Following oil extraction in the wetland of the Yellow River Delta, heavy metal contamination of coastal saline-alkaline soil, especially with cadmium (Cd), has become a serious environmental problem in some regions. Biochar application has been proposed to remedy Cd-contaminated soil, but the remediation effect is related to preparation conditions of biochar (e.g., pyrolysis temperature and raw material) and soil properties. The invasive plant, Spartina alterniflora, produces a high amount of biomass, making it suitable for biochar production in coastal China. We investigated the effect of S. alterniflora-derived biochar (SDB) pyrolyzed at four temperatures (350, 450, 550, and 650 °C) crossed with three addition ratios (1, 5, and 10%) and control on Cd contamination of coastal saline-alkaline soil. Pyrolysis temperature affected pH, surface area, and functional groups of SDB. SDB markedly improved soil pH and soil organic matter, but the degree of improvement was affected by pyrolysis temperature and addition ratio. SDB significantly altered available Cd content in soil, but reduced it only at low pyrolysis temperatures (350 and 450 °C). Available Cd content had a positive correlation with soil pH (R² = 0.298, P < 0.01), but was not related to salinity and soil organic matter content. Thus, SDB pyrolyzed at 350 °C with 5% addition was optimal for passivating Cd in coastal saline-alkaline soil, since available Cd content in soil decreased mostly (by 26.9%). These findings act as a reference for the development of an application strategy for SDB to ameliorate Cd-contaminated coastal saline-alkaline soil.

Cadmium (Cd²⁺), as the primary contaminant in Chinese soils, is dangerous to human health and ecological security. Invasive plant Spartina alterniflora in Chinese coastal wetlands presents a promising feedstock for biochar, which is an efficient adsorbent for heavy metal removal. S. alterniflora harvested in summer, autumn and winter were pyrolyzed to produce biochars. We analyzed the effects of harvest time and desalination of feedstock on biochar properties and Cd²⁺ adsorption capacity in aqueous solution. Biochars were characterized by pH probe, elemental analyzer, SEM, BJH, BET, and FTIR, and the Cd²⁺ concentrations were measured using AAS. Except pH (9.85-10.95) and nitrogen contents (0.71-1.59%), other biochar properties had no linear correlations with harvest time. Biochars produced from feedstock harvested in autumn had the highest carbon contents (73.25%) and lowest functional groups diversity (CC and -CH_x). The pH and carbon contents (64.44-73.25%) were increased by desalination treatment. The surface area (0.48-2.27 m²/g), total pore volume (0.0015-0.0055 mL/g), mesopore volume (0.0015-0.0052 mL/g), and Cd²⁺ adsorption capacities (16.29-32.34 mg/g) were affected by desalination treatment, and the effects varied with harvest time. Biochars produced from desalted feedstock harvested in summer and untreated feedstock harvested in winter showed higher surface area, porosity, and Cd²⁺ adsorption capacity. Moderate salt contents (1.5-3.0% in chloride content) in feedstock promote the formation of biochars with higher surface area and porosity.