Removal of Reactive Red 1 dye from aqueous solution using gypsum

In order to establish the capacity of gypsum in the removal of dyes from aqueous solutions, the adsorption behaviour of Reactive Red 1 (RR1) from aqueous solution onto gypsum was investigated under several different operating parameters namely, contact time, adsorbent dosage, pH, particle size, and adsorbate dosage, at ambient temperature. The study employed the batch experiment method and utilized two different particle sizes (150 µm and 300 µm) of the adsorbent. Two portions of each particle size were subjected to two different treatments (acid and base) while a third portion remained untreated. Acidic medium favoured the dye removal at an optimal pH of 2 and an optimal contact time of 25-35 minutes. Optimal adsorption was also recorded at an adsorbent dosage of 0.8 g. Under optimal conditions, 89.8 % adsorption was recorded with the 150 µm particle size of the adsorbent in the acid-treated medium. Both Langmuir and Freundlich isotherms could be used to describe adsorption of the dye, with the value of the separation factor of the Langmuir isotherm (RL) lying well in between 0 and 1. The Freundlich isotherm intensity parameter (1/n) also fell in between 0 and 1- indicating that both isotherm processes are favourable. The experimental data fitted well to the pseudo-second-order kinetics model with R2>0.99 indicating that chemisorption was the rate-controlling step in the adsorption of the dye. The smaller particle size (150 µm) adsorbed higher amount of the dy