Synergistic inhibition of green rust crystallization by co-existing arsenic and silica

Arsenic and silica are known inhibitors of the crystallization of iron minerals from poorly ordered precursor phases. However, little is known about the effects of co-existing As and Si on the crystallization and long-term stability of mixed-valence Fe minerals such as green rust (GR). GR usually forms in anoxic, Fe 2+ -rich, near-neutral pH environments, where they influence the speciation and mobility of trace elements, nutrients and contaminants. In this work, the Fe 2+ -induced transformation of As- and/or Si-bearing ferrihydrite (FHY) was monitored at pH 8 ([As] initial = 100 μM, Si/As = 10) over 720 h. Our results showed that in the presence of As( iii ) + Si or As( v ) + Si, GR sulfate (GR SO 4 ) formation from FHY was up to four times slower compared to single species system containing only As( iii ), As( v ) or Si. Co-existing As( iii ) + Si and As( v ) + Si also inhibited GR SO 4 transformation to magnetite, contrary to systems with only Si or As( v ). Overall, our findings demonstrate the synergistic inhibitory effect of co-existing Si on the crystallization and solid-phase stability of As-bearing GR SO 4 , establishing an inhibitory effect ladder: As( iii ) + Si > As( v ) + Si > As( iii ) > Si > As( v ). This further highlights the importance of GR in potentially controlling the fate and mobility of As in ferruginous, Si-rich groundwater and sediments such as those in South and Southeast Asia. Co-existing silica prolongs the stability of mixed valence iron minerals like green rust and magnetite, effectively immobilizing arsenic in anoxic environments.