Thermodynamics and Adsorption Efficiencies of Maize Cob and Sawdust for the Remediation of Toxic Metals from Wastewater

doi:10.4236/gep.2013.12004

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Journal of Geoscience and Environment Protection

2013. Vol.1, No.2, 18-21

Published Online October 201 3 in SciRes (http://www.scirp.org/journal/gep) http://dx.doi.org/10.4236/gep.2013.12004

Thermodynamics and Adsorption Efficiencies of Maize Cob and

Sawdust for the Remediation of Toxic Metals from Wastewater

Muhammad B. Ibrahim

Department of Pure and Industrial Chemistry, Bayero University, Kano, Nigeria

Email: [email protected]

Received July 2013

The thermodynamics and sorption efficiencies for the remediation of Cr, Ni and Cd from their aqueous

solutions using Maize Cob (MC) and Sawdust (SD) in a batch system are reported. Efficiencies were

judged based on parameters such as sorbent weight, initial adsorbate loading concentration, pH and sur-

face area. Shimadzu AA650 Double Beam Atomic Absorption/Flame spectrophotometer was employed to

study concentration differences before and after the adsorption process. Parameters such as ΔH, ΔS and

ΔG were determined. On MC, ΔH varied as 1466.59, 1271.21 and 1347.70 kJmol−1 for Cr, Ni and Cd re-

spectively. While on SD it varied as −566.85, 256.32 and 888.77 kJmol−1 respectively for the same order

of metal ions. The three ions were found to be chemisorbed onto MC, while on SD Cr and Ni were phy-

sisorbed and Cd remains chemisorbed as suggested by Freundlich isotherm.

Keywords: Adsorbate; Adsorbent; Maize Cob; % Removal; Sawdust; Wastewater

Introduction

Unlike organic pollutants which are biodegradable, heavy

metals like Cr(VI), Ni(II), Cd(II) etc. are not biodegradable;

and their increasing concentration in the environment is detri-

mental to a variety of living species. Excessive ingestion of

these metals by humans can cause accumulative poisoning, can-

cer, nervous system damage and ultimately death. This forms

the basis for the increasing researches with a view to remedying

their levels in the environment; and also the growing concern

by governmental agencies for the regulation of the discharge of

these metals into the environment. Different methods for the

removal of toxic metals from aqueous systems have been re-

ported by different workers amongst which adsorption onto natural

adsorbents have proven to be an efficient and inexpensive op-

tion for removal of heavy metals from wastewater (Rafika et al.,

2009; Kehinde et al., 2009; Khan et al., 2004). Maize cob is

mainly composed of lingocellulose materials having relatively

large surface areas that can provide intrinsic adsorptive sites to

many substrates and inherently adsorb waste chemicals such as

dyes and cations in water due t o columbic interaction and phy si-

cal adsorption (Sun & Shi, 1998). According to Shukla et al.

(2002) the cell walls of sawdust mainly consist of cellulose and

lignin, and many hydroxyl groups, such as tannins or other

phenolic compounds which are all active ion exchange com-

pounds. Lignin, the third major component of the wood cell

wall, is a polymer material, which is built up from the phenyl-

propane nucleus, i.e. an aromatic ring with a three-carbon side

chain.

Materials and Methods

The water used throughout this work was initially distilled

and then passed through a deionizer. Analar grade reagents

were employed for the preparation of all stock solutions and

refrigerated. Fresh working standards were prepared daily by

appropriate dilution of the stock solutions. All glassware and

plastic containers were washed with detergents, rinsed with dis-

tilled water and then soaked in a 10% HNO3 solution for 24 h.

They were then washed with deionised water and dried in an

oven for 24 h at 80˚C (Todorovi et al., 2001). The adsorbent

employed in this work were maize cob and sawdust. Maize

cobs (MC) collected from local farm were cut into small pieces,

washed several times with water and air-dried. Similarly, hard-

wood sawdust (SD) of Mahogany (Khaya senegalensis) tree

collected from a local saw mill was air-dried in sunlight until

almost all the moisture evaporated. Then it was washed several

times with distilled water in order to remove the water soluble

tannins, after which it was dried in air and then in an oven at

80˚C. The two substrates were then ground to two particle sizes

(850 μm and powdered form) and were finally kept in plastic

containers for subsequent use.

