Comparison of arsenic(V) and arsenic(III) sorption onto iron oxide minerals: implications for arsenic mobility

Arsenic derived from natural sources occurs in groundwater in many countries, affecting the health of millions of people. The combined effects of As(V) reduction and diagenesis of iron oxide minerals on arsenic mobility are investigated in this study by comparing As(V) and As(III) sorption onto amorphous iron oxide (HFO), goethite, and magnetite at varying solution compositions. Experimental data are modeled with a diffuse double layer surface complexation model, and the extracted model parameters are used to examine the consistency of our results with those previously reported. Sorption of As(V) onto HFO and goethite is more favorable than that of As(III) below pH 5-6, whereas, above pH 7-8, As(II) has a higher affinity for the solids. The pH at which As(V) and As(III) are equally sorbed depends on the solid-to-solution ratio and type and specific surface area of the minerals and is shifted to lower pH values in the presence of phosphate, which competes for sorption sites. The sorption data indicate that, under most of the chemical conditions investigated in this study, reduction of As(V) in the presence of HFO or goethite would have only minor effects on or even decrease its mobility in the environment at near-neutral pH conditions. As(V) and As(III) sorption isotherms indicate similar surface site densities on the three oxides. Intrinsic surface complexation constants for As(V) are higher for goethite than HFO, whereas As(III) binding is similar for both of these oxides and also for magnetite. However, decrease in specific surface area and hence sorption site density that accompanies transformation of amorphous iron oxides to more crystalline phases could increase arsenic mobility.     

Mechanisms of selenate adsorption on iron oxides and hydroxides

Selenate (SeO4(2-)) is an oxyanion of environmental importance because of its toxicity to animals and its mobility in the soil environment. It is known that iron(III) oxides and hydroxides are important sorbents for SeO4(2-) in soils and sediments, but the mechanism of selenate adsorption on iron oxides has been the subject of intense debate. Our research employed Extended X-ray absorption fine structure and attenuated total reflectance-Fourier transform infrared spectroscopies to determine SeO4(2-) bonding mechanisms on hematite, goethite, and hydrous ferric oxide (HFO). It was learned that selenate forms only inner-sphere surface complexes on hematite but forms a mixture of outer- and inner-sphere surface complexes on goethite and HFO. This continuum of adsorption mechanisms is strongly affected by both pH and ionic strength. These results suggest that adsorption experiments should be conducted on several different iron oxides and over a wide range of reaction conditions to accurately assess the reactivity of oxyanions on iron oxides.     

Sorption of ciprofloxacin and oxytetracycline zwitterions to soils and soil minerals: influence of compound structure

Oxytetracycline (OTC) zwitterions sorbed to a greater extent than ciprofloxacin (CIP) zwitterions onto goethite and soils with moderate-to-low effective cation exchange capacities (ECEC < 10 cmol(c)/kg) because adjacent pairs of hydroxyl groups on the OTC molecule (absent in CIP) facilitated greater surface complexation to soil metal oxides and aluminosilicate edge sites. CIP sorbed to a higher extentthan OTC onto aluminosilicates and onto soils with "high" ECEC values (>10 cmol(c)/kg). The sorption of heterocyclic compounds structurally similar to CIP indicated that both positive charge localization on the cationic amine and the extent of charge delocalization to the heterocyclic ring influenced molecular orientation within the montmorillonite interlayers, van der Waals interactions, and the potential for sorption. The sorption of compounds structurally similar to OTC revealed that greater positive charge localization on the cationic amine facilitated sorption to montmorillonite, whereas ortho substituted anionic and cationic groups on a zwitterionic molecule resulted in unfavorable Coulombic interactions between the anionic moiety and the negatively charged surface and hindered sorption. Thus, greater CIP zwitterion sorption to aluminosilicates and "high" ECEC soils resulted from greater distance between the anionic and cationic groups, which maximized Coulombic attraction to the surface.     

