Individual and Competitive Adsorption of Negatively Charged Acid Blue 25 and Acid Red 1 onto Raw Indonesian Kaolin Clay

Removal of negatively charged acid blue 25 (AB25) and acid red 1 (AR1) from their aqueous single and binary solute solutions by adsorption onto raw Indonesian kaolin was investigated. The effect of contact time, initial concentration, temperature, and the molar ratio of AB25 and AR1 in batch adsorption mode was determined. It was found that the individual and co-adsorption of AB25 and AR1 followed pseudo-second-order kinetics with mass transport being a combination of intraparticle and film diffusion. The adsorption isotherm data were consistent with the Langmuir model, implying that the adsorption of the synthetic dyes occurs on a homogeneous monolayer of the basal surfaces of kaolinite particles. Thermodynamic parameters suggested that the adsorption is spontaneous and exothermic. The maximum capacity of the co-adsorption of AB25 and AR1 was 9.02 and 7.67 mg g⁻¹ lower compared to their individual adsorption (16.5 and 12.8 mg g⁻¹, respectively). This confirms that adsorption of the two anionic synthetic dyes is competitive during co-adsorption. This competitive effect is important to consider in the simultaneous removal of multiple synthetic dyes from wastewater.