Adsorption capacity of iron oxyhydroxide-coated brick for cationic metals and nature of ion–surface interactions

The adsorption of six metal ions (Cd²⁺, Cu²⁺, Fe²⁺, Ni²⁺, Pb²⁺ and Zn²⁺) from an aqueous solution by a new composite – iron oxyhydroxide (ferrihydrite) deposited onto pre-activated brick grains – was investigated under dynamic experimental conditions. Breakthrough curves were obtained under specified fixed-bed-column conditions, and the determined adsorption capacity (molar basis), Q, followed the order of QPb(II) > QCd(II) ≈ QZn(II) > QFe(II) > QNi(II) > QCu(II). An attempt was made to correlate the ionic characteristics of these cationic metals with adsorption performance, and data revealed how strongly metal cations were attracted to negatively charged brick sites via electrostatic interactions. In addition to electrostatic attractions, a process implicating outer-sphere complexes, ≡ S⁻Na(H₂O)₆⁺ and inner-sphere complexes, (≡ S⁻)₂Me^{2 +} (Me = metal), was suggested. ESEM/EDS analysis showed that cations were preferentially sorbed by ferrihydrite, rather than other hydroxyl groups in brick clays. A correlation also was found between the hydration free energy of Me²⁺ ions and hydrated radius, raising the possibility of water molecules being implicated in this adsorption mechanism. To confirm this hypothesis, ²³Na and ¹H MAS NMR were applied to our samples in order to examine the bonding states of sodium and water molecules and their dynamics at the material surface.