Algerian Journal of Chemical Engineering 02 (2020) 66–75 Algerian Journal of Chemical Engineering ISSN:2773-3068 Journal homepage: http://www.ajce.periodikos.com.br/ * Corresponding author. Tel.: 07065654046 E-mail address:bridgetidoko0803@gmail.com http://dx.doi.org/ 10.5281/zenodo.5670200 Original Article Kinetics, equilibrium and thermodynamics studies of congo red dye adsorption from aqueous solution onto activated cowpea (Vigna unguiculata) husk Abdullahi Muhammad Ayuba and Bridget Idoko* Department, of Pure and Industrial Chemistry, Bayero University, Kano, Nigeria ARTICLE INFO ABSTRACT Article history: Received 14 June 2021 Revised 09 September 2021 Accepted 10 October 2021 Adsorption of Congo red (CR) dye from aqueous solution onto activated cowpea husk (ACPH), a low-cost agricultural waste material in a batch process was investigated. Adsorption was studied as a function of amount of adsorbent, pH, initial dye concentration and time. It was found that adsorption capacity varied linearly with the amount of adsorbent, initial CR concentration with time. The results show that maximum adsorption capacity was obtained at the optimum levels of contact time (24.3157mg/g at 60 minutes), adsorbent dose (24.532mg/g at 0.1g), initial dye concentration (407.2787mg/g at 500mg/L) and pH (24.26mg/g at 1.5). Adsorption equilibrium data were represented by isotherm, kinetics and thermodynamics models. Three isotherm models namely Langmuir, Freundlich and Temkin were tested and adsorption was found to fit well into Langmuir model relatively better than others. The maximum loading capacity (q m ) of the adsorbent for Congo red obtained from the Langmuir isotherm model is 263.16 mg/g. The kinetic data was well described by the pseudo- second order kinetic model with the correlation coefficients (R 2 ) value of 0.994. The adsorption process was found to be thermodynamically endothermic and spontaneous. The negative value of ΔS (-0.00053J/mol.k) infer that the randomness decreases at the adsorbent/adsorbate interface during the adsorption process. FTIR and SEM analyses of the adsorbent suggest that adsorption of the dye was through an electrostatic interaction between the functional groups present in the dye and those on the surface of the adsorbent. . Keywords: Adsorption Congo Red Isotherms Kinetics Vigna unguiculata 1. Introduction Dyes are coloured substances that can be applied to various substrates (textile materials, leather, paper, hair) from a liquid in which they are completely, or at least partly, soluble. Man has made use of dyes since prehistoric times, and in fact, the demand and the usage of dyes have continuously increased. However, the presence of dyes even in trace quantities is very undesirable in aqueous environment as they are generally stable to light and oxidizing agents, and are resistant to aerobic digestion [1]. This is partly due to the realization that contamination of aquatic environment by dyes causes reduction in the growth of algae due to obstruction of light required for photosynthesis, which subsequently leads to ecological imbalance in the aquatic ecosystem [2]. Congo red (C 32 H 22 N 6 Na 2 O 6 S 2 : disodium 4-amino-3-[4- [4-(1-amino-4-sulfonato-naphthalen-2-yl) diazenylphenyl]phenyl]diazenyl-naphthalene-1-sulfonate) is a secondary diazo dye that is used in textile, paper, and leather industries. It is a water-soluble dye and can be used as a pH indicator due to its colour change from blue to red at pH 3.0 – 5.2. The coloured effluents which are discharged into water cause environmental pollution with harmful effects on aquatic life [3]. Many researchers have studied the removal of this carcinogenic dye through adsorption process using various adsorbents of plant origin including: coconut residual fiber [4], Phoenix dactylifera date stones and Ziziphus lotus jujube shells [5], Solanum tuberosum and Pisum sativum peels [6], common Beach (Fagus sylvatica L.) [7], Ulva lactuca biomass [8], Mango leaves [9], Litchi peel biochar [10], Pomelo peel [11], Brewer’s grains [12], Magnolia-leaf [13], Pineapple peel [14], Banana peel [15], Walnut shell [16], Pine bark [17],