Pierre Brassard

A feasible set for chemical speciation problems

Abstract Numerical methods applied to solve chemical speciation problems often do not converge at the desired global minimum. The risk of failure can be reduced by starting calculations on the residual map in a region where convergence is assured. This is possible because the topography of the residual map for all ideal chemical equilibrium problems is predictable and contains three important regions: a region of mostly steep gradient where the simulation assumes an excess of mass in the system; another region of mass deficit with areas of low to zero gradients and, in between, a channel where total mass is conserved. The channel constitutes a feasible set and always contains the global minimum. Although apparently an advantage in guiding the descent towards the solution, the feasible set channel causes a major problem for convergence by hiding the global minimum from a starting point in either excess or deficit regions. In addition, the steep slopes of the channel cause most line search algorithms to overshoot several times before converging. The best approach to maximize both convergence and speed is to start in the mass excess region with a Levenberg–Marquardt method using a cubic polynomial line search and, in the small number of cases where convergence fails, to use the slower and robust simplex method.

Metal-sediment interaction during resuspension

A calibrated shaker was used to resuspend sediments from Hamilton Harbour, Ontario at different shear stresses. The flocs formed by resuspension were irreversible and increased in size and number with increasing shear. Individual particle size was constant at about 0.1 µm for the different flocs and independent of shear, but agglomeration size increased with shear. H-ion exchange as a function of sediment concentration fits a Langmuir type function well. Exchange of total Ba, Ca, Cd, Cr, Cu, Mg, Ni, Pb and Zn with H+ were determined for the different shear regimes which covered quiescence to gale conditions. A simple lumped one intrinsic constant and fixed capacitance electrostatic model is used to fit the exchange data for each divalent metal.

Characterization of multiple functional groups on kaolinite

Abstract Multiple proton functional group conditional binding constants ( K ′ i ) and their concentrations ( C i ) are determined from detailed acid/base titration data. The C - K ′ information is obtained for kaolinite by assuming that the distribution can be approximated by cumulative independent mono protic groups. Linear programming optimization techniques are used to Fit the data. In addition, the electrode calibration is optimized in the data fit. This adjustment is important for high and low pHs. Discrete concentration dependent pXs of about 3.4, 4.5, 6.7 and 9.8 are determined for a reference kaolinite. These correspond reasonably well to designations made by Wehrli et al. (1990, Aquatic Sciences, Vol. 52, pp. 1–31 ) to A10H2 and AlOH proton reactions at edge and surface sites and to silanol exchange. There is an ionic strength effect for one site. Long and short reaction times and reversibility affect the results.

Dissolved Metal Concentrations and Suspended Sediment in Hamilton Harbour

Abstract Hamilton Harbour sediments are elevated in metal and organic contaminants, resulting from 75 years of industrial and municipal discharge. These contaminants could be released from the sediments during resuspension events caused by strong winds or dredging operations. We predict the metal concentrations in the water column that would be found during resuspension, using an equilibrium adsorption model. Confirmation of the model was made experimentally using small reactors with spikes of metal added to the aqueous phase. Four different Hamilton Harbour sites were sampled and used in the experiments. The time to reach steady state was determined as 2 days or less after the resuspension event. Results from the experiments yielded aqueous phase concentrations equivalent to or lower than concentrations in Hamilton Harbour water. We conclude that the concentration of metals in the water column of Hamilton Harbour is not elevated by resuspension of surficial sediments.

Chemical and microbial diagenesis of humic matter in freshwaters

The importance of microbial activity in changing the chemical properties of humic material in brown waters was measured as a function of pH, acid neutralization capacity (ANC), and pKa affinity spectrum from brownwater samples kept in aquaria and sterile flasks. Changes to the humic matter in non-sterile microcosms caused increases in pH and ANC and loss of strong organic acid pKa sites. Microbial activity was shown to be the main contributor to a decrease in acidity of humic material as samples kept under sterile conditions showed less change than non-sterile ones. Microbial processes were shown to be mostly due to sessile organisms on the walls of the aquaria and in sediments. Our data suggest that acid-base properties of brownwaters can be modified based on residence times, as well as by temperature in their basins. Assuming similar starting humic materials, microbial degradation may account for the lower acidity measured in lakes and large rivers, compared to headwaters.

