Equilibrium and kinetic partitioning of trace metals with respect to sorption processes on natural particles in South San Francisco Bay, California.

摘要

Laboratory microcosm experiments were carried out to assess the timescales associated with equilibrium partitioning of Cd, Ni, Cu, Zn and Pb with respect to sorption processes on natural particles from South San Francisco Bay, CA. These studies present a novel approach to tracing sorption kinetics and the evolution of equilibrium conditions using low abundance (−1% occurrence) stable isotopes of these metals as tracers in conjunction with high-resolution inductively coupled plasma mass spectrometry (HR-ICPMS). Sorption kinetic rate constants were modeled from the concentration time series data, assuming pseudo-first-order kinetics. The best estimates of forward and backward kinetic rate constants for Cd, Ni, Cu, Zn and Pb respectively are K′ _f = 0.018, 0.03, 0.07, 0.12, 0.44 d−1 and k_b = 0.12, 0.13, 0.12, 0.15, 0.018 d−1. These results predict that sorption equilibria in the South Bay range from a month for Cd and Ni, to three weeks for Cu, to two weeks for Zn, to one week for Pb. These timescales are much slower than equilibrium times for metal sorption onto synthetic pure phase particles (on the order of seconds) and are similar to particle residence times in the water column of South San Francisco Bay, suggesting that these sorption reactions can be kinetically limited in natural systems.; Experimentally determined kinetic rate constants were used to calculate predicted equilibrium partitioning coefficients (K_Ds). The results from these studies find Cd and Pb to behave as end-member metals with respect to sorption processes in South San Francisco Bay, both in terms of determined kinetic constants and sorption equilibrium timescales, as well as in observed and predicted equilibrium partitioning K_Ds. Predicted equilibrium K_Ds closely matched observed (measured) K_Ds for these metals in the South Bay (within 0.2 log units). These data are significant as they identify the underlying kinetic controls on partitioning commonly observed for these metals in natural aquatic systems. The agreement between predicted and observed equilibrium partitioning also indicate that this overall approach to modeling sorption kinetics can successfully integrate the complexities of metal sorption to heterogeneous natural particles in a matrix of natural estuarine waters while assessing kinetic parameters.; A method development study was also undertaken to create an automated, flow injection procedure using a chelating resin in conjunction with HR-ICPMS to determine dissolved metal concentrations in estuarine waters. For the final study, this rapid method was then used to replace a more time and labor-intensive organic extraction technique required to pre-concentrate dissolved metals and remove the salt matrix of the estuarine samples prior to analysis with HR-ICPMS.