Anna Sophia Knox

Sequestering Agents for Active Caps—Remediation of Metals and Organics

This research evaluated organoclays, zeolites, phosphates, and a biopolymer as sequestering agents for inorganic and organic contaminants. Batch experiments were conducted to identify amendments and mixtures of amendments for metal and organic contaminant removal and retention. Contaminant removal was evaluated by calculating partitioning coefficients. Metal retention was evaluated by desorption studies in which residue from the removal studies was extracted with 1 M MgCl2 solution. The results indicated that phosphate amendments, some organoclays, and the biopolymer, chitosan, were very effective sequestering agents for metals in fresh and salt water. Organoclays were very effective sorbents for phenanthrene, pyrene, and benzo(a)pyrene. Partitioning coefficients for the organoclays were 3000–3500 L g−1 for benzo(a)pyrene, 400–450 L g−1 for pyrene, and 50–70 L g−1 for phenanthrene. Remediation of sites with a mixture of contaminants is more difficult than sites with a single contaminant because metals and organic contaminants have different fate and transport mechanisms in sediment and water. Mixtures of amendments (e.g., organoclay and rock phosphate) have high potential for remediating both organic and inorganic contaminants under a broad range of environmental conditions, and have promise as components in active caps for sediment remediation.

Amendments for the in situ remediation of contaminated sediments: evaluation of potential environmental impacts.

Active sediment caps represent a comparatively new technology for remediating contaminated sediments. They are made by applying chemically active amendments that reduce contaminant mobility and bioavailability to the sediment surface. The objective of this study was to determine if active cap amendments including organoclay, apatite, and biopolymers have the potential to harm benthic organisms. Methods included laboratory bioassays of amendment toxicity and field evaluations of amendment impacts on organisms held in cages placed within pilot-scale active caps located in Steel Creek, a South Carolina (USA) stream. Test organisms included Hyalella azteca, Leptocheirus plumulosus, Lumbriculus variegatus, and Corbicula fluminea to represent a range of feeding modes, burrowing behaviors, and both fresh and saltwater organisms. In addition to the laboratory and field assays, chemical extractions were performed to determine if the amendments contained harmful impurities that could leach into the ambient environment. Laboratory bioassays indicated that 100% apatite had minimal effects on Hyalella in freshwater and up to 25% organoclay was nontoxic to Leptocheirus in brackish water. Field evaluations indicated that pilot-scale caps composed of up to 50% apatite and 25% organoclay did not harm Hyalella, Lumbriculus, or Corbicula. In contrast, organisms in caps containing biopolymers died because of physical entrapment and/or suffocation by the viscous biopolymers. The extractions showed that the amendments did not release harmful concentrations of metals. These studies indicated that apatite and organoclay are nontoxic at concentrations (up to 50% and 25% by weight, respectively) needed for the construction of active caps that are useful for the remediation of metals and organic contaminants in sediments.

Interactions among phosphate amendments, microbes and uranium mobility in contaminated sediments

The use of sequestering agents for the transformation of radionuclides in low concentrations in contaminated soils/sediments offers considerable potential for environmental cleanup. This study evaluated the influence of three types of phosphate (rock phosphate, biological phosphate, and calcium phytate) and two microbial amendments (Alcaligenes piechaudii and Pseudomonas putida) on U mobility. All tested phosphate amendments reduced aqueous U concentrations more than 90%, likely due to formation of insoluble phosphate precipitates. The addition of A. piechaudii and P. putida alone were found to reduce U concentrations 63% and 31%, respectively. Uranium removal in phosphate treatments was significantly reduced in the presence of the two microbes. Two sediments were evaluated in experiments on the effects of phosphate amendments on U mobility, one from a stream on the Department of Energys Savannah River Site near Aiken, SC and the other from the Hanford Site, a Department of Energy facility in Washington state. Increased microbial activity in the treated sediment led to a reduction in phosphate effectiveness. The average U concentration in 1 M MgCl(2) extract from U contaminated sediment was 437 microg/kg, but in the same sediment without microbes (autoclaved), the extractable U concentration was only 103 microg/kg. The U concentration in the 1 M MgCl(2) extract was approximately 0 microg/kg in autoclaved amended sediment treated with autoclaved biological apatite. These results suggest that microbes may reduce phosphate amendment remedial effectiveness.

