Effect of pH and Calcium on the Adsorptive Removal of Cadmium and Copper by Iron Oxide–Coated Sand and Granular Ferric Hydroxide

Iron oxide–coated sand (IOCS) and granular ferric hydroxide (GFH) were used to study the effect of ${\mathrm{Ca}}^{2+}$ and pH on the adsorptive removal of ${\mathrm{Cu}}^{2+}$ and ${\mathrm{Cd}}^{2+}$ from groundwater using batch adsorption experiments and kinetic modeling. It was observed that ${\mathrm{Cu}}^{2+}$ and ${\mathrm{Cd}}^{2+}$ were not stable in synthetic waters. The extent of precipitation increased with increasing pH. Removal of ${\mathrm{Cu}}^{2+}$ and ${\mathrm{Cd}}^{2+}$ was achieved through both precipitation and adsorption, with IOCS showing higher adsorption efficiency. Increase of pH (from 6 to 8) resulted in a higher overall removal efficiency of both ${\mathrm{Cu}}^{2+}$ and ${\mathrm{Cd}}^{2+}$, with precipitation as predominant removal mechanisms at higher pH values, especially for ${\mathrm{Cu}}^{2+}$. An increase in ${\mathrm{Ca}}^{2+}$ concentration increased the precipitation of ${\mathrm{Cu}}^{2+}$ [as ${\mathrm{Cu}}_2{(\mathrm{OH})}_2{\mathrm{CO}}_3$ and ${\mathrm{Cu}}_3{(\mathrm{OH})}_2{({\mathrm{CO}}_3)}_2$] and ${\mathrm{Cd}}^{2+}$ [as $\mathrm{Cd}{(\mathrm{OH})}_2$ and ${\mathrm{CdCO}}_3$]. In addition, ${\mathrm{Ca}}^{2+}$ competes with ${\mathrm{Cu}}^{2+}$ and ${\mathrm{Cd}}^{2+}$ for surface adsorption sites on IOCS and GFH, and reduces their adsorption capacity. The kinetic modeling revealed that the adsorption of ${\mathrm{Cd}}^{2+}$ onto IOCS is a complex process, with limited contribution of chemisorption that increases in the presence of ${\mathrm{Ca}}^{2+}$.