Adsorption of Congo Red Dye from Aqueous Solution using ZnO and Al2O3/ZnO Composite: Isotherm, Kinetic and Thermodynamic Data

Herein, an adsorption study of congo red (CR) dye onto ZnO and Al2O3/ZnO is reported. ZnO was prepared using conventional chemicals by co-precipitation method and Al2O3 used in the composite was prepared through recycling of aluminium waste. Information about the materials was obtained through spectroscopic techniques. A batch adsorption method was used to obtain the adsorption data from which isotherm, kinetic and thermodynamic parameters were obtained. The result of the adsorbents characterisation revealed the expected properties of the prepared materials. The adsorption capacities at 250 mg/L of CR dye were 24.33 mg/g and 24.57 mg/g for ZnO and Al2O3/ZnO respectively. The isotherm study of the adsorption process revealed that Langmuir model fitted best the adsorption data with monolayer adsorption capacities (qm) of 27.67 mg/g and 33.39 mg/g for ZnO and Al2O3/ZnO respectively. The adsorption was rapid within the first 15 min and the equilibrium was reached at 45 min. The kinetic study followed a pseudo-second-order model with the rate constant of 0.049 and 0.093 g.mg.min for ZnO and Al2O3/ZnO respectively. Experimentally, the process was endothermic and was supported by the positive values of enthalpy (ΔH) with positive values of entropy (ΔS). The change in free energy (ΔG) is negative at all temperatures studied, indicating spontaneity but more spontaneous for Al2O3/ZnO than ZnO. The adsorption of CR dye from an aqueous solution onto ZnO as an adsorbent can be slightly improved upon by the introduction of Al2O3 to form Al2O3/ZnO composite. DOI: https://dx.doi.org/10.4314/jasem.v26i3.10 Open Access Article: (https://pkp.sfu.ca/ojs/) This an open access article distributed under the Creative Commons Attribution License (CCL), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Impact factor: http://sjifactor.com/passport.php?id=21082 Google Analytics: https://www.ajol.info/stats/bdf07303d34706088ffffbc8a92c9c1491b12470 Copyright: Copyright © 2022 Bello et al Dates: Received: 10 January 2022; Revised: 22 February 2022; Accepted: 15 March 2022

Water is an indispensable resource that is essential for life development (Lellis et al., 2019). Its contamination or pollution poses a serious threat to the environment and consequently affects plants, animals and human lives (Gupta and Saleh, 2013). Water contamination and poor sanitation are linked to the transmission of diseases such as cholera, typhoid, dysentery and hepatitis A (Ashbolt, 2004). Water pollutants are mainly organic and inorganic pollutants that are discharged from industrial effluents and sewage into the water bodies (Wasewar et al., 2020). Organic pollutants are primarily of carbon and hydrogen compounds which include dye, phenolic compounds, petroleum, surfactants, pesticides, and pharmaceuticals (Chowdhury et al., 2016). Dye is one of the major contaminants of great concern because of its complex structures which make them more resistant to biodegradation (Abdullah et al., 2018;Iqbal et al., 2021). Synthetic dyes are increasingly used in manufacturing operations such as leather, textiles, cosmetics, paper, printing, electroplating, plastics, and pharmaceuticals as industrial civilization progresses (Zhang et al., 2018). They find their way into water bodies as a result of indiscriminate discharge of the effluents of the aforementioned industries. Water contamination by dye-laden effluent is easily observed due to its colour even at very little concentration. Dyes can also irritate the skin and induce allergic dermatitis. Several of them are carcinogenic and mutagenic to aquatic creatures (Meroufel et al., 2013). Congo red (CR) dye is a water-soluble anionic dye that is commonly used in the textile, paper, printing, leather, and plastic industries. Its principal negative effects are irritation of the eyes and skin (Malik et al., 2020).  Long-term negative effects of ingestion of congo red dye are the destruction of the blood system, liver, and hematopoiesis, causing health problems such as breathing difficulties, diarrhoea, vomiting, and nausea (Munagapati and Kim, 2017). Among the conventional treatment methods of industrial effluent which include flocculation, coagulation and adsorption (Kannan and Meenakshisundaram, 2002), adsorption is often regarded as one of the most appealing techniques due to its great efficiency, ease of operation, and low cost, (Chikri et al., 2020;Dutta et al., 2021). The most commonly used adsorbent for the treatment of effluents is activated carbon. However, there is a high cost of production associated with the use of activated carbon (Ali et al., 2012). This has led to the investigation of different materials by researchers to be used as low-cost and effective adsorbents. Metallic oxides such as iron oxide, zinc oxide alumina, silica, clay and zeolite have been reported