Effect of aqueous Fe(II) on arsenate sorption on goethite and hematite

Biogeochemical iron cycling often generates systems where aqueous Fe(II) and solid Fe(III) oxides coexist. Reactions between these species result in iron oxide surface and phase transformations, iron isotope fractionation, and redox transformations of many contaminant species. Fe(II)-induced recrystallization of goethite and hematite has recently been shown to cause the repartitioning of Ni(II) at the mineral-water interface, with adsorbed Ni incorporating into the iron oxide structure and preincorporated Ni released back into aqueous solution. However, the effect of Fe(II) on the fate and speciation of redox inactive species incompatible with iron oxide structures is unclear. Arsenate sorption to hematite and goethite in the presence of aqueous Fe(II) was studied to determine whether Fe(II) causes substantial changes in the sorption mechanisms of such incompatible species. Sorption isotherms reveal that Fe(II) minimally alters macroscopic arsenate sorption behavior except at circumneutral pH in the presence of elevated concentrations (10?3 M) of Fe(II) and at high arsenate loadings, where a clear signature of precipitation is observed. Powder X-ray diffraction demonstrates that the ferrous arsenate mineral symplesite precipitates under such conditions. Extended X-ray absorption fine structure spectroscopy shows that outside this precipitation regime arsenate surface complexation mechanisms are unaffected by Fe(II). In addition, arsenate was found to suppress Fe(II) sorption through competitive adsorption processes before the onset of symplesite precipitation. This study demonstrates that the sorption of species incompatible with iron oxide structure is not substantially affected by Fe(II) but that such species may potentially interfere with Fe(II)-iron oxide reactions via competitive adsorption.     

A model-based evaluation of sorptive reactivities of hydrous ferric oxide and hematite for U(VI)

The sorption of uranyl onto hydrous ferric oxide (HFO) or hematite was measured by discontinuously titrating the suspensions with uranyl at pH 5.9, 6.8, and 7.8 under Pco2 = 10(-35)atm (sorption isotherms). Batch reactors were used with equilibration times up to 48 days. Sorption of 1 microM uranyl onto HFO was also measured versus pH (sorption edge). A diffuse double layer surface complexation model was calibrated by invoking three sorption species that were consistent with spectroscopic evidence for predominance of bidentate complexes at neutral pH and uranyl-carbonato complexes: > SOH:UO2OH(+1), (> SO)2: UO2CO3(-2), and (> SO)2:(UO2)3(OH)5(-1). The model was consistent with previously published isotherm and edge data. The model successfully predicted sorption data onto hematite, only adjusting for different measured specific surface area. Success in application of the model to hematite indicates that the hydrated surface of hematite has similar sorptive reactivity as HFO.     

Iron oxide surface-catalyzed oxidation of ferrous iron by monochloramine: implications of oxide type and carbonate on reactivity

The maintenance of monochloramine residuals in drinking water distribution systems is one technique often used to minimize microbial outbreaks and thereby maintain the safety of the water. Reactions between oxidizable species and monochloramine can however lead to undesirable losses in the disinfectant residual. Previous work has illustrated that the Fe(II) present within distribution systems is one type of oxidizable species that can exert a monochloramine demand. This paper extends this prior work by examining the kinetics of the reactions between Fe(II) and monochloramine in the presence of a variety of iron oxide surfaces. The identity of the iron oxide plays a significant role in the rate of these reactions. Surface area-normalized initial rate coefficients (k(init)) obtained in the presence of each oxide at pH approximately 6.9 exhibit the following trend in catalytic activity: magnetite > goethite > hematite approximately = lepidocrocite > ferrihydrite. The differences in the activity of these oxides are hypothesized to result from variations in the amount of Fe(II) sorbed to each of the oxides and to dissimilarities in the surface site densities of the oxides. The implications of carbonate on Fe(II) sorption to iron oxides are also examined. Comparing Fe(II) sorption isotherms for goethite obtained under differential carbonate concentrations, it is apparent that as the carbonate concentration (C(T,CO3)) increased from 0 to 11.7 mM that the Fe(II) sorption edge (50% sorption) shifts from a pH of approximately 5.8 to a pH of 7.8. This shift is hypothesized to be the result of the formation of aqueous and surface carbonate-Fe(II) complexes and to competition between carbonate and Fe(II) for surface sites. The implications of these changes are then discussed in light of the variable oxide studies.     

