Adsorption of cadmium on anatase nanoparticles-effect of crystal size and pH.

The adsorption and desorption of Cd(2+) to large and nanometer-scale anatase crystals have been studied to determine the relationship between heavy metal adsorption properties and anatase particle size. A solvothermal method was used to synthesize very fine anatase nanocrystals with average grain sizes ranging from 8 to 20 nm. On a surface area basis, it was found that large and nanometer-scale anatase particles had similar maximum Cd(2+) adsorption capacities, while their adsorption slopes differed by more than 1 order of magnitude. The particle-size effect on adsorption is constant over a pH range of 4-7.5. The desorption of Cd(2+) from both particle sizes is completely reversible. The adsorption data have been modeled by the Basic Stern model using three monodentate surface complexes. It is proposed that intraparticle electrostatic repulsion may reduce the adsorption free energy significantly for nanometer-sized particles.

More realistic soil cleanup standards with dual-equilibrium desorption.

The desorption of contaminants from soils/sediments is one of the most important processes controlling contaminant transport and environmental risks. None of the currently adopted desorption models can accurately quantify desorption at relatively low concentrations; these models often overestimate the desorption and thus the risks of hydrophobic organic chemicals, such as benzene and chlorinated solvents. In reality, desorption is generally found to be biphasic, with two soil-phase compartments. A new dual-equilibrium desorption (DED) model has been developed to account for the biphasic desorption. This model has been tested using a wide range of laboratory and field data and has been used to explain key observations related to underground storage tank plumes. The DED model relates the amount of a chemical sorbed to the aqueous concentration, with simple parameters including octanol-water partition coefficient, solubility, and fractional organic carbon; thus, it is the only biphasic model, to date, that is based on readily available parameters. The DED model can be easily incorporated into standard risk and transport models. According to this model, many regulatory standards of soils and sediments could be increased without increasing the risks.

Desorption kinetics of neutral hydrophobic organic compounds from field-contaminated sediment.

The chemical release rates from a field-contaminated sediment (Lake Charles, LA) using Tenax desorption were studied. Two dichlorobenzenes (m-, p-), hexachlorobutadiene, and hexachlorobenzene were investigated. Contrary to reports that sorption rates are inversely related to K(OW), the slow desorption rates were found to be similar for the four compounds. The data were modeled by a two-compartment irreversible adsorption and radial diffusion model. Desorption kinetics from the first irreversible compartment can be modeled by radial diffusion and assume an irreversible adsorption constant and soil tortuosity of 4.3. The desorption half-life is approximately 2-7 days. Desorption from the second irreversible compartment is very slow (half-life of approximately 0.32-8.62 years) presumably caused by entrapment in soil organic matter that increases the constrictivity of the solid phase to chemical diffusion. From the kinetic data, it is deduced that the diffusion pore diameter of the second irreversible compartment is approximately equal to the critical molecular diameter. The mass of chemicals in this highly constrictive irreversible compartment is approximately one-fourth of the maximum irreversible, or resistant, compartment. The slow kinetics observed in this study add additional support to the notion that the irreversibly sorbed chemicals are 'benign' to the environment.

Microbial removal of wastewater organic compounds as a function of input concentration in soil columns.

The fate of six organic compounds during rapid infiltration of primary wastewater through soil columns was studied. Feed solutions were prepared which contained all six compounds in individual concentrations ranging from 1 to 1,000 micrograms/liter and were applied to separate soil columns on a flooding-drying schedule. Feed solutions and column effluents were analyzed for the compounds by XAD resin (Rohm and Haas Co.) extraction and gas chromatography-mass spectrometry during each of three successive inundation cycles. Breakthrough profiles of o-phenylphenol were relatively consistent during the test, with fractional breakthrough (mass output/mass input) being independent of input concentration. Consistent profiles were also observed for 2-(methylthio)benzothiazole, although fractional breakthroughs were higher at lower input concentrations, indicating that removal processes were operating less efficiently at these levels. The behavior of p-dichlorobenzene was similar to that of 2-(methylthio)benzothiazole after the first inundation cycle, with the exception that increased fractional breakthroughs were observed at the highest input concentration as well. Microbial adaptation was evident for benzophenone, 2-methylnaphthalene, and p-(1,1,3,3-tetramethylbutyl)phenol, as indicated by increased removal efficiencies during successive inundation cycles, especially at the higher input concentrations. Column effluent concentrations of the latter two compounds were independent of input concentrations during the final stage of the test. Microbial activity and adaptation were confirmed for several of the compounds by using isotopes and measuring the extent of mineralization in batch tests with soil from one of the columns.(ABSTRACT TRUNCATED AT 250 WORDS)

Anaerobic inhibition of trace organic compound removal during rapid infiltration of wastewater.

When soil columns were operated aerobically on a flooding-drying schedule in a previous study, good removals were observed for several organic compounds at concentrations ranging from 1 to 1,000 micrograms per liter in primary wastewater. In this study, fractional breakthroughs of most compounds increased substantially once operating parameters were modified and the soil became anaerobic. These results imply that microbial removal of trace organic compounds can be inhibited if anaerobic conditions develop during rapid infiltration of wastewater.

Mineralization kinetics: a constant composition approach.

A new method is described for studying, reproducibly, the kinetics of crystallization of minerals under conditions of constant solution composition even at very low supersaturations. For calcium phosphates the method provides direct evidence for octacalcium phosphate as the precursor to hydroxyapatite precipitation at physiological pH.

The seeded growth of calcium phosphates on dentin and predentin.

The in vitro mineralization of predentin (PD) and of the organic component of dentin (OD) has been studied in a supersaturated solution of calcium phosphate at 37 C and pH=7.4 at total calcium and phosphate concentrations close to those in vivo. The mineralization of OD can be divided into at least three periods based upon the morphology and the specific surface area of the mineral phase formed. Dissolution of the newly precipitated mineral on OD revealed a stoichiometry close to that for octacalcium phosphate. In contrast, PD was ineffective as a crystal nucleator. The mineralization of the inorganic component of dentin (ID) and whole dentin (D) has also been examined.