Study On The Structure And Chemical Durability Of Sulfur-Containing Iron Phosphate Glass Waste Forms

Sulfur is one of the main elements in certain high-level waste（HLW）from China.It is difficult to deal with those HLW due to the high volatilization of sulfur at high temperatures.Phosphate glass shows the advantages of low melting temperature and high waste loading,so they are good matrices for vitrifying"insoluble"elements in HLW,especially,iron phosphate glass has been widely studied due to its excellent stability.As the widely used borosilicate glass,sulfur is difficult to incorporate into the structure of iron phosphate glass.In order to investigate the immobilization behavior of sulfur in iron phosphate glass,this paper explored the hosting of sulfur in iron phosphate glass,the influences of component changes,melting temperature and network polymerization degree on the loading capacity of sulfur,structure and stability of the waste forms.Moreover,the effect of neodymium（Nd）on the structure and chemical stability of sulfur-containing iron phosphate waste forms was investigated.By optimizing of the chemical composition and preparation parameters,iron boron phosphate waste forms with high loading capacity of sulfur and excellent stability were obtained.The main conclusions are as follows:（1）Due to the volatilization of sulfur,the“actual solubility”of sulfur in 60P₂O₅–27Fe₂O₃–13Na₂O（mol%）iron phosphate glass is extremely small,with the maximum of0.08 wt%（in terms of SO₃）,and sulfur is in the form of SO₄^2-located in the interstitial position in the phosphate glass network.During the melting process of the glasses,most of the sulfur is volatilized between 450 and 600°C,and the“actual solubility”of sulfur decreases with the increase of melting temperature.Moreover,the“actual solubility”of sulfur is positively correlated with degree of depolymerization,but the depolymerization induced by Na₂O addition cannot achieve an obviously increasing solubility of sulfur in iron phosphate glasses,with a maximum of only 0.12 wt%.In the glasses,the Q¹ phosphate groups are decreased,the chemical stability is reduced with the incorporation of Na₂O.When the content of Na₂O is 35 mol%,the normalized leaching rate of Na reaches up to 5g·m^-2·d^-1 for 7 days.（2）The“actual solubility”of sulfur in 46P₂O₅–14Fe₂O₃–10B₂O₃–10Na₂O–20Ba O（wt%）glass is up to 2.73 wt%（in terms of SO₃）.The increase of the loading capacity of sulfur is due to the decrease of P₂O₅ content in the component,which reduces the volatilization of sulfur by decreasing the reaction with ammonium at 450℃and increases the complexity of the glass network,which increases the space to accommodate sulfur.Although the glass is fully amorphous,there is a great tendency to crystallize.The network structure of the sulfur-containing iron-boron phosphate glass is mainly constituted of Q¹ group,a small amount of Q⁰ and Q² group.With the increasing content of sulfur,Q⁰ and Q¹ groups first increase and then decrease,Q² groups first decrease and then increase,[BO₃]conversing to[BO₄],and accordingly T_g first increase and then decrease.When the sulfate addition is less than 3 wt%（in terms of SO₃）,the glass network is polymerized,and if more than 3 wt%of sulfate is added,depolymerization of the glass network occurs.It is proved that the normalized leaching rates of B and P are both below 10^-3g·m^-2·d^-1 after 28 days.（3）Incorporating Nd₂O₃ be conducive to the loading of sulfur in 43P₂O₅–14Fe₂O₃–10B₂O₃–10Na₂O–20Ba O–3SO₃（wt%）glass.The addition of Nd₂O₃ is less than or equal to 2wt%,the“actual solubility”of sulfur in the glass increases from 1.38 wt%to 1.46 wt%（in terms of SO₃）;while with more than 2 wt%of Nd₂O₃,the“actual solubility”of sulfur further increases from 1.46 wt%to 1.92 wt%although monazite crystals precipitate in the glass.It is obtained that Nd“enters”into the glass network by forming Nd–O–P bonds interconnected with phosphate groups.With the increase of Nd₂O₃,Q⁰ group and B–O–P bond in the phosphate glass increase and the glass network is enhanced.After the precipitation of the monazite crystalline phase,the T_g,exothermic peak and crystallization tendency of the glass phase in the waste forms decrease.All the waste forms,with or without crystalline phase,show good chemical stability,the elemental normalized leaching rates of B,P and Nd after28 days are less than 3.734×10^-4,3.8866×10^-4 and 9.4243×10^-6 g·m^-2·d^-1,respectively.Moreover,the concentration of Nd in the leaching solutions for the waste form containing 8wt%Nd₂O₃ is below the detection limit(0.0960 mg·L^-1)of ICP-OES.