IN SITU URANIUM STABILIZATION BY MICROBIAL METABOLITES

Microbial melanin production by autochthonous bacteria was explored in this study as a means to increase U immobilization in U contaminated soil. This article demonstrates the application of bacterial physiology and soil ecology for enhanced U immobilization in order to develop an in situ, U bio-immobilization technology. We have demonstrated microbial production of a metal chelating biopolymer, pyomelanin, in U contaminated soil from the Tims Branch area of the Department of Energy (DOE), Savannah River Site (SRS), South Carolina, as a result of tyrosine amendments. Bacterial densities of pyomelanin producers were >10(6) cells per g wet soil. Pyomelanin demonstrated U complexing and mineral binding capacities at pH 4 and 7. In laboratory studies, in the presence of goethite or illite, pyomelanin enhanced U sequestration by these minerals. Tyrosine amended soils in a field test demonstrated increased U sequestration capacity following pyomelanin production up to 13 months after tyrosine treatments.

Long-Term Performance of a Constructed Wetland for Metal Removal

Constructed wetlands have the ability to economically remove pollutants from water and retain them in sediment. This paper describes the long-term performance of a constructed wetland for metal removal, including the efficiency of metal removal, and the retention of metals in the wetland sediment. It is based on four years of data collected from the A-01 wetland treatment system, a surface flow wetland planted with Schoenoplectus californicus (giant bulrush). The system is designed to remove Cu and other metals from the A-01 National Pollution Discharge Elimination System (NPDES) outfall effluent at the Savannah River Site near Aiken, SC. Copper, Zn, and Pb concentrations in water were usually reduced 60 to 80% during passage through the A-01 treatment system. Most of the metal removed by the wetland cells was accumulated in the two top layers of the substrate; i.e., the floc and organic layers. This gradient was strongly correlated with percent organic matter, pH, and the concentration of all metals. These results showed that most metals in the A-01 wetland sediments behaved similarly: their concentrations decreased as sediment depth increased.

INNOVATIVE IN-SITU REMEDIATION OF CONTAMINATED SEDIMENTS FOR SIMULTANEOUS CONTROL OF CONTAMINATION AND EROSION

New technologies are needed that neutralize contaminant toxicity and control physical transport mechanisms that mobilize sediment contaminants. The last 12 months of this comprehensive project investigated the use of combinations of sequestering agents to develop in situ active sediment caps that stabilize mixtures of contaminants and act as a barrier to mechanical disturbance under a broad range of environmental conditions. Efforts focused on the selection of effective sequestering agents for use in active caps, the composition of active caps, and the effects of active cap components on contaminant bioavailability and retention. Results from this project showed that phosphate amendments, some organoclays, and the biopolymer, chitosan, were very effective at removing metals from both fresh and salt water. These amendments also exhibited high retention (80% or more) of most metals indicating reduced potential for remobilization to the water column. Experiments on metal speciation and retention in contaminated sediment showed that apatite and organoclay can immobilize a broad range of metals under both reduced and oxidized conditions. These studies were followed by sequential extractions to evaluate the bioavailability and retention of metals in treated sediments. Metal fractions recovered in early extraction steps are more likely to be bioavailable and were termed the Potentially Mobile Fraction (PMF). Less bioavailable fractions collected in later extraction steps were termed the Recalcitrant Factor (RF). Apatite and organoclay reduced the PMF and increased the RF for several elements, especially Pb, Zn, Ni, Cr, and Cd. Empirically determined partitioning coefficients and modeling studies were used to assess the retention of organic contaminants on selected sequestering agents. Organoclays exhibited exceptionally high sorption of polycyclic aromatic hydrocarbons as indicated by a comparison of K{sub d} values among 12 amendments. These results suggested that organoclays have high potential for controlling organic contaminants. Measured partitioning coefficients were used to model the time required for a contaminant to penetrate sediment caps composed of organoclay. The results showed that a thin layer of highly sorptive organoclay can lead to very long migration times, perhaps longer than the expected lifetime of the contaminant in the sediment environment. A one-dimensional numerical model was used to examine the diffusion of metals through several cap material based on measured and assumed material and transport properties. These studies showed that active caps composed of apatite or organoclay have the potential to delay contaminant breakthrough due to diffusion by hundreds of years or more compared with passive caps composed of sand. Advectively dominated column experiments are currently underway to define effective sorption related retardation factors in promising amendments for various hydrophobic organic compounds. Upon completion of these experiments, advection transient models will be used to estimate the time required for the breakthrough of various contaminants in caps composed of different experimental materials. Biopolymer products for inclusion in active caps were evaluated on the basis of resistance to biodegradation, sorption capacity for organic and inorganic contaminants, and potential for erosion control. More than 20 biopolymer products were evaluated resulting in the selection of chitosan/guar gum cross-linked with borax and xanthan/chitosan cross-linked with calcium chloride for inclusion in active caps to produce a barrier that resists mechanical disturbance. A process was developed for coating sand with cross-linked biopolymers to provide a means for delivery to the sediment surface. Properties of biopolymer coated sand such as carbon fraction (indicating biopolymer coverage), porosity, bulk density, and biodegradability have been evaluated, and experiments are currently underway to assess the resistance of biopolymer coated sand to erosion. Although the ability of active cap materials to remediate contaminants has been emphasized in this study, it is also important to ensure that these materials do not have deleterious effects on the environment. Therefore, promising amendments were evaluated for toxicity using 10 day sediment toxicity tests, the standardized Toxicity Characteristic Leaching Procedure (TCLP), and measurement of metal concentrations in aqueous extracts from the amendments. Metal concentrations were below TCLP limits, EPA ambient water quality criteria, and other ecological screening values These results showed that apatite, organoclay, and biopolymer coated sand do not release metals. The sediment toxicity tests indicated that apatite and biopolymer coated sand are unlikely to adversely affect benthic organisms, even when used in high concentrations.

