EQUILIBRIUM AND KINETICS OF COLOUR ADSORPTION FROM TEXTILE WASTEWATER BY A NOVEL ADSORBENT

Activated carbons were produced from a local variety of mango seed by carbonizing the inner seed before activation with ZnCl2 using an impregnation ratio of 1:4. Laboratory batch studies were conducted in order to generate data for kinetic and equilibrium modelling of adsorption of colour by the produced mango seed endocarp activated carbon (MSEAC) for the purpose of determining the mechanism and rate controlling steps of the adsorption process. Langmuir, Freundlich and Tempkin Isotherms were adopted. While the kinetic models used were pseudo first order kinetic, pseudo second order kinetic, Intra-particle diffusion and Elovich models. The pseudo second order model gave the highest coefficient of determination (R 2 ) of 0.999 (indicating goodness of fit) compared to other kinetic models tested. This suggests that chemisorption is the rate-limiting step in this biosorption system. Langmuir isotherm fitted experimental data better compared with the other tested models(R 2 =0.9987), implying that the adsorption process is based on a monolayer adsorption. Chi-squared test performed on all the models confirmed the goodness of fit of the generated data to the Langmuir and pseudo-second order over the other isotherm and kinetic models because they had the least caculated Chisquared (χ 2 Cal) values of 5.2 and 3.2 respectively. It was recommended that these two models are reliable tools for predicting the mechanism and rate of adsorption of colour from textile wastewater by the produced MSEAC.


INTRODUCTION
Activated carbon has been used worldwide as an effective adsorbent for removing colour and chemical contaminants from water and waste water.Currently, Nigeria imports commercial carbon at high cost for water treatment which puts significant burden on the water treatment budget.A review of literature revealed many attempts on the use of agricultural wastes/by-products as adsorbents for water/wastewater treatment.The adsorption of phosphate by Terminalia catappa based activated carbon was studied by Ochonogor et al. (1999).Activated carbon was produced from Rosa canina and applied in the removal of dye from wastewater (Gurses et al., 1999).Rice husk and corncobs based activated carbon was produced and characterized by Aloko and Adebayo (2007) and used to remove phosphates from water.Jatropha husk activated carbon was used by Namasivayam et al. (2007) for the removal of anions, heavy metals, organics and dyes from water.High removal efficiencies were achieved by mango seed shell activated carbon in the removal of colour and phenol from wastewater (Akpen et al. 2011(Akpen et al. , 2014]]. However, scanty information is available on the use of mango seed endorcarp as adsorbent, though the potential for its use has been recognised by some researchers (Elizalde-Gonzalez &Hernandez-Montoya, 2007, Kwagher andAdejoh, 2012).In Nigeria, mango seed litter around streets especially in the suburban areas and they constitute environmental nuisance.The utilization of these seeds for wastewater treatment will have cost saving and waste management advantages.The aim of this study is to investigate the mechanism and nature of adsorption of colour from wastewater by mango seed activated carbon.The preparation, characterization and the optimum conditions for the removal of colour by MSEAC are reported elsewhere (Akpen et al., 2017).

Production of Mango Seed Endocarp Activated Carbon (MSEAC)
Mango seeds were collected from Mangar District of Bokkos Local Government Area of Plateau State, Nigeria, air-dried and broken to expose the endocarp which was further air dried.The air dried mango seed endocarps were carbonized in a muffle furnace at a temperature of 500 0 C for 2 hours and allowed to cool for 3 hours before activation with ZnCl 2 solution.The ZnCl 2 solution was prepared by dissolving 100 g of anhydrous ZnCl 2 in 100 mL of distilled water.400g of the carbonized material were mixed with the prepared ZnCl 2 solution to obtain an impregnation ratio of 1:4.That is 1 part of ZnCl 2 to 4 parts of carbonized mango seed endocarp by mass.

