Adsorption Characteristics Of Fluoride On Calcined MgAlFe-CO₃Layered Double Hydroxides

Fluorine is one of the essential elements for human health, and a moderate amount of fluoridecan prevent dental caries, but excess intake of fluoride would lead to various diseases such asdental and skeletal fluorosis. Adsorption is one of the most extensively used treatment methods forremoving fluoride from water. The adsorption capacity of the conventional adsorbents is generallylow, so it is an urgent task to develop a new and efficient adsorbent with low-cost. Layered DoubleHydroxides （LDHs） are a class of layered anionic clays and their calcined products showed goodadsorption potential for toxic and hazardous anions.In this paper, on the basis of M2+/M3+mole ratio of2, adding different proportions of Fe, aseries of MgFeAl-CO3LDHs were prepared by co-precipitation method, calcined at500℃for3h,to get the calcined samples and characterized using XRD, SEM, ICP-OES and BET. The effect ofadsorbent dosage, adsorptive time, pH value of adsorbate solution, fluoride ion concentration andcoexisting ions factor on the adsorption capacity of the adsorbents were investigated. The kineticdata, equilibrium data of adsorption studies were processed. The regeneration of the adsorbent wasstudied. The results are as follows:1) Characterization of The prepared samples: The samples are similar with the structure ofLDHs and closer to the actual composition and the amount of raw materials. With the increase ofFe/Al molar ratio, the interlayer spacing increased slightly. Calcined at500℃for3h, layerstructure of the LDHs disappeared. SEM shows sample grain diameter is about50nm. Themorphology of the LDHs didn’t change much after iron doped. Calcined at500℃, particlesagglomerated seriously. Specific surface area of iron-doped LDHs is greater than no iron-dopedLDHs and after calcined, the situation was exactly the the opposite.2) The experimental results demonstrate that maximum fluoride removal was obtained at a pH4.0⁸.0and it took4h to attain equilibrium. The adsorbent dose was0.5g·L-1. Kinetics adsorptionprofiles offered excellent fit with pseudo-second-order model. The intraparticle diffusion modelindicates there are two-stage adsorption processes, in the first stage, the fluoride adsorption occursat the surface, in the second stage the adsorption goes into the hole. The adsorption data fitted wellinto the Langmuir equation. With the increase amount of iron, the adsorption of fluoride ironincreased first and then decreased, because Iron doped increased the interlayer distance of theLDHs, which benefit to fluoride to come into the interlayer, however, excessive iron would destroy memory effect, and lead to low adsorption capacity. The best molar ratio of Al and Fe is2and the adsorption capacity can reach71.94mg·L-1. The adsorption mechanism involves theadsorption of fluoride ions on CMAF occurs by the reconstruction of the layered structure.3) Effects of coexisting ions on adsorption of fluoride was investigated and the descendingorder of the influence is CO₃2-> PO₄3-> SO₄2->NO₃-≥Cl-. After regenerated by calcining methodthree times, CMAF-2still has defluoridation properties, but with the increase of the regenerationtimes, the amount of fluoride sorption decreasesd gradually.4) After the adsorption of fluoride, CMAF were characterized and XRD analysis showed thatthe samples recovered its layer structure partly. the SEM showed after the adsorption of fluoride,the CMAF-0showed typical layered structure, while the layered structure of CMAF-2is irregular.Both are very different to the morphology of the calcined samples. The adsorption of fluoride onCMAF mainly depended on the "memory effect", which is anion came into the interlayer toreproduce the layered structure. Iron-doped could inhibit the the releasing of Al, when the Fe/Almolar ratio is greater than1:2, almost no aluminum ion released, so iron-doped could improve thestability of the adsorbent and produce no seconday pullotion.As a new type of adsorbent for removing fluoride, CMAF has the following characteristics:easy preparation, obvious removal of fluoride ions attained at low dose with high adsorption rate,solid and liquid dividing easily and no secondary pollution produced.