Transport modeling of copper and cadmium with linear and nonlinear retardation factors

The predictive accuracy of using the one-dimensional advection–dispersion equation to evaluate the fate and transport of solute in a soil column is usually dependent on the proper determination of chemical retardation factors. Typically, the distribution coefficient (K_d) obtained by fitting the linear sorption isotherm has been extensively used to consider general geochemical reactions on solute transport in a low-concentration range. However, the linear distribution coefficient cannot be adequately utilized to describe the solute fate at a higher concentration level. This study employed the nonlinear equilibrium-controlled sorption parameters to determine the retardation factor used in column leaching experiments. Copper and cadmium transportation in a lateritic silty-clay soil column was examined. Through the explicit finite-difference calculations with a third-order total-variation-diminishing (TVD) numerical solution scheme, all results of the theoretical copper and cadmium breakthrough curves (BTCs) simulated by using the Freundlich nonlinear retardation factors revealed good agreement with the experimental observations.     

Transport of Sr2+ and SrEDTA2- in partially-saturated and heterogeneous sediments

Strontium-90 has migrated deep into the unsaturated subsurface beneath leaking storage tanks in the Waste Management Areas (WMA) at the U.S. Department of Energy's (DOE) Hanford Reservation. Faster than expected transport of contaminants in the vadose zone is typically attributed to either physical hydrologic processes such as development of preferential flow pathways, or to geochemical processes such as the formation of stable, anionic complexes with organic chelates, e.g., ethylenediaminetetraacetic acid (EDTA). The goal of this paper is to determine whether hydrological processes in the Hanford sediments can influence the geochemistry of the system and hence control transport of Sr(2+) and SrEDTA(2-). The study used batch isotherms, saturated packed column experiments, and an unsaturated transport experiment in an undisturbed core. Isotherms and repacked column experiments suggested that the SrEDTA(2-) complex was unstable in the presence of Hanford sediments, resulting in dissociation and transport of Sr(2+) as a divalent cation. A decrease in sorption with increasing solid:solution ratio for Sr(2+) and SrEDTA(2-) suggested mineral dissolution resulted in competition for sorption sites and the formation of stable aqueous complexes. This was confirmed by detection of MgEDTA(2-), MnEDTA(2-), PbEDTA(2-), and unidentified Sr and Ca complexes. Displacement of Sr(2+) through a partially-saturated undisturbed core resulted in less retardation and more irreversible sorption than was observed in the saturated repacked columns, and model results suggested a significant reservoir (49%) of immobile water was present during transport through the heterogeneous layered sediments. The undisturbed core was subsequently disassembled along distinct bedding planes and subjected to sequential extractions. Strontium was unequally distributed between carbonates (49%), ion exchange sites (37%), and the oxide (14%) fraction. An inverse relationship between mass wetness and Sr suggested that sandy sediments of low water content constituted the immobile flow regime. Our results suggested that the sequestration of Sr(2+) in partially-saturated, heterogeneous sediments was most likely due to the formation of immobile water in drier regions having low hydraulic conductivities.     

Strontium migration in a crystalline medium: effects of the presence of bentonite colloids

The effects of bentonite colloids on strontium migration in fractured crystalline medium were investigated. We analyzed first the transport behaviour of bentonite colloids alone at different flow rates; then we compared the transport behaviour of strontium as solute and of strontium previously adsorbed onto stable bentonite colloids at a water velocity of approximately 7.1·10(-6)m/s-224m/yr. Experiments with bentonite colloids alone showed that - at the lowest water flow rate used in our experiments (7.1·10(-6)m/s) - approximately 70% of the initially injected colloids were retained in the fracture. Nevertheless, the mobile colloidal fraction, moved through the fracture without retardation, at any flow rate. Bentonite colloids deposited over the fracture surface were identified during post-mortem analyses. The breakthrough curve of strontium as a solute, presented a retardation factor, R(f)~6, in agreement with its sorption onto the granite fracture surface. The breakthrough curve of strontium in the presence of bentonite colloids was much more complex, suggesting additional contributions of colloids to strontium transport. A very small fraction of strontium adsorbed on mobile colloids moved un-retarded (R(f)=1) and this fraction was much lower than the expected, considering the quantity of strontium initially adsorbed onto colloids (90%). This behaviour suggests the hypothesis of strontium sorption reversibility from colloids. On the other hand, bentonite colloids retained within the granite fracture played a major role, contributing to a slower strontium transport in comparison with strontium as a solute. This was shown by a clear peak in the breakthrough curve corresponding to a retardation factor of approximately 20.     

