Jim Braven

The formation of humic coatings on mineral particles under simulated estuarine conditions : a mechanistic study

Abstract The adsorption of three humic substances isolated from the River Dodder (Eire) and one commercial humic acid (Aldrich) by six minerals was studied and found to be influenced by mineral type, the nature of the humics and by the solution characteristics. Adsorption was heavily pH-dependent, with lower uptake at higher pH values due to increased ionization of the humic substances. The amount of humic substances adsorbed was in the order: Aldrich humic acid > Dodder humic acid > Dodder fulvic acid > Dodder hydrophilic macromolecular acid corresponding to a decreasing aromatic carbon content and increasing O/C ratio. Desorption experiments showed Aldrich humic acid was the most difficult to be desorbed, followed by Dodder humic acid, Dodder fulvic acid then Dodder hydrophilic macromolecular acid. Humic uptake was found to increase with ionic strength due to the neutralization of charges on both mineral surfaces and humic substances, and more importantly, the compression of diffuse double layers; this also showed that van der Waals attraction is an important part of the interactive forces between humic substances and mineral particles. The amount of humic substances adsorbed varied with mineral type, particle size and surface area, with aluminium oxide having the highest capacity. The effect of temperature on humic adsorption was shown to vary with humic type and two types of reaction mechanism have been proposed responsible for the humic-clay interactions.

Survival of polycyclic aromatic hydrocarbons during diesel combustion

The application of a radiotracer technique to investigate the sources of polycyclic aromatic hydrocarbons (PAH) in diesel exhaust emissions is described. In separate experiments, 14 C-radiolabeled naphthalene, 2-methylnaphthalene (2-MeNp), fluorene, pyrene, and benzo[a]pyrene (B[a]P) were each added to diesel fuel, which was combusted in a 2-L direct injection Perkins Prima diesel engine. Exhaust samples were collected using a novel exhaust gas sampling device designed for sampling organic species in automobile exhaust emissions. Survivals for these PAH were 0.87% for fluorene, 0.54% for 2-MeNp, 0.47% for naphthalene, 0.17% for pyrene, and 0.04% for B[a]P. A linear relationship was observed between the extent to which individual PAH survived combustion and the energy level of the lowest unoccupied molecular orbital (LUMO) of the molecule. LUMO energy levels for each molecule were calculated from Huckel molecular orbital theory. The relationship observed in the current experiment suggests that, forthese PAH and under steady-state engine conditions, it is the chemical kinetics of reactions occurring in the combustion chamber as opposed to thermodynamic stabilities that determine the extent of PAH survival during diesel combustion. Using this relationship, it should be possible to predict the extent of PAH survival in diesel emissions from a knowledge of the PAH composition of the fuel.

An accurate and stable nitrate-selective electrode for the in situ determination of nitrate in agricultural drainage waters

A field evaluation of a novel nitrate-ion selective electrode (ISE) was undertaken by continuous immersion over a period of 5 months in agricultural drainage weirs. The nitrate sensor N,N,N-triallyl leucine betaine was covalently attached to polystyrene-block-polybutadiene-block-polystyrene (SBS) using a free radical initiated co-polymerisation, to produce a rubbery membrane which was incorporated into a commercially available electrode body. A measurement unit was constructed comprising the nitrate-ISEs, a reference electrode and a temperature probe connected through a pre-amplifier to a data-logger and battery supply. A temperature correction algorithm was developed to accomodate the temperature changes encountered in the drainage weirs. The nitrate results obtained with the ISEs at hourly intervals compared very favourably (R2 = 0.99) with those obtained with laboratory automated chemical determinations made on contemporaneous samples of drainage in a concentration range 0.47-16 ppm nitrate-N. The ISEs did not require re-calibration and no deterioration in performance or fouling of the membrane surface was observed over four months of deployment.

