Study of the interactions of dissolved organic matter with zinc ion and the impact of competitive metal ions (Ca2+ and Mg2+) by in situ absorbance.

The bioavailability and toxicity of zinc to aquatic life depend on dissolved organic matter (DOM), such as Suwannee River Fulvic Acid (SRFA), which plays an important role in the speciation of zinc. This study examined reactions of SRFA with zinc at different concentrations from pH 3.0 to 9.0, and competitive binding of calcium/magnesium and zinc to SRFA at pH 6.0, using in situ absorbance. Interactions of Zn2+ with SRFA chromophores were evidenced by the emergence of features in Zn-differential spectra. Among all Zn2+-SRFA systems, dominant peaks, located at 235, 275 and 385 nm, and the highest intensity at 235 nm indicated the replacement of protons by the bound Zn2+. The Zn2+ binding with SRFA could be quantified by calculating the changes of the slopes of Zn-differential log-transformed absorbance in the wavelength range of 350-400 nm (denoted as DS350-400) and by comparing the experimental data with predictions using the Non-Ideal Competitive Adsorption (NICA-Donnan) model. DS350-400 was correlated well with the bound Zn2+ concentrations predicted by NICA-Donnan model with or without Ca2+ or Mg2+. Ca2+ and Mg2+ only affect intensity of the Zn-differential and Zn-differential log-transformed absorbance, not shape. In situ absorbance can be used to gain further information about Men+-DOM interactions in the presence of various metals.