Phosphate Adsorption Onto Thermally Dehydrated Aluminate Cement Granules

In China,most of the lakes and reservoirs are faced with the problem of eutrophication.Phosphorus is the limiting factor of eutrophication in water body.Therefore,the control of phosphorus content in water body is the key to prevent eutrophication.The adsorption method has been widely concerned due to its simple operation,no secondary pollution and can recovery and utilization of phosphate resources.However,it remains some demerits,such as low adsorption capacity and high cost of adsorbents.In this study,a new granular activated adsorbent（THACs）was prepared,which originated from the aluminate cement and was thermally activated at different temperatures.The physical and chemical properties of THACs were analyzed and the optimal activation temperature（600℃）was determined according to the phosphate adsorption capacity and loss rate of mass of THACs.Phosphate adsorption properties and adsorption mechanism onto T600 was discussed and the application prospect of T600 as a new type of cheap and efficient phosphate adsorbent was studied.The results of the study are as follows:（1）After thermally activation,there are obviously changes of physical and chemical properties of hydrated aluminate cement.The dehydration of hydration products of aluminate cement samples gradually occurs with the increase of temperature.The surface of samples become more rough and porosities increase.After calcination at900℃,the surface of sample is cracked.The surface area and the pore volume of samples fluctuate along with the increase of activation temperature,and T600 and T700get larger surface area up to 24.05 m²/g and 24.79 m²/g,respectively.The peak intensity of Al-O first increase and then decrease with the increase of temperature,and reach the highest at 600℃.（2）The adsorption capacities of phosphate onto THACs fluctuate with the increase activation temperature and those of T600 and T700 are 49.1 mg/g and 50.1mg/g respectively,which are larger than other THACs.The loss rate of mass of T600and T700 are 6.6%and 11.4%,respectively.600℃is the optimal activation temperature of hydrated aluminate cement through comprehensive consideration.（3）T600 has a fast initial adsorption rate for phosphate,and adsorption capacity reaches 23.7 mg/g within 30 min.The adsorption capacity of phosphate onto T600decrease with the increase of pH.The phosphate adsorption on T600 was better under neutral and acidic conditions.T600 has a good selectivity for phosphate adsorption and the order of the influence of coexisting ions on phosphate adsorption is CO₃^2-,SO₄^2-,NO₃^-.The adsorption of phosphate onto T600 follows the inner-sphere complex mechanism since the adsorption capacity was almost not affected by the ionic strength.（4）The adsorption process of phosphate onto T600 could be better described by the pseudo-second-order model and isothermal adsorption process accords with Freundlich isotherm model.Phosphate precipitates occurred after phosphate adsorption,illustrating that phosphate ion successfully adsorbed onto the adsorbent surface and phosphate precipitation occurred in the adsorption process.Both physical adsorption and chemical adsorption occurred in the adsorption process and it was dominated by chemical adsorption,mainly through inner-sphere complexion and phosphate precipitation on the surface of the adsorbent.