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Calcium carbonate (CaCO3)-based nanoadsorbents have gained widespread attention due to their superiority in removing toxic heavy metal ions. Considering that the inherent structure of the material determines its performance, it is important to fully understand the effect of different crystals of CaCO3 on the removal of heavy metals. On the other hand, the difficult separation of nanoadsorbents during in situ remediation may cause the potential risk of secondary pollution by the remaining adsorbents. Herein, the magnetic calcium carbonate adsorbents with nanostructure (MCCR-X, X represent the annealing temperature) were fabricated with a tunable crystal structure of CaCO3 by adjusting the annealing temperature. Results from batch experiments revealed the crystal structure (CaCO3)-dependent adsorption performance of MCCRs toward Pb-(II) and Cd-(II). MCCR-350 with an aragonite phase showed the outstanding adsorption toward both Pb-(II) and Cd-(II) with maximum adsorption capacity of 1179 and 821 mg/g, and MCCR-RAW with a vaterite/aragonite phase also has similar adsorption properties. However, MCCR-550 with the calcite phase displayed a superhigh adsorption toward Pb-(II) only but very weak adsorption toward Cd-(II) with a capacity of 1350 and 30 mg/g in the initial concentration of 300 mg/g. The findings indicated that the efficiently tunable removal of the heavy metal ion Pb-(II) or Cd-(II) by MCCRs can be realized by regulating the crystal phase of CaCO3. X-ray photoelectron spectroscopy technology (XPS) was employed to clarify the possible mechanism of the crystal phase of CaCO3-dependent adsorption ability for Pb-(II)/Cd-(II) removal by MCCRs. This work provides an understanding of the crystal-structure-related adsorption properties of CaCO3 and also gives guidance for designing a CaCO3-based adsorbent for removal/recovery of heavy metal ions. [ABSTRACT FROM AUTHOR]

Solution polymerization synthesized alt-resistant superabsorbent poly (acrylic acid-acrylamide/fly ash) composites. The mass ratio of acrylic acid (AA) to acrylamide (AM), the concentration of crosslinker, the neutralization degree (ND) of AA, and the polymerization temperature were investigated by single-factor method. Optimized conditions for the synthesis of poly (acrylic acid-acrylamide/fly ash) (PAA-AM/FA) are, as following: m (AA)/m (AM) is 1.5, the content of crosslinker N, N-methylenebisacrylamide. (MBA) is 0.7%, neutralization degree of AA is 70%, polymerization temperature is 70 °C, and fly ash (FA) content is 50%. The prepared PAA-AM/FA demonstrated superior water absorption performance. The absorption capacities of PAA-AM/FA for pure water and 0.9% NaCl solution were found to be 976 g·g−1 and 81 g·g−1, respectively. Furthermore, PAA-AM/FA was found to have excellent adsorption capacity (148 mg·g−1) for Rhodamine B in water. Fourier Transform-Infrared Spectroscopy (FT-IR), Thermogravimetric Analysis (TGA), and Scanning Electron Microscopy (SEM) characterized the prepared materials. Results showed that fly ash was incorporated into the macromolecular polymer matrix and played a key role in improving the performance of the polymer composites. [ABSTRACT FROM AUTHOR]