Copper (II) complexation in northern California rice field waters: an investigation using differential pulse anodic and cathodic stripping voltammetry

Differential pulse anodic stripping voltammetry (DPASV) and competitive ligand equilibration-cathodic stripping voltammetry (CLE-CSV) demonstrated that≥99% of the dissolved copper in five rice field waters, one river water and a catchment basin water collected at sites in northern California is complexed by natural organic matter. Concentrations of natural copper complexing ligands (CL) ranged from 81 to 426 nM determined by CLE-CSV using the competitive ligand 8-hydroxyquinoline and from 156 to 1374 nM using DPASV. Experimental values for conditional stability constants (with respect to free Cu2+) of natural copper–organic ligand complexes (log K′CuL) fell in the range 10.2–11.2 using DPASV and 11.1–11.5 using CLE-CSV. DPASV analyses revealed evidence of organic matter adsorption to the electrode surface at low dissolved organic carbon (DOC) concentrations (i.e. 200 μg l−1 DOC for a 10-min deposition period), for some but not all water samples. Examination of lyophilized, filtered rice field water using scanning electron microscopy (SEM) and pyrolysis gas chromatography mass spectrometry (pyr-GC/MS) did not reveal differences that could be associated with surface active material. Uncomplexed copper accounts for only minor amounts of total copper in rice field waters and natural ligands are expected to influence its effectiveness as an agrochemical.