supplied by Marketech International,USA) was used as an adsorbent material without any pre-treatment. Carbon aerogels are composed of covalently bonded nanometer sized particlesthat are arranged in 3-D network and possess highporosity and high surface area. Carbon aerogels maybe produced in solid shapes, powder, and sheet forms. These are new and emerging adsorbent

materials. Carbon aerogels having high porosity and high surface area, provided excellent treatment efficiency in a cost-effective manner for the purification of effluents/waste waters.

(c) Mustard Husk

Mustard husk was collected during the harvest season from the nearby villages of Jaipur district, Rajasthan, India. The mustard husk was used as an adsorbent material without any pre-treatment.

Screening studies were conducted to enable comparative evaluation of the various adsorbents for the removal of cadmium from aqueous/wastewater solutions. The adsorbents studied were: untreated and treated granular activated carbon (GAC), untreated and treated sawdust, weathered coal, tree bark, carbon aerogel, and mustard husk. Experiments for screening studies were carried out using stoppered conical flasks containing 100mL of 5mg/L cadmium test solution and Ig adsorbent materials. The flasks containing the test solution and adsorbent were placed in a thermostatic mechanical shaker for 24h contact time at35°C.

On the basis of the results of screening studies the most promising adsorbents selected were treated GAC, carbon aerogel, and mustard husk, which were used for further experiments to study the effect of other parameters.

The experiments were carried out in a phased manner as given below:

Phase I-Effect of initial concentration Phase 2- Effect of adsorbent dosage Phase 3- Effect of contact time Phase 4- Effect of pH

Experimental conditions for phase 1,2,3, and 4 are as follows:

Volume of sample taken for each experiment

pH range

Initial Cd(II) concentration range (mg/L)

(Based on analysis of actual plant effluent samples)

Contact time (h)

Adsorbent dose (mg/100mL)

=

100 mL

=

2-12

=

1 to 5

=

24,48, 72

=

0.5, 0.8, 1, and 1.2 The results of these studies were used to obtain the optimum conditions for maximum cadmium removal from aqueous solution.

Tree Sawdust

412 J SCI IND RES VOL 63MAY 2004

Table 1- Results ofscreening studies for selection of adsorbents

Initial Cd Cone. :5mg/L, Contact time: 24 h,Adsorbent dose: Ig,Sample volume: 100 mL

Adsorbents : GAC Sawdust

bark Weathered

Coal

per centage removal : 30 50 52

IAdsorbent dose - 19/100 mL; Contact time - 48 h

BatchExperiments

Batch adsorption studies were carried out using stoppered conical flasks containing required amount of test solution and adsorbent material. The flasks containing the test solution and adsorbent were placed in a thermostatic mechanical shaker for the required time period at 3S°C. The contents were centrifuged and filtered through Whatman filter paper No. 41. The unadsorbed cadmium in the filtrate was estimated by atomic absorption spectrophotometer (AAS) using flame method.

The per cent' cadmium removal was calculated using Eq. (1).

... (1)

where Co = Initial Cd concentration of test solution, mg/L, C;

=

Final Cd concentration of test solution, mg/L.

Results and Discussion

The results of the experiments carried out for the removal of cadmium from the synthetic samples using low cost adsorbents are discussed subsequently.

Effect ofthe Nature ofAdsorbent

Eight different adsorbents namely, untreated and treated granular activated carbon, untreated and treated sawdust, mustard husk, weathered coal, tree bark and carbon aerogel were used in the experiments.

Table 1gives the per centage removal of cadmium by various adsorbents. From Table 1,it can be seen that the per centage removal is appreciably better with treated GAC, carbon aerogel and mustard husk.

In view of the poor cadmium removal efficiencies with sawdust, untreated granular activated carbon, treated sawdust, weathered coal and tree bark, it was decided to continue the remaining sets of experiments with carbon aerogel, treated GAC, and mustard husk as adsorbents.

