Groundwater is one of the major sources for drinking water supply worldwide. Conventional iron removal via aeration-filtration produces about 72,802 t of iron sludge annually in the Netherlands alone. Iron sludge comprises low-density flocs of little to no commercial value. The current study explored a novel concept for iron removal, namely anoxic iron sulfides formation in a fixed bed continuous flow reactor. Iron sulfides usually form dense structures and offer a wider range of re-use applications. A packed bed up-flow column reactor filled with pyrite granules was fed iron and sulfide containing solutions. Produced solids were analyzed applying X-ray diffraction analysis, Raman spectroscopy, digital microscopy, scanning electron microscopy and energy dispersive X-ray spectroscopy. Rapid iron sulfides formation was observed after < 10 min. The formed minerals were partially retained by the pyrite granules. The molar ratio of removed Fe(II) to removed S(-II) equaled up to 0.76 ± 0.16 mol Fe(II)_rem/(mol S(-II)_rem). Our results show that iron sulfides formation can present an interesting alternative to iron removal via aeration-filtration due to its compact particle sizes and fast formation rates.

Groundwater (GW) makes up roughly half of the global drinking water supply. Conventional iron removal in GW treatment produces approximately 10,000t/d of iron sludge. Iron sludge consists of low-density flocs with low to no commercial value and causes frequent energy intensive backwashing of the rapid sand filter. This study aimed to explore the novel concept of iron removal via iron sulfides formation. Iron sulfides are usually forming dense structures and offer a wider range of re-use applications.
To investigate this, an up-flow column reactor filled with pyrite seeding crystals was built and fed with iron and sulfide containing solutions. Flushed out formed solids were investigated with X-ray diffraction analysis and Raman spectroscopy. Seeding crystals were analyzed with scanning electron microscopy with energy dispersive X-ray spectroscopy.
This study observed rapid mackinawite formation after a few minutes. Mackinawite was likely retained by electrostatic adhesion on the pyrite seeding crystals. The molar ratio of removed iron to removed sulfide equaled up to 0.8 ± 0.2.
Mackinawite formation can present an interesting alternative to conventional iron removal, due to (i) its compact size, (ii) fast formation rates and (iii) possibly simple removal mechanism via electrostatic adhesion. Furthermore, in-situ formed mackinawite has the potential to simultaneously treat a wide range of pollutants ranging from toxic metals and metalloids such as arsenic over organic contaminants and nutrients such as nitrate and phosphate. Moreover, in-situ electrochemical dosing of sulfide by sulfate reduction might present a chemical-free solution for this approach. These potential synergies should be addressed in further investigations.