Investigating The Effect Of Cations On Molecular Fractionation Of Dissolved Organic Matter On Ferrihydrite

Dissolved organic matter?DOM?represents the organic matter that can dissolve in natural water or soil solution,and is ubiquitous in the environment.The DOM molecules are rich in highly diversified active binding site such as the hydroxyl and carboxyl functional groups,which participate in many geochemical processes such as soil carbon cycle,co-ordination complexation reaction,redox reaction,photo-degradation,is the most important active component in soil system.Ferrihydrite,as the precursor of other stable iron minerals in soil environment,can bind to other substances?minerals,heavy metals?in soil and control the migration,transform and storage of these materials due to its special physicochemical properties?high surface area,amorphous,and unsaturated coordination sites?.The unsaturated site on the surface of the ferrihydrite will interact with hydroxyl and carboxyl functional groups on DOM.So far,many studies have focused on interactions between ferrihydrite and DOM.The adsorption of DOM on ferrihydrite can not only affect the characteristics and conversions of ferrihydrite,but also affect the properties and dynamics of DOM,thus affecting the geochemical processes that involve in DOM.Therefore,understanding the influence of environmental conditions on the interactions between ferrihydrite and DOM at molecular level is of great significance for predicting the carbon cycle in the environment.In this study,we aim to elucidate the effects of different dissolved cations(Na⁺,Ca²⁺,Cu²⁺)on the fractionation of DOM on ferrihydrite using optical methods and electrospray Fourier transform ion cyclotron resonance mass spectrometry?ESI-FT-ICR-MS?.Results showed that the adsorptive fractionation of DOM on ferrihydrite were affected by the adsorptive content of DOM,molecular composition and types of cations.The adsorptive fractionation of DOM exhibited a concentration-dependent behavior,and enhanced with the increase of DOM adsorption.In addition,the affinity of DOM molecules for ferrihydrite was affected by the acidity?carboxyl,?phenol?hydroxyl?and hydrophobicity?CH₂ groups?of molecules.The adsorption of DOM on ferrihydrite exhibited the strongest and weakest molecular fractionation under the Na⁺-and Ca²⁺-containing background electrolytes,respectively.The Ca²⁺and Cu²⁺can interact with the biding sites of DOM,which reduce the adsorption of phenolic compounds with high molecular weight,oxygen number and unsaturation.Meanwhile,the selective adsorption of DOM by ferrihydrite was affected by the cation-bridging flocculation.