Effects of bacterial biofilms and iron oxide on copper distribution in a simulated marine environment : an implication to the sediments in the Pearl River estuary

Abstract

In recent years, rapid economic growth and urban development in the Pearl River estuary region have led to excessive discharge of waste into the estuarine and coastal environment. The average heavy metal concentrations in the sediments have generally increased. A comprehensive understanding of the fate and transport of heavy metals is essential to improving water quality management and pollutant mitigation in the Pearl River estuary. In order to improve the conditions for future development in the region, sequential extraction techniques were first used to determine the speciation of heavy metals in sediments collected from six coastal sites in Hong Kong. Different behaviors of trace metal partitioning onto the sediments reflected the degrees of pollution of a selected area. Higher metal concentrations of Cu (202 mg/kg) and Zn (225 mg/kg) were found in the sediment core sample collected at Kellette Bank. Tab Harbour and Cheung Sha Wan also have high levels of lead, copper and zinc in the sediments. The heavy metals have high bioavailabilities if they are associated with the exchangeable, carbonate and reducible phases of the sediments. Slightly over 20% of total Cu and Cr existed in readily available forms in Peng Chau and Kellette Bank. At most of the sampling sites, over 13% of Cu existed in the exchangeable and carbonate forms. A significantly higher percentage of Cu, Pb and Zn were associated with the three readily available fractions. Hence, the environmental concern on remobilization of Cu, Pb and Zn is more immediate than other metals. Metal concentrations increased with decreasing sediment particle size. Also, metal concentrations in the sediments decreased with increasing profile depth. The levels of Pb and Zn associated with phases II and III and the level of Cu associated with phases II and IV of the surface sediments increased sharply. These metals represented the pollutants which were recently introduced into the area as a result of the rapid economic and industrial development. Since these metals were bound to the readily available phases of the surface sediments, metal remobilization could be a concern. Thus copper was selected as the target metal for further experiments. In the second part of this project, a continuous-flow biofilm reactor system was used for studying copper interactions with iron oxides and biogenic materials in a well-controlled laboratory environment. The reactor environment was intended to simulate controlling biotic and abiotic elements of an estuarine system in order to study the distribution and speciation of copper under controlled conditions, but not to mimic a complete natural environment. The biofilm and the hydrous iron oxide components represented the dominant organic and metal oxide phases controlling copper speciation, phase distribution and toxicity in estuarine environments. The reactor consisted of a mixing chamber and a biofilm chamber that contained glass slides for biofilm attachment. The reactor provided controlled physical and chemical conditions, and the use of a chemical-defined artificial aquatic environment allowed the calculation of heavy metal speciation with a chemical equilibrium program (MINEQL+). Pseudomonas atlantica, a common marine bacterium, was employed as a test bacterial strain because of its ability to grow as a biofilm on glass surfaces in the defined medium. This bacterium affected heavy metal distribution in the reactor by adsorbing metal to both attached and suspended cells. The presence of surface coatings of adherent bacteria contributed to significant adsorption of copper to the reactor surfaces. Biofilms with 170 - 284 mequiv COD/m2 of total oxidizable material increased copper adsorption relative to bare glass by a factor of five. Attached cells in the biofilms contained significant quantities of extracellular organic material contributed to additional copper adsorption. A greater effect would be expected in natural aquatic systems. These results are an indication of the importance of the biofilm effect on copper distribution. The bacterium provides a potential model to evaluate the physical and chemical interactions between copper and the biophase in marine environments. Copper distribution in the biofilm reactor was determined experimentally. The copper adsorption to biofilms was found to fit Langmuir isotherm. Pretreatment of glass slides with colloidal iron also significantly increased the copper adsorption relative to bare glass. Copper adsorption to adsorbed iron also fit a Langmuir isotherm. The isotherms obtained in this research can be used to estimate the fate of copper in natural aquatic environments. It was also found that copper binding to the composite glass/iron/biofilm surface could be predicted by summing the predicted copper adsorption to each component using Langmuir adsorption isotherms. Significant inhibitions to the bacterial growth were observed when the copper concentrations were above 0.5 ppm in the artificial seawater medium. However, the presence of glycine and EDTA significantly reduced the toxic effect of copper on bacterial growth. Complexation of copper with glycine and EDTA rendered the copper ions less available to the bacterial cells. In the present investigations, the effects of bacterial biofilms and iron oxide on metal distribution in a simulated marine environment were retrieved. The experimentally determined Langmuir isotherm parameters for copper absorption to biofilms and iron oxide, qmax and b, can provide a basis for predictions of the distribution and speciation of copper in a marine environment. Models can also be developed to simulate the copper adsorption to natural suspended material based on the summation of adsorption to individual surface components present in natural suspended material.