Long-term immobilization of cadmium and lead with biochar in frozen-thawed soils of farmland in China.

The problem of potentially toxic elements (PTEs) in farmland is a key issue in global pollution prevention and control and has an important impact on environmental safety, human health, and sustainable agricultural development. Based on the climate background of high-latitude cold regions, this study simulated freeze-thaw cycles through indoor tests. Different initial conditions, such as biochar application rates (0%, 1%, 2%) and different initial soil moisture contents (15%, 20%, 25%), were set to explore the morphological changes in cadmium (Cd) and lead (Pb) in soil and the response relationship to the changes in soil physicochemical properties. The results indicate that soil pH decreases during freeze-thaw cycles, and soil alkalinity increases with increasing biochar content. Freeze-thaw cycles caused the total amount of PTEs to have a U-shaped distribution, and the amount of PTEs in the soluble (SOL) and reducible (RED) fraction increased by 0.28-56.19%. Biochar reduced the amount of Cd and Pb migration in the soil, and an increase in soil moisture content reduced the availability of Cd and Pb in the soil. Freezing and thawing damaged the soil structure, and biochar reduced the fractionation of small particle aggregates by enhancing the stability of soil aggregates, thereby reducing the soil's ability to adsorb Cd and Pb. In summary, for farmland soil remediation and pollution control, the application of biochar has a certain ability to optimize soil properties. Considering the distribution of PTEs in the soil and the physicochemical properties of the soil, the application of 1% biochar to soil with a 20% moisture content is optimal for regulating seasonally frozen soil remediation.

Preparation of durable magnesium oxysulfate cement with the incorporation of mineral admixtures and sequestration of carbon dioxide.

To reduce the consumption of energy and raw materials caused by the production of Portland cement and enhance the carbon dioxide sequestration of building materials, this paper aims to manufacture durable and green magnesium oxysulfate cement based on incorporating mineral admixtures (fly ash (FA) or ground granulated blast-furnace slag (GGBFS)) and CO2 curing treatment. Compressive strength, flexural strength, resistance to water and wetting-drying cycles of magnesium oxysulfate (MOS) were evaluated. Phase compositions and microstructures of typical samples were measured by X-ray diffraction (XRD), differential scanning calorimetry (DSC-TG), and scanning electron microscope (SEM) techniques. The results showed that mechanical strength and strength retention after wetting-drying treatment of MOS cement was increased by the sequestration of carbon dioxide. Both FA and GGBFS could improve the water resistance due to restrainting the phase conversion of MgO into Mg(OH)2. However, the addition of FA or GGBFS deteriorated the compressive strength of MOS cement samples after wetting-drying treatment, owing to the formation of more magnesium hydroxide crystals and decomposition of 5 Mg(OH)2·MgSO4·7H2O (5·1·7 phase). Furthermore, about 5% carbon dioxide can be captured by MOS cement paste during 24 h accelerated carbonation treatment. Therefore, the incorporation of mineral admixtures and sequestration of carbon dioxide was suggested as an effective method in manufacturing the highly durable and cleaner magnesium oxysulfate cement.