Size and elemental analyses of nano colloids in deep granitic groundwater: Implications for transport of trace elements

Are nano-colloids controlling rare earth elements mobility or is it the opposite? Insight from A4F-UV-QQQ-ICP-MS

Rare earth element (REE) mobility in the environment is expected to be controlled by colloids. Recent research has detailed the structure of iron-organic colloids (Fe-OM colloids), which include both large colloids and smaller nano-colloids. To assess how these nano-colloids affect REE mobility, their interactions with REE and calcium (Ca) were investigated at pH 4 and 6. Using Asymmetric Flow Field Flow Fractionation (A4F) combined with UV and Triple Quadrupole Inductively Coupled Plasma Mass Spectrometry (QQQ-ICP-MS), Fe-OM nano-colloids were separated from bulk Fe-OM colloids and their REE and Ca content were analyzed. Without REE and Ca, nano-colloids had an average diameter of approximately 25 nm. Their structure is pH-dependent, with aggregation increasing as pH decreases. At high REE loadings (REE/Fe ≥ 0.05), REE induced a size increase of nano-colloids, regardless of pH. Heavy REE (HREE), with their high affinity for organic matter, formed strong complexes with Fe-OM colloids, resulting in large aggregates. In contrast, light REE (LREE), which bind less strongly to organic molecules, were associated with the smallest nano-colloids. Low REE loading did not cause noticeable fractionation. Calcium further enhanced the aggregation process at both pH levels by neutralizing the charges on nano-colloids. These findings indicate that REE can act as aggregating agent controlling their own mobility, and regulating colloid transfer.

Water-dispersible colloids facilitate the release of potentially toxic elements from contaminated soil under simulated long-term acid rain

The release behaviors of potentially toxic elements (PTEs) associated with water-dispersible colloids (WDCs) in contaminated soils are of considerable public concern. However, little information is available on the size distribution and elemental composition of WDCs and their effects on the release of PTEs in contaminated soils under long-term acid rain. Here, a quantitative accelerated aging leaching test was conducted to evaluate the long-term release risks of PTEs from four contaminated agricultural soil types exposed to acid rain. Asymmetric flow field-flow fractionation (AF4), scanning transmission electron microscopy-energy dispersive spectroscopy (STEM-EDS) and ultrafiltration were used to clarify the size distribution and elemental composition of WDCs containing PTEs. Solution dynamics of successive leaching indicate high release potential for As, Cd, and Pb depending on soil properties under long-term (∼65 years) acid rain. Both ultrafiltration and AF4 analysis show that As in leachate was mainly in the "truly dissolved" fraction, while Pb, Cu, Cd and Fe were predominantly in the colloidal fraction and their percentages increased with increasing extraction time by acid rain. AF4-UV-ICP-MS and STEM-EDS reveal that nanoparticles at 1-7 nm most likely composed of organic matter (OM)-Fe/Al(/Si) oxides composite were the main carriers of Pb, Cu, As and Cd. Lead was also verified in Fe-oxide colloids at 34-450 nm in the first extracts but disappeared in the tenth extracts. This indicates that WDC-bearing PTEs become smaller as leaching proceeds. The study indicates the quantitative description and size-resolved understanding of WDC- and nanoparticle-bound PTEs in leachates of contaminated soils subjected to long-term acid rain.

Influence of Soil Colloids on Ni Adsorption and Transport in the Saturated Porous Media: Effects of pH, Ionic Strength, and Humic Acid

Natural colloids are widely distributed in soil and groundwater. Due to their specific characteristics, colloids can actively involve various transport contaminants, resulting in a complicated fate and the transport of heavy metals to the environment. This study investigated the effects of soil colloids on the adsorption and transport of Ni2+ in saturated porous media under different conditions, including pH, ion strength (IS), and humic acid (HA), because these indexes are non-negligible in the fates of various organic or inorganic matters in the subsurface environment. The results indicate that Ni2+ adsorption by soil colloids slightly increased from 17% to 25% with the increase of pH from 5.5 to 7.5 at the IS of 30 mmol·L−1, whilst it significantly reduced from 55% to 17% with the increase of IS from 0 to 30 mmol·L−1 at a pH of 5.5. Both Langmuir and Freundlich models can fit the adsorption isotherms of Ni2+ on soil colloids and quartz sand. According to the column experiment, the presence of soil colloids increased the initial penetration rate, but could not increase the final transport efficiency of Ni2+ in the effluent. The presence of soil colloids has weakened the effect of IS on Ni2+ transport in the sand column. Moreover, this experiment implies that HA remarkably decreased the Ni2+ transport efficiency from 71.3% to 58.0% in the presence of soil colloids and that there was no significant difference in the HA effect on the Ni2+ transport in the absence of soil colloids.

