Sorption of veterinary pharmaceuticals in soils: a review

Veterinary pharmaceuticals (VPs) are used in large amounts in modern husbandry. Due to their use pattern, they possess a potential for reaching the soil environment. To assess their mobility in soil, the literature on sorption of chemicals used as VPs is reviewed and put into perspective of their physicochemical properties. The compilation of sorption coefficients to soil solids (Kd,solid) demonstrates that these chemicals display a wide range of mobility (0.2 < Kd,solid < 6,000 L/kg). Partition coefficients for association of tetracycline and quinolone carboxylic acid VPs to dissolved organic matter (Kd,DOM) vary between 100 and 50,000 L/kg. The variation in Kd,solid for a given compound in different soils can be significant. For most of the compounds, the variation is not considerably lower for the organic carbon-normalized sorption coefficient Koc. In addition, prediction of log Koc by log Kow leads to significant underestimation of log Koc and log Kd,DOM values. This suggests that mechanisms other than hydrophobic partitioning play a significant role in sorption of VPs. A number of hydrophobicity-independent mechanisms such as cation exchange, cation bridging at clay surfaces, surface complexation, and hydrogen bonding appear to be involved. These processes are not accounted for by organic carbon normalization, suggesting that this data treatment is conceptually inappropriate and fails to describe the sorption behavior. Moreover, prediction of log Koc based on the hydrophobicity parameter log Kow is not successful.     

Sorption of three tetracyclines by several soils: assessing the role of pH and cation exchange

Tetracyclines (TCs) are widely used in veterinary medicine for treatment and prevention of disease and are present in animal waste products. Detection of TCs in soil, sediments, and water, and the growing concern of their potentially adverse effect on natural ecosystems have resulted in a need to understand their behavior in aqueous soil systems. TCs have multiple ionizable functional groups such that at environmentally relevant pH values, they may exist as a cation (+ 0 0), zwitterion (+ - 0), or a net negatively charged ion (+ - -), which complicates predicting their sorption, availability, and transport. We investigated the sorption of oxytetracycline (OTC), tetracycline (TC), and chlortetracycline (CTC) by several soils varying in pH, clay amount and type, cation exchange capacity (CEC), anion exchange capacity (AEC), and soil organic carbon in 0.01 N CaCl2, 0.001 N CaCl2, and 0.01 N KCI. All three TCs are highly sorbed, especially in acidic and high clay soils. When normalized to CEC, sorption tends to decrease with increasing pH. A sorption model in which species-specific sorption coefficients normalized to pH-dependent CEC (Kd+00, kd+-0, and kd+--) and weighted by the pH-dependent fraction of each species fit the data well across all soils except for a soil rich in gibbsite and high in AEC. Resulting kd+00 values were more than an order of magnitude larger than values for either kd+0 and kd+--values such that kd+00 alone described most of the sorption observed as a function of pH for eight soils that varied in their mineralogy and pH (pH ranged from 4 to 8).     

The sorption of iodide by soils as influenced by equilibrium conditions and soil properties

The sorption of iodide was reduced when soil was dried before equilibration with an iodide solution. With undried soils, sorption continued for > 48 h, maximum sorption occurred at pH values < 5 but a secondary sorption peak occurred at pH 8.5 to 9.0, particularly with a soil containing a high level of organic matter. Temperature had only a small effect on sorption over the range 10 to 35 °C. Maximum values for the sorption of iodide by two surface soils (0 to 10cm) at pH 6.6 to 6.8, assessed with a soil: solution ratio of 1:10, an equilibrium time of 40 h and at room temperature, were 25 and 6 fig I/g soil, respectively. The amounts of iodide sorbed by these soils, and by soils taken from successive 10 cm layers to a depth of 40 cm at the same two sites, were closely related to the contents of organic matter in the soils but not to contents of iron or aluminium oxides or of clay. Treatment of the surface soils with hydrogen peroxide to destroy organic matter greatly reduced the sorption of iodide at the pH of about 5.5 that resulted from the treatment. The removal of iron and aluminium oxides with Tamm reagent also resulted in a marked reduction in sorption at pH < 5. The results indicate that sorption was due in part to soil organic matter and in part to iron and/or aluminium oxides. At pH > 6, organic matter appeared to be the major sorbing constituent but under more acid conditions the oxides appeared to be increasingly important.     

