(418h) Computational and Experimental Investigation of Cu2+ and Pb2+ Adsorption to Hydrothermal Char

Heavy metal contamination to the environment is a significant concern that can accumulate from aqueous streams such as urban runoff, industrial wastewater, sewage effluents, agricultural pesticides and fertilizers. Removing these heavy metal contaminants in a sustainable manner is of utmost importance to both human health and environmental safety. Biomass derived adsorbents are an abundant, renewable and low-cost substrate effective at adsorbing many common heavy metal contaminants from aqueous media. Specifically, hydrochar produced from hydrothermal carbonization of glucose, a thermochemical process at elevated pressure and temperature of 180 °C for 12 hours has shown promise in Cu²⁺ and Pb²⁺ uptake after KOH activation. Hydrochars are tunable materials based on their treatment and an understanding of the structure is necessary to engineer an optimal material for heavy metal uptake. Structural information of glucose hydrochar was provided by Yuan, S. et al. and utilized to produce 20 potential discrete oxygen containing adsorption binding sites.

To better understand the structure property relations for hydrochar metal uptake, the adsorption of Cu²⁺ and Pb²⁺ to adsorption binding sites were computationally modeled through density functional theory (DFT) calculations. All DFT calculations were performed on WebMO using the Becke, 3-parameter, Lee-Yang-Parr (B3LYP) exchange-correlation functional and the Los Alamos National Laboratory 2 Double-Zeta (LANL2DZ) basis set. Binding site favorability was computationally assessed using the potassium exchange energy (KEE), or the enthalpy required to exchange a potassium cation bound to the hydrochar with a heavy metal cation, Cu²⁺ or Pb²⁺. The average KEE for Cu²⁺ is -86 kJ/mol and the average KEE for Pb²⁺ is -78 kJ/mol, suggesting energetically favorable binding of these cations. Of the binding site types considered, phenolate containing sites have the most negative KEEs indicating more favorable adsorption than other binding sites containing carboxylate, carbonyl, or furan moieties. For most binding sites, Cu²⁺ has a more negative exchange energy, indicating adsorption of Cu²⁺ is more favorable than Pb²⁺.

Copper and lead equilibrium adsorption capacities of the hydrochar were experimentally validated using copper nitrate and lead nitrate solutions ranging in concentration from 0 to 500 ppm and subsequently measuring the removal of copper or lead from the solution after mixing with hydrochar for 24 hours. Fitting the data to a Langmuir isotherm, the hydrochar was found to have a maximum adsorption capacity of 85 mg/g for copper, and 60 mg/g for lead. Langmuir isotherm fits were compared to alternative models including Freundlich, Sips, Multi-Langmuir, Distributed Activation Energy, and Multi-Distributed Activation Energy. Due to the DFT simulation results indicating that different types of binding sites are more energetically favorable than others, adsorption isotherm models with multi-site features are of particular interest. These adsorption model considerations for heavy metals on hydrochar have a direct impact on engineering hydrochars with optimal metal uptake performance.

References:

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Yuan, S.; Brown, A.; Zheng, Z.; Johnson, R. L.; Agro, K.; Kruse, A.; Timko, M. T.; Schmidt-Rohr, K. Glucose Hydrochar Consists of Linked Phenol, Furan, Arene, Alkyl, and Ketone Structures Revealed by Advanced Solid-State Nuclear Magnetic Resonance. Solid State Nuclear Magnetic Resonance 2024, 134, 101973. https://doi.org/10.1016/j.ssnmr.2024.101973.