Kinetics studies on the adsorption of Methyl Orange and Metanil Yellow onto bottom ash: a comparative account

Rapid industrialization and technological amplification are the major reasons for the inlet of pollutants into water systems, especially coloring materials (Mittal et al. 2009; Chequer et al. 2013; Gupta et al. 2015). The discharge of inefficiently treated textile effluent into aqueous bodies adversely affects the aquatic ecosystem by lowering photosynthetic activity and light penetration (Vijayakumar et al. 2012). Some textile dyes are carcinogenic and mutagenic, which can cause acute impairment in humans such as malfunction of the vital organs like liver, kidney, brain, reproductive system and most importantly the central nervous system (Laing 1991; Tsai et al. 2004; Qadeer & Akhtar 2005). Several chemical, physical and biological processes have been employed for the treatment of textile effluent, which include coagulation, chemical oxidation, photocatalytic degradation, microbial biodegradation, membrane filtration, catalysis, membrane separation, ozonation and advanced oxidation processes (de Souza et al. 2010; Moghaddam et al. 2010; Saracino et al. 2018; Kalia et al. 2019). However, most of these technologies are not cost effective or efficient enough in treating textile waste water. The adsorption process therefore seems to have high potential for wastewater treatment in industries because of its ease and efficiency in the elimination of pollutants and also due to its cost effectiveness.

Some cheaper materials have been investigated for the effective degradation of toxic dyes from waste waters which offer an advantage over the most widely used adsorbent, activated carbon, for example orange and banana peels, teak bark, neem leaves, and agricultural residues (Kehinde et al. 2009; Santhi et al. 2016; Uddin et al. 2017; Odoemelam et al. 2018; Li et al. 2019). The research presented in the paper highlights the performance of an industrial waste, bottom ash, as an adsorbent and compares kinetics and mechanistic parameters of two hazardous textile azo dyes for adsorption over this material.

The novelty of the current research work lies in evaluation of the adsorption capacity of a thermal power-plant waste, bottom ash, for the removal of Metanil Yellow and Methyl Orange from waste water, employing kinetic investigations.

Methyl Orange (C₁₄H₁₄N₃NaO₃S; molecular weight 327.33) and Metanil Yellow (C₁₈H₁₄N₃NaO₃S; 375.38), water soluble azo dyes were acquired from Merck. Other necessary reagents used for the experimentation were of analytical reagent (AR) grade. The test solutions were prepared using distilled water and the pH of the experimental solutions was regulated using a pH meter (Hanna Instruments, model no. HI 8424). The change in absorbance values of the dye solutions was assessed spectrophotometrically using a UV-Vis spectrophotometer (Systronics, model no. 117). Table 1 presents the chemical properties of the dyes under investigation.

Grey-black colored granular and porous bottom ash was procured from Bharat Heavy Electrical Limited (BHEL), Bhopal. A Philips SEM 501 scanning electron microscope and Philips X-ray diffractophotometer using nickel-filtered CuKα radiation were employed for gathering information regarding the morphology and nature of the adsorbent. Further physical analysis of the adsorbent was carried out by using specific gravity bottles, surface area analyzer and mercury porosimeter.

Bottom ash was first cleansed with distilled water and was left for drying. The dehydrated material was further treated with hydrogen peroxide (H₂O₂) for 24 h to expel the organic impurities. The adsorbent was then washed again with distilled water and was kept in a hot air oven for the optimum time to remove moisture. Further bottom ash was activated by keeping it in a furnace maintained at 500 °C followed by sieving to get the desired particle size. The activated adsorbent was then preserved in desiccators for further use.

Adsorption kinetics was investigated by understanding the influence of factors like the effect of sieve sizes 36, 100 and 170 BSS mesh, adsorbent dosage (0.01–0.05 g), adsorbate concentration (1 × 10⁻⁵ to 10 × 10⁻⁵ M), contact time (30–330 min) and temperature (30–50 °C) on the rate of adsorption of the two azo dyes. Kinetics during the adsorption of the dyes onto bottom ash was examined at known dye concentrations in the pH range 2–10 and at different temperatures. Two separate 100 mL volumetric flasks containing 25 mL of test solutions having concentration 10 × 10⁻⁵ M for each adsorption system were employed at pH 3. Each solution was filtered after a fixed interval of time and was analyzed spectrophotometrically.

Linear graphs of t/q_t against t were plotted from which values of k₂ and q_e were obtained from the intercept and slope of the graph.

B_t values (B_t is the time constant after time t) obtained from Reichenberg's table assisted in plotting the graphs to distinguish the type of diffusion during the adsorption process (Reichenberg 1953).