Modeling tetracycline antibiotic sorption to clays

Sorption interactions of three high-use tetracycline antibiotics (oxytetracycline, chlortetracycline, tetracycline) with montmorillonite and kaolinite clays were investigated undervaried pH and ionic strength conditions. Sorption edges were best described with a model that included cation exchange plus surface complexation of zwitterion forms of these compounds. Zwitterion sorption was accompanied by proton uptake, was more favorable on acidic clay, and was relatively insensitive to ionic strength effects. Calcium salts promoted oxytetracycline sorption at alkaline pHs likely by a surface-bridging mechanism. Substituent effects among the compounds in the tetracycline class had only minor effects on sorption edges and isotherms under the same solution pH and ionic strength conditions. At low ionic strength, greater sorption to montmorillonite than kaolinite was observed at all pHs tested, even after normalizing for cation exchange capacity. These results indicate that soil and sediment sorption models for tetracyclines, and other pharmaceuticals with similar chemistry, must account for solution speciation and the presence of other competitor ions in soil or sediment pore waters.     

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.     

Surface complexation of ferrous iron and carbonate on ferrihydrite and the mobilization of arsenic

Surface complexation models are commonly used to predict the mobility of trace metals in aquifers. For arsenic in groundwater, surface complexation models cannot be used because the database is incomplete. Both carbonate and ferrous iron are often present at a high concentration in groundwater and will influence the sorption of arsenic, but the surface complexation constants are absent in the database of Dzombak and Morel. This paper presents the surface complexation constants for carbonate and ferrous iron on ferrihydrite as derived for the double-layer model. For ferrous iron the constants were obtained from published data supplemented by new experiments to determine the sorption on the strong sites of ferrihydrite. For carbonate the constants were derived from experiments by Zachara et al., who employed relatively low concentrations of carbonate. The double-layer model, optimized for low concentrations, was tested against sorption experiments of carbonate on goethite at higher concentration by Villalobos and Leckie, and reasonable agreement was found. Sorption was also estimated using linear free energy relations (LFER), and results compared well with our derived constants. Model calculations confirm that sorption of particularly carbonate at common soil and groundwater concentrations reduces the sorption capacity of arsenic on ferrihydrite significantly. The displacing effect of carbonate on sorbed arsenate and arsenite has been overlooked in many studies. It may be an important cause for the high concentrations of arsenic in groundwater in Bangladesh. Sediments containing high amounts of sorbed arsenic are deposited in surface water with low carbonate concentrations. Subsequently the sediments become exposed to groundwater with a high dissolved carbonate content, and arsenic is mobilized by displacement from the sediment surface.     

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.     

Sorption of Sb(III) and Sb(V) to goethite: influence on Sb(III) oxidation and mobilization

Antimony is an element of growing interest for a variety of industrial applications, even though Sb compounds are classified as priority pollutants by the Environmental Protection Agency of the United States. Iron (Fe) hydroxides appear to be important sorbents for Sb in soils and sediments, but mineral surfaces can also catalyze oxidation processes and may thus mobilize Sb. The aim of this study was to investigate whether goethite immobilizes Sb by sorption or whether Sb(III) adsorbed on goethite is oxidized and then released. The sorption of both Sb(III) and Sb(V) on goethite was studied in 0.01 and 0.1 M KClO4 M solutions as a function of pH and Sb concentration. To monitor oxidation processes Sb species were measured in solution and in the solid phase. The results show that both Sb(III) and Sb(V) form inner-sphere surface complexes at the goethite surface. Antimony(III) strongly adsorbs on goethite over a wide pH range (3-12), whereas maximum Sb(V) adsorption is found below pH 7. At higher ionic strength, the desorption of Sb(V) is shifted to lower pH values, most likely due to the formation of ion pairs KSb(OH)6 degrees. The sorption data of Sb(V) can be fitted by the modified triple-layer surface complexation model. Within 7 days, Sb(III) adsorbed on goethite is partly oxidized at pH 3, 5.9 and 9.7. The weak pH-dependence of the rate coefficients suggests that adsorbed Sb(III) is oxidized by 02 and that the coordination of Sb(III) to the surface increases the electron density of the Sb atom, which enhances the oxidation process. At pH values below pH 7, the oxidation of Sb(III) did not mobilize Sb within 35 days, while 30% of adsorbed Sb(III) was released into the solution at pH 9.9 within the same time. The adsorption of Sb(III) on Fe hydroxides over a wide pH range may be a major pathway for the oxidation and release of Sb(V).     