Effects of water content on reactive transport of 85Sr in Chernobyl sand columns

It is known that under unsaturated conditions, the transport of solutes can deviate from ideal advective-dispersive behaviour even for macroscopically homogeneous porous materials. Causes may include physical non-equilibrium, sorption kinetics, non-linear sorption, and the irregular distribution of sorption sites. We have performed laboratory experiments designed to identify the processes responsible for the non-ideality of radioactive Sr transport observed under unsaturated flow conditions in an Aeolian sandy deposit from the Chernobyl exclusion zone. Miscible displacement experiments were carried out at various water contents and corresponding flow rates in a laboratory model system. Results of our experiments have shown that breakthrough curves of a conservative tracer exhibit a higher degree of asymmetry when the water content decreases than at saturated water content and same Darcy velocity. It is possible that velocity variations caused by heterogeneities at the macroscopic scale are responsible for this situation. Another explanation is that molecular diffusion drives the solute mass transfer between mobile and immobile water regions, but the surface of contact between these water regions is small. At very low concentrations, representative of a radioactive Sr contamination of the pore water, sorption and physical disequilibrium dominate the radioactive Sr transport under unsaturated flow conditions. A sorption reaction is described by a cation exchange mechanism calibrated under fully saturated conditions. The sorption capacity, as well as the exchange coefficients are not affected by desaturation. The number of accessible exchange sites was calculated on the basis that the solid remained in contact with water and that the fraction of solid phase in contact with mobile water is numerically equal to the proportion of mobile water to total water content. That means that for this type of sandy soil, the nature of mineral phases is the same in advective and non-advective domains. So sorption reaction parameters can be estimated from more easily conducted saturated experiments, but hydrodynamic behaviour must be characterized by conservative tracer experiments under unsaturated flow conditions.     

Adsorption and transport of arsenate in carbonate-rich soils: coupled effects of nonlinear and rate-limited sorption

The transport and fate of arsenate in carbonate-rich soil under alkaline conditions was investigated with multiple approaches combining batch, sequential extraction and column experiments as well as transport modeling studies. Batch experiments indicated that sorption isotherm was nonlinear over a wide range of concentration (0.1-200 mg L(-1)) examined. As(V) adsorption to the calcareous soil was initially fast but then continued at a slower rate, indicating the potential effect of rate-limited sorption on transport. Column experiments illustrated that transport of As(V) was significantly retarded compared to a non-reactive tracer. The degree of retardation decreased with increasing As(V) concentration. As(V) breakthrough curves exhibited nonideal transport behavior due to the coupled effects of nonlinear and rate-limited sorption on arsenate transport, which is consistent with the results of modeling studies. The contribution of nonlinear sorption to the arsenate retardation was negligible at low concentration but increased with increasing As(V) concentration. Sequential extraction results showed that nonspecifically sorbed (easily exchangeable, outer sphere complexes) fraction of arsenate is dominant with respect to the inner-sphere surface bound complexes of arsenate in the carbonate soil fraction, indicating high bioavailability and transport for arsenate in the carbonate-rich soils of which Fe and Al oxyhydroxide fractions are limited.     