The speciation, transformation kinetics and fate of spiked Pu (IV) in highly saline groundwater

The mobility of plutonium (Pu) in groundwater is dependent of its speciation distribution and transformation. The speciation and transformation kinetics of Pu(IV) and its colloids in highly saline groundwater have, however, been rarely studied. In the present study, groundwater (Ionic strength 1 M) from Dunhuang region, NW China, was collected for investigating the speciation, transformation kinetics and fate of spiked Pu (IV) with aging time. The results showed that ~99% of the spiked Pu (IV) (over initial concentration c₀ range 2.5 × 10^-10-7.8 × 10^-7 mol·L^-1) was easily associated with the natural colloids and transformed into relatively unstable Pu pseudo-colloids in 1 day, which then gradually deposited and/or adsorbed on the container walls with aging. The suspended Pu pseudo-colloids decreased in similar exponential models, with rate equations r(t) = -3.1 × 10^-10e^{- t/4} and -1.3 × 10^-8e^-/3 for c₀ = 1.25 × 10^-9 mol·L^-1and 4.17 × 10^-8 mol·L^-1, respectively. The chemical speciation of the suspended colloidal Pu was dominated by "Fe/Mn Oxides" at the early time, while "Carbonates" with slower depositing rate (r(t) = -6.9 × 10^-12e^{- 0.149t}) dominated it (~82%) at equilibrium state. Whatever the c₀ was, the concentration of dissolved Pu (i.e., the apparent solubility of Pu) kept at 0.7 × 10^-11 mol·L^-1 over aging. The valence of dissolved Pu was dominated by Pu(IV) at early time, while Pu(V + VI) would become dominant (~95%) at equilibrium state with transformation rate of r(t) = -92.9e^{- t/16.6} + 96.9. The equilibrium times of Pu deposition (and/or adsorption), speciation transformation of the suspended colloidal Pu, and valence change of the dissolved Pu were 30 d, 80 d and 120 d, respectively. The kinetic process for each Pu species could be well fitted with exponential model. These results suggest that the majority of released Pu(IV) into highly saline groundwater will be easily associated with natural aquatic colloids and then become immobile in short time due to deposition (and/or adsorption) onto the environmental medium, but potential migration risk caused by stable suspended Pu colloids cannot be ignored.

Stability, transport and ecosystem effects of graphene in water and soil environments

Graphene nanomaterials (GMs), such as graphene oxide (GO) and reduced graphene oxide (rGO), have been widely applied in various fields. Due to the rapid increase in production and application, the inevitable release of GMs into water and soil environments poses potential health and ecosystem risks. Upon exposure, the behavior, transport, and fate of GMs may be altered after interacting with the relevant environmental conditions. GMs can affect the microbial communities as well. Thus, it is imperative to understand the interaction between the GMs and the environmental systems for predicting their risks. For this purpose, this review highlights the influence of the most relevant environmental factors on the stability, aggregation, and transformation of GMs in aquatic environments. Moreover, the transport of GMs and microbial communities changes have also been presented based on the recent findings. To the best of our knowledge, this review covered most of the recent related studies and will allow for accurate predictions of the fate and risks associated with GMs. In consideration of the diversity of GMs and the complexity of environmental factors, further studies should be focused on their inherent properties and amicable development.

Effects of surfactants on graphene oxide nanoparticles transport in saturated porous media

Transport behaviors of graphene oxide nanoparticles (GONPs) in saturated porous media were examined as a function of the presence and concentration of anionic surfactant (SDBS) and non-ionic surfactant (Triton X-100) under different ionic strength (IS). The results showed that the GONPs were retained obviously in the sand columns at both IS of 50 and 200mmol/L, and they were more mobile at lower IS. The presence and concentration of surfactants could enhance the GONP transport, particularly as observed at higher IS. It was interesting to see that the GONP transport was surfactant type dependent, and SDBS was more effective to facilitate GONP transport than Triton X-100 in our experimental conditions. The advection-dispersion-retention numerical modeling followed this trend and depicted the difference quantitatively. Derjaguin-Landau-Verwey-Overbeek (DLVO) interaction calculations also were performed to interpret these effects, indicating that secondary minimum deposition was critical in this study.