Indian Journal of Engineering & Materials SciencesVol. 4, December 1997, pp.254-260

Removal of heavy metal ions from water usingsawdust-based activated carbon

C Raji, G N Manju & T S Anirudhan*

Department of Chemistry, University of Kerala, Kariavattom, Trivandrum 695 581, India

Received 31 March 1997; accepted 7 October 1997

The removal of Pb(II), Hg(II) and Cd(II) by activated carbon prepared from bicarbonate-treatedrubber wood sawdust has been found to be concentration, pH and temperature dependent. Thepercentage adsorption of metal ions increased with decrease in initial concentration of metal ions,increase in adsorbent dosage and temperature. The optimum pH range for the removal of ~b(II) andHg(II) is 4-8 whereas maximum uptake for Cd(II) is in the pH range 5-9. The applicability of Lagergrenkinetic model has also been investigated. The equilibrium data at different temperatures fit well with theLangmuir isotherm. Thermodynamic constant (Ko) and standard free energy (t.GO), enthalpy (M?) andentropy (&5°) changes were calculated for predicting the nature of adsorption. The surface mass transfercoefficient as a function of initial adsorbate concentration has been determined. Desorption studiesrevealed that spent adsorbent can be regenerated and reused by 0.2 M HC!.

The presence of heavy metals in the environmentcan be detrimental to a variety of living species.Therefore, the elimination of heavy metals fromwater and wastewater is important to protect publichealth. Many reports have appeared on thedevelopment of activated carbon from cheaper andreadily available materials for the removal ofmetals from water':'. Activated carbon derivedfrom rice husk', coconut shell", waste slurrygenerated from fertilizer plant', peanut huW' andwaste tea leaves" has been successfully employedfor the removal of heavy metals from aqueoussolutions. Our search for a cheap readily availablestarting material has identified sawdust as apotentially attractive material for the production ofactivated carbon for water treatment. Sawdust is acheap raw material and in abundant supply. Thehuge deposit of sawdust around sawmills posesproblems in its disposal. Besides its utilization inpreparing packing materials it has also beenrecently used in water pollution control"!". Thepresent investigation aims at using bicarbonatetreated activated carbon prepared from sawdust, alow cost and highly effective adsorbent for theremoval of hea vy metals namely Pb(II), Hg(II) andCd(I1) from aqueous systems.

*For correspondence

Experimental ProcedureAll the chemicals used were of analytical

reagent grade and were obtained from BDH.Sawdust carbon (SDC) was prepared by treatingone part of sawdust of rubber wood (Heveabrazeliansist with 1.8 parts by weight ofconcentrated sulphuric acid and keeping it in an airoven at 150°C for 24h. The carbonized materialwas washed with distilled water and dried at!05°e. The product, SDC was repeatedly soakedin I% sodium bicarbonate until effervescenceceased and finally soaked in I% sodiumbicarbonate solution overnight to remove anyresidual acid". The material, bicarbonate treatedsawdust carbon (BSDC) was then washed withdistilled water, dried at 105°C and sieved to -80+230 (ASTM).

In batch experiments, 0.1 g of BSDC indifferent stoppered bottles was shaken with 50 mLaqueous solution of metal ions of differentconcentrations at various temperatures and pH fordifferent time intervals in a shaking thermostatwith a speed of 125 rpm. At the end ofpredetermined time intervals, the solution wascentrifuged and the concentration of sorbate wasdetermined. Lead and cadmium were estimated byAtomic Absorption Spectrophotometer. Mercury

RAJI et al.: SAWDUST-BASED ACTIVATED CARBON 255

-- ,. -._. ..----." ..------.~.*Table I-Effect of initial concentrati. for the adsorpuo» of metal ;, :",!-SO'~!' nSDC

Initial Amount adsorbedconcn, Pb(I1) Hg(lI) C:d(lI)mg/L mg/g % rng/g

-01 mg/g %1f,I

10. 4.93 98.6 4.8G 97.2 4.76 95.125 10.91 87.3 9.75 78.0 9.37 75.150 20.18 80.3 19.08 76.3 17.75 71.0100 38.19 76.4 35.15 70.0 33.00 66.0200 71.47 71.5 60.70 60.7 60.11 60.1400 126.29 63.1 108.80 54.4 100.40 50.2500 151.20 60.5 124.50 49.8 iD2.53 41.0

was determined spectrophotometric ally using eosinmethod I I.

