Reaction Mechanisms of Simultaneous Removal for Nitrate and Phosphate in Groundwater Using Ca-citrate Complex

Ca-citrate complex has been proposed as a substance capable of simultaneously removing both nutrients (i.e., nitrate and phosphate), which may lead to serious environmental issues such as eutrophication. Citrate serves as a carbon source for denitrification process, and calcium precipitates to form phosphate minerals. The purpose of this study was to comprehend the mechanisms involved in the removal of nitrate and phosphate. In this study, column experiments were conducted to simulate the simultaneous removal of nitrate and phosphate. Upon injecting the Ca-citrate complex into the column, both nutrients were eliminated. In the process, bacterial communities in the soil and effluent were investigated to identify removal mechanism for nitrate. The bacterial communities significantly differed between the soil and the effluent. The bacteria in the soil (Bacillus, Enterobacter, and Arthrobacter) were primarily involved in processes of NO₃ or NO₂ reduction, while those in the effluent (Pelosinus, Azospirillum, and Pseudomonas) carried out the complete denitrification process. These results suggested an active denitrification process in the column, resulting in the complete removal of nitrate to N₂ gas. Meanwhile, in order to identify the removal mechanism of phosphate, phosphate minerals for the soil samples after the reactions with Ca-citrate complex were observed using electron probe micro analysis (EPMA). In the raw soil, silicate minerals were abundant, including quartz and plagioclase feldspar group such as anorthite, oligoclase, and orthoclase. Silicon (Si), oxygen (O), and aluminum (Al) were abundantly distributed throughout the scanned area, supporting that the presence of silicate minerals. On the other hand, for the soil after 24 hours of the reaction, phosphate minerals found included hydroxyapatite (Hap), calcium-deficient hydroxyapatite (CDHA), and amorphous calcium phosphate (ACP). The Ca/P molar ratio was a range of 1.61–1.66, supporting that the phosphate was removed by precipitation of Hap. Moreover, CDHA and ACP serve as intermediaries in Hap crystallization, indicating that the experimental environment was in Hap formation phase. After 120 hours, anapaite was observed in the soil, which is a type of Ca-Fe-P-mineral. In this study, the experimental condition was a highly reducing environment, as indicated by the changes of iron concentration. Ultimately, phosphate was precipitated with calcium and iron. The EPMA results indicate that Ca-citrate can remove the phosphate by precipitation of phosphate minerals (e.g., hydroxyapatite, amorphous calcium phosphate, and anapaite). This study concluded that the simultaneous removal mechanisms for nitrate and phosphate involved denitrification and precipitation by the Ca-citrate complex.