Phosphate and illite colloid pose a synergistic risk of enhanced uranium transport in groundwater: A challenge for phosphate immobilization remediation of uranium contaminated environmental water.

The phosphorus-containing reagents have been proposed to remediate the uranium contaminated sites due to the formation of insoluble uranyl phosphate mineralization products. However, the colloids, including both pseudo and intrinsic uranium colloids, could disturb the environmental fate of uranium due to its nonnegligible mobility. In this work, the transport pattern and micro-mechanism of uranium coupled to phosphate and illite colloid (IC) were investigated by combining column experiments and micro-spectroscopic evidences. Results showed that uranium transport was facilitated in granular media by forming the intrinsic uranyl phosphate colloid (such as Na-autunite) when the pH > 3.5 and CNa+ < 10 mM. Meanwhile, the mobility of uranium depended greatly on the typical water chemistry parameters governing the aggregation and deposit of intrinsic uranium colloids. However, the attachment of phosphate on illite granule increased the repulsive force and enhanced the dispersion stability of IC in the IC-U(VI)-phosphate ternary system. The non-preequilibrium transport and retention profiles, HRTEM-mapping, as well as TRLFS spectra revealed that the IC enhanced uranium mobility by forming the ternary IC-uranyl phosphate hybrid, and acted as the coagulation preventing agent for uranyl phosphate particles. This observed facilitation of uranium transport resulted from the formation of intrinsic uranyl phosphate colloids and IC-uranyl phosphate hybrids should be taken into consideration when evaluating the potential risk of uranium migration and optimizing the in-situ mineralization remediation strategy for uranium contaminated environmental water.

Stability and aggregation of nanoscale titanium dioxide particle (nTiO2): Effect of cation valence, humic acid, and clay colloids.

Fate and transport of engineered nanoscale titanium dioxide (nTiO2) have received much attention during the past decade. The aggregation and stability of nTiO2 in water with complicated components, however, have not been fully examined. The objective of this paper is to determine the individual and synergistic effect of cation valence, humic acid, and clay colloids on nTiO2 stability and aggregation, and elucidate the related mechanisms. We conducted systematic laboratory experiments to determine nTiO2 stability and aggregation in NaCl and MgCl2 solutions, both in the absence and presence of humic acid and illite colloids. Results showed that Mg2+, in comparison to Na+, could make the zeta potential of nTiO2 more positive, and shift the point of zero charge of nTiO2 (pHpzc,TiO2) towards higher pH. We also found that nTiO2 are destabilized by illite colloids at pH < pHpzc,TiO2 through formation of illite-nTiO2 hetero-aggregates, but are not interfered by illite colloids at higher pH. HA was found to make nTiO2 stable via electrostatic and steric effects, both in the absence and presence of illite colloids. Calculated interaction energy based on DLVO theory revealed that instability of the nTiO2 suspensions is mainly caused by primary minima, and that secondary minima normally do not destabilize the suspension, even though they are found to promote aggregation.

Influence of natural organic matter in porous media on fine particle transport.

Although extensive research has been conducted to understand the effects of dissolved organic matter (DOM) on fine particle transport, less attention has been paid to natural organic matter (NOM) in the transport medium (i.e., immobile rock and sediment grains). The objective of this study is to elucidate the roles of NOM in the transport medium in mediating particle transport. We conducted experimental and modelling study on the transport of nanoscale titanium dioxide (nTiO2) and illite colloid in columns packed with quartz sand under water-saturated conditions. Peat moss was used as an example NOM and packed in some of the columns to investigate its influence on particle transport. Experimental results showed that NOM may either increase or decrease particle transport depending on the specific conditions. NOM in the transport medium was found to attract particles and reduce particle mobility when the energy barrier between particle and NOM is low or non-existent. NOM also adsorb to Fe and Al oxyhydroxides and promote the transport of negatively-charged particles at low pH. Partial dissolution of NOM releases DOM, and the DOM adsorbs to and increases the transport of positively-charged particles. Additionally, NOM changes pore water pH, which influences particle mobility by affecting the interaction energy between the particle and transport medium. Modelling results showed that the deposition sites provided by peat moss are very heterogeneous, and the NOM from peat moss may reduce particle deposition rate by adsorbing to the particle and/or transport medium. Findings from this study demonstrate that NOM in the transport medium not only changes property of the medium, but also may alter water chemistry. Therefore, the role of NOM in mediating particle transport is complicated and dependent on the property of the particle, NOM, and mineralogical composition of the medium.