利用砷酸根(AsO3－4)与钠明矾石结构中硫酸根(SO2－4)之间的类质同象取代形成砷钠明矾石固溶体是实现砷固化/稳定化的1种新方法.本文以硫酸钠(Na2 SO4)、硫酸铝(Al2(SO4)3.18H2 O)和砷酸钠(Na3 AsO4.12H2 O)分别作为Na、Al和As源,研究溶液初始n(As/(As＋S))aq对砷钠明矾石合成的影响,同时采用XRD、ICP、SEM-EDS、FT-IR和TG-DSC等手段对其沉淀物进行表征分析.结果表明,当n (As/(As＋S))aq＝0~0．178时,沉淀物为钠明矾石相,砷最大键合率为8％；当n(As/(As＋S))aq＝0．185时,沉淀物为钠明矾石＋无定形相＋砷铝石三相混合物,砷在钠明矾石相中的键合率有所提高,可达14％.AsO3－4在钠明矾石结构中的键合机制为,AsO3－4直接取代结构中 SO2－4,两者的电荷差异通过结构中羟基(OH－)的质子化进行补充.随着AsO3－4对钠明矾石相中 SO2－4取代量的增加,晶胞参数 c、V 呈规律性轻微膨胀,原因归于 As-O 半径(0．1682 nm)大于S-O 半径(0．1473 nm).
Arsenate-for-sulfate substitution in natroalunite has been a new method for arsenic immobilization/stabilization.Precipitation of arsenical natroalunite was synthesized using sodium sulfate (Na2 SO4 ),aluminum sulfate (Al2(SO4)3.18H2O)and sodium arsenate (Na3AsO4.12H2O)as sources of Na,Al and As,respec-tively.The effect of (As/(As+S))aq molar ratio in the initial aqueous medium was investigated.The synthetic precipitates were characterized by XRD,ICP,SEM-EDS,FT-IR and TG-DSC.The results indicated that arse-nic precipitated almost exclusively as natroalunite,and arsenic incorporation in natroalunite increased with the (As/(As+S))aq increased in the (As/(As+S))aq molar ratio range 0 to 0.178.The maximum arsenic substitu-tion observed in the natroalunite structure was 8% molar.When n(As/(As+S))aq=0.185,a mixture of natroalunite,amorphous phase and mansfieldite was formed and the arsenic incorporated up to 14% molar in the natroalunite phase.Moreover,mechanism of arsenic incorporation was thought that AsO3-4 directly substi-tuted for SO2-4 in the natroslunite structure,the difference between AsO3-4 and SO2-4 charge maintained balance by appropriate amounts protonation of hydroxyl (OH-).It also elucidated that the c and V unit cell parameters expanded slightly with AsO3-4 -for-SO2-4 substitution increased in the natroalunite crystal structure,due to the larger As-O distance (0.1682 nm)than S-O distance (0.1473 nm).
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