Impacts of Shewanella putrefaciens strain CN-32 cells and extracellular polymeric substances on the sorption of As(V) and As(III) on Fe(III)-(hydr)oxides

We investigated the effects of Shewanella putrefaciens cells and extracellular polymeric substances on the sorption of As(III) and As(V) to goethite, ferrihydrite, and hematite at pH 7.0. Adsorption of As(III) and As(V) at solution concentrations between 0.001 and 20 μM decreased by 10 to 45% in the presence of 0.3 g L(-1) EPS, with As(III) being affected more strongly than As(V). Also, inactivated Shewanella cells induced desorption of As(V) from the Fe(III)-(hydr)oxide mineral surfaces. ATR-FTIR studies of ternary As(V)-Shewanella-hematite systems indicated As(V) desorption concurrent with attachment of bacterial cells at the hematite surface, and showed evidence of inner-sphere coordination of bacterial phosphate and carboxylate groups at hematite surface sites. Competition between As(V) and bacterial phosphate and carboxylate groups for Fe(III)-(oxyhydr)oxide surface sites is proposed as an important factor leading to increased solubility of As(V). The results from this study have implications for the solubility of As(V) in the soil rhizosphere and in geochemical systems undergoing microbially mediated reduction and indicate that the presence of sorbed oxyanions may affect Fe-reduction and biofilm development at mineral surfaces.     

Kinetics of oxytetracycline reaction with a hydrous manganese oxide

Tetracycline antibiotics comprise a class of broad spectrum antimicrobial agents finding application in human therapy, animal husbandry, aquaculture, and fruit crop production. To better understand the processes affecting these antibiotics in soils and sediments, the kinetics of oxytetracycline transformation by a hydrous manganese oxide (MnO2) were investigated as a function of reactant concentration, pH, and temperature. Oxytetracycline was rapidly degraded by MnO2. Initial reaction rates exhibited pronounced pH-dependence, increasing as pH decreased. Reaction of oxytetracycline with MnO2 was accompanied by generation of Mn(II) ions, suggesting oxidative transformation of the antibiotic. At pH 5.6, apparent reaction orders for oxytetracycline and MnO2 were 0.7 and 0.8. Reaction order with respect to H+ was 0.6 between pH 4 and 9. Initial reaction rates increased by a factor of approximately 2.4 for 10 degrees C temperature increases; the apparent activation energy (60 kJ x mol(-1)) was consistent with a surface-controlled reaction. Reactivity of tetracycline antibiotics toward MnO2 increased in the following order: rolitetracyline oxytetracycline < or =tetracycline approximately meclocycline < chlortetracycline. The initial rate of chlortetracycline degradation by MnO2 was substantially larger than that of the other tetracycline antibiotics investigated. MnO2 reactivity toward oxytetracycline decreased with time; a retarded rate equation was used to describe oxytetracycline reaction with MnO2 under declining rate conditions. This study indicates that natural manganese oxides in soils and sediments are likely to promote appreciable degradation of tetracycline antibiotics, and that reaction rates are strongly dependent on reaction time scale and solution conditions.     

Sorption of Am(III) onto 6-line-ferrihydrite and its alteration products: investigations by EXAFS

For the long-term performance assessment of nuclear waste repositories, knowledge about the interactions of actinide ions with mineral surfaces such as iron oxides is imperative. The mobility of released radionuclides is strongly dependent on the sorption/desorption processes at these surfaces and on their incorporation into the mineral structure. In this study the interaction of Am(III) with 6-line-ferrihydrite (6LFh) was investigated by EXAFS spectroscopy. At low pH values (pH 5.5), as well at higher pH values (pH 8.0), Am(III) sorbs as a bidentate corner-sharing species onto the surface. Investigations of the interaction of Am(III) with Fh coated silica colloids prove the sorption onto the iron coating and not onto the silica substrate. Hence, the presence of Fh, even as sediment coating, is the dominant sorption surface. Upon heating, Fh is transformed into goethite and hematite as shown by TEM and IR measurements. The results of the fit to the EXAFS data indicate the release of sorbed Am(III) at pH 5.5 during the transformation and likely a partial incorporation of Am into the Fh transformation products at pH 8.0.     

