Biochar from Rice Straw for Cu2+ Removal from Aqueous Solutions: Mechanism and Contribution Made by Acid-Soluble Minerals

Biochar is a promising material for removing metal ions from water and soil through adsorption. In this study, rice straw was pyrolyzed to prepare biochars at 300 °C (RSBC300), 500 °C (RSBC500), and 700 °C (RSBC700) in an oxygen-limited atmosphere. The biochars were used in batch experiments for adsorption of copper (Cu) ions in aqueous solution. The influence of various environmental conditions, including solution pH, solid-to-liquid ratio, contact time, and environmental temperature, on Cu removal were systematically investigated. To explore the adsorption mechanisms and ascertain the contribution of acid-soluble minerals, adsorption kinetics and isotherm analyses, Fourier transform infrared spectroscopy (FTIR), and X-ray photoelectron spectroscopy (XPS) were performed. The results showed that the removal rate reached 99.6% when the experimental condition was a solid-to-liquid ratio of 1 g L −1 , pH of 6.0, and the initial Cu concentration of 30 mg L −1 . The pseudo-second-order model and the two-compartment kinetic model well fitted with the Cu adsorption kinetics process onto RSBC700, whereas the Freundlich, Temkin, and Dubinin–Radushkevich (D–R) models best described the Cu adsorption isotherm process on RSBC700. Higher temperatures improved Cu removal from solution with the maximum adsorption capacity being 52.5 mg g −1 at 45 °C. The influence of co-existing Ca 2+ and Mg 2+ on Cu removal by RSBC700 was limited, whereas tetracycline exhibited some inhibition effect. The role of acid-soluble minerals in biochar for Cu removal cannot be ignored, especially in treatment using RSBC500 (contribution rate at 47.2–57.1%). The mechanism underlying Cu removal by rice straw biochar involved electrostatic interaction, complexation, cation–π interaction, and precipitation. Therefore, high temperature-derived rice straw biochar can be expected to an adsorbent to alleviate Cu pollution in wastewater.