Transport of anthracene and benz(a)anthracene through iron-quartz and three aquifer materials in laboratory columns

In groundwater systems, dissolved natural organic matter (NOM) can influence the mobility of organic contaminants by altering the contaminant behavior in water and solid phases. The transport of anthracene and benz(a)anthracene (B(a)A) was studied in the presence and absence of NOM and/or soil organic matter (SOM) in column experiments. The results show that sorption are related to the properties of polycyclic aromatic hydrocarbons (PAHs), NOM and SOM. In the Fe-quartz media, the amount of NOM (20 mg/l) in solution had a little effect on increasing the apparent solubility of anthracene and countering increased anthracene sorption. In the natural (Bemidji) soil, Suwannee river fulvic acid (SRFA, 20 mg/l) and Suwannee river humic acid (SRHA) in water did not compete with SOM for anthracene, indicating that SOM has higher partition efficiency for anthracene. It was also observed that slow diffusion through an organic phase apparently caused most of the observed tailing in column breakthrough curves (BTCs). Even though the fOC of washed Bemidji sediment was very low, the transport of B(a)A was retarded significantly, however, and the transport of B(a)A was shown to be facilitated by dissolved NOM.     

Cesium migration in saturated silica sand and Hanford sediments as impacted by ionic strength

Large amounts of 137Cs have been accidentally released to the subsurface from the Hanford nuclear site in the state of Washington, USA. The cesium-containing liquids varied in ionic strengths, and often had high electrolyte contents, mainly in the form of NaNO3 and NaOH, reaching concentrations up to several moles per liter. In this study, we investigated the effect of ionic strengths on Cs migration through two types of porous media: silica sand and Hanford sediments. Cesium sorption and transport was studied in 1, 10, 100, and 1000 mM NaCl electrolyte solutions at pH 10. Sorption isotherms were constructed from batch equilibrium experiments and the batch-derived sorption parameters were compared with column breakthrough curves. Column transport experiments were analyzed with a two-site equilibrium-nonequilibrium model. Cesium sorption to the silica sand in batch experiments showed a linear sorption isotherm for all ionic strengths, which matched well with the results from the column experiments at 100 and 1000 mM ionic strength; however, the column experiments at 1 and 10 mM ionic strength indicated a nonlinear sorption behavior of Cs to the silica sand. Transport through silica sand occurred under one-site sorption and equilibrium conditions. Cesium sorption to Hanford sediments in both batch and column experiments was best described with a nonlinear Freundlich isotherm. The column experiments indicated that Cs transport in Hanford sediments occurred under two-site equilibrium and nonequilibrium sorption. The effect of ionic strength on Cs transport was much more pronounced in Hanford sediments than in silica sands. Effective retardation factors of Cs during transport through Hanford sediments were reduced by a factor of 10 when the ionic strength increased from 100 to 1000 mM; for silica sand, the effective retardation was reduced by a factor of 10 when ionic strength increased from 1 to 1000 mM. A two order of magnitude change in ionic strength was needed in the silica sand to observe the same change in Cs retardation as in Hanford sediments.     

Development of a continuously stirred flow cell for investigating sorption mass transfer

This paper introduces a new reversible-flow design for a continuously stirred reactor used to study sorption mass transfer in soil and solvent systems. The stirred reactor has potential advantages over conventional packed column or batch reactors because it isolates intraparticle sorption rate limitations from advective-dispersive transport, yet allows changes to flux through the reactor for analysis of sorption kinetics under dynamic conditions. Previously, stirred reactors have often failed due to clogging of sediment on the effluent frit. The reverse-flow backwashing design allows longer life and higher confidence in maintaining mixed conditions than previous designs. Mass transfer 'rate coefficients estimated from stirred and column experiments are compared; both techniques produced results consistent with a published correlation. The data also show that fitted sorption mass transfer coefficients can be strongly dependent on the choice of equilibrium partition coefficient (i.e. batch or first-moment derived values), and that the conventional two-site sorption kinetics model fails to accurately predict sorption mass transfer in the presence of changing solvent velocity through the reactor.     