Kinetics of reductive dissolution of hematite by bioreduced anthraquinone-2,6-disulfonate

The reductive dissolution of hematite (alpha-Fe2O3) was investigated in a flow-through system using AH2DS, a reduced form of anthraquinone-2,6-disulfonate (AQDS), which is often used as a model electron shuttling compound in studies of dissimilatory microbial reduction of iron oxides. Influent flow rate, pH, and Fe(II) and phosphate concentrations were varied to investigate the redox kinetics in a flow-through reactor. The hematite reduction rates decreased with increasing pH from 4.5 to 7.6 and decreased with decreasing flow rate. The rates also decreased with increasing influent concentration of Fe(II) or phosphate that formed surface complexes at the experimental pH. Mineral surface properties, Fe(II) complexation reactions, and ADDS sorption on hematite surfaces were independently investigated for interpreting hematite reduction kinetics. AH2DS sorption to hematite was inferred from the parallel measurements of AQDS and AH2DS sorption to alpha-Al2O3, a redox stable analog of alpha-Fe2O3. Decreasing Fe(ll) and increasing AH2DS sorption by controlling flow rate, influent pH, and Fe(II) and phosphate concentrations increased the rates of reductive dissolution. The rates were also affected by the redox reaction free energy when reductive dissolution approached equilibrium. This study demonstrated the importance of the geochemical variables for the reductive dissolution kinetics of iron oxides.     

Coprecipitation of arsenate with metal oxides. 2. Nature, mineralogy, and reactivity of iron(III) precipitates

Coprecipitation of arsenic with iron or aluminum occurs in natural environments and is a remediation technology used to remove this toxic metalloid from drinking water and hydrometallurgical solutions. In this work, we studied the nature, mineralogy, and reactivity toward phosphate of iron-arsenate coprecipitates formed at As(V)/Fe(III) molar ratios (R) of 0, 0.01, or 0.1 and at pH 4.0, 7.0, and 10.0 aged for 30 or 210 days at 50 degrees C and studied the desorption of arsenate. At R = 0, goethite and hematite (with ferrihydrite at pH 4.0 and 7.0) crystallized, whereas at R = 0.01, the formation of ferrihydrite increased and hematite crystallization was favored over goethite. In some samples, the morphology of hematite changed from rounded platy crystals to ellipsoids. At R = 0.1, ferrihydrite formed in all the coprecipitates and remained unchanged even after 210 days of aging. The surface area and chemical composition of the precipitates were affected by pH, R, and aging. Chemical dissolution of the samples showed that arsenate was present mainly in ferrihydrite, but at R = 0.01, it was partially incorporated into the structures of crystalline Fe oxides. The sorption of phosphate on to the coprecipitates was affected not only by the mineralogy and surface area of the samples but also by the amounts of arsenate present in the oxides. The samples formed at pH 4.0 and 7.0 and at R = 0.1 sorbed lower amounts of phosphate than the precipitates obtained at R = 0 or 0.01, despite the former having a larger surface area and showing only a presence of short-range ordered materials. This is mainly due to the fact that in the coprecipitates at R = 0.1 arsenate occupied many sorption sites, thus preventing phosphate sorption. Less than 20% of the arsenate present in the coprecipitates formed at R = 0.1 was removed by phosphate and more from the samples synthesized at pH 7.0 or 10.0 than at pH 4.0. Moreover, we found that more arsenate was desorbed by phosphate from a ferrihydrite on which arsenate was added than from an iron-arsenate coprecipitate, attributed to the partial occlusion of some arsenate anions into the framework of the coprecipitate. XPS analyses confirmed these findings.     