Automatic continuous river monitoring of nitrate using a novel ion-selective electrode

On-site continuous measurement of nitrate in the River Taw situated in Devon (UK) was performed automatically for two months using a recently developed ion-selective electrode (ISE). A non-submersible electrode support system was developed and a field instrument was constructed comprising the nitrate-ISEs, a reference electrode and a temperature probe connected through a pre-amplifier to a data-logger and battery supply. The nitrate-ISE was constructed using a commercially available electrode body with a membrane incorporated into the tip. This rubbery membrane contained the sensor molecule (N,N,N-triallylleucine betaine chloride, 6.5% m/m) covalently bound to polystyrene-block-polybutadiene-block-polystyrene, SBS, (43.5% m/m), with 2-nitrophenyloctyl ether (2-NPOE) as mediator (40% m/m). The Nernstian slope was -59.1 mV per decade over a linear range of 1400-0.07 mg L(-1) nitrate-N. The limit of detection was 0.007 mg L(-1) nitrate-N with a selectivity coefficient for nitrate against chloride (k(pot)NO3-, Cl-) of 3.4 x 10(-3). The speed of response was less than a minute over the linear Nernstian range. The lifetime in the laboratory exceeded 5 months with no potentiometric drift over the linear Nernstian range. A temperature correction algorithm was also used to accomodate the temperature changes encountered in the river. The river nitrate results obtained with the ISEs at hourly intervals compared very favourably (R2 = 0.9) with those obtained with laboratory automated chemical determinations made on contemporaneous river samples over a concentration range 0.55-2.00 mg L(-1) nitrate-N. The ISEs did not require re-calibration and no deterioration in performance or fouling of the membrane surface was observed during 40 days.

Development of a sensitive nitrate-selective electrode for on-site use in fresh waters

Twenty seven membranes hot pressed from mixtures of a variety of quaternary ammonium salt sensor molecules and mediators in Krynac polymer have been evaluated as nitrate-selective electrode membranes suitable for use in fresh waters. The best performance was obtained from a covalently bound triallyldecylammonium nitrate sensor having a Nernstian slope of –57.7 mV decade–1 over a nitrate range of 1 × 10–1 to 1.2 × 10–5 mol dm–3 NO3– with a limit of detection of 8.8 × 10–6 mol dm–3 NO3–.Membrane performance remained unchanged for at least six months use. Interfering ions, temperature and pH dependency studies are reported . Selectivity towards chloride at the 10–2 mol dm–3 level was measured as kpotNO3–,Cl– 1 × 10–4.

Nitrate-selective electrodes with polymer membranes containing immobilised sensors

Several ion-selective electrodes for the determination of nitrate have been fabricated using polymer membranes containing covalently bound quaternary ammonium salts as sensor groups. Of the two sensor groups compared triallylbutylammonium bromide gave superior results to triallylethylammonium bromide. Hot-pressed membranes gave superior performance to solvent-cast membranes, producing more robust, cross-linked membranes into which a mediator group could be introduced. The best electrode was fabricated with a membrane containing triallylbutylammonium bromide (5.6% m/m), 2-nitrophenyl octyl ether (39.7% m/m) and dicumyl peroxide (7.3% m/m) in poly(acrylonitrile-butadiene) using hot pressing at 150 °C for 10 min at a pressure of 15 tons on a 5 in diameter ram. The cross-link density was 1.7 × 10–5 mol cross-link cm–3. The response to nitrate was Nernstian in the range 1 × 10–1–1 × 10–4M nitrate, with a limit of detection of 3 × 10–5M. The selectivity for chloride ([graphic omitted]) was 1.2 × 10–2 and for a range of other ions was comparable to commercially available electrodes. The covalently bound sensor electrode offered superior mechanical strength.

Changes in dissolved free amino acid composition in sea water associated with phytoplankton populations

Experiments investigating the way in which natural phytoplankton cells affect the composition of dissolved free amino acids in sea water are described. Evidence is presented which demonstrates an active release of amino acids by two living phytoplankton species into the environment. A similar process is not caused by dead cells of the same species.