Cesium migration in Hanford sediment: a multisite cation exchange model based on laboratory transport experiments

Cs+ transport experiments carried out in columns packed with uncontaminated Hanford formation sediment from the SX tank farm provide strong support for the use of a multisite, multicomponent cation exchange model to describe Cs+ migration in the Hanford vadose zone. The experimental results indicate a strong dependence of the effective Cs+ Kd on the concentrations of other cations, including Na+ that is present at high to extremely high concentrations in fluids leaking from the Hanford SX tanks. A strong dependence of the Cs+ Kd on the aqueous Cs+ concentration is also apparent, with retardation of Cs+ increasing from a value of 41 at a Cs+ concentration of 10(-4) M in the feed solution to as much as 282 at a Cs+ concentration of 5x10(-7) M, all in a background of 1 M NaNO3. The total cation exchange capacity (CEC) of the Hanford sediment was determined using 22Na isotopic equilibrium exchange in a flow-through column experiment. The value for the CEC of 120 microeq/g determined with this method is compatible with a value of 121.9 microeq/g determined by multi-cation elution. While two distinct exchange sites were proposed by Zachara et al. [Geochim. Cosmochim. Acta 66 (2002) 193] based on binary batch exchange experiments, a third site is proposed in this study to improve the fit of the Cs+-Na+ and Cs+-Ca+ exchange data and to capture self-sharpened Cs+ breakthrough curves at low concentrations of Cs+. Two of the proposed exchange sites represent frayed edge sites (FES) on weathered micas and constitute 0.02% and 0.22% of the total CEC. Both of the FES show a very strong selectivity for Cs+ over Na+ (K(Na-Cs)=10(7.22) and 10(4.93), respectively). The third site, accounting for over 99% of the total CEC, is associated with planar sites on expansible clays and shows a smaller Na+-Cs+ selectivity coefficient of 10(1.99). Parameters derived from a fit of binary batch experiments alone tend to under predict Cs+ retardation in the column experiments. The transport experiments indicate 72-90% of the Cs+ sorbed in experiments targeting exchange on FES was desorbed over a 10- and 24-day period, respectively. At high Cs+ concentrations, where sorption is controlled primarily by exchange on planar sites, 95% of the Cs+ desorption was desorbed. Most of the difficulty in desorbing Cs+ from FES is a result of the extremely high selectivity of these sites for Cs+, although truly irreversible sorption as high as 23% was suggested in one experiment. The conclusion that Cs+ exchange is largely reversible in a thermodynamic sense is supported by the ability to match Cs+ desorption curves almost quantitatively with an equilibrium reactive transport simulation. The model for Cs+ retardation developed here qualitatively explains the behavior of Cs+ in the Hanford vadose zone underneath a variety of leaking tanks with differing salt concentrations. The high selectivity of FES for Cs+ implies that future desorption and migration is very unlikely to occur under natural recharge conditions.     

Determination of bromacil transport as a function of water and carbon content in soils

This study was conducted to determine the significance of bromacil transport as a function of water and carbon content in soils and to explore the implications of neglecting sorption when making assessments of travel time of bromacil through the vadose zone. Equilibrium batch sorption tests were performed for loamy sand and sandy soil added with four different levels of powdered activated carbon (PAC) content (0, 0.01, 0.05, and 0.1%). Column experiments were also conducted at various water and carbon contents under steady-state flow conditions. The first set of column experiments was conducted in loamy sand containing 1.5% organic carbon under three different water contents (0.23, 0.32, and 0.41) to measure breakthrough curves (BTCs) of bromide and bromacil injected as a square pulse. In the second set of column experiments, BTCs of bromide and bromacil injected as a front were measured in saturated sandy columns at the four different PAC levels given above. Column breakthrough data were analyzed with both equilibrium and nonequilibrium (two-site) convection-dispersion equation (CDE) models to determine transport and sorption parameters under various water and carbon contents. Analysis with batch data indicated that neglect of the partition-related term in the calculation of solute velocity may lead to erroneous estimation of travel time of bromacil, i.e. an overestimation of the solute velocity by a factor of R. The column experiments showed that arrival time of the bromacil peak was larger than that of the bromide peak in soils, indicating that transport of bromacil was retarded relative to bromide in the observed conditions. Extent of bromacil retardation (R) increased with decreasing water content and increasing PAC content, supporting the importance of retardation in the estimation of travel time of bromacil even at small amounts of organic carbon for soils with lower water content.     

