Subsurface Transport of As, Se, Cu, and Pb Contaminants in Association with Soil and Biosolid Nano- and Macro- Colloid Fractions

Research Article

Austin J Hydrol. 2014;1(1): 13.

Subsurface Transport of As, Se, Cu, and Pb Contaminants in Association with Soil and Biosolid Nano- and Macro- Colloid Fractions

Karathanasis AD*, Ghezzi JL, Wendroth O, Matocha CJ, Unrine J and Thompson YL

Department of Plant and Soil Sciences, University of Kentucky, USA

*Corresponding author: Karathanasis AD, Department of Plant and Soil Sciences, University of Kentucky, N-122K Ag. Science North, Lexington, KY, 40475, USA

Received: May 19, 2014; Accepted: May 29, 2014; Published: May 30, 2014


This study investigated the potential of nanocolloid and macrocolloid fractions separated from selected soils and biosolids to co-transport two anionic (Se, As) and two cationic (Cu, Pb) contaminants through intact soil monoliths. The soil colloids represented smectitic, kaolinitic, and mixed mineralogical compositions while the biosolid colloids were derived from an aerobically digested municipal sewage sludge. Selected characterizations of the colloids included particle size (DLS, TEM, SEM), SA, settling stability, EC, pH, CEC, OC, zeta potential, Kf, and mineralogy. Leaching experiments involved the elution of nano- and macrocolloid suspensions mixed with 2 mg L-1 contaminants through soil monoliths during a four pore volume leaching cycle. In spite of similar contaminant sorption affinities and notable nanocluster formation during the transport, nanocolloids were eluted in greater quantities than macrocolloids and co-transported higher metal contaminant loads for both anionic and cationic contaminants. Significantly higher contaminant loads were mobilized in association with the colloids over the control contaminant solution, with soluble forms dominating the anionic and colloid-bound forms the cationic contaminants. The enhanced elution of both soluble and sorbed contaminant loads in the presence of nanocolloids is attributed to preferential flow, size exclusion and ion exclusion mechanisms. The findings of this study emphasize the importance of considering multiple physicochemical and mineralogical parameters in contaminant transport models in order to accurately assess environmental pollution risks and develop efficient remediation strategies.

Keywords : Nanoparticles; Particle size; Colloid composition; Colloid migration; Contaminant transport behavior


BTCs = Break Through Curves; CEC = Cation Exchange Capacity; TEM = Transmission Electron Microscopy; EDS = Energy Dispersion Spectrum; SEM = Scanning Electron Microscopy; OC = Organic Carbon; XRD = X-ray Diffraction; TG = Thermogravimetric; D = Depth; H = Height; PVC = Polyvinyl Chloride; EC = Electrical Conductivity; DI = De-Ionized; M = Molar; ICP-MS = Inductively Coupled-Mass Spectroscopy; ANOVA = Analysis of Variance; LSD= Least Statistical Difference; SAS = Statistical Analysis System; PZC = Point of Zero Charge; DOC = Dissolved Organic Carbon; TOC = Total Organic Carbon; DLS = Dynamic Light Scattering; C/Co = Influent to Eluent Concentration Ratio; PV = Pore Volumes; Kf = Freundlich Sorption Coefficient.


Recent studies have shown that environmental nanoparticles with their high surface area and reactivity may enhance the transport of contaminants in both surface waters and through soil media into the groundwater [1-3]. Nanoparticle mobilization may be influenced by hydraulic gradients and preferential flow paths within the soil, particle size and morphology, competitive sorption-desorption processes with soluble ions and organic functional groups, pore size distribution, as well as dispersion-flocculation phenomena [2,4-7]. The transport of nanoparticles in soil environments is controlled by Brownian motion rather than gravitational settling. Nanoparticle mobility may be limited by sequestration within micropores, coagulation into larger size aggregates that may lead to extensive straining even within macropores, and sorption or physical attachment to non-mobile particles. Nanoparticle sorption at the air-water interface may also affect their transport in the unsaturated zone [2].

While soil transport studies have shown that nanoparticles and colloids are capable of leaching through soil horizons, the likelihood of particle movement through both the soil root and vadose zone in most cases seems unlikely. However, under certain conditions when there is a large influx of water during storms or during snow melt events, a significant number of nanoparticles could migrate from the soil vadose zone to the groundwater [8]. Previous studies have suggested that nanoparticles are vehicles for the movement of heavy metals and other contaminants in surface and shallow subsurface environments [2,3,5,6]. This process involves complex biogeochemical interactions, including physicochemical sorption and precipitation reactions. Some groundwater nanocolloids have demonstrated enhanced transport of contaminants via high sorption affinities for aquifer solids [5]. Studies have shown that radionucleotides can be transported over several kilometers via nanoparticles in groundwater over short time periods, defying thermodynamic predictions [1,2,6]. Leaching experiments with contaminant-colloid suspensions of 220- 1050 nm in diameter through undisturbed soil monoliths produced eluted nanocolloids with a mean diameter range of 50-120 nm and a significant load of metal contaminants [3,4].

