Ionic strength reduction and flow interruption enhanced colloid-facilitated Hg transport in contaminated soils

Salinity and total suspended solids control mercury speciation in a tidal river: Comparisons with a photochemical mercury model

Daytime volatilization of gaseous elemental mercury (Hg(0)aq) is a significant mechanism for mercury removal from aquatic systems and potentially limits the production and bioaccumulation of methylmercury. Changes in incoming solar radiation (in the ultraviolet range), dissolved organic matter, salinity, and total suspended particles were investigated concurrently with several mercury species (Hg(0)aq, dissolved total mercury (THg), easily reducible mercury (ERM), and mercury associated with total suspended solids (THgTSS)) during daylight hours near the mouth of a hypertidal river. There were no predictable temporal changes observed for Hg(0)aq in unfiltered surface water. Hg(0)aq ranged from 0 to 12 pg L-1, THg ranged from 0 to 492 pg L-1, ERM ranged from 13 to 381 pg L-1, and THgTSS ranged from <1.58 ng g-1 to 261.32 ng g-1. The range of Hg(0)aq predicted by the empirical model was similar to measured ERM concentrations, but it was shown that ERM did not significantly predict in-situ photoreducible Hg(II) (Hg(II)RED). Production of Hg(0)aq appears to largely be suppressed by suspended solids, which limits ultraviolet radiation transmission through surface water. Comparison of these results to an empirical model developed for this site to predict Hg(0)aq indicates that significantly more mercury is available for photoreduction near the mouth of the tidal river, and that Hg(II) will likely photoreduce quickly when TSS levels decrease with ocean mixing.

Occurrence of dissolved black carbon in source water and disinfection byproducts formation during chlorination

Dissolved black carbon (DBC), the water-soluble component of black carbon, which is formed by incomplete combustion of fossil fuels or biochar, takes up about 10% of dissolved organic matter (DOM) in river water. However, the distribution of DBC in water environment especially in source water is not clear and as an important component of DOM, whether DBC can produce disinfection byproducts (DBPs) like other DOM during disinfection remains unknown. In this study, the DBC concentrations in seventeen source water samples from East China were measured. The concentrations of DBC in the source water samples ranged from 60 to 270 μg/L, which were positively correlated with UV₂₅₄ absorbance and chemical oxygen demand. The levels of DBC in wet season were higher than that in dry season. The average concentrations of DBC in different types of source water samples followed the order of reservoir > canal > lake > river. DBC could only be removed by 20% during the simulated coagulation, and further generate different categories of DBPs during chlorination, among which the concentrations of haloacetic acids (HAA) were the highest. The results indicated that DBC widely distributes in source water and is an important precursor of HAAs and THMs during chlorination.

Heavy metal transport driven by seawater-freshwater interface dynamics: The role of colloid mobilization and aquifer pore structure change

Co-transport of colloidal substances and pollutants is a pivotal link that significantly affects the environment of coastal groundwater. The effect of colloid mobilization and aquifer pore structure change on heavy metal transport driven by seawater-freshwater interface dynamics is not fully understood. In this study, packed column experiments were conducted to model the seawater intrusion (SWI) and freshwater replenishment (FWR) processes using a sampled medium from a coastal sandy aquifer. Hydrodynamic, hydrochemical variables, and heavy metal (Pb, Cu, Cd) transport during the propagation of the seawater-freshwater interface were tested and analyzed. During the SWI stage, cation exchange induced heavy metal liberations, and it developed peak concentrations synchronized with the seawater-freshwater interface at the pore volume of 1.00. The colloid-facilitated transport for heavy metals was the predominant mechanism in the FWR stage, characterized by a peak release lagging the interface propagation by approximately 0.5 pore volumes. Because the colloidal fraction was mobilized during aquifer desalination, it lagged behind the decline of the salinity gradient. Furthermore, Derjaguin-Landau-Verwey-Overbeek (DLVO) calculations explained that the replenishment decreased the depth of the secondary energy minimum of the colloids; meanwhile, the thickness of the electrical double layer increased from 0.63 nm to 10.14 nm, resulting in a repulsive energy barrier up to 3,213 kT. The transport of colloids led to a reduction in porosity from 18.16% to 2.28% of the total immobile domain. At these times, the dimension of the transported colloids evolved, showing a size-selective transport and therefore regulating the total emission fluxes of the heavy metals. These mechanisms were proposed to be incorporated in colloid filtration theory for targeting the coastal scenario.

