不同水分下水铁矿在土壤中稳定性变化对砷移动性的影响

Effects of instability of ferrihydrite on arsenic chemical behavior in soil under different water regimes

为合理利用水铁矿,将其作为土壤砷污染修复的一种高效稳定的钝化剂,采集湖南石门县三个不同乡镇的土壤进行室内培养试验,采用连续提取法和薄膜梯度扩散技术(D iffusive gradient in the thin films technique,DGT),研究不同水分模式下外源添加的水铁矿在土壤中稳定性变化过程中对砷在土壤中移动性的影响。结果表明,水铁矿在三种不同水分管理模式[100%SWHC(Soilwaterholdingcapacity,SWHC)、干湿交替(D ry/wetcycle,DWC)和30%SWHC]下培养33d后均发生转化,其中在100%SWHC条件下转化速率最快,在三种土壤中的转化率达到16.50%~22.78%,在30%SWHC和DWC条件下最大转化率仅为8.77%和10.96%。添加外源水铁矿后在三种水分管理模式下两种砷污染土壤中活性态砷浓度均显著降低,并且在100%SWHC条件下降幅最大,在中高浓度的砷污染土壤中降低程度分别达到42.08%和60.75%。但随后在100%SWHC和DWC条件下培养22d后,活性态砷的浓度均出现微弱的上升趋势。在100%SWHC和DWC两种水分条件下,两种砷污染土壤中弱结晶态铁氧化物结合态砷(F1-As)的含量均显著增加,并且随着水铁矿的转化而逐渐向更稳定的形态(F2-As)迁移,在30%SWHC条件下F2-As含量没有增加。研究表明,水铁矿在土壤高水分条件下稳定性最差,其次为土壤的干湿交替条件。在两种砷污染土壤中施加水铁矿修复过程中都具有潜在的砷释放风险,在农业生产中应合理调控土壤水分以保证水铁矿对砷的吸附效率。

In order to utilize ferrihydrite as an efficient and stable amendment of arsenic(As)-contaminated farmland,three typical red soils were sampled in three towns in Shimen Region,Hunan Province,and were used for the indoor soil incubation experiment.Sequential extraction and Diffusive gradients in the thin-films technique(DGT)were employed in this research to study the effect of soil moisture on the stability of ferrihydrite in soils.The results determined the transformation rate of ferrihydrite(0.1%W/W)in three soils(one uncontami⁃nated soil and two contaminated soils)using three soil water management regimes[100%SWHC(Soil water holding capacity),DWC(Dry/wet cycle)and 30%SWHC].Ferrihydrite transformed the fastest at 100%SWHC(16.50%~22.78%in the three soils)while the maximum transformation at 30%SWHC and DWC was only 8.77%and 10.96%,respectively.The concentration of DGT labile As was significantly decreased with all the treatments of the two contaminated soils,especially at 100%SWHC,where the concentration of labile As was re⁃duced 42.08%and 60.75%in mid and high As concentration soils,respectively.Using the 100%SWHC and DWC regimes,the concentra⁃tion of labile As increased as a result of ferrihydrite transformation/dissolution.After ferrihydrite addition,the content of arsenic in Fraction 1(F1-As)and Fraction 2(F2-As)both increased under 100%SWHC and DWC,while no transformation of As fractionation was observed in 30%SWHC.The study indicates that ferrihydrite is instable at 100%SWHC and DWC water regimes and As has the potential to be rere⁃leased to the environment during the remediation period.It is important to regulate soil moisture at a proper level to ensure ferrihydrite sta⁃bility.