The experimental conditions for the desorptionof metal onto BSDC were similar to that of batchsorption tests. An extend agitation of (for 6 h)metal-laden BSDC with 0.2 M HCI at 30°C wasused to elute metal ions into solution. Forregeneration, the sameprocedure was followed forthree cycles. After each cycle, the adsorbent waswashed with distilled water and dried.

Results and DiscussionThe FTIR spectra of BSDC was obtained

on a Shimadzu FTIR model 1801. The spectrashows weak and broad peaks in the region1780-1580 cm'. The band at 1720 ern"corresponds to a normal carbonyl group" while theone at 1605 ern" may be attributed to conjugatedhydrogen bonded carbonyl groups as suggested byHallum' and Drushel \3. The data clearly indicatesthe presence of some surface groups in theadsorbent materials. X-ray spectra (Philips 1710diffractometer) of BSDC does not show any peakthereby indicating the amorphous nature of thematerial. Physical and surface properties of BSDCwere carefully determined by standardmethods 10,14. The characteristics of the adsorbentare: apparent density, 1.33 g/L; surface area, 217.3rrr'/g; moisture content, 3.8%; porosity, 0.68 mUg;cation exchange capacity, 2.54 meq/g; iodinenumber, 720 and pHzpc6.4.

Fig.l shows the removal of Hg(II), Pb(II), andCd(II) as a function of carbon dosage by SDC andBSDC. It is evident that for the quantitativeremoval of 25 mg/L metal ions in 50 mL, aminimum carbon dosage of 200 mg BSDC or 600mg SDC for Pb(II),300 mg BSDC or 800mg SDC

for Hg(II), and 500 mg BSDC or 1000 mg SDC forCd(II) is required. The data clearly show thatBSDC is more effective than SDC for the removalof metal ions (3.0 times for Pb(II) , 2.7 times forHg(II) and 2.0 times for Cd(II)). This may be dueto the higher porosity and moderate ion exchangecapacity ofBSDC compared to soc".

The dependence of the process of Pb(II), Hg(II)and Cd(II) removal from different initialconcentrations (10-500 mg/L) by BSDC isillustrated in Table 1. At low concentrations

HQ(l1) :

!-~~:~*-"* 4--.---f

I

1 I I I I~.-'---'---"----W

Cd(11)100

eo

60f/4020

O~~-L~~~~~~+-~~o 2 4 6 8 () 12 14

Adsorbent dose, g/L

Fig. I-Effect of adsorbent dose for the adsorption of metalions on SDC (x) and BSDC (0). Conditions : initialconcentration, 25 rng/L; pH, 6; ionic strength, 0.01 M;temperature, 30°C; equilibrium time, 4h

256 INDIAN 1. ENG. MATER. SCI., DECEMBER 1997

(below 10 mg/L) adsorption was 95-97%. Thissuggested that BSDC can remove most of themetals from water if their concentrations are below10 mg/L. The examination of the data also revealsthat at a fixed adsorbent dose the amount adsorbedincreased with the concentration of solution, butthe percentage adsorption decreased: In the case oflower concentrations, the ratio of initial number ofmoles of metal to the available surface area is low,however, at higher concentrations the availablesites of adsorption become fewer and subsequentlythe percentage removal of metal ions depends uponinitial concentrations.

