Biosorption of Cd(II) by live and dead cells of Bacillus cereus RC-1 isolated from cadmium-contaminated soil.

The present study investigated the biosorption capacity of live and dead cells of Bacillus cereus RC-1 for Cd(II). The biosorption characteristics were investigated as a function of initial pH, contact time, and initial cadmium concentration. Equilibrium biosorption was modeled using Langmuir, Freundlich and Redlich-Peterson isotherm equations. It was found that the maximum biosorption capacities calculated from Langmuir isotherm were 31.95 mg/g and 24.01 mg/g for dead cells and live cells, respectively. The kinetics of the biosorption was better described by pseudo-second order kinetic model. Desorption efficiency of biosorbents was investigated at various pH values. These results indicated that dead cells have higher Cd(II) biosorption capacity than live cells. Furthermore, zeta potential, transmission electron microscopy (TEM), scanning electron microscopy (SEM) coupled with energy dispersive X-ray (EDX), and Fourier transform infrared spectroscopy (FTIR) studies were carried out to understand the differences in the Cd(II) biosorption behavior for the both biosorbents. The bioaccumulation of Cd(II) by B. cereus RC-1 was found to depend largely on extracellular biosorption rather than intracellular accumulation. Based on the above studies, dead biomass appears to be a more efficient biosorbent for the removal of Cd(II) from aqueous solution.

Prediction of the fate and transport processes of atrazine in a reservoir.

The fate and transport processes of a toxic chemical such as atrazine, an herbicide, in a reservoir are significantly influenced by hydrodynamic regimes of the reservoir. The two-dimensional (2D) laterally-integrated hydrodynamics and mass transport model, CE-QUAL-W2, was enhanced by incorporating a submodel for toxic contaminants and applied to Saylorville Reservoir, Iowa. The submodel describes the physical, chemical, and biological processes and predicts unsteady vertical and longitudinal distributions of a toxic chemical. The simulation results from the enhanced 2D reservoir model were validated by measured temperatures and atrazine concentrations in the reservoir. Although a strong thermal stratification was not identified from both observed and predicted water temperatures, the spatial variation of atrazine concentrations was largely affected by seasonal flow circulation patterns in the reservoir. In particular, the results showed the effect of flow circulation on spatial distribution of atrazine during summer months as the river flow formed an underflow within the reservoir and resulted in greater concentrations near the surface of the reservoir. Atrazine concentrations in the reservoir peaked around the end of May and early June. A good agreement between predicted and observed times and magnitudes of peak concentrations was obtained. The use of time-variable decay rates of atrazine led to more accurate prediction of atrazine concentrations, while the use of a constant half-life (60 days) over the entire period resulted in a 40% overestimation of peak concentrations. The results provide a better understanding of the fate and transport of atrazine in the reservoir and information useful in the development of reservoir operation strategies with respect to timing, amount, and depth of withdrawal.

Impact of COD/N ratio on nitrous oxide emission from microcosm wetlands and their performance in removing nitrogen from wastewater.

Constructed wetlands (CWs) are considered to be important sources of nitrous oxide (N(2)O). In order to investigate the effect of influent COD/N ratio on N(2)O emission and control excess emission from nitrogen removal, free water surface microcosm wetlands were used and fed with different influent. In addition, the transformation of nitrogen was examined for better understanding of the mechanism of N(2)O production under different operating COD/N ratios. It was found that N(2)O emission and the performance of microcosm wetlands were significantly affected by COD/N ratio of wastewater influent. Strong relationships exist between N(2)O production rate and nitrite (r=0.421, p<0.01). During denitrification process, DO concentration crucially influences N(2)O production rate. An optimal influent COD/N ratio was obtained by adjusting external carbon sources for most effective N(2)O emission control and best performance of the CWs in nitrogen removal from wastewater. It is concluded that under the operating condition of COD/N ratio=5, total N(2)O emission is minimum and the microcosm wetland is most effective in wastewater nitrogen removal.

Characterization and micellization of rhamnolipidic fractions and crude extracts produced by Pseudomonas aeruginosa mutant MIG-N146.

Two representative rhamnolipidic fractions, RL-F1 and RL-F2, produced by the P. aeruginosa mutant strain MIG-N146, were separated and chemically characterized by TLC, HPLC-MS, and FTIR. The RL-F1 fraction is predominantly mono-rhamnolipid homologues with a high content of one or two fatty acid moieties. The RL-F2 fraction is mainly composed of di-rhamnosyl moieties with two hydrophobic tails. Micellization behavior was investigated to assess the physicochemical properties of the surfactants, RL-F1, RL-F2, and crude rhamnolipidic extracts. The variations in morphology of micelle formation and growth were examined by dynamic light scattering measurements as a function of surfactant concentration. Critical micelle concentration (CMC), average minimal surface tension (gamma(CMC)), saturated surface excess (Gamma(m)), mean surface area per molecule (S), and adsorption efficiency (pC(20)) were determined from the surface tension profiles and compared for the three surfactant systems. It was found that micelle growth was significantly enhanced by increasing rhamnolipid bulk concentration, which was most probably accompanied with an aggregate shape transition. Well-separated multi- or bi-modal distributions of particle size were observed in RL-F2 and the crude extracts solutions. The results of this study demonstrate that molecular architecture of different surfactant compositions profoundly influences the performance of rhamnolipidic surfactants.

