Oxidation of Deep Well Saline Groundwater Generates the Precipitation of Ferrous Sulfide (FeS)

Hydrogen sulfide (H₂S) gas can be traced once the groundwater is pumped out from a deep well which is located <10 km from the costal line. The groundwater contains 5.1 ± 0.1 ppt of salinity which is classified as saline groundwater. The initial color of the groundwater is green yellowish. After 40 s exposed to the oxygen, its colour suddenly turned to black and become sludgy. Afterwards, the black colour turns to partially cloudy after 8 h being exposed to the oxygen, subsequently, the H₂S gas vanishes along with the disappearance of the black colour. Hence, from this reaction, this study aims to investigate the cause of the black precipitate formation which comes from the oxidation of the deep well saline groundwater. Based on the XRD and DSC results, the black precipitate is a troilite mineral (FeS). The elements that contained in the groundwater mostly originated from the seawater. The fast precipitation is caused by the Cl⁻ content which is increasing the oxidation rate. The increase of Fe²⁺ is caused by the weathering process during the travel of the groundwater through the aquifer. Meanwhile, \( {\text{SO}}_{4}^{2 - } \) is decomposed by microorganism to produce S²⁻ and this causes the reaction of Fe²⁺ and S²⁻ to form FeS despite in saline condition.