May 21, 2007 Acadia University's :: Home  |  News   |  Site Search  |  E-mail Search  |  A-Z  |  Directories  |  Contact
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John Murimboh

Ph.D. (Carleton University)
B.Sc. (McMaster University)

Assistant Professor
Department of Chemistry
Acadia University
6 University Avenue
Elliott Hall, Rm. 217
Wolfville, Nova Scotia, B4P 2R6

Tel: 902-585-1172
Fax: 902-585-1114












































































































































































Research Interests

Current Research Projects
1. Trace metal speciation and bioavailability in natural waters
2. Development of novel in situ techniques and methods for chemical speciation
3. Analysis of air particulate matter by Scanning Electron Microscopy
4. Mitigation of acidic waters: effects on the speciation of aluminum


Previous Research Projects
5. Characterization of petroleum hydrocarbons in contaminated soils


1. Trace metal speciation and bioavailability in natural waters

The delicate balance between the role of trace metals as micronutrients and toxicants plays a crucial role in the maintenance of life in natural systems. Trace metals exist in a variety of chemical forms in the aquatic environment, ranging from free metal ions to small inorganic complexes to large complexes with natural organic matter (NOM) — each with its own unique properties. The biological availability, and hence toxicity, of metals in aquatic systems is strongly dependent on its chemical speciation. Knowledge of the metal distribution among their different physical and chemical forms (i.e. chemical speciation) is therefore essential for predicting their fate and environmental impacts.

Formed by the decomposition of decaying plant and animal matter, NOM, such as humic and fulvic acids, plays a key role in controlling chemical speciation and metal bioavailability through the regulation of free metal ion concentrations, which has been proposed as an indicator of the biological impact of metals in natural waters. Humic substances are composed of a wide variety of molecules of many differing sizes with many ways to orient themselves by twisting, bending, compressing and expanding. They are loosely held together by weak forces in a colloidal state. Changes in the ambient conditions (e.g. pH, ionic strength, type of metals present, degree of metal loading) can result in fragmentation and rapid rearrangement of the humic colloids. As a result, humic substances have been described as dynamic supermixtures of chemically and physically heterogeneous components, whose metal binding properties are influenced by environmental factors. Knowledge about the nature of the interactions between humic substances and metal ions is therefore essential for improving our understanding of the mechanisms of that determine the transport, and the fate and bioavailability of trace metals in natural waters.

Metal species are characterized by their physicochemical parameters, including dissociation rate constants (kinetic reactivity), stability constants (thermodynamic stability), diffusion coefficients (mobility), free metal ion concentrations and ligand concentrations. The techniques being employed to investigate trace metal speciation in aqueous environmental samples include equilibrium-based techniques, such as Pseudopolarography; as well as kinetic-based techniques, such as Diffusive Gradients in Thin Films (DGT) and Competing Ligand Exchange Methods.

A proposed structure of a humic acid molecule (H.R. Schulten, M. Schnitzer, Naturwissenschaften, 1993, 80, 27) showing a wide range of functional groups (e.g. phelolic, carboxylic, salicylic, phthalate, nitrogen-bearing groups) that can bind to metal ions. The brown colour of many freshwaters is due to the presence of humic and fulvic acids, which help to moderate metal toxicity through the complexation of metal pollutants.


2. Development of novel in situ techniques and methods for chemical speciation

This research program is directed at the development of sophisticated models and in situ sample devices for the long-term monitoring and interpretation of trace metal speciation and bioavailability in the aquatic environment. The sampling devices will mimic the diffusive layer at the biological membranes of aquatic biota to provide an in situ, time-averaged estimate of bioavailable metal concentrations. Development of in situ approaches for multi-element speciation techniques in natural waters constitutes an important advance towards overcoming artifacts that plague laboratory-based techniques such as contamination, analyte loss and sample transformation during sample collection, handling and storage.

The in situ sampling device will consist of a four-layer system: 1) a metal binding layer composed of a solid-phase ion exchange resin (e.g. Chelex 100), 2) a porous membrane to hold the ion exchange resin, 3) a well-defined, stagnant boundary layer (e.g. deionized water) which controls mass transport to the binding layer, and 4) a size-selective outer membrane for size fractionation. Labile metals in the bulk solution diffuse through the stagnant boundary layer and are concentrated in the resin. Quantification of labile species is important because lability can be correlated with metal bioavailablility. The sampling device is based on the coupled diffusion of the metal complex, ML, and the free metal ion, M, from the sample medium through a stagnant boundary layer, where only M is bound by the receiving phase. This shifts the equilibrium between M and ML, inducing a steady-state concentration gradient within the boundary layer. ML is ‘labile’ when the formation and dissociation kinetics are sufficiently fast that the total metal flux towards the surface of the binding phase is controlled exclusively by the coupled diffusion of M and ML. Metal complexes not able to dissociate within the boundary layer are ‘non-labile’. The thickness of the boundary layer controls the analytical timescale of measurement; hence, it is the critical parameter in defining the metal species that are measured. Metal species are characterized by the physicochemical parameters that define lability (and hence bioavailability) at an interface: dissociation rate constants (reactivity), stability constants (thermodynamic stability), diffusion coefficients (mobility), and free metal ion concentration. Labile metals concentrated by the sampling device are eluted from the resin with 1 M nitric acid. The metal concentrations in the eluate can be determined by Graphite Furnace Atomic Absorption Spectroscopy, Inductively Coupled Plasma – Mass Spectrometry, or Anodic Stripping Voltammetry.