The experimental data for the pH effect on theadsorption of Pb(II), Hg(II),and Cd(II) (Fig. 2) areobtained at 100 mg/L. The percentage adsorptionof metal ions increased with an increase in pH upto a certain value and then decreased with furtherincrease of pH. The maximum adsorption tookplace around pH range 4-8 for Pb(II) and Hg(II)and 5-9 for Cd(II). The effect of pH can beexplained in terms of pHzpc of the adsorbent. ThepHzpc of BSDC was found to be 6.4, and below thispH surface charge of the sorbent is positive. On theother hand at pH less than 6.4 the predominantmetal species (M2+ and M(OHn are positivelycharged and therefore the uptake of metals in thepH range 2-6.4 (PHzpJ is a H+_M2+ exchangeprocess explained by other authors as well":" inmetal cation adsorption with activated carbons. Ithas also been shown by Cullen and Siviour" that

--------------lIi,

~ 60

0: Pb(lllX: H\j(II)t:,: ceun20

pH

Fig. 2-Effect of pH for the adsorption of metal ions onBSDC [Pb(I1) (0), Hg(II) (x), Cd(II) (t.)). Conditions: ionicstrength, 0.01 M; initial concentration, 100 mg/L; adsorbentdose, 2 g/L; equilibrium time 4 h

carbonaceous material 'on treatment with sodiumbicarbonate had the characteristics of a weak acidion exchange resin in the sodium form.

The surface of BSDC has COH and CO asmajor hydroxo groups". The adsorption reaction,for metal ions, in acidic pH can be described by thefollowing expressions:2(COH)+M2+ <=>(CO')2M2++2H (l)2(COH)+M(OHr<=>(CO')2Molr+2H+ (2)or2(CO),+M2+<=>(CO')2M2+2(CO,I)+MOH+ <=>(CO')2MOH+

(3) -(4)

The chemical bonding is resulted from the sharingof free electron pair between the surface oxygenatom and the metal atom or the formation of an 0-M(II) bonding".

An increase in pH above pHzpc shows a slightincrease in adsorption in which surface of theadsorbent is negatively charged and the sorbatespecies are still positively charged'":". Asadsorbent surface is negatively charged as well, theincreasing electrostatic attraction between positivesorbate species and adsorbent particles would leadto increased adsorption of metal ions. This is inaccordance with the earlier observations 19. Decr-ease in removal of metal ions at lower pH isapparently due to the higher concentration of H+ions present in the reaction mixture which competewith the M2+ions for the adsorption sites of BSDC.Decrease in adsorption at higher pH is due to theformation of soluble hydroxy complexes'Y".

The adsorption of metal ions on BSDC followsthe first-order adsorption rate expression ofLagergren".

klog(qe - q) = log c, - _ad_t ... (5)2.303

where qe and q (both in mg/g) are the amount ofmetal ions adsorbed at equilibrium and at any timet, respectively. A linear plot of log (qe-q) vs. t at

Table 2--Rate constants for the adsorption of metal ions ontoBSDC at different temperatures

Temp. k ad values (min -I)

°C Pb(/l) Hg(II) Cd(II)

30 2.22x1O'2 1.58 X 10'2 1.I8x 10'2

40 2.37 X 10'2 1.76 X 10'2 1.44 X 10'2

50 2.60 X 10'2 1.96 X 10'2 1.61 X 10'2

60 2.90 x10-2 2.19x10'2 1.71 x l O"

RAJI et al.: SAWDUST-BASED ACTIVATED CARBON 257

0·4

0·2

o-0·2- o· 4 l------'---'--L.-'------'----'----'----'L---'.--'..-L..---'----l

-;;. 1·2 HQ(JI)

I 1.0..!! 08

~ 0·1\

0·4

0·2

o-0· 2 l------'----'----L-'------'----'----'----'L---'.--'..~~

~r~o 20 40 60 80 100 120

Time,min

Fig. 3-Lagergren plots for the adsorption of metal ions onBSDC. Conditions: initial concentration 50 rng/L pH 6'adsorbent dose, 2.0 g/L; temperature, 30°cex), 40°C ce):50°C (6), 60°C (0)

4r-------------------------~PIlOI)

2

HQ(II)

:~I,Y//

~ .._,___'---'-I __ ~IL-_--'-'--...J1----'-------1151

Cd (\I)

