The role of counter ions in nano-hematite synthesis: Implications for surface area and selenium adsorption capacity

Abstract

Nano metal oxides are of interest for aqueous selenium (Se) remediation, and as such, nano-hematite (nα-Fe2O3) was examined for use as a Se adsorbent. The effect of surface area on adsorption was also studied. nα-Fe2O3 particles were synthesized from Fe(NO3)3 and FeCl3 via forced hydrolysis. The resulting particles have similar sizes, morphologies, aggregate size, pore size, and PZC. The nα-Fe2O3 from FeCl3 (nα-Fe2O3-C) differs from the nα-Fe2O3 from Fe(NO3)3 (nα-Fe2O3-N) with a ∼25±2m(2)/g greater surface area. Selenite Se(IV) adsorption capacity on nα-Fe2O3 has a qmax ∼17mg/g for the freeze-dried and re-suspended nα-Fe2O3. The Δqmax for nα-Fe2O3 from Fe(NO3)3 and FeCl3 that remained in suspension was 4.6mg/g. For selenate Se(VI), the freeze-dried and re-suspended particles realize a Δqmax= 1.5mg/g for nα-Fe2O3 from Fe(NO3)3 and FeCl3. The nα-Fe2O3 from Fe(NO3)3 and FeCl3 that remained in suspension demonstrated Se(VI) Δqmax=5.4mg/g. In situ ATR-FTIR isotherm measurements completed for Se(VI) at a pH 6 suggest that Se(VI) forms primarily outer-sphere complexes with nα-Fe2O3 synthesized from both salts.

Enhanced Adsorption of Selenium Ions from Aqueous Solution Using Iron Oxide Impregnated Carbon Nanotubes

“…For example, the attachment of iron oxide particles onto the surface of CNTs may provide ample adsorption sites for the selenium ions to interact with. Also, it is well known that surfaces of metal oxides nanoparticles in aqueous solution are covered with hydroxyl groups [18]. Therefore, anion adsorption occurs by positive adsorbent surface charge (less negative sign compared to anion).…”

“…Therefore, anion adsorption occurs by positive adsorbent surface charge (less negative sign compared to anion). Generally, increasing the pH causes decrease in the adsorbent surface charge and, accordingly, decreases in the adsorbent capacity [18, 39, 54]. As a result, when pH is increased the adsorbent surface is negatively charged (more negative sign) and leads to repulsion between negatively charged adsorbent particles and selenium anions.…”

“…The decline in the rate of adsorption at initial concentrations higher (as can be seen from the slope of the trend line) than 30 ppm (Figure 8) may be attributed to the saturation of all adsorption sites on the surface of CNTs. The higher adsorption capacity at higher Se concentration may be due to increase in the mass transfer (driving force) of selenium ions towards the iron oxide impregnated CNTs surfaces [18]. The highest adsorption capacity was about 88 mg/g at an initial Se concentration of 40 ppm as shown in Figure 8.…”

“…The authors reported a maximum removal of 95% of Se (VI) ions at pH ~ 2. While it is not easy to compare the maximum capacity of an adsorbent for the Se removal given the varied preparation methods, costs, and experimental condition, a recent study using a new iron oxide nanoparticles reported a higher removal rates compared with literature reported values [18]. Having said that, the values are still small and the search for other adsorbents is still required for cost effective Se removal from water.…”

Bimetallic Oxide Nanohybrid Synthesized from Diatom Frustules for the Removal of Selenium from Water

“…Recent study on selenium removal using MgO sheets showed adsorption capacity of Se(IV) 103.52 mg/g and Se(VI) 10.58 mg/g [22]. Se(IV) Langmuir adsorption capacity of MNP@hematite is 25.0 mg/g, MNP core 15.3 mg/g, magnetic nanoparticle-graphene oxide 23.8 mg/g [81], Al(III)/SiO2 20.4 mg/g [82], a-Fe2O3 17.9 mg/g [83], Fe/Si coprecipitates 17.4 mg/g [20], MIO/MWCNTs 13.1 mg/g [21], CuFe2O4 14.1 mg/g [19], and iron manganese oxide 6.57 mg/g [15].…”