Synthesis and characterization of zinc-hexacyanoferrate composite beads for controlling the ammonia concentration in low-temperature live seafood transports.

A new water treatment technology is presented for extending the longevity and increasing the maximal bio-load of container-bound, lucrative live seafood transportations. The technology is designed for removing ammonia and minimizing the bacterial concentration that develop in the water during the transport. This paper focuses on the characteristics of self-synthesized polyether-sulfone (PES) coated Zn-HCF composite beads, which have a high adsorbing capacity for NH4+ in seawater and constitute the heart of the developed technology. Adsorption isotherms show that the operational capacity of the composite material (PES = 20% w/w) at NH4+ concentration of 10 mgN/L at 3.5 °C is ∼3 mgN/g Zn-HCF. The kinetics of the PES-coated beads were shown to be considerably slower than the bare Zn-HCF, but since the retention time in the transport is long (many days), this does not detract from the effectiveness of the adsorption. Simulation experiments with and without live fish showed that the adsorbing material behaved as expected during a 21-d trip and that it did not have any effect on the fish. Repeated adsorption/regeneration (3 and 6 M NaCl) tests proved the composite material's stability and ion-exchange robustness. Electrooxidation of the ammonia in the exhausted regeneration solution was carried out with high efficiency and the treated solution could be used effectively in the following chemical regeneration step. The cost of a treatment unit installed in a 40-foot container was estimated at $40,000 and the ROI at 6 to 12 months.

Biochar-Assisted Iron-Mediated Water Electrolysis Process for Hydrogen Production.

The biochar-assisted water electrolysis process for hydrogen gas production is reported. The H2 generation is performed in a divided electrolysis cell in which the hydrogen evolution reaction occurs on a cathode and ferrous iron oxidation on an anode. Electrochemically produced Fe(III) species are reduced back to ferrous form in a reaction with biochar concentrated in a packed-bed column through which an acidic anolyte (FeCl3) solution is continuously recirculated. During the operation of the proposed process with commercial charcoal, the oxidation of carbon resulted in an accumulation of oxygen-containing groups on the carbon surface that leads to charcoal deactivation. Thermal treatment of the charcoal at 250, 350, and 450 °C in a nitrogen atmosphere resulted in reactivation of carbon, and the best results (≈80% reactivation) were achieved after 3 h of treatment at 450 °C. Nine successful cycles of electrolysis-charcoal regeneration were performed in this study. A ≈98% current efficiency for hydrogen production was achieved at a current density of 50 mA/cm2. Much higher current densities can be obtained using the proposed technique as the anodic process of ferrous iron oxidation is decoupled from the carbon oxidation process. The CO2 production rate achieved in this study was up to 98% of a stoichiometric value proposed for the iron-mediated carbon-assisted water electrolysis process.

When Salt-Rejecting Polymers Meet Protons: An Electrochemical Impedance Spectroscopy Investigation.

Polymeric membranes are widely used for salt removal, but mechanism of ion permeation is still insufficiently understood. Here we analyze ion transport in polymers relevant to desalination, dense aromatic polyamide Nomex and cellulose acetate (CA), using impedance spectroscopy, focusing on the effects of the salt type, concentration and pH. The results highlight the role of proton uptake in ion permeation. For Nomex the exceptionally high affinity to proton results in a power-low scaling of conductivity with salt concentrations with an unusual exponent 1/2. The results for CA suggest dominance of pore transport, with pore charge increasing with decreasing pH, which contradicts previous view of CA as a weakly acidic polymer and points to proton uptake as possible pore-charging mechanism. The observed effects may have far-reaching consequences in desalination, as even at neutral pH they may both enhance and suppress salt permeation and affect pH changes.

Predicting the Rejection of Major Seawater Ions by Spiral-Wound Nanofiltration Membranes.

Seawater nanofiltration (SWNF) generates a softened permeate stream and a retentate stream in which the multivalent ions accumulate, offering opportunities for practical utilization of both streams. This study presents an approach to simulation of SWNF including all major seawater ions (Na(+), Cl(-), Ca(2+), Mg(2+), and SO4(2-)) based on the Nernst-Planck equation, and uses it for permeate and retentate streams composition prediction. The number of degrees of freedom in the system was reduced by assuming a very high ionic permeability for Na(+), which only weakly affected the other parameters in the system. Two alternatives were examined to analyze the importance of concentration dependence of ion permeabilities: The assumption of constant ion permeabilities resulted in a reasonable fit with experimental data. However, for the permeate composition the overall fit was significantly improved (P < 0.0001) when the permeabilities of Ca(2+) and Mg(2+) were allowed to depend on the ratio of their total concentration to Na(+). This type of dependence emphasizes the strong interaction of divalent ions with the membrane and its effect on the membrane fixed charge through screening or charge reversal. When this effect was included, model predictions closely matched the experimental results obtained, corroborating the phenomenological approach proposed in this study.