Adsorption and co-precipitation behavior of arsenate, chromate, selenate and boric acid with synthetic allophane-like materials

Abstract

Pollution caused by boric acid and toxic anions such as As(V), Cr(VI) and Se(VI) is hazardous to human health and environment. The sorption characteristics of these environmentally significant ionic species on allophane-like nanoparticles was investigated in order to determine whether allophane can reduce their mobility in the subsurface environment at circum-neutral pH condition. Solutions containing 100 or 150 mmol of AlCl 3 . 6H 2 O were mixed to 100 mmol of Na 4 SiO 4 and the pH were adjusted to 6.4±0.3. The mineral suspensions were shaken for 1 hr and incubated at 80 o C for 5 days. Appropriate amounts of As, B, Cr and Se solutions were added separately during and after allophane precipitation. The results showed that As(V) and boric acid can be irreversibly fixed during co-precipitation in addition to surface adsorption. However, Cr(VI) and Se(VI) retention during and after allophane precipitation is mainly controlled by surface adsorption. The structurally fixed As(V) and boric acid were more resistant to release than those bound on the surface. The sorption characteristics of oxyanions and boric acid was also influenced by the final Si/Al molar ratio of allophane in which Al-rich allophane tend to have higher uptake capacity. The overall results of this study have demostrated the role of allophane-like 3 nanoparticles and the effect of its Si/Al ratio on As, B, Cr and Se transport processes in the subsurface environment.

Removal of barium, cobalt, strontium and zinc from solution by natural and synthetic allophane adsorbents

“…Furthermore, one can see that 5-10 min is enough to achieve adsorption equilibrium under the experimental conditions used in this study, which indicates that physical adsorption rather than ion exchange contributes mainly to the removal of metal ions by natural and synthetic allophane adsorbents. This finding is in line with previous experimental results, where allophane has been successfully used in the remediation of aqueous solutions that were contaminated with, for example, heavy metals cations [15,22,33,35], metal (oxy)anions [18,23,30], heterocyclic organic components [10] and anionic surfactants and organic acids [17,23]. …”

“…In addition, gibbsite and quartz were identified in this sample, which matches with the XRD results. The main lattice vibration bands of the allophanes are located between 1000-980cm -1 and at 870 cm -1 due to Si-O-(Si) or Si-O-(Al) vibrations and Si-OH groups [19,30,31]. The IR spectra of SynAllo-1 and SynAllo-2 are similar to those reported for natural allophane, which shows that proto-imogolite allophane or imogolite structures did not form under the given experimental conditions [19,25,32].…”

“…As expected, the removal efficiency strongly decreased and the time to reach adsorption equilibrium increased at decreasing pH (Table 1). This is because the ≡Al-OH° and ≡Si-OH° groups are more protonated under acidic conditions and potential binding sites are less available to retain metal ions [30].…”

“…However, the allophane surface exhibits a positive charge under acidic conditions and thus the metal ion uptake is limited due to repulsive electrostatic forces below the point of zero charge (PZC) value [30]. Thus, net adsorption increases significantly towards alkaline conditions, close to or above the PZC value, which is 7.8 to 9.4 for synthetic allophanes and 5.5 to 6.5 for natural allophanes [21,30,31]. The presence of trace amounts of goethite on the allophane surface could further affect the surface (charge) properties of NatAllo.…”

Synthesis and Identification of Allophane from Waste Blast Furnace Slag

“…Some researchers reported that As(V) and boric acid were adsorbed by formation of inner-sphere complex with exposed aluminol groups on the allophane surface while Cr(VI) and Se(VI) were adsorbed by formation of outersphere complex. 4,5) In this study, we have developed a synthesis process of allophane utilizing the undissolved residue of BFS by acid treatment. Identification of the synthesized allophane was performed by X-ray diffraction (XRD) for long range order, Fourier transform infrared (FT-IR) spectroscopy for short range order and transmission electron microscopy (TEM) for morphology.…”