Adsorption of Cd²⁺ from Aqueous Solution by Modified Rice Husk-derived Biochars

To improve Cd²⁺ adsorption ability of biochar in aqueous solution, NaOH- and FeCl₃-modified biochars (NBC1, NBC2 and FBC1, FBC2) were prepared based on two kinds of original rice husk-derived biochars (BC1, BC2). Moreover, Cd²⁺ adsorption performances of the six materials were explored by the kinetics and isothermal adsorption experiments. Results show that Cd²⁺ equilibrium adsorption capacities of BC1 and BC2 were 14.76 and 13.72 mg·g^-1 under the condition of Cd²⁺ concentration 50 mg·L^-1 and biochar dosage 1 g·L^-1, respectively. NaOH-modification enhanced the Cd²⁺ adsorption ability of rice husk-derived biochars consistently and significantly, and the equilibrium adsorption capacities of NBC1 and NBC2 were 3.26 and 2.47 times that of BC1 and BC2, respectively. However, FeCl₃-modification had little effect on Cd²⁺ adsorption of the biochars, and the equilibrium adsorption capacities of FBC1 and FBC2 were only 1.03 and 0.74 times that of BC1 and BC2. The process of Cd²⁺ adsorption onto the original and modified rice husk-derived biochars was more consistent with the pseudo-second-order model and the Langmuir model, and it was assumed that the adsorption mainly occurs in monolayers. The maximum adsorption capacities of NBC1 and NBC2 (131.58 and 98.04 mg·g^-1, respectively) were significantly higher than that of FBC1 and FBC2 (30.30 and 23.26 mg·g^-1, respectively). The reason for Cd²⁺ adsorption capacity enhancement of NaOH-modified biochars is that NaOH-modification increased the amount of alkaline oxygen-containing functional groups on the surface of biochars, which enhances biochars' cation exchange and precipitation capacity with Cd²⁺.