Zinc-sulfur and cadmium-sulfur association in metalliferous peats: evidence from spectroscopy,distribution coefficients,and phytoavailability

Peat deposits can concentrate chalcophilic metals such as Zn and Cd by biogeochemical processes; as a result, there is a possibility that the solubility, mobility, and bioavailability of these metals could increase when such deposits are drained and cropped, initiating oxidation of organic matter and sulfides under aerobic conditions. We use spectroscopic, chemical, and bioassay approaches to characterize high Zn (88-15,800 mg kg(-1)), Cd (0.55-83.0 mg kg(-1)), and S (3.52-9.54 g kg(-1)) peat soils collected from locations in New York State and Ontario that overlie Silurian-age metal-enriched dolomite bedrock. Total and KNO3-extractable trace metals were determined by ICP emission spectrometry, and labile Cd and Zn were measured in the KNO3 extracts by anodic stripping voltammetry. A greenhouse bioassay with maize and canola was conducted to determine the bioavailability and toxicity of the soil Zn and Cd. The electronic oxidation states of sulfur in the peat soils were determined by X-ray absorption near edge spectroscopy (XANES) and Zn and S distribution in soil particles by energy-dispersive X-ray absorption (EDX) spectroscopy. Sulfur-XANES analyses show that a high percentage (35-45%) of the total soil S exists in the most reduced electronic oxidation states (such as sulfides and thiols), while <5% exists in the most oxidized forms (such as sulfate). EDX analyses indicate a microscopic elemental association between Zn and S in these soils. Despite the EDX evidence of close association between Zn and S in soil particles, conventional X-ray diffraction analyses of the bulk soils did not detect a mineral phase of sphalerite (ZnS) in any of the soils. The distribution coefficients (Kd) for Zn and Cd increased with soil pH and indicated stronger Cd retention than Zn in the peats. The results of the bioassaytests showed that most of the high-Zn soils were very phytotoxic, with plant shoot Zn levels exceeding 400 mg kg(-1). Conversely, Cd concentrations in the plant shoots were generally below 2 mg kg(-1), showing a tendency toward low Cd phytoavailability relative to Zn. The information gained from the spectroscopic analyses (S-XANES and EDX) was used to explain the macroscopic observations (Cd and Zn Kd values and phytoavailability data) in these peat soils; we conclude that sulfur biogeochemical cycling may play an important role in Zn and Cd retention in these organic soils.     

Surface complexation of copper(II) on soil particles: EPR and XAFS studies

The interactions of transition metals with natural systems play an important role in the mobility and the bioavailability of these metals in soils. In this study, the adsorption of copper(II) onto natural soil particles was studied as a function of pH and metal concentration. The retention capacity of soil particles was determined at pH 6.2 to be equal to 6.7 mg of copper/g of solid. The Langmuir and Freundlich isotherm equations were then used to describe the partitioning behavior of the system at different pH values. A combination of EPR, extended X-ray absorption fine structure (EXAFS), and X-ray absorption near-edge structure (XANES) spectroscopies was used to probe the Cu atomic environment at the soil particles/aqueous interface. The spectroscopic study revealed that copper(II) ions are held in inner-sphere surface complexes. It also revealed that Cu was in an octahedral coordination with first-shell oxygen atoms. A weak tetragonal distortion was pointed out due to the Jahn-Teller effect, with a mean Cu-Oequatorial bond distance of 1.96 A and a Cu-Oaxial bond distance of 2.06 A. A detailed analysis of the spectroscopic data suggested that Cu(II) was bonded to organic matter coated onto the mineral fraction of soil particles.     

