The Adsorption Characteristics Of Phosphate To Hydrated Ordinary Portland Cement

Ordinary portland cement is a common building material in our lives.Overseas studies have shown that hydration of ordinary portland cement can be effective in the adsorption of phosphate in water,however,a similar report has not been found in domestic.In order to develop a new,low-cost and effective adsorbent for phosphate removal to prevent or control the increasingly serious eutrophication of natural waterways,this paper demonstrated the adsorption behavior of phosphate to hydrated ordinary portland cement in water,and made a thermodynamic analysis on the adsorption behavior.The results are as follows:(1) The adsorption equilibrium could not be reached when the new-hydrated particle of ordinary portland cement was not washed by deionized water for a long term, even the addition of phosphate reached up to 100mg/L.It results from a chemical reaction between phosphate and numerous alkaline matters in hydrated ordinary portland cement.After near three months washing by deionized water,the pH value of the solution with hydrated ordinary portland cement particle could be stable at 7.95 from the value of 11.76 in the beginning.(2) The results of dynamic analysis on the adsorption of phosphate to hydrated ordinary portland cement revealed that the adsorption rate decreased gradually and the adsorption equilibrium might take eight days to reach.Characteristics of adsorption rate obeyed the second quasi-dynamic model.(3) The behavior of phosphate adsorbed on the particle of hydrated ordinary portland cement could be fitted as Freundlich isotherm very well.The adsorption capacity,which was more than the most of reported adsorbent for phosphate,could be 9.46mg/g and 14.44mg/g at 20℃and 4℃,respectively.(4) It showed that the adsorption of phosphate to hydrated ordinary portland cement is a spontaneous process since the adsorption Gibbs free energy was -3.01 kJ / mol.On the other hand,the result of the low adsorption heat(-4.84 kJ / mol) suggested that the whole adsorption reaction was mainly physical and exothermic process.(5) The fact that a large number of the adsorption of phosphate to the particle of hydrated ordinary portland cement could be attributed to the physical adsorption on particle surface and the chemical reaction between variety components of internal cement particle and the phosphate adsorbed.