Kinetics, Thermodynamics and Isothermal Modelling of Liquid Phase Adsorption of Methylene Blue onto Moringa Pod Husk Activated Carbon

Abstract

This study focused on the liquid phase sorption of methylene blue (MB) using low cost agro-waste from moringa pod husks. Moringa pod husk was carbonized at 450°C for 30 min, which was later activated with 0.1 M phosphoric acid. The physico-chemical properties of moringa pod activated carbon (AMP) were determined. The results of the physicochemical parameters are: moisture content (13.6 ± 0.02), ash content (2.61 ± 0.11), point of zero charge (pH ZPC) (7.2) and bulk density (0.6 g L-1). The sample was also characterized using Brunaeur Emmett Teller (BET), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX) and Fourier-transform infrared spectroscopy (FTIR). The BET surface area obtained for AMP is 1340.234 m 2 g-1. The effect of optimized parameters such as initial concentration, contact time, pH, adsorbent dosage, and temperature on methylene blue removal was investigated. The obtained maximum monolayer adsorption capacity value (q max) is 9.5785 mg g-1. The three adsorption isotherm models, namely Langmuir, Freundlich and Temkin, were employed to describe the fitness of equilibrium data. The Langmuir equation fitted the adsorbent system better with a R 2 value of 0.9958. The pseudosecond-order kinetic equation also fitted the data well. Thermodynamic studies showed that the AMP-MB adsorption system is spontaneous and endothermic as a negative and positive value was obtained for G 0 (-0.460 KJ mol-1) and H 0 (4.482 KJ mol-1), respectively.

A critical review with emphasis on recent pieces of evidence of Moringa oleifera biosorption in water and wastewater treatment

“…For the removal of Diuron, the value of the activation enthalpy (Δ H °) is <40 kJ/mol, which allows us to suggest that the uptake process follows a physisorption mechanism (de Bezerra et al 2020 ). The same behavior was observed in Adebayo et al ( 2019 ). The authors observed the increase in the randomness degree during the adsorption process, which confirmed the physisorption nature of the sorption mechanisms (Adebayo et al 2019 ).…”

“…The same behavior was observed in Adebayo et al ( 2019 ). The authors observed the increase in the randomness degree during the adsorption process, which confirmed the physisorption nature of the sorption mechanisms (Adebayo et al 2019 ). A decrease in the percentage removal of Cr(IV) from 91.40 to 37.93% using Moringa stenopetala seed powder (MSSP) has been observed as temperature increased from 293 to 353 K. Similar observation has been made on adsorption capacities of Cr(VI) ion when the adsorption capacity of MSSP for Cr(VI) was decreased from 1.37 to 0.57 mg/g with increasing temperature from 293 to 353 K. This fall of the adsorbent metal uptake capacity with an increase in temperature might be due to desorption caused by an increase in the available thermal energy.…”

“… Reddy et al ( 2010b ) MOB 30.4 Langmuir The kinetic studies revealed that the biosorption process followed the pseudo-second-order kinetic model. Reddy et al ( 2011 ) MOS 29.6 Langmuir No indication of the mechanism in this work Marques et al ( 2012 ) MOS 412.3 Langmuir Physisorption nature Çelekli et al ( 2019 ) MOH 14.7 Langmuir/sips Physisorption nature (ΔH° < 40 kJ/mol) de Bezerra et al ( 2020 ) MOS 23.1 - No indication of the mechanism in this work but confirm the surface heterogeneity Araújo et al ( 2010a ) MOF 5.6 Langmuir No indication of the mechanism in this work Gautam et al ( 2020a ) MOB 25.2 Langmuir No indication of the mechanism in this work Mnisi and Ndibewu ( 2017 ) MOP and MOH 9.6 Langmuir No indication of the mechanism in this work Adebayo et al ( 2019 ) MOS 238 Langmuir SEM-EDX analysis confirmed an exchange of Mg(II) and K(I) for Cu(II) on MOS and the binding energy for the ion exchange mechanism is 8 to 9 kJ mol −1 Acheampong et al ( 2011 ) SMOS 90.3 % for 25 mg/L of methylene blue - At a pH value of 6.5, the carboxylic groups are deprotonated and are negatively charged. These negatively charged carboxylate ligands are likely to attract the cationic dye species.…”

“…Bearing in mind that this plant MO, especially its seeds MOS, as a good coagulant, is still being used till the current days (Taiwo et al 2020 ; Vega Andrade et al 2021 ). But usually, all the parts of this plant are used as natural biosorbents (Reddy et al 2010a , 2012 ; Vieira et al 2010 ; Acheampong et al 2011 ; Mnisi and Ndibewu 2017 ; Adebayo et al 2019 ; de Oliveira et al 2019 ; Soliman et al 2019 ; Verma et al 2020 ).

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“…Tables 2 and 3 demonstrate that the nature and intrinsic characteristics of MO leaves, seeds, and husks have an effect on biosorption; habitually, the researchers apply pretreatment processes to modify or affect some of their properties inherent in biomaterials in order to promote the biosorption process. According to these researchers, the pretreatment processes that have been frequently practiced by various research groups include grinding, boiling, autoclaving, and acid/alkali processing (Reddy et al 2010a , b , 2011 , 2012 ; Araújo et al 2010a ; Acheampong et al 2011 ; Marques et al 2012 ; Raj et al 2013 ; Mnisi and Ndibewu 2017 ; Adebayo et al 2019 ; Çelekli et al 2019 ; de Oliveira et al 2019 ; Gautam et al 2020a ).…”