Modeling kinetics of Cu and Zn release from soils

Kinetics of Cu and Zn release from soil particles was studied using two surface soils with a stirred-flow method. Different solution pH, dissolved organic matter (DOM) concentrations, and flow rates were tested in this study. A model for kinetics controlled sorption/desorption reactions between soils and solutions was globally fit to all experimental data simultaneously. Results were compared to a model that assumes local instantaneous equilibrium. We obtained one unique set of model parameters applicable to different pH, dissolved organic carbon (DOC), and flow conditions. We included DOM complexation of copper ions, which decreased their sorption. The effect of pH was included by assuming proton competition with metal ions for binding sites on soil particles. These results provide the basis for developing predictive models for metal release from soil particles to surface waters and soil solution.     

Solubility and sorption by soils of 8:2 fluorotelomer alcohol in water and cosolvent systems

The solubility and sorption by five soils of 8:2 fluorotelomer alcohol (FTOH) were measured from water and cosolvent/ water solutions. Aqueous solubility and soil-water distribution coefficients (Kd,w, L kg(-1)) were extrapolated from cosolvent data using a log-linear cosolvency model and compared to direct aqueous measurements. Liquid chromatography tandem mass spectrometry with electrospray ionization was employed to analyze the 8:2 FTOH in solutions and soil extracts. The cosolvent-extrapolated water solubility is 0.224 mg L(-1), in good agreement with the measured value of 0.194 mg L(-1). All sorption isotherms were generally linear regardless of cosolvent composition or soil organic carbon (OC) content. Kd,w values extrapolated from cosolvent data were similar but consistently higher than those measured in aqueous solutions. The latter was hypothesized to be due to dissolved OC (DOC) in the aqueous slurries. An average log KDOC of 5.30 was estimated and supported by DOC and Kd,w measurements at two soil-water ratios. Sorption appeared to be driven by hydrophobic partitioning with a log KOC value of 4.13 +/- 0.16. Irreversible sorption was also observed and appeared to be related to OC content, with the extraction efficiency reduced from 85% to 45% with increasing contact time from 3 to 72 h for the highest OC soil.     

Sorption of oxytetracycline to iron oxides and iron oxide-rich soils

The sorption interactions of oxytetracycline with goethite, hematite, and two iron oxide-rich soils were investigated using batch sorption experiments. Oxytetracycline sorption coefficients for goethite and hematite increased with pH to maximum values at pH approximately 8. The sorption edge shape and desorption treatments were consistent with a surface complexation mechanism and could be described by the interaction of divalent anion species with the oxide surface. Oxytetracycline sorption to Georgeville and Orangeburg Ultisol soils decreased with pH. Chemical digestion treatments were used to deduce that soil sorption occurred by complexation to oxide coatings on clay and quartz grains. These results indicate that sorption models must consider the interaction of oxytetracycline, and other similar ionogenic compounds, with soil oxide components in addition to clays and organic matter when predicting sorption in whole soils.     

Hydrophilic and hydrophobic sorption of organic acids by variable charge soils: effect of chemical acidity and acidic functional group

Sorption of organic acids by variable-charge soil occurs through both hydrophilic and hydrophobic sorption. In this study, the effect of chemical acidity and the type of acidic functional group on the relative contribution of hydrophilic and hydrophobic processes to sorption by a gibbsite-dominated and a kaolinite-dominated variable-charge soils was quantified by measuring sorption isotherms from different electrolytes (CaCl2, Ca(H2PO4)2, and KCl). The A1 soil is dominated by gibbsite whereas the DRC soil is primarily kaolinite. The organic acids investigated include five chlorinated phenols (pentachlorophenol, 2,3,4,6-tetrachlorophenol, 2,4,6-trichlorophenol, 2,4,5-trichlorophenol, and 2,4-dichlorophenol) with pKa values ranging from 4.69 to 7.85 and two acidic herbicides (2,4-D (pKa = 2.8) and prosulfuron (pKa = 3.76)) that contain carboxyl and urea functional groups, respectively. Anion exchange of chlorinated phenols and prosulfuron on both variable-charge soils as well as 2,4-D sorption on the A1 soil was linearly correlated to chemical acidity. The effective positive surface charge [AEC/(AEC + CEC)] and the anionic fraction of the organic acid in solution, which are both pH-dependent, were sufficient to estimate the contribution of anion exchange to organic acid sorption except for 2,4-D sorption by DRC soil. The latter was much greater than would be predicted from the pKa of 2,4-D. Calcium bridging between silanol edge group and 2,4-D was hypothesized and corroborated by differences in sorption measured from KCl and CaCl2 solutions. For predicting contributions from hydrophobic processes, a log-log linear relationship between pH-dependent octanol-water (Kow(pH)) and organic carbon-normalized sorption coefficients (Koc(pH)) appeared adequate.     

