Abstract
Equilibrium, kinetic, thermodynamic aspects, and mechanism of congo red (CR) adsorption onto rice husk treated with effluent from cotton pre-treatment (ERH) are examined by altering the initial dye concentration (0.1, 0.2, 0.3, 0.4, and 0.5 g/L), contact time (0 to 1440 min), temperature (298, 323, and 343 K), and adsorbent dosage (10, 20 and 40 g/L). When 10 and 20 g/L adsorbent dosages are applied, the treated rice husk adsorbs the CR following the Langmuir model, while at 40 g/L ERH, the adsorption follows the Freundlich model. A maximum of 149 mg CR per gram ERH is adsorbed with 10 g/L ERH at 70 °C. This adsorption capacity is one of the better ones found in the literature. The calculated Dubinin–Radushkevich activation energy lower than 8 kJ/mol, indicates the physical nature of CR adsorption. The adsorption kinetic follows a pseudo-second-order kinetic model because R2 for all tested samples is higher than 0.999. The activation energy (Ea) varies from 0.045 to 40.1 kJ/mol, while the isosteric heat of adsorption (ΔHX) varies from 1.06 to 36.0 kJ/mol. Ea and ΔHX lower than 40 kJ/mol and 80 kJ/mol, respectively, confirm the physical adsorption of CR onto ERH. The other thermodynamic analysis indicates spontaneous and endothermic adsorption. These results showed the applicability of the circular economy concept in the effort to obtain an efficient adsorbent without wasting additional chemicals and energy that could be used to create a continuous column-mode process of rice husk modification and purification of colored effluent from the textile industry.
Highlights
Direct dyes are water-soluble colorants for dyeing cellulosic fabrics when good lightfastness is required, but washing-fastness is not crucial (Broadbend 2001)
The present study explored equilibrium, kinetics, thermodynamics, and the adsorption mechanism of Congo red (CR) onto rice husk modified with cotton scouring effluent (ERH), by the concept of circular economy
The influence of adsorbent dosage, temperature, and initial dye concentration on the adsorption ability of the sorbent has to be inter-correlated to obtain a clear picture of the sorption phenomena for each sorbate-sorbent system
Summary
Direct (substantive) dyes are water-soluble colorants for dyeing cellulosic fabrics when good lightfastness is required, but washing-fastness is not crucial (Broadbend 2001). In order to improve its adsorption capacity, the rice husk was subjected to physical grounding (Vadivelan & Kumar 2005), incineration (Alam et al 2020), chemical treatments with acids (Abdelwahab et al 2005), ethylenediamine (Ong et al 2007) and bases (Chowdhury et al 2011; Chowdhury et al 2012) These modifications are effective but require an investment in expensive equipment that operates at high temperatures under pressure. Scanning electron microscopy-energy dispersive X-ray (SEM-EDX) was used to observe the occurred changes after this modification The results of these analyzes showed that the treatment with effluent from alkaline scouring effectively removed the lignin-silicone shield from the rice husk surface. Such modified rice husk has an exceptional absorption capacity higher than other chemically modified rice husks found in the literature
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