Measuring the energy of phosphate and iron adsorption at the metal oxide-liquid interface using flow-adsorption microcalorimetry

摘要

Phosphate and Fe(II) sorption by metal oxides continue to gather attention for the purpose of improving our knowledge of water quality issues pertaining to excessive nutrient loading and Fe(II)-mediated reductive transformation of organic and inorganic pollutants. In order to examine surface modifications caused by phosphate and Fe(II) uptake, flow-adsorption microcalorimetry (FAMC) was used to probe the interface of boehmite following phosphate retention and ferrihydrite, titanium dioxide, goethite, and boehmite after Ca 2+ and Fe(II) treatment.;Phosphate adsorbed exothermically to boehmite with a molar heat of adsorption (DeltaHads) of +20.9 kJ/mol, as well as evidence that other secondary reactions involving phosphate precipitation were operating as the surface concentration of phosphate approached the sorption capacity. Desorption experiments conducted at pH 10 confirmed that 90% of surface-bound phosphate was irreversibly retained, while ∼10% was easily dislodged due to competition from CO32-/OH- or mobilized during dissolution of the newly formed amorphous phosphate-precipitate.;A novel gpulse-pulseh flow-adsorption microcalorimetry (PP-FAMC) technique combined with Cl--selective electrode measurements was used to demonstrate that phosphate sorption onto boehmite temporarily reduced surface positive charge without imposing changes to the molar enthalpy, Delta Hexc of Cl-/NO3- exchange (3.2 kJ/mol) and also show that the point at which phosphate adsorption transitioned to surface precipitation occurred at a surface density of ∼150 mumol/g.;Thermal signals generated during the treatment of pristine and carbonate-treated ferrihydrite with 2-morpholinoethanesulfonic acid (MES), demonstrated that the heat of sorption of MES, estimated to be +3.1 kJ/mol, was reversible and dominated primarily by electrostatic interactions.;Additionally, the molar enthalpy, DeltaHads of Ca2+ sorption was directly related to the K+-derived cation exchange capacity in the order ferrihydrite ≈ TiO2 > goethite > boehmite while Fe(II) bonded stronger to TiO2 relative to ferrihydrite, goethite, and boehmite. Comparison of energies of Fe(II) sorption onto Fe-oxides to oxides that do not contain structural Fe, this dissertation is the first to calorimetrically demonstrate that the combined rate of Fe(II) sorption and interfacial electron transfer (IET) increases with adsorbed Fe(II) concentration and does not appear, at least at low and extremely high coverage, to be linearly related to the rate of secondary phase formation.