Binding Properties Of Heavy Metals To Fulvic Acid Before And After Fractionation On Ferrihydrite: Effects Of Phosphate

Dissolved organic matter(DOM)is a widely distributed and highly active component in natural environment,which is involved in many biogeochemical processes including adsorption/desorption,redox reaction,precipitation/dissolution,and microbial metabolism.Ferrihydrite is a common iron oxide with large specific surface area and developed pore structure in soil and sediment.The adsorptive fractionation of dissolved organic matter(DOM)at the mineral-water interfaces is an important geochemical process to affect the chemical compositions and properties of DOM.Phosphate,a widely existent oxyanion in the natural environment,can compete with DOM for adsorption to iron(oxy)hydroxide surfaces,and thus may change the compositions and reactivity of DOM during the adsorptive fractionation process.Therefore,clarifying the influencing factors of DOM adsorption fractionation on the minerals,especially the competitive effects of phosphate and DOM,is helpful to deepen our understanding of the environmental behaviors of DOM and heavy metals,and is of great significance for predicting carbon cycle in natural environment as well as the risk assessment and control of heavy metal pollution.In this study,we investigated the effect of the adsorptive fractionation of fulvic acid(FA)at the ferrihydrite-water interfaces with or without phosphate on the optical compositions and the binding properties of FA by employing spectroscopic methods combined with two-dimensional correlation spectroscopy(2DCOS)analysis and potentiometric titration coupled with WHAM 7 model calculation.Our results indicated that the presence of phosphate increased the concentration of total organic carbon,aromatic components,chromophores,acidic groups(e.g.,carboxylic and phenolic groups),and fluorescence fractions in the FA samples after adsorptive fractionation on ferrihydrite,indicating that phosphate would compete with these organic components in FA for the binding sites on the surface of ferrihydrite,and thus affecting the partition of these FA components at the ferrihydrite-water surfaces.The 2DCOS analysis revealed that phosphate competed with the FA components in the order as followed: fulvic-like fractions(350 nm)> humic-like fractions(460 nm)or fulvic-like fractions(305 nm).Furthermore,the presence of phosphate during the adsorptive fractionation of FA on ferrihydrite enhanced the Cu or Pb binding ability of FA,but could not change the binding sequence of Cu or Pb to FA components(for Cu: fulvic-like fractions(305 nm)> fulvic-like fractions(350 nm)> humic-like fractions(460nm;for Pb: humic-like fractions(400 nm)> fulvic-like fractions(350 nm),and humic-like fractions(400 nm)> humic-like fractions(460 nm).Our results contribute to understanding the metal binding characteristics of DOM under the influence of complex interactions among DOM,minerals and inorganic constituents.