Simultaneous Removal of Nutrients by Geopolymers Made From Industrial By-Products

Abstract

An effective way to recover phosphate and ammonium from contaminated waters is of great demand. Nutrients can be reused and applied to land as valuable fertilizers. Composite adsorbents were prepared from industrial waste materials and calcined natural clay. The ability of the new adsorbents to simultaneously remove phosphate and ammonium from diluted solutions was evaluated. Paper mill sludge or blast furnace slag together with kaolinite clay were used as raw materials to produce inorganic polymers by alkaline activation. All raw materials and composites have been characterized by XRF and XRD. The influence of clay and waste material in the adsorbent composition, the adsorbent dose, and time of adsorption characteristics have been investigated at static conditions by bench-top tests. For the best identified composition (metakaolin and blast furnace slag composite), the phosphate adsorption increases from 0.05 mg-P/g for pure clay up to 8.5 mg-P/g for composite with blast furnace slag content of 60 wt.%, while a decrease on the ammonium sorption capacity from 15 mg-N/g to 7 mg-N/g is observed. Phosphate removal was enhanced when ammonium was present, while ammonium removal was slightly varied whether phosphate ions in the system or not. In case of ammonium, ion exchange is the likely mechanism of removal, whereas in the case of phosphate surface precipitation in form of hydroxyapatite appears to occur.