Effects of harvest time and desalination of feedstock on Spartina alterniflora biochar and its efficiency for Cd2+ removal from aqueous solution

Abstract

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 –CHx). 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.