Accessible Surface Area, Not Layer Chemistry, Governs Pb(II) Uptake by Carbonate Layered Double Hydroxides: A Single-Protocol Comparison of Mg2Al–CO3, Mg2[FeAl]–CO3 and Zn2[FeAl]–CO3

The removal of Pb(II) ions from aqueous solution by sorption onto layered double hydroxides (LDHs) is widely documented, yet the reported capacities range from about 60 to about 120 mg g−1. This dispersion is usually ascribed to the layer chemistry, but this attribution cannot be tested because the published values were obtained under widely different operating conditions; this work supplies the missing reference member of a single-protocol series. Carbonate-intercalated Mg2Al–CO3 was prepared by coprecipitation at constant pH, characterized by XRD, SEM–EDX, N2 sorption, DSC and pHPZC measurement, and evaluated as a function of sorbent dose, pH, contact time, initial concentration and temperature. Equilibrium was reached within 60 min; the kinetics followed the pseudo-first-order model; the Redlich–Peterson and Langmuir equations best fitted the isotherm (monolayer capacity, 59.1 mg g−1); and the uptake was spontaneous and endothermic (ΔH° = 11.1 kJ mol−1). These results were then compared with those previously published by our group for two isostructural phases, Mg2[FeAl]–CO3 and Zn2[FeAl]–CO3, prepared and tested under strictly identical conditions; no new experiments were performed on these solids, whose values are quoted for comparison only. Replacing half of the Al(III) with Fe(III) doubled qm to 118.8 mg g−1, whereas further replacing Mg(II) with Zn(II) lowered it to 87.9 mg g−1. Normalized to the BET surface area, however, the three capacities converge on 1.55 ± 0.07 mg m−2: the layer composition acts on Pb(II) uptake only through the accessible surface area it generates, not through the intrinsic reactivity of the surface. Sorbent design should therefore target textural disorder rather than a particular cation pair.