Zinc coordination to multiple ligand atoms in organic-rich surface soils

We report on the solid-phase speciation of naturally occurring Zn in metalliferous organic-matter-rich surface soils. Synchrotron-based studies were used to probe elemental distribution and associations in soil particles (micro-XRF) together with the mineralogy (micro-XRD) and Zn bonding environment (Zn-micro-XANES) at the micrometer-scale level. The average bonding environment of Zn was also probed for bulk soils using XANES. We found the distribution of elements within soil particles to be heterogeneous; however, some elements are consistently co-located. While conventional XRD analyses of whole soils did not identify any Zn mineral phase, synchrotron-based-micro-XRD analyses indicated that sphalerite (ZnS) is present in a particle from a wetland soil (soil labeled G3). Linear combination fit (LCF) analyses of XANES spectra collected for bulk soils (Zn-XANES) and microm-regions (Zn-micro-XANES) within soil particles suggest Zn bonds to oxygen-, nitrogen-, and sulfur-functional groups in these sulfur-, nitrogen-, and zinc-rich organic surface soils. The XANES spectra of all bulk soils and of all microm-regions except for the wetland soil (G3), where ZnS was the most significant constituent, were best fitted by the Zn-arginine reference compound and therefore seems to indicate Zn bonding to nitrogen. Thus, these results provide compelling evidence of the formation of highly covalent Zn-organic bonds in the organic-rich surface soils that were studied. This may explain in part why metal partition coefficients (Kd) are generally higher in organic soils, and why the toxic thresholds for total metal concentrations are higher in organic than in mineral soils.     

Extensive sorption of organic compounds to black carbon, coal, and kerogen in sediments and soils: mechanisms and consequences for distribution, bioaccumulation, and biodegradation

Evidence is accumulating that sorption of organic chemicals to soils and sediments can be described by "dual-mode sorption": absorption in amorphous organic matter (AOM) and adsorption to carbonaceous materials such as black carbon (BC), coal, and kerogen, collectively termed "carbonaceous geosorbents" (CG). Median BC contents as a fraction of total organic carbon are 9% for sediments (number of sediments, n approximately 300) and 4% for soils (n = 90). Adsorption of organic compounds to CG is nonlinear and generally exceeds absorption in AOM by a factor of 10-100. Sorption to CG is particularly extensive for organic compounds that can attain a more planar molecular configuration. The CG adsorption domain probably consists of surface sites and nanopores. In this review it is shown that nonlinear sorption to CG can completely dominate total sorption at low aqueous concentrations (<10(-6) of maximum solid solubility). Therefore, the presence of CG can explain (i) sorption to soils and sediments being up to 2 orders of magnitude higher than expected on the basis of sorption to AOM only (i.e., "AOM equilibrium partitioning"), (ii) low and variable biota to sediment accumulation factors, and (iii) limited potential for microbial degradation. On the basis of these consequences of sorption to CG, it is advocated that the use of generic organic carbon-water distribution coefficients in the risk assessment of organic compounds is not warranted and that bioremediation endpoints could be evaluated on the basis of freely dissolved concentrations instead of total concentrations in sediment/soil.     

Variation in phenanthrene sorption coefficients with soil organic matter fractionation: the result of structure or conformation?