Methyl arsenic adsorption and desorption behavior on iron oxides

In virtually all Earth surface environments, methylated forms of arsenic can be found. Because of the widespread distribution and toxicity of methyl-arsenic compounds, their adsorption by soil minerals is of considerable interest. The objective of this study was to compare the adsorption and desorption behavior of methylarsonic acid (MMAsV), methylarsonous acid (MMAsIII), dimethylarsinic acid (DMAsV), dimethylarsinous acid (DMAsIII), arsenate (iAsV), and arsenite (iAsIII) on iron oxide minerals (goethite and 2-line ferrihydrite) by means of adsorption isotherms, adsorption envelopes, and desorption envelopes (using sulfate and phosphate as desorbing ligands). MMAsIII and DMAsIII were not appreciably retained by goethite or ferrihydrite within the pH range of 3 to 11, while iAsIII was strongly adsorbed to both iron oxides. MMAsV and iAsV were adsorbed in higher amounts than DMAsV on goethite and ferrihydrite at all pH values studied. MMAsV and iAsV exhibited high adsorption affinities on both goethite and ferrihydrite from pH 3 to 10, while DMAsV was adsorbed only at pH values below 8 by ferrihydrite and below 7 by goethite. All arsenic compounds were desorbed more efficiently by phosphate than sulfate. MMAsV, iAsV, and DMAsV each exhibited adsorption characteristics suggesting specific adsorption on both goethite and ferrihydrite. Increased methyl substitution resulted in both decreased adsorbed arsenic at low arsenic concentrations in solution and increased ease of arsenic release from the iron oxide surface.     

Influence of calcium carbonate on U(VI) sorption to soils

The high stability of calcium uranyl carbonate complexes in the circumneutral pH range has a strong impact on U(VI) sorption in calcareous soils. To quantify this influence, sorption of U(VI) to soils in the presence of naturally occurring calcium carbonate was investigated by conducting batch experiments in which either U(VI) concentration or solution pH was varied. Two soils containing different calcium carbonate concentrations were selected, one from Oak Ridge, TN, and another from Altamont Pass, CA. The results show that the presence of calcium carbonate in soils strongly affects U(VI) sorption. Higher concentrations of soil calcium carbonate lead to a pronounced suppression of the pH-dependent sorption curve in the neutral pH range because of the formation of a very stable neutral complex of calcium uranyl carbonate in solution. A surface complexation model considering both strong and weak sites for ferrihydrite and ionizable hydroxyl sites for clay minerals was compared with experimental results, and U(VI) binding parameters were reasonably estimated. Fair agreement was found between the model predictions and sorption data, which span a wide range of U(VI) concentrations and pH. The results also show that appropriate solution-to-solid ratios need to be used when measuring distribution coefficients in calcareous soils to avoid complete CaCO3 dissolution and consequent dilution of calcium uranyl carbonate complexes.     

Effects of surface sorption on microbial degradation of glyphosate

Sorption may affect the bioavailability and biodegradation of pesticides in soils. The aim of this study was to test the effect of surface sorption on microbial utilization of the herbicide glyphosate as a source of phosphorus, nitrogen, or carbon. We added goethite to a humus soil to manipulate the soil's glyphosate sorption capacity. The addition of glyphosate generally either decreased microbial CO2 production or produced no effect. Additions of glyphosate, in combination with glucose and N, did not change the respiration rate in comparison with the same treatment but without glyphosate. In contrast, glyphosate additions combined with glucose and P decreased microbial growth, whereas the combination with goethite counteracted the negative effect. The different treatments were examined using attenuated total reflectance Fourier transform (ATR-FTIR) spectroscopy; the results suggest that glyphosate was de-carboxylated in the sorbed state. Stimulating microbial growth by the addition of glucose and nitrogen resulted in further oxidation of glyphosate and only phosphate was detectable on the goethite surface after 13 days incubation. Our results show that sorbed glyphosate is microbially degradable, and it retards microbial activity. This study emphasizes the importance of combining quantitative measurements with a molecular-level examination, to better understand biogeochemical processes.