3

2

a ._..._.....l__._--1~____'_ __ _'_____ L__.l __ __l

a 180 360

Ce,mg/L

720

Fig. 4-Langmuir plots for the adsorption of metal ions onBSDC [30°cex), 40°C (e), 50°C (6), 60°C (0))

Table 3--Langmuir constants and thermodynamic parameters for the adsorption of metal ions onto BSDC

Temp., Langmuir constants Thermodynamical parameters°c QO, mg/g b, Llmg Ko 6Go, kllmol

Pb(II) Hg(II) Cd(II) Pb(I1) Hg(II) Cd(JI) Pb(lI) Hg(II) Cd(II) Pb(I1) Hg(I1) Cd(II)

30 262.1 189.5 182.2 0.0068 0.0074 0.0060 2.374 1.801 1.342 -2.178 -1.481 -0.74040 269.6 203.5 201.7 0.0094 0.0083 0.0063 3304 2.229 1.654 -3109 -2.086 -1.30950 274.1 219.6 216.3 0.0107 0.0097 0.0074 4367 3.117 2.153 -3.958 -3.053 -2.05960 290.5 237.9 232.9 0.0127 0.0'100 0.0087 5.910 4.167 2.989 -4.919 -3.951 -3.032

30, 40, 50, and 60°C (Fig. 3) indicates the validityof the above equation for the present system. Theadsorption rate constant, kad, has been determinedfrom the slope and are presented in Table 2.Similar studies regarding kinetics of equilibriumfor the metal adsorption on carbon surface has alsobeen performed by Namasivayam and Periasamy",The Arrhenius plot of In kad vs. liT was found to belinear. The values of energy of activation, E.calculated from the plots were found to be 7.46kJ/mol for Pb(II), 9.20 kJ/mol for Hg(II) and 10.19kJ/mol for Cd(II). The energies of activation showan increase in magnitude with increasing ionicsize. It is because of the cation with larger ionicsize that causes more hindrance to its entry into the

solid phase and also retards its mobility within thepores of carbon particles".

The adsorption data for the studied range fitsrearranged Langmuir isothermc, 1 c,-=-+- ...(6)s, QOb Qt:'

where g> (mg/g) and b (Llmg) are the Langmuirconstants related to the adsorption capacity andenergy of adsorption respectively. The linear plotsof Cfq, vs c, (Fig. 4) at 30, 40, 50, and 60°Csuggests the applicability of Langmuir isotherm.The values of g> and b were determinedgraphically and confirmed by regression analysis(Table 3). The adsorption capacity QO, increases

258 INDIAN 1. ENG. MATER. SCI., DECEMBER 1997

with increase In temperature suggests theendothermic nature of adsorption. The equilibriumparameter, RL has been calculated from therelationship" .

1RL = ... (7)

1+bCo

where Co is the initial concentration' (mg/L) and bis the Langmuir constant (Llmg). The values of RLshown in Table 4 indicate that adsorption of metalions on BSDC is a favourable process because RLvalues lies between 0 and 1.

The mass transfer coefficient for the adsorptionof Pb(II), Hg(II) and Cd(II) ions on BSDC wasdetermined using equation of Mckay et al. 24.

I lCt 1) I (mKL 1+ mKL) .n-- = rn - BStCo 1 + mKL 1 + mKL mKL . L s

... (8)Where Co is the initial adsorbent concentration(mg/L), Ct is adsorbate concentration (mg/L) aftertime t, m is the mass of adsorbent per unit volumeof particle free adsorbate solution (g/L), KL is theLangmuir constant (obtained by multiplying QOand b), BL is the mass transfer coefficient (cmls)

-I

-2

HOW)

-4x---L----~---L~~L_ __~-0·25

~.J -1·00 r...~~."_I~-1·15

I -2·50>--0' L

'-:~ -3-25r_~.~-~--L-~~_~_0.25,f r

I ceun I

:::~-~:J!-2·50~ <, •

l ~ ----.'''''a-3-25 ~ ~.

