Robert P. Gambrell

Arsenic in wetland vegetation: Availability, phytotoxicity, uptake and effects on plant growth and nutrition

In wetland surface sediments of Louisiana, arsenic (As) concentrations are elevated because of a wide use of inorganic arsenicals as cotton desiccants and of organic arsenicals as herbicides in rice-producing areas. Beside this, As levels are even higher in the region of produced water discharge associated with petroleum hydrocarbon recovery operations. The uptake, potential bioavailability and phytotoxicity of As to an important wetland plant species, growing in the vicinity of produced water discharge sites, were studied. The effects caused by As chemical form and concentration on growth, tissue concentrations and distribution of As and nutrient elements were studied in Spartina alterniflora growing in hydroponic conditions. A 4×4 factorial experiment was conducted with treatments consisting of four As chemical forms [arsenite, As(III); arsenate, As(V); monomethyl arsonic acid, MMAA; and dimethylarsinic acid, DMAA] and four As concentrations (0, 0.2, 0.8 and 2.0 mg As l−1). Arsenic phytoavailability and phytotoxicity were primarily determined by the As chemical form present in the nutrient solution. MMAA was the most phytotoxic species to this marsh grass. Regardless of the chemical form, an As level in the nutrient solution of 0.2 mg l−1 was safe or caused no toxic effects for this marsh grass (it did not reduce plant growth or interfere with plant nutrition). In fact, inorganic arsenicals significantly increased total dry biomass production at an application rate of 0.2 mg l−1. Arsenic availability followed the trend DMAA≪MMAA<As(V)<As(III). Root and shoot As concentrations significantly increased with increasing As application rates (all four species) to the rooting medium. Inorganic arsenicals and MMAA were mainly accumulated in roots, while DMAA was readily translocated to shoots. Arsenic chemical form and concentration significantly affected macro- and micro-nutrient concentrations in plant tissue. Plants treated with As(V) had an improved growth compared to control plants; this seemed to be associated to an increase in plant P concentrations. Organic arsenicals caused the highest Na root concentrations and simultaneously the lowest plant K levels (antagonism K–Na). A significant increase in leaf Ca concentrations was found when organic As species were applied; this could have been due to the protective action of this nutrient to metals and metalloids toxicity. Inorganic arsenicals significantly increased the concentrations of B (root), Cu (root) and Cu (shoot). The high phytotoxicity of the MMAA treatments could have been related to the significant reductions in the concentrations of several essential macronutrients P, K, Ca and Mg and micronutrients B, Cu and Fe.

Chemical availability of mercury, lead, and zinc in Mobile Bay sediment suspensions as affected by pH and oxidation-reduction conditions.

The significance of the organic compounds detected in this indoor environment warrants comment. Actual concentrations of the various substances are relatively small, in part because total levels of suspended particulates are low (between 10 and 14 pg/m3) (6). E x h of the identified organics contributes on the order of O.Olo’0 of this figure. The important features of this study are the spectrum of organics identified and the information this spectrum reveals. Phthalate esters have been observed in numbers and relative abundances that appear greater than those of outdoor samples (7,9); the detected alcohols and phosphates have not been reported in ambient aerosols (10) . A number of these compounds are used as additives rather than entities, and yet they have become significant constituents of an indoor aerosol. The salient point is that stable substances introduced into a confined environment are likely to accumulate in that environment. Materials used in such situations should be chosen accordingly.

Effect of dissolved oxygen on chemical transformations of heavy metals, phosphorus, and nitrogen in an estuarine sediment☆

