Quantitative mechanisms of cadmium adsorption on rice straw- and swine manure-derived biochars

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.