Environmental impact of ongoing sources of metal contamination on remediated sediments.

A challenge to all remedial approaches for contaminated sediments is the continued influx of contaminants from uncontrolled sources following remediation. We investigated the effects of ongoing contamination in mesocosms employing sediments remediated by different types of active and passive caps and in-situ treatment. Our hypothesis was that the sequestering agents used in active caps and in situ treatment will bind elements (arsenic, chromium, cadmium, cobalt, copper, nickel, lead, selenium, and zinc) from ongoing sources thereby reducing their bioavailability and protecting underlying remediated sediments from recontamination. Most element concentrations in surface water remained significantly lower in mesocosms with apatite and mixed amendment caps than in mesocosms with passive caps (sand), uncapped sediment, and spike solution throughout the 2520h experiment. Element concentrations were significantly higher in Lumbriculus variegatus from untreated sediment than in Lumbriculus from most active caps. Pearson correlations between element concentrations in Lumbriculus and metal concentrations in the top 2.5cm of sediment or cap measured by diffusive gradient in thin films (DGT) sediment probes were generally strong (as high as 0.98) and significant (p<0.05) for almost all tested elements. Metal concentrations in both Lumbriculus and sediment/cap were lowest in apatite, mixed amendment, and activated carbon treatments. These findings show that some active caps can protect remediated sediments by reducing the bioavailable pool of metals/metalloids in ongoing sources of contamination.

Properties and Function of Pyomelanin

Melanin pigments are the most common pigments produced in nature and these complex biopolymers are found in species of all biological kingdoms. There are several categories of melanins which include eumelanins, pheomelanins and allomelanins. Eumelanins and pheomelanins are produced from oxidation of tyrosine or phenylalanine to odihydroxyphenylalanine (DOPA) and dopaquinone. Pheomelanin results from cysteinylation of DOPA. Allomelanins include a heterogeneous group of polymers that include pyomelanin. Melanin biochemistry and synthesis has been reviewed previously (Plonka and Grabacka 2006). This chapter will focus on the properties and function of pyomelanin and their potential utility in biotechnology. Pyomelanin originates from the catabolism of tyrosine or phenylalanine (Lehninger, 1975) (Fig. 1). Complete breakdown of tyrosine to acetoacetate and fumarate requires the enzymes 4-hydroxyphenylpyruvic acid dioxygenase (4-HPPD) and homogentisic acid oxidase (HGA-oxidase). In the absence of HGA-oxidase, or if homogentisic acid (HGA) production exceeds that of HGA-oxidase activity, HGA is over-produced and excreted from the cell (Yabuuchi and Ohyama 1972; Ruzafa et al. 1994; Katob et al. 1995). Autooxidation and selfpolymerization of HGA then results in pyomelanin. In addition, deletion of the gene that encodes for HGA-oxidase results in hyper production of pyomelanin while deletion of the gene that encodes for 4HPPD results in the inability to produce pyomelanin (Coon et al. 1994; Ruzafa et al. 1995). In humans with loss-of-function mutations in HGA-oxidase, pyomelanin (also known as alkapton or ochronotic pigment) forms in the urine due to the spontaneous auto oxidation of excess HGA (Beltran-Valero de Bernabe, et al. 1999). This condition is known as alkaptonuria in humans and can result in arthritis in adults. Pyomelanin production in microorganisms often is associated with numerous survival advantages and was first characterized in bacteria among numerous species of the genus Pseudomonas (Yabuuchi & Ohyama 1972). Since then several fungi and a number of bacteria, especially in the ┛ Proteobacteria have been shown to produce pyomelanin.

