Optimised removal of methylene blue using live and dead algae: A box–Behnken design study

Release of dyes into the aquatic environments has intensified water pollution, and conventional water treatment is inadequate for dye removal. This study evaluates the potential of marine macroalgae ( Fucus sp., Gracilaria sp., and Ulva sp.) to remove methylene blue, a dye widely used in medical and textile sectors, among others. Both live and dry biomass were tested on water of different salinity. Distinct patterns of removal were observed, with both biomass forms showing high removal efficiencies. To understand the effect of key parameters and optimise the removal, the response surface methodology with a Box-Behnken design was used, focused on dry Fucus sp. and live Ulva sp. Parameters included macroalgae dosage (2, 6, and 10 g/L), dye concentration (5, 27.5, and 50 mg/L), and salinity (1, 15.5, and 30 PSU). Models revealed that macroalgae dosage and salinity significantly influence removal rates. Optimisation showed that live Ulva sp. removes up to 92.3 % of the dye in 6 h, while dry Fucus sp. achieves 95.7 % removal in just 0.5 h. Although dry biomass acts more rapidly, live biomass offers practical advantages for industrial applications, such as easy separation and CO 2 consumption. These findings highlight the potential of macroalgae as an eco-friendly solution for dye removal. • Higher dye removal at salinity 1 for all species, with dried biomass reaching equilibrium faster. • Living Ulva sp. and dried Fucus sp. selected for optimization due to high removal efficiency. • Algae mass and salinity drive removal, while dye concentration is key for living biomass. • Under optimal conditions, dried Fucus removed 93.7 % in 0.5 h; living Ulva, 95.7 % in 6 h.