Water with low ionic strength recovers the passivated birnessite-coated sand reactivity towards lincomycin removal.

The ionic strength of infiltration water changes with the seasonal alternation of irrigation sources. In this study, reactivity changes of birnessite-coated sand with the fluctuations of ionic strength of infiltration water (i.e. from groundwater to rainwater) and the involved mechanism were investigated through column experiments. Birnessite-coated sand was less reactive in groundwater than in rainwater because of the higher cation content and higher pH of groundwater. The cations in the groundwater were adsorbed on birnessite-coated sand and then desorbed in presence of a dilute aqueous solution represented by rainwater. The reactivity of the passivated birnessite-coated sand was recovered instantaneously, and approximately one-third of the pristine reactivity was restored. During recovery, Na+ desorption and lincomycin (LIN) removal both exhibited a two-stage reaction pattern. The LIN removal correlated with Na+ desorption (r = 0.99) so that the reactive sites that were binding 5.602 µmol of Na+ became available for 1 µmol of LIN removal. These results suggest that the reactivity of manganese oxides toward organic contaminant is associated with the ionic strength of infiltration water and indicate that the partial reactivity can be naturally restored.

Removal of lincomycin from aqueous solution by birnessite: kinetics, mechanism, and effect of common ions.

The removal of lincomycin (LIN) from aqueous solution by birnessite was investigated by batch experiments. When the dosage of birnessite is 500 mg L-1 and the initial concentration of LIN is 15.5 µmol L-1, more than 90% of LIN was removed within 240 min at pH 4.90. Under different conditions, the reactions were well fitted with the second-order model (R2 > 0.95). The removal kinetics and the reaction mechanism were described. The presence of cations (e.g., K+, Ca2+, Mg2+, Fe2+, and Mn2+) inhibited the removal of LIN by birnessite, following the order: Mn2+ > Fe2+ > Ca2+ > Mg2+ > K+ ≈ Na+, due to the sorption of cations on birnessite, companying with the electron transfer and precipitation of oxides (for Mn2+ and Fe2+). The addition of Cu2+, SO42-, or NO3- improved the reactions. The presence of Cu2+ could oxidize antibiotics, and the repulsion between SO42-or NO3- and birnessite might disperse the birnessite suspensions during the reactions. Mn(IV) and Mn(III) were the core Mn species that play an important role in LIN removal. These findings will help to understand the removal process of LIN and illustrate the influence of cations and anions on the removal of similar pollutants by birnessite.