Department of Earth Sciences, Memorial University of Newfoundland, St. John's, Newfoundland & Labrador A1B 3X5, Canada.
Department of Earth Sciences, Memorial University of Newfoundland, St. John's, Newfoundland & Labrador A1B 3X5, Canada.
Sci Total Environ. 2016 Jan 15;541:579-589. doi: 10.1016/j.scitotenv.2015.09.116. Epub 2015 Oct 3.
The fate and transport of nano-scale or micro-scale titanium dioxide particles (nTiO2) in subsurface environments are strongly influenced by the stability of nTiO2 and their attachment to sediment grains. nTiO2 may carry either positive or negative charges in natural water, therefore, environmental factors such as pH, humic substances, and Fe oxyhydroxide coatings on sediment grains, which are known to control the stability and transport of negatively-charged colloids, may influence nTiO2 in different manners. The objective of this study is to investigate the effects of pH and humic acid (HA) on the stability and attachment of nTiO2 to sand at HA concentrations that are relevant to typical groundwater conditions, so that mechanisms that control nTiO2 immobilization and transport in natural systems can be elucidated. Stability and attachment of nTiO2 to quartz sand and Fe oxyhydroxide coated quartz sand are experimentally measured under a range of HA concentrations at pH5 and 9. Results show that at pH5, negatively-charged HA strongly adsorbs to positively-charged nTiO2 and Fe oxyhydroxide, which, at low HA concentrations, partially neutralizes the positive charges on nTiO2 and Fe oxyhydroxide, and therefore decreases the repulsive electrostatic forces between the surfaces, resulting in nTiO2 aggregation and attachment. At high HA concentrations, adsorbed HA reverses the surface charges of nTiO2 and Fe oxyhydroxide, and makes nTiO2 and Fe oxyhydroxide strongly negatively charged, resulting in stable nTiO2 suspension and low nTiO2 attachment. At pH9, HA, nTiO2, and Fe oxyhydroxide are all negatively charged, and HA adsorption is low and does not have a strong impact on the stability and attachment of nTiO2. Overall, this study shows that changes in surface charges of nTiO2 and Fe oxyhydroxide coating caused by HA adsorption is a key factor that influences the stability and attachment of nTiO2.
纳米或微尺度二氧化钛颗粒（nTiO2）在地下环境中的命运和迁移强烈受 nTiO2 的稳定性及其与沉积物颗粒的附着的影响。nTiO2 在天然水中可能带正电荷或负电荷，因此，环境因素如 pH 值、腐殖质和沉积物颗粒上的铁氢氧化物涂层等，这些因素已知控制带负电荷胶体的稳定性和迁移，可能以不同的方式影响 nTiO2。本研究的目的是研究 pH 值和腐殖酸（HA）对 nTiO2 在与典型地下水条件相关的 HA 浓度下稳定和附着于砂的影响，以便阐明控制 nTiO2 在自然系统中固定和迁移的机制。在 pH 值为 5 和 9 的一系列 HA 浓度下，实验测量了 nTiO2 对石英砂和铁氢氧化物涂覆的石英砂的稳定性和附着。结果表明，在 pH 值为 5 时，带负电荷的 HA 强烈吸附带正电荷的 nTiO2 和铁氢氧化物，在低 HA 浓度下，部分中和 nTiO2 和铁氢氧化物上的正电荷，从而降低表面之间的排斥静电力，导致 nTiO2 聚集和附着。在高 HA 浓度下，吸附的 HA 反转 nTiO2 和铁氢氧化物的表面电荷，使 nTiO2 和铁氢氧化物带强负电荷，导致 nTiO2 稳定悬浮和 nTiO2 附着率低。在 pH 值为 9 时，HA、nTiO2 和铁氢氧化物均带负电荷，HA 吸附量低，对 nTiO2 的稳定性和附着影响不大。总的来说，本研究表明，HA 吸附引起的 nTiO2 和铁氢氧化物涂层表面电荷的变化是影响 nTiO2 稳定性和附着的关键因素。
