Silicate Hindering In Situ Formed Ferric Hydroxide Precipitation: Inhibiting Arsenic Removal from Water

The effects and dominant mechanism of silicate hindering arsenic removal with in situ formed ferric hydroxides were investigated. Zeta potential analysis showed that 10 mg/L silicate as Si obviously decreased ζ potential from −11.9 mV (pH 8.61) to −29.9 mV (pH 8.84) for ferric hydroxides; more significant effects was observed at higher pH levels. Photometric dispersion analysis indicated that silicate apparently hindered ferric hydroxides precipitation and aggregation, with more apparent influence at elevated pH values. Furthermore, in high pH ranges silicate led to filtered iron concentration increase. These adverse effects resulted in a significant decrease of arsenic removal with in situ formed ferric hydroxides, especially in higher pH ranges. Sequential filtration analysis showed the presence of aqueous arsenic species in liquid phase, demonstrating that silicate increasing repulsive charges between ferric precipitates and decreasing surface sites available for arsenic, rather than contributing to the formation of tiny ferric colloids with adsorbed arsenic, was more significant in silicate hindering arsenic removal by ferric hydroxides.