Resuspension and Redistribution of Sediments in Hamilton Harbour

Abstract The size distribution of sediments in Hamilton Harbour is a result of both natural and human processes. The objective is to predict the behavior of sediment particles in the harbor, assuming wind driven resuspension to be the major natural factor controlling grain size distribution. In this approach hydraulic sorting of sediments was assumed to be entirely predictable from 10 years of wind direction and velocity data used to calculate fetch over the extent of the harbor. Bottom velocities were estimated assuming a universal velocity defect law and the direction of bottom currents were assumed to parallel the depth contours. Although simplistic in many regards, the model returns adjustment parameters that are consistent with accepted values and is sufficiently articulated to be used as a reference to evaluate the cause of sediment transport in the harbor. Known local disturbances associated with human activity in the harbor could be predicted by mapping the differences between sediment distribution and actual core data from surficial sediments. A large part of the sediment in the deeper part of the harbor however contained more silt and clays than predicted. We assume this excess to be caused by a uniform disturbance equivalent to a 4.8 cm/sec bottom current and results from the combination of small bottom currents and other transfers of energy from the mixed layer. Historical records show that this situation existed long before the industrial period. Man-made disturbances on the overall distribution of bottom sediments are therefore less important compared to changes due to agricultural practices at the turn of the century.

Dialysis membrane to prevent cadmium ion specific electrode fouling.

We present a simple method of protecting a Cd(II) ion-selective electrode from fouling using a dialysis membrane. Drift due to fouling is inevitable during continuous exposure of the electrode to the bulk solution. It presents a major problem in the case of automated titrations because recalibration is not possible. The drift is due to natural organic matter present in the sediment, and the membrane prevents its accumulation on the electrode surface. This was verified by calibrating a Cd electrode exposed to sediment for varying times, both with and without a membrane covering. There are two types of time dependent biases. A primary drift occurs without exposure to sediments and is probably caused by oxidation of the surface. A more important effect due to natural organic matter accumulation causes additional drift and deteriorates the Nernstian response of the electrode. From these results, it is possible to predict the uncertainty associated with measurements of binding constants and surface site densities for a Langmuir model. In general, the biased electrode overestimates the total metal concentration, reduces the value of the stability constants and increases estimates of maximum surface site densities.

Acid-base analysis of dissolved organic matter in surface waters

A mono-protic multi-site model is developed to obtain a pK(acid) — concentration distribution. Dense and equal interval pH data are required for an accurate characterization. A computer driven titrimetric system is used to obtain the data. The technique is applied to dissolved organic carbon (> 15 mg C L-1) samples from the Kejimkujik region, Nova Scotia. A calculation shows that the acidic (pH=4.6) dystrophic waters can result from mixing 15 mg C L-1 of the organic acids with an initial inorganic system of about 75 peq L-1 alkalinity.

On the Ca2+ dependence of non-transferrin-bound iron uptake in PC12 cells.

Non-transferrin-bound iron (NTBI) uptake has been reported to follow two pathways, Ca2+-dependent and Ca2+-independent (Wright, T. L., Brissot, P., Ma, W. L., and Weisiger, R. A. (1986) J. Biol. Chem. 261, 10909–10914; Sturrock, A., Alexander, J., Lamb, J., Craven, C. M., and Kaplan, J. (1990) J. Biol. Chem. 265, 3139–3145). Studies reporting the two pathways have ignored the weak interactions of Ca2+ with the chelator nitrilotriacetate (NTA) and the reducing agent ascorbate. These studies used a constant ratio of total Fe2+ to NTA with and without Ca2+. We observed Ca2+ activation of NTBI uptake in PC12 cells with the characteristics reported for other cells upon using 1 mm ascorbate and a constant ratio of total Fe2+ to NTA with or without Ca2+. However, Ca2+ did not affect NTBI uptake in solutions without NTA. We then determined conditional stability constants for NTA binding to Ca2+ and Fe2+ by potentiometry under conditions of NTBI uptake experiments (pH, ionic strength, temperature, ascorbate, total Fe2+, and total Ca2+concentrations). In solutions based on these constants and taking Ca2+ chelation into account, Ca2+ did not affect NTBI uptake over a range of free Fe2+concentrations. Thus, the Ca2+ activation of NTBI uptake observed using the constant total Fe2+ to NTA ratio was because of Ca2+-NTA chelation rather than an activation of the NTBI transporter itself. It is suggested that the previously reported Ca2+ dependence of NTBI uptake be re-evaluated.

Iron promotes cadmium binding to citrate

Iron‐cadmium interactions are important in cadmium toxicity. Dietary iron supplements may decrease cadmium retention after oral cadmium exposure but the underlying mechanism is not known. Using a CdS/AgS ion selective electrode to measure [Cd2+] in physiological saline solution at pH 7.4, we show that Fe2+ promotes Cd2+ binding to citrate thereby decreasing the availability of free Cd2+. This suggests the formation of high molecular weight Cd2+‐Fe2+‐citrate complexes. We confirm this suggestion by showing that 109Cd2+ is retained by 1 kDa cut off filters when present with total 50 μM Fe2+ plus 1 mM citrate but not when present with citrate alone. The formation of high molecular weight complexes may prevent Cd2+ absorption. As citrate is part of the diet, we suggest that these iron‐cadmium interactions may contribute to the protective effect of iron against cadmium toxicity.