Life Span of Biopolymer Sequestering Agents for Contaminant Removal and Erosion Resistance

The objective of this paper is to report the development and life span of cross-linked biopolymers that remove contaminants, resist biodegradation over long periods of time, and resist erosion in dynamic aquatic environments. Biopolymers are polymeric compounds produced by living organisms (e.g., microorganisms, plants, crustaceans). They have repeated sequences that vary broadly in chemical composition including a variety of repeating functional groups (such as carboxyl, hydroxyl, amino, etc.). This makes them reactive and subject to cross-linking. Therefore, biopolymers, a great molecular weight compounds with repeated sequences, may have high opportunity for chemical interaction with other compounds. Depending on their functional groups, biopolymers can bind metals, organic contaminants, or soil particles and form interpenetrating cross-linking networks with other polymers. The ability of biopolymers (cross-linked or not) to bind a large variety of metals is supported by many studies (Chen et al., 1993; Etemadi et al., 2003; Knox et al., 2008 a, b). The capacity of alginate as a crosslinked product (calcium alginate) for Cr(VI) uptake was demonstrated by Fiol et al. (2004), who obtained an uptake of 86.42 mmol of Cr(VI) per L of wet sorbent volume using grape stalk wastes encapsulated into calcium alginate. The Cr(VI) removal ability of cross-linked calcium alginate was also shown by Araujo and Teixeira (1997), and its ability to bind Cu was shown by Chen et al. (1990 and 1993) and Wan et al. (2004). The removal of Cu, Cr, and As from treated wood onto the biopolymers, chitin and chitosan, was shown by Kartal and Iamamura (2004). The use of biopolymers based on elastine-like polypeptides for the selective removal of Hg was reported by Kostal et al. (2003), who also reported their potential for binding and removal of other metals such as As and Cr. Recently, the use of a similar elastin-like polypeptide composed of a polyhistidine tail was exploited as a metalbinding biopolymer with high affinity toward Cd by Prabhukumar et al. (2004). Knox et al. (2007 and 2008 a, b) showed that biopolymers (with and without cross-linking) have the ability to sequester a large variety of metals (e.g., Cu, Pb, Cd, As, Cr, Zn, and Ni) and organic contaminants (e.g., phenanthrene and pyrene).

Removal capacity and chemical speciation of groundwater iodide (I−) and iodate (IO3−) sequestered by organoclays and granular activated carbon

Radioiodine (present mostly as 129I) is difficult to remove from waste streams or contaminated groundwater because it tends to exist as multiple anionic species (i.e., iodide (I-), iodate (IO3-) and organo-iodide) that do not bind to minerals or synthetic materials. In this work, the efficacy of organoclay OCB and OCM, and granular activated carbon (GAC) as sorbents to bind I- and IO3- from artificial groundwater (AGW) was examined. These sorbents were highly effective at removing I- and IO3- from AGW under oxic condition, with the adsorption capacity up to 30 mg I/g sorbent. Based on X-ray spectroscopy measurements, I- was bound to organic ligands in organoclays OCB and OCM, but when GAC was exposed to I- in groundwater, the sequestered I species was molecular I2. For IO3- interacting with organoclay OCB and GAC, the adsorbed I species remained being IO3-, but when organoclay OCM that contains both quaternary amine and sulfur was exposed to IO3-, the sulfur compound would reduce IO3- to I- that was then bound to organic ligands. Thus, the inexpensive and high-capacity organoclays and GAC may provide a practical solution for removing 129I contaminant from environmental systems and liquid nuclear wastes.