Rapid, Selective, and Chemical-Free Removal of Dissolved Silica from Water via Electrosorption: Feasibility and Mechanisms.

The unavoidable and detrimental formation of silica scale in engineered processes necessitates the urgent development of effective, economic, and sustainable strategies for dissolved silica removal from water. Herein, we demonstrate a rapid, chemical-free, and selective silica removal method using electrosorption. Specifically, we confirm the feasibility of exploiting local pH dynamics at the electrodes in flow-through electrosorption, achieved through a counterintuitive cell configuration design, to induce ionization and concomitant electrosorption of dissolved silica. In addition, to improve the feasibility of silica electrosorption under high-salinity solutions, we developed a silica-selective anode by functionalizing porous activated carbon cloths with aluminum hydroxide nanoparticles (Al(OH)3-p-ACC). The modification markedly enhances silica sorption capacity (2.8 vs 1.1 mgsilica ganode-1) and reduces the specific energy consumption (13.3 vs 19.8 kWh kgsilica-1). Notably, the modified electrode retains remarkable silica sorption capacity even in the presence of high concentrations of co-occurring ions (up to 100 mM NaCl). The mechanisms underlying the superior silica removal stability and selectivity with the Al(OH)3-p-ACC electrode are also elucidated, revealing a synergistic interaction involving outer-sphere and inner-sphere complexation between dissolved silica and Al(OH)3 nanoparticles on the electrodes. Moreover, we find that effective regeneration of the electrodes may be achieved by applying a reverse potential during discharge, although complete regeneration of the modified electrodes may necessitate alternative materials or process optimization. We recommend the adoption of feedwater-specific designs for the development of future silica-selective electrodes in electrosorption capable of meeting silica removal demands across a wide range of engineered systems.

Sous Vide-Inspired Impregnation of Amorphous Titanium (Hydr)Oxide into Carbon Block Point-of-Use Filters for Arsenic Removal from Water.

Carbon block filters, commonly employed as point-of-use (POU) water treatment components, effectively eliminate pathogens and adsorb undesirable tastes, odors, and organic contaminants, all while producing no water waste. However, they lack the capability to remove arsenic. Enabling the carbon block to remove arsenic could reduce its exposure risks in tap water. Inspired by Sous vide cooking techniques, we developed a low-energy, low-chemical method for impregnating commercially available carbon block with titanium (hydr)oxide (THO) in four integrated steps: (1) vacuum removal of air from the carbon block, (2) impregnation with precursors in a flexible pouch, (3) sealing to prevent oxygen intrusion, and (4) heating in a water bath at 80 °C for 20 h to eliminate exposure and reactions with air. This process achieved a uniform 13 wt % Ti loading in the carbon block. Our modified carbon block POU filter efficiently removed both arsenate and arsenite from tap water matrices containing 10 or 100 µg/L arsenic concentrations in batch experiments or continuous flow operations. Surprisingly, the THO-modified carbon block removed arsenite better than arsenate. This innovative method, using 70% fewer chemicals than traditional autoclave techniques, offers a cost-effective solution to reduce exposure to arsenic and lower its overall risk in tap water.

Spin-Selective Oxygen Evolution Reaction in Chiral Iron Oxide Nanoparticles: Synergistic Impact of Inherent Magnetic Moment and Chirality.

Electron spin polarization is identified as a promising avenue for enhancing the oxygen evolution reaction (OER), which is the bottleneck that limits the energy efficiency of water-splitting. Here, we report that both ferrimagnetic (f-Fe3O4) and superparamagnetic iron oxide (s-Fe3O4) catalysts can exhibit external magnetic field (Hext)-induced OER enhancement, and the activity is proportional to their intrinsic magnetic moment. Additionally, the chirality-induced spin selectivity (CISS) effect was utilized in synergy with Hext to get a maximum enhancement of up to 89% improvement in current density (at 1.8 V vs RHE) with a low onset potential of 270 mV in s-Fe3O4 catalysts. Spin polarization and the resultant spin selectivity suppress the production of H2O2 and promote the formation of ground state triplet O2 during the OER. Furthermore, the design of chiral s-Fe3O4 with synergistic spin potential effect demonstrates a high spin polarization of ∼42%, as measured using conductive atomic force microscopy (c-AFM).

Free-base porphyrin polymer for bifunctional electrochemical water splitting.

