A novel approach for methylene blue removal by calcium dodecyl sulfate enhanced precipitation and microbial flocculant GA1 flocculation

•A novel approach for the efficient removal of MB was proposed.•SDS solubilization and Ca2+ effect were combined to reduce the solubility of MB.•MB precipitated from aqueous solution by adsorbing onto the Ca(DS)2 particles.•The Ca(DS)2 particles adsorbed MB were flocculated by MBFGA1.

A novel approach was proposed for the removal of methylene blue (MB), a soluble cationic dye, from aqueous solution by calcium dodecyl sulfate (Ca(DS)2) enhanced precipitation which was based on the solubilization of sodium dodecyl sulfate (SDS) on MB and Ca2+ effect on SDS micelles, and microbial flocculant GA1 (MBFGA1) flocculation. The independent and interactive effects of factors, such as SDS, Ca2+ and MBFGA1 dosages, on the MB removal and interaction between SDS and Ca2+ were investigated. The response surface methodology (RSM), environmental scanning electron microscope (ESEM) and energy dispersive spectrometer (EDS) analysis were employed to discuss the interaction mechanism between SDS and Ca2+, and MB removal mechanism. The results showed that MB and SDS removal efficiency could reach 98.63% and 88.97%, respectively, with pH (10), MB (50 mg/L), SDS (8 mM), Ca2+ (5 mM) and MBFGA1 (4 mL/L). Under the optimal conditions, residual SDS and Ca2+ concentrations in the upper phase were 0.88 mM and 1.27 mM, respectively, which reached the Ksp of Ca(DS)2. The concentration consumption ratio between SDS and Ca2+ was 2.0. The interaction between SDS and Ca2+ was depended on the SDS-Ca2+ concentration ratio in aqueous solution rather than the CMC of SDS. When Ca2+ concentration was relatively sufficient, SDS micelles containing the solubilized MB (SDS-MB micelles) would disassemble to generate MB loaded Ca(DS)2 particles (CDS-MB particles) which would be flocculated by MBFGA1. Whereas, when SDS concentration was superfluous relatively, SDS micelles formed in the upper phase would redissolve the CDS-MB particles in flocs.

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