The removal and desorption of two toxic dyes from aqueous solution by hydroxylated hematite sand: Kinetics and equilibrium

Abstract

The adsorption/desorption properties of hematite sand (HS) for removal of two toxic dyes have been tested in aqueous solution by batch process. The  raw sand and HS were characterized. Toluidine blue (TB) and crystal violet (CV) were selected as a toxicity model. The effect of adsorbent dose, contact  time, pH solution and ionic strength was investigated. The dye adsorption was influenced significantly by the presence of inorganic salt (KCl). The best  fitted equilibrium and kinetic model for two dyes were found to be Langmuir and pseudosecond order, respectively. Intra-particle diffusion is not the only speed-controlled step. The maximum monolayers capacities of TB and CV were 1.60×10^-5 and 2.65×10^-5 mol.g^-1, respectively with the optimal  equilibrium time of 60 min at room temperature and pH 10. In the batch desorption process, 0.2 M NH4Cl achieved better performance for both dyes  desorption and the HS can be effectively reused for four cycles consecutively.