Sorption of phenanthrene to varying soil types was investigated to better understand sorption processes. Humic acid and humin fractions were isolated from each soil sample, and sorption coefficients were measured by batch equilibration. Samples were characterized by carbon analysis and 13C cross polarization magic angle spinning (CP/ MAS) nuclear magnetic resonance (NMR) spectroscopy. Measured organic carbon-normalized sorption coefficients (Koc) of the fractions were greater in all cases when compared to the soils. The humin fractions exhibited greater Koc values than did source samples, suggesting that fractionation may reorganize organic matter in humin resulting in an increased availability of and/or more favorable sorption domains. Mass balance calculations revealed that the sum of sorption to the fractions is greater than sorption to the whole sample. The greatest difference between sorption values was found to occur with the mineral soils, suggesting that clay minerals influence the physical conformation of soil organic matter (SOM) and availability of sorption domains. The mass balance, sorption data, and a lack of consistent trends between observed Kco values and solid-state 13C NMR data suggest that the physical conformation of SOM and chemical characteristics both play important roles in sorption processes.     

Importance of black carbon to sorption of native PAHs, PCBs, and PCDDs in Boston and New York harbor sediments

The solid-water distribution ratios (Kd values) of "native" PAHs, PCBs, and PCDDs in Boston and New York Harbor sediments were determined using small passive polyethylene samplers incubated for extended times in sediment-water suspensions. Observed solid-water distribution coefficients exceeded the corresponding f(oc)Koc products by 1-2 orders of magnitude. It was hypothesized that black carbon (fBC), measured in the Boston harbor sediment at about 0.6% and in the New York harbor sediment at about 0.3%, was responsible for the additional sorption. The overall partitioning was then attributed to absorption into the organic carbon and to adsorption onto the black carbon via Kd = f(oc)Koc + f(BC)K(BC)C(w)n-1 with Cw in microg/L. Predictions based on published Koc, K(BC), and n values for phenanthrene and pyrene showed good agreement with observed Kd,obs values. Thus, assuming this dual sorption model applied to the other native PAHs, PCBs, and PCDDs, black carbon-normalized adsorption coefficients, K(BC)S, were deduced forthese contaminants. Log K(BC) values correlated with sorbate hydrophobicity for PAHs in Boston harbor (log K(BC) approximately 0.83 log gamma w(sat) - 1.6; R2 = 0.99, N= 8). The inferred sorption to the sedimentary BC phase dominated the solid-water partitioning of these compound classes, and its inclusion in these sediments is necessary to make accurate estimates of the mobility and bioavailability of PAHs, PCBs, and PCDDs.     

Sorption and dissipation of testosterone, estrogens, and their primary transformation products in soils and sediment

Concern over the potential negative ecological effects of steroid hormones from human- and animal-derived wastes has resulted in an increased interest regarding the mobility and persistence of these compounds in the environment. Batch experiments were conducted to examine the simultaneous sorption and dissipation of three reproductive hormones (testosterone, 17beta-estradiol, and 17alpha-ethynyl estradiol) in four midwestern U.S. soils and one freshwater sediment. Sorption isotherms were generated by measuring aqueous concentrations and by extracting the sorbed parent chemical or transformation products (e.g., estrone, androstenedione). Apparent sorption equilibrium is reached within a few hours. Measured sorption isotherms for the three parent chemicals and their principal transformation products were generally linear. Average organic carbon normalized sorption coefficients (K(oc)) resulted in standard deviations of less than 0.2 log units and were consistent with reported aqueous solubilites and octanol-water partition coefficients, indicating hydrophobic partitioning as the dominant sorption mechanism. Large log K(oc) values (approximately 3-4) suggest that leaching from soils will be limited, runoff of soil- and land-applied biosolids are the most likely inputs into surface waters, and that a significant fraction of these compounds will be associated with sediments. Half-lives for hormone dissipation in the aerobic soil and sediment slurries estimated assuming pseudo first-order processes ranged from a few hours to a few days with testosterone having the shortest half-life.     

