Considerations on the use of carbon and nitrogen isotopic ratios for sediment fingerprinting.

Carbon and nitrogen stable isotopic ratios are increasingly used in sediment fingerprinting studies. However, questions remain regarding tracer conservativeness during sediment transport and other error considerations. We investigate conservativeness processes, including carbon oxidation and nitrogen mineralization, using experiments. We also test how other considerations impact the isotopic ratios including algae accrual into temporary sediment deposits in the river, the physical loss of organic matter via disaggregation, concentration dependent mixing, and time-varying isotopic ratios of sediment sources. Results show all processes and considerations can change isotope abundance, however, significance varied. Carbon oxidation, nitrogen mineralization and upland seasonality of sediment sources did not significantly change isotopic ratios. Algae accrual, concentration dependency mixing, physical loss of organic matter during transport, and seasonality of the in-stream sediment source significantly changed the isotopic ratios for the conditions tested. Fertilization significantly impacted the stable carbon isotopic ratio in one case considered. Results from sediment fingerprinting simulations and testing how well the virtual mixture fits the mass balance equation agreed with significance results for tracer changes, and some uncertainty considerations changed fractional contribution of sources by as much as 50%. A noteworthy recommendation is the mean isotopic ratios of sediment sources should be separated by at least 1‰ to lessen tracer conservativeness concerns in fingerprinting simulation. We recommend concentration dependent mixing becomes the accepted practice when using isotopic ratios, however, we warn against using particle size corrections. We recommend the loss of organic matter during disaggregation be accounted for in fingerprinting estimates. We recommend algae accrual in in-stream sediment deposits should either be accounted for or in-stream sediment should be treated as a time-varying source in sediment fingerprinting simulations. Finally, we recommend both the carbon and nitrogen isotopic ratio should be tested as potential tracers because the two tracers performed similarly when testing how well the virtual mixture fits the mass balance equations.

Selenium removal by activated alumina in batch and continuous-flow reactors.

Selenium removal by activated alumina (AA) in batch and continuous-flow reactors was investigated in this study. The adsorption kinetics fitted pseudo-first as well as pseudo-second order models with equilibrium time for Se(VI) and Se(IV) adsorption of 8 and 12 hr, respectively. A significant higher adsorption capacity for Se(IV) than Se(VI) was observed in isothermal adsorption experiments. The adsorption isotherms of Se(VI) and Se(IV) agreed well with both Langmuir and Freundlich adsorption models. The results also showed that Se(VI) adsorption was adversely affected by NaHCO3 concentrations, while Se(IV) adsorption was not due to the selectivity of activated alumina for anions. In the continuous-flow reactor packed with AA and inoculated with Shigella fergusonii strain TB42616 under a hydraulic detention time of 3 days, approximately 74% and 70% Se(VI) were removed after 40 days at influent concentrations of 10 and 50 mg/L, respectively. Effluent concentrations of Se(IV) from the bioreactor were insignificant due to the combination of bioactivities and adsorption. The AA-packed bioreactor is promising in both Se(VI) and Se(IV) removal as better removal efficiency may be accomplished by increasing the liquid retention time. PRACTITIONER POINTS: The adsorption capacity for Se(IV) of activated alumina was significantly higher than that for Se(VI). Se(VI) adsorption was inhibited by NaHCO3 , while Se(IV) adsorption was not. Se(IV) formed by biological Se(VI) reduction was adsorbed by activated alumina in continuous-flow reactors. Both Se(VI) and Se(IV) were significantly removed by activated alumina-packed continuous-flow reactors.

Selenium reduction by a defined co-culture of Shigella fergusonii strain TB42616 and Pantoea vagans strain EWB32213-2.

Selenium reduction was evaluated with pure batch cultures of Shigella fergusonii strain TB42616 (TB) and Pantoea vagans strain EWB32213-2 (EWB), respectively. A two-stage process, from Se(VI) to Se(IV) and then from Se(IV) to Se(0), was observed. The second stage of reduction, from Se(IV) to Se(0), was observed as the rate-limiting step resulting in accumulation of the more toxic Se(IV). In order to facilitate Se(VI) reduction and reduce Se(IV) accumulation, the Se(VI)-reducing strain TB was co-cultured with a Se(IV)-reducing strain EWB. Although Se(VI) reduction rate was not affected, Se(IV) reduction was significantly enhanced with low Se(IV) accumulation in the defined co-culture. Effects of culture composition as well as nitrate and arsenate on Se(VI) reduction were also investigated. A co-culture composition of 10:1 (EWB:TB) ratio was observed to achieve the best total selenium reduction. In addition, nitrate at 50 mg/L was observed to inhibit Se(IV) reduction but not Se(VI) reduction, while arsenate at 200 mg/L exhibited slight inhibition on both Se(VI) and Se(IV) reduction.

