Study On Adsorption And Recovery Of Phosphate By Magnetic Hydrotalcite-Like Compounds In Aqueous Solution

The phosphorus resource of our county will turn to be in extremely shortage after 2025. Meanwhile, its release in a large amount to water body, occurring mainly as phosphate in municipal and industrial wastewaters and agricultural runoff has destroyed the ecological balance of water environmental system, and caused eutrophication, resulting in deterioration of water quality. Thus, pursuing of technologies for phosphorus removal and recovery from wastewaters can not only improve the water quality and lighten the contamination, but also realize the phosphorus recycling and sustainability, relieve the deficit of phosphorus resources.Recently, a number of studies have been focused on adsorption for phosphate removal and recovery. However, the big disadvantage that the suspended solid (SS) contained in the feed water accumulates in and clog the fixed adsorbent bed has greatly confined wide application of it. If fluidized–bed adsorption was adopted, the recovery of adsorbent will turn out to be the key problem. In this paper, magnetic hydrotalcite-like compounds adsorbent which has good ability of phosphate adsorption and magnetic separation and recovery was originally prepared by coprecipitating on the micron Fe3O4 powder surface.Firstly, a series of magnetic hydrotalcite-like compounds (HTALs) such as Mg-Al, Zn-Al, Ca-Al, Mg-Fe, Zn-Fe and Ca-Fe were synthesized in this paper by using coprecipitation method on the micron Fe3O4 powder surface. Adsorption of phosphate by these hydrotalcite-like compounds (HTALs) was investigated. The nature and content of di- and trivalent cations in HTALs had strong influence on the adsorptive capacity. The Mg-Al HTALs with Mg/Al molar ratio of 2.0 showed the highest adsorptive capacity. The pH value had certain influence on the adsorption process, and pH 5⁷ was determined as the better pH working range of the magnetic adsorbent. The experimental results indicate that the adsorption isotherm fitted to L3 type of istotherm. The kinetic studies showed that the order of fitness of different kinetic models followed as the pseudo second-order equation>the pseudo first-order equation> Elovich equation. The affinity of the magnetic adsorbent toward common anions is in the following order: SO₄^2- >CO₃^2- >Cl^->NO₃^-. The desorption and regeneration performance of the adsorbent showed that as for adsorbent with phosphate adsorptive capacity of 83.6mgP/g, a 10wt% NaOH solution desorbed effectively at 92%, and a 10wt% NaCl solution regenerated most effectively. Almost all phosphate in the exhausted desorption solution were precipitated by addition of CaCl₂ according to the Ca/P molar ratio 3.0. The confirmatory test for the adsorbent using in real wastewater treatment also showed its effectivity.Based on the above research on the adsorption ability, the whole phosphate recovery process was established as a three section flow which contained phosphate removal section, adsorbent desorption and regeneration section and phosphate recovery section, in which the desorption solution and regeneration solution can be recycled and reused by supplying additional chemicals. Besides, a jacket magnetic separation reactor was designed and further used in the simulation experiments for the phosphate recovery process. The simulation results showed that each of the batch and continuous operation had their advantages. The batch operation can increase the adsorptive capacity while the continuous operation can avoid the inconvenience of separation between the powder adsorbent and the suspended solids. However, no matter which operation method was chose, multilevel processing should be adopted to guarantee the effluent water quality and meet the related water quality standards. Ideal recovery product, hydroxylapatite and calcium hydroxide was obtained in simulation of the phosphate recovery process with both simulated water and sludge liquor.The characterization analysis verified that the adsorbent had Fe₃O₄ nucleus and can be separated and recovered by magnetic separation technology. The magnetic adsorbent can effectively adsorb phosphate in aqueous solution and can also be easily desorped and regenerated. The thermal decomposition consisted of three steps. The first step and endothermic peak at about 111℃corresponded to the loss of the external surface physisorbed water and the interlayer water molecules. The second step and weak endothermic peak corresponded to mass losses due to the removal of the interlayer anions such as hydroxyl ions, carbonate ions and the chloride ions etc. The third distinct endothermic peak can be assigned to the dehydroxylation of the hydroxide layer. The BET specific surface area of the magnetic hydrotalcite-like compounds adsorbent was 20.3863 m2/g. The hysteresis loops in the nitrogen adsorption-desorption isotherm corresponded to typical flat plate slit-shaped pore which just accorded with the structure property of HTALs. SEM and EDS spectra analysis showed that the HTALs adsorbent was non-homogeneous. The HTALs loaded on the external surface of the Fe₃O₄ particles was relatively excessive, thick and dense. In addition, the formation of a new kind of crystals was found in the SEM images of adsorbent with adsorptive capacity of 91.1mgP/g, which was further determined as mixed phosphate with the approximate molecular composition of Mg_12.38Al_6.01K_0.80H_0.64(CO₃)_10.32(PO₄)_7.79 through EDS analysis.The phosphate adsorption mechanism of magnetic HTALs was concluded as ion exchange, surface complexation and the formation of mixed phosphate based on the above characterization analysis plus the adsorption isotherm data. Schematic model of the adsorption process was also established.