Comparison of zinc complexation properties of dissolved natural organic matter from different surface waters

The zinc binding characteristics of natural organic matter (NOM) from several representative surface waters were studied and compared. NOM samples were concentrated by reverse osmosis. The samples were treated in the laboratory to remove trace metals. Square wave anodic stripping voltammetry (SWASV) was used to study zinc complexing properties of those NOM samples at fixed pH, ionic strength, and dissolved organic carbon (DOC) concentrations. Experimental data were compared to the predictions from the Windermere Humic Aqueous Model (WHAM) Version VI. At the same pH, ionic strength, and temperature, the zinc titration curves for NOM samples from different surface water sources tested in our study almost overlapped each other, indicating similarity in zinc binding properties of the NOM. A discrete two-site model gave good fits to our experimental titration data. Non-linear fitting by FITEQL 4.0 shows that the conditional zinc binding constants at the same pH are similar for NOM from different sources, indicating that zinc complexation characteristics of the NOM used in our study do not depend on their origin and one set of binding parameters can be used to represent Zn-NOM complexation for NOM samples from those different surface water sources representing geographically diverse locations. In addition, the total ligand concentrations (L1,T, L2,T, and LT) of all NOM show no observable gradation with increasing pH (L1,T=2.06±0.80 mmol/g carbon; L2,T=0.12±0.04 mmol/g carbon; LT=2.18±0.78 mmol/g carbon), while the conditional binding constants of zinc by NOM (logKZnLc) show a linear increase with increasing pH(logK1c(pH=6.0)=4.69±0.25; logK1c(pH=7.0)=4.94±0.10; logK1c(pH=8.0)=5.25±0.006; logK2c(pH=6.0)=6.29±0.13; logK2c(pH=7.0)=6.55±0.08; logK2c(pH=8.0)=6.86±0.023) with a slope of ca. 0.28, indicating the zinc-NOM complexes become more stable at higher pH. The WHAM VI predicted free zinc ion activities at high zinc concentrations agree with our experimental results at pH 6.0, 7.0, and 8.0. However, the zinc binding of these NOM samples is over estimated by WHAM VI at zinc concentrations below 10−6 M at pH 8.0.