Distribution, alkylation, and migration of mercury in soil discharged from the Itomuka mercury mine

The purpose of this study was to investigate the behavior of previously discharged mercury (Hg) released from the Itomuka Hg mine into the surrounding environment, especially into soil. Total-Hg (T-Hg), methylmercury (MeHg), and ethylmercury (EtHg) concentrations in the surface soil at eight sample sites around the mine were 3.8-64.2 mg/kg, 6.0-54.7 μg/kg, and undetected to 4.5 μg/kg, respectively. Core samples collected from seven of the eight sample sites showed that the vertical distribution of T-Hg was the highest in the surface soil layer and decreased rapidly in the lower layers. A strong positive correlation was observed between T-Hg and MeHg concentrations in the core samples; however, the slope of the regression line varied considerably for each core. This suggests that Hg and MeHg were not supplied from the atmosphere simultaneously, but rather that MeHg was produced on-site. Further, the formation of MeHg and EtHg in soil was considered in terms of the total organic carbon/total nitrogen ratio, which is a decomposition index of soil organic matter. The strong positive correlation between T-Hg and MeHg can be attributed to the migration of organic matter containing Hg species to the lower layers. There was no relationship between T-Hg and MeHg at the riverbed sample site because of the high T-Hg in the lower soil layers, suggesting that Hg was supplied by ore at this sample site. These assumptions of the formation change and migration of Hg in soil were supported by the results of the fractionation experiment and the elution test. To understand the current conditions in this area, measurements of Hg in the water, sediment, atmosphere, and plants were also conducted.

Release of soil colloids during flow interruption increases the pore-water PFAS concentration in saturated soil

Groundwater flow through aquifer soils or packed bed systems can fluctuate for various reasons, which could affect the concentration of natural colloids and per- and polyfluoroalkyl substances (PFAS) in the pore water. In such cases, PFAS concentration could either decrease due to matrix diffusion of PFAS or increase by the detachment of colloids carrying PFAS. Yet, the effect of flow fluctuation on PFAS transport or release in porous media has not been examined. To examine the relative importance of either process, we interrupted the flow during an injection of groundwater spiked with perfluorobutanoic acid (PFBA), perfluorooctanoic acid (PFOA), and bromide as conservative tracer through clay-rich soil, so that diffusive transport would be prominent during flow interruption. After flow interruption, the PFAS concentration did not decrease indicating an insignificant contribution of matrix diffusion. The concentration increased, potentially due to enhanced release of colloid-associated PFAS. Analysis of samples before and after flow interruption by particle size analysis and SEM confirmed an increase in soil colloid concentration after the flow interruption. XRD analysis of soil and the colloids proved that PFAS were associated with specific sites of the colloids. Due to a higher affinity of PFOA to soil colloids, the total PFOA concentration in the effluent samples increased more than PFBA after the flow interruption process. The results indicate that colloids may have a disproportionally higher role in the transport of PFAS in conditions that release colloids from porous media. Thus, fluctuations in groundwater flow can increase this colloid facilitated mobility of PFAS.

Impacts of Sediment Particle Grain Size and Mercury Speciation on Mercury Bioavailability Potential

Particle-specific properties, including size and chemical speciation, affect the reactivity of mercury (Hg) in natural systems (e.g., dissolution or methylation). Here, terrestrial, river, and marine sediments were size-fractionated and characterized to correlate particle-specific properties of Hg-bearing solids with their bioavailability potential and measured biomethylation. Marine sediments contained ∼20-50% of the total Hg in the <0.5 μm size fraction, compared to only 0.5 and 3.0% in this size fraction for terrestrial and river sediments, respectively. X-ray absorption spectroscopy (XAS) analysis indicated that metacinnabar (β-HgS) was the main mercury species in a marine sediment, whereas organic Hg-thiol (Hg(SR)₂) was the main mercury species in a terrestrial sediment. Single-particle inductively coupled plasma time-of-flight mass spectrometry analysis of the marine sediment suggests that half of the Hg in the <0.5 μm size fraction existed as individual nanoparticles, which were β-HgS based on XAS analyses. Glutathione-extractable mercury was higher for samples containing Hg(SR)₂ species than β-HgS species and correlated well with the amount of Hg biomethylation. This particle-scale understanding of how Hg speciation and particle size affect mercury bioavailability potential helps explain the heterogeneity in Hg methylation in natural sediments.

