Modelling adsorption of acid orange 7 dye in aqueous solutions to spent brewery grains

Abstract Acid orange 7 (AO7) is a monoazo acid dye currently used in paper and textile industries. The modelling and optimisation of AO7 adsorption to spent brewery grains (SBG), a by-product of the brewing process, was performed using response surface methodology (RSM). SBG was used dried or previously acid hydrolysed. Five central composite rotatable designs (CCRD), as a function of two variables, each, were followed, namely: (1) amount of SBG and hydrolysis time of SBG; (2) hydrolysis and adsorption times; (3) amount of SBG and adsorption time ( T ); (4) T and pH medium and (5) T and AO7 concentration. Hydrolysis of SBG showed not significantly increase its adsorption capacity for AO7 dye. Tri-dimensional response surfaces were fitted to the experimental data concerning the removal of dye obtained in CCRD-3 to CCRD-5. From CCRD-3, the best adsorption conditions (96% removal) were estimated for 5.1% (m/v) of SBG after 36 min contact. From CCRD-4, removals above 90% were observed for lower pH values and adsorption times higher than 30 min. An increase in AO7 adsorbed per unit weight of SBG was observed with increasing initial AO7 concentration. Also, SBG was reused in 12 consecutive batches, under previously optimised conditions. SBG residual adsorption activity can be well described by a first-order decay model. A maximum amount of adsorbed AO7 of 29 mg/g SBG is predicted from the cumulative curve.