复杂砷酸盐/亚砷酸盐—硫酸盐矿物的溶解热力学及其对砷在环境中迁移转化的影响

When the primary As-bearing sulphide minerals are exposed to the atmosphere and surface or groundwaters, oxidizing reactions cause formation of secondary arsenic minerals, including arsenate/arsenite–sulphate minerals, in the oxidized zone of metallic sulphide mineral deposits and in the acid mine drainages (AMD). In the present research program, the forming condition and the pH stability range of some complex arsenate/arsenite–sulphate minerals, such as arsenical natroalunite, beudantite, hidalgoite, tooeleite, sarmientite (zykaite /pitticite /bukovskyite) and As-bearing rhomboclase, will be firstly determined through precipitation/dissolution experiments. Based on the obtained experimental data, the thermodynamical parameters, such as the solubility products and free energies of formation of these arsenate/arsenite–sulphate minerals, will be calculated, and then the pH-Eh and pH-activity diagrams will be also generalized for the mineral-water interaction. Simultaneously, the dissolution mechanism and stability of the arsenate/arsenite–sulphate minerals in water will be studied in detail to recognize the reaction path and compositional evolution of the minerals and the aqueous solution. The research results will be helpful in understanding the transportation, transformation and distribution characteristics of the toxic trace elements including arsenic and in evaluating the long-term environmental effects of the chemical stabilization of the arsenic-containing solid wastes through the formation of the arsenate/arsenite–sulphate minerals.

当原生含砷硫化物矿物暴露于大气、地表水或地下水中时，通常被氧化而在硫化物矿床氧化带、酸性矿山废水中形成次生含砷矿物，如复杂砷酸盐/亚砷酸盐—硫酸盐矿物等。本项目拟通过沉淀/溶解实验，确定砷钠明矾石、砷铅铁矾、砷铅铝矾、水羟砷铁矿、硫砷铁矿、含砷板铁矾等几种常见的复杂砷酸盐/亚砷酸盐—硫酸盐矿物的形成条件及其稳定存在的pH值范围，并求得它们的基本热力学参数，如它们的溶度积和生成自由能，作出它们的pH-Eh和pH-活度相图；同时对不同复杂砷酸盐/亚砷酸盐—硫酸盐矿物在水环境中的溶解作用及其稳定性进行详细的研究，了解复杂砷酸盐/亚砷酸盐—硫酸盐矿物和水溶液在相互作用过程中的反应路径与演变，以及矿物表面化学组分与结构在矿物—水溶液作用过程中的变化，更好地掌握砷等有毒微量元素在自然界中的迁移、转化与分布的规律，同时预测含砷废物的化学固定过程中形成的各种矿物对环境影响的长期效应。

当原生含砷硫化物矿物暴露于大气、地表水或地下水中时，通常被氧化而在硫化物矿床氧化带、酸性矿山废水中形成次生含砷矿物，如复杂砷酸盐/亚砷酸盐—硫酸盐矿物等。本项目通过实验，确定它们的形成条件及其在水中的溶解机制，并计算出它们的溶度积（Ksp）和吉布斯生成自由能（ΔG°f）等，同时分析它们的形成对砷、铅等有害微量元素在环境中迁移转化的影响。在初始溶液pH=2和25°C条件下，随着AsO4的置换量从0%增加到67%，砷钠明矾石固溶体{[(Na1-x(H3O)x]Al3[(AsO4)x(SO4)1-x]2(OH)6-x}的log_Ksp从-81.21降低到-109.16，而ΔG°f则从-4714.49 kJ/mol降低到-5352.95 kJ/mol；砷铅铁矾[Pb0.35(H3O)0.40Fe3.09(AsO4)0.37(SO4)1.63(OH)6.00]和砷铅铝矾[Pb0.72(H3O)2.71Al2.26(AsO4)0.93(SO4)1.07(OH)6.00]的log_Ksp分别为-94.18±0.04和-73.82±0.11；水羟砷铁矿[Fe6(AsO3)4(SO4)(OH)4·4H2O]的log_Ksp和ΔG°f分别为-200.28±0.01和-5180.54±0.07 kJ/mol；碱式砷酸-硫酸铁[Fe(SO4)0.27(AsO4)0.73(OH)0.27·0.26H2O]的log_Ksp和ΔG°f分别为-23.04±0.01和-914.06±0.03 kJ/mol；板铁矾[(H5O2)1.434Fe0.855(SO4)2·2H2O]的log_Ksp和ΔG°f分别为-8.80±0.01和-2706.44±1.90 kJ/mol；含砷板铁矾[(H5O2)Fe[(AsO4)x(SO4)1-x]2·2H2O]的log_Ksp和ΔG°f分别为-10.03±0.03 ~ -8.89±0.01和-2680.51±2.05 ~ -2548.71±1.94 kJ/mol。研究结果有助于更好地掌握砷等有毒微量元素在自然界中的迁移、转化与分布的规律，同时预测含砷废物的化学固定过程中形成的各种矿物对环境影响的长期效应。