Magnesium Hydroxide Slurry Coagulation-Adsorption Performance for Reactive Orange Removal Assisted with PAM

Abstract

Magnesium hydroxide has been successfully used for alkaline reactive dye wastewater treatment. However, for wider range pH values, magnesium hydroxide slurry has a potential advantage due to its buffer action. Reactive orange in aqueous solution was removed by magnesium hydroxide slurry and non-ionic polyacrylamide (PAM) as coagulant aid. Magnesium hydroxide slurry was prepared, and the morphology and particle size were analyzed. The size of flocs and flocculation time have important influence during coagulation-adsorption experiment. Flocculant index (FI), floc size distribution, zeta potential, scanning electron microscopy (SEM), and Fourier transform infrared spectroscopy (FTIR) were used to study the coagulation performance and floc properties. The effects of Mg(OH)₂ dosage, PAM dosage, and different pH values on removal efficiency and settling time were investigated. The results show that charge neutralization, precipitation wrapping, and adsorption bridging are the main coagulation mechanisms. The optimal coagulation conditions were as follows: 200 mg/L magnesium hydroxide slurry was added after 3 s of rapid stirring at a speed of 250 rpm; 4 mg/L PAM was added before 15 s of beginning of slow stirring at 60 rpm; the removal efficiency of reactive orange could reach 91.5%. In the range of pH 5–12, magnesium hydroxide slurry shows very good performance for reactive orange removal and floc sedimentation.