Green synthesis of fly ash‐based zeolite Y by mixed alkali fusion method

Immobilization of Pb and Zn in Contaminated Soil Using Alumina-Silica Nano-Amendments Synthesized from Coal Fly Ash

To apply coal fly ash to the remediation of heavy-metal-contaminated soil, an alumina-silica nano-amendment (ASNA) was synthesized from coal fly ash and was used for the immobilization of lead and zinc in contaminated soil. The investigation on the synthesis of the ASNA shows that the ASNA can be obtained under a roasting temperature of 700 °C, a ratio of alkali to coal fly ash of 1.2:1, and a molar ratio of silicon to aluminum of 1:1. The ASNA could increase the soil pH and cation exchange capacity (CEC) and decrease the bioavailability of Pb and Zn. When the ASNA addition increased from 0 to 2%, the bioavailability (extracted by CaCl₂) of Pb and Zn decreased by 47% and 72%, respectively. Moreover, the addition of the ASNA facilitated the transformation of Pb from a reducible fraction to oxidizable and residual fractions and Zn from an exchangeable fraction to a residual fraction. The correlation analysis and cluster analysis verify that the ASNA modulates the chemical speciation of heavy metals by increasing the soil's CEC and pH, thereby immobilizing heavy metals. It is expected that this study can provide a new method for the remediation of Pb- and Zn-contaminated soil.

Sustainable Management of Salt Slag

The management of salt slag, a waste from the secondary aluminum industry, is associated with huge environmental concerns due to the risk of atmospheric pollution (emission of toxic gases), groundwater contamination (high salt content that can percolate and cause an increase in salinity) and soil unavailability (large extensions required for disposal). Therefore, the development of a sustainable process for its treatment and recovery is of the utmost importance. In this work, a two-step process for the valorization of salt slag was developed that rendered zeolite as the main added-value product and NaCl and NH3 as byproducts. First, salt slag was hydrolyzed at 90 °C and at a solid/water ratio of 1/3. More than 90% of salt and ~90% of ammonia were recovered. In a second step, the hydrolyzed slag was completely transformed into a NaP zeolite under mild hydrothermal conditions. The zeolite exhibited specific surface area (17 m2 g−1), cation exchange capacity (2.12 meq g−1) and zeta potential (−52 mV) values that represent good characteristics for use in the removal of metal ions from aqueous effluents. The transformation of salt slag into zeolite can be considered a sustainable process with a high contribution to the circular economy.