Effective As(III) Removal by A Multi-Charged Hydroacid Complex Draw Solute Facilitated Forward Osmosis-Membrane Distillation (FO-MD) Processes

Review of Hybrid Membrane Distillation Systems

Membrane distillation (MD) is an attractive separation process that can work with heat sources with low temperature differences and is less sensitive to concentration polarization and membrane fouling than other pressure-driven membrane separation processes, thus allowing it to use low-grade thermal energy, which is helpful to decrease the consumption of energy, treat concentrated solutions, and improve water recovery rate. This paper provides a review of the integration of MD with waste heat and renewable energy, such as solar radiation, salt-gradient solar ponds, and geothermal energy, for desalination. In addition, MD hybrids with pressure-retarded osmosis (PRO), multi-effect distillation (MED), reverse osmosis (RO), crystallization, forward osmosis (FO), and bioreactors to dispose of concentrated solutions are also comprehensively summarized. A critical analysis of the hybrid MD systems will be helpful for the research and development of MD technology and will promote its application. Eventually, a possible research direction for MD is suggested.

Recovering nutrients and rejecting trace organic compounds in human urine by a forward osmosis-membrane distillation (FO-MD) hybrid system

Urine is a major source of reclaimed water and fertilizer. Urine treatment involves two key processes: the recovery of nutrients and the rejection of trace organic compounds (TOrCs). In this study, we investigated the rejection of TOrCs and the recovery of nutrients in human urine using a seawater-driven forward osmosis and membrane distillation (FO-MD) hybrid system. Three 24 h experiments were conducted at draw solution temperatures of 30, 40, and 50 °C. The average rejection rates of cations, anions, and dissolved organic carbon were more than 93.7% and 79.5% in the FO-MD system and FO side, respectively. Ten types of TOrCs were detected in the feed solution, whereas none were detected in the product water, indicating that the TOrCs were completely rejected. The precipitates, i.e., the recovered nutrients in the FO side, were extremely close to magnesium ammonium phosphate (struvite, MgNH₄PO₄·6H₂O), according to their electron microscopic images, elemental composition, and X-ray diffraction spectra, and it was estimated that approximately 85% of the nutrients in the feed solution were recovered. The rejection and recovery efficiencies were unaffected by the draw solution temperature. These results indicate the potential for the sustainable use of FO-MD-based treatments for human urine.

Crosslinked poly(ionic liquid)s as selective receptors for Cr(VI) - Counter anion effect and application in treating drinking water and tannery effluents

A series of crosslinked poly(ionic liquid)s (C-PILs), with five different counter anions, were synthesised and investigated for their Cr(VI) binding properties. C-PIL with chloride as the counter anion (C-PIL-Cl) showed the highest sorption capacity (2.96 ± 0.03 mmol/g), while bis(trifluoromethanesulfonyl)imide led to minimum uptake. The chromium binding properties of C-PIL-Cl were investigated in detail, which revealed remarkable Cr(VI) selectivity against other anions. It could reduce the Cr(VI) concentration from 980 ppb to 28 ppb in a typical drinking water sample, which is below the WHO prescribed acceptable limit. The polymer could also remove about 90% of Cr(VI) from synthetic tannery effluent. SEM-EDX studies clearly revealed the chromium uptake and regeneration of the chromium sorbed polymer to be through anion exchange at the binding sites. Quantitative elution of the bound chromium from C-PIL-Cl was feasible with 1 M NaOH and 1 M NaCl solution. While the polymer regenerated with NaCl showed sorption properties similar to the virgin polymer, regeneration with NaOH led to reduced capacity. Microscopic investigations showed no effect on polymer morphology during desorption. The sorption kinetics followed pseudo second order and showed poor fit with pseudo first order model.

Removal of anionic hexavalent chromium and methyl orange pollutants by using imidazolium-based mesoporous poly(ionic liquid)s as efficient adsorbents in column

Poly(ionic liquid)s (PILs) are attractive for their various applications, but the use of porous PILs have rarely been reported in anionic pollutants removal via ion-exchange by column. Herein, we report a serial of crosslinked imidazolium-based mesoporous PILs with Cl^- and Br^- as anions for hexavalent chromium (Cr(VI)) and methyl orange (MO) removal. Among them, PDVIm-Cl-SCD, from the free-radical polymerization of a dicationic monomer (N,N'-methylene-bis(1-(3-vinylimidazolium)) chloride, DVIm-Cl) and further supercritical carbon dioxide drying (SCD), displayed a very high sorption capacity (328.2 mg g^-1 at 25 °C) and excellent utilization of adsorption sites (UOA, 86.2%) towards Cr(VI), and an unprecedentedly high sorption capacity (1615.0 mg g^-1 at 25 °C) with a UOA of 67.4% to MO. Moreover, PDVIm-Cl-SCD also exhibited a broad pH range, excellent regeneration and remarkable reusability. Regarding to Cr(VI) removal, the volume of saturated KCl aqueous used for regenerating the Cr(VI) saturated PDVIm-Cl-SCD column (7.5-9.5 mL) was much less than the volume of treated Cr(VI) solution (160-200 mL). For MO removal, the volume of saturated NaCl solution used for regenerating the MO saturated PDVIm-Cl-SCD column (10.5-13.5 mL) was also much less than the volume of treated MO solution (220-235 mL), implying the great potential of PDVIm-Cl-SCD in sustainable wastewater treatment.