Adsorption studies
Batch kinetic studies were carried out by adding a fixed amount of MSEAC of 0.8 g (size 150 µm-850 µm) into 500 mL flasks containing 100 mL of textile wastewater obtained from Angel Spinning Textile Industry, Kano.The flasks were agitated in a flocculator at 90 rpm at room temperature (± 34 0 C) at preset time intervals of 10 min, 20 min, 30 min, 40 min, 50 min, 60 min and 70 min respectively.The initial concentration of colour, pH, and carbon dose adopted were respectively 512 Pt-Co units, 7.6 and 0.8g.Colour concentrations were measured by a DR 2000 UV spectrophotometer (HACH Company, USA) according to the standard method specified in the manual.The amount of colour adsorption per unit mass of adsorbent, q t (Pt-Co/g) was calculated according to Equation ( 1 where C e (Pt-Co) is the liquid-phase concentration of colour at equilibrium.

Adsorption Isotherm Modelling
The equilibrium adsorption isotherm is one of the most important approach to understand the mechanism of adsorption systems.The Langmuir (1918), Freundlich (1906) and Temkin (Temkin and Pyzhev, 1940) isotherm equations were used to interpret the experimental data.
Langmuir isotherm theory is based on the assumption that adsorption is on a homogeneous surface, i.e., the surface consists of identical sites, equally available for adsorption and with equal energies of adsorption, and that the adsorbent is saturated after one layer of adsorbate molecules forms onto the surface The linearized form of the Langmuir adsorption isotherm equation is expressed as in Equation ( 3 where, q = mass of solute adsorbed/mass of adsorbent; q n = mass of solute adsorbed/mass of adsorbent for a complete monolayer, c = concentration of solutes in solution; and K a = constant related to enthalpy of adsorption (Langmuir (1918).The Langmuir constants; are obtained by plotting c/q against c.The slope, a =1/ q n and intercept, b = 1/ K a q n .
The Freundlich isotherm equation is an empirical expression that encompasses the heterogeneity of the surface and the exponential distribution of sites and their energies and is expressed as: where, C is the equilibrium concentration of the solute (mg/L) and q is the equilibrium adsorption capacity (mg/g).K F and 1/n are constants representing the adsorption capacity and the intensity of the adsorption respectively and can be calculated from the linear plot of ln q versus ln C. The slope (1/n) measures the surface heterogeneity.Heterogeneity becomes more prevalent as 1/n gets closer to zero (Valix et al., 2004).
The Temkin isotherm equation assumes that the heat of adsorption of all the molecules in the layer decreases linearly with coverage due to adsorbent-adsorbate interactions, and that the adsorption is characterized by a uniform distribution of the binding energies, up to some maximum binding energy.The linearized Temkin isotherm is given in Equation ( 6 where, B = RT/b.K T is the equilibrium binding constant (L/mg) corresponding to the maximum binding energy and b is a constant related to the heat of adsorption.R is the gas constant (8.314J/mol K) and T is the absolute temperature (K).A plot of q versus ln C enables the determination of the isotherm constants K T and b from the intercept and slope respectively.

