Paul R. Grossl

Rapid Kinetics of Cu(II) Adsorption/Desorption on Goethite.

The kinetics of Cu[sup 2+] adsorption/desorption on goethite ([alpha]-FeOOH) was evaluated using the pressure-jump (p-jump) relaxation technique. This technique provides both kinetic and mechanistic information for reactions occurring on millisecond time scales. A double relaxation event was observed for Cu[sup 2+] adsorption/desorption on goethite. The rate of these relaxations ([tau]) decreased with an increase in pH, along the adsorption edge. The mechanism ascribed to the relaxations is the formation of a monodentate innersphere Cu[sup 2+]/goethite surface complex. The calculated intrinsic rate constant for adsorption (k[sub 1][prime]int) was 10[sup 6.81] L mol[sup [minus]1] s[sup [minus]1] and was about 2 orders of magnitude larger than the intrinsic rate constant for desorption (k[sub 1][prime]int = 10[sup 4.88] L mol[sup [minus]1] s[sup [minus]1]). Using results from this study and others, it was established that the rate of adsorption of divalent metal cations on goethite was directly related to the rate of removal of a water molecule from the primary hydration sphere of a particular divalent metal cation. 30 refs., 7 figs., 1 tab.

Iodate and iodide effects on iodine uptake and partitioning in rice (Oryza sativa L.) grown in solution culture

In the Xinjiang province of western China, conventional methods of iodine (I) supplementation (i.e, goiter pills and iodinated salt) used to mitigate I deficiencies were ineffectual. However, the recent addition of KIO3 to irrigation waters has proven effective. This study was conducted to determine the effects of I form and concentration on rice (Oryza sativa L.) growth, I partitioning within the plant, and ultimately to assist in establishing guidelines for incorporating I into the human food chain. We compared IO3− vs. I− in order to determine how these chemical species differ in their biological effects. Rice was grown in 48 L aerated tubs containing nutrient solution and IO3− or I− at 0, 1, 10, or 100 μM concentrations (approximately 0, 0.1, 1, and 10 mg kg−1 I). The IO3− at 1 and 10 μM had no effect on biomass yields, and the 100 μM treatment had a small negative effect. The I− at 10 and 100 μM was detrimental to biomass yields. The IO3− treatments had more I partitioning to the roots (56%) on average than did the I− treatments (36%), suggesting differences in uptake or translocation between I forms. The data support the theory that IO3− is electrochemically or biologically reduced to I− prior to plant uptake. None of the treatments provided sufficient I in the seed to meet human dietary requirements. The I concentration found in straw at 100 μM IO3− was several times greater than seed, and could provide an indirect source of dietary I via livestock feeding on the straw.

Evaluation of contaminant ion adsorption/desorption on goethite using pressure jump relaxation kinetics

The adsorption/desorption of contaminant ions on soil constituents such as metal oxides occurs within milliseconds. The rapid kinetics of Cu e+ and arsenate adsorption/ desorption on goethite (α-FeOOH) were investigated using the pressure-jump (p-jump) relaxation technique, which provides rate constants and mechanistic information for fast reactions. Results of p-jump experiments at 25°C revealed that both Cu2+ and arsenate were specifically adsorbed on goethite. The divalent copper ion formed an inner-sphere monodentate surface complex, while arsenate formed an inner-sphere bidentate surface complex with goethite. In both cases, the rate constants for the adsorption reactions exceeded those for desorption, indicating that the desorption of Cu2+ and arsenate from the goethite surface was the rate-limiting process. Pressure jump relaxation techniques can be used to predict the adsorption behavior of heavy metals in soil environments.

Elemental allelopathy: processes, progress, and pitfalls

Allelopathic interference between plants has generally been discussed in terms of the production of toxic complex biochemicals; however, complex biochemicals may not be the only substances plants use to interfere with one another. It has also been suggested that inorganic elements may be used in an allelopathic manner. If, through phytoenrichment or root exudates, a plant is able to increase the bioavailable levels of a particular element and tolerate the levels better than its neighbors, it can produce an allelopathic effect. Elemental allelopathy has been implicated as the cause for the success of a number of invasive weeds, including Acroptilon repens, Tamarix spp., Halogeton glomeratus, Salsola iberica, and Mesambryenthemum crystallinum. Phytoenrichment of elements can occur through hyperaccumulation and litter deposition and by altering rhizosphere chemistry. Reported cases of elemental allelopathy have involved three types of elements: heavy metals and soluble salts in terrestrial systems and elemental S in aquatic systems. For the most part, studies that have reported elemental allelopathy have been inconclusive. In order to prevent overreaching conclusions in the study of biochemical allelopathy, criteria were set that can be adapted to the study of elemental allelopathy. Of the studies reviewed, the most common criteria left uninvestigated were whether the plant was actually responsible for changing the concentration of the element and whether the increased levels of an element negatively affected other species. If the study of elemental allelopathy is to avoid the same problems often associated with the study of biochemical allelopathy, these criteria should be included in investigations of elemental allelopathy.

