Jingke Sima

The conversion of waste biomass into biochar through inert pyrolysis represents a promising strategy for carbon sequestration. However, biochar production is often accompanied by the release of small molecular chemical substances during pyrolysis, and the resulting biochar is susceptible to environmental degradation. To enhance the carbon retention rate of biochar during pyrolysis and its stability in the environment, this study explored the incorporation of various metal soluble salts (CaCl₂, Ca(H₂PO₄)₂, MgCl₂, FeCl₃) and clay minerals (quartz, goethite, bentonite, albite) with two types of waste biomass (phragmites and goldenrod) for pre-treatment to enhance both carbon retention and stability in the resulting biochar. Furthermore, to elucidate the regulatory mechanisms of minerals on biochar structural formation, the three primary components of raw biomass—hemicellulose, cellulose, and lignin—were individually mixed with the minerals at a ratio of 1:5 (mineral/biomass, w/w) to produce biochars for a comparative analysis. The experimental results demonstrated that metal soluble salts, particularly Ca(H₂PO₄)₂, exhibited a superior performance in enhancing biochar’s carbon retention compared to clay minerals. Specifically, Ca(H₂PO₄)₂ treatment resulted in a remarkable 15% increase in the carbon retention rate. Through K₂Cr₂O₇ oxidation simulating soil aging conditions, Ca(H₂PO₄)₂-treated biochar showed approximately 12% greater stability than the untreated samples. This enhanced stability was primarily attributed to the formation of stable chemical bonds (C–O–P and P–O), which facilitated the preservation of aromatic carbon structures and small molecular compounds including sugars, alcohols, and ethers. Mechanistic investigations revealed that Ca(H₂PO₄)₂ significantly influenced the pyrolysis process by increasing the activation energy from 85.9 kJ mol⁻¹ to 156.5 kJ mol⁻¹ and introducing greater reaction complexity. During the initial pyrolysis stage (2PO₄)₂ catalyzed depolymerization, ring-opening, and C–C bond cleavage in hemicellulose, enhanced cellulose depolymerization, and side-chain cleavage in lignin phenylpropanes. In the intermediate temperature range (300–400 °C), Ca(H₂PO₄)₂ facilitated carboxylate nucleophilic addition reactions and promoted cyclization to form aromatic carbon structures. The innovative aspect of this work is that minerals can increase both the yield and carbon retention rate of biochar. Furthermore, it reveals the mechanisms underlying the improvements in pyrolysis, providing a scientific basis and theoretical foundation for better displaying the carbon sequestration potential of biochar in future applications.

Microplastics (MPs) inevitably undergo multiple aging processes during their life cycle in the environment. However, the information regarding the mechanical fragmentation behavior of MPs remained unclear, including the changes in the intrinsic properties of aged MPs, the measurement of aging degree, the underlying mechanism, and the interaction with heavy metals. Here, MPs (PS, PP, PET) were aged by crushing (-CR) and ball-milling (-BM) to simulate mild and severe mechanical fragmentation, respectively. Our results indicated that mechanical fragmentation significantly affected the morphology of MPs. The aging degree of MP-BM was deeper compared to MP-CR owing to smaller particle size, larger specific surface area, poorer heat resistance, better hydrophilicity, and richer oxygen-containing functional groups. The carbonyl index (CI) and O/C ratio were used to measure the aging degree of the two mechanical aging treatments. Besides, the mechanism was proposed and the discrepancy between the two treatments was elaborated from three aspects including the excitation energy source, reaction interface, and reaction dynamics. Furthermore, the extrinsic properties of MPs altered with the increase of aging degree; specifically, the adsorption capacities of heavy metals were enhanced. Meanwhile, it was unveiled that the CI value and O/C ratio played a vital role in estimating the adsorption ability of heavy metals. The findings not only reveal the mechanical fragmentation behavior of MPs but also provide new insights into the assessment of the potential risks of the aged MPs via chemical indexes.

Application of biochar to treat heavy metal polluted wastewater has received increasing attention; however, the immobilization ability of pristine biochar for metal ions is still limited. In this study, phosphate tailing was co-pyrolyzed with sawdust and peanut shell to acquire phosphorus-rich biochars with high removal rates for Cd, Zn, Pb, and Cu. Meanwhile, the improvement mechanisms by phosphate tailing were clarified by XRD, FTIR, SEM-EDS, BET-N₂, and model fitting. Results showed that after phosphate tailing impregnation, surface area of sawdust, and peanut shell biochars increased from to 11.6 m² g⁻¹, and from 43.5 to 53.4 m² g⁻¹, respectively. Functional groups of -COOH and CO₃²⁻ on biochar increased and the P₂O₇⁴⁻ newly generated. Besides, large amounts of Ca(PO₃)₂ and Ca₂P₂O₇ crystals were detected in biochar ash. As for sawdust biochar, loading of phosphate tailing raised the sorption rates of Cd, Zn, Pb, and Cu by 0.35, 0.61, 1.10, and 2.64 times, respectively, as for peanut shell biochar, it was raised by 0.12, 0.47, 0.11, and 1.98 times, respectively. The sorption isotherms by phosphate tailing-loaded biochars were better fitted to Langmuir (R² = 0.85–1.00) than Freundlich model (R² = 0.58–0.91). Heavy metals could bind with -OH and -COOH on phosphate tailing-loaded biochars, meanwhile generated phosphorus-rich precipitation with PO₃⁻ and P₂O₇⁴⁺, including Cd₂P₂O₇, Cd(PO₃)₂, Zn (PO₃)₂, Pb (PO₃)₂, Pb₂P₂O₇, Cu(PO₃)₂, and Cu₂P₂O₇. This study proposed an innovative method to produce phosphorus-rich biochars by loading phosphate tailing for highly efficient removal of heavy metals from water bodies, and also realized the resource utilization of phosphate tailing, which was of great significance to reduce environmental pollution.