镅在北山地下水中赋存形态及溶解度分析

使用PHREEQC软件及OECD/NEA出版的镅热力学数据,利用相关矿物的沉淀饱和指数,计算了镅在北山地下水中的赋存形态,探讨了pH值、方解石、CO₂分压及腐殖酸对镅的形态分布与溶解度的影响。结果表明,在近中性条件下镅主要以AmCO⁺ ₃和 AmSiO(OH)²⁺₃形式存在,在碱性条件下易形成AmOHCO₃·0.5H₂O(cr)和Am(OH)₃(cr)沉淀,因此偏碱性的北山地下水有利于阻滞镅的迁移。 围岩中存在的方解石对镅的溶解度和迁移性影响较复杂,在pH 6～8条件下,方解石能微弱地降低镅的溶解度;在pH 8～9.7时,方解石会提高Am(CO₃)^-₂离子的浓度,进而提高镅的溶解度,增强其可迁移性;而在pH 9.7～11时,方解石有利于AmOHCO₃·0.5H₂O(cr)沉淀形成,从而降低镅的溶解度。当固定pH为7.56时,镅的溶解度随二氧化碳分压升高先降低而后增大,在10^-2.25 bar时达最低值。由于易与腐殖酸形成络合物,在弱酸性至弱碱性条件下腐殖酸能提升镅的溶解度,从而增强其可迁移性,应引起重视。

Speciation and solubility of americium in Beishan groundwater

Based on the thermodynamic datapublished by OECD/NEA and the estimated saturation indices for the precipitation of secondary minerals, the influence of pH, calcite, CO₂, and humic acid (HA) on the speciation distribution and solubility of Americium (Am) in Beishan groundwater were investigated using PHREEQC software. The results indicated that Am mainly occurred as AmCO⁺₃ and AmSiO(OH)²⁺₃ at nearly neutral condition, whereas AmOHCO₃·0.5H₂O(cr) and Am(OH)₃(cr) became predominantly at alkaline condition. Therefore, alkaline Beishan groundwater is favorable for the retardation of Am. Calcite exhibited a complex effect on the solubility and mobility of Am. At pH 6—8, it can decrease the solubility of Am slightly; at pH 8—9.7, the formation of anionic Am(CO₃)^-₂ was favorable thus the solubility and mobility of Am increased in presence of calcite; whereas as pH increased to 9.7—11, calcite can decrease the solubility of Am because of the formation of AmOHCO₃·0.5H₂O(cr). When pH was fixed at 7.56, the solubility of americium decreased firstly and then increased subsequently with increasing CO₂ pressure, and reached a minimum value at 10^-2.25 bar. The results also indicated that humic acid had strong affinity towards americium, and from nearly acidic to alkaline conditions it could increase the solubility and hence the mobility of americium.