Copper binding by dissolved organic matter: I. Suwannee River fulvic acid equilibria

A cupric ion-selective electrode measured free Cu in solutions of Suwannee River fulvic acid (FA) in a series of 30 titrations carried out both at variable and at constant (5.14, 7.00, 8.44) pH. Total Cu varied 0.1–100 μM, dissolved organic carbon (DOC) 1–10 mg C/l, Ca and Mg 0–10 mM, and ionic strength 0.005–0.1. Copper complexation by FA is first order in DOC for 1–10 mg C/liter, and variable-order in pH. Increasing Ca ++ or Mg ++ from 0 to 10 mM slightly increases Cu ++ activity, while increasing ionic strength from 0.005 to 0.1 significantly increases Cu ++ activity. An empirical N- site model was calibrated using a pooled set of six titrations with varying pH and DOC. Five binding components of varying proton dependence predict Cu binding by FA over a range of pH, DOC and total Cu in two verification tests of the model parameters. Parameters in this and other models tested are only empirical constructs.