Surface volatilization modeling of (semi-)volatile hydrophobic organic compounds: The role of reference compounds

Volatilization of hazardous hydrophobic organic compounds is often observed in many water, wastewater and soil treatment (bio)processes. Several models have been developed to quantify and predict gas-liquid pollutant transfer, being the proportionality coefficient model (PCM) one of the most commonly used, particularly in wastewater treatment. The PCM is based on the use of oxygen as a reference compound, which has a low resistance to the transfer in the gas phase. However, this resistance might be important for (semi-)volatile organic compounds - or (semi-)VOCs, which may render the use of the PCM model inaccurate. This study proposes an experimental methodology and a modeling approach for the use of the two-reference compound model (2RCM) that considers both the liquid-side and the gas-side resistances, by using water and oxygen as references. Results showed that the 2RCM predicts more accurately the overall mass transfer coefficients than the PCM for a VOC and two semi-VOCs tested in this study. In addition, the 2RCM was found to be a more robust method to estimate mass transfer coefficient of any compound and its use can be extrapolated to all substances. Finally, the relevance and limitations of both models was established.

Role of configurational flexibility on the adsorption kinetics of bivalent bispecific antibodies on porous cation exchange resins

The adsorption kinetics of a monoclonal antibody (mAb) used as a reference and of bivalent bispecific antibodies (BiSAb) on a macroporous cation exchanger is studied experimentally by examining the transient patterns of bound protein within the particles using confocal microscopy for a range of protein concentrations, buffer concentrations and pH, and temperatures. The mAb adsorption kinetics is controlled by pore diffusion and conforms to the classical shrinking core model. While the mAb adsorption rate increases with temperature, the ratio of effective and free solution diffusivity, D_e /D₀, remains constant and has a value of 0.20. The BiSAb's structure is comprised of scFv domains that are genetically fused to a framework IgG through flexible peptide linkers which results in conformational flexibility leading to multiple binding forms with varying affinity for the adsorbent surface. As a result, adsorption of the BiSAbs shows complex patterns of total bound protein within the particles. These BiSAb adsorption patterns are influenced by buffer ionic strength, pH, and temperature in unique ways. Sharper intraparticle profiles are observed for conditions where the binding strength is greater (lower buffer concentration and/or pH) or when the protein is chemically crosslinked to restrict configurational flexibility. Temperature affects the BiSAb pore diffusivity as well as the interconversion kinetics. While the effects of temperature on BiSAb transport are also described by a constant D_e /D₀ = 0.15, the temperature also affects the rate of interconversion between binding forms leading to faster equilibration at higher temperatures. A phenomenological model indicates that the interplay of pore diffusion and adsorption with the kinetically limited interconversion between binding forms is responsible for the experimental trends.

Building an operational framework for selective nitrogen recovery via electrochemical stripping

Recovering nitrogen from wastewater can simultaneously fulfill the roles of traditional removal technologies such as nitrification-denitrification and fertilizer production processes such as Haber-Bosch. We have recently demonstrated a proof-of-concept for selective recovery of the fertilizer ammonium sulfate via electrochemical stripping, a combination of electrodialysis and membrane stripping. In this study, we furthered electrochemical stripping from concept to informed practice by investigating the effects of influent concentration (30, 300, and 3000 mg N/L), catholyte temperature (15, 23, and 35 °C), and gas permeable membrane choice on electrochemical nitrogen removal and recovery. We also proposed and validated a nitrogen mass transport model for the experimental results, providing mechanistic rationale behind observed effects of varying operating parameters. While changing operating parameters did affect performance, electrochemical stripping exhibited robust performance over a range of realistic ambient temperatures, three gas permeable membranes, and three orders of magnitude of influent concentrations. Practically, these results demonstrate that electrochemical stripping is viable across a range of waste streams and resilient to fluctuations in temperature and nitrogen concentration; they also establish operational trade-offs between residence time and energy consumption. As a result of this work, electrochemical stripping continues to mature from concept to practice and provides lessons for developing other resource recovery technologies.

Modeling mass transfer for adsorptive removal of Pb(II) onto phosphate modified ordered mesoporous carbon (OMC)

Phosphate modified ordered mesoporous carbon (MOMC-NP) has been synthesized and proven to be an effective adsorbent for Pb(II) removal from aqueous solutions. However, the key application components of the mass transfer operations and diffusion coefficient have not been determined. In this study, a modified Finite Bath Diffusion Control Model was mathematically developed containing a constant related to the radius of the adsorbent particle and the fractional attainment of adsorption. The adsorption experiments were conducted under various initial Pb(II) concentrations ranging from 60 mg L^-1 to 100 mg L^-1. The results suggested that the modified Finite Bath Diffusion Control Model was more applicable to the experimental data than the original Finite Bath Diffusion Control Model. The average value of the diffusion coefficient (λD¯) obtained from the modified finite bath diffusion control model was 1.63 × 10^-2 cm² s^-1 indicating the effective diffusivity in the adsorption of Pb(II) on MOMC-NP. Overall, the modified Finite Bath Diffusion Control Model exhibited the precise description and simulation of the mass transfer kinetics for Pb(II) adsorption onto MOMC-NP. Therefore, the modified Finite Bath Diffusion Control Model could be effectively used to investigate the mass transfer kinetics of the adsorption process.