Effectof Initial Cadmium Concentration

Figure 1 compares the per cent removal of cadmium with increasing initial cadmium concentration in aqueous solutions, using treated GAC, carbon aerogel and mustard husk for 1 g

Treated Husk

Mustard Aerogel

Carbon

cap earl

sho cad su ads pre fun are

Treated GAC

59.1 86.4

57 59.8 60

§:¹²⁰l

~ 100~1=---~----~--~

i80~ ;::

~ 60

1

i

⁴⁰

1

,.. ~ 2°l

_{~ 0 ---,- ~ -rr-r- --,--}

1 2 3 4 5

Initial Cd(lI) Cone,mg/L

Etll 1

Val

It

cei

ae do s11 in T

__ T,utedGAC __ c.rtIonMrOglll

...•....Mustanl husk

6

Fig. 1- Effect of initial concentration on percentage removalof cadmium for different adsorbents

re adsorbent dose and 48 h contact time. Treated GAC st

gives the best results, i.e., nearly 100 per cent removal of cadmium upto 4 mg/L level after which it decreases slightly to about 98 per cent for higher concentrations. Carbon aerogel is highly effectiveup to 1 mg/L after which the per centage removal decreases gradually to about 70 per cent at 5 mglL.

Mustard husk shows the lowest per centage removal at all initial concentrations. The above resultsmaybe explained on the basis of following facts:

At lower initial concentrations, sufficient active sites/surface area are available for the adsorptionof the cadmium molecules, therefore the fractional adsorption is independent of initial concentration.

Whereas, at higher concentrations the numberof cadmium molecules is relatively higher comparedto the availability of adsorption sites/surface area, hence the per centage removal of cadmium depends on the initial concentration and decreases with increasein initial concentration.

Differences in the extent of adsorption on treated GAC, carbon aerogel, and mustard husk are associated with the two main factors: (i) The availability of total surface area,which is in the following order : treated GAC (lOOO-1200m2/g)>

carbon aerogel (42Sm2/g) > mustard husk (O.27m2/g), and(ii)The presence of chemical functional groups such as -SNa/-SH, on the surface of treated GAC.

which has more chemical affinity for cadmium.The effectiveness of carbon aerogel in the removal of Cd(II) may be due to porosity as well as amoderate

c

aCi

ac c.

n c

ai

'bon

o

alof

"TAC oval

I it

~her

I up

ival

~/L.

IvaI be

ave of nal Dn.

of to ce he

e

MEENA etal.: ADSORPTION OF CADMIUM(II) IONS 413

capacityfor ion- exchange because of the presence of carboxylicand lactonic groups. The mustard husk had shownleast adsorptive potential for the removal of cadmium,which may be attributed to the lowest surface area because of minimal porosity. The adsorption in the case of mustard husk is due to the presenceof sulphur and oxygen, containing chemical functionalgroups on its surface rather than the surface areaand porosity.

Effect of Adsorbent Dose

Figure 2 shows the comparative behaviour of variousadsorbents with increasing adsorbent dosage.

It is observed that there is a sharp increase in per centage removal with adsorbent dose for carbon aerogel increasing from about 67per cent for 0.5 g doseto 97.7 per cent for 1.2 g. Mustard husk does not show much variation for low adsorbent dosage but increases gradually to 82 per cent for 1.2 g dose.

Treated GAC shows uniformly high per centage removal (86- lOOper cent) during adsorbent dosage studies.

It is apparent that the. per centage removal of cadmium increases with increase in the dose of adsorbent due to the increased availability of the active sites/surface area for the adsorption of the cadmium.Whereas, at lower adsorbent dosage the number of cadmium molecules is relatively higher, compared to availability of adsorption sites/surface area.

Effect ofContact Time

Figure3shows comparative per centage removal of Cd(II)by treated GAC, carbon aerogel,and mustard huskfor 1.0g dose/lOOmL Cd(II) solution of 3 mg/L initialconcentration at different contact times. It is observed that in cases of carbon aerogel, and mustard husk the per centage removal of cadmium is comparatively lower for 24 h contact time and increases gradually with increasing contact times. In thecase of treated GAC, the per centage removal efficiencies over the entire range of contact time are almost comparatively constant and are higher than thatfor the other adsorbents. Carbon aerogel shows lower removal per centage than treated GAC but the rise in per centage removal with increasing contact timeis steep, showing more than 80per cent removal beyond 48 h. On the other hand, per centage removal with mustard husk increases gradually with contact time,reaching 96 per cent removal at72 h.

As seen earlier the contact time required to attain equilibrium is dependent on the initial concentration

Contact time =48h; Initial Cd(lI) concentration =3mg/L

=-120

:co

.•.

100 0

c; 80

>0 E 60

e

Q)

40 JSCI

c

Q) 20

l:!