Effect of two organic amendments on norflurazon retention and release by soils of different characteristics

The influence of two organic amendments on norflurazon sorption-desorption processes in four soils with very different physicochemical characteristics was studied in laboratory experiments to evaluate the potential leaching of this pesticide through organic fertilized soils. Sorption-desorption experiments were performed on original soils and on a mixture of these soils with urban waste compost (SUW) and a commercial amendment from olive-mill wastes (OW), at a rate of 6.25% (w/w). These mixtures were used immediately after preparation and after aging for 2 months. Norflurazon was analyzed by using a HPLC method. Norflurazon retention in original soils was related not only to the organic matter (OM) content but also to mineral surfaces present in soils. Norflurazon sorption increases largely after amendment in soils with low OM content, but the addition of exogenous OM to soils with medium OM content and/or other available adsorptive surfaces did not significantly affect norflurazon sorption. Even in some cases pesticide sorption decreases, due to the blocking of the mineral and organic soil surfaces with the amendment added. Transformation of exogenous OM during incubation depends both on the amendment added and on the type of soil and can affect sorption-desorption behavior of the soils surfaces in different manner, due to the modification of their hydrophobic-hydrophilic characteristics. Norflurazon desorption from original soils showed hysteresis in all cases, but it was not affected or even decreased in amended soils. It was a nonexpected behavior, especially in sandy soil, since it is generally assumed that a higher sorption always implies a lower mobility in soils. Norflurazon sorption must be taking place on very low affinity sites on exogenous OM through weak bindings, from which the pesticide can be easily desorbed. The application to soils of the organic amendments used in the present study could not be accepted to reduce norflurazon losses due to leaching processes.     

Sorption of heterocyclic organic compounds to reference soils: column studies for process identification

In this study, the sorption behavior of a wide variety of N-, S-, and O-heterocyclic compounds (NSOs) to reference soils (Eurosoils 1-5) was characterized by a soil column chromatography (SCC) approach. The major goal was to identify the compound specific and environmental factors influencing sorption processes. The sorption of S- and O-heterocyclic compounds (thiophene, benzothiophene, 5-methylbenzo[b]thiophene, benzofuran, 2-methylbenzofuran, and 2,3-dimethylbenzofuran) was generally controlled by nonspecific interactions with soil organic carbon (OC). With regard to non-ionizable N-heterocyclic compounds, pyrrole, 1-methylpyrrole, and pyrimidine were hardly retarded in any soil. The sorption of indole, 2-hydroxyquinoline, and benzotriazole was dominated by specific interaction (e.g., complexation of surface-bound cations) rather than partition to soil OC. The sorption of ionizable N-heterocyclic compounds (quinoline, isoquinoline, quinaldine, 2-methylpyridine, and pyridine) can be described by a conceptual model including partitioning to soil OC, cation exchange, and an additional sorption process (probably surface complexation of the neutral species). Cation exchange was usually the dominant mechanism in the sorption of ionizable compounds if the protonated fraction of the compound exceeded 5%. Otherwise, surface complexation became dominant. Soil pH was the most important factor influencing the sorption of ionizable NSOs. Our study suggests that a fairly precise assessment of sorption in most soils can be expected for N-, S-, and O-heterocyclic compounds if the three sorption mechanisms are taken into accountwhere appropriate. Deviations from this behavior indicated special cases where additional soil specific properties (e.g., accessible surface, CEC, charge density) need to be considered such as for 2-methylpyridine and pyridine sorption to Eurosoil 1.     

