Can calcined oyster shells efficiently remove nitrate and phosphate of agricultural origin from groundwater in Ghana?

High levels of nitrate (NO₃^-) and phosphate (PO₄^3-) in water sources pose serious threats to both human health and the environment. In this study, the potential of calcined oyster shell (COS) as a low-cost and sustainable adsorbent for removing NO₃^- and PO₄^3- from groundwater in Ghana was investigated. Oyster shells were calcined at temperatures ranging from 200 °C and 800 °C, characterized using standard analytical techniques, and evaluated for their adsorption performance. The sample calcined at 600 °C (COS-600), predominantly composed of magnesian calcite (Ca₀.₉Mg₀.₁CO₃), exhibited the highest adsorption performance, with maximum adsorption capacities of 2.48 mg/g and 3.42 mg/g for NO₃^- and PO₄^3-, respectively. This enhanced performance was attributed to the improved surface and structural properties resulting from calcination. Adsorption was strongly influenced by solution pH, with higher removal observed under acidic conditions. Kinetic data followed the pseudo-second-order model (R² > 0.99 for NO₃^- and PO₄^3-), while equilibrium data were best described by the Langmuir isotherm (R² > 0.99 for NO₃^- and PO₄^3-), indicating monolayer adsorption governed primarily by chemisorption. Thermodynamic analyses further revealed that the adsorption process was spontaneous and endothermic. Regeneration experiments demonstrated the reusability of COS-600, highlighting its potential as a recyclable and eco-friendly material for treating nitrate- and phosphate-contaminated groundwater.