The spatial distribution of physical and chemical features along a flow path in an aquifer will also affect nanocolloid and associated contaminant transport [5]. Subsurface environments are not usually favorable for nanocolloidal deposition because of the electrostatic repulsion between the generally negatively charged nanocolloids and subsurface media. Furthermore, the surface charge (zeta potential) of nanocolloids or nanoparticles can become irrelevant to transport predictions due to spatial heterogeneity [5]. Aggregation, sorption or dissipation of surface charges will affect particle mobility and therefore the likelihood of contaminant transport [7]. However, nanocolloids because of their smaller size, high surface reactivity and contaminant sorption capacity, and prolonged stability in suspension are likely more potent contaminant transport vectors than macrocolloids in surface waters, unsaturated subsurface media, and in groundwater.

Although a considerable volume of research has been conducted on colloid-facilitated transport of pollutants, there is a limited knowledge on the role of natural nano- vs. macro-colloid particles in mediating transport of emerging contaminants such as Se, As, Cu and Pb in subsurface soil environments. The objectives of this study were to evaluate the potential of soil- and biosolid-derived nano- and macro-colloids to sorb and transport As, Cu, Pb, and Se contaminants through soil media. The study also addresses the effects of particle size and compositional differences among particles.

Material and Methods

Colloid generation and characterization

Three Kentucky soil Bt horizons were used to generate the mineral colloids: Caleast-variant (fine, smectitic, mesic mollic Hapludalf), Tilsit (fine-silty, mixed, mesic Typic Fragiudult), and Trimble (fineloamy, siliceous, mesic Typic Paleudult), referred to as smectitic, mixed, and kaolinitic nano- and macro-colloids, respectively. Biosolid nano- and macro-coloids were derived from aerobically digested municipal sewage sludge (Jessamine County, Kentucky). Centrifugation was used to fractionate colloids into two size classes (nanocolloids <100 nm and macrocolloids 100-2000 nm) using a Centra GP8R Model 120 centrifuge (Thermo IEC) in deionized water (resistivity of 1 μΩcm at 25°C) [3]. Sample suspensions were then diluted to 50 mg L-1 concentrations for additional analysis. Primary particle size of the nano- and macro-colloids was determined using TEM-EDS (JEOL 2010F, Tokyo, Japan) [9,10] and SEM-EDS ( Hitachi S-4300, Tokyo, Japan), respectively [11,12]. Dynamic light scattering was used to determine hydrodynamic diameters (dH) on a Malvern Zetasizer Nano ZS (Malvern, United Kingdom). Surface area was measured using the Ethylene Glycol Monoethyl Ether method. Settling kinetics experiments were used to determine the stability of the nano-and macro-colloid fractions over a 48 hour period in 50 mg L-1 suspensions containing 2 mg L-1 mixed contaminant concentrations. The suspended colloid concentrations were determined using a colorimetric procedure on a Molecular Devices Versa Max Microplate Reader at 450 nm [4]. Mineralogical characterizations were conducted using XRD and TG analyses on a Phillips PW 1840 diffractometer and PW 1729 x-ray generator (Mahwah, NJ), and a Thermal Analyst 2000 (TA Instruments) equipped with a 951 Thermo gravimetric Analyzer (DuPont Instruments), respectively [13,14].

Cation exchange capacity was determined using an adapted version of the ammonium acetate method. A Flash EA 1112 Series NC Soil Analyzer (Thermo Electron Corporation) with a Mettler Toledo MX5 microbalance was used to determine OC. A Denver Instruments Model 250 pH*ISE* electrical conductivity meter was used to measure pH and electrical conductivity (Arvada, CO). Macro- and nano-colloid affinity for the contaminants was evaluated from Freundlich sorption coefficients developed from mixed metal adsorption isotherms using duplicate colloid suspensions of 50 mg colloid L-1 spiked with 2 mg L-1 Cu, Pb, Se, and As. The Smoluchowski approximation was used to determine zeta potentials from electrophoretic mobilities in colloid suspensions with 0.001M NaCl background electrolyte using a Malvern Zetasizer Nano ZS (Malvern, United Kingdom). Selected characterization data of the colloid fractions are shown in Tables 1 and 2.

