| Adsorption/desorption of arsenate onto/from iron (hydr)oxides controls arsenicmorphology, migration and transformation in soil and aquatic environment. Arsenateadsorption onto well crystalline iron (hydr)oxides has been studied extensively,however, the researches about arsenate adsorption difference resulted from thedifference of structure of iron (hydr)oxides are not enough, especially the surfacereaction characteristics and mechanisms of arsenate adsorption onto weak crystallineiron (hydr)oxides that is widely distributed in environment. This research used thetechniques of X-ray diffraction (XRD), scanning electron microscopy (SEM), thermalgravimetric analysis method (TGA), fourier transform infrared spectroscopy (FT-IR)and pore size analysis, with goethite and hematite as reference, studied the superiorityof ferrihydrite for adsorbing arsenate, and it also discussed desorption characteristicsof arsenate from iron (hydr)oxides by low-molecular-weight organic acid.Compared with goethite and hematite, ferrihydrite had poorer crystallization,more physically adsorbed H2O, and larger specific surface area (285.3m2·g-1). Boththe rate and amount of arsenate adsorption onto ferrihydrite were much higher thanthat onto goethite and hematite. Three iron (hydr)oxides all showed a fast initialarsenate adsorption reaction followed by slow processes, and each stage fitted thefirst-order kinetic equation. Ferrihydrite had the highest adsorption rate in each stage,which was approximately one order of magnitude higher than that of goethite andhematite, and showed a significant slow diffusion reaction. At the30h afteradsorption reaction, the amount of arsenate adsorbed and OH-released was in theorder:737.5μmol·g-1(ferrihydrite)>>90.67μmol·g-1(goethite)>66.49μmol·g-1(hematite) and558.3μmol·g-1(ferrihydrite)>>61.67μmol·g-1(goethite)>53.33μmol·g-1(hematite). The arsenate adsorption onto iron (hydr)oxides included bothspecific adsorption and non-specific adsorption, and specific adsorption was mainly toform bidentate binuclear surface complexes. Arsenate adsorption capacity offerrihydrite was much higher than that of goethite and hematite. Arsenate adsorption isotherms for goethite and hematite agreed well with both Langmuir model andFreundlich model, but the former was better. According to the fitting results usingLangmuir model, the maximum amount of arsenate adsorption onto goethite andhematite was107.5μmol·g-1and92.72μmol·g-1, respectively. However, forferrihydrite, Freundlich model fitted better, which was related to its developedmicropore and higher heterogeneity of surface. The amount of arsenate adsorption byferrihydrite at maximum arsenate equilibrium concentration (2.399mmol·L-1) was901.7μmol·g-1. The desorption ability of three desorption solution was in the order:citric acid> oxalic acid> NaCl solution. Citric acid and oxalic acid had significantinduced effects on arsenate desorption from iron (hydr)oxides. Citric acid and oxalicacid could significantly promote desorption reaction with the mechanisms of ligandexchange and ligand-enhanced dissolution. However, for citric acid molecule hasmore ligand-OH than oxalic acid molecule, its desorption ability was stronger thanthe latter. Arsenate desorbed by citric acid and oxalic acid contained arsenic by bothspecific adsorption and non-specific adsorption, however, NaCl solution could onlydesorbed arsenic by non-specific adsorption. In all the three desorption solutions,ferrihydrite had the largest arsenate desorption amount and the lowest arsenatedesorption percentage. In the process of arsenate desorption, ferrihydrite was easier tobe dissolved in citric acid and oxalic acid solution than goethite and hematite. With itshigher arsenate adsorption capacity and its abundance, ferrihydrite plays a moreimportant role on geochemical behavior of arsenate in environment than goethite andhematite. |
PDF Full Text Request