Biochar has recently been received worldwide attention as a sorbent to remediate metals-contaminated soil and water. It can be produced from local biomass and at low cost compared to many alternative sorbents. The contemporary body of biochar literature reports on metal uptake by biochar without understanding the underlying metal adsorption reactions at the biochar surface. Surface complexation modelling (SCM) and isothermal titration calorimetry (ITC) are tools that can be used to study the underlying mechanisms and driving forces of metal adsorption at the biochar surface.<br/>In this investigation, we studied U(VI) uptake by two biochars produced from wheat straw (WS) and wood pine chip (WPC) that were produced by the Alberta Biochar Initiative (ABI).  Following thorough characterization of surface area, porosity, and bulk chemistry, potentiometric titrations were carried out to measure the proton (H<sup>+</sup>) reactivity of each biochar, to develop protonation models. Following, U adsorption experiments were carried out for each biochar as a function of pH and ionic strength, and the U(VI) binding stability constants calculated using a non-electrostatic surface complexation model. The results show that the biochars studied have significant proton reactivity and strongly adsorb U at circumneutral pH. To determine the surface coordination of U ions adsorbed to the biochar surfaces, synchrotron-based X-ray absorption spectroscopy (XAS) was employed. XAS investigations show short U-O distances, which is an indication of the formation of inner sphere surface complexes of U to biochar functional groups. ITC was used to measure the enthaplies of proton and uranyl adsorption onto biochar.  The results show that protonation reactions of WS and WPC produce exothermic enthalpies at all 3 sites of the non-electrostatic protonation model.  Uranyl adsorption onto biochar also produced exothermic enthalpies with heats become less exothermic as pH increased.  Our experimental approach provides constraints on the molecular-scale mechanisms involved in U-biochar adsorption reactions and also yields thermodynamic data that can quantitatively estimate the temperature dependence of proton and U adsorption reactions.<br/>

Analysis of U(VI) adsorption to biochar using x-ray absorption spectroscopy and isothermal titration calorimetry

253rd National Meeting (2017)

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