Colloidal stability and correlated migration of illite in the aquatic environment: The roles of pH, temperature, multiple cations and humic acid

The mobility and environmental risk of colloids and associated pollutants are dependent on their dispersion stability under various conditions. In this work, the stability and correlated migration of illite colloids (IC) were systematically investigated over a wide range of aquatic chemistry conditions. The results showed that IC was aggregation favorable at low pH, low temperature and high ionic strength. The critical coagulation concentration (CCC) of IC increased exponentially with increasing values of r/Z³, following the Schulze-Hardy and Hofmeister series. Humic acid (HA) greatly mitigated colloid aggregation since the attachment of HA on IC surface increased the steric hindrance and electrostatic potential, and the enhancement of stability was linearly correlated with the HA concentration. The Derjaguin-Landau-Verwey-Overbeek (DLVO) model revealed that the interaction force deriving from van der Waals forces and electrostatic double-layer energy evolved as the aquatic chemistry varied, and the reduction in repulsion force between particles facilitated the colloid collision and then aggregation. The migration of IC in the porous sand column was highly correlated with the dispersion stability and filtration effect, the agglomerated colloids were redispersed and released when conditions favored dispersion. The illite colloids acted as efficient carriers for Eu(III) transport. These findings are essential for improving the understanding of the geological fate of environmental colloids and associated radionuclides.

Mobility of Cellulose Nanocrystals in Porous Media: Effects of Ionic Strength, Iron Oxides, and Soil Colloids

Understanding the dispersivity and migration of cellulose nanocrystals (CNCs) in porous media is important for exploring their potential for soil and water remediation. In this study, a series of saturated column experiments were conducted to investigate the coupled effects of ionic strength, iron oxides (hematite), and soil colloids on the transport of CNCs through quartz sand and natural soils (red earth and brown earth). Results showed that CNCs had high mobility in oxide-free sand and that iron oxide coating reduced the mobility of CNCs. An analysis of Derjaguin-Landau-Verwey-Overbeek interactions indicated that CNCs exhibited a deep primary minimum, nonexistent maximum repulsion and secondary minimum on hematite-coated sand, favorable for the attachment of CNCs. The maximum effluent percentage of CNCs was 96% in natural soils at 5 mM, but this value decreased to 4% at 50 mM. Soil colloids facilitated the transport of CNCs in brown earth with larger effect at higher ionic strength. The ionic strength effect was larger in natural soils than sand and in red earth than brown earth. The study showed that CNCs can travel 0.2 m to 72 m in porous media, depending on soil properties, solution chemistry, and soil colloids.

Effect of sodium concentration on mobilization and fate of trace metals in standard OECD soil

The effect of different Na concentrations on the fate of trace metals (Cd, Cu, Ni, Zn) in standard OECD soil was evaluated by performing soil leaching column experiments. Five Na concentrations added in synthetic irrigation water (0, 1, 5, 10, 50 mM) were studied in order to evaluate the fate of the metals contained in both the irrigation water leachate and the soil layer. In all experiments, metals mostly accumulated on the top soil layer (0-0.5 cm), at variable concentrations according to the Na content in the artificial irrigation water. Nevertheless, concentration peaks of metal contamination occurred at different sampling time in the soil leachates depending on the metal and on influent water sodicity. Peaks of metals in the leachate appeared simultaneously with the release of organic matter and/or release of Al, suggesting significant involvement of colloids in metals transport. Sodium concentration (10-50 mM) was demonstrated to highly reduce colloidal mobilization leading to the accumulation of more than 95% of the influent metal in the top soil layer. Conversely, low Na concentrations (1-5 mM) favored colloidal transport leading to the recovery of metals in the soil leachates.