All batch sorption analyses were carried out at room tem-

perature (30˚C ± 2˚C) by shaking various amounts of the ad-

sorbents (2 - 8 g) with 100 cm3 of the aqueous solutions of the

adsorbates (in a screw capped Erlenmeyer flasks) with initial

loading concentrations ranging from 20 - 60 mg/L on an Innova

4000 shaker from New Brunswick Scientific at a speed of 290

rpm for a period of 1 h. Immediately after which, the samples

were separately filtered using Whatman No. 1 filter paper and

the filtrates collected in polyethylene bottles were taken for

AAS measurements for the residual adsorbate concentration

using Shimadzu AA650 double beam atomic absorption/flame

spectrophotometer. All assays were replicated and only mean

values are presented. pH adjustments of the adsorbate solutions

where achieved by using .5 M HCl and .5 M NaOH solutions as

required.

Results and Discu ssion

The affinities of the two substrates to the three adsorbates

show a gradual increase from the lowest amount (2 g) to the

M. B. IBRAHIM

highest (8 g) as shown in Figure 1, a trend which can be attrib-

uted to the increase in surface active sites as the adsorbent dose

is increased (Zhou et al., 2011). Similarly, from the figure saw-

dust shows higher affinity for the adsorbates, but Cd, compared

to maize cob due to, among other factors, that it contains vari-

ous organic compounds (lignin, cellulose and hemicellulose)

with polyphenolic groups that could bind heavy metal ions

through diﬀerent mechanisms (Wan Ngah & Hana ﬁah, 2008;

Abdel-Ghani et al., 2007).

Increase in adsorbate loading concentration has dual effects

on the removal of the ions (Figure 2) such that at some lower

concentrations the % adsorption increases with increase in con-

centration but it drops at higher concentrations. This phenome-

non according to Adie et al. (2012) and Ibrahim and Jimoh,

(2008) arises because at low loading concentration of metal

ions, more binding sites are available, but as concentration in-

creases the number of ions competing for available binding

sites in the adsorbent increased. Also, at higher concentration,

most of the ions are left unabsorbed due to saturation of the

adsorption sites; and the ratio of surface active sites to ion con-

centration decreased with increasing metal ion concentration

and so ion removal reduced.

The adsorption envelope as presented in Figure 3 shows that

pH affects the solubility of metals in solution and also the ad-

sorption behavior of ions on the functional groups of the ad-

sorbents. The adsorption of Ni2+ and Cd2+ onto the two sub-

strates increases from lower pH to a higher pH as a result of

lowered competition between

( )

and the metallic ions at

the later condition. However, a reverse phenomenon was ob-

served in the case of Cr(VI) adsorption for which it is higher at

lower pH, a case similar to what has been reported elsewhere in

the literature (Omar & Al-Itawi, 2007; Kehinde et al., 2009).

Also the figure showed an increase in adsorption from MC to

SD indicating variation in the surface active site of the two

substrates.

In Figure 4 the effects of increase in surface area of the ad-

sorbents on their adsorption efficiencies was observed. In all

cases the efficiency increased from the granular to the pow-

dered form of the adsorbent.

A plot of the linear form of the Freundlich isotherm, lnqe =

lnKF + alnCe is presented in Figure 5, in which the slope a =

1/n where n is the adsorption energetic and heterogeneity factor ,

Figure 1.

Variation of % adsorption with weight of adsorbent.

Figure 2.

Variation of % adsorption with adsorbate loading concentration.

Figure 3.

Variation of % adsorption with pH of the adsorbate solution.

100

120

0 1 2 3 4 5 6 7 8 9

% Adsorption

Weight of Adsorbent (gm)

Cr (MC)

Ni (MC)

Cd (MC)

Cr (SD)

Ni (SD)

Cd (SD)

100

120

010 20 30 40 50 60 70

% Adsorption

Adsorbate Loading Concentration (mg/L)

Cr (MC)

Ni (MC)

CD (MC)

Cr (SD)

Ni (SD)

CD (SD)

100

120

0246810 12

% Adsorption

pH of Adsorbate Solution

Cr (MC)

Ni (MC)

Cd (MC)

Cr (SD)

Ni (SD)

Cd (SD)

M. B. IBRAHIM

Figure 4.

Variation of % adsorption with weight of the powdered adsorbent.

Figure 5.

Freundlich adsorption isotherm.

representing the deviation from linearity of the adsorption.

