Assessing the contribution of Good's buffers to laboratory experiments on Fe(II)_aq-induced ferrihydrite transformation

Numerous previous experiments on Fe(II)_aq-induced Fe(III) oxide transformation used Good's buffers to maintain pH. However, Good's buffer has previously been found to directly complex with the mineral surface and the metals in solution, and indirectly influence the interactions between Fe(II)_aq and Fe(III) oxides by changing the mineral surface properties. So far, to what extent the presence of buffer unintentionally affects the experimental results remains unclear. Herein, this study evaluated the potential contribution of three typical Good's buffers, MES, MOPS and HEPES, to Fe(II)_aq-induced ferrihydrite transformation using batch experiments with 10 mM ferrihydrite, 1 mM Fe(II)_aq, 0–50 mM buffer and 0–0.1 M NaCl at pH 6.0–7.5. The results showed none of the tested buffers could adsorb on ferrihydrite. The extents and products of ferrihydrite transformation in the presence of 0–50 mM buffer were similar at any given pH within 24 h. However, the extents of ferrihydrite transformation with 10–50 mM buffer were significantly (p < 0.05) higher than those with 0–1 mM buffer after 72 h. The enhanced transformation with higher buffer concentrations can be attributed to the elevated ionic strength, which raised initial surface loading of Fe(II) on ferrihydrite. The comparison between experiments with and without background electrolyte (NaCl) further confirmed that high ionic strength could promote ferrihydrite transformation with the effect becoming more and more evident over time. Our data systematically assessed the contribution of Good's buffers to laboratory experiments on Fe(II)_aq-induced Fe(III) oxide transformation and provide important reference for similar experiments in the future.