The adsorbent is constituted of the following: 15% moisture, 45.4% SiO₂, 10.3% Al₂O₃, 9.7% Fe₂O₃, 15.3% CaO and 3.1% MgO. The scanning electron microscope photograph of activated bottom ash (Figure 1) revealed the surface and pore properties of the adsorbent along with its adsorptive nature. Differential thermal analysis (DTA) curves indicated that bottom ash was thermally stable and showed negligible weight loss even at high temperatures. The presence of mainly alumina (Al₂O₃), gypsum (CaSO₄.2H₂O), beverite (Pb(Cu,Fe,Al)₃(SO₄)₂(OH)₆), borax (Na₂B₄O₇.10H₂O) and kaolinite (2(Al₂Si₂O₅(OH)₄)) was confirmed by the d-spacing values provided by the X-ray spectrum of the adsorbent. Infrared spectrophotometric (Figure 2) study of bottom ash exhibited a sharp absorption band in the region of 3,700–3,500 cm^–1. The bands at 3,467, 2,930, 2,676, 1,502, 1,097 and 790 cm^–1 indicated the presence of laumonite, amber, mulite, azurite, bavenite and kaolinite (2(Al₂Si₂O₅(OH)₄)) in the adsorbent.

In order to optimize the amount of adsorbent for removal, process tests were performed under predetermined conditions and the rate of the adsorption process was found to increase with the increase in the adsorbent dosage. Results showed that on increasing adsorbent dosage from 0.01 g to 0.05 g, adsorption of Methyl Orange increased from 11 × 10⁻⁵ g to 18 × 10⁻⁵ g from 30 to 50 °C respectively whereas Metanil Yellow adsorption increased from 42 × 10⁻⁵ g to 55 × 10⁻⁵ g at 50 °C. The optimum dosage for both the systems was found to be 0.05 g, as above this dose not much obvious adsorption was obtained. The statistical data obtained during investigation are presented in Table 2. It was also noted that with the rise in temperature, adsorption decreased, indicating an exothermic nature for both the studied systems. Further, the half-life of the process was evaluated for affirming the dependence of the rate of adsorption over different dosages of adsorbent (Sekar et al. 2004; Mittal et al. 2007). Investigations revealed the half-life for 0.05 g of adsorbent of 100 BSS mesh as 12.31 h for Methyl Orange and 8.64 h for Metanil Yellow.

Contact time studies assisted in calculating the adsorption capacities of the dye at different time intervals with fixed adsorbent dosage (0.05 g) at three distinct temperatures (30, 40 and 50 °C). It was shown that for the Metanil Yellow–bottom ash system, 4 h of contact time was enough to acquire about 50% adsorption (Figure 3), while for the Methyl Orange–bottom ash system, saturation during adsorption was obtained in 5 h (Figure 3).

The adsorption process for Methyl Orange–bottom ash and Metanil Yellow–bottom ash was investigated at concentrations ranging from 1 × 10^–5 M to 10 × 10^–5 M at a fixed pH and at different temperatures (30, 40 and 50 °C). The trend is confirmed from the calculated values of the amount adsorbed as percentage adsorption at higher concentration varied from 56.87% to 51.50% and 61.00% to 34.70% at 30 °C and 50 °C, respectively, for the Methyl Orange–bottom ash and Metanil Yellow–bottom ash systems.

The k_ad values obtained for the two adsorbate–adsorbent systems are given in Table 3. It was found that the rate constant decreased for Methyl Orange as the temperature increased from 30 to 40 °C and the remained constant. For Metanil Yellow, the rate constant decreased with increase in temperature from 30 to 40 °C and then increased in moving to 50 °C.

Figure 4 represents comparative graphical elucidation of the data for the pseudo-second-order model for both the systems. The results obtained for the amount of dye absorbed at equilibrium along with correlation coefficients of both the dye adsorption systems have been provided in Table 4.

Thus on the basis of the procured results, the pseudo-second-order rate expression successfully explained both the adsorption processes.

Based on the values of fractional attainment, B_t versus time plots were plotted to discriminate between the film diffusion and particle diffusion adsorption rates. For the Methyl Orange–bottom ash system, straight lines passed through the origin, while in case of Metanil Yellow the straight lines did not pass through the origin (Figure 5). The graphical plot indicated particle diffusion where external transport of the ions is less than the internal transport in the case of the Methyl Orange over bottom ash, while film diffusion dominates in the case of the Metanil Yellow–bottom ash system, where transport of the dye occurs on the surface of the adsorbent. The slopes of the B_t versus time graph (Figure 5) assisted in computing D_i values for both the adsorbates at different temperatures.

It was evident that there was an increase in mobility of ions since D_i increased with increase in temperature suggesting the diminishing of retarding forces on diffusion of ions with the rise in temperature. The values of D_o, E_a and ΔS^# were assessed using the above relations and are presented in Table 5.

It is clear from the kinetic studies that bottom ash acts as an effective adsorbent for the removal of both the soluble azo dyes from waste water. For both the adsorption processes (Methyl Orange–bottom ash and Metanil Yellow–bottom ash system) it is obtained that with increase in the amount of adsorbent the percentage as well as the rate of adsorption increases. The saturation in the adsorption for the Methyl Orange–bottom ash system and Metanil Yellow–bottom ash system was obtained in about 4 h and 5 h respectively. The second-order rate expression is applicable in both adsorption processes. Particle diffusion was operational in the case of Methyl Orange over bottom ash while film diffusion is operative in the case of the Metanil Yellow–bottom ash system.