Uptake Of Oxyanions Research Articles

Amorphous metal-organic framework UiO-66-NO2 for removal of oxyanion pollutants: Towards improved performance and effective reusability

Water pollution is one of the most significant environmental problems nowadays. The remediation of oxyanion pollutions is of great importance due to their recalcitrance and persistence in the environment as well as acute toxicities even at trace concentration levels. In this study, a water stable metal–organic framework (MOF), UiO-66-NO2, was prepared and deliberately amorphized in order that it will assume a more open and dynamic structure. The resulting amorphous UiO-66-NO2 (am-UiO-66-NO2) demonstrated a great efficacy in the removal of representative oxyanion pollutants in wastewater, i.e., arsenic, chromium and selenium species. It was found that the am-UiO-66-NO2 adsorbent exhibited enhanced adsorption capacities as well as an excellent reusability, which was less reported in literature. Notably, it was found that it could be effectively regenerated and reused whilst retaining more than 80% of its capacity after 8 cycles of applications. The amorphized MOF as well as its adsorption behaviors were thoroughly characterized and analyzed using X-ray photoelectron spectroscopy; the uptake of oxyanions was mainly due to the formation of Zr-O-M (M: oxyanions) complexes.

Behaviour of kaolinite intercalation compounds with selected ammonium salts in aqueous chromate and arsenate solutions

The removal of aqueous Cr(VI) and As(V) oxyanions from waters by different materials with sorption properties is of environmental importance. In this study, a methoxy-kaolinite derivative was intercalated with benzyltrimethylammonium (B1), tetramethylammonium (TMA), and benzyldimethylhexadecylammonium (B5) chlorides and the interaction of the obtained materials with oxyanions was examined. The PXRD (powder X-ray diffraction) and IR (Infrared spectroscopy) analyses indicated a monolayer arrangement of the B1 and TMA molecules in the interlayer space of the mineral, while a tilted arrangement was noticed in the case of B5. A complete or partial deintercalation of introduced molecules was observed in the reactions with aqueous solutions of Cr(VI) and As(V). In all studied systems a significant improvement of the oxyanions removal was observed as compared to the pure kaolinite. The highest uptake of oxyanions was noticed in the reaction with B5-intercalated material. This was due to precipitation of organic alkyl salts. The formation of alkylchromate was confirmed using FTIR spectroscopy. The lower uptake of oxyanions by the B1- and TMA-intercalated materials was due to lack of new solid precipitation and resulted from the ion-exchange of chlorides initially compensating the ammonia nitrogen charge. The experimental sorption isotherms for all the reactions were best represented by Langmuir equation. A gradual, two-step removal process of Cr(VI) and As(V) by B1- and TMA-intercalated materials was observed. In turn, the precipitation of alkyl salts in reaction with B5-intercalated material resulted in a rapid immobilization of the oxyanions. The kinetic data modelled using pseudo-second order equation showed very good agreement with experimental results.

Removal of Selenium and Arsenic Oxyanions Using Natural Goethite-Rich Iron Ore from Daitari, Orissa, India: Effect of Heat Treatment

Sorption capacities of raw and calcined goethite-rich iron ore (GRI) collected from Datari iron-ore mines in Orissa for the removal of As and Se oxyanions were investigated in this study. Thermally treated GRI at 300 °C (GRI-300), having the maximum surface area (49.5 m2/g) and pore volume (0.028 cm3/g), showed maximum uptake of As(III), As(V) and Se(IV) oxyanions from their aqueous solutions, which progressively decreased when the pH of the solution is increased. At pH 6.0 (approximately), 2.0 g/l GRI-300 dose could remove more than 90% of As(III), As(V) and Se(IV) from their aqueous solutions containing 0.215, 0.265 and 20 mg/l amounts of the compound, respectively. The presence of co-existing ions showed a significant decrease in the uptake of oxyanions by GRI-300 in (PO43- > Cl- > NO3-). The present set of data may be helpful to further asses the potential of readily available GRI to remove different toxic ions from contaminated waters.

Synthetic hydrotalcite-type and hydrocalumite-type layered double hydroxides for arsenate uptake

The objectives of this study were to (i) synthesize cost-effective layered double hydroxides (LDH) or anionic clays to remove arsenate from water and (ii) quantify arsenate uptake by LDH and understand the mechanisms of uptake. Hydrotalcite and hydrocalumite-type LDHs were synthesized by coprecipitation at room temperature or higher with different compositions of layers and interlayers. The kinetics study showed that anion exchange apparently attained a steady-state in the range of 8–16 h. The arsenate removal was 100% and 99.9% with nitrate form of hydrotalcite and hydrocalumite, respectively. Carbonate and chloride forms of hydrotalcite and chloride form of hydrocalumite removed 50–90% of arsenate from solution. The uptake capacities of hydrotalcites synthesized by different methods were also compared. Hydrotalcite-type LDH synthesized by coprecipitation method had greater uptake capacity than those synthesized by hydrothermal method because of smaller crystal size in the former. The uptake of oxyanions with calcined hydrotalcite-type LDH was higher than with commercially available uncalcined carbonate form of hydrotalcite-type LDH, as expected. Calcination of hydrotalcite-type LDH produced intermediate non-stoichiometric oxides, which underwent rehydration and regeneration of the structure with the incorporation of these oxyanions. In the presence of much larger concentrations of other anions, the uptake of arsenate was reduced but it was still selective on LDH. The results of uptake were confirmed by powder X-ray diffraction (XRD), Raman spectroscopy, and scanning electron microscopy (SEM).