Kinetically inert Cu in coastal waters

Area of Science:

• Environmental Chemistry
• Analytical Chemistry
• Marine Science

Background:

• Natural waters contain metals bound by strong ligands, often assumed to be reversible.
• Existing analytical methods struggle to differentiate between reversible and kinetically inert metal-bound compounds.
• Understanding copper speciation is crucial for aquatic ecosystem health.

Purpose of the Study:

• To investigate the nature of strongly bound copper in New England coastal waters.
• To differentiate between kinetically inert and reversibly bound copper species.
• To determine the role of colloidal material in copper binding.

Main Methods:

• Modified competitive ligand exchange adsorptive cathodic stripping voltammetry.
• Size fractionation using ultrafiltration with varying pore sizes (0.2 µm and 0.02 µm).
• Modeling of reversible copper complexes using conditional stability constants.

Main Results:

• Most apparently strongly bound copper (log K ≥ 13) was found to be kinetically inert.
• A significant fraction of inert copper was retained by a 0.02-µm filter, indicating sequestration in colloidal matter.
• Reversibly bound copper complexes had conditional stability constants of 10(10)-10(13), comparable to humic substances.
• Both inert and reversible fractions influenced free copper ion concentration ([Cu2+]).

Conclusions:

• Copper in coastal waters exists in both kinetically inert and reversible forms.
• Colloidal material plays a significant role in sequestering inert copper.
• Accurate speciation analysis requires methods that distinguish inert from reversible metal binding.
• Both forms of copper binding are critical for determining copper bioavailability and aquatic chemistry.