Brewers' spent grain in adsorption of aqueous Congo Red and malachite Green dyes: Batch and continuous flow systems

Highlights: • Brewers' spent grain adsorbs Congo Red and Malachite green dyes from water. • Contact time, dye concentration and adsorbent surface area important in the process. • Ionic strength and pH also influence the adsorption process. • Langmuir adsorption isotherm gave a better fit to the experimental data. • The adsorption process fitted pseudo-second order kinetics with R² close to 1. -- Abstract: Sorption of Congo Red (CR) and Malachite Green (MG) dyes currently used in pigments and clothing industries were investigated using brewers' spent grain (BSG) from a local brewery. Adsorption increased with a higher adsorbent weight and lower colorant concentrations. Accumulation of CR and MG was optimal at acidic pH and neutral pH respectively. Sorption decreased with an increase in temperature signifying an exothermic process. Batch adsorption data fitted better to Langmuir adsorption isotherm model and pseudo-second-order kinetics. Maximum monolayer coverage capacities ($Q_{O)})$ were found to be 2.55 mg/g for MG and 36.5 mg/g for CR dye. Column studies using BSG were also conducted for both dyes. Fixed bed breakthrough was fast with an increase in dye concentration, adsorbent surface area, and flow rate and with a decrease in column depth. BSG are effective, simple in design and inexpensive adsorbing material from renewable sources.