Dissolution of a lunar basalt simulant as affected by pH and organic anions

Abstract The dissolution of a lunar simulant (MLS-1) basalt was examined at 298 K; pH 3, 5, and 7; and in the presence of citrate and oxalate anions. The basalt was mined from an abandoned quarry in Duluth, Minnesota. The relative abundance of minerals in the basalt are plagioclase > pyroxene > olivine > ilmenite. The chemical composition and mineralogy of the basalt most closely resembles Apollo 11 Mare soil. For most of the experiments the order of major ion release (Mg, Ca, Fe, Al, and Si) from the MLS-1 basalt was controlled by the solubility of its mineral components. The amount of major ion release followed the order Fe ≈ Mg > Si > Al > Ca. Deviations in this release order were related to the effect of the treatments on the quantity of MLS-1 basalt dissolved and the precipitation of secondary minerals. For the pH experiments dissolution followed a two-stage process. The first stage was characterized by a rapid rate of release of the major ions into solution, followed by a slower, more linear rate during the second stage. Rate coefficients were calculated from the linear portions of the rate curves and were inversely related to pH. The first stage was attributed to the dissolution of ultrafine particles created during the sample grinding process. During the second stage, dissolution occurred on the larger mineral surfaces, at higher energy sites (i.e. dislocations, twinning planes, fluid inclusions, etc). For the organic anion experiments, dissolution followed a one-stage parabolic process. Dissolution was greater in the presence of the citrate anion compared to the oxalate anion and decreased with a decrease in concentration. It is proposed that the organic anions accelerate the dissolution of the MLS-1 basalt through chemisorption and subsequent disruption of metal-oxygen bonds. The rate-limiting step of the reaction involves the diffusion of the cation-organic complex formed at the mineral surface.

Kinetics of octacalcium phosphate crystal growth in the presence of organic acids

Octacalcium phosphate (OCP) is an important P solid phase in geochemical and biological systems and has been recognized as a precursor phase to the formation of thermodynamically more stable hydroxyapatite (HAP). Metastability of OCP with respect to HAP may be explained by precipitation kinetics and the influence of dissolved organic C (DOC) on crystal growth. Octacalcium phosphate precipitation was measured at pH 6.0 and 25°C in the absence and presence of organic acids commonly found in natural waters and soil solutions using a seeded crystal growth constant composition method. Rate constants for OCP precipitation were calculated from the following expression: Rate = kS(IAP18 − Ksp18)n, where k is the rate constant (L7 mol−6 m−2 s−1), S is OCP seed crystal surface area (m2 L−1), IAP = ion activity product, Ksp = OCP solubility constant (mol8 L−8), and n is the rate reaction order. The rate constant for OCP precipitation in the absence of organic acids was 1034.93·L7 mol−6 m−2 s−1. Humic, fulvic, tannic, and citric acids were added to OCP crystal growth experiments at total soluble (CTS) C levels ranging from 20 μM to 2 mM. Inhibition of OCP precipitation was nearly complete (99% ) in the presence of 1.0 mM CTS as humic acid. At the same level of CTS, OCP precipitation was inhibited by 97,88, and 68% in the presence of fulvic, citric, and tannic acids, respectively. Inhibition of precipitation is caused by adsorption of organic acids onto OCP surfaces blocking active crystal growth sites. The ability of organic acids to inhibit OCP crystal growth is related to their hydrophobicity, functional group content, size, geometry, and orientation on the crystal surface. Precipitation kinetics and crystal growth inhibition by organic acids may explain the metastability of dicalcium phosphate dihydrate (DCPD) and OCP with respect to thermodynamically more stable HAP often observed in geochemical environments.

Dissolution kinetics of a lunar glass simulant at 25 degrees C: the effect of pH and organic acids.

The dissolution kinetics of a simulated lunar glass were examined at pH 3, 5, and 7. Additionally, the pH 7 experiments were conducted in the presence of citric and oxalic acid at concentrations of 2 and 20 mM. The organic acids were buffered at pH 7 to examine the effect of each molecule in their dissociated form. At pH 3, 5, and 7, the dissolution of the synthetic lunar glass was observed to proceed via a two-stage process. The first stage involved the parabolic release of Ca, Mg, Al, and Fe, and the linear release of Si. Dissolution was incongruent, creating a leached layer rich in Si and Ti which was verified by transmission electron microscopy (TEM). During the second stage the release of Ca, Mg, Al, and Fe was linear. A coupled diffusion/surface dissolution model was proposed for dissolution of the simulated lunar glass at pH 3, 5, and 7. During the first stage the initial release of mobile cations (i.e., Ca, Mg, Al, Fe) was limited by diffusion through the surface leached layer of the glass (parabolic release), while Si release was controlled by the hydrolysis of the Si-O-Al bonds at the glass surface (linear release). As dissolution continued, the mobile cations diffused from greater depths within the glass surface. A steady-state was then reached where the diffusion rate across the increased path lengths equalled the Si release rate from the surface. In the presence of the organic acids, the dissolution of the synthetic lunar glass proceeded by a one stage process. The release of Ca, Mg, Al, and Fe followed a parabolic relationship, while the release of Si was linear. The relative reactivity of the organic acids used in the experiments was citrate > oxalate. A thinner leached layer rich in Si/Ti, as compared to the pH experiments, was observed using TEM. Rate data suggest that the chemisorption of the organic anion to the surface silanol groups was responsible for enhanced dissolution in the presence of the organic acids. It is proposed that the increased rate of Si release is responsible for the one stage parabolic release of mobile cations and the relatively thin leached layer compared to experiments at pH 3 and 5.

