THERMODYNAMIC MODELING OF METAL IMMOBILIZATION BY BIOCHAR

• Author / Creator

  Alam, Md Samrat

• Biochar is a cost-effective sorbent owing to its low production cost and its ability to efficiently remove metals and organics from water. Developing mechanistic and predictive geochemical models of metals immobilization by biochar that are predictive across wide ranges of environmental conditions is important to understand its behavior toward metals in nature and to enhance its use for remedial application. Such models, requires detailed information about biochar surface chemistry and reactivity. The studies in this thesis provide critical insights into metal-biochar surface reactions at molecular-scale by answering questions including: (1) Can the adsorption of divalent cations and radionuclides at the biochar surface be predicted by the surface complexation modeling (SCM) approach across a wide range of water chemistry conditions? (2) Can the SCM approach predict metal adsorption in multi-sorbent systems involving biochar? (3) To what extent can biochar, whether by itself or as part of a composite material, reduce redox-sensitive metals such as Cr(VI)?
  Following thorough characterization of biochar, I employ two thermodynamic approaches, (i) surface complexation modeling (SCM) and (ii) isothermal titration calorimetry (ITC), supported by synchrotron based X-ray absorption spectroscopy (XAS), to develop predictive and mechanistic models of the binding of divalent metals (Ni(II) and Zn(II)) and a radionuclide (U(VI)) to biochar. The results show that Ni (II), Zn(II) and U(VI) adsorbed through proton-active -COOH and -OH functional groups on the biochar surface, as confirmed by FT-IR studies coupled with EXAFS. The SCM approach is able to accurately predict the adsorption of Ni(II), Zn(II) and U(VI) across a wide range of water chemistries and at varying adsorbent to adsorbate concentrations. The thermodynamic driving forces of protonation adsorption reactions on the biochar surface suggest that the biochar surface contains anionic oxygen ligands, and Ni(II), Zn(II) and U(VI)-biochar surface adsorption reactions have slightly exothermic to slightly endothermic enthalpies. Ni(II), Zn(II) and U(VI) form both inner- and outer-surface complexes with the biochar surface. The combined SCM-ITC approaches, supported by EXAFS, enhance our understanding of the molecular scale mechanisms of divalent cations and radionuclides adsorption to the surface of biochar. I also test a non-electrostatic SCM approach to predict the adsorption of Cd and/or Se in mixtures of biochar to two agricultural soils. The results show that the metal adsorption to biochar amended soils is successfully predicted by SCM the approach. However, it may be necessary to invoke ternary complexes to accurately predict metal removal from solution if organic ligands are present in solution. Generally, this approach could be useful to develop predictive models of metal distribution in biochar containing multi-sorbent systems. Moreover, I test the redox properties of biochar to reduce chromate from solution, and the efficiency of biochar as a composite material combined with magnetite nanoparticles to enhance Cr(VI) removal from aqueous solution. The results show that both adsorption and reduction of Cr(VI) is enhanced by magnetite nanoparticle – biochar composites compared to biochar and magnetite nanoparticles alone, suggesting a synergetic interaction between magnetite nanoparticle and biochar. Cr(VI) immobilization by magnetite nanoparticle – biochar composites occurs through adsorption and intraparticle diffusion followed by reduction. Synchrotron-based XAS shows that all of the Cr(VI) is reduced to Cr(III) and that the Cr(III)-bearing precipitates Cr(OH)3 and chromite (Cr2FeO4) are formed. This result has important implications for the use of magnetite nanoparticle – biochar composites as a low-cost and efficient material for treating chromate-containing solutions.

• Subjects / Keywords

  • Themodynamic modeling
  • Metal
  • Immobilization
  • Biochar

• Graduation date

  Fall 2018

• Type of Item

  Thesis

• Degree

  Doctor of Philosophy

• DOI

  https://doi.org/10.7939/R3RX93W09

• License

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