Q) Q, 0

0.4

~ Treated GAC

1

..•..Carbon aerogell

~ Mustard husk

1.2 1.4

0.6 0.8

Adsorbent dose, g/100 mL

Fig. 2- Effect of adsorbent dosage on percentage removal of cadmium for different adsorbents

Adsorbent dose - 1 g/100 mL;Initial Cd(lI) Concentration =^{3 mg/L}

120 100 80

40 20

O+---·~---~---.----~

o ^{24 48 72 96}

Time, h

Fig. 3- Effect of contact time on percentage removal of .cadmium fordifferent adsorbents

of cadmium. For the same concentration,' the per centage removal of cadmium increases with increase in contact time till equilibrium is attained. The optimal contact times to attain equilibrium with carbon aerogel, and mustard husk adsorbent were experimentally found to be about 48 h. In the case of treated GAC the optimal contact time was 24h (Figure 3).

Effect of pH

pH of the adsorptive solution is one of the most important parameters controlling the uptake of cadmium from aqueous solutions/wastewater by adsorbent. Figure 4 shows the effect of pH on cadmium removal efficiencies of all the three adsorbents.

The per centage adsorption increases with pH to attain a maximum at pH 4 and thereafter, it decreases with further increase in pH. The maximum removalof cadmium at pH 4 was found to be nearly 100, 80.6 and 70 per cent for treated GAC, carbon aerogel and mustard husk, respectively.

414 J SCI IND RES VOL 63MAY 2004

Initial Cd(U) concentration =^{3mil; Adsorbent dose}=^{1g/100 ml}

Contact time =48 h

~ 120

1

o 100 '0

~ 80 I

! ^60-1

E

⁴⁰

1

; 20~

~ I

~ 0.LI_~_~_~_~ -,---,--,

o

~ Treated GAC

I

-e-,Carbon aerogel

-+-Mustard husk

2 4 6 8 10 12 14

pH

Fig. 4 - Effect of pH onpercentage removal of cadmium for different adsorbents

There results clearly indicate that the solution pH has a considerable influence on the adsorption of

cadmium ions on the surface of the adsorbent,

because both the surface charge density of the adsorbent and charge of the cadmium ions present, depend on the pH.

Moreover, differences in the extent of adsorption are also associated with the chemical state of the metals in the adsorptive solutions. As observed subsequently, it determines the adsorptive species relevant to the adsorption process. The Ccf+ ions in aqueous solution may under go hydration, hydrolysis and polymerization'"; as shown below:

Cd2++nH20

=

Cd(H20)n2+

Cd(H20)n²+ =Cd2+(H20r-' (OHt +H+

nCd²+ +mH20

=

Cdn(OH)m(2n-m)++mH+

Cd2+ can form several hydrolysis products, which exist under widely varying conditions24,25. In dilute solutions, however the formation of Cd2+hydrolysis products occur at pH >6.This shows-that cadmium in the adsorptive solution is likely to be found as free ions in equilibrium with hydrated species. The hydration numbers for Cd2+have been determined by various physical techniques and the proposed value ranges between 12and 4.6 (ref. 23)

Lower pH results in the protonation of the adsorbent surface, which leads to the extensive repulsion of Cd2+ions. This results in a decrease in cadmium adsorption. With increase in pH from 2 to 4.0 the Cd exists as Cd(OH)2 in the medium and surface protonation of adsorbent isminimum, leading tothe enhancement of Cd2+ions adsorption.

Adsorption Isotherms

The adsorption studies were conducted at fixed initial concentration of cadmium by varying adsorbent

dosage. The equilibrium data obtained was analyzed in the light ofLangmuir and Freundlich isotherms,

The Freundlich equation isgiven bi^6:

x/m

=

K C^el/n,

Taking the logarithmic form of the equation logxlm

=

10gK+lIn 10gC^e,

Langmuir equation is given bi?

x/m

=

^{abCe/(l +bCe ),}

or,

x/m

=

(l/ab).(l/Ce + l/a),

where, xlm is the amount of cadmium adsorbed per unit mass of adsorbent in mg/g, C^eis the equilibrium concentration of cadmium in mg/L, Ke and n are Freundlich constants, ^'a' is a Langmuir constant which is a measure of adsorption capacity expressed in mg/g. 'b' is also Langmuir constant which is a measure of energy of adsorption expressed in Llmg.

The parameters 'a' and 'b' have been calculated from the slope and theintercept of the plots.

Figure 5 gives the Freundlich adsorption isotherm plot of logxlm vs 10gCe. The values of K, and 1111 obtained from intercept and slope of the plot are given in Table 2.

Figure ⁶ gives the Langmuir adsorption isotherm plot of mix vs l/C, . The essential characteristics of Langmuir isotherm can be described by a separation factor or equilibrium constant R^L, which isdefined as,

where, C; is the initial concentration of cadmium ( mg/L ) and b is Langmuir constant which indicates the nature of adsorption. The separation factor RL indicates the isotherm shape. and whether the adsorption is favourable or not, as per the criteria given subsequently.