Abstract The effect of dissolved O2 on the chemical transformations of Fe, Mn, Zn, Cu, Pb, Cd, NH4+ -N and P in an estuarine sediment suspension was investigated under laboratory conditions. Reduced sediment suspensions were constantly purged with a gas mixture containing 0.11%, 2.1% and 21% O2 at a flow rate of 20 ml min−1. Suspension samples were taken at various time intervals and extracted for different chemical fractions. The data indicate that increasing O2 concentrations in the gas mixture resulted in lower sediment pH and higher oxidation-reduction potential (redox potential) with time. Changes in redox potential and pH strongly modified the distribution of trace metals in various chemical fractions. The hydroxylamine hydrochloride extraction failed to selectively remove oxides and hydroxides of Mn without dissolving a substantial proportion of reactive Fe, as the recovery of Fe by this extractant was 8 to 10 orders of magnitude greater than Mn. The large recovery of Fe with Mn in this fraction makes it difficult to distinguish trace metals associated with Mn oxides from those associated with Fe oxides. A substantial fraction of total sediment Zn was associated with potentially bioavailable chemical forms, which may be mobilized to more readily available forms due to changes in the physicochemical properties of the sediment-water system. Significant correlation coefficients obtained between soluble Pb, Cd and Cu and different Fe fractions suggest a strong relationship between these trace metals and Fe compounds. Phosphorus and NH4+ -N concentrations decreased sharply with time as a result of increased redox potential levels in the 2.1% and 21% O2 levels. The effect of the 0.11% O2 treatment was negligible. This indicates that oxidized sediment conditions may be an important factor in regulating eutrophication by reducing the levels of P and N available for biota. This necessitates a careful study of changes in the dissolved O2 concentration (reflected by redox potential levels) in the sediment-water system.

Synthesis, Characterization, and Mechanical Properties of Red Mud-Based Geopolymers

A pilot study investigates the potential of reusing red mud, an abundant industrial waste produced from alumina refining by the Bayer process, by geopolymerization reactions with another solid waste, fly ash, and sodium silicate. Parameters involved in the synthesis, including red mud to fly ash ratio (values of 80/20, 50/50, and 20/80), presence of sand filler, curing duration (up to 28 days), and sodium silicate solution to solid mixture (consisting of red mud and fly ash) ratio, were examined to understand the extent and degree of geopolymerization. Unconfined compression testing was employed to assess the influence of these synthesis parameters on the mechanical properties of the end products, red mud–based geopolymers. The composition and microstructure were characterized by X-ray diffraction and scanning electron microscopy, respectively, which confirm the geopolymerization reactions. The mechanical properties, including strength, stiffness, and failure strain, were analyzed against the chemical compositions of the red mud geopolymers, such as Si/Al and Na/Si molar ratios. For the studied geopolymers, the unconfined compressive strength, ranging from 7 to 13 MPa, increases with the Si/Al ratio as in some types of portland cement. A higher Na/Si ratio appears to reduce the strength and stiffness but enhance the ductility. The results indicate that red mud geopolymers are a viable cementitious material that can be used in roadway construction. The engineering implications are discussed in terms of waste recycling, environmental benefits, and energy consumption.

Toxic chemicals and trace metals from urban and rural louisiana lakes: recent historical profiles and toxicological significance.

Sediment cores collected from lakes in rural and urban/industrial areas of Louisiana were dated using 137 Cs, sectioned, and analyzed for a wide range of pollutant chemicals deposited during the period 1950-1991. Mutagenicity testing also was performed on extracts from the core sections. Statistical and other comparisons of chemical data indicated that annual loadings of polycyclic aromatic hydrocarbons, chlorinated chemicals, and trace metals were not significantly different in the urban lake vs the rural lake over the historical period compared. Mutagenic activity was detected in both lakes, primarily in sediments deposited between 1955 and 1980, with minimal activity before and after that period. This was a time of widespread industrial and agricultural activity in Louisiana, before restrictions on chemical releases to the environment were instituted and enforced.

Factors affecting the adsorption of 2,4-D and methyl parathion in soils and sediments

Abstract The adsorption of methyl parathion and 2,4-dichlorophenoxyacetic acid (2,4-D) by 19 soil and sediment materials differing widely in their physical and chemical properties was investigated using a batch equilibration technique. Organic matter was the most important single factor affecting adsorption of methyl parathion and 2,4-D. Soil pH and cation exchange capacity were also reasonably well associated with 2,4-D adsorption whereas cation exchange capacity contributed significantly for methyl parathion. Data are presented which indicate that in soil and sediment materials where the organic matter content in less than 1 %, oxalate extractable Mn and Ca were associated with the adsorption of these synthetic organics. Additional work should be done in low organic matter soil materials to better quantify the relationship between adsorption and soil geochemical properties.

