title: Adsorption of phosphate from seawater on calcined MgMn-layered double hydroxides.
abstract: Adsorptive properties of MgMn-3-300 (MgMn-type layered double hydroxide with Mg/Mn mole ratio of 3, calcined at 300 degrees C) for phosphate were investigated in phosphate-enriched seawater with a concentration of 0.30 mg-P/dm3. It showed the highest phosphate uptake from the seawater among the inorganic adsorbents studied (hydrotalcite, calcined hydrotalcite, activated magnesia, hydrous aluminum oxide, manganese oxide (delta-MnO2)). The phosphate uptake by MgMn-3-300 reached 7.3 mg-P/g at an adsorbent/solution ratio of 0.05 g/2 dm3. The analyses of the uptakes of other constituents (Na+, K+, Ca(+, Cl-, and SO(2-)4) of seawater showed that the adsorbent had a markedly high selectivity for the adsorption of phosphate ions. Effects of initial phosphate concentration, temperature, pH, and salinity on phosphate uptake were investigated in detail by a batch method. The phosphate uptake increased slightly with an increase in the adsorption temperature. The adsorption isotherm followed Freundlich's equation with constants of logK(F)=1.25 and 1/n=0.65, indicating that it could effectively remove phosphate even from a solution of markedly low phosphate concentration as well as with large numbers of coexisting ions. The pH dependence showed a maximum phosphate uptake around pH 8.5. The pH dependence curve suggested that selective phosphate adsorption progresses mainly by the ion exchange of HPO(2-)4. The study on the effect of salinity suggested the presence of two kinds of adsorption sites in the adsorbent: one nonspecific site with weak interaction and one specific site with strong interaction. The effective desorption of phosphate could be achieved using a mixed solution of 5 M NaCl + 0.1 M NaOH (1 M = 1 mol/dm3), with negligible dissolution of adsorbent. The adsorbent had high chemical stability against the adsorption/desorption cycle; it kept a good phosphate uptake even after the repetition of the seventh cycle.
authors.affiliation: Health Technology Research Center, National Institute of Advanced Industrial Science and Technology (AIST), 2217-14 Hayashi-cho, Takamatsu 761-0395, Japan. chitrakar-ramesh@aist.go.jp
authors.forename: Ramesh
authors.initials: R
authors.lastname: Chitrakar
authors.forename: Satoko
authors.initials: S
authors.lastname: Tezuka
authors.forename: Akinari
authors.initials: A
authors.lastname: Sonoda
authors.forename: Kohji
authors.initials: K
authors.lastname: Sakane
authors.forename: Kenta
authors.initials: K
authors.lastname: Ooi
authors.forename: Takahiro
authors.initials: T
authors.lastname: Hirotsu
mesh_terms: D000327:Adsorption; D006358:Hot Temperature; D006863:Hydrogen-Ion Concentration; D006878:Hydroxides / Q000737:chemistry*; D008274:Magnesium / Q000737:chemistry*; D008345:Manganese / Q000737:chemistry*; D010710:Phosphates / Q000737:chemistry*; D012623:Seawater / Q000737:chemistry*; D012965:Sodium Chloride / Q000737:chemistry; D013499:Surface Properties
chemical_list: D006878:Hydroxides; D010710:Phosphates; D008345:Manganese; D012965:Sodium Chloride; D008274:Magnesium