Arsenic re-mobilization in water treatment adsorbents under reducing conditions: Part II. XAS and modeling study

Stable Magnetite@La-Fe Oxide Core-Shell Nanostructures Prepared via Lattice Lock for Reusable Extraction of Phosphate Anions

Stable magnetic core-shell nanostructures are developed by lattice locking lanthanide-iron (La-Fe) oxide shells with magnetite cores to prevent the release of La from the surfaces of the magnetite nanostructures. The resulting core-shell nanostructures demonstrate excellent outstanding regeneration performance and high adsorption capacity for phosphate (115 mg P·g-1). These nanostructures release minimal La from the magnetite core surfaces after adsorbent regeneration, with a La loss of only 20% compared to the control sample, Mag@La(OH)3. La3+ ions were released at concentrations ranging from 1 to 2.3 μg·L-1 at pH levels of 4 to 8, which is within the metal content range found in natural aquatic environments. These results demonstrate the high stability of the nanostructures after regeneration. Furthermore, the adsorbent exhibits high extraction capacity across a wide pH range of 4 to 10 and performs well even in the presence of interfering anions at phosphate-to-anion molar ratios of 1:5, 1:25, and 1:100. Microscopic and spectroscopic analyses reveal that the primary extraction mechanism of phosphate in the La-containing shells is surface precipitation. This approach not only improves the use of magnetic core-shell nanostructures as adsorbents but also demonstrates the creation of a broad range of stable magnetic functional materials for diverse applications.

Selective adsorption of arsenic over phosphate by transition metal cross-linked chitosan

The ability of transition metal chitosan complexes (TMCs) of varying valence and charge to selectively adsorb As(III) and As(V) over their strongest adsorptive competitor, phosphate is examined. Fe(III)-chitosan, Al(III)-chitosan, Ni(II)-chitosan, Cu(II)-chitosan, and Zn(II)-chitosan are synthesized, characterized via Attenuated Total Reflectance-Fourier Transform Infrared spectroscopy (ATR-FTIR) and X-ray Diffractometry (XRD), and their selective sorption capabilities towards As(III) and As(V) in the presence of phosphate are evaluated. It was found that the stability of the metal-chitosan complexes varied, with Al(III)- and Zn(II)-chitosan forming very unstable complexes resulting in precipitation of gibbsite, and Wulfingite and Zincite, respectively. Cu(II)-, Ni(II)-, and Fe(III)-chitosan formed a mixture of monodentate and bidentate complexes. The TMCs which formed the bidentate complex (Cu(II)-, Ni(II)-, and Fe(III)-) showed greater adsorption capability for As(V) in the presence of phosphate. Using the binary separation factor ∝ t / c , it can be shown that only Fe(III)-chitosan is selective for As(V) and As(III) over phosphate. Density Functional Theory (DFT) modeling and extended X-ray adsorption fine structure (EXAFS) determined that Fe(III)-chitosan and Ni(II)-chitosan adsorbed As(V) and As(III) via inner-sphere complexation, while Cu(II)-chitosan formed mainly outer-sphere complexes with As(V) and As(III). These differences in complexation likely result in the observed differences in selective adsorption capability towards As(V) and As(III) over phosphate. It is hypothesized that the greater affinity of Fe(III)- and Ni(II)-chitosan towards As(V) and As(III) compared to Cu(II)-chitosan is due to their forming less-stable, more reactive chitosan complexes as predicted by the Irving Williams Series.

Activation of natural halloysite nanotubes by introducing lanthanum oxycarbonate nanoparticles via co-calcination for outstanding phosphate removal

Efficient phosphate capture achieved via activating halloysite nanotubes to create functional alumina nanoparticles by anchoring La-based nanoparticles to tube surfaces.

Activating 2D nano-kaolinite using hybrid nanoparticles for enhanced phosphate capture

A natural 2D kaolinite host is activated by hybridizing with adsorptive La-based nanoparticles, demonstrating efficient phosphate capture.

An X-ray absorption study of synthesis- and As adsorption-induced microstructural modifications in Fe oxy-hydroxides

Synthetic adsorbents based on Fe oxy-hydroxides (FeOOH) prepared under a wide range of pH-values via intense oxidation conditions of FeSO4 as well as the As(III) and As(V) adsorption mechanism are investigated using X-ray absorption fine structure (XAFS) spectroscopies at the Fe- and As-K-edges. Synthesis in an alkaline environment promotes the face-connectivity of the Fe(O,OH)6 chains at the expense of edge- and corner-sharing linkage, which is consistent with the lower surface charge density and in turn with the lower arsenic adsorption capacity. Microstructural changes are also detected after As(V) adsorption onto FeOOH synthesized at pH 5.5: the ratio of face-/edge-sharing sites increases from approximately 0.4-0.7 as a function of the As(V)-loading. This modification of the polymeric Fe(O,OH)6 structure at higher As/Fe ratios is attributed to strong As(V) bidentate mononuclear ((2)E) and binuclear ((2)C) adsorption. In contrast, no alterations in the FeOOH microstructure were observed, possibly due to the weaker (2)E linkage of As(III).

