Trace metal speciation in complex aquatic environments : the copper, cadmium, ferrihydrite, phthalic acid and bacterial system

Abstract

Trace metal speciation in aquatic environments is inherently complex due to the large number of possible interactions with dissolved and particulate components. Adsorption onto iron oxyhydroxide and bacterial surfaces, as well as the formation of metal-ligand complexes can play important roles in controlling the fate and transport of trace metals in natural environments. The objective of this study is to describe and understand metal speciation and distribution in a complex biogeochemical system by incrementally increasing the complexity from simple binary systems to a dynamic quaternary system containing a trace metal, iron oxide and bacteria that are active and metabolizing an organic ligand. Copper, cadmium, and phthalic acid (H2Lp) adsorption onto ferrihydrite in binary systems was well reproduced using the diffuse layer model (DLM). The adsorption of H2Lp adsorption was analogous to that of inorganic diprotic acids in terms of the relationship between the adsorption constants and acidity constants. In ternary systems H2Lp caused Cu2+ or Cd2+ adsorption to be either enhanced (due to surface ternary complex formation) or inhibited (due to solution complex formation) depending on the conditions. The DLM could only describe the effect of H2Lp on metal ion sorption by including ternary complexes of the form ≡FeOHMLp (0), where ≡FeOH is a surface site and M is Cu or Cd. The relationship between binary metal adsorption constants and the ternary complex adsorption constants from this and previous studies suggest several properties of ternary complexes. First, ternary complex structures on both ferrihydrite and goethite are either the same or similar. Second, those cations having large adsorption constants also have large equilibrium constants for ternary complex formation. Third, ligands forming stronger solution complexes with cations will also form stronger surface ternary complexes but because of the strong solution complexes these ligands will not necessarily enhance cation adsorption. The bacterial strain Comamonas spp. was isolated from the activated sludge of a wastewater treatment plant. Comamonas spp. could effectively degrade H2Lp in the presence of Cd2+ and ferrihydrite and was therefore chosen to study the effect of H2Lp degradation on Cd2+ speciation. Proton, cadmium and H2Lp adsorption onto Comamonas spp. were measured. The Comamonas spp. titration curve is flatter than that of ferrihydrite, indicating a higher degree of site heterogeneity at the bacterial surface. Adsorption edges of Cd2+ adsorption onto Comamonas spp. occurred over about 4~5 pH units compared to those of ferrihydrite which occurred over ≈ 2 pH units on a dry weight basis. Comamonas spp. can accumulate a larger amount of Cd2+ than ferrihydrite especially under lower pH conditions. Proton and Cd2+ adsorption onto Comamonas spp. cells over a wide sorbent/sorbate and pH range was reasonably well described by a four site non-electrostatic model. The acid-base and Cd2+ adsorption behaviour of Comamonas spp. in this work were within the range of studies of bacteria adsorption. Phthalic acid adsorption onto inactive Comamonas spp. was negligible over a pH range of 3 to 8 and became significant only at pH < 3 where H2Lp was fully protonated. This is consistent with the proposed mechanism for ligand adsorption onto bacterial surfaces which involved a balance between hydrophobic interaction and electrostatic repulsion. The presence of H2Lp decreased Cd2+ adsorption onto Comamonas spp. due to competition for Cd2+ between the bacterial cell surface and the formation of solution complexes of Cd2+. This was accurately modelled with the Cd-Lp solution species indicating that no significant surface ternary interaction occurred between Cd2+, phthalic acid and Comamonas spp.. Cadmium adsorption onto ferrihydrite-Comamonas spp. mixtures was slightly less than the simple additive predicted adsorption of ferrihydrite plus Comamonas spp.. This suggests there is a weak interaction between ferrihydrite and Comamonas spp. and this interaction could be modelled by including a generic reaction between the ferrihydrite and Comamonas spp. surface sites. Cadmium distribution in a system of inactive Comamonas spp.-ferrihydrite in the absence and presence of H2Lp could be predicted by combining the ferrihydrite and bacteria models with the inclusion of the ferrihydrite-bacteria interaction. The effects of H2Lp degradation on Cd2+ distribution were investigated in dynamic systems with live bacteria. Results showed that Cd2+ adsorption in these dynamic systems was reasonably estimated with the model parameters developed in the proceeding experiments though uncertainty exists in the dynamic process with regards to H2Lp biodegradation products and changes in the bacteria population. This thesis was therefore able to provide a better understanding of metal speciation in complex and heterogeneous realistic environments by experimentally examining and modelling metal speciation and distribution in various systems with increasing complexity. This helps to bridge the gap of quantitative description of metal speciation from simple laboratory experiment systems to real world systems, both natural and engineered.