While from the intercept, KF is indicative of the relative adsorp-

tion capacity (mg1−n∙Ln∙g−1) of the adsorbent related to the

bonding energy. From the n-values in Table 1 it shows that the

adsorption of Cr, Ni and Cd ions onto MC and for Cd ion onto

SD (for which n < 1) is a chemisorption process, in other words,

a localised monolayer adsorption. However, those of Cr and Ni

onto SD (having n > 1) are favourable physisorption (multilayer)

adsorption processes. On the other hand, Figure 6 represents

the Langmuir plot of the adsorption process, and from Table 2

it can be understood that the adsorption of the three ions on MC;

and those of Cr and Ni onto SD cannot be explained by the

Langmuir isotherm, while that of Cd onto SD is linear indicat-

ing both chemisorption and physisorption are taking place at

the same rate.

The thermodynamicity of the adsorption process is outlined

in Table 3, from which the spontaneity (ΔS) and feasibility

(ΔG) of the adsorption of the three metal ions onto MC varied

as Cr > Cd > Ni. Whereas on SD the order is Cd > Ni > Cr.

Conclusion

The work highlighted the possibility of using the agricultural

waste for the removal of the metallic ions from aqueous solu-

tions with the adsorption nature varying from physical to che-

mical due to the differences existing in the binding nature of the

adsorbates onto the two substrates.

Table 1.

Numeric constants for the adsorption of the metal ions onto the adsor-

bents.

Adsorbent Ion Freundlich Langmuir

nF KF qm(mg/g) k(L/ mg )

Maize Cob

Cr .2967 .0103 500 −.0769

Ni .2319 .0025 500 −.0541

Cd .1872 .0018 100 −.1020

Sawdust

Cr 1.4514 .6949 −12.1951 −.1595

Ni 1.6556 .5236 −71.4286 −.0718

Cd .5627 .0596 .0000 .0000

Table 2.

Variation of RL for the various adsorb ents with increase in initial metal

ion concentration.

Langmuir Separation Parameter (RL)

Co(mg/L) Maize Cob Sawdust

Cr Ni Cd Cr Ni Cd

20 −1.8571 −12.333 −.9608 −.4565 −2.2941 1.0000

30 −.7647 −1.6087 −.4852 −.2641 −.8667 1.0000

40 −.4815 −.8605 −.3245 −.1858 −.5343 1.0000

50 −.3514 −.5873 −.2438 −.1433 −.3861 1.0000

60 −.2766 −.4458 −.1952 −.1167 −.3023 1.0000

100

120

02 4 68 10

% Adsorption

Weight of Powdered Adsorbent (g)

Cr (MC)

Ni (MC)

Cd (MC)

Cr (SD)

Ni (SD)

Cd (SD)

-1

-0 .9

-0 .8

-0 .7

-0 .6

-0 .5

-0 .4

-0 .3

-0 .2

-0 .1

-0.2 00.2 0.4 0.6 0.8 11.2 1.4

lnqe

lnC e

Cr (MC)

Ni (MC)

Cd (MC)

Cr (SD)

Ni (SD)

Cd (SD)

线性 (Cr (MC))

线性 (Ni (MC))

线性 (Cd (MC))

线性 (Cr (SD))

M. B. IBRAHIM

Figure 6.

Langmuir adsorption isotherm.

Table 3.

Thermodynamic parameters for the adsorption of the various metal ions.

Ions Maize Cob Sawdust

Ka ΔG

(Jmol−1) ΔH

(Jmol−1)

ΔS

(Jmol−1∙K−1) Ka ΔG

(Jmol−1) ΔH

(Jmol−1)

ΔS

(Jmol−1∙K−1)

Cr .0118 −3861.4 1466.5896 17.5841 .5708 6199.57 −566.8485 −22.3314

Ni .0062 −1167.32 1271.2106 8.0480 .0875 3055.09 256.3206 −9.2369

Cd .0143 −2977.67 1347.6994 14.2751 .0299 −406.07 888.7666 4.2734

Note: Conditions: 8 g Adsorbent, 20 mg/L metal ion concentration and 1 hr Agitation time.

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0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0.45

0.5

0510 15 20 25

Ce /qe

Cr (MC)

Ni (MC)

Cd (MC)

Cr (SD)

Ni (SD)

CD (SD)

线性 (Cr (MC))

线性 (Cd (MC))

线性 (Cr (SD))

线性 (Ni (SD))