Division of Soil and Water Management, Department of Earth and Environmental Science, KU Leuven, Kasteelpark Arenberg 20, B-3001 Heverlee, Belgium.
Division of Soil and Water Management, Department of Earth and Environmental Science, KU Leuven, Kasteelpark Arenberg 20, B-3001 Heverlee, Belgium.
J Colloid Interface Sci. 2018 Jun 15;520:25-32. doi: 10.1016/j.jcis.2018.03.010. Epub 2018 Mar 5.
Layered double hydroxides (LDH) of Mg/Al are anion exchangers that are candidate materials for phosphate (PO) recovery and recycling from waste streams. However, PO recycling in agriculture might be limited by incomplete desorption of PO from the minerals. This study was set up to identify the factors explaining irreversible PO sorption ("fixation") on LDHs by comparing the isotopic exchangeability (PO) with the PO desorption from LDH materials and from boehmite as a P fixing reference mineral. Six different Mg-Al LDH materials were synthesized, by varying the synthesis pH and exposing obtained materials to hydrothermal (HT) treatment. Phase pure LDH materials were obtained from syntheses at pH 10 and 12, while at pH 8 Al-rich phase impurities such as a boehmite or gibbsite were formed. Crystallite size increased significantly during HT treatment. The LDHs were first loaded with PO prior to PO isotopic exchange (0-20 days, 1 mM NaHCO) or PO desorption (0-20 days, NaHCO concentrations increasing from 0 to 20 mM). The isotopic PO exchangeability was 85-95% of total PO after 72 h in the phase pure LDHs with intercalated PO whereas this value was 40-54% in the presence of Al-rich phase impurities in non-HT treated materials and in boehmite. In contrast, the maximally desorbed PO fractions were only 55-63% for the phase pure LDHs, indicating that not all of the isotopically exchangeable PO can be desorbed. Samples at different stages of desorption (different initial NaHCO concentrations) were subjected to isotopic exchange after desorption. In the LDHs with PO intercalation, PO was increasingly less isotopically exchangeable as the initial NaHCO concentrations increased, while this trend was not observed for samples without intercalated PO. This suggests that PO becomes increasingly less accessible for isotopic exchange as the fraction binding sites occupied with HCO increases. The interlayer outward diffusion of PO might be increasingly rate limited upon HPO/HCO exchange, which explains the PO fixation in LDHs.
层状双氢氧化物（LDH）是镁/铝的阴离子交换剂，是从废物流中回收和再循环磷酸盐（PO）的候选材料。然而，PO 在农业中的再循环可能受到 PO 从矿物质中不完全解吸的限制。本研究旨在通过比较同位素交换性（PO）与 LDH 材料和作为 P 固定参考矿物的勃姆石的 PO 解吸，来确定解释不可逆 PO 吸附（“固定”）在 LDH 上的因素。通过改变合成 pH 值并使获得的材料暴露于水热处理（HT），合成了六种不同的 Mg-Al LDH 材料。在 pH 为 10 和 12 时获得了纯相 LDH 材料，而在 pH 为 8 时形成了富铝相杂质，如勃姆石或水铝石。在 HT 处理过程中，晶体尺寸显著增大。在进行 PO 同位素交换（0-20 天，1 mM NaHCO）或 PO 解吸（0-20 天，NaHCO 浓度从 0 增加到 20 mM）之前，先将 LDH 加载 PO。在具有插层 PO 的纯相 LDH 中，72 小时后 PO 的同位素交换率为总 PO 的 85-95%，而在未经 HT 处理的材料中存在富铝相杂质和勃姆石时，该值为 40-54%。相比之下，纯相 LDH 中最大解吸 PO 分数仅为 55-63%，表明并非所有可进行同位素交换的 PO 都可解吸。在不同的解吸阶段（不同的初始 NaHCO 浓度）的样品在解吸后进行同位素交换。在具有 PO 插层的 LDH 中，随着初始 NaHCO 浓度的增加，PO 的同位素交换性越来越差，而在没有插层 PO 的样品中则没有观察到这种趋势。这表明，随着与 HCO 结合的结合位点分数的增加，PO 变得越来越难以进行同位素交换。PO 向层间的外向扩散可能在 HPO/HCO 交换时受到越来越大的速率限制，这解释了 LDH 中的 PO 固定。
