Influence of ionic strength, electrolyte type, and NOM on As(V) adsorption onto TiO2.

As(V) adsorption onto a commercially available TiO2 (Degussa P25) in NaCl or NaClO4 at various concentrations (0.001-0.1 M) was investigated. The effect of natural organic matter (NOM) on As(V) removal through the adsorption by TiO2 was also examined. In either electrolyte, As(V) adsorption onto TiO2 increased with the increase of ionic strength under alkaline conditions (pH 7.0-11.0). Under acidic conditions (pH 4.0-6.0), the adsorption of As(V) onto TiO2 was insensitive to ionic strength in NaClO4 electrolyte but decreased with increasing ionic strength in NaCl electrolyte. The presence of 2-15 mg/L NOM as C significantly decreased the fraction of As(V) adsorbed onto TiO2 at pH 6.0 regardless of the initial As(V) concentration (1-15 microM). The measurement of zeta potential of TiO2 with and without As(V) suggests that the presence of As(V) can shift the point of zero charge (pH(pzc)) of TiO2 to a lower pH value. The overall data presented in this study suggest that As(V) can form both inner-sphere and outer-sphere complexes on TiO2 surface, and NOM is an important factor controlling As(V) adsorption onto TiO2.

Particle-scale investigation of PAH desorption kinetics and thermodynamics from sediment.

Dredged sediment from Milwaukee Harbor showed two primary classes of particles in the <2 mm size range: a lighter-density coal- and wood-derived fraction with 62% of total PAHs and a heavier-density sand, silt, and clay fraction containing the remaining 38% of the PAHs. Room-temperature PAH desorption kinetic studies on separated sediment fractions revealed slow desorption rates for the coal-derived particles and fast desorption rates for the clay/silt particles. The effect of temperature on PAH release was measured by thermal program desorption mass spectrometry to investigate the desorption activation energies for PAHs on the different sediment particles. Three activated diffusion-based models and an activated first-order rate model were used to describe the thermal desorption of PAHs for four molecular weight classes. PAH binding with the coal-derived particles was associated with high activation energies, typically in the range of 115-139 kJ/mol. PAHs bound to the clay/silt material had much lower activation energy, i.e., in the range of 37-41 kJ/ mol for molecular weight 202. Among the desorption models tested, a spherical diffusion model with PAHs located like a rind on the outer 1-3 microm region best described the PAH thermal desorption response for coal-derived particles. This internal PAH distribution pattern on coalderived particles is based on prior direct measurement of PAH locations at the subparticle scale. These studies reveal that heterogeneous particle types in sediment exhibit much different amounts and binding of PAHs. PAHs associated with coal-derived particles aged over several decades in the field appear to be far from reaching an equilibrium sorption state due to the extremely slow diffusivities through the polymer-like coal matrix. These results provide an improved mechanistic perspective for the understanding of PAH mobility and bioavailability in sediments.

Succession of phenotypic, genotypic, and metabolic community characteristics during in vitro bioslurry treatment of polycyclic aromatic hydrocarbon-contaminated sediments.

Dredged harbor sediment contaminated with polycyclic aromatic hydrocarbons (PAHs) was removed from the Milwaukee Confined Disposal Facility and examined for in situ biodegradative capacity. Molecular techniques were used to determine the successional characteristics of the indigenous microbiota during a 4-month bioslurry evaluation. Ester-linked phospholipid fatty acids (PLFA), multiplex PCR of targeted genes, and radiorespirometry techniques were used to define in situ microbial phenotypic, genotypic, and metabolic responses, respectively. Soxhlet extractions revealed a loss in total PAH concentrations of 52%. Individual PAHs showed reductions as great as 75% (i.e., acenapthene and fluorene). Rates of (14)C-PAH mineralization (percent/day) were greatest for phenanthrene, followed by pyrene and then chrysene. There was no mineralization capacity for benzo[a]pyrene. Ester-linked phospholipid fatty acid analysis revealed a threefold increase in total microbial biomass and a dynamic microbial community composition that showed a strong correlation with observed changes in the PAH chemistry (canonical r(2) of 0.999). Nucleic acid analyses showed copies of genes encoding PAH-degrading enzymes (extradiol dioxygenases, hydroxylases, and meta-cleavage enzymes) to increase by as much as 4 orders of magnitude. Shifts in gene copy numbers showed strong correlations with shifts in specific subsets of the extant microbial community. Specifically, declines in the concentrations of three-ring PAH moieties (i.e., phenanthrene) correlated with PLFA indicative of certain gram-negative bacteria (i.e., Rhodococcus spp. and/or actinomycetes) and genes encoding for naphthalene-, biphenyl-, and catechol-2,3-dioxygenase degradative enzymes. The results of this study suggest that the intrinsic biodegradative potential of an environmental site can be derived from the polyphasic characterization of the in situ microbial community.

Roadblocks to the implementation of biotreatment strategies.

The Department of Defense (DoD) has over 21,000 contaminated sites requiring some form of remediation. Contaminants on these sites include explosive compounds (i.e., TNT, RDX, HMX), chlorinated solvents (i.e., PCE, TCE, TCA), polycyclic aromatic hydrocarbons (i.e., benzo-a-pyrene), and polychlorinated biphenyls (i.e., aroclors). Current technology has centered around incineration, air stripping, and the use of activated carbon. Frequently, this technology is not cost effective nor publicly acceptable. Biotreatment offers a possible alternative. Biotreatment can cost effectively eliminate contaminants and avoid the use of harsh chemicals and physical treatments. However, special care must be employed to ensure that the proper remediation system is designed and engineered to optimize clean-up and minimize costs. Unfortunately, not all bioremediation efforts have been successful. In an attempt to develop bioremediation technology from the flask (bench scale) to the field (full-scale design), many scientists and engineers have failed to understand the phenomena that influence bioremediation. Issues such as additional mass transport mechanisms/limitations, the presence of multiple phases, spatial heterogeneities, and unfavorable factors for bacterial growth represent only a few of the phenomena that can limit or complicate biodegradation. Successful bioremediation requires a complete examination of the phenomena that can be observed as the scientist and engineer progress together from the bench to the field. An excellent way to examine these phenomena is by using the conceptual scales of observation: microscale, mesoscale, and macroscale. The microscale represents the level at which chemical/microbial species and reactions can be characterized independently of any transport phenomena. These activities are those occurring at the microbial cell level and generally are the focus of bench level work. The mesoscale is the level at which transport phenomena and system geometry are first apparent, with the exclusion of advective or mixing processes. This scale represents those activities that occur at the pore channel, soil particle or microbial aggregate level. The macroscale is the scale at which you have the ability to discern advective or mixing phenomena. These activities are generally associated on a site level and are the focus of the design engineer. The critical path as bioremediation technology is developed from flask to field is to observe and understand the phenomena that exert influence at each scale of observation so that its effects can be incorporated into the final remediation design.