RLValues RL>1 R^L

=

1 0< ^RL< 1 RL=O

Adsorption Unfavourable Linear Favourable Irreversible

The values of Langmuir constants ^'a', 'b' andRL are presented in Table 2. Since RLvalues lie between

... (2)

(3)

...(4) _Fi

Ta fo

... (5) Al

s

... (6)

MEENA et al.: ADSORPTION OF CADMIUM(II) IONS 415

Amount ofadsorbent =1.2g; contact time=48h 1.2

0.8

•Carbonaeroge~

•Mustard hUSkJ

oTreatedGAC ----_.

0.6 0.4

0.2

o

o ²

log(Ce)

3 4

Fig.5 - Freundlich isotherms of cadmium for various adsorbents

Table2 - Values of Langmuir and Freundlich isotherm constants foradsorption of cadmium

Amountof adsorbent = 1.2 g/100 mL; Contact time =48 h

Langmuir constants

Adsorbents a b R2 RL

mg/g Llmg

Treated GAC 104.78 1.10 0.96 0.16

Mustard husk 43.85 0.28 0.93 0.26

Carbon aerogel 400.80 0.35 0.99 0.25

Freundlich constants

Adsorbents Ke n R2

Treated GAC 0.73 2.68 0.69

Mustard husk ""1.62 3.63 0.97

Carbon aerogel 1040 5.32 0.86

o

and 1 for all three adsorbents studied, it is seen that theadsorption of cadmium is favourable.".

Adsorption capacity as indicated by value of ^'a' is seen to be maximum for carbon aerogel, i.e. 400.8 mg/g,followed by treated GAC with the adsorption capacity of 104.78 mg/g and mustard husk with a muchlower capacities of 43.85 mg/g. The energies of adsorption, as indicated by ^'b' are seen to be highest for treated GAC, followed by carbon aerogel and mustard husk, in that order. A comparison of the' Freundlich adsorption isotherms for the three adsorbents show, that n is maximum for carbon aerogel and minimum for treated GAC. Ke seen to be higher for mustard husk and least for treated GAC.

This gives a similar inference as that obtained from Langmuir isotherms,

On the basis of regression analysis of the experimental data on the adsorptive behaviour of cadmium on treated GAC, mustard husk and carbon aerogel, it may be inferred that the adsorption behaviour of cadmium on treated GAC is in good agreement with Langmuir model. These can be attributed to three main causes: (i) The formation of

Contacttime-48 h;Adsorbent dose - 1.2 9/100 mL

'10 Carbon aerogel

• Mustard husk

i~Treated GAC_

o ^{200 400 600 800 1000}

lICe, ppm-l

fig. 6- Langmuir isotherms of cadmium for various adsorbents

monolayer coverage on the surface of treated GAC with minimal interaction among molecules of substrate, (ii) Immobile and localized adsorption, and (iii) All sites having equal adsorption energies.

Whereas the adsorptive behaviour of cadmium on untreated mustard husk and carbon aerogel satisfies not only the Langmuir assumptions but also the Freundlich assumptions, i.e., multilayer formation on the surface of the adsorbent with an exponential distribution of site energy. The shapes of isotherms suggest that there are high-energy adsorption sites to favour strong adsorption at low equilibrium concentrations for the treated GAC, carbon aerogel and mustard husk.

Conclusions

Following conclusions are drain from the study.

(i) Treated GAC, carbon aerogel and mustard husk showed 100, 87 and 72 per cent, respectively, adsorptive removal of cadmium under optimized conditions of pH 4 and dosage 1 gllOO mL for 2 mg/L cadmium aqueous solutions in 48 h.

(ii) The adsorption is pH dependent and maximum adsorption occurs at pH 4,

(iii) The Langmuir model is found to be in good agreement with experimental data on the adsorptive behaviour of cadmium on treated GAC, whereas the experimental data on adsorptive behaviour of cadmium on mustard husk and carbon aerogel follow both Freundlich model and Langmuir models.

(iv) These experimental studies on low cost adsorbents would be quite useful in developing an appropriate technology for the removal of cadmium from contaminated plant effluents.

Acknowledgement

The authors are grateful to Sh. A K Kapoor, Director CEES, for providing encouragement and facilities for carrying out this work.

416 ^{J SCI IND RES VOL 63 MAY 2004}

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