Bench Scale Application of the Hybridized Zero Valent Iron Process for the Removal of Dissolved Silica From Water

Dissolved silica in water is notorious for precipitating on industrial equipment. Scale formation can occur within tubes of a boiler, heat exchangers, and cooling towers, turbine blades are susceptible to deposit formation, and reverse osmosis membranes are susceptible to glass like scaling all leading to reduced efficiency. Conventional chemical treatment methods such as hot lime, activated alumina, and MgO require heavy dosage of chemicals and have limited pH ranges for which removal is effective. A more robust and cost-effective dissolved silica removal technique is desirable.

The hybridized zero-valent iron (hZVI) process, now commercially available as Pironox?, uses zero-valent iron (Fe^0 ) as its main reactive media developed to remove heavy metals/metalloids, reactive oxyanions, and impurities from water/wastewater. The distinctive feature of this novel chemical treatment platform is the controlled formation of magnetite as the main iron corrosion product in the presence of aqueous Fe^2+. The hZVI system was shown to reduce dissolved silica from 70 mg/L to below 5 mg/L in a pilot scale demonstration for treating flue-gas desulfurization wastewater.

In this study bench scale tests were performed using a single stage, continuously stirred tank reactor to optimize the removal efficiency of dissolved silica (100 mg/L as SiO2) using the hZVI process. It was demonstrated the continuous formation of magnetite in an hZVI system played a key role in achieving high system performance with respect to dissolved SiO2 removal. Using ZVI grains with an average diameter of 5 microns optimal reagent dosages were determined to be 27.9 mg/L (0.5 mM) Fe^2+ and 10 mg/L (0.72 mM) to 15 mg/L (1.07 mM) NO3-N. With added Fe^2+ and nitrate at these dosages or higher removal efficiency was 88% to 99% over a broad range of pH 6.8 to 9.8 offering a more flexible approach to removing dissolved silica from water when compared to conventional treatment methods. Using the optimal reagents the hZVI system sustained dissolved silica removal with >95% efficiency over an extended period. It was also shown that increasing the reactor temperature from 25?C to 90?C did not attenuate dissolved SiO2 removal in an hZVI system.