| Phosphorus is not only an important element, causing eutrophication of natural water bodies, but also an unregenerate and irreplaceable resource to modern agriculture and industry. With the adoption of stringent phosphorus discharge limit of wastewater and the depletion of high-quality commercial phosphate rocks, phosphorus removal and recovery have been attracting increasingly great concern, and phosphorus recovery has been becoming one of the most important ways of phosphorus sustainability. However, in our country, the study on phosphorus recovery is just only in the initial stage. The synthetic tobermorite, a hydrothermally synthetic byproduct accompanied with the preparation of potassium salts from potassium feldspar, was employed to recover phosphorus from wastewater as seed material, which contains neither heavy metals nor phosphorus and can decrease chemical reagent costs by giving out OH- and Ca2+.1. The experimental results of no seeds and no alkali showed that pH decreased suddenly when the reaction passed through one hour, which indicated the reaction used much OH- in the solution. However, the experimental results of no alkali and adding seeds showed that pH changed a little, which indicated the seeds can provide lots of OH-. The Ca2+ concentration decreased continuously in the experiment of no seeds and no alkali, but the Ca2+ concentration kept balance when arrived at some concentration in the experiment of adding seeds and no alkali. All these results showed that the seeds can provide vast OH- and Ca2+.2. The experimental results showed that under the conditions of different initial concentration, no additional Ca2+, the seeds concentration of 2g/L and shaking speed of 170r/min, the higher initial phosphorus concentration is, the more Ca2+ concentration provided by the seeds is.3. The experimental results showed that pH value of the solution decreased faster with the increase of the initial concentration of phosphorus. In the other words, the higher concentration of phosphorus is, the faster reaction rate is. The reaction used lots of OH-.4. Batch experimental results showed the pH=8, Ca/P=2 (molar ratio), seed concentration of 2-2.4g/L and shaking speed of 140r/min were preferable. Under the optimized conditions, the seeds reuse experiments with the initial phosphorus concentration of 50mg/L showed that the value of pH increased gradually, the phosphorus concentration decreased gradually and the Ca2+ concentration after 5min stabilized relatively with the reaction progressing. The pH value, the Ca2+ concentration and the phosphorus concentration stabilized relatively in every reuse experiment. The synthetic tobermorite adsorbed phosphorus of sewage before 5 min, and the crystallization happened when the Ca2+ concentration stabilized (after 5min).5. With the thermodynamic analysis and calculation, the vaLue△Gθ<0 and the value△Sθ>0 benefited the crystal process. And the value△Hθ>0 showed the crystal process is endothermic reaction.6. With the dynamic analysis and calculation, the reaction order to the reaction temperature was confirmed by the differentiation. By drawing linear plot of n versus T, we got n=-0.1409T+45.792, R2=0.9457.7. The characteristics of the products were determined by Powder X-ray Diffraction (XRD), Fourier Transform Infrared (FTIR), Scanning Electron Microscopy (SEM) and EDS analytic techniques. The results showed that octacalcium phosphate(OCP) existed in the sample of 2h and then changed slice hydroxyapatite (HAP)with lower crystallization degree. |