as adsorbents for the removal of pollutants from water. Zinc oxide (ZnO) is a low-cost material and that can be produced on a large scale (Zhang et al., 2013). ZnO has received significant attention owing to its exclusive electrical, optical, and chemical properties (Nassef et al., 2020). It has been applied to the removal of dyes like methylene blue (Nassef et al., 2020), reactive blue and acid black (Khoshhesab and Souhani, 2018), congo red (Nayak et al., 2020) from aqueous solutions. Similarly, alumina is a common adsorbent that is environmentally stable (Asim et al., 2019). It exhibits a large surface area, high mechanical properties, and good resistance to heat (Reyes-lópez et al., 2021). Adsorption of some dyes such as methylene blue , diresul black (Asim et al., 2019), orange G (Banerjee and Chattopadhyaya, 2017), orange 7 (Khosla et al., 2013) among others using alumina has been reported. This background information about the adverse effects of dye on environmental health together with the wide application of zinc oxide and alumina towards dye removal from aqueous solution instigated this present study. Herein, we report the adsorption study of a composite of zinc oxide and alumina (Al2O3/ZnO) as an adsorbent in comparison with zinc oxide alone.

MATERIALS AND METHODS
Materials: Aluminium cans were collected as municipal waste for the preparation of alumina. All chemicals used were of analytical grade. Zinc nitrate, sodium hydroxide, hydrochloric acid and congo red dye were purchased from BDH chemicals.
Preparation of ZnO: A modified method reported by Romadhan et al., (2016) and Suntako, (2015) was adopted. In a typical preparation, 250 ml of 0.5 M of zinc nitrate hexahydrate solution was measured and transferred into a beaker with a magnetic bar placed inside, 1 M of sodium hydroxide was added stepwisely until a pH of 13 was reached. This reaction was stirred constantly on a magnetic stirrer for 30 min at room temperature. The mixture was filtered, the residue was washed severally with aqueous ethanol and dried in an oven to obtain zinc oxide (ZnO) powder.
Preparation of Al2O3 from aluminium can: A slightly modified method reported by Chandra Shil, (2016) was employed. Herein, the colour inscription on the can was removed and then cut into smaller pieces. A 1 g of the pieces of aluminium was weighed and transferred into a beaker. This aluminium was leached with 100 ml of 6 M HCl in a conical flask on a magnetic stirrer in which Hydrogen gas (H2) was evolved.
The obtained aluminium trichloride was filtered to remove the thin layer which was used to coat the aluminium, the filtrate was placed on a magnetic stirrer and 100 ml of 3 M NaOH was added step-wisely to form whitish precipitates of aluminium trihydroxides 2Al(OH)3. The obtained precipitates were filtered and washed with deionized water. It was then dried and calcined at a temperature of 450 o C for 2 hr to yield aluminium oxide (Al2O3) in an electric furnace.
Preparation of Al2O3/ZnO composite: The composite was prepared by adopting a sol-gel method (Lakhera et al., 2015) using the earlier prepared Al2O3 and ZnO materials. A colloidal solution was formed by mixing 95 wt.% of ZnO and 5 wt.% of Al2O3 in deionized water. Each of the materials was dissolved separately in 50 ml of deionized water, stirred for 30 min and was added together with constant stirring on a magnetic stirrer at room temperature for 45 min. The mixture was filtered, washed and dried to obtain the Al2O3/ZnO composite.
Batch adsorption experiments: A batch adsorption method was employed (Abdus-Salam and Adekola, 2018). In acquiring typical adsorption data, 0.2 g of adsorbent was contacted with 20 ml of CR dye in a 50 ml conical flask reactor. The reactors were agitated on a mechanical shaker for a period of 2 hr at 25 o C. At equilibrium, the supernatant liquids were filtered and the filtrates were analyzed using UV
Where is the quantity adsorbed (mg/g), is the quantity adsorbed at any time t (mg/g), and are the initial and equilibrium concentration respectively (mg/L), m is the mass of adsorbent (g) and V is the volume of adsorbate (L) Isotherm study: The following isotherm models were used: Langmuir, Freundlich, Temkin and Dubinin-Radushkevich models.  Where qe and qt (mg/g) are the amounts of CR dye adsorbed at equilibrium and at time t (min), respectively; k1 (min -1 ) is the adsorption rate constant for pseudo-first-order; k2 (g m/g min) is the rate constant of pseudo-second-order adsorption; ki (mg/g.min 0.5 ) is the intra-particle diffusion rate constant; C is the intercept related to the thickness of the boundary layer; α (mg/g.min) is the initial sorption rate and β (g/mg) is related to the extent of surface coverage and activation energy for chemisorption. Kinetic study: Four kinetic models were employed for the treatment of the data, which are pseudo-first-order (PFO), pseudo-second-order (PSO), intra-particle diffusion (IPD) and Elovich models.