Synthesis of polyamine flocculants and their potential use in treating dye wastewater.

Polyamine flocculants were synthesized by the polycondensation of dimethylamine and epichlorohydrin, in which organic amines, e.g. 1,2-diaminoethane, were used as modifying agents. Different products were obtained by varying the reaction parameters, such as the molar ratio of epichlorohydrin to dimethylamine, the amount of 1,2-diaminoethane and reaction temperature. The polyamine flocculants were characterized by transmission electron microscope (TEM). Their flocculation performance was evaluated with simulated dye liquor and actual printing and dyeing wastewater. The behavior of the flocculants was compared with that of inorganic coagulant, polyaluminum chloride (PAC). The experimental results show that polyamine with the highest viscosity and cationicity could be prepared under following conditions: an epichlorohydrin to dimethylamine molar ratio of 1.5, a reaction temperature of 70 degrees C, a 3% content of 1,2-diaminoethane in the total reaction monomers and a reaction time of 7h. Polyamine polymers can, as flocculants for treating simulated and actual dye wastewater, remove color and COD efficiently. The rate of color removal from reactive red liquor, reactive blue liquor and reductive yellow liquor reached as high as 96%, 97% and 96%, respectively. The highest efficiency of color removal and COD removal from polyamine for treating dye wastewater was 90% and 89%, respectively.

A two-dimensional model for simulating the transport and fate of toxic chemicals in a stratified reservoir.

A two-dimensional reservoir toxics model is essential to establishing effective water resources management and protection. In a reservoir, the fate of a toxic chemical is closely connected with flow regimes and circulation patterns. To better understand the kinetic processes and persistence and predict the dissipation of toxic contaminants in the reservoir during a spill or storm runoff event, a toxics submodel was developed and incorporated into an existing laterally integrated hydrodynamics and transport model. The toxics submodel describes the physical, chemical, and biological processes and predicts unsteady vertical and longitudinal distributions of a toxic chemical. The two-dimensional toxicant simulation model was applied to Shasta Reservoir in California to simulate the physico-chemical processes and fate of a volatile toxic compound, methyl isothiocyanate (MITC), during a chemical spill into the Sacramento River in 1991. The predicted MITC concentrations were compared with those observed. The effect of reservoir flow regimes on the transport and fate of the toxic substance was investigated. The results suggested that the persistence of MITC is significantly influenced by different flow regimes. Methyl isothiocyanate is more persistent in the reservoir under an interflow condition due to reduced volatilization from deep layers than under an overflow condition. In the overflow situation, the plume moved more slowly toward the dam and experienced greater dissipation. This analysis can assist in toxic spill control and reservoir management, including field sampling and closure of water intakes.

Modeling flow and sediment transport in a river system using an artificial neural network.

A river system is a network of intertwining channels and tributaries, where interacting flow and sediment transport processes are complex and floods may frequently occur. In water resources management of a complex system of rivers, it is important that instream discharges and sediments being carried by streamflow are correctly predicted. In this study, a model for predicting flow and sediment transport in a river system is developed by incorporating flow and sediment mass conservation equations into an artificial neural network (ANN), using actual river network to design the ANN architecture, and expanding hydrological applications of the ANN modeling technique to sediment yield predictions. The ANN river system model is applied to modeling daily discharges and annual sediment discharges in the Jingjiang reach of the Yangtze River and Dongting Lake, China. By the comparison of calculated and observed data, it is demonstrated that the ANN technique is a powerful tool for real-time prediction of flow and sediment transport in a complex network of rivers. A significant advantage of applying the ANN technique to model flow and sediment phenomena is the minimum data requirements for topographical and morphometric information without significant loss of model accuracy. The methodology and results presented show that it is possible to integrate fundamental physical principles into a data-driven modeling technique and to use a natural system for ANN construction. This approach may increase model performance and interpretability while at the same time making the model more understandable to the engineering community.

River temperature sensitivity to hydraulic and meteorological parameters.

A sensitivity analysis is performed to evaluate river temperature variations in response to changes in hydraulic and meteorological conditions. The effects of instream flow, river geometry, and weather factors on daily mean and daily maximum river temperatures are quantified by analytical solutions to a simplified model. The influence coefficient method is used to determine river temperature sensitivity. The sensitivity analysis presents quantitative evidence that river temperatures are more sensitive to instream flowrate, upstream inflow temperature, air temperature, humidity and solar radiation than to other parameters including wind speed and channel geometry and morphometry. It is found that the sensitivity of river temperatures to flow is as significant as that to weather. Daily maximum river temperature is more sensitive to flowrate than daily mean temperature. Adapting the concept of 'diminishing returns', a critical instream flowrate is identified, which divides high and low sensitivity of water temperatures to flowrate. The critical flowrate can be used to determine practically achievable and economically feasible flow requirements for summer river temperature control. The sensitivity results can assist in streamflow management and reservoir operation for protections of habitat and aquatic environment.