The superior adsorption capacity of boron-nitrogen co-doping walnut shell biochar powder for Au(III), Pt(IV), and Pd(II)

In recent years, low-cost and eco-friendly biomass materials have become the focus of research for the purposes of removing poisonous noble metals waste. In particular, the role of walnut shell biomass material in the adsorption of heavy metals has garnered significant attention, an example being the boron-nitrogen co-doped walnut shell biochar powder (B-N-WSBP) which can be synthesized through one-step pyrolysis process. Various methods and analyses, including scanning electron microscope (SEM), transmission electron microscope (TEM), X-ray diffraction (XRD), infrared spectroscopy (FTIR), Raman spectroscopy (Raman), Brunauer-Emmett-Teller specific surface area analysis (BET), X-ray photoelectron spectroscopy (XPS), and thermal gravimetric analysis (TGA), were used to characterize the morphology, structure, and physical properties of prepared B-N-WSBP. In this paper, different experimental conditions affecting adsorption including adsorption dose, solution pH, ionic strength, time, and temperature were investigated. Experimental results showed the B-N-WSBP achieved maximum adsorption rates of Au(III), Pt(IV), and Pd(II) of 246.96 mg g(-1), 108.8 mg g(-1), and 44.78 mg g(-1), respectively. The results further suggested that the adsorption kinetics are combined with a quasi-two-stage kinetic model. The adsorption isotherm equilibrium data were aligned with the Langmuir isotherm model, which indicated that the mechanism of reaction between B-N-WSBP and the noble metal ions belongs to chemical and single-layer adsorption.