Thermodynamic study: The variation in the adsorption equilibrium constant with temperature is usually studied by applying Van't Hoff equation, which is presented in table 3.

RESULTS AND DISCUSSION
Characterisation: Surface functional group: The FTIR spectra of ZnO and Al2O3 are presented in Fig.  1. The surface of the prepared individual ZnO and Al2O3 is characterised by similar functional groups with a slight shift. For ZnO (a), the metal-oxygen bond which suggests Zn-O is observed at 894 and 461 cm -1 . The broad band at 3464 cm -1 is due to O-H stretching of the absorbed water molecule (Sowri Babu et al., 2013). The peak observed for Al2O3 (b) at 3450 cm -1 is due to O-H stretching of the absorbed water molecule. Al-O bonds of boehmite are attributed to peaks at 992, 552 and 455 cm -1 (Liu et al., 2012). The peak at 1634 and 1642 cm -1 for ZnO and Al2O3 respectively indicate -OH bending (Gupta et al., 2011). Similarly, the peaks at 1384 and 1453 cm -1 for ZnO and Al2O3 respectively are likely due to CO2 absorption. Surface morphology and elemental composition: The surface morphologies of ZnO, Al2O3 and Al2O3/ZnO are presented in Fig. 2. Roughly small particles characterised the surface of ZnO while that of Al2O3 showed bigger flake-like particles. The composite, however, showed agglomeration of particles resulting in a big rigid particle on its surface. The elemental compositions as revealed by energy dispersive x-ray (EDX) showed the expected elemental compositions in ZnO and Al2O3. The presence of Al and Zn are also observed in the composite (Al2O3/ZnO).
Adsorption studies: Effect of concentration and isotherm study: The adsorption capacities of ZnO and Al2O3/ZnO increased with an increase in the concentration of CR dye as presented in Fig. 3. The increase in quantity adsorbed by the two adsorbents is due to the increase in the gradient between the adsorbents and the dye molecules as the concentration increases (Akkaya Sayʇili, 2015). Both adsorbents competed favourably for the removal of CR dye molecules from aqueous solutions at a lower concentration between 50 mg/L and 150 mg/L. However, Al2O3/ZnO composite material showed little improvement over ZnO between 200 mg/L and 250 mg/L of CR dye. The maximum adsorption capacities at 250 mg/L are 24.33 mg/g (97.28%) and 24.57 mg/g (97.35%) for ZnO and Al2O3/ZnO respectively. The adsorption of congo red (CR) dye onto ZnO and Al2O3/ZnO can be best described by Langmuir isotherm with the coefficient of correlation (R 2 ) of 0.9972 and 0.9681 respectively ( Table 4). The order of fitness of the adsorption data to the isotherm model is Freundlich < Dubinin-Radushkevich < Temkin < Langmuir for the two adsorbents. The fitness of the adsorption data to the Langmuir model suggests monolayer adsorption (Banerjee and Chattopadhyaya, 2017). The estimated monolayer maximum adsorption capacity (qm) showed that Al2O3/ZnO performed better than ZnO. The adsorption of CR dye onto the adsorbents is favourable as revealed by the value of the dimensionless factor, RL. Value of RL between 0 and 1 is an appropriate condition (Batool et al., 2018). The heat of adsorption from the Temkin model, 5.860 and 7.594 J/mol for ZnO and Al2O3/ZnO respectively suggest that the adsorption of CR dye is physisorption. The position is also supported by the value of adsorption energy (E) obtained for ZnO (2.461 kJ/mol) and Al2O3/ZnO (2.104 kJ/mol) from the Dubinin-Radushkevich model. The extent of energy between 0 and 8 kJ/mol is said to be physisorption while the adsorption is chemisorption when it ranges between 8 and 16 kJ/mol (Das et al., 2014). Comparing the values of qs (mg/g) which is related to the theoretical adsorption capacity, it is also observed that Al2O3/ZnO performed better than ZnO. The Freundlich model suggests that a heterogeneous surface of the adsorbents cannot be ruled out of the process as it also gave significant R 2 values.