Modeling arsenite oxidation by chemoautotrophic Thiomonas arsenivorans strain b6 in a packed-bed bioreactor.

Arsenic is a major toxic pollutant of concern for the human health. Biological treatment of arsenic contaminated water is an alternative strategy to the prevalent conventional treatments. The biological treatment involves a pre-oxidation step transforming the most toxic form of arsenic, As (III), to the least toxic form, As (V), respectively. This intermediate process improves the overall efficiency of total arsenic removal from the contaminated water. As (III) oxidation by the chemoautotrophic bacterium Thiomonas arsenivorans strain b6 was investigated in a fixed-film reactor under variable influent As (III) concentrations (500-4000 mg/L) and hydraulic residence times (HRTs) (0.2-1 day) for a duration of 137 days. During the entire operation, seven steady-state conditions were obtained with As (III) oxidation efficiency ranging from 48.2% to 99.3%. The strong resilience of the culture was exhibited by the recovery of the bioreactor from an As (III) overloading of 5300±400 mg As (III)/L day operated at a HRT of 0.2 day. An arsenic mass balance revealed that As (III) was mainly oxidized to As (V) with unaccounted arsenic (≤4%) well within the analytical error of measurement. A modified Monod flux expression was used to determine the biokinetic parameters by fitting the model against the observed steady-state flux data obtained from operating the bioreactor under a range of HRTs (0.2-1 day) and a constant influent As (III) concentration of 500 mg/L. Model parameters, k=0.71±0.1 mg As (III)/mg cells h, and K(s)=13.2±2.8 mg As (III)/L were obtained using a non-linear estimation routine and employing the Marquardt-Levenberg algorithm. Sensitivity analysis revealed k to be more sensitive to model simulations of As (III) oxidation under steady-state conditions than parameter K(s).

Arsenite oxidation by immobilized cells of Alcaligenes faecalis strain O1201 in a fluidized-bed reactor.

Arsenic(III) oxidation was evaluated in a continuous-flow fluidized-bed reactor (FBR) with Alcaligenes faecalis strain 01201 immobilized in gel beads. The FBR was operated under 300 mg/L citrate and a range of influent As(III) concentrations (75 to 3000 mg/L) at short hydraulic retention times (1.06 to 3.17 hours). The pH and temperature in the FBR were maintained at optimal growth conditions for strain O1201 (pH 7 and 30 degrees C) throughout the study. A total of 10 quasi-steady-state operating conditions were obtained after 54 days of operation under an As(III) concentration of 441 mg/L (10 000 mg/L/d loading rate), with As (III) removal efficiency ranging from 76% to near complete. Material balance analysis over the FBR revealed that the difference between the cumulative influent As (III) and the sum of cumulative effluent As(III) and As(V) was insignificant. The major mechanism of As(III) removal from the FBR is biological oxidation to As(V).

Modeling hexavalent chromium removal in a Bacillus sp. fixed-film bioreactor.

A one-dimensional diffusion-reaction model was developed to simulate Cr(VI) reduction in a Bacillus sp. pure culture biofilm reactor with glucose as a sole supplied carbon and energy source. Substrate utilization and Cr(VI) reduction in the biofilm was best represented by a system of (second-order) partial differential equations (PDEs). The PDE system was solved by the (fourth-order) Runge-Kutta method adjusted for mass transport resistance using the (second-order) Crank-Nicholson and Backward Euler finite difference methods. A heuristic procedure (genetic search algorithm) was used to find global optimum values of Cr(VI) reduction and substrate utilization rate kinetic parameters. The fixed-film bioreactor system yielded higher values of the maximum specific Cr(VI) reduction rate coefficient and Cr(VI) reduction capacity (kmc = 0.062 1/h, and Rc = 0.13 mg/mg, respectively) than previously determined in batch reactors (kmc = 0.022 1/h and Rc = 0.012 mg/mg). The model predicted effluent Cr(VI) concentration well with 98.9% confidence (sigmay2 = 2.37 mg2/L2, N = 119) and effluent glucose with 96.4 % confidence (sigmay(w)2 = 5402 mg2/L2, N = 121, w = 100) over a wide range of Cr(VI) loadings (10-498 mg Cr(VI)/L/d).