45879-G2
Stabilization of Metal-Sulfide Nanoparticles by Natural Organic Matter
This on-going research seeks to identify interactions occurring between dissolved humic materials and nanoparticulate metal-sulfides in the natural environment. Natural petroleum deposits contain impurities such as sulfur and trace metals that can interfere with the production and utilization of petroleum-based products. When metals such as Zn, Hg, Cd, and Cu are present during diagenesis of carbonaceous material, they are likely to be in the form of metal-sulfides. Nanoscale metal-sulfide particles (<100 nm diameter) are formed as intermediates of precipitation reactions in sediment and soil porewater. These species can persist at the nanoscale if further crystal growth and aggregation are kinetically hindered. This ACS PRF grant supported research activities with the following objectives: 1) Investigate the occurrence of Zn-sulfide nanoparticles in waters surrounding hydrothermal vent habitats; 2) Understand the importance of natural organic acids for promoting ZnS and HgS nanoparticles. Both of these objectives focused on potential surface interactions occurring between the particles and common natural organic acids that are substrates for early diagenesis processes. In the first objective, the funds contributed to a larger project related to the PI’s research on metal-sulfide speciation at deep-ocean hydrothermal vents. This study sought to identify the importance of Fe and Zn for controlling sulfide speciation in Eastern Lau Spreading Center (ELSC), located in the Lau Basin in the southern Pacific Ocean. Results of field sampling from a cruise in June 2005 indicated that vent fluid chemistry in the Lau Basin is unique from other well-studied vents in the Pacific Ocean. In particular, our results indicated north-to-south variability in vent fluid composition with regards to the ratio of total Fe to total Zn in the fluids from high temperature vents (>250°C). With the high Zn concentrations relative to Fe, our speciation calculations indicated that sphalerite (ZnS(s)) precipitation was thermodynamically favorable, whereas in many samples precipitation of mackinawite (FeS(s)) was not favorable. Furthermore, laboratory analyses of several filtered vent fluids indicated that ZnS nanoparticles were present. Anodic stripping voltammetry indicated that a fraction of the Zn(II) was in a strongly coordinated species, and dissolved organic carbon (needed to surface-cap nanoparticles and prevent particle growth/aggregation) was present in micromolar levels in the fluids). This study provides an example of naturally-occurring ZnS nanoparticles. The results were published in 2008 in Geochemical Transactions. The second objective of this project is to identify molecular-scale interactions occurring between dissolved natural organics and metal-sulfide nanoparticles as they precipitate in the aquatic environment. Nanoparticles of ZnS and other metals are known to occur in the environment; however, the processes that keep these particles at the nanoscale are not well understood. We conducted laboratory experiments to investigate the kinetics of metal-sulfide particle growth in water containing humic substances and simple organic acids. Using dynamic light scattering to monitor the size of particles over time, we observed that humic acids and thiol-containing organics (cysteine and thioglycolic acid) were capable of slowing growth of ZnS and HgS particles precipitating in solution. Under solutions conditions that represent the natural environment (e.g., metal and sulfur concentrations, pH, ionic strength), these organic acids were able to stabilize nanoscale particles (< 100 nm diameter) for periods of hrs to days. This research is noteworthy because it identifies a potential mechanism under environmentally relevant conditions that would enable metal-sulfide particles to persist at the nanoscale in natural aquatic systems. These findings offer a new perspective to the large body of research on NOM-colloid interactions by suggesting a thiol specific NOM binding mechanism that is unique to metal-sulfide particles. The results of the ZnS work were recently accepted for publication in Environmental Science & Technology.