Research on Forward Osmosis Membrane Technology Still Needs Improvement in Water Recovery and Wastewater Treatment

Forward osmosis (FO) has become an evolving membrane separation technology to recover water due to its strong retention capacity, sustainable membrane fouling, etc. Although a good deal of research has been extensively investigated in the past decades, major challenges still remain as follows: (1) the novel FO membrane material properties, which significantly influence the fouling of the FO membranes, the intolerance reverse solute flux (RSF), the high concentration polarization (CP), and the low permeate flux; (2) novel draw solution preparation and utilization; (3) salinity build-up in the FO system; (4) the successful implementation of the FO process. This work critically reviews the last five years’ literature in development of the novel FO membrane material, structure in modification, and preparation, including comparison and analysis on the traditional and novel draw solutes coupled with their effects on FO performance; application in wastewater treatment, especially hybrid system and integrated FO system; fouling mechanism; and cleaning strategy as discussed in the literature. The current barriers of the research results in each hotspot and the areas that can be improved are also analyzed in detail. The research hotspots in the research and development of the novel membrane materials in various countries and regions have been compared in recent years, and the work of variation in pop research hotspots in the past 10 years has been analyzed and the ideas that fill the blank gaps also have been proposed.

A pH-responsive supramolecular draw solute that achieves high-performance in arsenic removal via forward osmosis

A pH-responsive, charge switchable piperazine derivative, 1,4-bis(3-propane- sulphonate sodium)-piperazinediethanesulfonic acid disodium-sulfate (4), has been designed via a stepwise synthesis and proposed as a draw solute to remove arsenics (As^III, As^V) from water through forward osmosis (FO). Having multiple sulfonic groups, 4 generates a high osmotic pressure and produces a water flux as high as 58.4 LMH at a dilute concentration (0.24 M), surpassing most of the existing draw solutes in water transfer rate under the similar experimental conditions. Compound 4 at 0.24 M yields a water flux of 52.9 LMH with a 100% As^V rejection, and 57.8 LMH with a 96.0% As^III rejection when 50 ppm As^V or As^III as the corresponding feed, manifesting the best performance in arsenic removal and concurrent water recovery efficiency. Remarkably, being a polymeric configuration in water, 4 has a negligible solute loss in the FO process. 4 can be readily regenerated for reuse in FO by precipitation from its solution through acidification. The abundance in ionic groups and the pH-responsive property coupled with a supramolecular configuration make 4 an ideal draw solute for FO wastewater treatment.

Pressure-driven and thermally-driven membrane operations for the treatment of arsenic-contaminated waters: A comparison

The presence of arsenic in water beyond the admitted limits is becoming an important concern for many Countries. Methods usually employed for the arsenic removal from water are based on coagulation followed by filtration, ion-exchange, adsorption. Drawbacks like the use of chemicals and the production of sludges (in case of coagulation) have, however, to be mentioned. Membrane operations, based on the features of membrane materials involved in the separation, do not need chemicals and are easy to scale-up, due to their modularity. In this contribution, the potential of membrane operations for the treatment of arsenic-polluted water is presented and discussed. In particular, two classes of membrane operations are illustrated and compared, the pressure-driven, like Nanofiltration (NF) and Reverse Osmosis (RO) and the thermally-driven ones, like Membrane Distillation (MD).

Synthetic draw solutes for forward osmosis: status and future

Forward osmosis (FO) has developed rapidly over the past decade. The development of draw solutes, a key component of FO processes, has also progressed remarkably. A wide range of synthetic draw solutes have been explored in recent years. Synthetic draw solutes exhibit superiority over the conventional draw solutes obtained commercially in terms of lower reverse solute fluxes and less energy consumption in draw solute recycling. However, there are still some big challenges for synthetic draw solutes, such as complicated synthetic procedures, low water fluxes, severe concentration polarization (CP) and decreased water recovery efficiency when recycled draw solutes are reused in FO. These challenges are also the current research focus on the exploration of novel draw solutes. This article aims to review the recent progress especially on synthetic draw solutes. Their design strategies, synthesis routes and FO performance are assessed. Some representative applications involving the synthetic draw solutes-facilitated FO processes are exemplified. The advantages and disadvantages of the existing synthetic draw solutions are evaluated. The challenges and future directions in exploring novel draw solutes are highlighted.