Effectiveness of termite hill as an economic adsorbent for the adsorption of alizarin red dye

The adsorption of alizarin red (AR) dye onto termite hill sample (THs) was investigated. Prior to the adsorption studies, the elemental, morphological, surface and structural properties of THs were examined by modern analytical methods. Instrumental analysis showed that the homogenous microstructured THs are comprised of iron oxide, silica oxide, and alumina as major components. Experiments showed that the adsorption capacity of AR decreases with increasing pH and initial AR concentrations, and increases with increasing contact time, stirring speed and temperature. The equilibrium study obeyed the Langmuir adsorption model and the kinetics followed the pseudosecond-order model. About 95.0% AR reduction (1.425 mg/g) was achieved when 0.8 g of THs was mixed with 30 mL of 40 mg/L AR solution for 120 min at 400 rpm and a pH of 2. Thermodynamic study suggested that AR adsorption onto THs is spontaneous at higher temperatures of 323 K and above (ΔG values are negative). However, ΔG are positive at lower temperatures of 293–313 K, which implies that the adsorption process is not spontaneous at these temperatures. This study showed that THs could be used as alternative, low-cost, natural adsorbents for the removal of dyes from wastewater. This is an Open Access article distributed under the terms of the Creative Commons Attribution Licence (CC BY 4.0), which permits copying, adaptation and redistribution, provided the original work is properly cited (http://creativecommons.org/licenses/by/4.0/). doi: 10.2166/wrd.2018.026 ://iwaponline.com/jwrd/article-pdf/9/1/83/523048/jwrd0090083.pdf Olushola S. Ayanda (corresponding author) Simphiwe M. Nelana Eliazer B. Naidoo Department of Chemistry, Vaal University of Technology, Vanderbijlpark 1900, South Africa E-mail: osayanda@gmail.com Olushola S. Ayanda Habibat Adubiaro Oluwapese T. Ebenezer Nanoscience Research, Department of Industrial Chemistry, Federal University Oye Ekiti, P.M.B 373, Oye Ekiti, Ekiti State, Nigeria Olusola S. Amodu Department of Chemical Engineering, Lagos State Polytechnic, P.M.B. 21606, Lagos, Nigeria Godwin O. Olutona Department of Chemistry and Industrial Chemistry, Bowen University, Osun State, Nigeria


INTRODUCTION
In numerous industries, such as cosmetics, textiles, cotton, paper, plastics, leather, pharmaceuticals and food, dyes are used to colour the products (Afkhami & Moosavi ).
Consequently, large amounts of coloured wastewater are generated. The presence of dyes in wastewaters raises tremendous concerns: the presence of dyes in water, even at very low concentration, might lead to reduction in the aesthetic value of water. The degradation by-products of some organic dyes are potentially toxic, carcinogenic, mutagenic and allergenic to marine life (Chung & Cerniglia ).
Besides, the presence of these dyes in the environment may lead to collapse of some dyeing industries due to non-compliance with quality standards specified by the ISO 14000 certification. Alizarin is an anthraquinone originally derived from the root of the madder plant (Ghaedi et al. ). Synthetic dyes, such as Alizarin Red S (AR-S), belong to the most durable dyes; they cannot be completely degraded by general chemical, physical and biological processes (Gautam et al. ). This is attributed to their complex aromatic structures, which give them physicochemical, thermal and optical stability. Different methods such as adsorption, coagulation, advanced oxidation, and membrane separation are used in the removal of dyes from wastewater (Gupta ). Freundlich isotherm models, and that approximately 83.0% of AR-S was removed at lower temperature. Ramesh et al. () studied the adsorption of AR dye onto calcium hydroxide as low-cost adsorbent. It was reported that calcium ions have very strong affinity and bind effectively to AR dye during staining of biomaterials. The authors also reported that the adsorption of AR dye on calcium hydroxide is endothermic and occurs spontaneously.
It was stated that the adsorption process is effective at pH 12 (60 C) and the kinetic parameters indicate that the adsorption phenomena is of monolayer type and fit well the pseudo-second-order rate equation.
Abdus-salam & Buhari () studied the adsorption of AR dye onto activated carbon from mango seed; the adsorption of AR dye was described as rapid for the first 15 min of agitation with 86.90% removal, and the adsorption equilibrium was achieved in 90 min of agitation with 90.44% AR dye removal. The adsorption data fitted well to the pseudo-second-order kinetic models and Langmuir isotherm.
Furthermore, Ghaedi et al.  used multi-walled carbon nanotubes as adsorbents for the kinetic and equilibrium study of the removal of AR-S. The use of activated carbon, nanomaterials and composites for the treatment of wastewater is relatively expensive when compared with natural adsorbents or wastes. Therefore, the exploration of readily available natural adsorbents that will effectively remediate wastewater with or without the initial treatment processes is necessary.
Termites are insects belonging to the Isoptera order, with around 2,800 species known worldwide (Araujo et al.