I ,

-4.001 ! I !. -:::::::-.

o 40 80 120 160 200

Time,min

Fig. S-Mckay et al. plots for the adsorption of metal ions atdifferent concentrations on BSDC [25 mg/L (x), 50 mg/L (e).75 rng/L (D), 100 mg/L (0»). Conditions: ionic strength.0.01 M; pH, 6.0; adsorbent dose, 2 g/L; temperature, 30°C

and S, is the outer surface of adsorbent per unitvolume of particle free slurry (ern"). The value ofS" was calculated using the method describedearlier". The straight line plots of In {CICo-

[I/(l+mKL)]} vs. t (Fig. 5) suggest the validity ofequation for the present system. The BL valuesdetermined from the slope and intercept of theplots were found to be 1.34x 10.4 crnls for Pb(II),1.09xlO-4 cm/s for Hg(II) and 0.74 xlO-4 cmls forCd(U) for an initial concentration of 25.0 mg/L.These values indicate that the velocity of metalstransport from the liquid phase to solid phase israpid enough to suggest the use of this adsorbentfor the treatment of wastewaters enriched in Pb(II),Hg(II) and Cd(II) 10ns25

.

Since the external mass transfer coefficient is animportant design and kinetic parameter, it is usefulif BL can be correlated against any particularsystem variable, in this case, initial metalconcentration. Logarithmic analysis of the datayields a straight line represented by the followingequation (Fig. 6). .log BL=log X+y log Co ... (9)Eq. (9) may be expressed in the general form bythe following equation

BL=4.86x 10-4 Co·O.40 for Pb(II) (10)BL=3.43xl0-4 Co·035 for Hg(II) (11)BL= 1.72x 10-4 Co·

027 for Cd(II) (12)Thermodynamical parameters were calculated

from the variation of the thermodynamicequilibrium constant (Ko) with change intemperature (Table 3). K; for the adsorptionreaction was determined by the method of Khanand Singh" by plotting In (qjCe) vs q; and

,--'I

-+L

.....--._.-._----------

x : PIlI").:!i9I1t)0: C4(11)

Fig, 6--Plots of log BL vs. log Co for the adsorption of metalions on RSDC [Pb(II) (x), Hg(II) (e), Cd(lI) (0»).Conditions: ionic strength, 0.0 I M; pH, 6.0, adsorbent dose, 2g/L

RAJI et a/.: SAWDUST-BASED ACTIVATED CARBON ,259

extrapolating to zero qe (Fig.7). The standard freeenergy (tlGO), enthalpy (M?) and entropy changes(llS') were calculated using the followingequationstlGo=-RT In «, (13)

MO sn:InK = --- (14)

o R RTA plot of In K; versus liT was found to be linear.M? and tlSO as calculated from the slope andintercept of the plots were found to be 28.06kl/mol and 90.04 J/mol/K for Pb(II), 26.70 kJ/moland 82.87 J/mollK for Hg(JI) and 25.18 kl/mol and75.38 J/mol/K for Cd(II) respectively. The positivevalues of M? confirm the endothermic nature of

2,0 0 POOl)

1,0

o

-1·Ob--.....L-·-.1--..l....-----1..~~J::::::,~~

1,6 0

0·2

-0·4

-1·0

-I ,6:---:"::-----:-l~_~~---=:tc~~.L:::::"'---,-Jo 45 90 136 180 225 270

Qe,mQ/g

Fig. 7-Plots of In q/C, vs q, for the adsorption of metal ionson BSDC [30°C(x), 40°C ee), 50°C(~), 60°C (0)].Conditions: ionic strength, 0.01 M; pH, 6,0; adsorbent dose, 2g/L; equilibrium time, 4 h

adsorption and suggest the possibility of weakbinding of adsorbate species with active surfacesites of adsorbent". The positive values of llS'suggest the increased randomness at the solid-solution interface during the adsorption at alltemperatures. The negative values of tlGo (Table3) indicate the feasibility of the process andspontaneous nature of adsorption, the order ofpreference being Pb(II) > Hg(II) > Cd(II).

