Journal of Geoscience and Environment Protection
2013. Vol.1, No.2, 18-21
Published Online October 201 3 in SciRes (
Copyright © 2013 SciRes.
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
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 kJmol1 for Cr, Ni and Cd re-
spectively. While on SD it varied as 566.85, 256.32 and 888.77 kJmol1 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
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
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
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
Copyright © 2013 SciRes.
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 dierent 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
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.
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)
010 20 30 40 50 60 70
% Adsorption
Cr (MC)
Ni (MC)
Cr (SD)
Ni (SD)
0246810 12
% Adsorption
pH of Adsorbate Solution
Cr (MC)
Ni (MC)
Cd (MC)
Cr (SD)
Ni (SD)
Cd (SD)
Copyright © 2013 SciRes.
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 (mg1nLng1) 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.
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-
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
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
02 4 68 10
% Adsorption
Weight of Powdered Adsorbent (g)
Cr (MC)
Ni (MC)
Cd (MC)
Cr (SD)
Ni (SD)
Cd (SD)
-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
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))
Copyright © 2013 SciRes.
Figure 6.
Langmuir adsorption isotherm.
Table 3.
Thermodynamic parameters for the adsorption of the various metal ions.
Ions Maize Cob Sawdust
(Jmol1) ΔH
(Jmol1K1) Ka ΔG
(Jmol1) ΔH
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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0510 15 20 25
Ce /qe
Cr (MC)
Ni (MC)
Cd (MC)
Cr (SD)
Ni (SD)
线性 (Cr (MC))
线性 (Cd (MC))
线性 (Cr (SD))
线性 (Ni (SD))