Deposition and mobilization of viruses in unsaturated porous media: Roles of different interfaces and straining.

The vadose zone is the first natural layer preventing groundwater pollution. Understanding virus transport and retention in the vadose zone is necessary. The effects of different interfaces and mechanisms on virus transport and retention were investigated by studying Escherichia coli phage migration in laboratory-scale columns under unsaturated conditions. The E. coli phage was used as a model virus. Colloid filtration theory, extended Derjagin-Landau-Verwey-Overbeek theory and two-site kinetic deposition model were used to calculate fitted parameters and interaction energies to assess virus retention at different interfaces. The collector diameters and the size of E. coli phages in the influent and effluent were compared to assess the effect of straining. The results indicated that the roles of solid-water interfaces (SWIs) and air-water interfaces (AWIs) in retaining E. coli phages are strongly controlled by the moisture content and hydrochemical conditions. Decreasing the moisture content and increasing the ionic strength (IS) of the suspension increased E. coli phage retention. At suspension ISs of 0.01 or 0.03 M and various moisture contents, E. coli phages were mainly retained at the SWIs rather than AWIs. When the IS was increased to 0.06 M, the viruses were strongly retained by becoming attached to both SWIs and AWIs. The role of straining in virus retention could not be ignored. Viruses were retained more at the SWIs and less straining occurred under acidic conditions than under neutral or alkaline conditions. This was mainly because of the effects of the pH and IS on surface charges and the model virus particle size. This study has important implications for modeling and predicting virus transport in soil affected by rainfall, snowmelt, and human activities (e.g., irrigation and artificial groundwater recharging).

Co-transport behavior of ammonium and colloids in saturated porous media under different hydrochemical conditions.

To investigate co-transport behavior of ammonium and colloids in saturated porous media under different hydrochemical conditions, NH4+ was selected as the target contaminant, and silicon and humic acid (HA) were selected as typical organic and inorganic colloids in groundwater. Column experiments were then conducted to investigate the transport of NH4+ colloids under various hydrochemical conditions. The results showed that because of the different properties of colloidal silicon and HA after combining with NH4+, the co-transport mechanism became significantly different. During transport by the NH4+-colloid system, colloidal silicon occupied the adsorption sites on the medium surface to promote the transport of NH4+, while humic acid (HA) increased the number of adsorption sites of the medium to hinder the transport of NH4+. The co-transport of NH4+ and colloids is closely related to hydrochemical conditions. In the presence of HA, competitive adsorption and morphological changes of HA caused NH4+ to be more likely to be transported at a higher ionic strength (IS = 0.05 m, CaCl2) and alkalinity (pH = 9.3). In the presence of colloidal silicon, blocking action caused the facilitated transport to be dependent on higher ionic strength and acidity (pH = 4.5), causing the recovery of NH4+ to improve by 7.99%, 222.25% (stage 1), and 8.63%, respectively. Moreover, transport increases with the colloidal silicon concentrations of 20 mg/L then declines at 40 mg/L, demonstrating that increased concentrations will lead to blocking and particle aggregation, resulting in delayed release in the leaching stage. Graphical abstract.

Different roles of silica nanoparticles played in virus transport in saturated and unsaturated porous media.

Because of the complexity of contaminants infiltrating groundwater, it is necessary to study the co-transport of contaminants in the vadose and saturated zones. To investigate the role of inorganic colloids in the transport of biocolloids through porous media, a series of experiments were performed using columns packed with sand. The Escherichia coli phage (E. coli phage) was used as the model virus and silica as the model colloid in this study. The model virus exhibited a higher degree of attachment when compared with silica under similar experimental conditions. Under unsaturated flow conditions, the degree of virus retention was higher than in the corresponding saturated flow case, regardless of the presence of silica. Mass recovery and breakthrough curve data showed that silica hindered virus transport in saturated porous media. The model virus exhibited a higher degree of retention in the presence of silica. This could be related to pore structure changes caused by aggregated virus-silica particles located within the pores of the sand. Conversely, the suspended virus retained at the air-water interface provided new retention sites for other colloids; the retention was observed to be higher in the presence of colloidal silica in the saturated columns. In the corresponding unsaturated experiments, silica was observed to play the opposite function with respect to virus transport, which demonstrated that silica facilitated virus transport in the presence of unsaturated porous media. Capillary forces were stronger than the virus-silica interactions, and inhibited the aggregation of particles. Suspended silica competes with the virus for sorption sites because of a high affinity for the air-water interface. This competition inhibits virus retention by electrostatic repulsion of like-charged particles, and concomitantly facilitates virus transport under unsaturated conditions.

Impacts of leachate of landfill on the groundwater hydrochemistry and size distributions and heavy metal components of colloids: a case study in NE China.

Colloids associated with heavy metals are ubiquitous in contaminated groundwater; waste accumulation at imperfectly sealed landfills can produce large amounts of leachate with colloids and heavy metal contaminants, which can pollute the downstream groundwater. In this study, three sites in a landfill were sampled to reveal heavy metal particle size distributions and their chemical compositions. The > 220 nm particle sizes were the predominant size in the downstream groundwater, while the < 10 nm particle sizes were the predominant size in the upstream groundwater. Total Fe increased from 35.5 µg/L in the upstream groundwater to 107 µg/L in the downstream groundwater. This increase was attributed to the enhanced migration and accumulation of colloids in the aqueous phase. The elements and the colloid size distribution in the landfill indirectly reflected the composition and degradation of the waste. Colloids played a key role in distribution of both solid particles and aqueous contaminants in the landfill. The results of this study will contribute to the knowledge of the effect of different contaminants in the vicinity of landfills without appropriate sealing systems.

Fe-colloid cotransport through saturated porous media under different hydrochemical and hydrodynamic conditions.

To investigate the effect of different colloids on Fe migration in saturated porous media under different hydrochemical and hydrodynamic conditions, experiments were performed using colloidal silicon (inorganic) and colloidal humic acid (HA, organic), which are representative of the colloids in groundwater. Transport of Fe with and without colloid was investigated by column experiments using various porous media, colloid concentrations, ionic strengths (ISs), cation valences, and flow rates. The results show that colloidal silicon promotes and colloidal HA inhibits Fe transport, which is mainly because of their different bonding ratio, bonding modes with Fe and opposite surface charges between Fe-colloidal silicon and Fe-colloidal HA. Almost 100% of HA binds to Fe through the deprotonated functional groups, whereas only 13.3% of colloidal silicon binds to Fe, which is by electrostatic forces. Cotransport is also dependent on the hydrochemical and hydrodynamic conditions. For the Fe-colloidal silicon system, increasing the colloid concentration and flow rate, and decreasing the IS enhances Fe transport. Compared with colloidal silicon concentration = 10 mg/L, flow rate = 0.25 mL/min, and IS = 0.05 with CaCl2, a higher colloidal silicon concentration (20 mg/L), a higher flow rate (0.50 mL/min), and a lower IS (<0.0005 M) increase Fe recovery by 1.69%, 94.49% and 38.92%, respectively. Fe migration is also different in different porous media. For the Fe-colloidal HA system, Fe recovery decreases by 81.46% as the colloidal HA concentration increases from 0 to 20 mg/L. The type of porous medium and flow rate conditions have the same effects on Fe-colloidal HA transport as for colloidal silicon, although the electrical conditions have the opposite effect. With increasing IS, Fe-colloidal HA transport is enhanced because of competitive adsorption of the cations and Fe to colloidal HA and the porous medium.