Microscale speciation of arsenic and iron in ferric-based sorbents subjected to simulated landfill conditions

During treatment for potable use, water utilities generate arsenic-bearing ferric wastes that are subsequently dispatched to landfills. The biogeochemical weathering of these residuals in mature landfills affects the potential mobilization of sorbed arsenic species via desorption from solids subjected to phase transformations driven by abundant organic matter and bacterial activity. Such processes are not simulated with the toxicity characteristic leaching procedure (TCLP) currently used to characterize hazard. To examine the effect of sulfate on As retention in landfill leachate, columns of As(V) loaded amorphous ferric hydroxide were reacted biotically at two leachate sulfate concentrations (0.064 mM and 2.1 mM). After 300 days, ferric sorbents were reductively dissolved. Arsenic released to porewaters was partially coprecipitated in mixed-valent secondary iron phases whose speciation was dependent on sulfate concentration. As and Fe XAS showed that, in the low sulfate column, 75-81% of As(V) was reduced to As(III), and 53-68% of the Fe(III) sorbent was transformed, dominantly to siderite and green rust. In the high sulfate column, Fe(III) solids were reduced principally to FeS(am), whereas As(V) was reduced to a polymeric sulfide with local atomic structure of realgar. Multienergy micro-X-ray fluorescence (ME-μXRF) imaging at Fe and As K-edges showed that As formed surface complexes with ferrihydrite > siderite > green rust in the low sulfate column; while discrete realgar-like phases formed in the high sulfate systems. Results indicate that landfill sulfur chemistry exerts strong control over the potential mobilization of As from ferric sorbent residuals by controlling secondary As and Fe sulfide coprecipitate formation.

Fabrication, characterization, and application of a composite adsorbent for simultaneous removal of arsenic and fluoride

Coexisting arsenic (As) and fluoride (F) in groundwater poses severe health risks worldwide. Highly efficient simultaneous removal of As and F is therefore of great urgency and high priority. The purpose of this study was to fabricate a novel composite adsorbent and explore the mechanism for concurrent removal of As(V) and F at the molecular level. This bifunctional adsorbent with titanium and lanthanum oxides impregnated on granular activated carbon (TLAC) exhibits a pronounced As(V) and F adsorption capacity over commercially available iron- and aluminum-based adsorbents for synthetic and real contaminated groundwater samples. Synchrotron-based X-ray microfluorescence analysis demonstrates that La and Ti were homogeneously distributed on TLAC. Extended X-ray absorption fine structure spectroscopic results suggest that As(V) formed bidentate binuclear surface complex as evidenced by an averaged Ti-As bond distance of 3.34 Å in the presence of F. Adsorption tests and Fourier transform infrared spectroscopy analysis indicate that F was selectively adsorbed on lanthanum oxides. The surface configurations constrained with the spectroscopic results were formulated in the charge distribution multisite complexation model to describe the competitive adsorption behaviors of As(V) and F. The results of this study indicate that TLAC could be used as an effective adsorbent for simultaneous removal of As(V) and F.

Remediation of organic and inorganic arsenic contaminated groundwater using a nanocrystalline TiO2-based adsorbent

A nanocrystalline TiO2-based adsorbent was evaluated for the simultaneous removal of As(V), As(III), monomethylarsonic acid (MMA), and dimethylarsinic acid (DMA) in contaminated groundwater. Batch experimental results show that As adsorption followed pseudo-second order rate kinetics. The competitive adsorption was described with the charge distribution multi-site surface complexation model (CD-MUSIC). The groundwater containing an average of 329 microg L(-1) As(III), 246 microg L(-1) As(V), 151 microg L(-1) MMA, and 202 microg L(-1) DMA was continuously passed through a TiO2 filter at an empty bed contact time of 6 min for 4 months. Approximately 11,000, 14,000, and 9900 bed volumes of water had been treated before the As(III), As(V), and MMA concentration in the effluent increased to 10 microg L(-1). However, very little DMA was removed. The EXAFS results demonstrate the existence of a bidentate binuclear As(V) surface complex on spent adsorbent, indicating the oxidation of adsorbed As(III).