Zhu Ying, Liu Jingjing, Goswami Omanjana, Rouff Ashaki A, Elzinga Evert J
Department of Earth & Environmental Sciences, Rutgers University, 101 Warren Street, Newark, NJ, 07102, USA.
Geochem Trans. 2018 Jan 25;19(1):3. doi: 10.1186/s12932-018-0048-5.
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.
我们通过批量动力学实验和同步加速器铁 K 边扩展 X 射线吸收精细结构（EXAFS）分析相结合的方法，研究了腐殖质（HS）在 pH 为 7.5 时对铁（II）在氧化铝和粘土吸附剂上吸附的影响。在缺氧的粘土和氧化铝悬浮液中，用不同类型的 HS（腐殖酸，HA；或富里酸，FA）和含量（0、1 和 4 重量%）以及不同的铁（II）和 HS 添加顺序（共吸附和预涂覆实验，其中铁（II）吸附质分别在添加 HS 时和之后添加），监测了 4 个月过程中铁（II）的吸附情况。在氧化铝悬浮液中，HS 的存在减缓了铁（II）吸附的动力学，但对平衡时水溶液中铁（II）的浓度影响有限（如果有影响的话）。EXAFS 分析表明，在含 HA 和不含 HA 的体系中，铁（II）-铝（III）层状双氢氧化物（LDH）相的沉淀是铁（II）吸附的主要方式。这些结果表明，HS 减缓了氧化铝悬浮液中铁（II）的沉淀，但不影响形成的次生铁（II）-铝（III）-LDH 相的组成或稳定性。HS 对铁（II）-铝（III）-LDH 沉淀的干扰归因于形成了有机铝络合物，HS 限制了铝掺入次生层状铁（II）氢氧化物的可用性。在粘土体系中，HA 的存在导致主要的铁（II）吸附产物从铁（II）-铝（III）-LDH 变为含少量结构铝的铁（II）层状硅酸盐。这归因于 HA 对铝的络合作用，以及粘土悬浮液中溶解硅的存在使得层状硅酸盐沉淀。在较低 HA 水平下，铁（II）沉淀机制的变化并未影响铁（II）吸附的速率，这表明该体系中铁（II）-铝（III）-LDH 形成的抑制作用被增强的铁（II）层状硅酸盐沉淀所抵消。在较高 HA 水平下铁（II）吸附速率降低归因于 HA 对粘土表面或其附近次生铁（II）矿物生长的表面掩蔽或中毒作用。我们的结果表明，HS 在控制还原土壤中铁（II）沉淀的动力学和产物方面起着重要作用，其影响受土壤矿物学、HS 含量和 HS 性质的调节。需要进一步开展工作来评估层状铁（II）氢氧化物在天然还原环境中的重要性。