Water splitting is considered a promising approach for renewable and sustainable energy conversion. The development of water splitting electrocatalysts that have low-cost, long-lifetime, and high-performance is an important area of research for the sustainable generation of hydrogen and oxygen gas. Here, we report a metal-free porphyrin-based two-dimensional crystalline covalent organic polymer obtained from the condensation of terephthaloyl chloride and 5,10,15,20-tetrakis(4-aminophenyl) porphyrin which is stabilized by an extensive hydrogen bonding network. This material exhibits bifunctional electrocatalytic performance towards water splitting with onset overpotentials, η, of 36 mV and 110 mV for HER (in 0.5 M H2SO4) and OER (in 1.0 M KOH), respectively. The as-synthesized material is also able to perform water splitting in neutral phosphate buffer saline solution, with 294 mV for HER and 520 mV for OER, respectively. Characterized by electrochemical impedance spectroscopy (EIS) and chronoamperometry, the as-synthesized material also shows enhanced conductivity and stability compared to its molecular counterpart. Inserting a non-redox active zinc metal center in the porphyrin unit leads to a decrease in electrochemical activity towards both HER and OER, suggesting the four-nitrogen porphyrin core is the active site. The high performance of this metal-free material towards water splitting provides a sustainable alternative to the use of scarce and expensive metal electrocatalysts in energy conversion for industrial applications.

Superparamagnetic MOF@GO Ni and Co based hybrid nanocomposites as efficient water pollutant adsorbents.

A series of highly efficient adsorbents were developed using Ni3(BTC)2 and Co3(BTC)2 metal-organic frameworks (MOFs) and Fe3O4 magnetic nanoparticles (MNPs) to functionalize graphene oxide (GO). XRD results show high crystallinity of the prepared nanomaterials and the successful decoration of Ni3(BTC)2 and Co3(BTC)2 MOFs over the GO substrate (BTC = benzene-1,3,5-tricarboxylic acid). SEM and TEM imaging show the successful formation of nanoscale MOFs and Fe3O4 MNPs over GO. IR spectroscopy supports the characterization and successful preparation of the Fe3O4/MOF@GO hybrid composite nanoadsorbents. The prepared composite nanoadsorbents were used to sorb Methylene Blue (MB) as a model for common organic pollutants in water and common ions (Na+, Ca2+, Mg2+, SO42-, SiO32-) from a brackish water model. The adsorbed concentration at equilibrium of MB of the prepared composite nanoadsorbents increases by an average of 30.52 and 13.75 mg/g for the Co and Ni composite, respectively, when compared to the MOFs parent materials. The adsorbed amount of sulfate ions increases by 92.1 mg/g for the Co composite and 112.1 mg/g for the Ni composite, when compared to graphene oxide. This adsorption enhancement is attributed to suppressed aggregation through increased dispersive forces in the MOFs due to the presence of GO, formation of nanoscale MOFs over the GO platform, and the hindering of stacking of the graphene layers by the MOFs. Leaching tests show that the release of Co and Ni ions to water is reduced from 105.2 and 220 mg/L, respectively, in the parent MOF materials to 0.5 and 16.4 mg/L, respectively, in the composite nanoadsorbents. These findings show that the newly developed composite nanoadsorbents can sorb organic pollutants, and target sulfate and silicate anions, which makes them suitable candidates for water and wastewater treatments.

Silica Removal Using Magnetic Iron-Aluminum Hybrid Nanomaterials: Measurements, Adsorption Mechanisms, and Implications for Silica Scaling in Reverse Osmosis.

Composite magnetic aluminum hydroxide at iron oxide nanomaterials, Al(OH)3@Fe3O4, with a well-defined core-shell structure, were used as pretreatment adsorbents for the removal of silica in brackish water. The Al(OH)3 outer shell confers silica adsorption capacity, and the superparamagnetic Fe3O4 core allows material separation and magnetic recovery. The as-prepared nanomaterials (2 g L-1) remove ∼95 and ∼80% silica from Si-rich solutions with 0.5 and 2 mM initial silica concentrations, respectively. Regeneration under basic conditions was evaluated, and post-adsorption treatment with 0.05 M NaOH yielded optimal material reusability. After four regeneration cycles, the Al(OH)3@Fe3O4 nanomaterials retain their magnetic property while still being able to remove ∼40% silica from solutions at an adsorbent concentration of 2 g L-1. The mechanism of silica adsorption onto the surface of the nanomaterials was probed using several spectroscopic techniques. ATR-FTIR (attenuated total reflection-Fourier transform infrared) integrated with two-dimensional correlation analysis shows that silica species vary from Q2 to Q4 with adsorption time corresponding to silica polymerization. 29Si solid-state NMR spectra show an upfield chemical shift displacement of the Q2 signal, which indicates the formation of Q4 units, suggesting silica polymerization onto the Al(OH)3 shell. In addition, a laboratory-scale reverse osmosis setup was used to evaluate Al(OH)3@Fe3O4 as pretreatment materials for silica removal. Results show that silica scaling was significantly alleviated, and water recovery was enhanced when feed waters were pretreated with the magnetic nanomaterials. Taken together, our study highlights the promise of magnetic Al(OH)3@Fe3O4 nanomaterials in treating brackish water and achieving higher water recovery for inland desalination.