title: Improving and testing geochemical speciation predictions of metal ions in natural waters.
abstract: The ability of WHAM VII and NICA-Donnan models to predict free-ion activities of Cu in natural waters was examined from two perspectives, (i) the presence of EDTA and NTA contaminants, (ii) the need to improve estimates of HA and FA concentrations. Potentiometric responses of a Cu(II) ion-selective electrode were investigated in five assays containing dissolved organic matter (DOM) isolated from a series of polluted (urban) and relatively unpolluted (upland) streams in northern England. The [Cu]/[DOC] ratio in these assays spanned an environmentally realistic range of ∼1-500 μmol/g. Reasonably good agreement between measured and predicted Cu(2+) activities was obtained with both WHAM VII and NICA-Donnan models, assuming 65% of DOM as fulvic acid and including the measured EDTA and NTA concentrations, but generally the models overestimated the activities by a factor of ∼2. In contrast, the models over-predicted the Cu(2+) activities by up to 2 orders of magnitude at low [Cu]/[DOC] ratios in urban waters if anthropogenic ligands were not included in the model simulations. Three-dimensional fluorescence excitation-emission matrix (EEM) spectroscopy was used to measure the functional properties of the isolated DOM and to estimate the fractions of FA and HA present. Using these fractions in the models gave improvements in predictions compared to the 65% FA assumption, as shown by higher correlations, reduced error and reduced bias. These results highlight various issues with the use of the available speciation models for predicting free ion concentrations in natural waters, such as the use of the Biotic Ligand Model (BLM) for the derivation of environmental standards. It is clearly necessary to measure EDTA and NTA in waters with urban influences, while fluorescence measurements offer the possibility of appreciably improving the accuracy of predictions.
authors.affiliation: Lancaster University, Lancaster Environment Centre, Lancaster, LA1 4YQ, United Kingdom. Electronic address: imad.a.ahmed@gmail.com.
authors.forename: Imad A M
authors.initials: IA
authors.lastname: Ahmed
authors.affiliation: Lancaster University, Lancaster Environment Centre, Lancaster, LA1 4YQ, United Kingdom.
authors.forename: John
authors.initials: J
authors.lastname: Hamilton-Taylor
authors.affiliation: Lancaster University, Lancaster Environment Centre, Lancaster, LA1 4YQ, United Kingdom.
authors.forename: Magdalena
authors.initials: M
authors.lastname: Bieroza
authors.affiliation: Lancaster University, Lancaster Environment Centre, Lancaster, LA1 4YQ, United Kingdom.
authors.forename: Hao
authors.initials: H
authors.lastname: Zhang
authors.affiliation: Lancaster University, Lancaster Environment Centre, Lancaster, LA1 4YQ, United Kingdom.
authors.forename: William
authors.initials: W
authors.lastname: Davison
mesh_terms: D001578:Benzopyrans; D003300:Copper / Q000737:chemistry*; D004492:Edetic Acid; D004739:England; D004784:Environmental Monitoring / Q000379:methods*; D006812:Humic Substances / Q000032:analysis*; D008956:Models, Chemical; D011199:Potentiometry; D045483:Rivers / Q000737:chemistry*; D013050:Spectrometry, Fluorescence
chemical_list: D001578:Benzopyrans; D006812:Humic Substances; D003300:Copper; D004492:Edetic Acid; C005023:fulvic acid
keywords: Chemical speciation; Dissolved organic matter; EDTA; Fluorescence; Freshwater; Ion selective electrode; NICA-Donnan; PARAFAC; WHAM