Magnesium And Aluminium Layered Double Hydroxides: Modification, Granulation And The Adsorptive Removal Of Phosphorus And Nitrogen Anions

Powder magnesium and aluminium layered double hydroxides (Mg-Al LDH) is modifiered and granulated (MG Mg-Al LDH) by powder mixture, slurry mixture, deposition and dipping methods with magnesium compounds. After modification and granulation, the fixed bed can not be accumulated and cloged by the adsorbents. PO43-, P3O105-, NO2- and NO3- are removed from aqueous solution, sewage and seawater by Mg-Al LDH and MG Mg-Al LDH with batch adsorption and column experiments. It is shown that MG Mg-Al LDH is an effective adsorbent and water quality can satisfy with the first degree of integrated wastewater discharge and seawater standards. So this work is very useful for the practical application of Mg-Al LDH and the removal of phosphorus and nitrogen anions from different aquatic system.The rigidity of MG Mg-Al LDH is very high (there is no powder appeared after dipped into water for more than two weeks). And the surface area of MG Mg-Al LDH made by deposition method is 233.34 m2·g-1,40% higher than granulated Mg-Al LDH. The mole ratio of magnesium to aluminium on MG Mg-Al LDH is also higher than granulated Mg-Al LDH. The modification and granulation of Mg-Al LDH is carried out by hydroxyl group and ion exchange.The adsorption isotherms of PO43-, P3O105-, NO2- and NO3- on MG Mg-Al LDH are belonged to H type. The adsorption of PO43-, P3O105- and NO2- on MG Mg-Al LDH are more fitted to Langmuir isotherm equation, but NO3- on MG Mg-Al LDH is more fitted to Freundlich isotherm equation. The maximun adsorption capacity of PO43-, P3O105-, NO2- and NO3- is 34.84,29.41,22.52 and 17.49 mg·g-1, respectively. It is 29% to 37% higher than granulated Mg-Al LDH. The adsorption processes are fitted well to pseudo-second kinetic models. The adsorption rate contants is K2 (PO43-)>K2 (P3O105-)>K2 (NO2-)≈K2 (NO3-). The removal of PO43-, P3O105- and NO2- is not influenced by initial pH of 4.0 to 10.0, but the removal rate of NO3- is decreased with increasing pH.The breakthrough curves, breakthrough time and capacity are influenced by adsorbents, bed depth, flow rate, initial concentration and initial pH. The breakthrough time and capacity are increased with increasing bed depth, decreasing flow rate and initial concentration. The adsorption processes are controlled by film and intraparticle diffusion. The adsorption of PO43- is favorate to weak alkline environment, NO3- and NO2- are favorated to weak acid environment. P3O105- is not influenced by initial pH of 6 to 9. The exhausted MG Mg-Al LDH can be desorbed by 0.1 mol·L-1 NaOH and 3 mol·L-1 NaCl, and regenerated by 25 w/V% MgCl2. The regeration rate of PO43-, P3O105-, NO3-, NO2- is 126.24%,71.86%,120.67% and 1514.61%, respectively.P3O105-, PO43-, NO3- and NO2- can be removed simultaneously from aqueous solution by MG Mg-Al LDH and PO43- is priority. PO43-, NO3- and NO2- from sewage can be removed by MG Mg-Al LDH to satisfy with the first degree of integerited wastewater discharge standard. PO43- can be removed from seawater in Qindao by MG Mg-Al LDH to satisfy with the first degree of seawater quality.