We studied the effects of humic substances (HS) on the sorption of Fe(II) onto Al-oxide and clay sorbents at pH 7.5 with a combination of batch kinetic experiments and synchrotron Fe K-edge EXAFS analyses. Fe(II) sorption was monitored over the course of 4 months in anoxic clay and Al-oxide suspensions amended with variable HS types (humic acid, HA; or fulvic acid, FA) and levels (0, 1, and 4 wt%), and with differing Fe(II) and HS addition sequences (co-sorption and pre-coated experiments, where Fe(II) sorbate was added alongside and after HS addition, respectively). In the Al-oxide suspensions, the presence of HS slowed down the kinetics of Fe(II) sorption, but had limited, if any, effect on the equilibrium aqueous Fe(II) concentrations. EXAFS analyses revealed precipitation of Fe(II)-Al(III)-layered double hydroxide (LDH) phases as the main mode of Fe(II) sorption in both the HA-containing and HA-free systems. These results demonstrate that HS slow down Fe(II) precipitation in the Al-oxide suspensions, but do not affect the composition or stability of the secondary Fe(II)-Al(III)-LDH phases formed. Interference of HS with the precipitation of Fe(II)-Al(III)-LDH was attributed to the formation organo-Al complexes HS limiting the availability of Al for incorporation into secondary layered Fe(II)-hydroxides. In the clay systems, the presence of HA caused a change in the main Fe(II) sorption product from Fe(II)-Al(III)-LDH to a Fe(II)-phyllosilicate containing little structural Al. This was attributed to complexation of Al by HA, in combination with the presence of dissolved Si in the clay suspension enabling phyllosilicate precipitation. The change in Fe(II) precipitation mechanism did not affect the rate of Fe(II) sorption at the lower HA level, suggesting that the inhibition of Fe(II)-Al(III)-LDH formation in this system was countered by enhanced Fe(II)-phyllosilicate precipitation. Reduced rates of Fe(II) sorption at the higher HA level were attributed to surface masking or poisoning by HA of secondary Fe(II) mineral growth at or near the clay surface. Our results suggest that HS play an important role in controlling the kinetics and products of Fe(II) precipitation in reducing soils, with effects modulated by soil mineralogy, HS content, and HS properties. Further work is needed to assess the importance of layered Fe(II) hydroxides in natural reducing environments.

中文翻译：

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腐殖质对Fe（II）在氧化铝和粘土上的吸附作用。

通过分批动力学实验和同步加速器Fe K-edge EXAFS分析，我们研究了腐殖质（HS）对Fe（II）在pH 7.5的氧化铝和粘土吸附剂上的吸附作用。在4个月的过程中监测了在缺氧粘土和用多种HS类型（腐殖酸，HA或黄腐酸，FA）和水平（0、1和4 wt％）修正的氧化铝悬浮液中Fe（II）的吸附，并且具有不同的Fe（II）和HS添加顺序（共吸附和预涂覆实验，其中分别在HS添加和之后添加山梨酸Fe（II））。在氧化铝悬浮液中，HS的存在减慢了Fe（II）吸附的动力学，但对平衡的Fe（II）水溶液浓度的影响有限（如果有的话）。EXAFS分析显示，在含HA和不含HA的系统中，Fe（II）-Al（III）层状双氢氧化物（LDH）相的沉淀是Fe（II）吸附的主要方式。这些结果表明，HS减慢了氧化铝悬浮液中的Fe（II）沉淀，但不影响形成的次级Fe（II）-Al（III）-LDH相的组成或稳定性。HS对Fe（II）-Al（III）-LDH沉淀的干扰归因于有机铝配合物HS的形成，限制了Al掺入第二层Fe（II）-氢氧化物的可用性。在粘土体系中，HA的存在导致主要的Fe（II）吸附产物从Fe（II）-Al（III）-LDH变为含少量结构Al的Fe（II）-页硅酸盐。这归因于医管局对铝的络合，与粘土悬浮液中溶解的Si的存在相结合，可使页硅酸盐沉淀。Fe（II）沉淀机理的变化不影响较低HA水平下Fe（II）的吸附速率，表明该体系对Fe（II）-Al（III）-LDH形成的抑制作用被增强Fe（II）-页硅酸盐沉淀。高HA含量的Fe（II）吸附率降低归因于HA在粘土表面或附近的次生Fe（II）矿物生长引起的表面掩盖或中毒。我们的结果表明，HS在控制还原性土壤中Fe（II）沉淀的动力学和产物方面起着重要作用，其作用受土壤矿物学，HS含量和HS性质的调节。需要进一步的工作来评估在自然还原环境中层状Fe（II）氢氧化物的重要性。