High Sorptive Removal of Borate from Aqueous Solution Using Calcined ZnAl Layered Double Hydroxides

Borate uptake studies were carried out over both as-synthesized and calcined ZnAl layered double hydroxides (LDHs) containing carbonate or chloride as interlayer anions, with different Zn/Al atomic ratios (2, 3, and 4). Chloride-containing materials in both as-synthesized and calcined forms showed higher borate uptake than carbonate-containing forms. A maximum borate uptake of 31 mgB/g was achieved for the 400 degrees C heat-treated Zn2Al-Cl LDH material (referred here as Zn2Al-Cl-CLDH). The mechanism of borate uptake for heat-treated materials involves reconstruction as elucidated from PXRD, FT-IR, and SEM measurements. Interestingly, solid state transformation of tetraborate to monoborate anion occurred in the interlayer on drying for longer time. At a ratio of 4 g Zn2Al-Cl-CLDH per liter of 100 mgB/L solution, the uptake of borate reached 97% further, and at 5 mgB/L solution the borate concentration reduced below the value of 0.5 mgB/L recommended by the World Health Organization (WHO), using 1 g/L ratio of sorbent. The borate uptake adsorption isotherm fitted well to a monolayer Langmuir adsorption model. The influence of various parameters such as the mass of adsorbent, reaction time, borate concentration, temperature, and pH of the medium was studied. Constant borate uptake was observed over the pH range 3-7 owing to the buffering nature of LDH. The influence of the copresence of other anions on borate uptake by Zn2Al-Cl-CLDH depended on the charge to radius ratio of the anion. Cyclic adsorption-desorption studies revealed that the material was recyclable. High uptake capacity along with recyclability of this material suggests the promise as a sorbent for wastewater remediation of borate.