Idaho National Laboratory, PO Box 1625, MS 6188, Idaho Falls, ID 83415-6188, USA.
Geochem Trans. 2011 Oct 26;12(1):8. doi: 10.1186/1467-4866-12-8.
Induced precipitation of phosphate minerals to scavenge trace elements from groundwater is a potential remediation approach for contaminated aquifers. The success of engineered precipitation schemes depends on the particular phases generated, their rates of formation, and their long term stability. The purpose of this study was to examine the precipitation of calcium phosphate minerals under conditions representative of a natural groundwater. Because microorganisms are present in groundwater, and because some proposed schemes for phosphate mineral precipitation rely on stimulation of native microbial populations, we also tested the effect of bacterial cells (initial densities of 105 and 107 mL-1) added to the precipitation medium. In addition, we tested the effect of a trace mixture of propionic, isovaleric, formic and butyric acids (total concentration 0.035 mM).
The general progression of mineral precipitation was similar under all of the study conditions, with initial formation of amorphous calcium phosphate, and transformation to poorly crystalline hydroxylapatite (HAP) within one week. The presence of the bacterial cells appeared to delay precipitation, although by the end of the experiments the overall extent of precipitation was similar for all treatments. The stoichiometry of the final precipitates as well as Rietveld structure refinement using x-ray diffraction data indicated that the presence of organic acids and bacterial cells resulted in an increasing a and decreasing c lattice parameter, with the higher concentration of cells resulting in the greatest distortion. Uptake of Sr into the solids was decreased in the treatments with cells and organic acids, compared to the control.
Our results suggest that the minerals formed initially during an engineered precipitation application for trace element sequestration may not be the ones that control long-term immobilization of the contaminants. In addition, the presence of bacterial cells appears to be associated with delayed HAP precipitation, changes in the lattice parameters, and reduced incorporation of trace elements as compared to cell-free systems. Schemes to remediate groundwater contaminated with trace metals that are based on enhanced phosphate mineral precipitation may need to account for these phenomena, particularly if the remediation approach relies on enhancement of in situ microbial populations.
从地下水中提取痕量元素的诱导沉淀是受污染含水层修复的一种潜在方法。工程化沉淀方案的成功取决于生成的特定相、形成速度及其长期稳定性。本研究的目的是研究在代表天然地下水条件下磷酸钙矿物的沉淀。由于地下水中存在微生物，并且一些提议的磷矿沉淀方案依赖于对天然微生物种群的刺激，我们还测试了添加到沉淀介质中的细菌细胞（初始密度为 105 和 107 mL-1）的影响。此外，我们还测试了痕量丙酸、异戊酸、甲酸和丁酸混合物（总浓度 0.035 mM）的影响。
在所有研究条件下，矿物沉淀的总体进展相似，最初形成无定形磷酸钙，并且在一周内转化为非晶态羟基磷灰石（HAP）。细菌细胞的存在似乎会延迟沉淀，尽管在实验结束时，所有处理的沉淀总体程度相似。最终沉淀物的化学计量以及使用 X 射线衍射数据进行的 Rietveld 结构精修表明，有机酸和细菌细胞的存在导致 a 和 c 晶格参数增加，细胞浓度越高，变形越大。与对照相比，细胞和有机酸处理中 Sr 被吸收到固体中的量减少。
我们的结果表明，在痕量元素螯合的工程化沉淀应用中最初形成的矿物可能不是控制污染物长期固定的矿物。此外，与无细胞系统相比，细菌细胞的存在与 HAP 沉淀延迟、晶格参数变化以及痕量元素掺入减少有关。基于增强磷酸盐矿物沉淀的修复受痕量金属污染地下水的方案可能需要考虑到这些现象，特别是如果修复方法依赖于原位微生物种群的增强。