Soil monolith preparation and characterization

Twenty-two intact soil monoliths (D-18x H-30 cm) representing the Bt horizon of an Ashton soil series (Fine-silty, mixed, active, mesic Mollic Hapludalfs) were encased in PVC columns and sealed with Poly-U-Foam to decrease preferential flow. Four extra monoliths were collected for characterization. Soil bulk density (Db) was determined from triplicate oven dried cores collected with a bulk density probe. Hydraulic conductivity was determined on measurements taken at 10 minute intervals for one hour at upper and lower boundaries set first at -10 cm and then at -5 cm prior to leaching experiments. A representative monolith sample was air dried, ground, homogenized and analyzed for mineralogy, pH, EC, OC and CEC using the methods described in the colloid chemical characterization section. Particle size analysis was completed using the pipette method.

Colloid leaching experiments

Colloid-contaminant suspensions of 2 mg L-1 As, Cu, Se and Pb with 50 mg L-1 colloid were infused through duplicate columns using an unsaturated, steady state, unit gradient, downward percolation experiment with upper and lower boundaries set at -5 cm, referring to a hydraulic conductivity of 5.57 mm per hour, representing a 10- day Kentucky rainstorm with a 2-year frequency of reoccurrence. Infusions occurred over four continuous pore volumes. Soil Measurement Systems infiltrometers attached to baseplates at the top of the monolith controlled the upper boundary while a marriote device at the bottom of the monolith controlled the lower boundary. Collection vials allowed sample collection at the outlet of the marriote device at the lower boundary. Control monoliths were infused with DI Water solutions of 2 mg L-1 As, Cu, Se and Pb. Infusion of a 0.02 M solution of KBr acted as a conservative tracer. Leaching experiments were conducted under controlled temperature conditions (200 C).

Eluted colloid concentrations were determined using a colorimetric procedure on a Molecular Devices Versa Max Microplate Reader at 450 nm alongside a standard colloid curve. The pH and EC were measured using a Denver Instruments Model 250 pH*ISE*electrical conductivity meter (Arvada, CO). DOC was determined on 20 mL samples acidified with 50 μL of concentrated HCl on a Flash EA 1112 Series NC Soil Analyzer (Thermo Electron Corporation) with a Mettler Toledo MX5 microbalance. Eluted colloid mineralogy was determined using XRD and TG analysis and checked against the composition of colloids from the stock suspension. This comparison allowed assessment of colloid contamination from the column matrix and preferential filtration of specific minerals [4,15].

Total, soluble, and sorbed metals were analyzed using a Millipore filtration system set up with 0.025 µm nitrocellulose filters. Blanks consisted of 30 mL of double-deionized water passed through and analyzed for As, Cu, Pb, and Se. Aliquots (15 ml) of eluent samples were filtered and analyzed for soluble As, Se, Cu and Pb. Finally, 15 mL of 1N trace metal grade nitric acid was passed through the filter and analyzed for sorbed metals. Filtered samples were preserved with 1% nitric acid, stored in polyethylene vials, and analyzed within 24 hours via ICP-MS.


Significant differences between means were tested using ANOVA (SAS PROC GLM) and the Fisher’s protected LSD in SAS 9.3 (SAS Institute Inc., Cary, NC, USA). The statistical significance level used was α = 0.05.

Results and Discussion

Soil monolith characteristics

Leaching experiments were conducted with soil monoliths representing depths of 5 cm to 35 cm below the surface with a neutral pH (7.07), 1.45% OC, and 42% porosity (Table 3). The pH of the monoliths (7.07) was considerably higher than the colloid suspensions (4.92 to 5.39) (Tables 2 and 3). The monoliths had a loam texture, and a CEC (8.12), typical of a mixed clay mineralogy (Table 3). The CEC of the monoliths was comparable to that of the mixed and kaolinitic colloid suspensions, but significantly lower than that of the smectitic and biosolid colloids (Tables 2 and 3). The loam texture, porosity, and hydraulic conductivity suggested adequate potential for subsurface flow pathways for transport (Table 3). Contaminant sorption affinity by the soil monolith matrix as indicated by Freundlich coefficients (Kf) followed the sequence Pb > Cu > Se > As. However, all values (except for the Kf(As) of the biosolid nanocolloids) were lower than those of the colloid fractions suggesting little competition during the contaminant transport process (Table 3).