Attempt to model laboratory-scale diffusion and retardation data

Different approaches for measuring the interaction between radionuclides and rock matrix are needed to test the compatibility of experimental retardation parameters and transport models used in assessing the safety of the underground repositories for the spent nuclear fuel. In this work, the retardation of sodium, calcium and strontium was studied on mica gneiss, unaltered, moderately altered and strongly altered tonalite using dynamic fracture column method. In-diffusion of calcium into rock cubes was determined to predict retardation in columns. In-diffusion of calcium into moderately and strongly altered tonalite was interpreted using a numerical code FTRANS. The code was able to interprete in-diffusion of weakly sorbing calcium into the saturated porous matrix. Elution curves of calcium for the moderately and strongly altered tonalite fracture columns were explained adequately using FTRANS code and parameters obtained from in-diffusion calculations. In this paper, mass distribution ratio values of sodium, calcium and strontium for intact rock are compared to values, previously obtained for crushed rock from batch and crushed rock column experiments. Kd values obtained from fracture column experiments were one order of magnitude lower than Kd values from batch experiments.     

Column studies on transport of deicing additive benzotriazole in a sandy aquifer and a zerovalent iron barrier

Benzotriazole (BTA), a chemical with wide industrial applications, is a typical additive in deicer/anti-icer used at airport. To achieve a better understanding of the transport behaviour and environmental fate of BTA, laboratory column studies have been performed on subsoil samples from Oslo Airport, Gardermoen. To explore possibilities for aquifer remediation, BTA behaviour was also studied in a column of granular zerovalent iron (Fe(0)). The subsoil column study demonstrates a very limited retardation of BTA. Consecutive loadings of BTA of the subsoil column showed no change of the break-through curve (BTC) and complete desorption was observed. The sorption behaviour of BTA to metallic iron (Fe(0)) was rather complex. Considerable retardation was observed in the Fe(0) column and repeated BTA loading resulted in an earlier break-through. Between 20% and 50% of the input concentration was retained permanently in the iron (Fe(0)) column. The BTA sorption to metallic iron was found to be enhanced by chloride which lowered the break-through concentration (i.e the C/C(0) plateau). The fraction of BTA remaining in the iron column was found to vary with the flow rate, indicating a time dependant multilayer sorption mechanism. The steady increase in the amount of adsorbed BTA to the iron column during loading corresponds to a rather strong bonding of 4-15 BTA layers to the iron surface. A very slow desorption of BTA was observed; even after flushing with 753 pore volumes of BTA free water, 7.5% of the BTA remained in the column. A geochemical model was developed based on PHREEQC-2 to simulate the sorption and transport of BTA in the tested materials. The BTA sorption was modelled with Freundlich sorption isotherms, as earlier determined in batch experiments. A slight adjustment of the Freundlich parameters was required to fit the observed column break-through. However, our model was not able to simulate the long-term retainment of BTA in the granular iron columns. The simulations confirm the high mobility of BTA in groundwater aquifers and suggest that zerovalent iron could be used to retain a BTA plume, although oxidation of the sorbent might reduce the long-term performance of such a remediation scheme and slow desorption has been observed.     

Biodegradation of petroleum hydrocarbon vapors: laboratory studies on rates and kinetics in unsaturated alluvial sand