Tefluthrin Sorption to Mineral Particles: Role of Particle Organic Coatings

Abstract The sorption of tefluthrin was studied on “pure” clay minerals and those that had been coated with aquatic humic substances over a mass percent carbon range of 0.02 to 2.15. Tefluthrin sorption onto humic-coated minerals was significantly greater than on to the clean minerals and increased with increasing quantities of sorbed humic substances. Humic acid, the most aromatic coating, was the strongest sorbent, followed by fulvic acid, hydrophilic macromolecular acid and natural coatings on estuarine suspended particles. This shows the significant impact of humic coatings on the sorptive capacity of mineral particles. The sorption was linear, also consistent with the operation of a partition process. The partition coefficient normalised to organic carbon (Koc) after deduction of the contribution from the clean mineral, ranged from 120000 to 770000 and was highest for the most aromatic humic acid fraction.

Nitrate-selective electrodes containing immobilized ion exchangers within a rubbery membrane with controlled cross-link density

A range of ion-selective electrodes have been produced for the determination of nitrate using polymeric membranes containing immobilized quaternary ammonium salts as the ion exchanger. The use of acrylonitrile–butadiene copolymer with a 19% acrylonitrile content as the polymer matrix produced electrodes with excellent [graphic omitted] values of 1.1 × 10–3 but with short lifetimes. Varying the cross-link density of the membranes from 0.6 × 10–5 to 11.0 × 10–5 mol cross-link cm–3 did not produce any significant difference in electrochemical behaviour. Two novel ion exchangers, triallyloctylammonium bromide and triallyldodecylammonium bromide, were successfully synthesized and fabricated into membranes. Triallyloctylammonium bromide gave the longest lifetime membranes and the best electrode was fabricated from the acrylonitrile–butadiene copolymer with a 50% acrylonitrile content and 6.5% m/m triallyloctylammonium bromide, 7.2% m/m dicumyl peroxide and 39.7% 2-nitrophenyl octyl ether. The electrode had a lifetime in excess of 500 d and the response to nitrate was Nernstian in the range 1 × 10–1–1 × 10–4 mol dm–3 of nitrate. The limit of detection was 4.5 × 10–5 mol dm–3 and the selectivity coefficient for chloride over nitrate [graphic omitted] was 5.3 × 10–3. The immobilized ion exchanger membrane electrode offered superior lifetime and mechanical strength.

Novel synthesis and characterisation of ruthenium tris (4-methyl-4′-vinyl-2,2′-bipyridine) complexes

Abstract The synthesis of the ligand 4-methyl-4′-vinyl-2,2′-bipyridine is discussed. Methods previously reported presented problems in terms of yield and purity of a precursor. Traces of starting material (4,4′-dimethyl-2,2′-bipyridine) contaminated the product and this contaminant carried through to the synthesis of 4-methyl-4′-vinyl-2,2′-bipyridine. Thus, upon complexation to form ruthenium tris (4-methyl-4′-vinyl-2,2′-bipyridine) bis (hexafluorophosphate), the bis-, mono- and non-vinyl complexes were also present. Electropolymerisation of ruthenium tris (4-methyl-4′-vinyl-2,2′-bipyridine) bis (hexafluorophosphate) to produce a high quality polymer film requires the maximum number of vinyl ligands per molecule of complex. The relative percentages of complexes within the mixture can be calculated using proton NMR. This method is based upon the integration values of the vinyl resonances of the tris- and bis-compounds. However, a simple mathematical model has been developed that can predict the percentage composition of the mixture prior to complexation by taking into account the purity of 4-methyl-4′-vinyl-2,2′-bipyridine. The novel syntheses of 4-(2-hydroxypropyl)-4′-methyl-2,2′-bipyridine, 4-methyl-4′-(E-prop-2-enyl)-2,2′-bipyridine and ruthenium tris (4-methyl-4′-(E-prop-2-enyl-2,2′-bipyridine) bis (hexafluorophosphate) are also described. Full spectral data for all compounds, together with novel data for ruthenium tris (4-methyl-4′-vinyl-2,2′-bipyridine) bis (hexafluorophosphate), are reported.