Indian Journal of Chemical Technology Vol. 9. November 2002. pp. 499-503
Articles
Activated carbon from parthenium as adsorbent: Adsorption of Hg(II) from aqueous solution
K Kadirvelu" C Sivasankarib , M Jambuligamb & S Pattabhia
*
"Department of Environmental Science, bDepartment of Chemistry , PSG College of Arts and Science. Coimbatore 641 014. India
Received 27 JUl1 e 200 I; revised received 10 April 2002; accepTed 5 AugllsT 2002
Activated carbon (AC) prepared from parthenium was used to remove Hg(II ) from aqueous solution by adsorption technique under varying conditions of agitation time, metal ion concentration, adsorbent dose and pH. Adsorption equilib­ rium reached within 165 min for all concentrations studied (10 to 50 mg/L). Adsorption is dependents on solution pH. Hg (II ) concentration, carbon concentration and contact time. Adsorption followed both Langmuir and Freu ndlich isotherm models. The adsorption capacity was found to be 10 mg/g of AC at initial pH of 5.0 at 30+2°C for the particle size of 125- 250flm. The percent removal increased with pH from 2 to 6 and remained constant up to pH 10.0.
Environmental contamination due to heavy metal is
caused by several industries, metal plating, mining,
and radiator manufacturing, and by agricultural proc­
ess such as fertilizers and fungicidal sprays I . The
presence of mercury in the environment is a major
concern because of its toxicity and threat for human
life and for the environment, especially when toler­
ance level is exceeded2 . Mercury is generally consid­
ered to be one of the most toxic metals found in envi­
ronment' . Once mercury enters the food chain, larger
accumulation of mercury compounds takes place in human and animals. According to the Bureau of In­
dian Standards (BIS) the tolerance limit for Hg(II) for
discharge into land surface waters is IOf-lg /L 4 and for
drinking water, If-lg/L5. Mercury causes damage to the
central nervous system and chromosomes, impairment
of pulmonary function and kidney , chest pain and
dysphoeao. Hence, it is essential to remove Hg (II)
from wastewaters. The general treatment methods
employed for removing mercury from
industrial wastewaters include sulphide precipitation,
ion exchange, alum and iron coagulation, activated
carbon adsorption, electro-deposition, ultrafilteration and various biological process 7.
The above methods for removal of Hg(II) from wastewater are not economical in the Indian context.
"' For correspondence (E- mail ps!!cas@ llld2.vs n1.nct.i.!!; Fax 0422-575(22)
Some works of low-cost, non-conventional adsorbents have been carried out for heavy metal removal. Ad­ sorbents used include waste such as coir pith carbonx. coconut tree sawdust carbon lJ, peanut hull carbon 10, waste rubber II , agricultural wastes 12. ", car-
b . d d l4 b ' d . IS b' olllze woo , car onlze waste newspnnt ., ttu- minous coal and fly ash 17.
The objective of the present study was to explore the feasibility of using parthenium plant as raw mate­ rial for preparation of acti vated carbon for the re­ moval of Hg(II).
Experimental Procedure Adsorbent
The parthenium plant was collected in and around Coimbatore city, Tamil Nadu State, India and made
into small pieces and dried in sunlight. The dried
patthenium was used for carbon preparation by mix­ ing I part of parthenium and 1.5 part of concentrated
H2S04 and keeping it at l20±5°C for 24 h. The car­
bonized material was then washed with distilled water
to remove the free acid and soaked in I % sodium bi­
carbonate solution for overnight to remove any resid­ ual acid. Thi s material was then washed with distilled water and dried at I05°e. The particle size of 125-250 f-lm was used for Hg (II) removal studies. Character­ istics of parthenium activated carbon are presented in Table I. All the chemicals used were of analytical reagent grade and were obtained from BDH, E-Merk.
SDs and/or Ranbaxy. All the solutions were prepared in distilled water.
