Crystallization Behaviour of Iron-Hydroxide Sulphates by Aging under Ambient Temperature Conditions

Efficient adsorption of Cr(VI) in acidic environment by nano-scaled schwertmannite prepared through pH regulation: characteristics, performances, and mechanism

Acidic Cr(VI)-containing wastewater has received increasing attention in recent years. Schwertmannite is a suitable adsorbent for its acid resistance and good adsorption ability. However, it shows poor Cr(VI) adsorption performance under acidic conditions. Herein, inspired by the fast neutralization-mineralization process of acid mine drainage (AMD) triggered by alkaline rocks, a novel nano-scaled schwertmannite (Sch-2.7) with high Cr(VI) adsorption capacity was synthesized at constant pH of 2.7 via adding OH^-. Compared with common schwertmannite (Sch), appropriate OH^- effectively improved mineral yield (the precipitation efficiency of Fe: 96.75% vs. 29.93%), specific surface area (65.1 m²/g vs. 18.9 m²/g), surface group content, and further Cr(VI) adsorption ability of Sch-2.7. The maximum adsorption capacity was 54.17 (pH = 3), 61.59 (pH = 4), and 66.5 mg/g (pH = 5) for Sch-2.7, whereas only 20.35, 24.51, and 27.17 mg/g for Sch. On average, the former was 2.53 times higher than the latter. Temperature and coexisting ions had little influences on the sorption process of Sch-2.7. The mechanism analysis demonstrated that the Cr(VI) removal by Sch-2.7 was a more thermodynamic favorable process due to abundant reactive-active components on Sch-2.7 for adsorption reaction. This work provided new insight into performance optimization and application potential on Cr(VI) removal of schwertmannite.

Role of biogenic Fe(III) minerals as a sink and carrier of heavy metals in the Rio Tinto, Spain

Oxidation of sulfide ores in the Iberian Pyrite Belt region leads to the presence of extremely high concentration of dissolved heavy metals (HMs) in the acidic water of the Rio Tinto. Fe(II) is microbially oxidized resulting in the formation of suspended particulate matter (SPM) consisting of microbial cells and Fe(III) minerals with co-precipitated HMs. Although substantial amount of HM-bearing SPM is likely deposited to river sediment, a portion can still be transported through estuary to the coastal ocean. Therefore, the mechanisms of SPM formation and transport along the Rio Tinto are important for coastal-estuarine zone. In order to reveal these mechanisms, we performed diurnal sampling of Rio Tinto water, mineralogical and elemental analysis of sediment from the middle course and the estuary of the river. We identified two divergent but interrelated pathways of HM transfer. The first longitudinal pathway is the transport of SPM-associated metals such as As (6.58 μg/L), Pb (3.51 μg/L) and Cr (1.30 μg/L) to the coastal ocean. The second sedimentation pathway contributes to the continuous burial of HMs in the sediment throughout the river. In the middle course, sediment undergoes mineralogical transformations during early diagenesis and traps HMs (e.g. 1.6 mg/g of As, 1.23 mg/g of Pb and 0.1 mg/g of Cr). In the estuary, HMs are accumulated in a distinct anoxic layer of sediment (e.g. 1.5 mg/g of As, 2.09 mg/g of Pb and 0.04 mg/g of Cr). Our results indicate that microbially precipitated Fe(III) minerals (identified as ferrihydrite and schwertmannite) play a key role in maintaining these divergent HM pathways and as a consequence are crucial for HM mobility in the Rio Tinto.