Abstract
Desirable cost and removal effectiveness of the adsorbents can be obtained by the use and modification of agricultural waste products. Therefore, in this study rice husk (RH), rice husk char (RHC) and chemically modified rice husk char (CMRHC) were prepared and used as adsorbents for the removal of Congo Red dye from aqueous media under different experimental conditions. It was observed that the adsorption of Congo Red dye was not only affected with the types of adsorbents used but also with other experimental variables such as time, adsorbent dose, dye concentration, pH and temperature. Thermodynamics study indicated that the adsorption of Congo Red dye on the three adsorbents is spontaneous and exothermic process. Kinetics data showed that the adsorption of Congo Red dye on these adsorbents follow pseudo-second order model better than the pseudo-first order model. Equilibrium adsorption data were tested with Langmuir, Freundlich, and Temkin adsorption isotherm models. It was observed that the equilibrium data of RH and RHC were found best fit to the Langmuir adsorption isotherm model whereas that of CMRHC was best fit to the Freundlich and Temkin models. CMRHC showed better efficiency compared to both RH and RHC.
 
 Bull. Chem. Soc. Ethiop. 2020, 34(1), 41-54.
 DOI: https://dx.doi.org/10.4314/bcse.v34i1.4
Highlights
Nowadays high quantity of dye effluents are discharged into the aquatic environment due to very high speed of industrial growth in many countries
Comparative analysis of the three adsorbents indicated that chemically modified rice husk char (CMRHC) has greater adsorption capacity even at high concentration of dye in the aqueous media. These results indicated that thermal modification of rice husk enhanced its adsorption properties which could be further increased by chemical modification to its maximum
Comparative adsorption studies showed that maximum adsorption of Congo Red dye at pH 4 on the surface of RH and rice husk char (RHC) were investigated as 88.7% and 92.3%, respectively, whereas CMRHC could remove 98.9% of the dye at pH 6
Summary
Nowadays high quantity of dye effluents are discharged into the aquatic environment due to very high speed of industrial growth in many countries. Dyes are used in many industries, like food, leather, plastics, mineral processing, paper, rubber, cosmetics, pharmaceutical, and textile [2]. Most of the dyes are stable to degradation by any chemical or biological means. Various methods including chemical-coagulation, ozonation, activated carbon adsorption, biological treatment and catalytic reduction are used to control the dye-waste effluents [6,7,8]. Congo Red dye is an anionic, diazo, water soluble direct dye having maximum absorption at 520 nm; this is mostly used in textile, paper, printing, leather and plastic industries and causes irritation to eyes and skin as its main side effects [9]. An attempt was made for the removal of Congo Red dye by adsorption approach while using Solanum tuberosum (potato) peels and Pisum sativum (pea) peels as adsorbents [9]
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