Adsorption behavior of methylene blue and its congeners on a stainless steel surface

Methylene blue and its congeners as model dyes were adsorbed onto stainless steel particles at different ionic strengths, pH values, and ethanol contents, and the adsorption mechanism was investigated. A Fourier transform infrared spectroscopy (FTIR) analysis of the dyes adsorbed on the stainless steel plate was carried out to determine the orientations of the adsorbed dyes on stainless steel surface. The adsorption isotherms for all the dyes tested were approximated by a Langmuir equation (Q = Kq_m C/(1 + KC)) in most cases except under strongly basic conditions. From the ionic strength and ethanol content dependencies of the K value in the Langmuir equations, both the electrostatic and hydrophobic interactions were indicated to contribute to the adsorption of the dyes at neutral pH. By comparing the K and q_m values for the methylene blue congeners and with the aid of the FTIR analyses, it was found that the kind of substituent groups at most positions of the polyheterocycles of methylene blue strongly affects the adsorption behavior, particularly the area occupied by an adsorbed dye molecule, the affinity for the stainless steel surface, and the orientation of the adsorbed dye molecule on the stainless steel surface.