Effect of contact time and kinetics study:
The effect of contact time between the adsorbate (100 mg/L of CR dye) and the adsorbents (ZnO and Al2O3/ZnO) is presented in Fig. 4. The adsorption of CR dye onto ZnO and Al2O3/ZnO was rapid at the initial stage of the process (0 -15 min). Thereafter, the rate of adsorption slowed down towards equilibrium at 45 min. The adsorption sites of the adsorbents were readily available at the initial stage thereby making the removal process faster and slowed down when the adsorption sites have been saturated (Banerjee and Chattopadhyaya, 2017;Kurniawati et al., 2021). At 15 min, 8.53 mg/g (85.32%) and 9.26 mg/g (92.57%) of CR molecules were adsorbed onto ZnO and Al2O3/ZnO respectively. The kinetics study using four models (Table 5) shows that pseudo-second-order (PSO) with the highest R 2 values for both adsorbents best described the adsorption kinetics, suggesting physicochemical interaction between adsorbate and adsorbent. Also, the calculated quantities adsorbed (qe, cal) are in closed value with the one obtained experimentally (qe, exp). The rate constant k2 indicates that the adsorption process is faster onto Al2O3/ZnO (0.093 min -1 ) than ZnO (0.049 min -1 ). The higher the rate constant k2, the faster is the reaction (Liu et al., 2019). The good fitting showed by the pseudo-first-order (PFO) model is suggesting that the physical interaction of the adsorbate and adsorbent cannot be overlooked (Bello et al., 2021). The intra-particle diffusion (IPD) model plot did not pass through the origin (Fig. not shown) but the confirmation of its existence is the significant R 2 values for ZnO (0.7235) and Al2O3/ZnO (0.7449). Consequently, intra-particle diffusion is not the only rate-controlling step for the adsorption of CR dye onto the adsorbents (Yakout and Elsherif, 2010). Elovich model which is used to describe chemisorption (Wu et al., 2009) showed the least but moderate R 2 values suggest that chemical interaction cannot be excluded in the process.
Effect of adsorbent dosage: The determination of the best performing adsorbent dosage is important to reduce the wastage of adsorbent and for optimum process efficiency (Iqbal et al., 2021). of the adsorption sites as the quantity of the adsorbent was increased while the concentration of dye molecule remained constant were the likely causes of the reduction (Etim et al., 2016;Zhu et al., 2017).   Effect of temperature and thermodynamic study: The effect of temperature on the adsorption of CR dye (100 mg/L) onto ZnO and Al2O3/ZnO composite is presented in Fig. 6. It was observed that the quantity adsorbed increased with an increase in temperature from 25 to 45 o C, indicating an endothermic process. The endothermic nature of the adsorption process is attributed to an increase in the kinetic energy of the dye molecules and they moved faster towards the adsorption sites (Jiang et al., 2018). A similar observation for adsorption of CR dye onto adsorbent derived from shrimp shells was reported by Zhou et al., (2018). The thermodynamic parameters obtained from van't Hoff plot are presented in Table 6. The thermodynamic study of the adsorption process agrees with the endothermic nature observed in the experiment with positive enthalpy change (ΔH) of 78.9 kJ/mol and 40.94 78.9 kJ/mol for ZnO and Al2O3/ZnO respectively. The lower ΔH for adsorption of CR onto the composite is an indication of its enhanced performance over ZnO. This is supported by the spontaneity of the reaction as suggested by more negative values of ΔG for Al2O3/ZnO than ZnO. However, both are spontaneous within the range of temperature studied. The positive degree of disorderliness (ΔS) for both adsorbent indicates that the adsorption is controlled by the entropy effect (Zhou et al., 2018). Conclusion: The removal of congo red dye from aqueous solution using the prepared ZnO and Al2O3/ZnO composite was achieved by adsorption process. The isotherm parameters suggested physisorption and monolayer interaction between the adsorbate and the adsorbents. Kinetic and thermodynamic data indicated physicochemical, surface interaction, intra-particle diffusion and spontaneous adsorption process. The Al2O3/ZnO composite showed a slight improvement over ZnO alone, indicating the impact of the Al2O3.