Predictions of natural attenuation of volatile organic compounds (VOCs) in the unsaturated zone rely critically on information about microbial biodegradation kinetics. This study aims at determining kinetic rate laws for the aerobic biodegradation of a mixture of 12 volatile petroleum hydrocarbons and methyl tert-butyl ether (MTBE) in unsaturated alluvial sand. Laboratory column and batch experiments were performed at room temperature under aerobic conditions, and a reactive transport model for VOC vapors in soil gas coupled to Monod-type degradation kinetics was used for data interpretation. In the column experiment, an acclimatization of 23 days took place before steady-state diffusive vapor transport through the horizontal column was achieved. Monod kinetic parameters Ks and vmax could be derived from the concentration profiles of toluene, m-xylene, n-octane, and n-hexane, because substrate saturation was approached with these compounds under the experimental conditions. The removal of cyclic alkanes, isooctane, and 1,2,4-trimethylbenzene followed first-order kinetics over the whole concentration range applied. MTBE, n-pentane, and chlorofluorocarbons (CFCs) were not visibly degraded. Batch experiments suggested first-order disappearance rate laws for all VOCs except n-octane, which decreased following zero-order kinetics in live batch experiments. For many compounds including MTBE, disappearance rates in abiotic batch experiments were as high as in live batches indicating sorption. It was concluded that the column approach is preferable for determining biodegradation rate parameters to be used in risk assessment models.     

Fate of seven pesticides in an aerobic aquifer studied in column experiments

The fate of selected pesticides (bentazone, isoproturon, DNOC, MCPP, dichlorprop and 2,4-D) and a metabolite (2,6-dichlorobenzamide (BAM)) was investigated under aerobic conditions in column experiments using aquifer material and low concentrations of pesticides (approximately 25 microg/l). A solute transport model accounting for kinetic sorption and degradation was used to estimate sorption and degradation parameters. Isoproturon and DNOC were significantly retarded by sorption, whereas the retardation of the phenoxy acids (MCPP, 2,4-D and dichlorprop), BAM and bentazone was very low. After lag periods of 16-33 days for the phenoxy acids and 80 days for DNOC, these pesticides were degraded quickly with 0.-order rate constants of 1.3-2.6 microg/l/day. None of the most probable degradation products were detected.     

Bioremediation of strontium (Sr) contaminated aquifer quartz sand based on carbonate precipitation induced by Sr resistant Halomonas sp

Contamination of aquifers or sediments by radioactive strontium (⁹⁰Sr) is a significant environmental problem. In the present study, microbially induced calcite precipitation (MICP) was evaluated for its potential to remediate strontium from aquifer quartz sand. A Sr resistant urease producing Halomonas sp. was characterized for its potential role in bioremediation. The bacterial strain removed 80% of Sr from soluble-exchangeable fraction of aquifer quartz sand. X-ray diffraction detected calcite, vaterite and aragonite along with calcite-strontianite (SrCO₃) solid solution in bioremediated sample with indications that Sr was incorporated into the calcite. Scanning electron micrography coupled with energy-dispersive X-ray further confirmed MICP process in remediation. The study showed that MICP sequesters soluble strontium as biominerals and could play an important role in strontium bioremediation from both ecological and greener point of view.     

Application of the method of temporal moments to interpret solute transport with sorption and degradation

In this note, we applied the temporal moment solutions of [Das and Kluitenberg, 1996. Soil Sci. Am. J. 60, 1724] for one-dimensional advective-dispersive solute transport with linear equilibrium sorption and first-order degradation for time pulse sources to analyse soil column experimental data. Unlike most other moment solutions, these solutions consider the interplay of degradation and sorption. This permits estimation of a first-order degradation rate constant using the zeroth moment of column breakthrough data, as well as estimation of the retardation factor or sorption distribution coefficient of a degrading solute using the first moment. The method of temporal moment (MOM) formulae was applied to analyse breakthrough data from a laboratory column study of atrazine, hexazinone and rhodamine WT transport in volcanic pumice sand, as well as experimental data from the literature. Transport and degradation parameters obtained using the MOM were compared to parameters obtained by fitting breakthrough data from an advective-dispersive transport model with equilibrium sorption and first-order degradation, using the nonlinear least-square curve-fitting program CXTFIT. The results derived from using the literature data were also compared with estimates reported in the literature using different equilibrium models. The good agreement suggests that the MOM could provide an additional useful means of parameter estimation for transport involving equilibrium sorption and first-order degradation. We found that the MOM fitted breakthrough curves with tailing better than curve fitting. However, the MOM analysis requires complete breakthrough curves and relatively frequent data collection to ensure the accuracy of the moments obtained from the breakthrough data.     

