Removal efficiency of arsenate and phosphate from aqueous solution using layered double hydroxide materials: intercalation vs. precipitation

Adsorption behaviours of arsenate and phosphate over Mg-based and Ca-based layered double hydroxide (LDH) adsorbents have been examined in kinetics and thermodynamics. Removal of these anions from aqueous solution follows the Lagergren first-order and/or pseudo-second-order model, and the adsorption isotherm is well fitted with either the Langmuir or the Freundlich model. Structure analysis of used LDH adsorbents reveals that two processes, i.e. intercalation and precipitation, are responsible for the anion removal. Adsorption over the Mg-based LDH adsorbent occurs by way of intercalation into the interlayer spacing while that over the Ca-based adsorbent occurs by means of precipitation of dissolved Ca2+ with the anion. More particularly, we have found that As(V) at a concentration below 10 mg/L can be very efficiently removed through intercalation into the interlayer of reconstructed MgAl-LDHs, with less than 0.010 mg/L of As left in solution. We have also noted that phosphate at [P] up to 100 mg/L can be quickly and effectively removed through precipitation with CaAl-Cl-LDH, giving rise to similar to 0.1 mg/L of P left in solution with the maximum adsorption amount up to 135 mg/g. Therefore, these two LDH materials (calcined Mg3Al-CO3-LDH and uncalcined Ca2Al-Cl-LDH) are potential cost-effective adsorbents for arsenate and phosphate, respectively.