Enhancing the affinity of Pb(II) to the metastable ferrihydrite with the presence of gallic acid and anoxia condition.

Naturally widespread ferrihydrite is unstable and often coexists with complex ions, such as the heavy metal ion Pb(II). Ferrihydrite could fix Pb(II) by precipitation and hydroxyl adsorption, but release Pb(II) with mineral aging. Gallic acid plays an important role in influencing the geochemical behavior of ferrihydrite-Pb, and anoxia is one of the factors influencing the transformation of mineral. This study investigated the effects of Gallic acid and anoxia on the migration and distribution of Pb(II) in ferrihydrite-Pb co-precipitates. XRD, FT-IR, SEM, XPS were employed to explore the internal interactions. The results showed that Gallic acid could promote Pb(II) to enter the mineral and inhibit the release of Pb(II). The fixation of Pb(II) could be achieved under anoxia by passivating ferrihydrite. Gallic acid could formed ferrihydrite-gallic acid-Pb ternary complexes with ferrihydrite-Pb co-precipitates, which improved the affinity of ferrihydrite to Pb(II) and promoted the ability of ferrihydrite to fix Pb(II). The anoxia allowed the Fe(II) produced by reductive dissolution of ferrihydrite to be retained for longer time, thus catalyzed the production of goethite from ferrihydrite and passivating ferrihydrite to inhibit the aging of ferrihydrite. In addition, acid environments caused most of Pb(II) to be released into solution through competition with hydrogen ions. Pb(II) in alkaline environment led to Pb(II) immobilization by entering the interior of mineral. The findings of this study provide references for better understanding the environmental behavior of Pb(II) during ferrihydrite transformation.