Numerical modeling of humic colloid borne americium (III) migration in column experiments using the transport/speciation code K1D and the KICAM model

The humic colloid borne Am(III) transport was investigated in column experiments for Gorleben groundwater/sand systems. It was found that the interaction of Am with humic colloids is kinetically controlled, which strongly influences the migration behavior of Am(III). These kinetic effects have to be taken into account for transport/speciation modeling. The kinetically controlled availability model (KICAM) was developed to describe actinide sorption and transport in laboratory batch and column experiments. Application of the KICAM requires a chemical transport/speciation code, which simultaneously models both kinetically controlled processes and equilibrium reactions. Therefore, the code K1D was developed as a flexible research code that allows the inclusion of kinetic data in addition to transport features and chemical equilibrium. This paper presents the verification of K1D and its application to model column experiments investigating unimpeded humic colloid borne Am migration. Parmeters for reactive transport simulations were determined for a Gorleben groundwater system of high humic colloid concentration (GoHy 2227). A single set of parameters was used to model a series of column experiments. Model results correspond well to experimental data for the unretarded humic borne Am breakthrough.     

Effect of cell physicochemical characteristics and motility on bacterial transport in groundwater

The influence of physicochemical characteristics and motility on bacterial transport in groundwater were examined in flow-through columns. Four strains of bacteria isolated from a crystalline rock groundwater system were investigated, with carboxylate-modified and amidine-modified latex microspheres and bromide as reference tracers. The bacterial isolates included a gram-positive rod (ML1), a gram-negative motile rod (ML2), a nonmotile mutant of ML2 (ML2m), and a gram-positive coccoid (ML3). Experiments were repeated at two flow velocities, in a glass column packed with glass beads, and in another packed with iron-oxyhydroxide coated glass beads. Bacteria breakthrough curves were interpreted using a transport equation that incorporates a sorption model from microscopic observation of bacterial deposition in flow-cell experiments. The model predicts that bacterial desorption rate will decrease exponentially with the amount of time the cell is attached to the solid surface. Desorption kinetics appeared to influence transport at the lower flow rate, but were not discernable at the higher flow rate. Iron-oxyhydroxide coatings had a lower-than-expected effect on bacterial breakthrough and no effect on the microsphere recovery in the column experiments. Cell wall type and shape also had minor effects on breakthrough. Motility tended to increase the adsorption rate, and decrease the desorption rate. The transport model predicts that at field scale, desorption rate kinetics may be important to the prediction of bacteria transport rates.     

Impact of woodchip biochar amendment on the sorption and dissipation of pesticide acetamiprid in agricultural soils

Pyrolysis of vegetative biomass into biochar and application of the more stable form of carbon to soil have been shown to be effective in reducing the emission of greenhouse gases, improving soil fertility, and sequestering soil contaminants. However, there is still lack of information about the impact of biochar amendment in agricultural soils on the sorption and environmental fate of pesticides. In this study, we investigated the sorption and dissipation of a neonicotinoid insecticide acetamiprid in three typical Chinese agricultural soils, which were amended by a red gum wood (Eucalyptus spp.) derived biochar. Our results showed that the amendment of biochar (0.5% (w/w)) to the soils could significantly increase the sorption of acetamiprid, but the magnitudes of enhancement were varied. Contributions of 0.5% newly-added biochar to the overall sorption of acetamiprid were 52.3%, 27.4% and 11.6% for red soil, paddy soil and black soil, respectively. The dissipation of acetamiprid in soils amended with biochar was retarded compared to that in soils without biochar amendment. Similar to the sorption experiment, in soil with higher content of organic matter, the retardation of biochar on the dissipation of acetamiprid was lower than that with lower content of organic matter. The different effects of biochar in agricultural soils may attribute to the interaction of soil components with biochar, which would block the pore or compete for binding site of biochar. Aging effect of biochar application in agricultural soils and field experiments need to be further investigated.