Abstract
In this study, lignin was isolated from rice (Oryza sativa L.) husk using alkaline extraction method and was used as an adsorbent for aqueous solutions of methyl orange (MO) and bromothymol blue (BTB). The equilibrium removal rate of MO was found to be at 61.4%, with experimental equilibrium adsorbate uptake, qe, of 1.23 mg·g-1, achieved at 150 minutes contact time. For BTB, the equilibrium removal rate was found to be at 78.3%, with an experimental qe of 1.57 mg·g-1, achieved at 150 minutes contact time. Pseudo-first order (PFO) and pseudo-second order (PSO) kinetic models were then used to investigate the kinetics of the adsorption process. Both MO and BTB on lignin were found to follow a PSO kinetic model, with rates of 6.84 x 10-3 g·mg-1·min-1 and 0.69 g·mg-1·min-1, respectively. Langmuir, Freundlich, Dubinin-Radushkevich (DR), Brunauer-Emmett-Teller (BET), Flory Huggins (FH), and Temkin adsorption isotherm models were then used to determine the appropriate equilibrium adsorption model for both substrates. Based on the calculations performed, the Temkin model best described the adsorptive removal of both MO and BTB. Based on the Temkin model, the adsorption processes of both MO and BTB were found to occur spontaneously, with equilibrium rate constants of 0.083 L·mg-1 and 0.012 L·mg-1, respectively.
Highlights
The discharge of various toxic chemicals from industrialization and urbanization leading to water pollution is a global concern nowadays
The sample was subjected to alkaline pre-treatment with 4.0 M NaOH to enable extensive delignification, which is expected to cause swelling, which results in an increase in the surface area, decrease in the polymerization and crystallinity ratios, and separation of the structural bonds between lignocellulosic components (Kucharska et al, 2018)
The large absorption area implied that the isolated alkali lignin was abundant in O–H group
Summary
The discharge of various toxic chemicals from industrialization and urbanization leading to water pollution is a global concern nowadays. Among the different hazardous chemicals, the presence of toxic dyes in wastewaters from the effluents of textile, rubber, paper, leather, plastics, cosmetics, and printing industries causes severe damage to the environment even at low concentrations. Because dyes are highly colored substances, they affect the visibility and recalcitrance and generate several problems for ecosystem wastewater such as reduced sunlight. Resistance to photochemical and biological attack, and toxic or even mutagenic and carcinogenic degradation products. Removal of these dye pollutants is necessary for both health and environmental considerations (Subbaiah and Kim, 2016; Agarwal et al, 2016)
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