We quantified and investigated mechanisms for Cd2+ adsorption on biochars produced from plant residual and animal waste at various temperatures. Ten biochars were produced by pyrolysis of rice straw (RB) and swine manure (SB) at 300-700 °C and characterized. The Cd2+ adsorption isotherms, adsorption kinetics, and desorption characteristics were studied via a series of batch experiments, and Cd2+-loaded biochars were analyzed by SEM-EDS and XRD. The total Cd2+ adsorption capacity (Qc) increased with pyrolysis temperature for both biochars, however, rice straw-derived biochars had greater Qc than swine manure-derived biochars; hence, the biochar derived from rice straw at 700 °C (RB700) had the largest Qc, 64.4 mg g-1, of all studied biochars. Cadmium adsorption mechanisms in this study involved precipitation with minerals (Qcp), cation exchange (Qci), complexation with surface functional groups (Qco), and Cd-π interactions (Qcπ). Both the pyrolysis temperature and feedstock affected the quantitative contributions of the various adsorption mechanisms. The relative percent contributions to Qc for Cd2+ adsorption by RB and SB were 32.9-72.9% and 35.0-72.5% for Qcp, 21.7-50.9% and 20.4-43.3% for Qci, 2.2-14.8% and 1.4-18.8% for Qco, and 1.4-3.1% and 3.0-5.8% for Qcπ, respectively. For biochars produced at higher pyrolysis temperatures, the contributions of Qcp and Qcπ to adsorption increased, while the contributions of Qci and Qco decreased. Generally, Qcp dominated Cd2+ adsorption by high-temperature biochars (700 °C) (accounting for approximately 73% of Qc), and Qci was the most prominent mechanism for low-temperature biochars (400 °C) (accounting for 43.3-50.9% of Qc). Results suggested that biochar derived from rice straw is a promising adsorbent for the Cd2+ removal from wastewater and that the low-temperature biochars may outperform the high-temperature biochars for Cd2+ immobilization in acidic water or soils.
Keywords: Adsorption mechanism; Biochar; Cadmium; Desorption; Pyrolysis.