One-step uranium extraction and brine desalination via adsorptive pervaporation by graphene-oxide scaffold membranes

The ocean reserves nearly four billion tons of uranium, providing an inexhaustible supply of nuclear energy if the limits of ultralow U(VI) concentration (3.3 µg·L^-1) are addressed. Membrane technology is promising to make this happen by simultaneous U(VI) concentration and extraction. Herein, we report a pioneering adsorption-pervaporation membrane for efficient enrichment and capture of U(VI) along with clean water production. A bifunctional poly(dopamine-ethylenediamine) and graphene oxide 2D scaffold membrane was developed and further crosslinked by glutaraldehyde, capable of recovering over 70% U(VI) and water from simulated seawater brine, which validates the feasibility of one-step water recovery, brine concentration, and uranium extraction from seawater brine. Moreover, compared with other membranes and adsorbents, this membrane exhibits fast pervaporation desalination (flux: 153.3 kg·m^-2·h^-1, rejection: >99.99%) and excellent uranium capture properties of 228.6 mg·m^-2 benefiting from plentiful functional groups provided by embedded poly(dopamine-ethylenediamine). This study aims to provide a strategy for recovering critical elements from the ocean.

Sustainable disposal of seawater brine by novel hybrid electrodialysis system: Fine utilization of mixed salts

Sustainable seawater brine treatment demands an essential paradigm shift for effective recovery of resources and high value utilization of mixed-salts. Here, a novel hybrid electrodialysis (ED) system was proposed that integrated an innovative hybrid selective ED (HSED) and a developed selective bipolar membrane ED (SBMED). The HSED process allowed simultaneous recovery of major divalent cations and anions from seawater brine when NaCl was selectively enriched. Then, the impure NaCl-rich stream was fed directly into the SBMED process for acid/base preparation without any purification pretreatment. Detailed analysis of the HSED process showed that increasing unit voltage from 2.33 V to 2.67 V would improve the removal ratio of Ca²⁺, Mg²⁺ and SO₄^2- from 54.7%, 41.4% and 13.3% to 78.9%, 76.6% and 32.1%, respectively. In addition, the increment of initial concentration of product streams promoted the transport of various ions from the feed and middle compartments. The fine utilization performance, in terms of ionic removal ratio and fractionation ratio of divalent ions in the HSED process, was more limited by the initial concentration of product streams. Furthermore, the SBMED stack was found to have nearly identical performance over five cycles, indicating that the presence of a trace amount of hardness cations did not induce scaling. The current study thus provided a novel suitable strategy with a perspective of fine utilization for practical applications in sustainable disposal of seawater brine.

Recovery of sodium sulfate from seawater brine using fractional submerged membrane distillation crystallizer

Seawater reverse osmosis (SWRO) brine contain many valuable resources. In this study, fractional-submerged membrane distillation crystallizer (F-SMDC) was used to recover sodium sulfate (Na₂SO₄) from SWRO brine. The concentration/temperature gradient (CG/TG) in the reactor enhanced water recovery utilizing MD and Na₂SO₄ crystallization via a crystallizer. Crystals were not obtained at the bottom section of the F-SMDC due to: firstly, calcium sulfate crystallization occurring on the membrane surface; and secondly, low temperature-sensitivity solubility component such as NaCl exerting a negative influence. In order to obtain supersaturation, a sulfate-rich scenario was created in the reactor through the addition of the following three components: Na₂SO₄, MgSO₄ and (NH₄)₂SO₄. When Na₂SO₄ and MgSO₄ were added, a larger concentration was observed at the top section, resulting in a low concentration gradient (CG) ratio, i.e. around 1.7. Conversely, the addition of (NH₄)₂SO₄ achieved faster Na₂SO₄ crystallization (VCF 1.42) at the bottom section with a greater CG ratio of more than 2.0. Total water recovery ratio of 72% and 223.73 g Na₂SO₄ crystals were successfully extracted from simulated SWRO brine using laboratory scale F-SMDC.

Rubidium extraction from seawater brine by an integrated membrane distillation-selective sorption system

The ultimate goal of seawater reverse osmosis (SWRO) brine management is to achieve minimal liquid discharge while recovering valuable resources. The suitability of an integrated system of membrane distillation (MD) with sorption for the recovery of rubidium (Rb⁺) and simultaneous SWRO brine volume reduction has been evaluated for the first time. Polymer encapsulated potassium copper hexacyanoferrate (KCuFC(PAN)) sorbent exhibited a good selectivity for Rb⁺ sorption with 10-15% increment at 55 °C (Langmuir Q_max = 125.11 ± 0.20 mg/g) compared to at 25 °C (Langmuir Q_max = 108.71 ± 0.20 mg/g). The integrated MD-KCuFC(PAN) system with periodic membrane cleaning, enabled concentration of SWRO brine to a volume concentration factor (VCF) of 2.9 (65% water recovery). A stable MD permeate flux was achieved with good quality permeate (conductivity of 15-20 μS/cm). Repeated cycles of MD-KCuFC(PAN) sorption with SWRO brine enabled the extraction of 2.26 mg Rb⁺ from 12 L of brine (equivalent to 1.9 kg of Rb/day, or 0.7 tonne/yr from a plant producing 10,000 m³/day brine). KCuFC(PAN) showed a high regeneration and reuse capacity. NH₄Cl air stripping followed by resorcinol formaldehyde (RF) resin filtration enabled to recover Rb⁺ from the desorbed solution.