Industries have been fingered as the major institutions since the last four decades,
responsible for diverse environmental and human catastrophes; hence they are a major
contributor to environmental degradation and pollution. Environmental pollution due to
appreciating industrial activities is unquestionably some of the most important problems
of this century (Ladwani et al., 2012).
Tannery wastewater is highly polluted in terms of the content of its Biological Oxygen
Demand (BOD), Chemical Oxygen Demand (COD), Suspended Solids (SS), nitrogen,
conductivity, sulphate, sulphide and chromium (Mondal et al., 2005) and in most developing
countries tannery effluents are discharged directly into sewers or water bodies without
treatment [1,2]. High BOD content of an effluent affects the survival of gill breathing animals
of the receiving water body and high COD value indicate toxic State of the wastewater,
including the presence of biologically resistant organic substances. High level of ammonia
(NH3
) is toxic to aquatic organisms and excess nitrogen will cause eutrophic condition. The
high salinity and Total Dissolved Solids of effluents may result in physiologically stressful
conditions affecting some species of aquatic organisms due to alterations in osmotic
conditions. Changes in the ionic composition of water can also eliminate some species
while promoting population growth of others [3]. Chrome Tan liquor is greenish in color
and highly toxic and acidic with pH 3.1 to 6.0.The waste contains a high concentration of
trivalent chromium ranging from 100 to 5500 mg/L. Haxavalent chromium is not generally
present in the waste chrome liquor because of the reducing agent used and one bath process
utilized [4].
The manufacture of leather has evolved into a significant source of livelihood in many
industrialized and developing countries, [5] reported an estimated production capacity
of 1.8 billion metric tons of leather yearly, with a larger part of the product processed in
Africa and Asia, factored by the high labour intensity involved the manufacture of leather.
Procedures employed by most developing countries in leather tanning is still at the
traditional level and are not technologically compactable or designed for use with chemical
and water. As estimated, the total wastewater discharge from tanneries runs to about 400
million m3
each year. The industry has been painted negatively in the society owing to its
high pollutant composition, health and environmental implication is a negative to the sitting
of the leather industry. The difficulty in treating tannery effluent owes it to the complex
nature of the wastewater produced, leading to various environmental regulations and laws
in many developing countries being passed especially in the last ten years.
Adsorption is a phenomenon that occurs on the surface or pores of a solid (sorbent), it is characterized by the available surface area and is a function of
the partial pressure (or concentration in aqueous solutions) of a
chemical species (solute) [6]. Adsorption occurs if the attractive
force between the solute and Adsorbent is greater than the cohesive
energy of the substance itself. Adsorption isotherms are important
for the description of how adsorbates will interact with an Adsorbent
and are critical in optimizing their use [7]. Correlation of equilibrium
data using a theoretical or empirical equation is useful for adsorption
data interpretation and prediction. Several mathematical models are
applicable in describing experimental data of adsorption isotherms.
Langmuir adsorption isotherm, originally developed to describe
gas–solid-phase adsorption onto activated carbon, has traditionally
been used to quantify and contrast the performance of different biosorbents. In its formulation, this empirical model assumes monolayer
adsorption (the adsorbed layer is one molecule in thickness), with
adsorption can only occur at a finite (fixed) number of definite
localized sites, that are identical and equivalent In its derivation,
Langmuir isotherm refers to homogeneous adsorption and can be
represent as Equation.
(1)Where KL
(L/mg) and qm are Langmuir isotherm constants [8].
Freundlich isotherm is the earliest known relationship
describing the non-ideal and reversible adsorption, not restricted to
the formation of monolayer. This empirical model can be applied to
multilayer adsorption, with non-uniform distribution of adsorption
heat and affinities over the heterogeneous surface. This model has
been expressed either as a non-linear (Eq. 2) or linear (Eqn. 3) form
of the model:
KF and F n are Freundlich constants obtained from the
intercept and slope of the linear plot of log qe vs log Ce
. They are
empirical constants which are indicators of sorption capacity and
adsorption intensity, respectively [8,9].
The Temkin isotherm assumes that the heat of adsorption of all
the molecules increases linearly with coverage. The linear form of
this isotherm can be given by:
R is universal gas constant (8.314J/molK), T is absolute
temperature (K) and bT is Tempkin isotherm constant. The slopes
and intercept are obtained from the graphical plot against ln [10,11].
The linear form of the Dubinin- Radushkevich isotherm can be
expressed as:
Where qD
is the theoretical saturation capacity (mg g-1
), B is a
constant related to mean free energy of adsorption per mole of the
adsorbate (mol2
/J
2
) and ε is the polanyi potential which is related to
equilibrium is given by:
Where, R is the Universal gas constant (8.314 J/mol/K) and T is
the temperature in Kelvin. E the mean sorption energy is calculated
using the following relation in Equation. 7:
Based on this energy of activation one can predict whether an
adsorption is physisorption or chemisorptions. It is general applied
to express the adsorption process occurring onto both homogeneous
and heterogeneous surfaces, the plot of ln (qe
) vs ɛ2
is used to test the
suitability of the data to this model [12].
BET isotherm is a theoretical equation, most widely applied in
the gas–solid equilibrium systems. This is a more general, multi-layer
model. It assumes that a Langmuir isotherm applies to each layer
and that no transmigration occurs between layers. It also assumes
that there is equal energy of adsorption for each layer except for the
first layer. Its extinction model related to liquid–solid interface is
exhibited as Equation. 8:
Where CBET, Cs
, qs
, and qe
are the BET adsorption isotherm (mg
/L), adsorbate monolayer saturation concentration (mg/L), the
theoretical isotherm saturation capacity (mg/g) and equilibrium
adsorption capacity (mg/g), respectively. As CBET and CBET (Ce
/Cs
) are
much greater than 1, the equation is simplified as Equation. 9 [13,14]
Among several Authors, [15] worked on Removal of Chromium
from Tannery Effluent Using Bio-adsorbents. On a similar quest,
[16] investigated the adsorptive removal of chromium ions from tan
liquor using. The Neem Sawdust (NS) gave removal efficiency of 84%
at contact time of 120 minute [16]. We now consider a reclamation
approach by regenerating spent commercially available activated
carbon for a possible reuse in Cr uptake from the Nigerian Institute of
Leather and Science Technology (NILEST) Zaria, Nigeria. This present
work also investigated pollution load of chrome tan- wastewater
and arrayed five isotherm models for interpretation of the sorption
phenomenon [17].