Articles
Table l--Characteristics of activated parthenium carbon Parameter Value
pH( I % solution ) Moisture content (%) Ash content (%) Apparent density (g/ml) Water soluble matter (%) Acid soluble matter (%) Methylene blue adsorption (mg/g) Surface area (m2/cm)
Batch mode adsorption studies
6.9 3.8 3.9 0.5
265.0
An adsorbate stock solution of 1000 mg/L Hg (II) was prepared by dissolving 1.3S40 g of mercuric chloride in double distilled water acidified with S mL of concentrated HN03 to prevent hydrolysis and made up to 1000 mL. This stock solution was diluted to re­ quired concentration for obtaining standard solution containing 10 to 40 mg/L of Hg (II) . Batch mode ad­ sorption studies were carried out with 100 mg of ad­ sorbent and SO mL of Hg (II) solution of desired con­ centration at initial pH of 6.0 in 100 mL conical flasks and were agi tated at ISO rpm for predetermined time intervals at room temperature (30±2°C) in a mechani­ cal shaker. At the end of agitation time, the adsorbate and adsorbent were separated using centrifugation at SOO rpm and estimated spectromatically using rho­ damine 6G 1s
. Effect of carbon concentration on per­ cent removal was studied with carbon concentration from 2S-300 mg/SOmL while maintammg Hg (II) concentration at 10, 20 and 30 mg/L. Lang­ muir isotherm study was carried out with different initial concentrations of Hg (II) from 10 to 40 mg/L while maintaining the adsorbent dose at 100 mg/SOmL. Freundlich isotherm was obtained from the effect of carbon concentrations on Hg (II) removal. Effect of pH on Hg (II) removal was studied using 100 mg of carbon dose and Hg (II) concentration of 10 mg/L. To correct any adsorption of Hg (II) on containers, control experiments were carried out in duplicate. There was no adsorption by container walls. Experiments were carried out in duplicate. The maximum deviation was within 3%.
Results and Discussion
Effect of agitation time and initial Hg (II) concen­ tration 'on Hg (II) adsorption
Fig. 1 shows the effect of agitation time on the re­ moval of Hg (II) by parthenium carbon. The removal
SOO
100.---------=-a:::;~~~F~ii
Time (min)
Fig . l--Effect of agitation time and initial Hg(lJ) concentration on Hg(IJ) adsorption
1· 0
-0·6
o
Agitation ti~ (min)
Fig. 2--Plot of log(q,-q) versus time (I)
increases with time and attains equilibrium within 16S min for the all the concentrations studied (10 to 40 mg/L). The curves were single, smooth and continu­ ous till the saturation of Hg (II) on activated carbon surface.
Adsorption kinetics The adsorption kinetics of Hg (II) adsorption on
parthenium carbon follows first order rate expression given by Lagergren 19.
... (I)
Kadirve lu el (I/.: Activated carbon from parthenium as adsorbent: Adsorption of Hg(II ) from aqueous solution Articles
where, Kat! (llmin) is the rate constant of adsorbent, q and q" are the amount of Hg (II) adsorbed (mg/g) at time t (min) and equilibrium time. Linear plots of log- 10 (qe-q) versus t (Fig. 2) show the applicability of above equation for parthenium carbon. The Kat! values at different initial metal ion concentrations were cal­ culated from the slope of the linear plots and are pre­ sented in Table 2. The Kat! values were comparable with recently reported values for Hg (II) removal by acti vated carbonss,lO.
Effect of carboll dosage 011 Hg (II) removal Fig. 3 shows the effect of carbon dosage on Hg (II)
adsorption. The Fig. 3 shows that the removal of Hg (II) increases with increase in carbon concentration. It can be seen from removal of Hg (II) from 1000 mL of 10,20 and 30 mg/L Hg (II) required 3.0, 3.5 and 4.0 g of carbon, respectively.
Adsorption isotherms The Langmuir isotherm was applied for adsorption
equilibrium of Hg (II) onto patrhenium carbon20.
C,o I C" -=--+- ql' Qoh Qo
... (2)
where, Ce is the equilibrium concentration (mg/L), qe is the amount of Hg (II) adsorbed (mg/g) , Q" and bare Langmuir constants related to adsorption capacity and energy of adsorption, respectively. The linear plot of C/q" versus Ce shows that the adsorption follows Langmuir isotherm model for Hg (II) adsorption (Fig. 4). The value of Q" and b were calculated from the slope and intercept of the plot, respectively. The values obtained were Q" = 10 mg/ g, b = 0.1 l/min. The adsorption capacity of parthenium carbon is equal to that of GAC (12.38 mg/g) and rice hull carbon (8.76 mg/g)S.2 1.