Adsorption Kinetics Modelling
The kinetics of adsorption data was processed to understand the dynamics of adsorption process in terms of the order of rate constant.The rate constants of chemical adsorption of colour from the textile wastewater were determined using the pseudo-first order, pseudo-second order, Elovich (Chien and Clayton, 1998), and the intra-particle diffusion (Weber and Morris, 1963) models.
The pseudo-first-order rate expression based on solid capacity is generally expressed as follows (Lagergren, 1898) where, is the rate of adsorption, q e is the amount of adsorbate adsorbed at equilibrium (mg/g), q t is the amount adsorbed at any time t (mg/g), k 1 is the rate constant of first order adsorption (l/minutes).After integration and applying boundary conditions, t = 0 to t and q t = 0 to q t ; Equation (7) becomes: ..................
Values of adsorption rate constant (k 1 ) for the adsorbate are determined from the straight line plot of log (q e -q t ) against t.A high value of the coefficient of determination is an indication that the rate of removal of the adsorbate by the adsorbent follows the pseudo-first-order equation.
The pseudo-second-order equation is also based on the sorption capacity of the solid phase.It predicts the behaviour over the whole range of data.Furthermore, it is in agreement with chemisorption being the rate controlling step and is expressed as (Ho et al., 2000) Equation ( 14) does not have the disadvantage of the problem with assigning an effective q e .If pseudo-second order kinetics is applicable, the plot of t/q t against t of Equation ( 14) should give a linear relationship from which q e and h values will be determined from the slope and intercept of the plot respectively and there is no need to know any parameter beforehand (Ho and McKay, 1998).k 2 is then determined from Equation ( 13).
The Elovich model is mainly applicable for chemisorption kinetics.The equation is often valid for systems in which the adsorbing surface is heterogeneous (Sivakumar and Palanisamy, 2009) To simplify the Elovich equation, Chien and Clayton (1998) assumed that αβt>>1 and integrating Equation ( 15) for the boundary conditions, qt = 0 at t = 0 and q t = q t at t = t yields in linear form: where, α is the initial adsorption rate (mg/g minutes) and β is the desorption constant and is related to the extent of surface coverage and the activation energy for chemisorptions (g/mg).q t , q e and t are as defined under Equation ( 7).A plot of q t against ln t gives a linear trend with a slope of (1/ β) and an intercept of 1/ β ln (α β).
The most commonly used technique for identifying the mechanism involved in the adsorption process is by using intra-particle diffusion model as proposed by (Weber and Morris, 1963) where, K d is the intra-particle diffusion rate constant.q t and t are as defined under Equation ( 7).If intra-particle diffusion occurs, then a plot of q t against t 1/2 will be linear and the line will pass through the origin if intra-particle diffusion was the only rate limiting parameter controlling the process.Otherwise, some other mechanism such as external mass transfer is also involved.Values of I give an idea about the thickness of the boundary layer.

Model Verification
To test the reliability of the developed models for prediction of the adsorption process, a chi-squared (χ 2 ) test was performed according to Equation (18) thus: .......... (18) where, N = independent observations in the sample and k =1 is the population parameter.N = 7 for both the kinetic and equilibrium data.The results of the chisquared test are presented in Table 4.

Characteristics of Textile Wastewater
The results from the characterization of Textile waste water are presented in Table 1.The concentrations of BOD and COD were higher than values specified by National Environmental Standards Regulation and Enforcement Agency (NESREA).Other parameters analysed were within acceptable limits specified by NESREA except, the five-day dissolved oxygen concentration.

Adsorption Isotherms Results
The equilibrium adsorption isotherm is one of the most important data to understand the mechanism of adsorption systems.Values of all the isotherm constants as obtained are shown in Table 2 along with the values of the coefficient of determination (R

2
).The Langmuir model fitted the experimental data better than the other isotherm models given the higher values of R 2 obtained.Conformation of the experimental data to the Langmuir isotherm indicates the formation of a monolayer of colour molecules at the outer surface of the mango seed endocarp activated carbon.It further implies that, the adsorbed molecules cannot migrate across the surface or interact with neighbouring molecules (an indication of chemisorption).Similar observations were reported by Bello et al. (2010) on the adsorption of acid orange 10 dye onto treated sawdust and Namasivayam and Kavitha (2002)] on the adsorption of Congo red dye on treated sawdust from coir pith.

Adsorption Kinetics Results
The intra-particle diffusion plot presented in Fig. 1 indicates that the linear plot for a wide range of contact times did not pass through the origin.This deviation from the origin may be due to the variation of mass transfer in the initial and final stages of adsorption as reported by (Sivakumar and Palanisamy, 2009).This indicates that although intra-particle diffusion was involved in the adsorption process, it was not the sole rate-controlling step.This also confirms that adsorption of colour on the adsorbent was a multi-step process, involving adsorption on the external surface and diffusion into the interior.The kinetic parameters for the adsorption of colour from the textile wastewater were calculated as presented in Table 3.The coefficient of determination (R 2 ) for the pseudo-second-order kinetic model was higher than the other kinetic models tested implying that the adsorption system studied obeys the pseudo second-order kinetic model.This suggests that the rate-limiting step in these biosorption systems may be chemisorptions involving valence forces through sharing or exchange of electrons between adsorbent and the adsorbate as reported by (Ho and McKay, 1999).Langmuir isotherm c/q = c/q n + k a /q n q n = 588.

Table 1 :
Characteristics of Textile Wastewater

Table 2 :
Isotherm parameters for the removal of colour by MSEAC. 2)