Mercury speciation, transformation, and transportation in soils, atmospheric flux, and implications for risk management: A critical review

Mercury (Hg) is a potentially harmful trace element in the environment and one of the World Health Organization's foremost chemicals of concern. The threat posed by Hg contaminated soils to humans is pervasive, with an estimated 86 Gg of anthropogenic Hg pollution accumulated in surface soils worldwide. This review critically examines both recent advances and remaining knowledge gaps with respect to cycling of mercury in the soil environment, to aid the assessment and management of risks caused by Hg contamination. Included in this review are factors affecting Hg release from soil to the atmosphere, including how rainfall events drive gaseous elemental mercury (GEM) flux from soils of low Hg content, and how ambient conditions such as atmospheric O₃ concentration play a significant role. Mercury contaminated soils constitute complex systems where many interdependent factors, including the amount and composition of soil organic matter and clays, oxidized minerals (e.g. Fe oxides), reduced elements (e.g. S^2-), as well as soil pH and redox conditions affect Hg forms and transformation. Speciation influences the extent and rate of Hg subsurface transportation, which has often been assumed insignificant. Nano-sized Hg particles as well as soluble Hg complexes play important roles in soil Hg mobility, availability, and methylation. Finally, implications for human health and suggested research directions are put forward, where there is significant potential to improve remedial actions by accounting for Hg speciation and transportation factors.

Colloid mobilization and heavy metal transport in the sampling of soil solution from Duckum soil in South Korea

Colloid mobilization is a significant process governing colloid-associated transport of heavy metals in subsurface environments. It has been studied for the last three decades to understand this process. However, colloid mobilization and heavy metal transport in soil solutions have rarely been studied using soils in South Korea. We investigated the colloid mobilization in a variety of flow rates during sampling soil solutions in sand columns. The colloid concentrations were increased at low flow rates and in saturated regimes. Colloid concentrations increased 1000-fold higher at pH 9.2 than at pH 7.3 in the absence of 10 mM NaCl solution. In addition, those were fourfold higher in the absence than in the presence of the NaCl solution at pH 9.2. It was suggested that the mobility of colloids should be enhanced in porous media under the basic conditions and the low ionic strength. In real field soils, the concentrations of As, Cr, and Pb in soil solutions increased with the increase in colloid concentrations at initial momentarily changed soil water pressure, whereas the concentrations of Cd, Cu, Fe, Ni, Al, and Co lagged behind the colloid release. Therefore, physicochemical changes and heavy metal characteristics have important implications for colloid-facilitated transport during sampling soil solutions.

Facilitated transport of cadmium with montmorillonite KSF colloids under different pH conditions in water-saturated sand columns: Experiment and transport modeling

This study investigated the impact of pH on the migration of cadmium(II) ions (Cd²⁺) in relation to montmorillonite KSF colloids through a water-saturated sand column (WSSC). The sorption isotherms of Cd²⁺ on colloids and sand at pH values of 3, 6, and 8 were characterized by batch experiments. Cd²⁺ sorption by colloids and sand fit well with the Freundlich model. In the column experiments, increasing the pH increased the retardation factors and K_F values of Cd²⁺ both with and without the presence of the colloids. The amount of Cd²⁺ sorbed onto the montmorillonite KSF colloids in the column effluent increased from 0.29 to 0.97 mg as the pH increased. The colloid increased Cd²⁺ mobility and acted as a carrier at a high solution pH. The increasing level of Cd²⁺ sorbed on colloids as the pH increased resulted in a long tailing of the breakthrough curve (BTC) of the total Cd, indicating that the total Cd was controlled by rate-limited reactions. These findings indicate that when the solution pH was greater than the point of zero charge (PZC) of the colloids (pH > 6), the system tended to follow a nonequilibrium two-site (TSM) model rather than an equilibrium (CD_eq) model. This implies that the PZC of the colloids in the groundwater system is the main factor in predicting facilitated Cd²⁺ transport.