MATERIALS AND METHODS
Chemicals, termite hill samples, and instrumentation AR ( Figure 1) was purchased from PS Park Scientific Limited, Northampton, UK. Stock solution containing 1,000 mg/L AR dye was prepared by dissolving 1.0 g of AR dye in 1,000 mL of deionized water, and stored in the dark at 4 C. Working solutions were prepared daily using serial dilution. The TH sample was mined at Oye-Ekiti, Ekiti State, Nigeria. The sample was air-dried in the laboratory at room temperature. The sample was placed in an oven at 60 C for 3 hr before crushing and grinding in a mortar. It was sieved with a mesh of 0.5 mm pore size. The ash content, moisture content and the pH of the sample (in 50 mL distilled water) were carried out and reported elsewhere (Ayanda et al. ). After each of the experiments, an aliquot was withdrawn and a UV/visible spectrophotometer was used to determine the concentration of AR (λ max ¼ 450 nm). The percentage AR dye removal was calculated with Equation (1) and the amount of AR dye adsorbed (q e ; mg of AR dye per g THs) was calculated using Equation (2).
where C o and C e (mg/L) are the initial and equilibrium concentrations of the AR dye solution, respectively, V (mL) is the volume of the solution and W (g) is the mass of THs used.

Kinetic, equilibrium and thermodynamics models
The linearized form of the pseudo-first-and pseudo-secondorder kinetic models are presented in Equations (3) and (4), respectively.
where q e is the equilibrium amount of AR dye adsorbed per unit mass of THs (mg/g), q t is the amount of AR dye adsorbed per unit mass of THs at time t (mg/g), k 1 and k 2 are the pseudo-first-and pseudo-second-order adsorption rate constants, respectively, and t is time (min).
For the adsorption process obeying the pseudo-firstorder kinetic model, a plot of log 10 (q e À q t ) against t will give a straight line graph with the q e (mg/g) and k 1 obtained from the intercept and slope of the graph (Equation (3)), respectively. However, a straight line graph of a plot of t/q t against time t (Equation (4)) is an indication that the adsorption process follows the pseudo-second-order kinetic model, and k 2 and q e (mg/g) are calculated from the intercept and slope of the plot, respectively.
The linearized form of the Langmuir () and Freundlich () models are presented in Equations (5) and (6), respectively.
where C e , q e , q m and n are the equilibrium concentration of given as Equation (7). An isotherm is favourable if 0< Conversely, for an adsorption process obeying the Freundlich isotherm, the plot of log 10 q e against log 10 C e will be linear with K F and n obtained from the intercept and slope, respectively. A value of n between 1 and 10 indicates a favourable adsorption process.

Elemental investigation of THs
The EDS spectrum of THs is as shown in Figure 3. are kaolinite and quartz.

FTIR examination
As shown in Figure 4, the wavenumbers 3,620.51 cm À1 and 3,693.04 cm À1 are typical absorption bands for kaolin (Saikia & Parthasarathy ). The band at 3,620.51 cm À1 shows the presence of inner hydroxyls in the THs while the bands at 3,693.04 cm À1 could be assigned to Al-OH or The BET surface area of THs was recorded as 35.36 m 2 /g; this is higher than the surface area of the uncalcinated THs (28.40 m 2 /g) reported by Fufa (). The difference in the surface area might be due to differences in the sizes of the THs particles resulting from different methods of sample preparation. As expected, the BET surface area of treated or calcined THs will be higher.

Effect of adsorbent dosage and initial concentration
The       The effect of temperature on AR dye adsorption onto THs was studied at temperatures ranging from 293 K to 339 K. It was observed that the higher removal due to increasing temperature (Figure 8) may be attributed to an increase in the mobility of the AR dye molecules. The increase in the percentage of AR dye adsorbed from 37.5% to 57.5% with increasing temperature may suggest an increasing accessibility of the AR dye molecules to the THs active sites. Thus, the adsorption process is endothermic.

Effect of contact time and stirring speed on AR dye adsorption
It was observed from Figure 9 that the percentage removal of the AR dye increases from 45.0% (0.68 mg/g) to 60.0%

Isotherm, kinetic and thermodynamic studies
The Langmuir isotherm is used to determine whether the adsorption process occurs through a monolayer formation.   Table 2.
The relation between R L and C o to represent the features of the Langmuir isotherm for THs is shown in Figure 12.
The R L for the initial AR concentrations are found in the range 0.00015-0.0015 which suggests a favourable adsorption of AR dye onto THs under the conditions of the experiment.
The Freundlich isotherm assumes that the dye uptake occurs on a heterogeneous surface by multilayer adsorption and that the amount of the adsorbed adsorbate increases with an increase in adsorbate concentration. The equilibrium adsorption plot shows that AR dye adsorption onto THs does not suit the Freundlich isotherm due to the low regression coefficient value of 0.8573 ( Figure 13). K F and n obtained are also presented in Table 2.
The pseudo-first-order and pseudo-second-order kinetics as tested on the adsorption of AR dye onto THs are presented in Figures 14 and 15, respectively. The adsorption kinetics followed the pseudo-second-order equation, having a higher regression coefficient value of 0.9729; this showed that chemisorption is the main rate limiting step.          obeyed the Langmuir adsorption model, while the adsorption kinetics followed the pseudo-second-order equation.
The thermodynamic parameters showed that the adsorption process is endothermic and spontaneous at 323 K and above. Thus, AR adsorption onto THs has good feasibility as a process at high temperature. The effectiveness of THs for the removal of AR dye (95.0% AR removal) may be credited to the elemental composition of THs, such as the presence of Fe, Si and Al oxides. The treatment of real dye wastewater with THs will be carried out in future to compare the results with the simulated dye solution, as the presence of additives may affect the efficiency of adsorption.