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

Abstract

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 (Q_c) increased with pyrolysis temperature for both biochars, however, rice straw-derived biochars had greater Q_c than swine manure-derived biochars; hence, the biochar derived from rice straw at 700 °C (RB700) had the largest Q_c, 64.4 mg g⁻¹, of all studied biochars. Cadmium adsorption mechanisms in this study involved precipitation with minerals (Q_cp), cation exchange (Q_ci), complexation with surface functional groups (Q_co), and Cd-π interactions (Q_cπ). Both the pyrolysis temperature and feedstock affected the quantitative contributions of the various adsorption mechanisms. The relative percent contributions to Q_c for Cd²⁺ adsorption by RB and SB were 32.9–72.9% and 35.0–72.5% for Q_cp, 21.7–50.9% and 20.4–43.3% for Q_ci, 2.2–14.8% and 1.4–18.8% for Q_co, and 1.4–3.1% and 3.0–5.8% for Q_cπ, respectively. For biochars produced at higher pyrolysis temperatures, the contributions of Q_cp and Q_cπ to adsorption increased, while the contributions of Q_ci and Q_co decreased. Generally, Q_cp dominated Cd²⁺ adsorption by high-temperature biochars (700 °C) (accounting for approximately 73% of Q_c), and Q_ci was the most prominent mechanism for low-temperature biochars (400 °C) (accounting for 43.3–50.9% of Q_c). 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.

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