Roles of cation valance and exchange on the retention and colloid-facilitated transport of functionalized multi-walled carbon nanotubes in a natural soil

Saturated soil column experiments were conducted to investigate the transport, retention, and release behavior of a low concentration (1 mg L^-1) of functionalized ¹⁴C-labeled multi-walled carbon nanotubes (MWCNTs) in a natural soil under various solution chemistries. Breakthrough curves (BTCs) for MWCNTS exhibited greater amounts of retardation and retention with increasing solution ionic strength (IS) or in the presence of Ca²⁺ in comparison to K⁺, and retention profiles (RPs) for MWCNTs were hyper-exponential in shape. These BTCs and RPs were well described using the advection-dispersion equation with a term for time- and depth-dependent retention. Fitted values of the retention rate coefficient and the maximum retained concentration of MWCNTs were higher with increasing IS and in the presence of Ca²⁺ in comparison to K⁺. Significant amounts of MWCNT and soil colloid release was observed with a reduction of IS due to expansion of the electrical double layer, especially following cation exchange (when K⁺ displaced Ca²⁺) that reduced the zeta potential of MWCNTs and the soil. Analysis of MWCNT concentrations in different soil size fractions revealed that >23.6% of the retained MWCNT mass was associated with water-dispersible colloids (WDCs), even though this fraction was only a minor portion of the total soil mass (2.38%). More MWCNTs were retained on the WDC fraction in the presence of Ca²⁺ than K⁺. These findings indicated that some of the released MWCNTs by IS reduction and cation exchange were associated with the released clay fraction, and suggests the potential for facilitated transport of MWCNT by WDCs.

Antimony release from contaminated mine soils and its migration in four typical soils using lysimeter experiments

Antimony (Sb) can pose great risks to the environment in mining and smelting areas. The migration of Sb in contaminated mine soil was studied using lysimeter experiments. The exchangeable concentration of soil Sb decreased with artificial leaching. The concentrations of Sb retained in the subsoil layers (5-25cm deep) were the highest for Isohumosol and Ferrosol and the lowest for Sandy soil. The Sb concentrations in soil solutions decreased with soil depth, and were adequately simulated using a logarithmic function. The Sb migration pattern in Sandy soil was markedly different from the patterns in the other soils which suggested that Sb may be transported in soil colloids. Environmental factors such as water content, soil temperature, and oxidation-reduction potential of the soil had different effects on Sb migration in Sandy soil and Primosol. The high Fe and Mn contents in Ferrosol and Isohumosol significantly decreased the mobility of Sb in these soils. The Na and Sb concentrations in soils used in the experiments positively correlated with each other (P<0.01). The Sb concentrations in soil solutions, the Sb chemical fraction patterns, and the Sb/Na ratios decreased in the order Sandy soil>Primosol>Isohumosol>Ferrosol, and we concluded that the Sb mobility in the soils also decreased in that order.

Kinetic modeling of antimony(III) oxidation and sorption in soils

Kinetic batch and saturated column experiments were performed to study the oxidation, adsorption and transport of Sb(III) in two soils with contrasting properties. Kinetic and column experiment results clearly demonstrated the extensive oxidation of Sb(III) in soils, and this can in return influence the adsorption and transport of Sb. Both sorption capacity and kinetic oxidation rate were much higher in calcareous Huanjiang soil than in acid red Yingtan soil. The results indicate that soil serve as a catalyst in promoting oxidation of Sb(III) even under anaerobic conditions. A PHREEQC model with kinetic formulations was developed to simulate the oxidation, sorption and transport of Sb(III) in soils. The model successfully described Sb(III) oxidation and sorption data in kinetic batch experiment. It was less successful in simulating the reactive transport of Sb(III) in soil columns. Additional processes such as colloid facilitated transport need to be quantified and considered in the model.