To determine the competition among variousmetal ions for the substrate binding sites, 50 mLcombined solution containing 50 mg/L each ofPb(II), Hg(II) and Cd (II) was shaken with O.lgBSDC for 4 h. After adsorption equilibrium wasreached, the contents centrifuged and supernatantwas analyzed for Pb(II), Hg(II), and Cd(Il). Theadsorption of these metals on the adsorbent wasfound as: Pb(II) = 21.23 mg/g, Hg(II) = 18.56mg/g and Cd(U) = 15.21 mg/g. It was observedthat amongst the cations used, adsorption of Pb(II)is highest followed by Hg(JI) and Cd(II). For ionsof the same valence, sorbent prefers the metal withhigher atomic number".

Table 5 summarizes the results of desorptionand regeneration of BSDC for Pb(II), Hg(II) andCd(II) removal. After two cycles the adsorptioncapacity ofBSDC was reduced by 9.6% and on theother hand recovery of Pb(II) ions in 0.2 M HClwas decreased from 98.7% in the first cycle to91.2% in the third cycle. The absolute adsorptioncapacities of the adsorbent for Hg(II) and Cd(II)removal after two cycles decreased by 8.0 and6.5% respectively. The recovery of Hg(II) ions in0.2 M HCI was decreased from 95.6% in the firstcycle to 89.4% in the third cycle, whereas in thecase of Cd(II) ions it was decreased from 90.9 to85.6%. The small fraction of adsorbed metals notrecoverable by regeneration presumably representsthe metals which are bound through stronger

Table 4--Equilibrium parameters for the adsorption of metal ions onto BSDC

Initial Pb(II) Hg(Il) Cd(Il)

concn 30°C 40°C 50°C 60 °C 30°C 40°C 50 °C 60°C 30°C 40 °C 50°C 60 °Crng/L

25 0,853 0.809 0.788 0,759 0.843 0.828 0.804 0.784 0.869 0,863 0.844 0,82150 0,744 0.769 0.650 0.611 0.729 0.706 0.672 0.645 0,769 0.759 0,731 0,696100 0,592 0.514 0.482 0.440 0.573 0.545 0.506 0.475 0,624 0,612 0.576 0.534200 0.421 0.346 0.318 0,282 0.401 0.375 0,339 0,313 0.454 0,441 0404 0,364

INDIAN J. ENG. MATER. SCI., DECEMBER 1997

Table 5--Desorption and regeneration data

No. of cycles Adsorption Desorption in 0.2 M Recovery in 0.2 Mmg/g % HCI, mg/g HCI,%

1 11.23 89.8 11.08 98.72 10.54 84.3 10.27 97.43 10.02 80.2 9.14 91.2

1 9.90 79.2 9.46 95.62 9.45 75.6 8.86 9383 8.91 71.2 7.96 89.4

I 9.02 72.1 8.19 90.92 8.55 68.4 7.54 88.23 8.20 65.6 7.02 85.6

260

Metal ions

Pb(lI)

Hg(lJ)

Cd(lI)

mteractions and as a result, the adsorptionefficiency is reduced in subsequent cycles. Thedata in Table 5 shows that spent BSDC can beeffectively regenerated by 0.2 M HC!.

ConclusionsThe present study shows that bicarbonate treated

sawdust carbon can be reused as an adsorbent forthe effective removal of metals namely Pb(II) ,Hg(II) and Cd(II), from aqueous solutions. Higheruptakes [98.6% of Pb(II), 97.2% of Hg(II) and95.1 % of Cd(II)] from aqueous solution is possibleusing activated carbon as an adsorbent providedthe initial concentration of metal ions is low. Theadsorption follows first-order kinetics, and data fitsthe Langmuir adsorption isotherm. pH has beenfound to be a master variable controlling theadsorption of metal ions on activated carbonsurface. Increasing adsorbent dosage andtemperature increased the adsorption. The datamay be useful in designing and fabrication of aneconomic treatment plant for the removal of heavymetal ions from wastewaters.

AcknowledgementTIle authors are thankful to the Head,

Department of Chemistry, Universit; of Kerala,Trivandrum for providing the laboratory facilities.

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