The essential characteristics of Langmuir isotherm can be expressed in terms of dimensionless separation factor of equilibrium parameter RL
I9 , which is defined
RL =( 1/1 +bCo) ... (3)
where, Co is the initial Hg (II) concentration mg/L and b is the Langmuir constant(Llmg). The parameter in­ dicates the shape of isotherm as follows
RL values between 0 and 1 (0.099-0.022) at different
Table 2-Kinetics constants for Hg (II ) adsorption
Hg (II) concen- qe (mg/g) tration
Adsorption rate constant
Concentration of Hg
Adsorbent dosage (mg/50mL)
2·0
1· 6
Ce (mg/L)
Type of isotherm
en S!
1 .2r---__________________________ ~
0·6
0·4
0·2
OL---~ ____ ~ __ ~ ____ ~ __ ~ ____ ~ 0·2 0·4 0· 6 0·8 1·0
x Fig. 5--Plot of log - versus log Cc
m
' ·2
concentration (10 to 40 mg/L) indicates favourable adsorption for Hg (II) onto parthenium carbon.
The Freundlich equation is widely used in the envi­ ronmental engineering practice to model adsorption of pollutants from an aqueous medium empirically22. The expression of Freundlich equation is given as
KfC l in qe= e (4)
The linear form of Freundlich equation is given by the following expression
log J()(X//II ) = log JOKf + lin log JOCe ... (5)
where, X is the amount of Hg (II) adsorbed at equilib­ rium (mg ), /J1 is the weight of adsorbent used (mg), Ce is the equil ibrium concentration of Hg (II) in solu­ tion (mg/L) and Kr & 11 are the constants incorporating all facto rs affecting the adsorption process (adsorption capacity and intensity of adsorption). Linear plots of the log X/11l versus log Ce show that the adsorption follows Freundlich isotherm (Fig. 5). The Freundlich constants Kr and II found have been reported. The constants fou nd were Kf 4.8 and n 2.7, for 20 mg/L Hg (11) concentration. According to Traybal23
, it has been shown using mathematical cal­ culations that n values between 1 and 10 represent the
502
100
80
beneficial adsorption. Effect of pH on Hg (II) removal
Fig. 6 presents the effect of initial pH on the re­ moval of Hg (II) by parthenium carbon. It can be shown by stability constant calculation that in the presence of chlorine the predominant species at pH>4.0 is HgCl 2 II. The formation of HgCl2 has been found to decrease the Hg (II) sorption onto a commer­ cial activated carbon Fs- 400 GAC8
. Accordingly , the Hg (II) adsorption decreased when the pH was low­ ered from 6.0 to 2.0 with dilute HCI for parthenium carbon and on increasing the pH fro m 2.0 to 6.0 it became quantitative over the pH range of 6.0 to I 1.0. This implies that Hg (OHh, species may be retained in the micro-pores of carbon by chemisorption in­ volving surface complexes l9
. The influence of pH on Hg (II) removal may be acidic pH conditions; both adsorbent and adsorbate are positively charge (M2+ and H+) and therefore the interaction is that of electro­ static repulsion8
, lo. Beside, the higher concentration of H+ ions for the surface adsorbing sites results in de­ crease in the removal of Hg (II). The concentration of Hg (II) remains constant resulting in increase in the removal of Hg (II ).
Conclusion
The objective of this work was to study the de­ pendence of adsorption on parthenium carbon in the case of Hg (II). Based on the results of this study, it is concluded that parthenium carbon is an effective low cost adsorbent for the removal of Hg (II) aqueous so­ lutions. In batch mode adsorption studies, the adsorp­ tion is dependent on the solution pH, initial Hg (II)
Kadirvelu el al.: Activated carbon from parthenium as adsorbent: Adsorption of Hg(lJ) from aqueous so lution Articles
concentration and carbon concentration. Adsorption followed both Langmuir and Freundlich isotherm models. The adsorption capacity was found to be IOmg/g of parthenium carbon at initial pH 5.0 to 30±2°C for the particle size of 125-250 !-un. The pres­ ent removal increases with pH from 2 to 6 and re­ mained constant up to pH 10.0.
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