Impact of manure-related DOM on sulfonamide transport in arable soils

Field application of livestock manure introduces colloids and veterinary antibiotics, e.g. sulfonamides (SAs), into farmland. The presence of manure colloids may potentially intensify the SAs-pollution to soils and groundwater by colloid-facilitated transport. Transport of three SAs, sulfadiazine (SDZ), sulfamethoxypyridazine (SMPD), and sulfamoxole (SMOX), was investigated in saturated soil columns with and without manure colloids from sows and farrows, weaners, and fattening pigs. Experimental results showed that colloid-facilitated transport of SMOX was significant in the presence of manure colloids from fattening pigs with low C/N ratio, high SUVA280nm and protein C, while manure colloids from sows and farrows and weaners had little effect on SMOX transport. In contrast, only retardation was observed for SDZ and SMPD when manure colloids were present. Breakthrough curves (BTCs) of colloids and SAs were replicated well by a newly developed numerical model that considers colloid-filtration theory, competitive kinetic sorption, and co-transport processes. Model results demonstrate that mobile colloids act as carriers for SMOX, while immobile colloids block SMOX from sorbing onto the soil. The low affinity of SMOX to sorb on immobile colloids prevents aggregation and also promotes SMOX's colloid-facilitated transport. Conversely, the high affinity of SDZ and SMPD to sorb on all types of immobile colloids retarded their transport. Thus, manure properties play a fundamental role in increasing the leaching risk of hydrophobic sulfonamides.

Mobilization and transport of metal-rich colloidal particles from mine tailings into soil under transient chemical and physical conditions

Exposed mine tailing wastes with considerable heavy metals can release hazardous colloidal particles into soil under transient chemical and physical conditions. Two-layered packed columns with tailings above and soils below were established to investigate mobilization and transport of colloidal particles from metal-rich mine tailings into soil under transient infiltration ionic strength (IS: 100, 20, 2 mM) and flow rate (FR: 20.7, 41, and 62.3 mm h(-1)), with Cu and Pb as representatives of the heavy metals. Results show that the tailing particles within the colloidal size (below 2 μm) were released from the columns. A step-decrease in infiltration IS and FR enhanced, whereas a step-increase in the IS and FR restrained the release of tailing particles from the column. The effects of step-changing FR were unexpected due to the small size of the released tailing particles (220-342 nm, being not sensitive to hydrodynamic shear force), the diffusion-controlled particle release process and the relatively compact pore structure. The tailing particles present in the solution with tested IS were found negatively charged and more stable than soil particles, which provides favorable conditions for tailing particles to be transported over a long distance in the soil. The mobilization and transport of Cu and Pb from the tailings into soil were mediated by the tailing particles. Therefore, the inherent toxic tailing particles could be considerably introduced into soil under certain conditions (IS reduction or FR decrease), which may result in serious environmental pollution.

Cotransport of bismerthiazol and montmorillonite colloids in saturated porous media

While bismerthiazol [N,N'-methylene-bis-(2-amino-5-mercapto-1,3,4-thiadiazole)] is one of the most widely used bactericides, the transport of bismerthiazol in subsurface environments is unclear to date. Moreover, natural colloids are ubiquitous in the subsurface environments. The cotransport of bismerthiazol and natural colloids has not been investigated. This study conducted laboratory column experiments to examine the transport of bismerthiazol in saturated sand porous media both in the absence and presence of montmorillonite colloids. Results show that a fraction of bismerthiazol was retained in sand and the retention was higher at pH7 than at pH 4 and 10. The retention did not change with ionic strength. The retention was attributed to the complex of bismerthiazol with metals/metal oxides on sand surfaces through ligand exchange. The transport of bismerthiazol was enhanced with montmorillonite colloids copresent in the solutions and, concurrently, the transport of montmorillonite colloids was facilitated by the bismerthiazol. The transport of montmorillonite colloids was enhanced likely because the bismerthiazol and the colloids competed for the attachment/adsorption sites on collector surfaces and the presence of bismerthiazol changed the Derjaguin-Landau-Verwey-Overbeek (DLVO) interaction energies between colloids and collectors. The transport of bismerthiazol was inhibited if montmorillonite colloids were pre-deposited in sand because bismerthiazol could adsorb onto the colloid surfaces. The adsorbed bismerthiazol could be co-remobilized with the colloids from primary minima by decreasing ionic strength. Whereas colloid-facilitated transport of pesticides has been emphasized, our study implies that transport of colloids could also be facilitated by the presence of pesticides.