Iodine toxicity in a plant-solution system with and without humic acid

Aqueous iodine (I2(aq)) is a potent disinfectant that is being evaluated as a soil sanitizer for agricultural fields and a water purification treatment for the International Space Station. Rice (Oryza sativa L.) plants were grown in solution culture containing different I compounds at approximately 0, 18, or 30 μM total I [I2(aq) + iodide (I−)] consisting of 0, 6, and 20 μM I as I2(aq), respectively. In addition, humic acid (HA) was added to half the treatments. Most I2(aq) was electrochemically reduced to the endpoint metabolite I− within 24 h with HA promoting the response. Plants receiving the highest dose of I2(aq), particularly those in treatments without HA, had the least growth and the greatest biomass I concentrations. Roots from both I2(aq) treatments without HA were periodically sampled for bacteria. Viable and direct caints of bacterial cell density declined with increasing I2(aq) concentrations within the first hour after treatment application. However, cell densities recovered within 96 hours and eventually surpassed the control treatment cell density. Additionally, the resulting high viable: direct count density ratio suggests that opportunistic species likely dominated the post I2(aq) environment.

Abiotic transformation of high explosives by freshly precipitated iron minerals in aqueous FeII solutions.

Zerovalent iron barriers have become a viable treatment for field-scale cleanup of various ground water contaminants. While contact with the iron surface is important for contaminant destruction, the interstitial pore water within and near the iron barrier will be laden with aqueous, adsorbed and precipitated Fe(II) phases. These freshly precipitated iron minerals could play an important role in transforming high explosives (HE). Our objective was to determine the transformation of RDX (hexahydro-1,3,5-trinitro-1,3,5-triazine), HMX (octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine), and TNT (2,4,6-trinitrotoluene) by freshly precipitated iron Fe(II)/Fe(III) minerals. This was accomplished by quantifying the effects of initial Fe(II) concentration, pH, and the presence of aquifer solids (Fe(III) phases) on HE transformation rates. Results showed that at pH 8.2, freshly precipitated iron minerals transformed RDX, HMX, and TNT with reaction rates increasing with increasing Fe(II) concentrations. RDX and HMX transformations in these solutions also increased with increasing pH (5.8-8.55). By contrast, TNT transformation was not influenced by pH (6.85-8.55) except at pH values <6.35. Transformations observed via LC/MS included a variety of nitroso products (RDX, HMX) and amino degradation products (TNT). XRD analysis identified green rust and magnetite as the dominant iron solid phases that precipitated from the aqueous Fe(II) during HE treatment under anaerobic conditions. Geochemical modeling also predicted Fe(II) activity would likely be controlled by green rust and magnetite. These results illustrate the important role freshly precipitated Fe(II)/Fe(III) minerals in aqueous Fe(II) solutions play in the transformation of high explosives.

Evaluation of elemental allelopathy in Acroptilon repens (L.) DC. (Russian Knapweed)

Although Acroptilon repens (L.) DC. (Russian knapweed) is known to concentrate zinc (Zn) in upper soil layers, the question of whether the elevated Zn has an allelopathic effect on restoration species has not been addressed. Experiments were conducted to investigate whether soils collected from within infestations of A. repens (high-Zn) inhibit the germination or growth and development of desirable restoration species, compared to soils collected adjacent to an A. repens infestation (low-Zn). Four bioassay species [Sporobolus airoides (Torrey) Torrey (alkali sacaton), Pseudoroegneria spicata (Pursh) A. Love (bluebunch wheatgrass), Psathyrostachys juncea (Fischer) Nevski (Russian wildrye) and A. repens] were germinated in a growth chamber and grown in a greenhouse in both soils and received treatments for the alleviation of Zn toxicity (P, Fe, Fe-oxide, and soil mixing) to isolate the effects of elevated soil Zn on plant performance. Percent germination, total plant biomass, tiller and stem number, inflorescence number, and tissue metal levels were compared among soil types and treatments for each species. There was no evidence from any of the indicators measured that high-Zn soils reduced plant performance, compared to low-Zn soils. Tissue Zn levels barely approached the lower range of phytotoxic levels established for native grasses. Older plants with longer exposure times may accumulate higher Zn concentrations. S. airoides and A. repens both had higher biomass in the high-Zn soil, most likely due to increased macronutrient (N and P) availability. As the Zn levels in the soils used in this study were much higher than any levels previously reported in soils associated with A. repens, it is unlikely that the elevation of soil Zn by A. repens will hinder germination or growth and development of desirable grasses during establishment.