Fate of petroleum hydrocarbons and toxic organics in Louisiana coastal environments

Numerous potentially toxic compounds are entering Louisiana’s inshore and nearshore coastal environments. To a large degree there is insufficient information for predicting the fate and effect of these materials in aquatic environments. Studies documenting the impact of petroleum hydrocarbons entering Louisiana coastal wetlands are summarized. Also included are research findings on factors affecting the persistence of petroleum hydrocarbons and other toxic organics (pentachlorophenol (PCP), 2,4-dichlorophenoxyacetic acid (2,4-D), creosote, etc.) in sediment-water systems. Sediment pH and redox conditions have been found to play an important role in the microbial degradation of toxic organics. Most of the hydrocarbons investigated degrade more rapidly under high redox (aerobic) conditions although there are exceptions (e.g., 1,1,1-trichloro-2,2-bis(4-chlorophenyl) (DDT) and polychlorobiphenyls (PCBs)). Some of these compounds, due to their slow degradation in anaerobic sediment, may persist in the system for decades.

Effects of ferric iron reduction and regeneration on nitrous oxide and methane emissions in a rice soil

A laboratory soil slurry experiment and an outdoor pot experiment were conducted to study effects of ferric iron (Fe(III)) reduction and regeneration on nitrous oxide (N(2)O) and methane (CH(4)) emissions in a rice (Oryza sativa L.) soil. The anoxic slurry experiment showed that enhancing microbial Fe(III) reduction by ferrihydrite amendment (40 mol Fe g(-1)) transitionally stimulated N(2)O production and lowered CH(4) production by 16% during an initial 33-day incubation. Increased regeneration of Fe(III) through a 4-day aeration period in the Fe-amended slurry compared to the control slurry reduced CH(4) emission by 30% in the subsequent 15-day anaerobic incubation. The pot experiment showed that ferrihydrite amendment (63 micromol Fe g(-1)) stimulated N(2)O fluxes in the days following flooding. The Fe amendment suppression on CH(4) emission was obscured in the early season but became significant upon reflooding in the mid- and late-seasons. As a result, seasonal CH(4) emission in Fe-amended pots was 26% lower than the control with a single 2-day drainage and 69% lower with a double 2-day drainage. The reduction in CH(4) emission upon reflooding from the Fe-amended pots was mainly attributed to the increased Fe(III) regeneration during drainage showing a mechanism of Fe(III) regeneration in mitigating CH(4) emission by short-term drainage in flooded soils.

Effect of Exopolymers on the Liquid Limit of Clays and Its Engineering Implications

An experimental study aimed at understanding the interactions between exopolymers exuded by microorganisms and clays, particularly kaolinite, is described. Two biopolymers–-xanthan gum, an anionic bacterial extracellular polysaccharide, and guar gum, a neutrally charged plant polysaccharide–-were used as exopolymer analogs. Liquid limits of a kaolinite clay were measured, with varied biopolymer concentrations and background cations (Ca2+, Na+, or K+) in the pore fluid, to study the influence of exopolymers on clay behavior. Results indicate that the liquid limit of kaolinite generally increases with the biopolymer concentration of the pore fluid as a result of increased viscosity, and the background cation present in the pore fluid alters the liquid limit. Five types of active nanoscale interactions between clay particles, cations, and biopolymers were adopted to interpret the results: biopolymer-induced aggregation of clay particles tends to decrease the liquid limit; polymer cross-linking caused by divalent cations significantly increases the biopolymer solution viscosity and hence the liquid limit; the formation of a clay–polymer interconnected network via cation bridging and hydrogen bonds increases the liquid limit; a reduction in the thickness of the electric double layer on the clay surface decreases the liquid limit; and preferred adsorption of monovalent cations over biopolymer molecules decreases the liquid limit. The variations in the liquid limit reflect the macroscopic response of these interactions competing with each other at the nanoscale. Engineering implications also are discussed on the basis of the experimental results and the clay–biopolymer interactions.

The effects of high levels of polycyclic aromatic hydrocarbons on sediment physicochemical properties and benthic organisms in a polluted stream

Abstract The effects of polycyclic aromatic hydrocarbons on benthic micro- and meiofaunal biomass was determined and related to selected sediment properties such as redox potential and organic matter content in a contaminated environment.