Adsorption-desorption of methylene blue dye onto marine sediments: Kinetics and equilibrium studies

The present study concerns the investigation of the sorption and desorption phenomena of methylene blue (MB), which is an organic molecule used as a synthetic dye in several industries. For that purpose, the batch equilibrium technique was applied by using three marine sediments that were collected from the North Aegean Sea (Greece) and more specifically from unpolluted coasts of Chios island (sample S1), Lesvos island (S2 and S3). The criterion for the selection of those substrates as adsorbent surfaces were based on both their difference in their textural analysis (that ranged between 1.68–34.54%) and the content of organic matter as well (that varied from 1.70 to 5.38%). The obtained experimental results showed that both Freundlich and Langmuir isotherm models could describe the kinetics of the occurred process, whereas the correlation regression coefficients for the fit to that models were calculated to be R2≥ 0.9004 and R2≥ 0.8487, respectively. Furthermore, acquired results indicated a positive correlation between the sorption of MB and the organic matter content of marine sediments. Desorption studies revealed that the studied dye in some cases was adsorbed very weakly on sediments tested and hence was easily desorbed with seawater, while on the contrary in other cases MB was adsorbed very strongly on studied matrices with lower quantities of seawater extractable amounts.