A steady-state concentration gradient of ML is induced within the boundary layer. Metal complexes are classified as labile, quasi-labile or non-labile depending on its ability to dissociate before reaching the binding phase. An array of Diffusive Gradients in Thin Film (DGT) devices, which are similar in design to the proposed sampling device.


3. Analysis of air particulate matter by Scanning Electron Microscopy

The widespread use of radionuclides in a wide range of industries such as petroleum engineering (well logging for oil exploration), the airline industry (to check welds and structural integrity) and medicine (cancer treatment and diagnostics) has lead to concerns that these sources can be used to create radiological dispersal devices (RDDs) for terrorist attacks. Scanning Electron Microscopy and Energy Dispersive X-ray Spectroscopy are being used to characterize the size, shape, morphology and chemical composition of airborne particulate matter from RDDs. Canada has initiated testing on ceramic materials, focusing on SrTiO3, CaTiO3 and CeO2. SrTiO3 is used to simulate Sr-90 used in Radioisotope Thermoelectric Generators (a generator which obtains power from radioactive decay), CaTiO3 is used to simulate SrTiO3 for outdoor testing, and CeO2 is used as a mechanical surrogate for UO2, PuO2 & AmO2.

This micro-analytical approach has the advantage of providing information about the behaviour and distribution of particulate material that cannot be obtained by traditional bulk analytical techniques. The threat from RDDs has been a topic of considerable debate over the past few years. Expert opinions on the risks from RDDs vary wildly. This research forms part of a multidisciplinary counter terrorism program with scientists from Defence Research and Development Canada, Health Canada, the University of British Columbia and Carleton University that will asses the risks from RDDs.

Ti element map for SrTiO3 particles

EDS spectrum for SrTiO3 particles

Mottled SrTiO3 particle


Partially Melted SrTiO3 particle

Mottled CaTiO3 particle

Wetted CeO2 particle

Secondary Electron Image of CeO2 and Si particles Backscatter Electron Image of CeO2 and Si particles



4. Mitigation of acidic waters: effects on the speciation of aluminum

Aluminum is known to contribute to fish mortality in lakes and rivers in Nova Scotia. Aluminum toxicity is dependant not only on the total dissolved Al concentration, but also on the Al speciation. Inorganic monomeric aluminum species (e.g. Al3+, Al(OH)2+ and Al(OH)2+) are reported to be toxic, and these forms predominate at low pH. Acid rain combined with low alkalinity have resulted in acidic rivers and lakes in Nova Scotia, thereby creating conditions that favour Al toxicity. Current methods for mitigation of acidic freshwaters involve addition of carbonate ions, usually done through the addition of lime (CaCO3) using dosers (lime titrating equipment). However, liming is expensive and must be applied continually in order to remain effective.

The addition of a novel carbonate-rich industrial waste product to acidic waters has been proposed as being an inexpensive and potentially more effective alternative to traditional liming practices, as the material is abundant, and the carbonate ions are available at twice the level of that in limestone. The proposed approach involves investigating the effects of introducing the new carbonate material into water samples collected from the Felix Mill Brook (Claire, NS) on the pH and aluminum speciation using ion exchange technique(s) as well as Diffusive Gradients in Thin Films (DGT).

Soil porewater samplers at the Felix Mill Brook.


5. Characterization of petroleum hydrocarbons in contaminated soils

Contamination of soils by petroleum hydrocarbons is a widespread environmental problem. The source of such contamination is primarily accidental release from gas stations and fuel depots. Gasoline and diesel fuel have deleterious effects on plant and animal life, and often leaches into local groundwater supplies, posing a potential hazard to human health. Contaminated soils can reduce the usability of land for development, and weathered petroleum residuals may stay bound to soils for years. The chemical composition of petroleum products is complex and often subject to temporal and spatial variations in the soil environment. Hence, little is known about their potential for health or environmental impacts. Total petroleum hydrocarbon concentrations serve as gross measures of petroleum contamination; however, more detailed information is required to assess the risk to human and ecosystem health.

The objectives of this research were to collect and analyze soils from six sites located throughout the Maritimes that are potentially contaminated with gasoline and/or diesel fuel. The study areas are sites of decommissioned fuel depots and service stations. Both field and laboratory analyses of hydrocarbon contaminants were performed in order to determine their concentrations and to identify the compounds present.

The Gastechor Hydrocarbon Surveyor (“Sniffer”) was used to measure total hydrocarbon concentrations in the soil based on catalytic combustion of volatile species. It is easy to use and portable, making it ideal for field measurements. For soils, hydrocarbon concentrations in the headspace of enclosed samples are directly proportional to concentrations in the soil. Solid Phase Microextraction – Gas Chromatography – Mass Spectrometry (SPME-GC-MS) was used to identify and quantify the hydrocarbon compounds present in the soil samples. SPME is a sensitive and relatively inexpensive sample preparation technique developed by Janusz Pawlyszyn (Waterloo). It uses non-exhaustive extraction to preconcentrate the sample in a thin fibre, made up of an inner core of fused silica, and an outer coating composed of a solid and/or liquid polymer sorbent. A thin, stagnant layer develops adjacent to the fibre, known as the boundary layer. Over time, the analyte is concentrated in the fibre, so that at equilibrium, the concentration in the fibre is greater than in the sample. Following extraction of the analyte, the fibre is inserted into a Gas Chromatograph (GC) injector port, and the sample is thermally desorbed onto the GC column for analysis.

Digging a hole

Taking "Sniffer" measurements

Gasoline Chromatogram

Diesel Chomatogram


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