Simultaneous determination and removal of Toluidine blue (TO) and basic blue (BB) dyes by dopamine-modified magnetic nanoparticles was considered. Nanomagnetic oxide coated with polydopamine (PDA) acid is introduced as a new and efficient adsorbent to remove cationic dyes in a binary mixture solution. Characterization of the Fe3O4 and Fe3O4@PDA nanoparticles was also supported by FT-IR, XRD, SEM, and VSM analysis. First-order derivative spectrophotometry is employed to simultaneously determine TO and BB in a binary mixture. Linear range for TO and BB in binary mixture obtained by least square equations (y= 0.00014x+ 0.00013 and y= - 0.00136x- 0.00487) were 2-20 mg L-1 and limit of detection 0.4 and 0.3 mg L-1, respectively. The effect of independent factors such as pH, dye’s initial concentration (C0), and adsorbent amount (m) on the dyes adsorption process was investigated. The response surface methodology was employed by running a central composite design (CCD) to obtain optimal values for dye removal percent. It leads to optimum conditions as follows: pH= 10˛ m=25 (mg), CTO=320 (mg L-1), CBB=350 (mgL-1), %RTO=85, %RBB=80. Kinetic data followed a pseudo-second-order model while equilibrium data were well described by the Freundlich model. Using Langmuir's isotherm, the maximum monolayer uptake capacity qmax for single system and binary systems, respectively, were 381 mg g-1 and 355 mg g-1 for BB, and 362 mg g-1 340 mg g-1 for TO. The negative value of △Go and the positive value of △Ho (12.52 kJ mol-1 for TO and 12.98 kJ mol-1 for BB) demonstrated the spontaneous and endothermic nature of both dyes' adsorption.