Proton buffering and metal leaching in sandy soils

Recent developments in acidification research focused on the leaching of metals from contaminated soil. In this paper the buffering of sandy soils upon acidification is studied in relation to the release of major (Al, Ca, Mg) and trace metals (Cu, Cd, Ni, Zn) from the soil reactive surface. The buffering process and the (de)sorption of metals are described with a mechanistic multisurface model, expressing the sorption onto different soil surfaces (organic matter, clay, Fe (hydr)oxides). The pH of sandy soil samples is predicted upon proton addition in combination with the behavior of major and trace metals. Acidification of contaminated sandy soil samples, with different pH levels and metal contents, is performed in a flow-through reactor by flushing the samples with acid solution. Acidification has taken place in successive steps of proton addition and followed by sampling. Prediction of pH upon acidification with a multisurface model gives satisfying results for all samples studied. The pH is modeled reasonably well between pH 6 and 4. Below pH 4 the predicted pH values are slightly too low, probably due to the buffering by Al-containing minerals (e.g., Al hydroxide), which are not included in the model. Desorption of major and trace metals upon pH decrease is, in general, predicted well, within a factor of 1-5 on a linear scale. Overall prediction of proton buffering in combination with desorption of metals in sandy soil samples, over a wide pH range and metal content, is done quite well for the studied metals with the multisurface model.     

Ambient gas/particle partitioning. 1. Sorption mechanisms of apolar, polar, and ionizable organic compounds

There remain several ambiguities in the literature regarding the dominating sorption mechanisms involved in gas/particle partitioning, particularly for polar and ionizable compounds. The various hypothetical mechanisms would depend differently on relative humidity (RH) and the presence of various aerosol components. Thus, in order to resolve these ambiguities, here we measured the RH-dependency of gas/particle partitioning constants, K(ip), for four diverse aerosol samples and a large set of chemicals covering apolar, polar, and ionizable organic compounds. In addition, we also removed the water-soluble components from two ambient particle samples to study how their presence influences sorption behavior. The measured K(ip) values collectively indicate that a dual-phase sorption mechanism is occurring, in which organic compounds partition into a RH-independent water-insoluble organic matter phase and additionally into a RH-dependent mixed-aqueous phase. All K(ip) values could be successfully fitted to a RH-dependent dual-phase sorption model. The trends in K(ip) data further support findings that the sorption behavior of ambient aerosol samples is different from raw mineral surfaces and soot.     

Chemical speciation of Zn,Cd, Cu,and Pb in pore waters of agricultural and contaminated soils using Donnan dialysis

Knowledge of trace metal speciation in soil pore waters is important in addressing metal bioavailability and risk assessment of contaminated soils. Numerous analytical methods have been utilized for determining trace metal speciation in aqueous environmental matrixes; however, most of these methods suffer from significant interferences. The Donnan dialysis membrane technique minimizes these interferences and has been used in this study to determine free Zn2+, Cd2+, Cu2+, and Pb2+ activities in pore waters from 15 agricultural and 12 long-term contaminated soils. The soils vary widely in their origin, pH, organic carbon content, and total metal concentrations. Pore water pM2+ activities also covered a wide range and were controlled by soil pH and total metal concentrations. For the agricultural soils, most of the free metal activities were below detection limit, apart from Zn2+ for which the fraction of free Zn2+ in soluble Zn ranged from 2.3 to 87% (mean 43%). Five of the agricultural soils had detectable free Cd2+ with fractions of free metal ranging from 59 to 102% (mean 75%). For the contaminated soils with detectable free metal concentrations, the fraction of free metal as a percentage of soluble metal varied from 9.9 to 97% (mean 50%) for Zn2+, from 22 to 86% (mean 49%) for Cd2+, from 0.4 to 32.1% (mean 5%) for Cu2+, and from 2.9 to 48.8% (mean 20.1%) for Pb2+. For the contaminated soils, the equilibrium speciation programs GEOCHEM and WHAM Model VI provided reasonable estimates of free Zn2+ fractions in comparison to the measured fractions (R2 approximately 0.7), while estimates of free Cd2+ fractions were less agreeable (R2 approximately 0.5). The models generally predicted stronger binding of Cu2+ to DOC and hence lower fractions of free Cu2+ as compared with the observed fractions. The binding of Cu2+ and Pb2+ to DOC predicted by WHAM Model VI was much strongerthan that predicted by GEOCHEM.